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      12-09-2011, 07:02 PM   #96
MFGJR
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Drives: 2007 Z4 M Coupe
Join Date: Sep 2008
Location: Richmond, VA

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I know a number of you out there have felt that squirmy feeling in the rear end when braking hard on the track.

I frequently experience it when braking into turns 10/11 at VIR, the infamous "Oak Tree" corner. For those unfamiliar with VIR, this section of the track involves a fast left hander followed by a short straight that goes downhill (where you gain speed FAST) and then it changes to uphill as you come up to the major braking zone for the slow right-hand hair pin. Because of the uphill braking, you can brake late and hard. It's here that the rear end often gets "happy"--not like it's coming around, just unsettled and uncomfortable. My instructors have sometimes thought it was because I was giving steering input while braking here, which I thought was BS and I've been anxious to have data to see what's really going on. Problem--I'm not going out any time soon.

I readily found the same squiggles in Randy's data from VIR. Here's an example:

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The green line is throttle/brake pedal input, and you can see in the highlighted section he's rapidly building braking force. The red line is lateral acceleration, and you can see the sharp swing of about a quarter G when the car gets unsettled. Speed when this begins is ~75 mph, having already decelerated from 90 and on the way down to ~ 50 at turn-in. You can see on the blue steering angle line that Randy's steering inputs aren't fluctuating by much at all before the twitch--but note that the angle wasn't at zero, but rather he has a constant input of 8 or 9 degrees right dialed in throughout the braking. I think this is what the car doesn't like.

The track is bending to the right very subtly here, leading up to the turn, and I think there's a subconscious need to get lined up parallel to the left-hand curbing before turning into the right hander at the end. I think I'll try ignoring that curbing, and just focus on braking in a straight line toward the correct turn in point, striving for zero steering input.

Having this kind of data really lets you play detective on these weird little situations!

Last edited by MFGJR; 12-10-2011 at 01:51 PM.. Reason: typo
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