DougR

Hi everyone,

I'm trying to choose spring-rates for a STaSIS Track Sport setup for my car. Ryan has provided terrific guidance on a whole host of issues, and now it's down to making the final call on spring-rates. I don't think I quite understand the handling impact of this tradeoff well enough though -- can anyone shed some light?

So here's the difficulty. If I want to dial out understeer, I can either increase spring-rate in the rear, or increase the stiffness of the rear anti-sway bar. By running a little lower spring rate with a bigger bar, in theory I ought to get a cushier ride on the street, but still get rid of my understeer.

My question is, what do I give up handling-wise compared to running higher spring-rates in the rear instead? For instance, will one setup give me greater trailing-throttle oversteer, and the other greater power-on oversteer?

I know that this isn't a life-or-death decision -- STaSIS will exchange springs for 90 days, I can always upsize or downsize my anti-sway bars, etc...but I'd really like to understand the problem a little better.

Cheers,

ryoung

There's very little trade-off until you get to extremes with a very stiff rear bar and very hard cornering.

The obvious advantage of the stiffer sway bar is that the rear suspension will be softer in 2-wheel bump, the bar just pivots, and you get a better ride from the rear.

The disadvantage is that handling becomes non-linear when the stiff rear bar results in lifting an inside rear tire. Instantaneously at this point, the rear roll center is no longer in the center of the car. Instead, the rear of the car pivots in an arc around the center of the outside rear tire patch, and any additional roll results in raising the rear of the car. And of course, lifting any wheel isn't good for acceleration with an AWD car.

I'm sure you've seen photos of this rear end jacking effect with FWD cars and stiff rear bars on the track. Tracks are normally very smooth, but this can happen at lower cornering g's on the street when only the inside rear tire goes over a dip or low spot in the road. As you can see, even with the stock S8 suspension, I'm already very close to lifting an inside rear tire.

<img src="http://pictureposter.audiworld.com/17157/turn8_lift.jpg">

DougR

So both approaches share some characteristics. They reduce traction in the rear, which could be viewed as a bad thing, but in this case that reduces my understeer (which I'm currently in favor of :-) Also, they reduce body roll, making the car more composed in quick transitions.

With a higher rear spring rate, body roll is reduced by the stiffer spring on the outside pushing back up against the car as it tries to squat down. I'm guessing that in a perfectly independent suspension, this wouldn't have any effect on the inside wheel. Why is traction in the rear reduced?? Is it because the stiffer spring doesn't soak up small irregularities in the surface as well, therefore slightly reducing the contact patch relative to a softer spring?

With a bigger rear sway bar instead of the higher rates, body roll is reduced because the side that wants to squat down has to pull the other side up at the same time. Traction is reduced in the rear because we get a smaller overall contact patch when the inside wheel leaves the ground altogether, and the height of the car in back increases. (and the real killer is the non-linearity of it...hmmm, giving back some of the composure you gain by reducing body roll.)

Am I on the right track??

Thanks a lot Randy, I've got a new way to think about the tradeoff!

ryoung

Stiffer rear springs or rear sway bar both do reduce total grip available from the rear tires, but they also INCREASE the total grip availble from the front tires!! As you know, the desireable result with our noseheavy cars is reduced understeer.

The reason this happens is essentially the same for both, but it's not exactly intuitively obvious. The lateral grip available from a tire is proportional to it's coefficient of friction. In the ideal world, the cf is constant; and if you increase the vertical load on a tire, the lateral grip increases in proportion. This is almost, but not quite, true with tires. If you double the vertical load on a tire, the available lateral grip does increase, but it doesn't quite double. The conclusion from this is that the car has the most grip when all four tires have the same weight distribution, 50:50 front-to-back and 50:50 left-to-right.

First, think about what happens to the load on the tires in a right-hand corner. For simplicity, assume the car has a perfect 50:50 weight distribution. At 1.0g of lateral acceleration, you might have something like 67% of the load on the left outside tires, and 33% on the inside. [See <A HREF="https://forums.audiworld.com/a8/msgs/36145.phtml">Weight Distribution Revisited</a>.] The total grip available from the tires has been reduced, but the car is perfectly balanced and neutral handling.

Now add either stiffer rear springs or a stiffer rear bar to the car. The increased spring rate in the rear increases the load on the outside rear tire, but the total front-to-rear or left-to-right load distribution can not change. The result is that the loads on the LF and RR tires are reduced, and the load on the RF tire increases. The improved load distribution across the front increases grip from the front tires, the poorer load distribution across the rear reduces their grip, and our former neutral car now oversteers.

If you can find a copy of "Racecar Engineering" July '03, the article "Tire Theory" discusses this and other issues in more detail.

Finally, Yes to your question "Is it because the stiffer spring doesn't soak up small irregularities in the surface as well?" In general and all else being "equal" (it never is), as long as suspension geometry is under control, softer springs do provide more grip. The effect is usually overwhelmed by changes in load distribution, but it's easier for soft springs to keep tires uniformly in contact with the road, which results in the best grip. Needless to say, shock tuning is critical, but that's another story :-)

ryoung

<A HREF="http://www.neohio-scca.org/clubinfo/comp_clinic/hand_out_reprints/LoadTransfer%20reduced%202.pdf">Load Transfer</a>, a pdf by William Mitchell of <A HREF="http://www.mitchellsoftware.com/">Mitchell Software</a>. If I had remembered that I had this link, I could have saved myself a lot of writing in the previous message.

DougR

Ok that makes sense (you still have the same 4 tires with about the same cf more or less planted on the ground.) You don't really reduce the traction on one end of the car, you *transfer* that traction to the other end.

But I'm missing a couple of fundamental points from your description. Why does increasing the spring rate in the rear increase the load on the outside rear tire? (Logically I can see why this is the case with a stiffer anti-sway bar.) I guess because it's being held up higher rather than sinking to the ground, there's more upward force being applied to accomplish this? And why can't the total front-to-rear and left-to-right load distribution change? Is this purely a function of CG, weight distribution, speed, and radius?

I suspect that these answers are incredibly obvious, but I'm not sure that I quite understand the assertions.

I see how if these 2 assertions are true, then it follows that load on the opposite front corner would increase, and the load on the 2 adjacent corners would decrease; and seeing how the tire's cf decreases as vertical load increases, I also see how this increases front cornering power.

So...running higher spring rates in the rear will reduce the traction of the rear not so much because a stiffly sprung wheel has slightly less traction than a more softly sprung one; but because the outside tire becomes even more loaded vertically, and it doesn't return the same amount of side load that is given up by the less lightly loaded (vertically) inside tire.

A larger rear sway bar reduces rear traction for this reason, but also because it might lift the inside tire completely off the ground, losing all of its side load. (Hmmmm....is the non-linearity here really all that severe?)

Thanks for taking the time to explain this -- even if it's covering the same ground, I find your explanation more approachable than Mitchell's!

ryoung

1. "You don't really reduce the traction on one end of the car, you *transfer* that traction to the other end."

This is true, you don't "lose" the rear traction, most (but not all) of the loss is gained at the front, but ... ***The sum total grip from all four tires is always highest when all four tires have an equal load.*** With our noseheavy Audi, though, the center of gravity of the car is biased toward the front, and the front tires have to resist most of the lateral forces. This is why our cars understeer, the front tires exceed their limits before the rears. And this is why stiffer rear springs and/or a rear bar helps -- the "excess" grip in the rear tires is transferred to the front tires to balance the car.


2. "Why does increasing the spring rate in the rear increase the load on the outside rear tire?"

During cornering, load is transferred to the outside tires from centrifugal force acting thru the center of gravity and resisted by the outside tires at the end of a lever arm equal to 1/2 of the track of the car. Since the car is supported by springs, it wants to roll. If the spring rate in only the rear is increased (with springs or a sway bar), the load carried by that corner is increased.

Said another way -- The load that gets transferred from the inside to the outside tires in a corner is only a function of the height of the cg, the track of the car, and speed. How much of the additional load is carried by the front or rear outside tire is only determined by the relative spring rates of the corners. Up to the point where an inside tire lifts, springs and sway bars are both just affecting spring rate.

As an aside, note that lowering the cg or widening the track of the car reduces load transfer during cornering, the tires carry more of an equal load, their total grip increases, and the maximum cornering speed increases.


3. "So...running higher spring rates in the rear will reduce the traction of the rear not so much because a stiffly sprung wheel has slightly less traction than a more softly sprung one; but because the outside tire becomes even more loaded vertically, and it doesn't return the same amount of side load that is given up by the less lightly loaded (vertically) inside tire."

Absolutely exactly correct!!


4. "A larger rear sway bar reduces rear traction for this reason, but also because it might lift the inside tire completely off the ground, losing all of its side load. (Hmmmm....is the non-linearity here really all that severe?)"

The rules don't change just because the car lifts an inside tire, but load transfer in the rear is now as far as it can go; 100% of the rear load is carried by the outside tire, zero by the inside tire. You're right, the non-linearity is not that severe, except perhaps on the track. Imagine trail braking into a corner -- the inside tire lifts, ABS tries to keep the tire spinning, any additional roll results in the rear of the car jacking up, and when you do get back on the gas, you've got to wait for that tire to get back on the ground before you can start transferring power.


5. One last thought -- Up to now, we've been talking about load distribution primarily during steady state corners. Load distribution is also important when braking into but most mportant of all when accelerating out of a corner. That perfect 50:50 weight distibution (of a RWD M3, for example) sounds good, but think about the load distibution and forces on the tire when you go to full throttle, accelerating out of a corner. The rear tires of the poor M3 are carring their normal lateral cornering loads, and now you add ALL the forces of acceleration. If not careful, too much throttle results in exceeding the limits of the rear tires, and a spin. Weight transfer to the rear helps, but it's not enough compared to the Audi, which now does have a near 50:50 front-to-rear weight distribution and AWD. The result is that the Audi can put down more power sooner, which is a huge advantage on track; any additional exit speed is carried all the way to the next corner. For a RWD car, the optimum weight dstribution is closer to 40:60 front-to-rear; i.e. a mid or rear engine car.

LCP

I'm running 550f/600r with a 19mm rear anti-sway bar on soft.

<a href="https://forums.audiworld.com/s4/msgs/1280131.phtml">My STaSIS odyssey</a>

In short, my <a href="https://forums.audiworld.com/s4/msgs/1284155.phtml">19mm bar was kept for ride quality</a>. There has been a complaint that the stiff rear end launches off of bumps in the pavement too harshly. I know someone who ditched his whole Stasis suspension because of this one issue. The stiffer the spring in the rear (compared to the front), the more the rear of the car will tend to launch off of bumps. Note as well, however, that by dialing down the rebound adjustment on the rear shocks you can reduce the amount the rear end tends to jump, but if you get the rear rebound too soft then the car tends to bounce/oscillate on relatively smooth pavement, and that can be nauseating.

Warren Wang

The higher spring rate you go, the less effective your sway bar becomes, so you might need to bump the size of that up as you go. However, Audis have the somewhat unique problem of needing to keep the inside rear planted, otherwise you'll lose power spinning the wheel.

Stiff rear suspension helps keep the inside front planted during track out. Like Randy's saying, you're not really reducing overall traction. I'm running 782 lb/in up front and 1231 lb/in in the rear of my race car, and I would only consider it neutral, not oversteery. Part of that is driver training, but hey, now I've seen the light.

Seeing as to how Stasis has done reasonably well in an ultra competetive series, you could ask them for their input. The easiest thing to do is to copy someone else's hard work. I did :P

Warren

Jay@Boston

Roughly what spring rate are you thinking of... 450, 500 for the front? I'm thinking of ditching my Stasis and going with softer rear springs and a thicker sway set at soft. Way I look at it, I rather give up some precision track setting for a slight improvement in daily street behavior (again, soft setting on the sway bar and I don't drive fast on the street... who can in Boston?). Who knows, I may end up with something I dislike. How much traction does the inside rear tire have anyway? All those pictures of the new S4 in car magazines with its inside rear tire up in the air... Audi engineers must have felt that its spring rate and sway bar set up was optimal for the cars handling (impressive .90 G) and comfort. I'm approacing this with very little-to-no technical facts and just common sense... will I really notice the difference on the track with "the perfect" setup? Probably not. Pursuing perfection has been very expensive ($6k+ on suspension parts and labor) and dissatisfying. Perfection... you'll never get there. It's my understanding that Stasis uses pretty hefty sway bars in their race cars. Dick Shine is a big advocate of thicker sways bar and Herb Adams (suspension guru) believes the thicker sway bar appoach is the simplest way and least expensive way to fine-tune over/understeer. I hope you had a good time at the Glen.