Track day - rear wiggle under heavy braking

Techapma

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I recently had my first track day in my car with an instructor at Harris Hill Raceway in San Marcos, Tx.

I’ve been through the track attack and FPRS second day in the 2015 Mustang GT.

Although my track experience is somewhat limited, my instructor and I were able to get up to speed pretty quickly.

I ran the car in track mode with the cup 2s set to 28 PSI cold. I drove the car to the track ~30 miles and the ambient temperature increased ~10 degrees F between leaving the house with cold 28 PSI tires and the first session in the track. Tire pressure was 30 PSI at the start of the first session and never got over 35 PSI at the end of any session.

The car has Steeda dual rate lowering springs and I had an alignment done to track specs the day before going to the track. I noticed the alignment didn’t feel quite right after picking the car up from the shop, but didn’t think it felt bad enough to scratch the track session. I took the car back to the alignment shop after the track session to be corrected. In hindsight, I should have gotten the alignment fixed before going to the track. Turns out the alignment was far more jacked up than I thought. The excessive front toe caused accelerated wear on the front tires.

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There was one particular section of track where the back of the car felt like it was wiggling under hard braking. The track in that stretch is downhill leading to an off camber right hand turn that goes back up hill. I was entering the braking zone at roughly 105 MPH in 4th gear and I was not downshifting. The wiggle wasn’t enough to cause the car to feel like it was going to spin, but it was distracting enough that it caused me to delay initiating the turn. I don’t recall feeling this sensation at the track attack, or in the 2015 GT on the second day. The instructor was not concerned by the wiggle, but I was.

The car has stock brakes, there were no ABS events, and the pedal never faded but the brakes did smell hot at the end of the 2nd and 3rd session. Each session was roughly 20 minutes.

Is the wiggle I experienced likely the result of my technique, the jacked up alignment, something wrong with the brakes, or is it normal?

 

Hack

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I've never had a Mustang wiggle under hard braking at the track. They are exceptionally stable. I would assume it's the alignment.

The assumption I'm making is that you are braking in a straight line and then getting off the brakes before starting to turn. If you are trying something fancy like trail braking, yes of course the rear will start to get looser under braking and you may get oversteer or spin the car out if you try to turn while braking, especially if you are braking hard.
 

Egparson202

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OP: Hack may be on to something. Can you describe the point in the braking/turning process where you felt the car wiggle?
 

K4fxd

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Alignment issue, it should be gone if the final specs are true.

Who aligned it? Chain, dealer or ??
 


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Techapma

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I was running the factory cup 2 tires. Tires had roughly 500 miles on them prior to the track event.

I was definitely straight line braking. Not trying anything fancy - just hard application, then melting off the pedal as the car slowed. The wiggle was during the hard portion of the brake application. I didn’t start my turn until the wiggle settled.
 
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Techapma

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The alignment was done by a performance shop that claims to specialize in track alignments. The tech that worked on my car the first day clearly didn’t have a clue. Their machine reads in degrees/minutes rather than fractional degrees. When he converted the Ford spec of .1 degrees toe in he converted it to 60 minutes instead of 6 minutes. He also claimed the rear camber wasn’t adjustable. When I took the car back, the owner smugly agreed the car didn’t feel right and redid the alignment himself. He did a better job, but still not perfect.

I won’t go back there for a number of reasons, but I don’t want to disparage them.
 

DonnieO

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Looks like it needs more rear camber. 1.3 isn't much
 

VictorH

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I know what you are talking about and I've experienced it another car. My car has has some suspension work and a good alignment, so I've not experienced it in the Mustang. My thoughts

1) Some people will tell you it's your braking technique, but I don;t buy that. I've ridden with enough pro and ex-pro drivers to know they absolutely jam on the brakes on initial application.
2) You didn't have much toe-in, in back which might be some of your problem, but I bet not all of it.
3) I also doubt it has anything to do with your suspension bushings or rear suspension parts (assuming no high-mileage worn out stuff).
4) On my other car that was doing this it was 90% fixed with a bit more aggressive rear pad and for almost all street cars there's very little rear brake bias. OEMs are all worried you're going to spin the car so they don't want too much rear brake bias. Especially if you get into ABS with ANY steering angle at all, the rear brake pressure will get dumped to zero and it's mostly along for the ride at that point. A bit more bite in the rear gets the car a bit more settled and reduces, just a bit, some of the weight shift to the front, not a lot, but a little bit.
5) Obviously, the tire pressures in back need to be proper. If they are too low, then you'll get some wiggle too.
 

2BigPups

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I was definitely straight line braking. Not trying anything fancy - just hard application, then melting off the pedal as the car slowed. The wiggle was during the hard portion of the brake application. I didn’t start my turn until the wiggle settled.
Ran into the same thing during the braking exercises at Mach 1 Track Attack. The instructor told me that I was "over-braking" the stock pads with too much initial pressure. Adjusted for the road course portion and had no issue.

Or, I was doing a track bedding of the brake pads during the braking exercise. LOL
 

Egparson202

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@Techapma, I had a similar experience with the Shelby for the first few events. Since then I’ve worked on alignment, bushings, shock settings and driver technique. It’s much improved but not entirely gone. Which leads me to @VictorH post…


i
I know what you are talking about and I've experienced it another car. My car has has some suspension work and a good alignment, so I've not experienced it in the Mustang. My thoughts

1) Some people will tell you it's your braking technique, but I don;t buy that. I've ridden with enough pro and ex-pro drivers to know they absolutely jam on the brakes on initial application.
2) You didn't have much toe-in, in back which might be some of your problem, but I bet not all of it.
3) I also doubt it has anything to do with your suspension bushings or rear suspension parts (assuming no high-mileage worn out stuff).
4) On my other car that was doing this it was 90% fixed with a bit more aggressive rear pad and for almost all street cars there's very little rear brake bias. OEMs are all worried you're going to spin the car so they don't want too much rear brake bias. Especially if you get into ABS with ANY steering angle at all, the rear brake pressure will get dumped to zero and it's mostly along for the ride at that point. A bit more bite in the rear gets the car a bit more settled and reduces, just a bit, some of the weight shift to the front, not a lot, but a little bit.
5) Obviously, the tire pressures in back need to be proper. If they are too low, then you'll get some wiggle too.
I like the points you made here. As it relates to front/rear bias I’m expecting to find out at my next event. I’ll be running the same compound front and rear. The guys at KNS Brake suggested it for Mustangs being tracked on 200TW tires as opposed to R-Comps. Based on the conversation, I expect it to make the car more stable under braking and to increase driver confidence at braking limits.
 
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NeverSatisfied

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I think it’s driver induced over braking.

Alignment looks fine for a starter.

These cars have substantial rear brake bias when running r comp tires, stiff suspension, and even more so when running mild front camber #s.

Switch to offset pad compounds front/rear after you burn up your current set.
 

Knockdown

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After 6 HPDE days at Sebring, I still haven’t figured out a good brake pad f/r combination. I’ve tried Hawk DTC60f/30r, DTC70f/60r and Gloc R12f/R10r looking to stay in the heat range with similar wear front to back. But I have also been changing tire compounds and springs during this time and also disabled the traction control for the last several events. Depending on the track, getting a 2 ton high powered car stopped for a 25 minute session is not trivial.
Check you pad wear f/r, turn off traction control, downhill off camber may need a bit more braking zone, select pads that stay in the temp range.
 

NeverSatisfied

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I run gloc r18/12. Very happy with the bias on pp brembos. Using vorshlag air deflectors.

I do burn through the rears faster than the fronts. First car I’ve ever had that does that.
 

Egparson202

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I run gloc r18/12. Very happy with the bias on pp brembos. Using vorshlag air deflectors.

I do burn through the rears faster than the fronts. First car I’ve ever had that does that.
That is curious. Forgive me if you already stated it, but what drive mode/advance track settings do you use? I ask because it’s possible the stability control systems are engaging rear brakes quite a lot without your knowledge.
 

 
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