Oversteer/understeer is more a characteristic than a "problem." Correct me if I'm wrong, but I think you're asking "why is there oversteer or understeer?"
There are four reasons: the contact patch of four tires to the ground.
The amount of grip each tire has depends on both the weight on the tire and its "stickiness." For a given set of tires, the force a tire can generate depends on the weight on that tire. The weight distribution to the tires can be altered so that greater or lesser grip can be attained at a specific tire or set tires. See Dreamin's table above (thanks, Azeem!). Keep in mind that all the above is important but the most important item is the tire pressure (for a specific set of tires). If your tire pressures are correct, you can maximize the grip that your suspension setup will allow.
When I switched to the Hotchkis setup, my red truck went from having understeer to excessive oversteer. After adjusting my front tire pressures, I adjusted the oversteer so that it was more neutral and predictable. Yet, Dreamin suspected that the rear springs would settle somewhat and soften up. They have and understeer has increased. To compensate for that, I can increase the front tire pressures.

Furthemore, awhittle brings up several good points one of them being driver skill. Improving driving skills is the best way to extract more performance from your L.

BTW, I've learned most of this stuff from Dreamin and Ruslow. They are GREAT sources of information, and way cool guys. I'm learning stuff from awhittle too!

Spike

See http://www.esbconsult.com.au/ogden/l...rs/phors02.htm for a more detailed description.

[This message has been edited by Spike Engineering (edited 06-22-2001).]

 

I also wrote this a few years ago for an autocross board. This may help

Andy

* * * * * * * * * * * * * *


I am going to try to clear up this whole idea of oversteer/understeer and how to adjust it with springs shocks and swaybars. Lots of charts can be found that state "to decrease understeer. . make your front bar stiffer". I will try to explain why this works. See http://www.wtrscca.org/tech.htm

The first thing to understand is the engineering term "coefficient of friction" (CoF) at least from a tire standpoint. A coefficient of friction of 1 means that for every pound that a tire is pushing down on the ground, that same can push sideways 1 pound. A coefficient of friction of 2 means that for every pound that a tire is pushing down of the ground, it can push sideways 2 pounds. Most high school physics books state that for any given combinations of two materials the coefficient of friction is fixed. For a normal street tire the CoF is about 1. This is why most street cars on real street tires all max out at about .95g. A couple things were left out of the books that make this whole issue more interesting that the book would lead you to believe.


In the real world, the CoF is a little adjustable. Tires actually stick a little better at you remove load from. Conversely the CoF gets a little less as the load is increased. The strange thing about tires is that the total load that you gain from the outside tire being pushed down harder is not quite as much as the load lost on the inside. Bottom line, as the loads go up, the cornering goes down.


So how does a swaybar improve anything? Sway bars are really messing with your head from two different directions. A chassis is connected to the world through first the suspension and then the tires. A tire has the most side grip when the tire is leaning in (negative camber) even when the chassis and suspension are pushing the tire in the positive direction. But the inside tire keeps leaning in the ever more negative direction, but if you think about it, that tire started out leaning in the wrong direction and is even
getting worse as the chassis rolls with load. As you increase the roll bar on this end of the car, the outside tire will go less positive (a good thing) but this tire will also get overloaded and it's CoF will go down. (a bad thing). So why do we add sway bars. We add roll stiffness to the end of the car that is biting to less underload the outside tire on that end of the car. What we are really doing is removing some of the load from the outside tire on the opposite end of the car. Remember that sway bars decrease roll and increase load on the outside tire of the end of the car that it is installed on. They are mostly used to balance the steady state (mid section of sweepers) cornering power.

So how does messing with shocks do anything? Remember that overloading a tire will make its CoF go down. How lets think of just on end of the car (rear). As you crack the wheel going into a turn, the chassis will all of a sudden roll to a new position, say 4 degrees at first and then settle down after the initial shock to say 3 degrees. While this is doing al this rolling the outside shock is quickly extending as it tries to pick up the inside tire. Lets assume that the shock is so stiff that it, actually for a moment, picks the inside tire off the ground. Then as this way hard extension valved shock picks up this tire the weight that was on it goes entirely to the outside tire. Bingo the CoF on the outside tire drops and that end of the car momentarily overloads and breaks loose. When the inside finally gets back to the ground, everything goes back to that balance that the swaybars were giving. Bottom line a shock can overload a tire, but for just a moment. You could do this with the outside tire and shock compression and get pretty much get the same thing accomplished, but guess what happens when you hit a bump in mid turn and the tire has to quick get up and over the bump. Bang, that tire overloads with all this compression damping and you go off into the weeds. (a bad thing) Remember you are going through a corner and that outside tire is already overloaded. It needs all the help it can get.

What about light wheels, independent suspensions and all that stuff? This is on of the most misunderstood parts of the car. Its really fairly simple. A tire hates to be shocked by changes in load. The more gradually you can bring the load up to maximum, the better. The problem with heavy suspension parts (including uprights, wheels, tires, 1/2 of the axles, 1/2 of the shocks among other things) have to be accelerated up and over the back side of whatever bump you hit. The lighter that all this stuff is the less the tire is overloaded as it does its work getting out of the way of the bump. The cool thing about independent suspensions is that the 40 or 50 lbs of the differential does not have to go over the bump. The tires are way happier. Some other time we can get into unsprung weight ratios.

Sway bars and chassis stiffness. One of the more difficult things to comprehend is that sway bars can only move weight. Weight never goes away, it just relocates. With a front sway bar, the weight moves from the outside front to the outside rear tire. But the same amount of weight will be moved from the inside rear tire and moved forward to the inside front tire. The total weight of the front of the car can not change only the tire that is carrying that weight. The center of weight will always be in the center of gravity. Those of you that have taken a Statics course, the sum of the moments is zero. The thing that is often assumed, is that the chassis is strong enough to transfer all this weight. In the real world, in stock cars this assumption can be way wrong. Have you ever noticed on some cars, adding a rear bar seams to not do much? Bingo, the chassis is flexing and the sway bar force is just twisting the chassis.

I have a simple test to check if this is the problem. PS be prepared to accept the answer if you do this. I sawed up a raced car chassis and started over one day after I did this. Get a digital level that reads out in 0.1 deg increments, and 2 4x4x 1 foot long boards. Find a level spot in the driveway and put the level on a front bulkhead in the engine compartment and zero the digital level. Then go to the back of the car and see if the back of the care is level or at least record how out of level it is. You may have to get creative to find a place to measure but be consistent so this is repeatable. You may want to magic marker the spots where you set the level. Next the fun part. Jack up the left front corner of the car and put one 4x4 under the tire. Next jack up the right rear tire and put a 4x4 under that tire. At his point the car is under about as much twist as it can possibility get under cornering and braking. At this point go measure the bulkheads and see how bad you are tristing the chassis. For the record our a-mod car at 800 lbs will twist about 1/16 of an inch in 16" sitting on three tires or about 3000 ft-lbs/deg

Andy

 

First off, Great thread CBQB!

Excellent posts Andy, Spike, and Azeem!

Gordon, nice synopsis. LOL

 

CBQB3

I bet that was more info than you were expecting. HEHE

Andy

------------------
Andy Whittle
White 2000
#2482 of 4966
Built 4/27/00

My Autocross car

 

Another good read on this subject is by Paul Van Valkenbugh, "Race car Engineering & Mechanics".

Mike

 

Andy,

Thanks for the info! By any chance, have you ever checked out the articles at http://www.esbconsult.com.au/ogden/l...hors/phors.htm ? It's a good read.

Spike

 

Lots of good info but a little hard to read. It's been a few years sence I was in engineering school. The autocross word knows him well. Have you guys read Allen Staniforth or Carrol Smith "Tune to Win"?

By the way, Any time I refer to a Stock car, that is a car as it came from the showroom new.

AW



[This message has been edited by awhittle (edited 06-22-2001).]

 

Understood. At least the articles are divided into a narrative followed by the physics. When I'm tired, I'll read just the first half to get the concepts. Some of the articles are interesting because he'll discuss Silverstate and braking at 200 mph, or hitting bumps at 165 mph.

Spike