Understanding Corner Weights


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One of the most important aspects of car setup is the static weight distribution and the cross-weight percentage. Why? Picture the following:

Your car is really fast in right-hand turns, but understeers in left turns. If you get the car neutral in left turns, it oversteers in right turns. The situation is frustrating. You’ve tried springs, shocks, different bars, neutralizing the anti-roll bar, and nothing seems to work. Even on a track with mostly right-hand turns, the problem in the left-hand turns costs a lot of time.

While several different setup parameters could have caused this situation, a likely cause is excessive cross-weight.

Static Weight Distribution

Static weight distribution is the weight resting on each tire contact patch with the car at rest, exactly the way it will be raced. This means the driver should be in the car, all fluids topped up, and the fuel load should be such that the car makes your minimum weight rule at the designated time-usually after a race. The car should be at minimum weight, using ballast as needed to make the proper weight.

When working with static weight distribution, we use two percentages to analyze the car's corner weights: Left weight percentage and rear weight percentage. These tell us all we need to know about the setup relative to the weight distribution. The left weight percentage is found by adding the LF weight to the LR weight and dividing the sum by the total weight.

The rear weight percentage is found in a similar manner: Add the LR and the RR weight together and divide the sum by the total weight. Many electronic scales will perform the calculations for you.

For road racing and autocrossing, the ideal left weight percentage is 50 percent. This makes the cornering force balanced from left to right and offers the best performance overall. However, many cars cannot make the 50 percent left-side weight percentage due to driver offset. Still, it is a worthwhile goal to strive for 50 percent left-side weight.

Rear weight percentage for road racing and autocrossing is less definite. The more power a car has, the more that static weight over the drive wheels helps acceleration off the corners. Additionally, it is much more difficult to change rear percentage much, since rear weight is mostly a design function. It still pays to be thoughtful about weight placement fore and aft in your car.

The only way to change the static weight distribution percentages is to physically move weight around in the car. Jacking weight will not alter the left side or the rear percentages.

Cross-weight Percentage

Cross-weight percentage compares the diagonal weight totals to the car's total weight. To calculate cross-weight percentage, add the RF weight to the LR weight and divide the sum by the total weight of the car. Cross-weight is also called wedge: If the percentage is over 50 percent, the car has wedge; if below 50 percent, the car has reverse wedge.

More wedge means that the car will likely understeer more in a left turn. The advantage to wedge is that the left rear tire carries more load, so the car drives off the turns better. But in a right turn, the opposite occurs and the handling is worse. In almost all cases, the loss of cornering performance in one direction is greater than the gain in the other direction.

On oval track cars, cross-weight is usually used in conjunction with stagger (where the right rear tire is larger in circumference than the left rear tire) to balance handling. More stagger usually loosens the handling in left turns, so more cross-weight is used to tighten it up. But stagger is not a good idea on a road course or autocross either, where the ideal is 50-percent cross-weight and no stagger.

One of the problems with cross-weight is that it will change the handling balance from a left to a right turn. This can make maneuvering in traffic difficult, even dangerous. On a road course, the cross-weight percentage should be very close to 50 percent, within a half-degree either way, to keep the handling balance similar in a right-hand turn compared to a left-hand turn. In the example at the beginning of the article, this was the problem: a cross-weight percentage that was less than 50 percent, and probably off by at least two percent.

One of the keys to obtaining a good setup is using the correct procedure to weigh your race car.

How to Weigh Your Race Car

Here are some points to remember when weighing your race car:

  • Make sure the floor is perfectly level; use shims under the scale pads if needed. Small angles can throw off your readings significantly.
  • Set tire pressures first.
  • Check stagger at each tire, even if using radials.
  • Put the driver weight in the car, preferably the driver.
  • Use a load of fuel for where you you want the car balanced, either at the start of the race, the end of the race or an average between the two.
  • Disconnect the shocks, when possible, and the anti-roll bars.
  • Use blocks the same height as your scale pads to move the car off the scales to make adjustments.
  • Bounce the car at each corner to free the suspension from any bind, then roll the car onto the scales.
  • Make sure the tires are centered on the scales.
  • Recheck air pressure often to assure ride heights stay consistent.

Setting Static Weight Distribution:

  • Check static weight before working on cross-weight.
  • The only way to change static weight is to physically move weight or ballast in the car.
  • To increase left-side weight, move weight as far to the left as possible.
  • To increase rear weight, move weight as far back as possible.
  • Move ballast first, since it's easier. Then move components like the battery or fuel cell.
  • It is best to get 50 percent left-side weight when possible.
  • Get the rear percentage as close to the manufacturer's specs as possible.

Setting Cross-weight:

  • Once static weight percentages are set, work on cross-weight percentages.
  • You cannot change the left or rear percentages by jacking weight around in the car, although this will change cross-weight.
  • Changing the ride height at any corner will change the cross-weight percentage.
  • If you raise the ride height at a given corner (put a turn in or add a round of wedge), the weight on that corner will increase, as will the weight on the diagonally opposite corner. The other two corners will lose weight.
  • If you lower the ride height at a given corner, that corner will lose weight as will the diagonally opposite corner. The other two corners will gain weight. This will not change the left-side or rear weight percentages.
  • To add weight to a given corner, raise the ride height at that corner or lower the ride height at an adjacent corner. For example, if your initial setup is 52 percent cross-weight, and you want 50 percent cross-weight, lowering the right front or left rear corner will decrease cross-weight percentage. You could also raise the left front or right rear ride heights to do the same thing.
  • It is best to make small changes at each corner, instead of a big change at one corner. This keeps the ride heights as close to ideal as possible. In the above example, to go from 52 percent to 50 percent cross-weight, try lowering the right front and the left rear one-half turn on the weight jack bolt or spring perch while raising the left front and right rear the same amount.
  • Always record the cross-weights and ride heights for reference at the race track in case changes are needed.
  • Measure control arm angles after each change. The angles are another way to set the suspension for the desired ride height and cross-weight percentage.
  • The distance from the ground to an inner suspension arm pivot point will also accomplish the above goal.
  • Remember that changes in stagger, tire pressures and springs will change the ride height and alter the cross-weight percentage.

Change at the Track:

  • Make small changes at the track, and make only one change at a time.
  • If the car understeers or oversteers in only one direction, check the cross-weight percentage.

One of the most important aspects of racing is having a good handling balance. Setting static weight distribution and adjusting cross-weight percentage is one way to assure good handling. Taking the time and making the effort always pay dividends.


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te72
te72 New Reader
1/3/18 11:33 p.m.

Thanks for posting this. I've read all the info before, in various places, but it's good to have a refresher. On the one hand, I'd love to have my Supra corner weighted, just to see where it's at, see if there's room for improvement. On the other hand, it drives really, REALLY good for being setup by this idiot behind the keyboard, and I really don't wanna spoil a good setup by chasing after a perfect setup... at least not yet.

 

Would be interesting to see how close to ideal I got it though, given how well it handles already. Basically so long as I don't completely overcook a corner entry, it does better than any car this heavy has any right to...

Justin
Justin None
5/15/18 2:07 p.m.

Good stuff. I've had my cars corner balanced a lot, but never really looked into the science of it. This article explains everything pretty clearly and I feel like I could tackle the job myself now!

Toebra
Toebra Dork
11/16/18 4:12 p.m.

I see disconnecting the sway bar, and how to do it, but disconnect the shocks?

Cloud9...68
Cloud9...68 New Reader
11/16/18 5:23 p.m.
Toebra said:

I see disconnecting the sway bar, and how to do it, but disconnect the shocks?

I had the same question.  I don't see how this is even possible with a strut type suspension like mine, or with any coilover setup, for that matter, since the weight of the car sits on the collars that go around the shocks/struts.

freetors
freetors Reader
11/16/18 7:52 p.m.

In reply to Toebra :

Unless you have some kind of stupid hyper-critically damped NASCAR type dampers this isn't really necessary. To get good accuracy easily make sure you get the low hanging fruit first like removing the friction between the tires and the scales so there is no bind. Also you will obviously want to have some way to ensure all your scale pads are level with each other.

te72
te72 Reader
11/16/18 9:05 p.m.

I would imagine that disconnecting the shocks is only applicable to setups where the shock and spring are separate, like a lot of solid axle cars, or Mk2 Supras in my experience.

iwannarace
iwannarace New Reader
11/16/18 9:36 p.m.

Funny. I dropped my integra off at edge to have this done today. looking forward to getting it back!

Stefan
Stefan MegaDork
11/16/18 10:44 p.m.

Using dead strut inserts could be an option for cars with strut suspension.

Cloud9...68
Cloud9...68 New Reader
11/17/18 12:42 p.m.

My big stumbling block on this subject is how to get accurate readings by removing all the friction/bind from the tires, sway bars, bushings, etc.  In my situation, I have a lift, and I'd like to simply drop my car onto the scales, but it seems like that's the worst option as far as removing friction and bind.  Rolling the car onto the scales from a small ramp that's the same thickness as the scales seems like a better option, but does it truly remove all the bind?  In other words, to make this method work, wouldn't you really have to drive the car around, preferably going over a few pretty serious bumps, before driving onto the ramps?  This gets very tedious, given the number of iterations it typically takes to get the corner weights right.

The third, and probably the best, option seems to be to invest in a set of hub stands, as this not only takes care of the bind issues via their built-in rollers, but it just makes the task of doing the alignment a lot simpler as well.  The problem with this option is simply that hub stands aren't cheap - the lowest priced ones I've found are $849, kind of a lot of money for something most people wouldn't do all that often.  

Maybe I'm over-thinking this, but I was amazed when I got my new scales, and let my car down on them for the first time.  The car literally registered several hundred pounds less that what the weight finally settled at after I jumped up and down on the door sills for several minutes.  Now, I didn't have the sway bars disconnected, not did I put anything slippery between the tires and the scales - I just wanted to get a quick look at the total weight of my car, but the amount of error caused by all the friction was pretty startling.

Interested in hearing peoples' opinions and (preferably) experiences.  

Stefan
Stefan MegaDork
11/17/18 1:22 p.m.

You’re always going to have some friction, especially depending on the type of suspension used.

Struts and trailing arms generally aren’t great in this case as they have a lot of inherent bind.  Using dead struts and lots of bearings will help.

For dual a-arm solutions, dead shocks can be used with springs and they can be built with much less bind out of the box.

Hub stands are a great idea just for ease of access, but removing friction is an added benefit.

Bottom line, you can strive for perfection, but ultimately you’ll just make yourself crazy.  Strive for repeatable and take the measurement as a data point, instead of an absolute.

Cloud9...68
Cloud9...68 New Reader
11/17/18 2:36 p.m.

Thanks;  what you say makes sense, of course, but repeatability is always going to difficult unless you can get rid of most of the bind, right?

 You've mentioned "dead struts" a couple of times - what do you mean by that?  A strut with its innards removed?  How would you make something like that?  Or do you just mean an old, worn-out strut? And what do you mean by "lots of bearings"?  Thanks.

A 401 CJ
A 401 CJ Dork
11/18/18 7:38 a.m.
Cloud9...68 said:

My big stumbling block on this subject is how to get accurate readings by removing all the friction/bind from the tires, sway bars, bushings, etc.  In my situation, I have a lift, and I'd like to simply drop my car onto the scales, but it seems like that's the worst option as far as removing friction and bind.  Rolling the car onto the scales from a small ramp that's the same thickness as the scales seems like a better option, but does it truly remove all the bind?  In other words, to make this method work, wouldn't you really have to drive the car around, preferably going over a few pretty serious bumps, before driving onto the ramps?  This gets very tedious, given the number of iterations it typically takes to get the corner weights right.

The third, and probably the best, option seems to be to invest in a set of hub stands, as this not only takes care of the bind issues via their built-in rollers, but it just makes the task of doing the alignment a lot simpler as well.  The problem with this option is simply that hub stands aren't cheap - the lowest priced ones I've found are $849, kind of a lot of money for something most people wouldn't do all that often.  

Maybe I'm over-thinking this, but I was amazed when I got my new scales, and let my car down on them for the first time.  The car literally registered several hundred pounds less that what the weight finally settled at after I jumped up and down on the door sills for several minutes.  Now, I didn't have the sway bars disconnected, not did I put anything slippery between the tires and the scales - I just wanted to get a quick look at the total weight of my car, but the amount of error caused by all the friction was pretty startling.

Interested in hearing peoples' opinions and (preferably) experiences.  

I don’t get this.  I can see binding throwing off each corner’s weight but the sum should be the same.  Or maybe I read the post wrong.  Did you mean it was off several hundred pounds per corner?  That seem like a lot.  

flatlander937
flatlander937 HalfDork
11/18/18 11:50 a.m.

Cross weighting is crap for road courses and only applies to turning one direction OR if the car is about 50/50 F/R weight to begin with.

Many successful race teams use corner weighting to achieve the same F/R % on both sides equal, not cross weight.  Andy Hollis does this.

 

So LF/LR = RF/RR is what you shoot for. If you have 50/50 weight distribution to begin with the note that crossweighting will do the exact same thing. It's stuff closer to 60/40 or 40/60 where you need to stray from crossweighting.

 

Then there is what I do for FWD stuff... That is forget the rear weights entirely, and just balance the fronts to be equal. My left rear is something like 150lbs heavier than the right rear, with both fronts even at ~740lbs each on a 2425lb FWD car.

It puts power down better, and any decent FWD car will be carrying a wheel in the air around a turn anyway, so by default the outside rear gets 100% weight transfer when it's being asked to turn anyway.

 

There are many ways to corner balance a car. Don't just blindly cross weight it thinking it's the "right" way. Then if it's for a particular road course, you will find several seconds optimizing for select turns and throwing any of the above methods out the window... A friend's Chump Car found 2 sec at VIR making it turn right better than left.

 

Go out and experiment with it.

Shaun
Shaun Dork
11/18/18 2:40 p.m.

Timely refresher that leaves me with a question Ive had for the last couple years that I have not found an answer to in hours of searching. I have a 96 civic with a d16/manual.  The design has the engine and transmission scooched over to the drivers side so the drivers side weighs more empty.  At the time Honda was still favoring their home market I reckon.  I weigh 220.  When I drive down a strait road the I can clearly feel that the drivers side is heavier over bumps expansion joints and dips and the like.  Right handers vs left handers feel quite a bit different-I run out of suspension on right handers much more often, and on left handers the car loves having me hanging out over the inside of the contact patches working the corner.  Right now I have (IIRC) 350 lb front and 200 lb rear springs (koni sport, eibach pro line, ST anti sway bars)  .  Besides the eventual move to an adjustable ride height set up,  I feel like I need 100 lbs or so more spring in the left front.  So 2.5" springs and experimenting is the way to go-  I think I know that much..  . Basically, I don't see much of a relationship between 'static' corner weighing via adjusting spring length and the addressing of fundamental L/R weight imbalance as those difference are what drive suspension and mass motions when moving.  Or am I missing something?

Cloud9...68
Cloud9...68 New Reader
11/18/18 3:58 p.m.

Yeah, I'm a little puzzled by my result when I let me car down on my new scales as well.  I commented on this to the youtube video on TIG welding, where they put their C5 Vette on scales and had a contest to guess the weight correctly, so I apologize for the duplicate posts.  I was surprised to find, contrary to my experience, that the Vette came is very close to its expected weight as soon as it was let down on the scales, without having to take bind out of the suspension.

Thinking about this a little, the weight measured by the scales is the sum of the force of gravity acting on the car's mass, and the force of the springs pressing downward.  Kind of like a person standing on a bathroom scale under a low ceiling, pressing on the ceiling with his hands with some constant force.  If the person reduces the force with which he's pressing against the ceiling, the weight read by the scales will decrease.  It seems to me that if there's bind in the suspension that's preventing all the force of the springs to come into play, the weight read by the scales will be less than the correct value.  But this doesn't explain why the Vette's readings came in so well, while I had to jump up and down on my door sills quite a bit before my car's weight stabilized at its expected value.  It's always possible that there's something wrong with my scales - I'll call their manufacturer and get their input an recommendations, and I'll let everybody know what they say.

freetors
freetors Reader
11/18/18 4:40 p.m.

In reply to Cloud9...68 :

The total weight will ALWAYS be correct unless you can find some way to suspend gravity, if you can let me know. It has to, it's just the laws of physics. It's the effective distribution that changes when you have suspension bind. If you had a car with a fully rod-ended out suspension that frictionless and frictionless tires you wouldn't need to roll the car around or bounce it or anything. It would just automatically settle.

When I weigh my cars I try not too overthink all this binding stuff. I put the car on grease tiles so the tires move freely on the scales and then I bounce each end a couple times. Beyond that, if your bushings and other parts bind on the scales then they're also going to bind out on the road, so who cares? Same goes for for swaybar endlinks. Are they adjustable? Disconnect and adjust later. Are they non-adjustable? Then just leave them attached. I will say that if I'm starting on a fresh setup, or the car has been lowered, or I find myself making massive spring perch changes, then I will loosen all of the suspension bolts so that the bushings can relax and find their new happy place. I retighten them after I've made all my adjustments. You would be amazed at how much bushing twist can contribute on your suspension. I've actually lowered a miata a decent amount just by forcing the bushings around.

Cloud9...68
Cloud9...68 New Reader
11/18/18 5:40 p.m.

In reply to freetors :

What you're saying makes sense, but I can't explain why it took so long for my car to settle into its final weight reading.  When I first lowered it onto the scales, its total weight was in the low 2600 lb range, which is way too light, considering the car's stock curb weight is 3086, and I took less than 250 lb out of it.  Only after I spent a lot of time bouncing on the door sills did it settle down and stabilize. 

Maybe there's some sort of bind in the scales.  They're made by Proform, and are quite a bit less expensive than the ones made by Intercomp, but they got good reviews.  I'm anxious to see what Proform says about my experience.

freetors
freetors Reader
11/18/18 7:18 p.m.
Cloud9...68 said:

In reply to freetors :

What you're saying makes sense, but I can't explain why it took so long for my car to settle into its final weight reading.  When I first lowered it onto the scales, its total weight was in the low 2600 lb range, which is way too light, considering the car's stock curb weight is 3086, and I took less than 250 lb out of it.  Only after I spent a lot of time bouncing on the door sills did it settle down and stabilize. 

Maybe there's some sort of bind in the scales.  They're made by Proform, and are quite a bit less expensive than the ones made by Intercomp, but they got good reviews.  I'm anxious to see what Proform says about my experience.

Proforms are cheap scales. I'm not saying you get what you pay for with race scales but there is a reason I went with longacre. I don't like the lower weight ratings and plastic scale pads of proform. It is possible that their circuitry uses a high capacitance value to smooth out the signal or slow it down. Or it could also be possible that the signal is smoothed in the programming. That's all total speculation though.

freetors
freetors Reader
11/18/18 8:44 p.m.

In reply to flatlander937 :

This was a very interesting post to me. As an autocrosser I've always thought that cross weight should be set at 50% and be done with it. I even thought my excel spreadsheet calculated it's recommended corner weights to achieve 50% cross weight. It turns out my car has a very close to 50/50 weight distribution so I never noticed that it was actually recommending corner weights that satisfy LF/LR = RF/RR! I plugged in a bunch of numbers into my calculator and it all checks out. Now that's pretty cool! I have now added the ratios so I can visually see what's happening. Here is a screenshot with some random-ass corner weights.

Stefan
Stefan MegaDork
11/18/18 11:21 p.m.
Cloud9...68 said:

Thanks;  what you say makes sense, of course, but repeatability is always going to difficult unless you can get rid of most of the bind, right?

 You've mentioned "dead struts" a couple of times - what do you mean by that?  A strut with its innards removed?  How would you make something like that?  Or do you just mean an old, worn-out strut? And what do you mean by "lots of bearings"?  Thanks.

Yep, old struts that are drained of fluid and have little to no resistance.

Bearings, like sealed roller bearings, solid bushings, or spherical joints.  Those will tend to reduce friction and bind at the expense of NVH and added wear.  For pure race cars this isn’t a consideration.  For street cars it certainly can be.

Cloud9...68
Cloud9...68 New Reader
11/18/18 11:48 p.m.

In reply to Stefan :

Ah, OK, thanks for the clarification.  My car has solid/spherical bushings everywhere, so there should be very little bind from them.

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