how to avoid flatting?

[pmajka]

I ran 1 auto cross last year in the fall....I flat spotted my rears. Daily driver...If i want to avoid flat spots , should I get a harder or softer tire.

i was running Kuhmo 712's they are rated at 280 AA A?

[RaceProEngineer]

Well, ASSUMING that you will flatspot your tires at each future autocross, we recommend the softest compound you can get. That way, at least your first run will be as fast as possible before you destroy another set of rubber.

Now, for a real answer to your true question, "How to avoid flatting?" see below:

[emcon5]

Originally posted by RaceProEngineer
Now, for a real answer to your true question, "How to avoid flatting?" see below:

That is the best advice I have seen in quite a while.

Tom

[RaceProEngineer]

It is unfortunate that you have compromised a set of tires, but hopefully with a little learning and a little practice you will be able to continue autocrossing with much better results.

You may already know this: Flat spotting is usually caused by locking the brakes while the car is at speed, so that the tire(s) stops rotating, and the rubber simply grinds off on the track in a single contact patch on each tire.

Furthermore, "locking" the brakes, or causing the tire(s) to cease rotation, actually loses traction and INCREASES stopping (or deceleration) distance, just like on an icy road.

On the track, the objective is to optimize deceleration, by using the brakes to their limit WITHOUT "locking". This is usually referred to as threshold braking. With some on-track experience, under heavy breaking, one can easily feel when a wheel or two has started to "lock", and the corrective action is to decrease brake pedal pressure EVER SO SLIGHTLY, until the offending tire(s) start rotating again.

With more experience, the driver can feel when a wheel IS ABOUT TO LOCK, and modulate accordingly.

A BRIEF lockup would not severely flatspot a street-track tire like the Kumho. However, once a real flat spot is present on a tire, it will lockup much more easily (sooner), and with that self-same contact patch grinding against the pavement, making the condition ever worse.

So, here are a few recommendations:

(1) Check, or have a Porsche guy check your FRONT brakes to be certain they are operating properly (pads in particular). Under load, all four wheels should have roughly the same stopping "power". The master cylinder, and brake lines may also need attention, depending.

(2) Bleed the brakes to ensure best pedal feel, and ability to modulate pressure.

(3) At the next autocross, recommend having an autocross instructor ride with you for a run ot two. He/she can advise on threshold braking. Incidentally, most new autocrossers use their brakes WAY TOO MUCH.

(4) Remember, a steadily increasing application of the brake pedal (never a "stab") leads to truly rapid deceleration in the same way that a steadily increasing application of the throttle leads to quick acceleration. "Squeeze" the brake pedal; "squeeze" the throttle.

(5) Finally, counteracting my wise-ass comment above, a harder compound tire, once locked, will not flat-spot as easily as a soft compound. The soft compound tire will slow the car more BEFORE it locks, however.

BUT - Practice these recommendations, pay very close attention to your braking technique, and you will have trouble with neither hard nor soft compounds, nor anything in between.

Ed LoPresti

[emcon5]

He flat spotted the rears, so I am guessing he spun, rather than from locking the brakes.

Excellent braking advice though.

Tom

[pmajka]

Thankyou that is good advice.

emcon5: you are correct, my lateral velocity had exceeded my forward velocity...but i still got around that cone...the run counted!


I guess to avoid this in the future, i need a set of track tires...

the wife wont be pleased.

Originally posted by pmajka



I guess to avoid this in the future, i need a set of track tires...

the wife wont be pleased.

New set of Hoosiers - $900

Having my company purchase and deliver direct to my mechanic - Priceless.

[RaceProEngineer]

Most wives and girlfriends do not "understand" track tires the way that we do, and no amount of "educating" will help the condition. We simply need to work around it.
-Ed

[pmajka]

We just need to explain that track tires are like the other 150 pair of shoes she has...

after all, they all have a purpose and she can only wear 1 pair at a time.

[byndbad914]

might want to check the Hoosier site - I dunno what size tires you need, but they have been closing out the R3S04s in various sizes for awhile now. I just ordered a set of tires less than two weeks ago - at half original price and shipping was included (though they charged a $7 "handling fee", but $7 to ship a set of tires from Indiana to SoCal was dirt cheap to me!). I bought last October was over $900 for the set of four, but $143 each last week. Check for your size in the DOT Road Race Radials.

The 05s they are coming out with have a more rounded edge, but they really only saw premature wear issues on the 17s and 18s according to the tech I talked to at Hoosier. the smaller diams didn't have quite the issue with the "squarer" edge of the 04s. FYI.

Oh yeah, they have an A3S04 too IIRC, those would be on closeout too maybe... I dunno what they are like, I just remember wanting the R3s for my racecar...

[RaceProEngineer]

The "A" compound (as in A3S04) is specifically for autocross and hillclimb. They come up to temperature super fast!

Talk about a S - O - F - T compound, this is it!

-Ed