adrial

For a given corner, with X amount of roll, X lat G's in X car...if you stiffen the springs you loose grip. A stiffer sprung car has less grip than a softly sprung car, ignoring the angle of the contact patch.

Lets use a 500lbs/in spring for example. With stiffer springs you have more variation in normal force (you go over a hole in the road that is say one inch deep, the spring extends one inch). F=kx, so now the spring is pushing back with 500lbs less force on the road. Do the same thing with a 100lbs spring...spring is pushing back with only 100lbs less force on the road. So the car with a 100lbs spring has more grip in that situation. Same theory applies to smaller holes/bumps in the road.

Of course that is all really just theory especially with these cars, because its all about keeping that tire flat. You could just run -5 degrees camber with a stock suspension, but you will be penalized in straight line grip.

Danno

"I strong disagree with Danno feelings that the straighs are were time is gained. Nope time is gained by not slowing down for the corners."

We're talking about the same thing, just describing it differently. Not slowing down for the corners makes you faster on the straights. You are still spending more time on the straights than in the corners. By cutting off more time on the straights, you make a bigger impact on your laptimes. But that depends upon the course as well. In a tight autocross with minimal number of straitghs, the corners makes a bigger difference.

How you save that time is different with each car. A low-power car will require as much momentum as possible with the highest cornering speeds possible to get as high straightaway speeds as possible. A high-powered car faces a lot of issues with drive-traction and spins the tyres halfway down each straight. This car would get fastest laptimes by making the straights as long as possible, thus starting the acceleration down the straights as early as possible. Then there's everything in between...

"It says in that chart that increasing tire pressure in the front and conversely lowring rear pressure gives more oversteer. Is that correct? Seems counter intuitive."

Every tyre has an operating range where it's most optimal. Let's say we've got a tyre that works well from 38-43psi hot. It will have more grip at the high end of that range, beyond which it will drop off rapidly. That's due to the stability of the tread. The higher pressure makes for a stiffer sidewall, less flexing and less squirming and the contact patch stays flatter for more grip. However, higher pressures than that ends up reducing the contact-patch surface-area too much and you end up overheating the centers.

So if you have 43psi front and 38psi rear, the front will have more grip and you'll end up with some oversteer. Conversely 38-front and 43-rear will give you understeer. But you have to identify this range. Let's say you're at 40/40psi and it's feeling a little sluggish. So you add 2psi to the front and it starts rotating better and feeling livelier. Then you add another 2psi and it all goes to sh*t and starts understeering again because you're overstressing the front tyre now. So it takes a lot of trial and error and record-keeping to identify the best pressure-range for any tyre.

But like Joe said, you want to set pressures to where each tyre gets maximum grip. This provides the highest amount of total grip. You don't want to adjust F/R cornering-balance by taking away grip from one end or the other, that'll just reduce total grip and slow you down. You adjust F/R grip by changing the split of the lateral-weight transfer. If you want more rear grip, transfer more weight to the front by using stiffer front springs/roll-bars. If you want more front grip, transfer more transfer weight to the rear.

"Now lowering just the rear of a 944 has in the effect of causing understeer. Not sure exactly why, but in every 944 I have driven it does. Lower rear = more understeer."

Yup, that has to do with the camber-change curve of the semi-trailing rear arms. As the rear suspension compresses it dials in more and more negative camber. Compare that with the front suspension that changes to more and more positive camber with compression. If you lower the rear, you dial in more negative-camber and increase the rear grip (-3.5 to -4.0 seems to work best on stock rear suspension). This ends up causing the rear to stick better than before and the car will understeer assuming the front grip remains the same.

"Even so my 944-spec noticalble has more understeer and feels less lively with full tank vs empty tank."

What are the changes in your laptimes between a full vs. empty tank? Even with more understeer on an full tank, do you go faster?

"For a given corner, with X amount of roll, X lat G's in X car...if you stiffen the springs you loose grip. A stiffer sprung car has less grip than a softly sprung car, ignoring the angle of the contact patch."

This is true, but from a different mechanism even though the effect is the same. This is what Carrol Smith advocates as well, use only stiff enough springs to just barely bottom the suspension on the biggest bumps you'll hit and no stiffer. Unsprung weight (wheels, tyres, 1/2 the suspension) will always follow the contours of the road. The weight pressing these parts onto the ground is always the weight of the car regardless of the spring-rates. It's just that under dynamic conditions of hitting bumps and holes, that stiff springs and dampers affect the sprung weight, the body of the car, in a detrimental way, it launches the body upwards, which then pulls the suspension upwards and lightens up the weight on teh contact patch. With a soft suspension, the body of the car remains flat and applies even weight to all four tyres, even under cornering over bumps.

With a stiff suspension, a necessary evil to keep the tires vertical on our economy suspensions, you end up with diminishing returns. You do gain more grip by keeping the tyres vertical, but only on perfectly smooth surfaces. As soon as you hit bumps and holes in the corners, the bump forces will get transmitted through the suspension more with a stiff suspension and throw the body of the car upwards more than with a soft suspension. This causes the body to be traveling in a higher arc after the bump and this reduces the weight pushing down on the tyres after the bump, you catch air like on those pictures Matt posted.

So it's a criss-cross catch-22 situation, you gain grip from keeping the tyres vertical, but you lose it in bump-control. Ideally, you want to be able to pick up the wheels going up the bump and put it down quickly after going over the bump and keeping air-time to a minimum. This is like the giant-slalom skiers, they want to have soft and absorbing knees to soak up all the bumps and keep their head moving parallel with the ground. Which is where double-wishbone suspensions comes in. They can keep the spring-rates and dampers soft to follow the contours of the road-surface, yet they can also keep the tyres vertical with the ground for maximum-grip regardless of the amount of body-roll.

Serge944

E aho La ula - Wider is better.

MTM

I am very skeptical about this notion that softer is always better for handling bumps. I know this is not really an objective argument, but I feel much more safe driving on bumpy mountain roads now that my suspension is a lot stiffer than before. The car feels much more connected to the ground now.

I believe that (as adrial said) after you go over a bump with soft springs, the downward force will not drop as much as it would with stiff springs, but the problem is that it will take longer before it is fully recovered. So if there are several bumps in a row, the stiff suspension may have almost lost contact with the ground, but it will have fully recovered well before the it hits the next one. On the other hand, with the soft suspension, you are just kinda floating the whole time because it never has enough time to fully recover.

This is how I picture it (NOTE: I just made this up, it's not based on any formulas or factual knowledge):


I think this is also the reason why soft suspension is so difficult to work with in autocross. When you want to change directions quickly, you want the weight transfer to occur very quickly as well. Otherwise any series of quick changes in direction (like a slalom) will not give enough time for the weight transfer to recover. Because of this, soft suspensions force you to drive very smoothly, without making any sudden changes in input. You pretty much have to slowly coast your way through the whole course. The extreme opposite of that can be seen in go-kart racing. The steering response in those things is absolutely amazing. Weight transfer occurs pretty much instantaneously, so you can change directions so fast you get bruises from bouncing back & forth in the seat (which has like zero padding).

Please keep in mind that I have not actually studied this at all. It is all purely speculation on my part.

Danno

You also have to separate the spring-rates vs. dampers. A lot of what you're talking about has to do with the damper valving... Personally, I think they make a bigger difference than the actual spring-rates themselves.

porsche924

Wow, a lot of interesting facts that have cleared some stuff up for me. My questions is, i have a 944s2 cabrio, i added a subwoofer and amp with a fairly large box to my trunk, the total weight is a good 70-100 lbs. i have noticed since i did this that i get more oversteer then i used to. personally i think it handles and performs better then it did before. does this make any sense, or is it just one of those it just feels that way things.

thanks
bobby

Danno

Well, it handles more neutrally and may feel better, but it shouldn't be any faster. The limitation before was the adhesion limit of the front-tyres. When you're at 100% the cornering limit and the front tyres start to slip, you may be at only 80% of the rear tyres (understeer). Then by adding extra weight in the rear, you reduce the grip of the rear so that when reaching the same 100% limit in front as before, the rears are now at 90%. The actual cornering speed should be be the same because it's still the front tyres that's the limitation. By reducing some of the rear traction, you may make the car steer and handle more neutrally, but you're not going to be faster at only 95% of the total traction as before.

M758

I have noticed this for both with fuel changes and if you lower the rear and re-aling back to your orginal rear camber settings. So static camber changes are really playing a partial role. Not sure about dynamic changes.

Nope I don't go faster. I typically feel it and I can't drive the car as hard in critical corners. I have never tracked it completly, but it is about .5 sec to 1 second. Remember some of that is just due to plain old weight. In order for me to get best lap times I need to balance the car on a knife edge in certain corners and be flat in others. Add in a little understeer can cause me to back off in some corners since I can't get the car to turn and in others it becomes more difficult to balance. In yet other corners it is harder to get car to transition from left turn to right turn (or vise versa). This means I can't get the car though quite as quickly. In order to understand what I am taking about you need to drive the **** out of a 944. You really need to slide the car all over the place to achieve best laps. Once you start doing this a little too much under or oversteer will cause you slow as it becomes harder to balance the car. If 944-spec ever feels in control you are simply going two slow.

Example at place you know. Turn 5 at Willow. Come down the hill get the back end to start rotating as drop down in the turn. Wait for the car to get pointed where you want it add power. Do it right and you carry big speed. Be alittle off and you backwards in the dirt. Bascly every lap you lose the back end and try to catch it just in the right spot. Here it is critical to know how much you pitch the car and still recover. Too much and spin, Too little and you don't turn.

That is just one example, but willow in general does not require much sliding. Speeds are too fast. Maybe in turn 9 and turn 1 too, but I have not gone fast enough there (only race there 4 times) to push it that hard. Cal Speedway has a bunch of side the back corners. Nearly the entire infield except for the two flat our chicanes.

M758

With respect spring stiffness.

Stiff springs in the racing world means the car react faster to input. The weight is transfered faster. This good since you can do more with the car. Soft springs = slow response.

Now on most street cars there is something call ride comfort. In race car you go a stiff as possible. If you had perfectly smooth surface you ditch the springs all together. Well tracks are not that smooth so you need to soften the suspension to handle the bumps. The bumpier the track the softer the sprigns needed to maintain road contact. In real race cars the camber changing effect of body roll is tiny effect since it is most dailed out with geomerty or in general the roll so tiny anyway.

Street cars are very different due that ride comfort. In general stiffer = faster a 944 is like 140 lbs/in so until you get to like 1400 lbs spring you really can't get too stiff (assuming the driver and dampers can control this).


MTM you are right on about autocross. In my stock springed 944 I need to plan ahead and give the car inputs BEFORE the turn so that when I get there the car is already set. This is especially true of slalom sections. Now this is not too bad if you can plan for it. However the down side is that it is harder balance since tiny adjustments take longer to infuence the car. The upside is that if you have well tuned butt you can feel the car being off a little sooner than in super stiff car simply because it moves so much.

A super stiff car can catch someone out easily since they feel nothing and the all of a sudden the car is gone? Then they wonder why. A soft car will being to roll alot before losing it. Once gone it takes more effort to get back however. With stiff car you can be late with reaction to sliding. It has to be now otherwise the car will be past that input. Nice thing is if you respond fast enough the car will do every thing you want as soon as you think about it.

L8 APEKS

This thread = Too smart for meeh.

It looks like I'll spend my suspension money on Advil instead...ugh!