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Racing environment brake performance

842 Views 9 Replies 6 Participants Last post by  IJM
So I had a specific question with brake performance: After a fast run down a drag strip, and you're going about 100mph-does anyone feel that when they get on the brakes hard (if you want to for fun, or realized you are about to miss the first left turn) that the car get a tad bit squirrly? Like, it requires a little bit of hanging on the steering wheel and some steering input.
I don't think my tires are losing traction, ABS does NOT kick in.....I'm just wandering what exactly is going on.
With aftermarket brakes does this happen? Is this not even happening and it's just my blood pumping?
It feels as if each corner isn't braking the EXACT same way. Nothing is broken or worn. I've replaced brake fluid before the recommended time intervals.
thanks for any input.
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Re: Racing environment brake performance (viasevenvai)

your trap terminal velocity is 100mph plus/minus and when you are hard on brakes, front end walks side-to-side felt through steering wheel... no abs intervention
at moderate to high road speeds chassis will slew laterally when:
(1) rotors may be warped from overheating/rapid cooling (thermal warping) and/or non-uniformly/incorrectly/excessively tightened wheel bolts (mechanical warping)
correctly retighten in sequence with accurate torque wrench all wheel bolts - all threads should be clean & dry - NO grease or anti-seize on threads
just before making run, preheat rotors - suddenly going hard on brakes from higher road speed may cause transient thermal warping, which may self resolve or may not self resolve
(2) one or both front tires have belt run-out
radial street tires or bias drag slicks??? big difference
first confirm "cold" tire pressure, second:
rotate tires left rear to left front and right rear to right front
also shave all tires to uniform rolling diameter
(3) your drive factory fitted floating brake calipers - if one or more bind and will not immediately self-align to rotor when brakes applied - chassis slewing can occur
(4) you have used excessive amount of tire grip solvent on tread just prior to making run - by end of run tread compound rehardened non-uniformly left to right tire resulting in slight traction difference
long story short - rotate tires back to front & preheat brakes just before run...
when this occurs on circuit car, driver(s) & mechanic foreman usually exchange sharp words...

Modified by iamnobody1 at 12:37 AM 10-18-2008
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excellent, thank you for the ideas!
come to think of it, i think my tire pressures are all different by probably 5 psi. I assumed the place that mounted my tires would put them all at the same psi, but they didn't and i only corrected the one that was looking flat........weird.
interesting stuff though
Re: (viasevenvai)

Vacuum boosted brake check valve, at high rpm it wont respond quickly enough........tap the brake before you get on the brakes hard to open it first.
P.S. drag racing is for wussies......try doing 100mph and have to make a corner at speed, that's for real men!
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Re: (gehr)

Quote, originally posted by gehr »
P.S. drag racing is for wussies......try doing 100mph and have to make a corner at speed, that's for real men!

and not just left turns either!

Back ON topic:
It seems like you have the answers you need, I'm curious as to how things work out for you as it may improve my road course braking.
How exactly do you guys suggest to preheat the brakes, just use them? I'm not familiar with drag strips, but I didn't think there really was a place before the track where you could get enough speed to really heat them...
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my understanding is preheating brakes is only good for the ceramic ones, since they operate best when hot.
the iron ones don't need preheating, unless you're worried about shocking them i guess?
Re: (viasevenvai)

if the track is somewhat unlevel and/or has grooves in it (think freeway) you will shimmy when hard of the brakes.
and yes, coming down from 100+ mph on the straightaway, car is squirrely and your brakes are maxed out you see the turn-in marker....turn turn tuuurrrnnn...modulate,..mooooodduuulllate....gas gas gassss....real men
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how high should the front tire pressure be for these types of maneuvers?
i'm guessing over 35psi, but i don't know if it should be 40 or above....
Re: (viasevenvai)

re: tire pressure - street drives - tire pressure & front/rear pressure bias is factory recommended in usa market - remember the ford bronco/rollover issues/tire pressures?
"drive thru" strip times using your daily drive is a special case - there is very little time to prep car before making run, exiting & driving over to friend's place for evening relaxation
for strip times you must adjust street radial tire pressures - increase by 4psi assuming 32-34psi normal road driving tire pressure and than reduce tire pressure after your run(s)
yes there is massive thermal shock to your rotors & pad surface
the preheating is for rotors, not street pads, for your strip runs
I also agree that a series of low pavement pressure ridges at terminal end of strip (from braked wheel torque transmitted into pavement causing crawling/cold flow) may be partly reponsible - however this is exactly the condition your abs system is designed to control
without glazing rotors with friction pad binding resin(s) try to use brakes at low speed during approach to lineup to keep rotors at least warm (say 125c) to reduce thermal shocking - warning of rotor glazing
also keep eye on driveshaft inside tripod universals as this off-road activity genuinely stresses oem tripods
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Re: Racing environment brake performance (viasevenvai)

Do you take the rear seats and spare tire out for these events? These cars tend to have the brake bias set to the rear to accommodate a fully loaded car. Many of the road course guys (myself included) use more aggressive pads up front to help move some of the brake bias forward. Then again, tire pressures could have a significant effect if they're off by quite a bit.
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