PCA SDR

Fontana was rough on tires and brakes. I corded two right rear tires from two different sets. So, I'm in search for at least one tire to make one of my sets complete for Pahrump. Does anyone have one Yokohama 280/650/18? I don't care about the compound.

Just sent you a PM

glenn_993 wrote:Fontana was rough on tires and brakes. I corded two right rear tires...

Interesting, Glenn. I, too, corded two right rear tires. Both at turn 9. My thought was that my right rear caliper was overheating because I did the damage near the end of each of two sessions. I'm interested in knowing if you had any other thoughts on why you had a similar experience.

Corded Right Rear? A 911 on the Oval? Makes sense to me - I also had significantly more wear on the Right Rear.
I have heard more camber on the right (especially rear) is called for at this track.

Running Las Vegas Speedway should have caused this asymetrical wear even more (20°banking), but I have only run that course in the rain (less wear, lower speed).

pdy wrote:Corded Right Rear? A 911 on the Oval? Makes sense to me - I also had significantly more wear on the Right Rear...

Thanks, Paul, but I see my post wasn't clear. I should have said I flat-spotted (to the cord) two right rears at turn 9. I think my right rear caliper was overheating. Any other ideas?

Why do you suspect the caliper?
-Are you thinking it overheated and then locked up? If so then wouldnt you have numerous flat spots?
-Are you thinking that back in the pits the overheated caliper affected the tire? Seems like it would affect the tire near the rim first, rather on the tread area.
Was your suspension bottoming out so the top of the tire hit the top of the wheel well causing it to temporarily lock up?
Just some wild guesses.

richard wrote:Why do you suspect the caliper?
-Are you thinking it overheated and then locked up? If so then wouldnt you have numerous flat spots?
-Are you thinking that back in the pits the overheated caliper affected the tire? Seems like it would affect the tire near the rim first, rather on the tread area.
Was your suspension bottoming out so the top of the tire hit the top of the wheel well causing it to temporarily lock up?
Just some wild guesses.

Thanks for the feedback, Richard. I was thinking the caliper overheated and locked onto disk because it didn't happen in either case until late in each run session. There was plenty of tire smoke coming off the right rear tire as broke into turn 9. And, there was only one flat spot because I eased into the pits after that first flat-spot each time. I'll work with Mark K. and see if we can figure it out, but that's just my best guess at this point.

Don,

Has that happened at any other track? Do you trail brake into that corner?

Just my theory, but the lock up on the right rear is more than likely because it's the unloaded wheel, coupled with the road being off camber there that would cause the lock up.

SDGT3 wrote:Don,

Has that happened at any other track? Do you trail brake into that corner?

Just my theory, but the lock up on the right rear is more than likely because it's the unloaded wheel, coupled with the road being off camber there that would cause the lock up.

+1. With the combination of a Braking Zone that turns, and being off-camber, and wanting to trail-brake into 9 instead of braking across the line to the very outside before turning in, I'd guess it's Peter's theory in action. Couple the improbability of trail-braking into 9 without ABS or a limited-slip diff.; plus heat derived high-inflated tires at the end of the run session causing slight crowning of the tire (less contact patch when overheated/over-inflated), plus a little adrenaline during competition driving, and you have the perfect storm of off-camber/unweighted/turning-while-braking/over-inflated tire pressure conditions that will presage lock-up and flat-spotting.

If you go to the DE in May, near the end of the run session try a different line during braking (brake in a straight line and skip the trail-braking) and turn in later (that's what I have to do in the SC). I think it will reduce the chances of inside wheel lock-up. See if the issue isn't caused by the conditions on the track rather than a mechanical malfunction.

Just my 2-cents.

First, I'd like to thanks the SDR family for coming through for me. Thanks Gunter.

Second, My tire problems were probably the result of turns 1 & 2. I should have swapped the tires from left to right after a couple of sessions (As Jae suggested )

Plus, one of my almost new Hoosier R100, developed a split/crack in the the sidewall. It was on the left rear outside sidewall. So I only did one lap during timing, to make it offical.

Don, could it be your rear brake proportioning valve? Carreras have one to limit the braking force on the rear and (hopefully) prevent the rear from locking.

More brainstorming:

WRT the caliper "overheating and locking up", consider that when braking, the pad is forced against the rotor. Heat is generated and moves into the rotor and pads. The heat going into the pads then goes into the caliper piston, the brake fluid, and the caliper itself. Depending on the air flow and the boiling point of the fluid (and other factors) you will probably see:
- the fluid boils and the brake pedal gets spongy , called brake fade, or
- the pads get so hot they lose their "friction", the pedal stays hard, but the brakes don't work, also called brake fade.
I don't see how the caliper would "lock up".

If one brake cylinder does not properly retract after the brakes are applied, causing the pad to continue rubbing on the rotor, this pad will heat up abnormally, fade sooner than the others, causing uneven braking action, causing the otherside to work more, which the proportioning valve isnt expecting, causing possible lock up. Jack up the rear and see if one side turns much harder than the other.

SG is probably rolling his eyes at this.

Peter/Dan, thanks for the feedback. I was wondering about the trail braking issue. Having you mention it adds to my suspicions that there may be something to it contributing to the problem.

Gordon, you may on to something, too. I had the right rear locking up on a left hander at an AX in the fall last year. I decided not to react to it until I put some good rubber on the car. So, with new tires, I went to Spring Mountain, Buttonwillow, an AX and Streets without a problem. I was thinking that the earlier problem was just bad rubber. Then, with new rubber at Fontana this month, I flat-spotted two right rears (ouch!) in turn 9.

So, it can't be coincidence that both flat-spottings occurred in turn 9. But, it is probably also not coincidence that it occurred both times late in the run session on a track which is hard on brakes.

My guess now is that I have a problem that causes the right rear caliper to overheat, relatively to the left rear. I had a problem all weekend with a soft pedal that brake bleeding 4 times did not resolve. But, in addition to the overheating caliper, the addtion of a little trail braking into turn 9 was just enough to allow the overheated right rear to lock and flat-spot the tires.

Good stuff. Thanks guys.

richard wrote:More brainstorming:...

Richard, just saw your post...you may on to something, too. If the right rear caliper is overheating, the braking there might just fade away, and not lock up.

Perhaps the source of the problem is a combination of 1) lack of balance and the left rear fading, 2) brake system problem that made brake modulation difficult, and 3) some trailbraking into turn 9.

Time to jack up the car and check out the calipers. Thanks for all the thoughts.

Don,

You need to check your read brake calipers first.
The pistons need to be at 20 degrees...
Look at this 356 brake article.
http://www.356registry.com/techinfo/rep ... rhaul.html

Look at the 8th picture down.
You need to make one of these tools.
In the picture shown with the tool; draw and arrow pointing down and that will indicate rotation direction of the rotor.
Pull your pads and check this first.
Read the article to find out why.
Pistons should also move in and out freely and dust boots should be intact.
If not, it's time for caliper overhaul.