Front End 'Wandering' Under Heavy Braking

[TexasPettey]

I just got back from a fun DE event at TWS in college station. I had a blast. Got some good video of me playing with a Gen3 and a 550RWHP Supra. The event was run clockwise, for those that know the track.

One thing that was a bit odd was that the front of the car seemed to wander a bit under heaving braking. It was most noticable on the front and back straight, braking hard at high speed. There was no pedal vibration, as if the rotors were warped. It was as if the front of the car was trying to move around unpredictably. I don't think I can come close to locking the tires at max pedal at 100+MPHM with my current setup, and there was clearly no flat spotting. Does anyone have some suggestions on what may be happening?

I've got a Gen1, with Aldans, 500/750 springs, stock rotors, BM3's, Tom's rear brake upgrade, cone brake duct kit, 17" Kumo v700 315/335, 2 deg neg camber, 1/16" toe out.

 

[FATHERFORD]

Do these cars have a problem with bump-steer?

 

[Steve 00RT/10]

Tom could answer this in more detail, but I have experienced the same thing. I have his upgrade, B3s, no prop valve, stock rotors. I believe he explained to me a couple years ago that it may have something to do with the car becoming a little unsettled under heavy braking if there are any uneven track surfaces ...at all. Adding the bigger calipers to the back (especially with the prop valve out) allows heavier braking before lock up thereby accentuating any car unbalance due to uneven road surface. Although my RF will still lock first, I now wear out my back pads as fast or faster (the 1st set) than the fronts.

Steve

 

[GTS Dean]

I'd say it's probably a combination of too much negative camber at the front and toe-out. 2 degrees is a little on the aggressive side for TWS. For better braking, the tires should stand straighter up. Like everything else in road racing, setup is a compromise for average conditions.

 

[Steve 00RT/10]

I'd say it's probably a combination of too much negative camber at the front and toe-out. 2 degrees is a little on the aggressive side for TWS. For better braking, the tires should stand straighter up. Like everything else in road racing, setup is a compromise for average conditions.

Would this same logic apply to my stock setting? It is definitely a little disconcerting when this happens. How would changing stock settings affect tire wear?

Thanks--Steve

 

[GTS Dean]

If you are talking about shock damping, then the front bump and rear rebound settings affect straight line braking.

If you are talking about ride height, then you will have dynamic (negative) camber gain as the car is lowered, or the wheels are jounced. The toe pattern changes from jounce to rebound as well.

 

[GTS Jeff]

You might need these, anti toe brackets

https://secure.fiftybelow.com/archerracing/index.cfm?page=item-detail&ID=VV-2&storeid=1&vtype=viper

 

[GTS Dean]

the driver's side won't fit if the rear frame strut brace recall has been installed.

 

[GTS Dean]

the driver's side won't fit if the rear frame strut brace recall has been installed.I think the toe brackets helped power down more than braking.

 

[FrankBarba]

Have you checked all of your front suspension. When was the last time you either turned your rotors or new rotors? Have you replaced any of your suspension pcs?

 

[Tom F&L GoR]

Lowering the car (with spring, adj shocks) increases camber and toe-out. Under braking the front gets lower again, so more camber and toe-out. Any little steering loads one side more than the other, so the tire contact patch changes size and dimension. It's quite dynamic, and you have an agressive alignment to start with.

On Lime Rock Park's straight (suffering Connecticut winters) the surface looks and feels smooth, but is uneven. Under heavy braking as one wheel rides a slow heave upward it will see more load and pressure onto the surface and can brake a little more than a wheel that is riding over a dipping slope. If you are at the edge of braking, the car will experience some yaw changes that (luckily) seem to average out and you still head in the desired direction. To me, though, this feels more like the car is trying to spin about the axis in the middle of the car rather than steering left or right.

What tire pressure were they hot? My Kuhmos seemed to want something in the middle 40's psi and they were a little floppy before I discovered that.

 

[GTS Dean]

I've got a Gen1,

And it's probably still riding on the original wishbone bushings. Change all of them. Now.

 

[Viper X]

In addition to the wavy surface, if all else checks out, I'd look at the tire pressures in your Kumhos.

I run the Kumho Ecsta V-700's too on my GTS and find that they are very sensitive to tire pressure differences from side to side. For example, if you have say 40 lbs hot in the right front and 30 lbs hot in the left front, you will have braking issues. Too low = floppy and wandering feeling - like you describe. I chunked a fresh, shaved rear last time out when I lowered the air pressure too much. Too high = not good either. I seem to loose some traction. I've found 36 to 37 psi hot best for my GTS and the tracks that I run. Tom seems to like middle 40's. Tire pressure preference seems to be trial and error along with driver preference.

Good luck,

Dan

 

[Tom F&L GoR]

for the record:

hot 45F/37R
cold 33F/28R

same side to side

 

[TexasPettey]

Thanks for all the input. Tire pressure is an easy change, as I was only 28PSI F&R cold on the Kumhos. Hot they were in the mid 30's. I'll try more pressure at the next event. I got some advice that the Kumhos like to be run lower, though I can't recall who gave me the advice at this point. I've only run on them twice and was running 36PSI cold on the first event. I felt like traction was not great up that high, and probably lowered it too much. This event we got a lot of rain, so I didn't get as much time to experiment. I was pushing much harder this time, and that's when I noticed the wandering.

The areas I was feeling it on the track are certainly rough. Even doing parade laps, there are a number of wavy areas in the pavement. The track takes a lot of abuse in those areas.

The suspension components are all new. Aldans and springs are ~4 months old. New poly pushings control arm bushings (Boy are they expensive for a Gen1!) and a new sway bar w/poly bushings at the same time. New poly motor mounts as well. Rotors and pads have seen 1 event, so they're fairly new.

 

[Chuck 98 RT/10]

The problem might be with the rear toe, not the front.