RF Chatter While Braking

[Lonewolf583]

I'm not sure what I can change given my setup. I've inspected the car and alignment...everything checks out, nothing loose.

Z28 kit, JPSS Caster Lock/SAI Plate/Billet Bushes, 275/35R18 Toyo RR
-2.5/-1.25 Camber, 3/16" Out, 1/8" In Toe F/R, Hot pressure 40/38 FR.

Upon straight line braking on asphalt during yesterdays autocross, I got some RF chatter that seemed to time exactly with ABS. LF did not do this. In my experience that's a symptom of low speed compression being too high.

I plan on adding more camber down the road once I get the rear in-check (need billet subframe bushes), what can I do with my non-adjustable damper setup to combat this? Wondering if I've got some cross caster or something since the LF does not chatter, the RF does. I don't have tools to measure caster so I haven't checked it. Just slapped in the JPSS locks and called it.

I'm grabbing at straws.

[Cornercarver]

Pretty sure the ABS can operate any one wheel individually. Maybe you have an issue with the brakes on the FL corner and its not getting as much brake pressure.

Otherwise I did caster bolts which is pretty much the same as your JPSS locks. I went from 6 degrees to 6.5 degrees on both sides, I highly doubt caster has anything to do with it. I say bleed the brakes and inspect the pads both sides.

What pads and fluid do you run?

[Lonewolf583]

You're probably right on individual brake modulation but I'll need to get go pro of the symptom. Feels more upsetting than just ABS.

I need to do stainless braided lines at some point, but I'm running just a generic Hawk HPS with Wilwood fluid...will need to check the exact fluid when I get back.

Just feel like it "turned on" after I got settled with this new setup, could just be coincidence.

[JusticePete]

With the extra weight of the driver, it is possible you have worn out or glazed the driver's side pad?

[Lonewolf583]

Wait did Pete just call me fat? :P

They have plenty of thickness but I'll have to pop 'em out an get a better look.

[JusticePete]

LOL

Cars driven with only a DRIVER most of the time will wear the brake pads on the fat side of the CAR more...

[Lonewolf583]

Seems like the original problem was a fluke at the time...really didn't happen anymore, but...this happened everywhere at my autocross today. New pads (GLOC R6) only seemed to aggrivate it.

Very pronounced front end chatter. Almost like wheel hop...my times are awful now and I can't brake well anymore.

[Norm Peterson]

Originally Posted by Lonewolf583

Wondering if I've got some cross caster or something since the LF does not chatter, the RF does. I don't have tools to measure caster so I haven't checked it. Just slapped in the JPSS locks and called it.

I'm grabbing at straws.

Didn't see this thread the first time.

Did you ever get caster checked? If not, this can be DIY-calculated from camber measurements taken with the wheels steered left and steered right the same number of degrees (commonly 20°, but other angles can be used).

I suppose that uneven pad bedding or new pads incompatible with old transfer layer could also be possible, as could rotors where the 'cheeks' are not uniform thickness all the way around.


Norm

[Lonewolf583]

I did end up checking when I had a shop correct my thrust angle...and they were within .2 or .3 deg of eachother, alot closer than I expected. (Need to dig up the sheet..it's hiding somewhere)

I pulled the rotors and calipers when doing the pad swap. Cleaned it all up and hit the rotors with 80 grit. Did braided lines and DIY cooling ducts at the same time.

I took some goPro footage pointed squarely at the front tire...I have yet to look at it but hoping it shows something. Had some other possible visual confirmation when I noticed some HEAVY thick black looks-like-ABS tire marks right inline with my prior run braking points...on BOTH sides of the car.

All I know is that it's made the car a basketcase during braking. Tons of chop and chatter that shakes the whole car considerably. Kept me up last night lol

[Russell James]

Does your tire pressure need to be that high? Sounds like your basketballing the tire. Which is usually either too high tire pressure or a shock/strut that has died.

I'd first try the same stop with the tires down around 32/30ish.

With high tire pressure, it also makes the contact patch real small... easy to hit abs.

Like Pete mentioned, the LF is going to have a lot more weight on it. Which will make the RF slide easier on a smaller contact patch/less weight.

Also, a good way to see if a shock/stut has died.... Have someone drive next to that side on the highway and watch that tire hit bumps. A dead strut on that corner will make the tire basketball after the bumps. Real easy to see from watching that corner from another vehicle. I see a lot of people driving down the highway with one tire bouncing like crazy after a bump, and from inside the car they probably have no idea that shock is dead.

[Lonewolf583]

Originally Posted by Russell James

Does your tire pressure need to be that high? Sounds like your basketballing the tire. Which is usually either too high tire pressure or a shock/strut that has died.

I'd first try the same stop with the tires down around 32/30ish.

With high tire pressure, it also makes the contact patch real small... easy to hit abs.

Like Pete mentioned, the LF is going to have a lot more weight on it. Which will make the RF slide easier on a smaller contact patch/less weight.

That's a good word for it! Neglected to mention I actually had it at 36/34 hot yesterday and that's what the tire wanted from a loaded zone perspective. I tried dropping more and it was rolling over too much. Not only that but the tire "looked low" @ 34 up front so I wouldn't be comfortable going lower.

If the Z28 struts are dead already that's super disappointing.

[olblue75]

Check for missing weights.

[Russell James]

Originally Posted by Lonewolf583

That's a good word for it! Neglected to mention I actually had it at 36/34 hot yesterday and that's what the tire wanted from a loaded zone perspective. I tried dropping more and it was rolling over too much. Not only that but the tire "looked low" @ 34 up front so I wouldn't be comfortable going lower.

If the Z28 struts are dead already that's super disappointing.

If the tire pressure didn't affect it, have someone watch that corner over highway bumps. A strut could have gone weak, or a bad strut mount will do that too... but usually noisy over bumps.

[Lonewolf583]

Originally Posted by Russell James

If the tire pressure didn't affect it, have someone watch that corner over highway bumps. A strut could have gone weak, or a bad strut mount will do that too... but usually noisy over bumps.

I'll see what I can make happen after the next event this Sunday. Will be pretty busy next couple of days.

Another thing I've considered is the ABS tuning parameters/stability. For these runs I left it in "Competitive Mode" which still keeps things in order-ish. I wonder if things will improve in "All Nannies Off" mode. Part of me wonders if the car is sensing incredible deceleration and is like holy crap he's stopping quickly hit the ABS.