1 2 3
fast_eddie_72
fast_eddie_72 SuperDork
4/21/12 10:50 a.m.

This is starting to drive me crazy.

Okay, I'm trying to figure out some basics. So I'll focus on one thing for a minute in hopes of understanding something. Though I suspect they are all inter-related and it won't be so simple. Anyhow...

Roll center. The front of my car is struts and I can't use roll center adjusters, so lowering the car has moved the roll center very low. Not exactly sure how low, but let's assume very, very low, maybe even underground.

Rear is a four link with panhard bar. Lowering the rear has lowered the roll center a little, but not a lot. For now, the bar is not level and the axle isn't in line with the car. Seems I should fix that. So I'm building an adjustable panhard bar, but it seems I should also lower the bracket on the axle to make the bar straight again. Basically the adjustable bar will just be for fine tuning - lowering the bracket will make the length pretty close to stock again. It will also lower the roll center, and it seems almost universally accepted that is a good thing. I see after market kits for popular cars that relocate the panhard bar for lowered cars by lowering the axle side bracket, and hear a lot of autocross discussions about lowering roll center. But, I've read article after article that say stuff like this:

"Generally speaking, a lower roll center will have less traction where a higher roll center has more traction. You can easily see this with foam tires, a place where we never use high roll center since it will make the car traction roll due to the increased traction capabilities of the higher roll centers."

http://losi.com/Articles/Article.aspx?ArticleID=1562

They're talking RC cars, but same physics, right? I've actually found a good deal of suspension basics on RC car sites.

So what gives? Should I lower the bracket on the axle side and lower the roll center or raise it on the chassis side and raise the roll center pretty much back to stock? Like I say, I'm pretty sure the answer is lower, but why so many references to that giving me less grip?

Is the answer, maybe, that they're assuming that you don't change spring rates and so you have a lot more roll and upping spring rates will prevent that to a good degree offsetting the negative side of the lower roll center?

Jesus, there are two dogs barking and my wife is playing some inane video link she got on facebook. I'm not sure if I'm making sense right now.

fast_eddie_72
fast_eddie_72 SuperDork
4/21/12 11:01 a.m.

Okay, now that I can think again-

This has something to do with the distance between the cg and the rc, right? And more distance means more leverage and more weight transfer, right? And more weight transfer means more grip on the tires seeing more weight, right? Or am I missing something?

Thanks to anyone who reads all this and can understand what the hell I think I'm saying.

emodspitfire
emodspitfire Reader
4/22/12 8:05 p.m.

Hey Eddie,

Search "roll center" in the GRM techtips section. Curmudugeon loaded a good sketch for figuring roll center on a Strut car. Lotsa good conversations in this thread.

Cars with live axle and Panhard bars (generally) have the rear roll center measured at the Panhard bar height at the vehicle centerline.

Get a copy of Puhns: "How to make your car handle". Lotsa good stuff for street base performance cars.

See you at the May event.

Rog

DaewooOfDeath
DaewooOfDeath Dork
4/22/12 8:23 p.m.

If you have a SUPER low front roll center, you probably do want to lower the rear pandhard bar so your roll axis inclination doesn't get too steep. That said, is there any reason you are running so low in the front? Strut suspensions with low RCs tend to have positive camber gain in addition to requiring a ton of spring and/or bar.

The rear suspension could actually yield a lot of adjustments. Not only can you move the panhard rod up and down, you can create adjustable brackets to lower or raise the longitudinal control arms on your four link.

This changes the leverage on the chassis. Aiming those arms downward will tend to make the car put down the power better, aiming them upwards will tend to increase power-on oversteer. Think of it in terms of doing wheelies. If the leverage point is aimed at the front tires, that will do a lot of weight transfer to the back. If it's aimed at the back seat, you will spin quicker.

Leverage is simply determined by where the top and bottom control arm vectors would intersect - I think.

BTW, your roll centers.

DaewooOfDeath
DaewooOfDeath Dork
4/22/12 8:36 p.m.
fast_eddie_72 wrote: Okay, now that I can think again- This has something to do with the distance between the cg and the rc, right?

The distance between the roll center and the center of gravity creates a lever arm. The amount of G force times the length of this lever interacts with your suspension springs and swaybars to determine how much body roll you will get. With a very low front roll center (which I would really, really try to avoid) and a tall, iron block engine, you probably have a long lever arm.

This means many bad things.

a) You are going to need a ton of spring or bar to control body roll - assuming you want to stay off the bumpstops.

b) You are going to have a gnarly roll center axis inclination, which will make your car unpredictable.

c) On a strut car, you are probably going to get a lot of positive camber gain on compression.

Basically, the suspension is going to be actively working against you.

And more distance means more leverage and more weight transfer, right?

Nope. No significant difference in the amount of weight transfer. It will just cause roll. Roll is bad not because it causes transfer (it does, but the amount is so tiny we can safely ignore it), but because it makes your car very slow to respond and because it screws your suspension geometry up every time you go round a bend.

And more weight transfer means more grip on the tires seeing more weight, right? Or am I missing something?

Yes and no. Grip is a function of normal force (the force pushing straight down on the tire) and coefficient of friction. So, the more weight you put on a tire, the more grip that individual tire will have. However, coefficient of friction decreases with load.

So, lets say you have a unicycle that weighs 200 lbs and the tire has a coefficient of friction of 1 at that weight. You have a max cornering force of 1 G because the 200 pounds of normal force times coefficient 1 equals 200 lbs of cornering force.

Now, lets say you increase the weight on your unicycle to 300 lbs. At this weight you get a coefficient of .8. This means that you get 40 extra pounds of cornering force, but that extra 40 pounds has to try and turn an extra 100 pounds. You can now only corner at .8 gs.

In general, you want to spread the weight as evenly as possible among your tires for maximum cornering and braking.

You want weight transfer during acceleration in a rwd car to the back wheels because it doesn't matter if you rob the front wheels of their traction in this situation.

Thanks to anyone who reads all this and can understand what the hell I think I'm saying.

No problem at all.

mazdeuce
mazdeuce Reader
4/22/12 9:15 p.m.

Ok, I can calculate my roll centers (strut car) at various heights. I can also do the geometry to figure out at what rate my camber goes away. My question is, how do I optimize this? Somewhere between stock and sitting on the bumps I should have the best answer, right? The best final answer is contact patch optimization, but how do I figure that out?

fast_eddie_72
fast_eddie_72 SuperDork
4/22/12 11:31 p.m.

Thanks for the input guys, some good info for me to follow up on.

I set the front ride hight by watching the control arms. They're almost parallel to the ground, but not quite. Still just a tick of travel before I'll start to gain camber. And I do have very high spring rates and a large sway bar to try to keep it from moving much. My thinking was I wanted to lower the CG as much as I could but not at the expense of positive camber. At the next event I'm going to put some wire ties on my struts to see how much the suspension does travel.

It may not be scientific, but I know when I look at the really fast FSP cars- they're all pretty low, even the ones with struts. But I really will have to get a handle on what my car is doing. I'm pretty much decided on getting some suspension software so I can try to model it all. I really don't understand why SCCA doesn't allow roll center adjusters in Street Prepared. They seem like exactly the kind of mod the class is all about.

In reply to mazdeuce:

Sounds like we're in about the same boat. But I think the answer to your question, at least with regard to camber, is with a tire pyrometer.

DaewooOfDeath
DaewooOfDeath Dork
4/22/12 11:46 p.m.
mazdeuce wrote: Ok, I can calculate my roll centers (strut car) at various heights. I can also do the geometry to figure out at what rate my camber goes away. My question is, how do I optimize this? Somewhere between stock and sitting on the bumps I should have the best answer, right? The best final answer is contact patch optimization, but how do I figure that out?

Optimize ...

What do you want to optimize for? Camber gain? Geometric roll resistance? Roll center?

You can optimize for camber gain by angling the control arms up at as severe an angle you can get away with. This will work great right up to the point where your car starts jacking.

Geometric roll resistance will be best when you have the RC and the COG as close together as possible. This will let you run surprisingly mild springs/bars.

Roll center is best when you have a mild slant down from the back RC to the front RC.

These optimizations probably won't play nicely together.

Lowering your strut car will, in all likelihood, compromise your camber curve, steepen your roll axis of inclination and decrease the geometric anti-roll. These problems are why my strut track car is less than an inch lower than stock - and my car had very high roll centers for a strut equipped sedan.

If you must lower it, and depending on application even if you don't lower it, roll center adjusters might not be a bad idea.

DaewooOfDeath
DaewooOfDeath Dork
4/22/12 11:49 p.m.
fast_eddie_72 wrote: Thanks for the input guys, some good info for me to follow up on. I set the front ride hight by watching the control arms. They're almost parallel to the ground, but not quite. Still just a tick of travel before I'll start to gain camber. And I do have very high spring rates and a large sway bar to try to keep it from moving much. My thinking was I wanted to lower the CG as much as I could but not at the expense of positive camber. At the next event I'm going to put some wire ties on my struts to see how much the suspension does travel. It may not be scientific, but I know when I look at the really fast FSP cars- they're all pretty low, even the ones with struts. But I really will have to get a handle on what *my* car is doing. I'm pretty much decided on getting some suspension software so I can try to model it all. I really don't understand why SCCA doesn't allow roll center adjusters in Street Prepared. They seem like exactly the kind of mod the class is all about. In reply to mazdeuce: Sounds like we're in about the same boat. But I *think* the answer to your question, at least with regard to camber, is with a tire pyrometer.

Something you can do to improve the camber curve is to buy camber plates and go completely negative. Then use crash bolts to stand the spindle back up. This will screw up your roll center slightly more, but the camber should make up for it.

As for swaybars, I've got an interesting but long read if you want it.

mazdeuce
mazdeuce Reader
4/23/12 6:52 a.m.

DoD, I'd love to read it.
What am I trying to optimize? I'm trying to balance contact patch under cornering using fairly sticky tires with overall ride quality ultimately. I understand that I can just spring it stiff enough to limit range of travel and then set camber and be done with it but that's not really what I'm going for. I'm trying to get a handle on the math that lets me make an educated decision about all of this. Lower? Yes, but how much and why? More roll bar? Sure, but what am I aiming for? I know it's not that easy or we'd all be running exactly the same setups, but I trying to learn.

DaewooOfDeath
DaewooOfDeath Dork
4/23/12 1:07 p.m.
mazdeuce wrote: DoD, I'd love to read it. What am I trying to optimize? I'm trying to balance contact patch under cornering using fairly sticky tires with overall ride quality ultimately. I understand that I can just spring it stiff enough to limit range of travel and then set camber and be done with it but that's not really what I'm going for. I'm trying to get a handle on the math that lets me make an educated decision about all of this. Lower? Yes, but how much and why? More roll bar? Sure, but what am I aiming for? I know it's not that easy or we'd all be running exactly the same setups, but I trying to learn.

The first thing I would do in your position is go out and look at how the control arms angle on your Mazda. If it's a typical strut car, they will be near flat or angled slightly up from the spindle to the chassis. Then calculate your roll center and take a guess for your COG.

http://robrobinette.com/cg_height_calc.htm

Here is where I'm going to depart from most in my suspension tuning preferences. I like high roll centers on struts. I like them because they build a lot of geometric anti-roll into the system and because they tend to result in a good camber curve.

So, what I would do if I were you is bump up the spring rates to the most you are comfortable with, keep the bars stock and lower it very little. If you have a low roll center from the factory, or if you want to lower it more than an inch or so, I would recommend roll center adjusters.

DaewooOfDeath
DaewooOfDeath Dork
4/23/12 1:13 p.m.

Here's part one, taken from the MR2 Owners Forum. According to Steve, you can basically turn these calculations around backwards for a FWD car.

We’ll take this a step at a time:

Ok, so its a pure competition car. No/little compromise. So the starting point is a target "total roll". As a general rule a strut car doesn't react well to body roll due to camber issues and lateral roll center movement. A target of 1.5~1.75 degrees total roll is typical. Total roll is a function of grip, wheel rate, moment arm length and roll stiffness. Keeping the car flat also minimized the severe bump steer issue the Mk1 chassis has.

We'll start with grip. Current generation STS tires are going to generate something in the 1.1g of grip, vs 1.3+ of say a Hoosier A6. So the STS car will require slightly less roll stiffness than a CSP car to achieve the same total roll. We need some roll to give the driver feedback and prevent the outside tires from being overloaded on turn-in and transition. Shocks will play a key roll here.

Next is ride height because that determines the length of the moment arm (the distance between the roll axis and CG). The CG acts through the moment arm (like a lever) to roll the car about the roll axis. The roll axis is the line drawn through the front and rear roll centers. The CG height is basically fixed in relation to the body but the roll axis is a function of the control arm and strut angle and therefore ride height. The roll axis height drops at a significantly higher rate than ride height. So we have to be careful about how low the car goes.

Before we can choose spring/bar rates we need to set the ride height to determine roll axis height and therefore moment arm length. Ideally, we want the roll axis to be 1 to 2 inches above ground with the rear at about 1" and the front at about 2". Since we won't be modeling the car in software, our target is to simply have the roll centers above ground level and that the front be above the rear so the roll axis is reclined toward the rear of the car.

So our starting point is to deal with the things that we can’t really change easily. Spring/bar rates are free so we can choose rates that work with the rest of the setup. Ride height is basically free so we can raise/lower the car to our advantage. What we can’t change as per the STS rules is the control arm geometry. So we need to set the control arm angle to optimize roll axis location and camber curve. In reality, its not control arm angle but instead the angle of the line between the inner control arm pivot bolt and the center of the outer ball joint. You can clearly see the front ball joint pivot is well above the centerline of the control arm. Same for the rear but somewhat less offset.

To achieve our previously stated geometry, we want the front virtual control arm (the line between the inner pivot and the ball joint center) to be roughly parallel to the ground. In the rear, the ball joint should be slightly above (maybe ¼”) the inner pivot. Unfortunately, this is likely to set the nose of the car noticeably higher than the rear. This may be adjusted later when we know the severity of the body’s rake. Actual ride height and roll center location will be effected by tire diameter. This virtual control arm angle is about the best compromise location to optimize the camber curve assuming less than 2 degrees of body roll. Its doesn’t address the bump steer problem but that will be addressed later.

-Steve Hoelscher

Part Two

Now that we have control arm angle (roll axis) set, we will address wheel rates. Realize that spring rates and wheel rates are related but not equal. The wheel rate is the result of the wheel’s mechanical advantage over the spring. This is expressed as a linkage ratio. As an example, the linkage ratio of a typical strut suspension is about 1.1:1. That is, the wheel moves 1.1” for every 1” of strut movement. Now this changes as the suspension moves through its range of travel and typically the mechanical advantage is at its maximum at full droop and goes down as the suspension compresses. While this is a generalization, it is basically the nature of the Mk1’s suspension. Its also important to remember that front and rear suspensions seldom have the same linkage ratio.

Its also important that we understand that a wheel rate includes the action of a swaybar if fitted. Swaybar rates are figured much the same. The bar’s wheel rate is the bar’s spring rate multiplied by the linkage ratio. Most Mk1 swaybars connect to the strut, so the linkage ratio is the same as that of the spring. And finally, the wheel rate is the wheel rate of the spring plus the wheel rate of the bar.

Now we return to the first installment and consider total roll rate. We targeted ~1.5 degrees of total roll. Now we need to figure a total roll rate that yields that amount of roll with the given roll axis height. Total roll rate is the total of the front and rear wheel rates. Front wheel rate + rear wheel rate = total roll rate. So how do we determine a total roll rate? I have developed my own basic starting point that is simple and effective. I have proven this method to work well for me with many different cars in both autocross and road racing. As noted previously, a favorite assumption of many people is to choose spring rates that equal corner weights. The problem with this method is it ignores actual vehicle dynamics. Most such setups would include a front (and sometimes a rear) swaybar. The front swaybar, if sized accordingly could provide enough additional front roll stiffness to give adequate handling balance. For our purposes here, we don’t care if its swaybar, spring rate or a combination of both, we are only concerned with the resulting wheel rate. Later we will decide on the split between spring and bar rates.

Back to determining total roll rates. I noted I had a simple formula for a starting point. That formula is: ½ the total vehicle weight divided by the inverse of the weight distribution. An example for a 2200 lbs car with 44/56 front to rear weight distribution (a rough estimate of your car's weight and distribution):

2200 / 2 = 1100 lbs

1100 * .56 = 616 lbs/in front wheel rate

1100 * .44 = 484 lbs/in rear wheel rate

The reason for inversing the weight ratio is to offset the car’s rear weight bias. If each end of the vehicle had the exact same level of grip, the rear would break away first because the higher weight would overcome the available traction earlier. Therefore a rear weight bias car will naturally oversteer therefore more front roll stiffness is necessary to counteract that natural tendency. How much front roll bias varies but all things being equal (and they never are) basing the offset in roll ratio on the inverse of the weight bias gets you pretty darned close.

Now you are probably stunned at the high wheel rates, but we aren’t through yet. First, remember the second paragraph regarding grip? STS tires don’t generate the same level of grip that A6 Hoosier do. As a result, lower spring rates are necessary to produce the desired amount of body roll. The difference in G loading is about 20~25%, so lets reduce our wheel rates by that amount

616 front * .8 = 492.8 lbs/in

484 rear * .8 = 387.2 lbs/in

To round off the rates: 500 lbs/in front and 400 lbs/in rear.

Now lets apply these to the MR2 chassis and its dynamics. Experience and testing are valuable here and I can apply my experience and make a couple of assumptions. First, that the low roll centers resulting from the current ride height and the high CG result in a long moment arm and a lot of body roll for a given wheel rate. Second, that the front has a longer moment arm than the rear. So we need a little more front roll stiffness and a little more total. Therefore we will bump the rates up slightly, especially in the front. I would think that using the wheel rates as spring rates would be enough of a bump and then add say 50 lbs/in to the front. The resulting spring rates:

550 lbs/in front

400 lbs/in rear.

Next we will address swaybars.

-Steve

Think about it this way. A mid/rear engined car is a fwd car turned around backwards. On a fwd car, they run softer front springs and bars and REALLY stiff rear springs and bars to get something like a normal handling balance. AND, to reduce inside front wheelspin on corner exit.

So if a mid/rear engined car is a fwd car backwards, why not just turn the fwd setup round backwards and put it on the mid/rear engined car?

Or think about it this way. Control body roll with the end opposite the weight/drivewheels. This creates mechanical grip on the drive axle. Now it doesn't matter which end has the drivewheels/weight.

Funny how sometimes the most obvious things aren't that obvious because its so far outside our existing pool of knowledge.

What I have posted above is basically the process I sat down and put together back in 1994 when I realized I wasn't making any progress using the "conventional" tuning methods (softer springs, big swaybars and stiff shocks). I derived and refinded this method over the following couple of seasons. I even put together a spreadsheet that would let me look at the entire setup on one page. Then I could change a spring rate or swaybar rate and see how that change rippled through the entire setup.

It was an eye opening experience for me. I could try lots of different combinations on paper, pick one to test and note the change in feel and performance. Then compare that to the spread sheet. It didn't take long to correlate results with changes and then make changes and predict results. Accurately. A very powerful too.

I now have a much more sophisticated method in use on the DP car. But then Prepared rules give you so much freedom you need a more sophisticated process. But my simple Lotus 123 spreadsheet was a revelation at the time.

The car was transformed in the space of a single season. After floundering around in the middle of the trophies for my first half dozen trips to Nationals, this process put me on the path that lead directly to my first Championship in '98. Now I don't credit this for my first Championship but it certainly put me in a position to win. Car setup isn't a substitute for learning to win. But you can't win if the car isn't there.

-Steve

Now that we have a starting point for individual wheel rates we can now decided how we want to divide the wheel rate between spring rate and swaybar rate. To make an informed decision we must understand the function of the swaybar and how it interacts with the springs. A swaybar is nothing more than a torsion bar (spring) that has either end attached to each wheel of a single axle. It’s the twisting action of the bar that is the torsion spring. Because it only applies when a one wheel of the axle moves independently of the other, it has only moderate effect on ride quality.

To understand the swaybar’s effect on the springs and chassis we must understand how the springs interact with the chassis as well. We already understand how they affect body roll but they also effect ride quality and that is what we will now address. There are three basic principles that apply. First is ride frequency. Ride frequency is the rate at which the chassis reacts to input, a bump in the road. Ride frequency is expressed as Hz. A soft ride frequency would be about 1 Hz or one cycle per second. A cycle being the car passing over a bump, the chassis reacting and then returning to its original state. A stiff ride would be a frequency of about 2 Hz and a very stiff ride would be 2.5 to 3 Hz. We could easily calculate ride frequency based on the sprung weight of the car (total weight carried by each axle – unsprung weight of each axle = sprung weight) and the wheel rate of the springs on each axle. However, for our exercise here its only necessary that we understand the concept. The final part of the concept is that each axle has its own ride frequency based on the sprung weight and wheel rate of each axle.

Ride frequency is used to determine the second principle favored speed. Favored speed is the road speed at which the two ends of the car return to their original state after the car passes over a bump. To achieve a positive favored speed (a speed greater than zero, and yes you can have negative favored speeds) the front ride frequency must be lower than the rear so that the front returns at the same time the rear does for a given speed. Achieving this effect will produce the best ride quality for that speed and desired stiffness. The favored speed can be set at any target speed by tuning the front and rear spring rates to achieve the ride frequencies that produce the effect at the target speed. This effect works for both stiff and soft rides at most any speed and is why your stiffly sprung car feels smoother as speed increases. Most manufacturers set the favored speed using spring rate to achieve a smooth ride at their desired target speed (usually between 50 and 70 mph), then use swaybars to tune the ultimate handling balance and body roll.

The final principle we need to understand is center of suspension. Center of Suspension (CoS) is the point on the chassis at which, if weight was applied, both ends of the car would compress exactly the same amount. As an example, if the front and rear springs had exactly the same wheel rate then you could push down exactly half way between the two axles and the front and rear would compress the exact same amount. If the front springs were softer than the rear, the center of suspension point would be moved rearward to accommodate the softer front spring rate. The weight of the car then acts through the center of suspension via the Center of Gravity (CG). The difference in the car’s CG and Center of Suspension is a moment arm (which is basically a lever) just as the difference in CG and roll center. So the CG acts through the moment arm to compress the suspension in reaction to a bump. If the CG is behind the CoS, then the rear suspension is compressed more for a given load. The inverse is also true, if the CG is ahead of the CoS, the front suspension is compressed more. And like body roll, the longer the moment arm (the greater the distance between the CoS and the CG), the more the suspension on that end is compressed for a given load.

To minimize excessive body movement in response to bumps, we want the center of suspension as close to the CG as possible. This minimizes the length of the moment arm and therefore the CG's leverage over the CoS. For the Mk1, the CG is near the rear axle so we would have to have front springs that were much softer than the rear to achieve a center of suspension anywhere near the CG. Then to achieve good ride quality we set a desired favored speed and then tune the spring rates to suite. For the Mk1, setting a favored speed of say 50 mph, the front springs would have to be slightly softer, or the rear springs slightly stiffer, that that which would locate the center of suspension exactly at the CG.

This is the method the manufacturers use to tune the ride quality and handling balance of their cars. When we consider what we learned about roll ratios in the earlier installments its easy to see why the manufacturers use a hefty swaybar and soft springs on the front of the Mk1 (or most cars for that matter). Now that we understand how and why Toyota setup the stock Mk1 we can determine where we need to go to improve the car. Clearly we are not as concerned with ride quality because we are setting up a competition car. However, it is necessary that the car be able to track smoothly over bumps so as to not upset the car enough to loose traction.

-Steve

Part Five

Now that we understand the function of the springs and how they interact with the body to determine ride quality and body roll we can now directly address the swaybars. To achieve our target body roll and roll ratio we have chosen specific wheel rates. However, the rates we have chosen would result in a very stiff ride if we used only springs to achieve that wheel rate. To soften the ride we need to lower the spring rates, especially in the front (to achieve a positive favored speed) however, that will upset our chosen roll ratio so we add back wheel rate by supplementing spring rate with swaybar rate.

From installment #2 we had chosen 550 lbs/in front and 400 lbs/in rear spring rates. Its easy to now choose a front spring rate that will produce a more comfortable ride. Say 300 lbs/in front springs would give us a positive favored speed and move the center of suspension closer to the CG of the car, just aft of the center point between the two axles. We can then add back the spring rate by installing a swaybar with a rate of 250 lbs/in.

250 lbs/in front bar + 300 lbs/in spring = total rate of 550 lbs/in.

That looks pretty easy. However, its hard to find an off the shelf swaybar that has exactly the rate you want. In reality, unless you want to fabricate a custom bar every time you want to test a different setup, you need to calculate the rates of the available bars and then determine how much spring rate you need to achieve the desired total rate. So lets say you measure your existing swaybar and find it has a rate of 200 lbs/in. The spring rate you would want is determined by subtracting the existing bar rate from the target spring rate.

550 lbs/in target rate – 200 lbs/in bar = 350 lbs/in springs.

Easy enough. But we have a problem here. Swaybars are not the dynamic equivalent of springs. A swaybar transfers load from the inside tire to the outside tire and thus reduce mechanical grip as they add spring rate. And the effect is not linear. The stiffer the bar in comparison to the springs, the greater the effect (loss of mechanical grip). To give an example of the effect, if we set our proposed STS2 car up using the target data we have assumed above using the target spring rates without any swaybar, the car should have good balance. However, if we achieved that same target spring rate using a front swaybar, the car would tend to understeer more than if we used only springs and no swaybar. And the greater percentage of the total front spring rate the bar accounted for, the more the car would understeer. I noted as much early in the thread.

We now must choose how much bar we want to use for our final setup. As the reader may know, I don’t use swaybars on my DP car. Nor did I use swaybars on my previous racecar, a DSP X1/9. For me it is far easier to manage the setup of the car without swaybars. I also prefer the feel of the car without swaybars. It has long been my thought that; because a swaybar reduces mechanical grip, why would you want to put anything on the car that reduces mechanical grip?

With this simple method, it is easy to compare the effect of the front bar by comparing the same total spring rate using just springs and no front bar to the same total spring rate incorporating a front bar. I have done extensive testing and have proven to my own satisfaction that the theory is in fact accurate. The same total front spring rate achieved using a front swaybar will produce more understeer than the same total front spring rate achieved using springs only. In addition, the effect of the front bar changes based on the level of grip the surface offers. As a result, the car does not have consistent balance from surface to surface or even from run to run as the tires heat up from and the surface cleans and heat up throughout the day. I have found that my no-swaybar setup is very consistent on different surfaces and conditions seldom if ever requiring any changes to setup. At most, a pound or two of air pressure is all that is needed to tune the balance even in the most extreme of conditions. In fact, I don’t even change the setup for rain. All I have to do is bolt on the rain tires and the car is fine.

If one chooses to use a front swaybar, the effect resulting from the loss of mechanical grip will have to be accounted for by softening the front springs enough to bring the balance back to neutral. Choosing the amount of swaybar to use is now easy and dependant on driver taste. If the driver prefers a smoother/softer ride, use a very stiff front swaybar and subtract the front bar rate from the total spring rate to determine the required front spring rate. Testing can then determine how much less spring rate is necessary to bring the handling balance back to neutral. One could also compromise and use a very soft front bar, thus minimizing the loss of mechanical grip.

-Steve

Part Six

Remember this number: 150%

So now is easy to see why one might choose, as I have, not to use any swaybars at all. I have spent more than 10 years working on this setup theory and testing the various permutations. Somebody else may not like the setup but I know its effective. The above method is exactly how I got to the point I am now. I came to this method by first using a spread sheet to aid in computing wheel rates and roll ratios accurately. I have since transitioned to a more scientific application of this theory using Susprog3d. I can now figure total roll based on lateral grip, ride height, CG height, roll axis and spring rates very accurately. I can also figure camber curves and roll center movement dynamically. A very powerful tool. But there is no substitute for testing and it is through testing that I have tailored my setup to suite my personal tastes. Now I will throw out much of what we have covered to this point.

I prefer a car that has slight understeer and is VERY stable in transition. I DO NOT want the car to feel loose in a slalom or fast transition. I know that a car with a significant rear weight bias, that is loose in transition, is slow. So my car has a lot of front roll stiffness to achieve my preference. So lets throw out all of the theory above and look at some simple data gathered from testing. Pulling off the swaybars and just sticking springs under the car, what works?

I decided I needed an objective comparison. I wondered if I could I derive a better, faster setup by pure testing alone, ignoring my theory. A number of test days at my favorite venue (Hunt Stage Field in Ozark, Alabama)

http://maps.google.com/maps?f=q&hl=...052872&t=k&z=14

where Wire Grass Region events are run and you can make all of the runs you want, when you want, enabled a lot of valuable data to be gathered. The basics can’t be ignored so the ride height must be set so the roll centers are close, as noted previously, and the camber curves have to be in a reasonable range. So those two elements determine ride height. What about springs? We need the car flat and we determined a total roll based on the weight of the car and potential grip. From installment #2:

2200 lbs car / 2 = 1100 lbs of total roll rate

So pulling data from my test notes. We need 1100 lbs of total roll stiffness less 20% for STS legal tires.

1100 x 80% = 880 lbs

So out came the box of springs and I started making runs. I have a spring inventory that covers a range from 300 lbs/in to 700 lbs/in in 50 lbs increments. I quickly start narrowing down the spring rate combination that produces the best balance. Once the spring rates are close, I start on roll center locations, ride height and camber curves. Then back to spring changes to further refine the balance and confirm the data. Final balance and dynamics are tuned with toe settings and tire pressure.

The testing results tell me that, for most mid-engined cars, making the front roll rate 150% of the rear is a good target. Therefore, given the above data:

350 lbs/in rear springs x 150% = 525 lbs front springs.

and

350 lbs/in rear springs + 525 lbs/in front springs = 875 lbs of total roll rate.

Close enough.

The next installment will cover shocks and dynamics.

-Steve

Tangent 1

Feel? Feel is subjective, its the clock is not.

One of the things you will have to come to terms with is how the car will feel. if you are used to stock class cars, or cars with big swaybars and soft springs you are in for something of a shock the first time you drive the car this way. While I can't speak to the feel of the Bridgestones I can for the Hoosier radials.

I don't know what you have been driving but the first time out with this kind of setup might be a shock. The amount of feedback is likely to overwhelm you. You are likely to be able to feel every tiny imperfection in the pavement. Every seam, crack, dip, gap, etc... I used to joke that I could run over a dime and tell if it was heads or tails.

It may take a few runs but you should adapt as you get acustomed to the sensation. Also, the car should feel more lively and precise. Turn-in should be improved along with the car's response to steering wheel inputs.

If you have any questions before then ask them now as I will be in Daytona for the Grand Am race Wednesday and Thursday. And if you are watching (Thursday night at 8:00pm eastern on Speed Channel) pull for the #26 GT Porsche of Gotham Competition.

I will eagerly await your report.

-Steve

Yes, swaybars should be used as a trim device only. Never the primary means of roll control. Most modern, purpose built, racecars use very light swaybars. Most are adjustable and are either on or off.

They are used as a method of adjustment to correct for changes in fuel load, tire wear and surface changes.

-Steve

I spent a lot of time working on this methodology. Figuring out all of the interactions and putting the elements together into a functioning package took a lot of work on the computer and testing.

When I first put a setup like this on my DSP car (back in '94) I got lots of funny looks and lots of people telling me it wouldn't work. That it couldn't work. Often by people I was beating.

Back then, the conventional wisdom was: Springs should be as soft as possible, that they only held the car up. It was the shocks and bars that were used to control body roll. That just didn't make sense to me. But, because I assumed I didn't know as much as the experienced guys (and I certainly didn't at the time) I put a big f'ing front swaybar on my car. It was awful. Then I put an even bigger front bar at the advise of a respected engineer. It was worse.

On the way home from Nationals that September, I decided I needed to better understand chassis tuning. A month later, I tested my first "all spring" setup just as a baseline to try and understand what "just springs" did and then add swaybars back into the equasion. That event was a revelation and I never put the bars back on. And never looked back.

-Steve

I loved that car (Fiat X19). It was a truely great racecar. My theories on the barless setup were developed, tested, refined and proven on that car.

That car won more than 30 National Tours, 4 National Championships and every single event it entered from the season opener of 1998 to Nationals of 2001 where I finished 2nd to Mark Daddio on that fatefull day of 9/11/01 when the event was stopped after day 1 of DSP (we ran 1st heat Tues). It then won every event entered though Nationals '03 where it finished 5th and was retired. So, from the beginning of the '98 season to Nationals of '03 the car won every event entered except the '01 Nationals where it finished 2nd. That is one of the best records in the history of the sport. Note that during that period of time DSP was one of the largest classes with more than 40 cars turning out at Nationals each year.

It was the ground work done on that car that is now being successfully applied to my DP Mk1 and verno-dub and micaceli's Mk1s.

I should also note that there are a few clubracers successfully running with my barless setups.

Just for fun, here is my DSP X1/9 from the '03 Nationals:

Part Seven

The more dependant the setup is on swaybars for tuning balance, the more changing conditions effect the car's balance and response.

A well balanced, all spring setup, with properly tuned shocks, will be very consistant from surface to surface and as conditions change.

One of the things I see regularly is SP and Prepared guys at big events diving under the car after each run changing the swaybar adjustments, shock settings, tire pressures, etc... trying to dial the car in. You will likely NEVER see me do that to my cars unless its a test session and I am working on testing setup changes. That's because my car is so consistant from surface to surface, hot, cold, sunny, rain, etc...

There's more, but it's about tuning around an LSD and shocks. Basically the same stuff Mike preaches, except Steve prefers to go without an LSD if at all possible.

Oh, almost forgot this.

This works equally well on full road courses. The one thing to keep in mind is that on a full road course, the shocks play a much bigger role in controlling body motion. You might find that because the shocks are at the limits of their capabilities that they may have trouble controlling body motion on the higher speed bumps.

I have run my autox setup on full road courses many times and found it to work just as well there at in autox. I have also used this setup on a number of track and road racing cars. In fact, the 2nd place finisher in H Prod at last fall's "Runoffs" was running my no bar setup.

I would recommend doing the 600f/350r version.

Trust me guys, it won't push. It really won't. You might find slightly more understeer in very high speed sweepers taken at full throttle. But that can be tuned out with toe settings.

-Steve

Matt B
Matt B Dork
4/23/12 2:02 p.m.

That's awesome you're also familiar with Steve's no-swaybar setup. I remember not-so-patiently waiting for him to post the next installment back when he wrote those on mr2oc.com. I've tried to talk to various racers about his theory, but I still get weird looks when I talk about not using sway-bars at all.

mazdeuce
mazdeuce Reader
4/23/12 3:29 p.m.

DoD, thanks a lot for that. I'm copying and printing it so I can read and ponder and doodle in the margins. Much appreciated.

DaewooOfDeath
DaewooOfDeath Dork
4/23/12 6:52 p.m.
Matt B wrote: That's awesome you're also familiar with Steve's no-swaybar setup. I remember not-so-patiently waiting for him to post the next installment back when he wrote those on mr2oc.com. I've tried to talk to various racers about his theory, but I still get weird looks when I talk about not using sway-bars at all.

It's funny how many times the guys I out-corner by A LOT at every turn tell me I'm doing it wrong.

Have you tried it on any of your cars? If so, I'd really like to compare notes.

DaewooOfDeath
DaewooOfDeath Dork
4/23/12 6:53 p.m.
mazdeuce wrote: DoD, thanks a lot for that. I'm copying and printing it so I can read and ponder and doodle in the margins. Much appreciated.

My pleasure.

Matt B
Matt B Dork
4/26/12 11:19 a.m.
DaewooOfDeath wrote: It's funny how many times the guys I out-corner by A LOT at every turn tell me I'm doing it wrong. Have you tried it on any of your cars? If so, I'd really like to compare notes.

Not yet, but I'm getting really close to making the plunge, perhaps this summer. My MR2 has been my daily driver for six years, so I haven't really wanted 750lb/in springs on the front if you know what I mean. I think we'll probably pick up an extra car this summer and I have the GC setup just waiting, just need a couple more springs (the REALLY stiff ones).

DaewooOfDeath
DaewooOfDeath Dork
4/26/12 4:49 p.m.

It really improved my car. Compared to stock, the steering feel got a hundred times better, the levels of grip went through the roof, the balance got better and the tire wear went way, way down.

The only downside is that if you hit a speedbump at the wrong speed, it will happily launch your rear seat passengers into the roof. Ask me how I know ...

kevlarcorolla
kevlarcorolla Reader
4/26/12 7:50 p.m.

I played around alot with bars/no bars on my composite bodied AE86 autox car,for sure bars were slower.The scratch built car that followed it was designed with no bars and it works pretty well,no intention of adding the weight of a bar to it.

Friend of mine went barless on his gze mr2 converted to turbo car with off the shelf koni's,my suggestion would be to spend the extra loot for valved dampers to match.His was pretty bouncy.

fast_eddie_72
fast_eddie_72 SuperDork
4/26/12 7:57 p.m.

Holy cow, did I ever miss a few important posts here!

Thanks everyone, especially DOD. Man. Lot of reading to do here. Hope I understand it all.

Curmudgeon
Curmudgeon MegaDork
4/26/12 8:34 p.m.

Before you go completely bonkers, you need to have a tuning tool. That tool is a pyrometer. I have a Longacre memory pyrometer, it's a wunnerful thing, but you can do the same thing with an infrared non contact pyrometer and a notepad. Use a sheet like this:

From personal experience running them back to back, here's the deal: the memory pyrometer tells you how hot the tire is under the tread. That's a good thing; you need to know how hot the tires are, if one is way hotter than the rest you need to investigate why.

The other thing you need to know is how hot the tire is at different points across the tread. That will tell you a lot; if the tire is way hotter on the outside than the inside, you don't have enough camber. If it's hotter inside than outside, the opposite is true. If the tire is hotter in the middle than the edges, the tire pressure is too high.

The Longacre will tell me all that, but the non contact gives me the same basic information, only it shows ~10 degrees cooler. It still shows what part of the tire is hottest. And it's a lot cheaper to start out with. Once you get to where you really need to know exactly how hot the tires are, grab a contact pyrometer.

Now you have a tuning tool that can help tell you what you need to do to optimize grip. Once you dial in your camber and tire pressures, then you start worrying about roll centers.

fast_eddie_72
fast_eddie_72 SuperDork
4/26/12 8:42 p.m.
Curmudgeon wrote: Before you go completely bonkers, you need to have a tuning tool. That tool is a pyrometer.

Another really useful post! I actually figured this out myself and got a pretty nice tire pyrometer.

But thank you for the great thoughts about using it right, and I love that sheet! I'll print it and grab a clip board.

Thanks!!

Curmudgeon
Curmudgeon MegaDork
4/26/12 9:01 p.m.

Well, hell. You are all set, then.

fast_eddie_72
fast_eddie_72 SuperDork
4/26/12 10:02 p.m.
Curmudgeon wrote: Well, hell. You are all set, then.

Scratching the surface, I'm afraid. But working on it!

DaewooOfDeath
DaewooOfDeath Dork
4/27/12 10:57 a.m.
kevlarcorolla wrote: I played around alot with bars/no bars on my composite bodied AE86 autox car,for sure bars were slower.The scratch built car that followed it was designed with no bars and it works pretty well,no intention of adding the weight of a bar to it. Friend of mine went barless on his gze mr2 converted to turbo car with off the shelf koni's,my suggestion would be to spend the extra loot for valved dampers to match.His was pretty bouncy.

I read your stuff over on the MR2 forums. Didn't know you went barless as well.

I guess that makes three known cases, Steve's mid engined car, your FR car, my FF car.

1 2 3
Our Preferred Partners
h38SpYej7KtzfgX1hXNBqSOhI6aEUlPCb7iV826UApnCZiXd6kXs5upS3XZhmZQa