PS2 Meltdown

[thstone]

Originally Posted by jaykay

These were cold pressures...the beads on the front were from my tires there was additional pickup as well

Thanks all for your thoughts

My experience in street tire pressures on the track is as follows:

Neutral balance: 32F and 34R hot. Start at 28F and 30R cold and adjust after each session as the tires heat up.

A little understeer: 30F and 34R hot. Start at 26F and 30R cold and adjust after each session as the tires heat up.

I personally like a little understeer still in the car on the track so I aim for 30/31F and 34R hot.

[insite]

track pressures all depend on driver, tire size, tire compound, alignment, ambient temp, track temp, track surface grade, relative humidity & about thirty other factors......

if you are using air (vs. nitrogen), your back tires will tend to increase about 6-10psi and your front tires will increase a little less; maybe 4-8 psi. at jaykay's cold pressures, he was likely well into the 40's hot. this will cause treadwear & temperature issues.

jaykay posted two pics; the first one shows some mild overheating toward the center of the tread. if he would have driven extended sessions, i think he would have started to see a bit of chunking here. the little blobs are just pickup from the track & are no big deal. the second picture looks fine to me.

tires are a funny thing. they will respond in a specific manner to a POINT. then, they will respond in the OPPOSITE manner. tires have a sweet spot for temp & pressure. within this sweet spot, adding pressure will increase the efficiency of the tire, resulting in MORE grip. a common misconception is that less air = more grip. this is only true when you are outside this sweet spot.

tires have two primary modes through which they develop grip. the first is friction; when you rub a material across another material, friction tries to prevent motion. the other is called mechanical keying. the tire actually deforms to the shape of the road surface. when it does this, the surface deformation acts almost like gears in a transmission. the tire and road are 'keyed' together. combined, friction & mechanical keying are referred to as 'stiction' in motorsports circles.

at any rate, pressures & temperatures effect the amount of keying vs. friction that will contribute to total stiction. when there is too much pressure in a tire, there is too much mechanical keying & the tire begins to pull apart (much like stripping gears in a transmission from too much torque). this will cause heat buildup, which will increase pressure & chunking. this cycle will continue until the tire fails. reducing the pressure reduces the keying & allows the tire to slide over the surface rather than chunk apart.

too little pressure = too little keying. the pressure is not adequate to deform the rubber to the road (lack of grip efficiency). in this case, the tire will slide over the road TOO easily; the tire will feel sloppy and the car will float a bit.

here's what happens when you run hard w/ too much pressure:

[insite]

btw, notice in the above photo that the chunking tends to occur on the outside shoulder of the tire AND on the center rib.

the outside shoulder abuse we would expect, but the center rib? this is actually expected. the reason is that excess pressure rounds the tread surface (like a donut), causing excessive wear at the center of the tread.

[BYprodriver]

BFG KD's ? are bad about chunking.

[insite]

those are KDW2's, and you're right.....they are pretty bad about chunking if pressures go even a LITTLE too high. that set isn't mine, but i destroyed a set of fronts in 20min not realizing they'd run up to 42psi.....if i keep them at 40psi or less, they never have that problem (even at full tread).

[thstone]

My PS2's never chunked like those in the pic's, even when the temps got up into the low 40's, but they did wear quickly on the outside shoulders, lost traction, and had a thick "greasy" feeling.