Need Advice: Track Day - Rear wheels like to dance during high-speed braking

ems657

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Hello guys,

I know there is quite a lot of experienced track enthusiasts on this forum, and please help me with a situation. I was a t Ridge Motorsports park yesterday (near Seattle). During high-speed braking (typically from 4th or 5th gear), the rear wheels just dance left and right. I talked to a couple instructors, and they suggested lowering the rear tire pressure. It only helped very little, and I don't want the pressure to be too low.

I come to this track quite often, and frankly also experienced the same thing in the past on the R. At first I thought it's the grooves on the pavement, but this becomes more obvious & happens at more braking zones as I pick up speed. Is this because R has very little weight at the back and has a front-biased braking power?

Thanks so much for your advice. I hate to see the car in front pull away from me because of this! :thumbsup:
  • 2018 CTR with 18,000 miles. Completely stock. No tune. Stock suspension setting.
  • Stock front and rear brake pads (~60% and 80% left)
  • 245/40/18 Bridgestone RE71R tires (~50 thread left). Running 32-33 psi hot front, 31-32 rear.
  • Ambient temp: 70-75F, sunshine and overcast.
  • Driver: 20 track day experience (about 8 on this track)



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ImTheStatMan

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In for comments. I ran my first HPDE with Project Mu Club Racers front pads and OEM rear pads. I noticed the same behavior and was wondering if it was a front to rear balance issue. Seems you are getting the same behavior with OEM pads all around though...
 

Lust

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What does your alignment look like?
 

roflitzjinno

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Logically it seems the rear is lifting during heavy braking. Could be just needing more downforce. Possibly a GT wing to help. The car is already front biased for the most part, heavy braking dumps all the weigh to the front and dives the nose and will lift the rear.

This used to happen to me in my S2000 with a non aero set up. Had more stability when I added a GT wing.
 

Shred

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Would bumping up the front PSI help? Instead of just lowering the rears, door is stamped at 35/33 front and rear so you might be able to bump the fronts up a few.
 

DRUSA

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The rear end wiggles under high speed, straight line braking OR they wiggle during trail-braking into a corner? The later sounds like the rear end rotating, which i would think you would want.
 

d15b7

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hi there. first thing i'd say is that you have the hot pressures too low. don't go by what the door card says; typically on my FWD cars that I'm tracking I'm looking at about 36-38 psi hot front (depending on which tires I am running) and about 34-36 hot rear.

next have a GOOD alignment shop do a four wheel alignment. go to a shop that does road race cars. now is the time to take out the camber pins up front and you'll need a realignment for that anyway. if it is moving a lot in the back that usually means too much toe out in the rear.

my CTR is rock solid under braking.

my road race EG civic with 450++ whp was a basket case under braking until I put big a APR wing on it (but I was braking from 160+ mph and I doubt you are getting anywhere near that). the CTR is very well balanced right out of the box; something is not right on your car (air pressure and alignment).

PS on my race EG civic, I start at about 28 psi cold on front and it grows to 38 psi hot after about 4-5 laps. but I drive that car HARD.
 
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ems657

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Thanks all for your great comments. Having more down force is a great idea. :thumbsup:

Oh, from my experience, the "dancing" actually happened during braking at straight way (not trailing braking). It feels that the more brake pressure I apply, the worse this gets.

Going back to the tire pressure, I purposely set it around 33 psi based on talking to a few folks at the track (with the same tires over the years), internet forum research and personal experience. I used to track an Infiniti G37 with RE71R, and 33 psi worked well from trial and error. I am listening to your thoughts :)

Thanks so much again !!👍
 

Lust

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Thanks all for your great comments. Having more down force is a great idea. :thumbsup:

Oh, from my experience, the "dancing" actually happened during braking at straight way (not trailing braking). It feels that the more brake pressure I apply, the worse this gets.

Going back to the tire pressure, I purposely set it around 33 psi based on talking to a few folks at the track (with the same tires over the years), internet forum research and personal experience. I used to track an Infiniti G37 with RE71R, and 33 psi worked well from trial and error. I am listening to your thoughts :)

Thanks so much again !!👍
Still need to know what your rear alignment looks like since squirm can be caused by toe changes.
 
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ems657

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Thanks, Lust. My car is completely stock, so the alignment is stock as well :)
 

AdamD19DFK8

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I haven't tracked but have done plenty of high speed braking from high up in 5th gear. On the stock setup I would get a bunch of that wiggly yaw feeling. Once I changed pads and got gyrodisc rotors it has pretty much disappeared. That was the first thing i noticed after the swap, how much more controllable it felt under high speed hard braking.
 

Lust

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Thanks, Lust. My car is completely stock, so the alignment is stock as well :)
Well my recommendation would be to get your alignment checked. I’ve seen several cars from the lot have wonky toe. I recommend 0 toe up front and about 1/16” total toe in for the rear.
 
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ImTheStatMan

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EEFC31C9-AD57-4BD0-BEBA-97D2C0F0B623.jpeg

These are the alignment specs I ran when I was having the same issue.

To Lust’s point, the before specs (I.e. the rear toe out) was the factory alignment.

Edit to clarify: I was experiencing the issue even on the “after” alignment.
 
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ApexEight

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Don't think an alignment itself will fix the issue. Pretty sure this is from changes in toe while the rear wheels are being unloaded during hard braking. I believe this is why Honda stiffened the rear lower B arm/bushing on the 2020s - less dynamic toe change. Interesting how one member noted elimination of the issue from upgrading rotors and pads - a big difference in brake bias could cause the issue.
 

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