Hasbro
Hasbro HalfDork
11/8/08 1:07 p.m.

Is that the golden number? I know there are lots of variables but could a fwd with a lower front weight - say 55/45 - be more optimum for cornering if other factors such as wider tires or driver style were introduced into the equation or is 60/40 the ideal to start from? The book I'm reading doesn't mention fwd. I'm referring to a track as opposed to autox.

iceracer
iceracer Reader
11/8/08 6:46 p.m.

As always a 50/50 is ideal. With FWD the 60/40 is normal since on most the engine is in front of the wheels. This induces natural under steer. There are things that can be done to help the problem.

Hasbro
Hasbro HalfDork
11/8/08 7:11 p.m.

Understood.

I was just wondering what anyone thought of going closer to the ideal - say 55/45 as opposed to 60/40 - and using larger front tires and a soft pedal through the turns. I had a great older Honda (78) that was somewhere around 55/45 and my EP3 is looking close those numbers If I pursue it enough and would weigh 23XX lbs, which is very light for an EP. Wouldn't this allow me to hit the corners faster and brake later? I'm looking at a lot of other things such as DC5R lcas etc. that would increase the track by almost 2" without wacking out the scrub radius and increasing castor/camber adjustability. Everything that has held the EP3 back (except for suspension travel). I believe the lighter weight (1) and better suspension (2) setup will get it that much closer to an early civic in the corners.

AngryCorvair
AngryCorvair Dork
11/9/08 10:46 a.m.

the answer, of course, is "it depends." there are many factors, besides weight distribution, that define the understeer/oversteer nature of a vehicle (aka the "understeer gradient" or USG).

in general, it is the non-linear nature of the friction-versus-weight behavior of automotive tires which allows us to change the handling of a vehicle with things like (fore-aft) weight placement and (side-side) anti-roll bars. for any total vehicle weight, maximum cornering force can be generated by four equally-loaded tires -- if the tires are all the same size. uneven weight distribution can be balanced with different tire sizes, different inflation pressures, alignment specifications, etc. i don't believe there's a magic weight distribution number that will automatically be better than another, because everything can be balanced with proper tire / wheel / spring / bar selection.

as far as being able to brake later, that's purely a function of how sticky your tires are and the balance of your brake system at the max decel that your tires will support.

are you in a professional field where your employer might pay for you to take a course from the Society of Automotive Engineers? i'm an assistant instructor for the SAE's "Applied Vehicle Dynamics" course, and in this class we spend three days alternating between classroom lectures and on-track exercises, to bridge the gap between textbook and track.

iceracer
iceracer Reader
11/9/08 11:43 a.m.

It is an experimental thing. There are so many factors that weight distribution is low on the list. It depends on how the car handles. One thing, less weight on the drive wheels reduces traction.
From experience with my ZX2-SR which is also a DD, a much stiffer rear sway bar and extra camber and caster on the front along with wider wheels and sticky tires makes a very good handling car.

Hasbro
Hasbro HalfDork
11/9/08 6:03 p.m.

Ok, thanks, Ice and Angry. I just wanted to take it a bit further to see if I was missing something. By (1)lowering the cg and weight, (2)improving castor/camber with a jdm DC5R lca and a couple of other things the major weakness left on the EP3 will be suspension travel. Some cars across the ponds have moved the connecting points and have better geometry/travel so it's been done but not sure about the details or if it's worth it.

Anyway, it looks as if it will come in around 57/43 which is a 5% shift to the rear.

Side note. Nothing to do with the topic but while I'm on here- The EP3 Civic Si (4.76 final drive) has an EPA of 22/28 mpg. I used to get around 32-34 but with some hypermiling techniques the last tank average was 42.6 and no highway! It makes driving on the street a lot more interesting.

Jay
Jay HalfDork
11/9/08 6:28 p.m.

My Elan is something like 64/36 but you'd never know it to drive it. A well-designed suspention can overcome 90% of the drawbacks in such a noseheavy design.

J

EPcivic
EPcivic New Reader
11/9/08 9:25 p.m.

The ideal weight distribution depends on a lot of factors. A lot of the decision depends on how much tire you have to work with. If you are using very limited grip tires, you may want to have more rear bias to get higher cornering speeds. The downside is it reduces acceleration capability, which IMHO is more important. If you can get enough tire, I think that the ideal bias is as much as possible on the front of the car. When I say enough tire, I mean a tire with enough grip to lift the inside front wheel. If you have enough grip to reach the physical limitations of the mass and width of the car, then you are just giving up acceleration by having more rear bias.

As an example, my 86 Civic EP racecar can corner at around 1.7g. This is the point where the whole inside lifts off the ground. That is the physical limit of the car's CG and width, but I have enough tire to genrate that much force on a good surface. I have intentionally moved as much weight as practical to the front of the car to reduce the polar moment and increase the acceleration traction. I am at around 70 / 30 F/R. In my case, the limiting factor is not the tire's ability to generate cornering force, it is the roll stability of the physics of the chassis. Even if the tires do saturate before the physical limit, they may still pick up more acceleration ability than they lose cornering ability as you add weight to the front.

If you can move weight around easily, I'd recommmend trying a few setups and see which is faster.

-Chris

Osterkraut
Osterkraut HalfDork
11/9/08 9:44 p.m.
iceracer wrote: As always a 50/50 is ideal. With FWD the 60/40 is normal since on most the engine is in front of the wheels. This induces natural under steer. There are things that can be done to help the problem.

The 50/50 thing being perfect is a tad incorrect. Look at the weight balances of racecars: more like 46/54.

GameboyRMH
GameboyRMH Dork
11/10/08 8:01 a.m.

Yeah having perfect weight balance or handling balance (front/rear bias) (edit: well really more handling balance...the weight balance itself will only become an issue if the handling balance is already near perfectly neutral) will cause the car to lose traction in a sudden and unpredictable manner. The closer the car is to perfect the more unpredictable it will be. I've heard that it's a good idea to keep a good 4-6% margin away from the "perfect zone"

Tyler H
Tyler H Dork
11/10/08 9:04 a.m.

If 50/50 is 'perfect,' should that weight distribution be achieved at full braking, full acceleration or sitting still?

GameboyRMH
GameboyRMH Dork
11/10/08 9:14 a.m.
Tyler H wrote: If 50/50 is 'perfect,' should that weight distribution be achieved at full braking, full acceleration or sitting still?

Sitting still.

iceracer
iceracer Reader
11/10/08 12:51 p.m.

As a for instance, NASCAR racers shift their weight distribution depending on the race track and not just side to side.

Per Schroeder
Per Schroeder Technical Editor/Advertising Director
11/10/08 2:41 p.m.

And interestingly, a few engineers I've talked to say that it's more important to get the weight on the front tires equal....rather than the cross weights. From there, you also don't have to worry about moving a bunch of weight from the front to the rear...moving the weight further towards the rear than the car's CG will actually increase the car's polar moment...making it turn slower.

Hasbro
Hasbro HalfDork
11/10/08 3:00 p.m.

^ Ahh, jeeez! My head hurts. The whole field is Mathematics and the Black Arts.

This is an excerpt from a similar thread I placed on another forum:

Thing is, drive train layout only comes in to the equation on corner exits. and no matter what you do with weight distribution a FWD car is still going to tend to understeer once you get back on the throttle coming out of a turn.

BTW, re read the section on tires and how they make traction. you've grasped the idea that to a tire weight = traction, but haven't seemed to get the concept of diminishing returns from weight. As in when weight is applied to a tire that tires traction output increases, but at a decreasing rate.

so, even though a 60/40 weight distribution would appear to give more traction on the nose, it in fact actually means a greater net reduction in traction overall. So give the same vehicle weight a car balanced at 50/50 is going to have more traction available to it for cornering then a car with 60/40.

So, in a perfect world you'd want a cars weight distribution to be 50/50 regardless of drive train layout. Problem is, there's no way your going to get that with a FWD car with out adding weight, which is counter productive. And in the end on a FWD car, your best bet is to simply reduce weight overall as much as possible and then tune the suspension to work with what ever weight distribution it ends up with. which as my friend puts it... means that your civic will have all the weight distribution of a lawn dart. But ultimately the car will always tend to understeer at its limits, what those limits are will be determined by overall weight & what tires you run among other things.

Jensenman
Jensenman SuperDork
11/10/08 3:12 p.m.

I dunno about all the mathematical stuff, but a guy here in SC runs a VERY fast FSP Rabbit. The car's a featherweight, the weight distribution is way nose heavy to begin with and he's a light guy, so he has gone to the 255/40-13 A6's with very good results. The only downside: he had to add power steering to combat the torque steer.

EPcivic
EPcivic New Reader
11/10/08 5:19 p.m.

Dennis Grant's website has some good reading in FWD setup:

http://farnorthracing.com/autocross_secrets.html

He agrees that you want the weight up front on a FWD car. I don't think he acknowledges the value of reducing the polar moment as much as I do. This is really the key to understanding why there are some cases where a FWD will be faster than a RWD.
Think about holding a set of barbells:

First, put the weights at the center, then twist the barbell about the center - should be easy, right?

Now put the same weights at the ends and twist about the center - much harder. You have to do a lot more work to twist it when the weights are at the ends.

Your car has to do the same. It takes more work to slalom or transition a car with a higher polar moment. That work is coming from the tires in the form of slip angle, and that slip is creating drag, which is slowing the car down. It also requires a higher yaw angle, which makes the car effectively 'wider' with respect to the course, further slowing it.

Another thing to keep in mind is that static weight distribution does not equal cornering weight distribution. You have to consider the roll centers / roll axis and all of that stuff that is hard to expain in a web forum.

-Chris

Salanis
Salanis SuperDork
11/10/08 5:30 p.m.
Osterkraut wrote: The 50/50 thing being perfect is a tad incorrect. Look at the weight balances of racecars: more like 46/54.

Don't aerodynamics factor into that though? At higher speeds, isn't there more aerodynamic force on the front of a car than the back? Effectively shifting the weight balance forward?

CivicSiRacer
CivicSiRacer Reader
11/12/08 10:01 a.m.

There is no way you are going to get a 55/45 weight in a FWD car unless you start adding balast which goes against most FWD cars (underpowered, no torque).

My 91 Civic Si corner weighed was 59/41 at 2080lbs. But it out-ran many higher hp cars at our local events. From memory the car was 1227lbs front and of course 853lbs rear. To get 50/50 you would need 400lbs rear. Making the car accelerate like crap

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