FixedWing

I am reading Carroll Smith’s “Tune to Win”.

To me, the best part of the book so is the “table of handling characteristic causes and effects” starting on page 135. This is the information I was looking for when I bought the book. But I find it very cryptic.

Ergo, I have a couple questions. Maybe others here can help me with them?

1) On page 136, Smith lists insufficient front damper bump resistance as a potential cause of corner entry understeer. But I would have expected that increasing bump resistance to have had the same effect as increasing the front anti-roll bar or increasing the front spring rate – i.e. to increase the front roll stiffness (which I would expect to increase understeer). So what am I missing? Is it because increasing the front damper resistance will allow the suspension to take a “set” sooner and therefore decrease the amount of time before the car can start to turn-in?

2) On reading through the book, it doesn’t seem as if Smith takes any account of longitudinal weight transfer and the changes that this would make to the handling of the car. For example, I might use the brakes on entry to a corner to get more downward force onto the front wheels so that I can reduce understeer and get a faster turn-in. Maybe in the snow in a tight corner I might even try to lighten up the back end of the car so that I can get some oversteer and make the car rotate. Is the reason Smith doesn’t seem to address this because he is talking mostly about formula cars and on those cars weight transfer would be a lot less significant than on a sedan?

The book is useful but I find it a bit dated now. Also, I am looking for a lot more information on tuning the suspension of a car. Can anyone suggest a different book that I should look at?

Thank you.

Stephen

RedlineMan

Hey;

Firstly, Smith is arguably the preeminent source for tuning info in my book. He can talk the high tech jargon of roll centers and so on, and spell it all out in the most technical of terms, and in the next sentence tell you you're a freakin bonehead if you follow this or that crack-pot theory. Great mind!

Specifically regarding your questions, yes he talks race cars and slants toward open wheel to some degree.

Compression damping is indeed a "possible" cause for understeer. If you slow for a corner, you transfer weight forward no matter what setup you have. It is only in the amount that there is room for debate. When that weight shifts forward, if there is nothing to push back, then you have no grip.

This "pushing back" is generally accomplished with spring rate, but compression damping also has an effect as well, PARTICULARLY in a car that does not HAVE a higher spring rate. IF you have a softer spring, then damping would be crucial. Even then, it is a POSSIBLE cause, although not the first thing I would think of.

Consider too that the track surface is rarley glass smooth, and so even small bumps in braking and turning zones that would deflect the suspension too much need consideration.

Damping curves are like tire pressures. The curve looks like a mountain. On the left side you are underinflated, and on the right, over inflated. Making adjustments depends on which side you are on. To get more grip on the underinfalted side you ADD air. To increase grip on the overinflated side, you REDUCE air pressure. Damping can be similar. If you are underdamped, you increase compression to "push back" on the chassis and/or push down harder on the tires, yielding more grip. If you are overdamped, the front is not resilient enough and therefore decreasing compression damping potentially offers more grip.

FixedWing

Thank you John for the reply.

So if I understand correctly, what you are saying is that it isn't about roll stiffness but rather about traction. So in other words, it isn't just about increasing bump resistence but rather about making the front end work better which might entail either increasing or decreasing bump resistence. Have I got that right?

Stephen

924RACR

I think you've summed up the Carroll Smith experienced quite well. He's got a lot of really great ideas and knowledge, and a fella could do a lot worse than to heed his advice; that said, you can drive yourself up the wall or create problems that shouldn't exist if you try to take his every word as gospel. A little digging should reveal that after all, this is a guy who just hates Porsches - and lets that colour his professional judgements on occasion.

But yes, there's plenty to learn. My two pieces of advice on suspension tuning. Get Fred Puhn's "How to Make Your Car Handle" - got a great pic of a 935 on the front. Perhaps more detailed than you ultimately need, but complete. Secondly, when trying to apply this stuff at the track, remember the following.

The typical problem all of the advice is trying to help you cure is an imbalance in the car; an area (both in terms of location on track and a mode of vehicle dynamics) where the car is no longer balance adequately for you in understeer/oversteer (US/OS). That imbalance occurs where one end of the car loses grip before the other; in US the front end loses grip first, etc. If you want to balance the car, taking a case of US, you have options to change the grip at the front or the rear - the hard part is knowing which option to take. But if you want to be faster, not just balanced, you have only one option.

Seems obvious to sit back and think about it now, but you have to improve the grip of the end that's losing it first if you want to go faster. Again, you're doing a balancing act, but do you want to raise one end up to the grip level of the other, or vice versa? So in our case of US, you need to improve grip on the front end, rather than removing grip from the rear. Everyone knows a stiffer rear bar will result in more OS, but that won't necessarily make the car do anything but slow down.

The next step is to figure out which of the many options you can take. This is where it's important to understand where on the track, in which cases, you are having a handling problem, and how bad. Some changes are big (eg spring rate), obviously, some are small (tire pressure), and you need time playing with your car (suspension) to understand them. You need to understand, in our example, if the car's got bad US all the time, which might indicate the need for a fundamental spring rate change, or if it's just part of the time, such as on corner entry or exit - dynamic situations that might indicate a need for damping setting changes. Don't forget aero effects too, of course.

Sometimes your choices will be made easier by being limited by the stuff on your car. Perfect example would be a car with Koni yellows; can only adjust in rebound, so you can only change shocks when a rebound adjustment is called for. So if I'm having corner-exit US, I can soften the front shocks to get a little more grip... but I'm unable to stiffen the rears for the same; the rears will only help me if I'm trying to tune corner entry.

Hope that helps...

RedlineMan

Yes, you've got it;

Roll stiffness is usually used in terms of lateral roll. Weight shift is usually governed by a combination of spring rate and ride heights. Damping is pretty much a 10/10ths addition to that.

Let me give you a real world example that will help. I have a friend/customer who had ( sad ending) a nice 930S. The suspension was stock, but a lot of the rest of the car wasn't. He also had big red brakes on it. He suffered from terrible front brake lock, and had a previous shop doing all kinds of mechanical stuff to try an solve that. I told him they were chasing their tails, and spending a lot of his money doing it.

When he approached upgrading the suspension system, I put together a proposal and got the nod over the other shop. I told him going in that a side benefit of this suspension upgrade (Leda coilovers with heavier springs [350F/500R]) was that his brake lock would dissappear. He was skeptical but eager for me to be right. I was, of course!

He had soft stock springs, and so had tremendous forward weight shift before, but he did not have enough spring pushing the tires into the road (which is a good way of thinking of it, by the way). In effect he had been too far to the soft (left) side of the graphed curve I mentioned previously. With the increased spring rate the forward shift was controlled, and utilized to increase front grip. In the instant weight shifted forward, he now had something pushing back, which loaded the tires immediately, not allowing them to brake loose.

Now, with the stiffer springs we had to pay some attention to the track surface, our damping rates, and in extreme cases, front-to-rear ride height. Anticipating some bump induced brake lock, I always set cars up with the rear higher than the front initially to defer to forward weight shift. However, the highly adjustable Leda dampers allowed us to tune out a lot of brake lock caused by bumpy braking zones. It was not a big problem, but we did have a lot more ways of dealing with it with the uprated suspension.

If you have discrete rebound adjustability, you can tune for bumpy surfaces by decreasing the rebound damping to help keep the tires on the ground. With Koni single adjust shocks (rebound only), you have to watch that you do not have them cranked up too high on a bumpy track. Not allowing the wheels to droop and stay in contact with the track surface doesn't do much for traction!

Either pitching the car up in the air too much because it is not resilient enough - which is what Smith was talking about re: possible causes of understeer - or not allowing the suspension enough droop to maintain road contact amounts to about the same thing. How the car handles the rest of the track governs which you adjust to solve this. In cases like the bumpy old Mt. Tremblant circuit, damping was not enough and you had to go all the way and use very soft springs to keep the tires on the road.

Clear as mud? ;-)

Geo

Actually, spring rates have zero impact on load transfer. IIRC, ride height does however.

RedlineMan

Hmmm...

Indeed it would have been more accurate for me to say that spring rates effect the REACTION to weight shift. Judging by the further text of my reply, that is quite obviously what I MEANT to infer.

dbanazek

This is what bilstein has to say, which seems more inline with what you first assumed would be correct, more bump on the front = more corner entry understeer. They of course are refering to circle track cars here.

Loose (Oversteer) from 0 to 90
Increase compression rate on front.
Decrease rebound rate on rear, or only on the left rear.

Tight (Understeer) from 0 to 90
Decrease compression rate on front, or only right front
Increase rebound rate on rear, or only on the left rear.

Loose (Oversteer) from 90 to 0
Decrease rebound rate on front.
Decrease compression rate on rear, or only on right rear.

Tight (Understeer) from 90 to 0
Increase rebound rate on front.
Increase compression rate on rear, or only on right rear.