Problems with tracking the Challenge Stradale

Will you be running the same size Toyo R888's as the stock ones? I'd love to hear your impressions on how it works with the Scuderia's suspension setup.

 

entry understeer is a completely different phenomenon than other types of understeer. ALL cars will understeer on entry if you go in too hot. obviously, this is because you are asking too much of the front tires, and the fix is to slow down. this is a deficiency in the specific car, as factors such as more front downforce or a different alignment can give more grip at entry and allow more entry speed. that said, we can completely ignore this aspect of understeer for our discussion.

steady state understeer, just for an example of one of the frustrating kinds of understeer, that is not "the driver's fault", is when you are just on maintenance throttle and the car is understeering -- either leaving you to miss the apex and possibly even the exit. this is the kind that drivers generally don't like but it is safe. it limits the mid-corner speed of the car to below its potential, and can be tweaked by changing the suspension. it is not "driver error" (going too fast), because the car in fact can be faster if more grip in the steady (but loaded) state can be moved to the front. it's not always the case that this can be achieved, as the rear of the car may be under-tired and adding more front grip might mean taking away more rear grip. but in general the easy things we can change are alignment settings, shock settings, and sway bar settings to make the car more neutral. not in a street car so much, but also we can change harder things like track width, ackermann angle and bump steer (and suspension geometry in general) to affect the balance of the car.

camber has a relatively minimal affect on balance but for a typical stock street car we are generally left with camber and tire pressure to affect balance. more tire and a wider front stance or track change is another easy "adjustment" that can be done which will have a bigger effect.

the suspension geometry and tuning (shocks, sway bars, etc) of a street car is designed to understeer in steady state or power-on conditions.

an easy mental exercise is to imagine skinny bicycle tires on the front. once you turn in (ignore entry understeer, as said above a different kind of phenomenon) you are extremely limited in mid-corner speed by the front tires. at exit, if you try to get on the throttle, the weight transfer will unload your front tires (at their 10/10 limit) and the car will understeer. change your suspension (e.g. shock settings) so that weight transfer to the rear is much slower, and it will take out some of the understeer. add more front tire, and it will take out some of the understeer. the standard driver's fix for this is to carry more brake into the corner and rotate the car on entry, to allow for a straighter exit. ie, trail braking. (which isn't used just to combat understeer, i just present it as one way to help understeer.)

You mention "correct" speed. Yes of course there is a "correct" speed where the car doesn't understeer. This is also called "too slow". When the car is not neutral (neutral = we run out of grip at both ends at the same time) there is more speed left and it is due to the suspension design and/or tuning. Going "too fast" here is not driver error, it is a car setup problem.

The skid pad is a great way to test a car's understeer/oversteer. As you exceed the limit, does the car push or get loose?

 

Again, yes they do. Street cars are designed to understeer. That is, virtually all street cars will lose front grip "well" before they lose rear grip.

The reason we have to mitigate understeer at all is because the car is designed to do so.

 

Are you saying, then, that when a perfectly set up racecar understeers that it's the fault of the engineer?

 

Discounting the issue that there is no such thing as the perfect setup (tire conditions change even during a single lap), yes.

There is a tuning and setup that you do for track conditions and driver style (smooth, twitchy, whatever) and the type that you do to setup the basic characteristics of the car. The engineer that i think you are referring to works on the former.

Because all corners are different, all racecars will understeer/oversteer depending on the corner. You tune for the most important corners and for the driver's style. That means some of the other corners will suffer.

If your racecar (ie, fully adjustable suspension) understeers everywhere, that is either the fault of the suspension design or, yes, of the engineer. well, ok it also could also be the fault that the driver can't correctly/adequately communicate what the car is doing back to the engineer.

A street car OTOH will understeer in nearly all track conditions, all tire conditions, etc. It's inherent in the design for safety reasons.

 

b-mak, if you could drive my FE formula car you would really understand the meaning of understeer. it cannot be tuned out due to the very restrictive class rules. you drive around it, ie you use whatever techiques to avoid the understeer, but that's just a driver adjustment to make the car go as fast as it can go. if the car could be changed to take out the understeer you could go faster.

i guess i understand what you're saying that the driver influences whether or not the car understeers, but just because i don't experience severe understeer doesn't mean it's not inherent in the car. my car doesn't understeer but that's because i drive a certain way to make up for the car. i mean, if you looked at my data you might say "i don't see understeer". but it wouldn't be correct to say the car does not understeer.

 

mousecatcher, you're almost at the conclusion. Keep your dialogue going.

 

actually, i already reached the conclusion back in post #15.

 

Thank you! This is the key to the entire thread:

 

If you mark out a constant radius circle and drive your car around it, increasing the speed every second revolution, eventually the car's tyres will break away at either the front or rear.
If the front goes first the car understeers. If the rear it oversteers.
Obviously transiant over or understeer can be induced by driver inputs etc but when speaking of a car's set up balance that pretty much describes it.
There's some good suspension books around and I'd really recommend getting one or two so you understand the basics of suspension set up.

 

Don´t get that, if i get on power to hard , to early I end up spinning, not getting understeer!

 

No kidding! How does this happen?

 

Rear wheel drive behaves differently than front wheel drive when the driver overloads the driven wheels.

 

Exactly.

 

Yes they fit.

 

I'm running a new set of 235/35-295/30 R888 this weekend on the stock wheels.

I have run the following combinations so far in the Scuderia: 235/35 - 295/30 A6, 235/35 - 315/30 A6, 295/30 - 315/30R19 (-4.85% ratio difference with the stock sizes), 295/30 - 345/30 A6. All of them with different variations on diameters compared to stock, but keeping them under what I guess is the tolerance in the TC/ABS/CST electronics, so far no issues. At all times I have tested the stock Corsa system against these combination of tires, the awful Corsa tire has been up to 6.5 secs slower on a 60 secs course compared to the A6 sizes that work the best for me right now.

When using R888, the biggest ever difference I saw between them (21 Heat cycles) and brand new A6 was just 2.5 secs. At $1100 per set, and being a street friendly compound, R888 are the best choice for performance, price and street/track ability on 19".

The number designation on the tire sidewalls is for reference only. I measured the stock Corsa 285/35R19 from my Scuderia, and the tire is wider than a 295 Hoosier A6, even though Hoosiers are among the widest tires available for a given number designation. At the same time, the front 235 Corsa is much narrower than the 235 Hoosier A6. The Scuderia stock Corsa should be labeled 225/305. Within the same brand and models there are variations, for instance the 235 A6 is wider than the 265 A6 on 19", the 255 A6 on 18" is wider than the 265 on 19", and it gets worse once you compare tires from multiple brands.

For track tires, the recommendation should be based on intended use and goals. For tires driven to the track on public streets I would choose R888 or MPSC only. For dedicated track tires, I would choose Michelin/Pirelli slicks on the Challenge sizes, A6/R6. These are the decent choices in 19". If you can run 18" wheels (Stradale can, Scuderia cannot), then add Yoko/Dunlop/GY slicks for dedicated track use, forget about the Challenge sizes and run wider tires, RA-1/NT01 for street/track. Sizes would depend on driver's choice and skills, changes to the suspension are not just recommended, they are required. I was riding on the bump stops with the stock low grip tires, this can be very dangerous at a racetrack.

 

6.5 seconds? I ran nearly dead dried out the corsas last week earlier in the day on the same course you did. My best time 43.7 iirc on bad shocks babysitter off huge pig of a 550M. Your 36:xx + 6.5 puts you almost equal to me. I don't know if any comparisons could be drawn but your car is way faster than mine! I don't think on fresh A6's I'd shave anywhere close to 6.5 seconds. Your A6's last week must have been toast.

 

My A6 from this past weekend were toast indeed, they had 2 practice days and three events on them. On fresh A6 I could have dropped another 2 secs.

Also, this course was a mid 30s course, so a difference of 6.5 secs over a 60 secs course between fresh A6 and Corsas, would be down to just 3.7 secs on this smaller course. Then we need to analyze each course independently, there was a decent straight from the first turn around to the start of the slalom, on such a long straight Corsa vs. A6 will provide the same level of grip, so you can eliminate the length of that element from the total time to compare the effective grip level, and we are probably down to a course of less than 30 secs on transitions.

OT: You guys had two of the nicest cars there, a NSX (Ayrton Senna tuned) and a 550 (a Mario Andretti favorite). You should come this Sunday to El Toro Airport in Irvine,CA. Porsche Club setups high speed courses on a high grip Airfield surface, this is the same spot where Ken Block's Subaru STI Rally video was filmed. I saw a Ferrari 360 Spyder registered for this Sunday, and a multitude of Exige S, Z06, new Viper ACR, 997 GT3, GT2, lots of fast cars running.

 

How and who do you register with? POC PCA? Its a drag ...strip auto-x...High speed blast down a long runway?