I recently received a set of R6's and I put my first autoX on them (the tires not my first) and I noticed that my car understeers more with the R6's then with my street tires. I think this is because a combination of having a rear spring rate that's to soft and only a 19mm RSB.

I already wanted to go the coilover route and larger rear sway bar but budget says otherwise on the coilovers I want for now.

So my question is does anybody have any reason why I shouldn't run the star specs out back and the R6's up front? The only rear draw back I can think of is that the R6's get much stickier as they heat up but I wasn't able to heat my rears anyways.

 

This would just be for the next event as a test, unless it turned out to be the magic perfect combo.

 

Are you a very experienced autocross? If so then you can try it but I can tell you from watching others put R compound tires on the fronts and purposely mounting street tires on the rears that the two ends of your car will have wildly different levels of grip and as a result you will be either controlling or fighting oversteer, never mind your former experience with understeer.

Do you have two drivers for your car? If not, then usually Hoosier A6 or A7 would have been the tire of choice for autocross, as R6 is for track or road coarse or 2 drivers.

Controllable oversteer can be a workable approach but only if you can control it. Sometimes it can just make driving a bit much to handle.

Normally if you have any understeer you can go for a stiffer rear bar or softer front bar, or soften the shocks in front, get wider tires and wheels in front, increase negative camber in front.

Also how you drive makes a difference. Watch corner entry speed and try not to late brake which are factors in excessive speed leading to understeer.

Sometimes a good test is to put one of the most experienced drivers from your area in your car and see if it understeers, often an experienced driver will know just how fast a tight corner will allow and there may be little to no understeer.

Do you have a Cooper S? What size tires?

 

Why did you go with the R6? Also, what tire pressures are you running?

The guys that used to autocross the mini in D Stock used to run stupid high rear pressures. Like 55psi to get the car to rotate if it's warm out.

 

I managed a deal where the R6's didn't end up costing me anything and there still stickier then any street tires I've ever run.

 

I'm the only driver for the car.

05 MCS, in SMF. My car is localy competitive in this class.

 

So is there any reason I shouldn't try this at the next event to see what happens?

 

Worked okay for some friends with a Fiesta ST. Take it easy on your first few runs to see how (un)balanced it is.

 

This ^^^^^^^^^^^^^^^^

On the track car I start @ 34 cold front, 44 cold rear and target 48 hot rear. I'd try at least +10 psi rear before mixing tire manufacturers / construction / compounds.

 

Sounds good. that's what I think I'll do.

 

I assume you mean that with Dunlop Star Specs you are locally competitive in SMF?

If so, then you don't have anything to loose by trying R6 on the front with lower tire pressure to help warm them up faster and Star specs on the rear with relatively high tire pressure.

The Star specs already have less grip and even worse with higher pressures. I'd try something like 36-38 psi and then adjust based on how much oversteer you get. But that may depend on your driving style and skill level. The benefit is you can more easily get the car to rotate around tight turns but this will not help you with a higher speed sweeper turn so be careful as you can get power oversteer under those conditions.

What are your courses like? long or short, tight or more high speed? Concrete or asphalt?

Hoosier R6 is more durable than A6 or A7 but it does not have the grip level of the softer compound. It can be used for long course high speed auto crossing with two drivers to help bring the temps up to optimal levels. With one driver or on a short course (45 seconds or less per run) then it would be hard to get the rubber warm enough to get good grip, then it is only a little better than having Extreme Summer tires which I think you are finding.

How much negative front camber are you running? Hoosiers need or do better with -3 to -3.5 degrees if you have that ability with adjustable front camber plates. If you are running less than -3 degrees then you aren't getting max grip from the tires, you are going to see that if you check tire temps during the event. It will show hot temps on the outer treads and cool on the inner treads. If that is the case (use a probe style tire pyrometer to check) then let some air out. However Hoosiers tend to like being run at higher tire pressures for R compound tires.

 

On R6's I'm competitive locally.
45 second courses and a mix of surfaces and surface conditions at our sites.

I don't know my camber number (no plates yet)
Up front my temps stay pretty consistent across the tire (last year on street tires)
Out back I do heat up the outsides a bit quicker. (again on street tires)

I do hope to have A6's next year but I got these R6's for a couple hours labor so I couldn't turn that down.

I have been playing with pressures but I think to get the rear loose enough I'll a least need a bigger RSB.

I enjoy playing with different setups I just like to run the ideas by others first. ie not running wrinkle walls on the back of my old TA with radial street tires up front.

 

For a 05 MCS with no front camber plates it's likely you are at -0.5 degrees or close to that. This is not good for using Hoosiers which are designed to be used with much more camber.

From tirerack on using Hoosiers-
http://www.tirerack.com/images/pdf/warranty/HOOSIER.pdf
"less than -2.5 degrees camber may result in more shoulder wear"

In any event, when using street tires, they are generally more forgiving when used with less negative camber. All of the R compound tires favor -2.5 or more camber to make the best use of the "entire" width of the tire.

If you are getting even tire temps across the width on street tires with -0.5 camber then you are not driving hard enough or you are checking the temps too much after your run (tire will cool down) or you are not using the tire pyrometer correctly (maybe). Infrared pyrometers are not sufficient because they read only the surface, you want the probe style and press it into the rubber to the same depth for each reading. Ideal is to check as soon as possible after your run, it there is a longish return trip to the grid then you may be cooling down too much. Tire temps fall off very fast once you finish.

Without checking tire temps you really don't know how much tire you are using on any one corner, each tire may be differently used depending on the course and your driving style so don't assume the pressures to be the same for any given wheel or axle. I often have four different pressures by the end of the 2nd run.

The goal is to maximize grip for all tires, each tire heats up at different rates, fronts get hot faster most of the time.

Another idea is to use lower pressures for the R6 to drive in the first run and try to slide them around a bit on purpose to heat up the tires so that in run 2 you will have more grip.

I usually find that for my first run when the tires are cold I have about 80% grip and tend to slide around, after the temps rise to about 120-130 degrees, handling gets better. If your temps are very high then you can also use water spray to cool them off back to 120 degrees.