Road Racing - Rear axle hop on braking




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Mercutio
12-30-2010, 09:23 PM
Toward the end of the autocross season, My 99 Z28 began axle hopping violently during hard braking. We all know about the front wheel bearing issue, but has anyone experienced this one? Are the two problems somehow related through the ABS system? Should I replace the rear ABS sensors, or is the problem elsewhere?


EchoMirage
12-31-2010, 07:34 AM
it has nothing to do with hubs or ABS. youre losing weight in the rear due to heavy braking and weight transfer to the front, causing the rear tires to lock and hop. the easiest and cheapest fix, besides changing your braking style, is swapping to a race compound pad in the rear.

jimmyblue
12-31-2010, 11:34 AM
Have you done things like LCA relocation and/or changing
the torque arm angle? Things you do for drag launch, if
taken too far, will steal from braking.


Glenn98ZM6
12-31-2010, 09:28 PM
Have you adjusted your rear rebound? Making it softer will allow weightto transfer to the nose helping wheel hop kick in.

And this is nothing new. 4th gens are well known to have this issue.

Mercutio
12-31-2010, 10:45 PM
This car is fully built for ESP autocrossing, with all of Sam Strano's recommended parts and 315 Hoosier A6s front and rear. It's got Hawk pads on all four corners, and the problem only showed up after about the 10th of 12 races this year. I had no problems with axle hop before that, which is why I'm a bit stumped. The only thing that changed on the car throughout the year was the amount of tire wear.

Glenn98ZM6
12-31-2010, 11:17 PM
Possibly a change in how hard you drive the car? Pushing a car farther than you have ever been able to before will allow "issues" to show.
Try making adjustments to resolve it. Wheel hop is a fact of life w/ a 4th gen. You can move its threshold for when it will appear, but you just cannot get rid of it w/ the OEM geometry.

EchoMirage
01-01-2011, 08:38 AM
only thing that will really help is a decoupled torque arm

Mercutio
01-04-2011, 05:59 PM
My tires were much worse at the end of the season than the beginning. No way I was driving the car harder when it started wheelhopping than I drove it when it wasn't wheelhopping. The Watts link is the Fays2. I don't know enough about torque arms to know whether that's decoupled or not. I'm new to F-bodies, and the suspension on these cars is still something I'm figuring out. Suspension mods are as follows:

Koni single adjustable front struts
Koni single adjustable rear shocks (4th gen style, adjustment at top)
Strano springs
35mm front sway bar
Hellwig adjustable rear sway bar
Strano offset front uca bushings
Fays2 Watts link
UMI adjustable torque arm
1LE bushings in rear control arms

If axle hop is just what these cars do, then that's that.

EchoMirage
01-05-2011, 08:12 AM
if you didnt buy a decoupled torque arm, then you dont have one. and they cost around $700. just an adjustable TA isnt decoupled, and they do nothing for handling. look into getting LCA relocation brackets also.

ninetres
04-02-2011, 11:40 PM
Oh how I love LS1 tech. This happned to my car today on the road course and it sounded like the F*&^ING rear end was going to bounce out from under the car. It lasted several solid seconds and scarred the shit outta me!

I feel a LITTLE better now but definitly need to find a way to get rid of it....I was on a serious lap ;)

SVThuh
04-03-2011, 05:14 AM
Contact Jason with Unbalanced Engineering.. A Google search can find him... Hes not a sponsor here so I can't post his info.

He engineered a very effective de-coupled torque arm and it works quite well, especially on all out AutoX cars.

My second choice for your setup would be the Global West Trac-Link.. Though that being said, the UE torque arm would likely be a better investment.

Glenn98ZM6
04-03-2011, 08:24 AM
There are ways to minimize the chance of wheel hop happening. Driving technique plays a big role. If you are not heal-toe downshifting, you need to start. Matching the rev's will reduce the sudden loading to the rear tires. Along w/ that, downshift as late as possible in the braking zone. In my CMC car, my down shift happens just in time to put my hand back on the wheel as I start my turn-in. Do it as late as possible. This will place your downshift lower in the rev range and will apply less compression braking to the rear tires.

My car runs a stock torque arm, no relocation brakets, and is about as low as any 4th gen needs to be. I use my prop valve to manage front to rear balance and very rarely I'll give it a 1/4 turn to stop what feels like wheel-hop trying to start.

Wheel hop can also be started by the track surface. Any ripples or small bumps will help it start. At Texas World Speedway the braking zone to T4 (going CCW) is know for this. I have to move my car over about 1/2 a car to find some smoother track. Of course I'm not using all the track and it is costing me some time, but it is costing me less than what wheel hop would. And wheel hop can break alot of parts. I have a ring gear in my gargae w/ 7 missing teeth that was the result of wheel hop. The splined part of the axle was twisted so much, you couldn't push it in to drop the C clip.

LS1-450
04-03-2011, 08:56 AM
Toward the end of the autocross season, My 99 Z28 began axle hopping violently during hard braking. We all know about the front wheel bearing issue, but has anyone experienced this one?


When's the last time you changed brake fluid? Cooked trapped front fluid could be causing the rears to clamp w/ more force than is normal. This can cause rear wheel hop &/or the rear to kick out on braking. Adding an adjustable proportioning valve is another solution. This will allow the brake bias, front to rear, to be adjusted under conditions where the rears start to hop. When hopping starts, reduce rear bias.

...(2) things

-change brake fluid after every second autocross & after every road course day.
-add adjustable proportioning valve to adjust brake bias for when adverse braking conditions occur. Allows for "on the fly" adjustability. Can make adjustments while @ the autocross event. Eliminates rear wheel locking.

One more thing, add front brake cooling ducts.

Glenn98ZM6
04-03-2011, 11:13 AM
I dont use/need cooling ducts on my CMC car w/ 12" brakes using C6 calipers.
The fluid you use will dictate if the fluid is the problem. If you use Castrol SRF, that is not the problem - trust me on this one. I bleed my fluid less than once a year. I almost never crack a bleeder.
If you are using cheep fluid, then the above advice is something I would have never thought of. Although, if I has boiled the fluid, the brakes would feel like ass long before I could get into a wheel hop issue.

Mercutio
04-10-2011, 09:07 PM
Hadn't noticed the new replies in this thread. Anyway, I put some new Hoosiers on it and had no problems at all with brake hop in two events. Today I ran a used set of tires at a local event and had wheel hop again, so the problem seems related to the amount of wear in the tires.

I haven't flushed the fluid in a year, it's worth doing again. And in my case, downshifting is not the issue because I so seldom need to shift in autoX.

ninetres
04-10-2011, 09:56 PM
Hadn't noticed the new replies in this thread. Anyway, I put some new Hoosiers on it and had no problems at all with brake hop in two events. Today I ran a used set of tires at a local event and had wheel hop again, so the problem seems related to the amount of wear in the tires.

I haven't flushed the fluid in a year, it's worth doing again. And in my case, downshifting is not the issue because I so seldom need to shift in autoX.

I was on a new set of 275/40/17 NT01s. And autoX and RR definitly upset the car differently. I am talking about HARD braking from 100+ mph.

I'll be back out next weekend (April 16th). No changes to the car other than trying to ease onto the brakes sooner before I stomp on them, and downshifting later.