I am mainly interested in information that relates to street tires (say 17" summer runflats..) and stock suspension/alignments. I am also interested in understanding how street tires behave in autotox and road course work.

I have been trying to educate myself about tire performance with the front drive Mini, as most of my driving experience has been with rear drive cars.

I have done my best to use the search function for most of the NAM forums. I have learned alot about rim size, offset, tire diameter, and wheel weights.

I chose to message at STX because street tires can be used in this class, and members here probably have more experience than others with tire pressure experimentation.

Tire pressures have me a little confused. I have made the observation that the pressures people use; are determined by tire sidewall stiffness and if there are any modifications to the suspension/alignment. (to name a few things..)

Here are my questions:

1. Please explain how "rotation" is used to get a better "line" during an autocross. ( I would guess rotation characteristics would not be that desired on a high speed road course..)


2. How could you adjust the tire pressure ratio to get rotation with street tires (say for 17" summer runflats..) and stock suspension? (I have seen all sorts of different inflation ratios on these forums...It must have something to do with tire type and the extent of suspension modification)
If a probe pyrometer was used; what temperatures would be considered too hot for summer runflats during autocross?
What is a good starting point for cold tire pressure using summer runflats during autocross?


3. What would be reasonable (cold) pressures for a performance drivers school (road course) on stock 205/45 x 17 summer runflats?

4. Do you have any opinions on tire pressures for good street performance using the summer 17" runflats? I have seen recommendations from those I trust anywhere from 32/32 to 38/38! 35/35 seems to be the most common. My 2005 doorsill plate states 38/38, and the manual states 33/33 for two people. Safety on higher speed freeway ramps...and in the rain would be two factors important to me.

I hope that by understanding the answers to these questions about stock runflats and a stock suspension, I can then start to understand why R-compounds, alignment changes, and sway-bars are used.

Please answer any of the questions that may interest you.

Thanks for your patience...

Steve

 

I'll take a shot, but I've got limited time this morning

1. Please explain how "rotation" is used to get a better "line" during an autocross. ( I would guess rotation characteristics would not be that desired on a high speed road course..)

Most cars, mini's included are designd with some degree of understeer. As you turn into a corner the car wants to go straight or push. Autoxers all have driving styles from setting the car up to be neutral or with varying degrees of oversteer. So rotation is used to get through the corner quicker not take a better line. A well set up Autox Mini is rotated with the throttle. It's a slight lift off the gas to unweigh the rear in the turn, then as the nose tucks in, back on the the gas to continue out. The worst case of this is called snap oversteer and should be avoided, but moderate, controllable oversteer is a good thing -- even on a road course - but more so on an autox course.

2. How could you adjust the tire pressure ratio to get rotation with street tires (say for 17" summer runflats..) and stock suspension? (I have seen all sorts of different inflation ratios on these forums...It must have something to do with tire type and the extent of suspension modification)
If a probe pyrometer was used; what temperatures would be considered too hot for summer runflats during autocross?
What is a good starting point for cold tire pressure using summer runflats during autocross?

With runflats you can either overinflate or underinflate the rear tires in relation to the front pressures. THere are a lot of variables - if tire pressures are your only adjustment for rotation - then you tend to adjust them more. BTW, nobody autocrosses on runflats very long. There are very good high performance summer tires out there made by Kumho, BFG, Goodyear, Falken, Hankook, etc. for STX or the street. Runflats are a compromise. I run Azenis on the street, and have a set of Blizzacks for the winter - no all season compromise for me.

Out of time - will try and continue later

David

 

Thanks so much David for your early morning reply! I was getting into your answers like a good book...:smile:

No need for immediate reply...just when you get a chance.


More about question 1:

So rotation is about "position"...I guess that is what I meant by a "better line".....

Does rotation getting the car quicker through a corner, have anything to do with less friction on the tires and not "fighting" the understeer that you mentioned?

I think I now understand how the technique of "throttle lift" is used..thanks.


More on question 2:

So you can overinflate or underinflate the rears! That is why I have seen so many setups that seem to conflict with eachother.

With the runflats (if that is the only adjustment for rotation); more pressure in the rear than the front for rotation....correct?

You will have to explain some theory to me here; why does inflating the rears more cause rotation....I just don't understand. Thanks. (with reardrive cars; I thought the solution was "add more air to make the tire stick better")

Yea...I read enough on these forums to realize how bad "runcraps" are.
I just wanted to use them in these questions because I have them on my car, and know that they are a "stiff" tire.

Thanks again for your time.

Steve

 

With my car, I'm amazed what a differerence 3-4 pounds more or less pressure in the rear tires makes. I'm running the old Falken Azenis on 16-inch SSR Competitions. I usually start out at about 35 lbs front and rear, and on most surfaces, at moderate air temperatures, this is great.

Last outing at FedEx, very hot conditions, my car was extremely loose on my first run (too much/too easy rotation). It was scary in the transitions that are so popular in WDCR autocross course designs, fine in the slow stuff.

I then bled 3-4 pounds out of the rears, and the balance was much better. Less rotation, more stability. Some times, that's all it takes.

On the OEM run flats, I would think this sort of tinkering is harder to achieve, because the sidewalls are so incredibly thick and stiff. This is another reason to get a set of "normal" street tires or a set of R-compounds, depending on your class. Handling becomes not only better but more "tunable."

 

I was not really planing to use the run-flats during an autocross school, it was more of a "theory" question. I was interested what someone would do with run-flats during autocross (and road course), because they are so "stiff".

Very interesting how your tires responded because of the heat. I wonder what the temperature on the rear tires was before you let out the air?

Could you explain the reason (theory) why letting out 3-4lbs made the car more stable?

Thanks again for the tire details from Fedex.

Steve

 

I observed those handling characteristics right off the starting line, first run, so I am thinking the optimal pressures were REALLY 36 front, 33 rears, more or less. So I had too much pressure at the rear and not enough traction. Lowering the pressures on the rears after first run fixed that.

If you are running STX and have modded your car, handling characteristics will also be a function of those changes. So how your handling responds to air pressure changes varies from car to car, since our mods are all different. YMMV.

 

JimInVirginia;

Thanks for the 36/33 update.

What was your impression on the effects of the Helix bar when you first put it on?

What is your opinion on a Helix bar for the street...would it give too much rotation on rainy streets if one was using the factory camber specs?

Steve

 

I put the bar on with the coilovers, so I can't isolate the impact of just adding the bar on its own. I'm sure if you added the bar and used the middle hole, it would be fine for the street.

 

Great Site called "Turn Fast" - I especially like the part on handling - it is written more for the road racer and not the autocrosser. It talks a lot about tire traction, pressures, and temperature.

Link

 

Great link, David. I bookmarked that one. It prompted me to recall this one, which is certainly germane to the present discussion:

http://www.wtrscca.org/tech.htm

Cheers...

 

Dpayne1;

Thanks for the link. The Turn Fast site is fantastic! A wonderful source of material.

Boy did I learn something. One should start out with less than street cold temps. on a road course! Somehow I thought that street cold temps. should be increased before driving on a road course (to make the sidewalls "stiffer" for performance... )

The section on front wheel drive tire temperatures and pressures is also very interesting. How the fronts may go up 9 degrees while the rears only
5 degrees.

Tire temps. first....then adjust hot pressures...then adjust slightly to get more or less oversteer. I like the statement; "that this may result in some strange looking cold temperatures for a front-drive car."

It seems that in a road race car you have to do some serious laps before the tires come up to their desired temperatures. That explains alot of the "incidents" that I have seen on SpeedVision.

Always be aware of outside and track temperature changes. I never considered how ambient temperatures could make such an influence on track "set-up"

Well can't wait until tomorrow to use my pyrometer on the Mini.

Maybe I will be able to answer my own questions about what tire pressure is best for the street. (don't worry...I'm not going to do anything dangerous) It will be at least a start in understanding my summer run-flats. It will also be interesting to see how close my numbers are to Mini's 33/33 cold.

JiminVirginia; thanks for the other website. I will have to study that one tomorrow!

I appreciate everyones assistance.

Steve

 

I'm Rodger, a JCW convertible owner in Montana. I'm autocrossing tomorrow and wanted to check out some tire pressure ideas.
Cheers,
Rodger