sjfehr

What's odd is how Porsche would go to such great lengths to design such well-balanced cars... then spec out stock wheels and tires that make 'em plow like a FWD. Probably just trying to keep people out of the ditch, while still allowing enthusiasts an easy upgrade path to the true design performance.We recently had a big discussion about this over at autocross.us that's worth a read if you're interested in this sort of thing:
http://www.autocross.us/forums/index...topic=4410&hl=

Short answer is that when you're cornering, you're never really going in a straight line, the rubber is bending and stretching, and you crab sideways a little, kinda like a sailboat or an airplane in a cross-wind. This is called "slip angle." Wider tires (due to the wide thin contact patch) allow grip at a greater slip angle than narrow tires before the rubber stretches too far and lets go of the pavement with the audible squeak/squealing you're no doubt familiar with. This gives you better overall cornering force.

In theory, you can run wider tires at lower pressures, as well, which increases the size of the contact patch (at decreased pressure and thus increased coefficient of friction), and increases overall traction.

FYI, I ran last autocross season OEM stock, but for this season, I'm putting bigger tires up front and increasing front negative camber to reduce understeer. I don't want to increase oversteer, as that would mean less overall traction. But I would like to decrease understeer. I'll let you know in a few weeks how well it worked

AudiOn19s

If we're speaking just on technique and not just setup changes, Keith Code's "Twist of the wrist" series is an outstanding read. Yes, they're geared towards motorcycles but the constant focus on contact patch management which is critical on a bike directly applies to our cars to a fair extent as well. They're some of my favorite books and I read them once a year as a refresher.

springgeyser

I'm speaking from my actual experience with zero suspension changes in an AX envrionment. The wider front tire reduces understeer and allows for more effective braking.

8.5"/10.5" rims
225/295 huge understeer. PS2
235/295 understeer. PS2
255/295 almost neutral. PS2
255/295 w GT3 bars. neutral PS2, R888

This is not even talking about overtiring a 255 tire on a 7.5" rim which I think you are referring to. My effective contact patch is much wider on a 8.5" rim from 225 to 255 tire.

sjfehr has it on the mark. BTW I am trying out a 265/285 combo TY which I expect to be loose (R1 size/diameter limitation). I still have my 255/295 r888 mounted if you local, you can slap it on and run some skidpad test to realize the huge difference what a 'little' the extra front width can do to balance the 996.

ArneeA

Just lift off the throttle.

JimB

This is way too complicated a topic to make much headway in a discussion thread but I will say the 996 comes nicely set up from the factory. Let's face it, a neutral car is really an unpredicable car. You never know if it will understeer or oversteer. A car set up to oversteer is a handful. I'm sure we all think loose is fast and we're all fast but let's get real. Very few can drive a loose car fast. A car with nice controllable understeer is a joy to drive fast.

Just my 2 cents.
Jim

p.s. if your 996 pushes like a pig, it ain't the car!

SirPzalot

I have never liked understeer. To me it is an uncontrolled slide. Now a car that tends to oversteer is a slide that can be controlled. As the rear steps out you can lift off the throttle leaving the car pointed at the apex, in effect tightening up the corner. Where as an understeer that is headed for disaster leaves one choice, get the tail out . If there wasn't room or time it can be ugly. That is why I prefer a set up that induces oversteer rather than understeer. So to be more specific, what sway bar change is needed to reduce understeer, making the car more neutral with a tendency towards oversteer ?

himself

I haven't put a calculator or serious brain power to this, but my first thought responses are below. My two premises are: contact patch is dependent only on weight and pressure. Friction is [mostly] dependent on tire compound and surface conditions.

There may be significant differences in handling due to sidewall thickness alone. Changing sidewall ratio can change the center of gravity (ride height), rake, and turn-in responsiveness.

Regarding slip angle and the crabbing tire: different compounds behave differently. I would guess that some manufacturers design tires to operate with particular dimensions contact patches such that the generalized concept may not apply. Further, for hard tires, like the PS2, I am not convinced that the "crabbing" theory holds up. With racing slicks, I'm fairly sure there is something going on requiring wider tires - perhaps to ensure integrity of the tire, but I haven't seen data or been convinced that it affects contact patch, slip angle, or friction characteristics. Some people say yes, other peole say no. Despite both "people" being educated they still disagree. I'm on the fence.

What is "effective tire patch"? And, the actual patch should be WIDER, since it has 30mm more width to start with. But that doesn't mean it has more contact patch area for the same car weight and air pressure.

I appreciate the offer, but I've actually driven your tire combination, and a *few* others. And I'm not convinced that (for my car) wider fronts descrease understeer. [In the last few months alone I have driven: Yokohama A005 slicks; Hoosier Slicks; Dunlop Slicks; Hoosier R6; Michelin Pilot Sport Cups; Toyo RA1; and Toyo R888]. I have both 8" and 8.5" fronts, with 10" rears. In my experience the REAL different is setup, not tire width.

I do recognize that for some cars all there is to adjust is tire width and pressure.

Like I said, I haven't studied this at all, so I may be way off - but I can't believe the answer is as simple as "wider is better."

-td

FWIW, my car has been through 3 full suspensions, 4 different types of sway bars (adjustable and not), adjustable and non-adjustable drop links, different alignments, ride heights, track widths (spacers), etc etc, so I really haven't been paying attention to tire width and handling.

springgeyser

The OP simply asked what minor change can be made to dial out some understeer. To me it all starts with tires assuming all other variables being fixed.

I am speaking what works for me, with your car do with whatever you want. No point quoting and being somewhat critical on what works for me. I only upgrade one area at a time to note the results before making further changes which has worked out nicely for me.

Neutral/tight/loose is relative on speed/driver anyways. Loose for one driver, might be tight for another. For AX I like it neutral (controlled slip angle for all 4 wheels), which of course in a real slow hairpin there is still understeer but in general neutral handling throughout most of the course at 45-55 mph. For DEs, I loosen the rear sway bar a notch to have a bit of understeer to minimize the 'pucker' effect.

There is a diminishing return going with a wider front tire past a certain point. I do not know what that threshold is currently. Hoosier is making a 255 a6, how many AX drivers in Stock classes will be upgrading from their 245s front tires?

JimB

To use oversteer to rotate your car prior to the apex you turn in early and trail brake hard. The heavy braking while turning shifts your weight forward reducing traction in the rear which induces oversteer. You better not plan on lifting to catch the car though or you will be in the weeds. The right thing to do is to transition from trail braking to fairly heavy throttle (depending on the corner) to shift the weight back to the rear taking the car from oversteer to neutral at the apex.

Because you come into the corner early and fast and brake very late this can be very fast but it's not for beginners. One false move and you are in a world of hurt.

If you really feel the need to change your car to help with this the best way to help the weight transfer. If you have 2 way adjustable shocks increase rebound in back allowing the rear to lift more under braking or decrease compression in front allowing the it to dip more or both.

If you don't have adjustable shocks do something to increase turn in like softening the front bar or stiffening the rear. The downside is that this will also increase oversteer under acceleration at and after the apex.

You can also play with toe. A little toe out in front and toe in in the rear will help. Or pressures, or.........

JimB

My only concern with putting really wide tires on front is getting them warm enough to stick. On the track or AX this might work but on the street too wide a front tire could actually increase understeer.

FWIW, when I raced a 996 I tried every possible tire combination and found a 245-315 R6 combo to work best. I had lots of suspension changes though.

JimB

Grip is almost entirely a function of the adhesion properties of the tire, the adhesion properties of the road or track surface, the size and shape of the contact patch and the amount of pressure (weight) between the tire and the track. Almost all changes to the car and all driving techniques affect one of these properties.

No matter what car you get in the one thing under your control is weight transfer. Some racer (whose name I can't remember at 2:43 am) said learning to drive fast is all about learning to move your car's weight to where it's needed.

I'm going to bed.

sjfehr

Does PSM come into play at all here as an electronic means to limit rotation? I've been leaving it on, as it doesn't seem to get in the way at all, and generally keeps me pointed in a straight line if I screw up.

Steelkilt

apart from car setup and a good corner balance/alignment, you could practise trailbraking (and early turn-in), it deals with the understeer by keeping the front wheels planted hence giving you traction in front and lets you rotate the rear end around the turn letting you brake later, even finishing your braking at the apex if you can get the timing right. some understeer isn't a bad thing cuz it lets you play with the oversteer inducing techniques

JimB

I'm certain it would but to be honest I've carefully avoided PSM in my cars and I would never drive a students car hard enough to find out.
Jim

AudiOn19s

Jim,
Good to see you back here in the "regular" 996 board. This is one of the best threads in weeks over here.

Andy