Real wheel hop during braking [Archive]

E30M30

04-28-2010, 11:03 PM

Hi

Finally I've driven my firebird at a track. It's a '67 firebird with hotckiss lowering springs with red koni dampers at the front. And multileafs with konis in the back. I'm running 2002 camaro brakes front and back with adj. valve for the rear brakes. At the end of the straight my rear wheels starter hopping all over the placed (quite scary). Unfortunately I'm running traction bars (I know they're not designed for corners) What should I do to stop the rear wheel dancing :) By the way the suspension is rock hard - should I adjust the dampers?

Best Regards from Denmark/Europe

stefan

MrQuick

04-28-2010, 11:10 PM

have you tried turning down the rear line pressure?

on a safe stretch of road too.

vince

E30M30

04-28-2010, 11:24 PM

Yeah I've adjusted the pressure so that the front wheels lock up first. To start with the rears locked up first, so I adjusted the pressure until they didn't

ArtosDracon

04-29-2010, 02:58 AM

I think two things are contributing to your problem. First off, you're probably braking harder on the track than when you first set up your rear brakes, so they're locking up earlier due to increased weight transfer. Secondly, the stiffness of the shocks combined with the angle of the leaf springs is causing an excess(90%+) of Anti-Squat in the rear suspension, which is only contributing to the first issue by preventing weight from settling back onto the rear axle.

My suggestion would be to get a rear swaybar if you don't all ready have one, then soften the rear so the weight can transfer back more quickly, but you'll likely also have to adjust the rear brake pressure down some as well.

Apogee

04-29-2010, 06:45 AM

Depending on how much adjustment you have left with your proportioning valve, you can also decrease your rear brake pad coefficient of friction. It's fairly common to run a less aggressive pad compound in the rear when tracking a car with race rubber. The stickier the front tires are, the more weight you'll transfer to the fronts and off of the rears. We have had a lot of 4th-gen F-body guys who have had similar issues.

Tobin
KORE3

E30M30

04-29-2010, 06:53 AM

My tires are not sticky.. 4 year old hankock tires. I'll try to loosen the proportion valve a bit more. I just hope decreasing brake pressure to a minimum isn't the only answer to my problem. I guess I'll have to find a remote area and start breaking :) By the way I'm using your brake lines Tobin - nice quality!

John Wright

04-29-2010, 07:05 AM

What rate springs are those multi-leafs?

Can you turn your traction bars around backwards?...LOL

parsonsj

04-29-2010, 08:02 AM

Unfortunately I'm running traction bars (I know they're not designed for corners) What should I do to stop the rear wheel dancing?You've answered your own question: the traction bars are causing the wheel hop. Take them off, and I'll bet your rear suspension becomes more compliant, and the wheel hop goes away.

jp

MrQuick

04-29-2010, 08:16 AM

didn't think of that John.

Maybe try a set up like this. After you explore all other possibilities.
https://www.pro-touring.com/forum/showpost.php?p=665608&postcount=9

What leafs are you running? If the traction bars are contributing perhaps you are getting some sort of axle wrap or resonation upon braking??
might be a stretch.
Vince

LSx_88_Ciera

04-29-2010, 11:06 AM

You've answered your own question: the traction bars are causing the wheel hop. Take them off, and I'll bet your rear suspension becomes more compliant, and the wheel hop goes away.

jp

Just when I started to think nobody picked up on that I came across your post.
I got a dollars to doughnuts bet that is the cause.

John Wright

04-29-2010, 11:34 AM

What kind of traction bar is this that we're talking about? A regular old slapper bar? Is it long enough to reach the spring eye or is the snubber hitting somewhere in the front half of the spring? Is there a J-bolt to keep the bar from digging into the pavement on decel or not?

How about the caltracs aren't they preloaded with springs in them...wouldn't that encourage the spring to wrap in the other direction during decel?

E30M30

04-30-2010, 04:40 AM

I don't know about the rate on the leafs, but they are extremely firm. The CD jumps around when I'm cruising :)

The traction bars are "slapper style" They don't hit the spring eye, but they are fairly close (2-3 inches I think)
They do have bolts to prevent them from reaching the ground
If I take the traction bars off. What about wheel hop during acc. and axle wrap?
Will Caltracs help or will they have a similar effect during braking.

Fascinating being on the other side of the world getting advice from you guys!

Thanks !!!

ArtosDracon

04-30-2010, 06:24 AM

I gotta say that I still do not think that the bars alone could be causing your issue, I am personally inclined towards the rear end unloading weight onto the front under heavy braking, more so than you might have experienced during your previous pressure setting sessions. I believe that to be a combination of extremely stiff springs and shocks.

I think this for two reasons:
#1 as you've attested the rear suspension is very stiff, likely indicating a very high AS percentage due to both design and components.
#2 LS camaros, whose brakes you're using, often have a similar problem simply due to the bias built into the system, the components themselves are good, but it's biased for a street driven car that doesn't see excessive weight transfer to the front as one would see in a racing scenario.

wendell

04-30-2010, 07:37 AM

Slapper bars aren't going to effect your braking in the slightest. It's basically a little pair of torque arms, decoupled in braking.

John Wright

04-30-2010, 07:52 AM

It's basically a little pair of torque arms, decoupled in braking.
....except that they do load the spring via the j-bolt to keep them from digging into the pavement when braking.

Skip Fix

05-01-2010, 08:49 AM

Some bars also have a braking strap to prevent the bar from diving too much. Looks almost like a spring clamp. In fact old Lakewood bars had one in the front and one in the rear.

Cdog

05-01-2010, 09:54 AM

Are you heel toe down shifting or stabbing the brakes to slow down the car? The three things to focus on are traction, shocks and brake bias. Tweak one thing at a time. Try the brake bias first. Run some laps. Then adjust your shocks if they can be adjusted. If that doesn't fix it play with tire pressures.

Be methodical or you'll never know what fixed your problem.

I'm assuming the 67 FB has the non staggered shocks like the camaro. I'd focus on staggering the shocks like on the 68&69's, that may be the easiest fix and eliminate the need for trac bars all together.

Lowend

05-01-2010, 11:48 AM

Question no one has asked:
Are your rear shocks staggered, or both on one side of the axel?

E30M30

05-05-2010, 01:04 AM

John Wright

05-05-2010, 03:09 AM

IIRC, the Global West springs have a beefy lower leaf to help prevent axle wrap.

Cdog

05-05-2010, 06:57 AM

Hi again

I have non staggered shocks - both in front of the axle

I talked to global west and DSE. Both suggest getting rid of the traction bars (Guess I'll just cut them off and reuse the shock plates) Other than that they gave me several different causes.

So I think I'll cut the bars off. Then try lowering the brake pressure to the rear brakes. If that doesn't help, I'll soften the shocks in the back and maybe buying a set of new global west/DSE rear springs..

I'll get back after the next track day!!

You can convert to the staggered shock set up for less than $250. I think DSE sells everything you need. Keep in mind during the 67 race season the non staggered shocks killed the Camaro teams. I'd bet if stiffer springs were a real fix the race teams would have gone that route instead of staggering the shocks. Just my opinion.