Locking rear brakes!

[Shaun C]

Not sure if this is the correct section to ask this but I need some help...

I've recently replaced my std diff with an LS to my "close to standard" race Giulia.

We decided to take off the rear ant-roll bar to further help the traction out of corners which I think is now the cause of the rear wheels locking up under heavy braking. I'm new to racing so I wasn't even sure what was happening as I haven't actually seen smoke or anything in the mirrors to confirm this, but the dashboard shudders so violently when this happens I can barely see the gauges - and so I'm told it's caused by locking the rear wheels.

I have new limit straps which I'm also told are standard length, which I originally thought could be lifting up the back axle (now that the anti roll bar isn't there) under heavy braking causing the wheels to lock.

Any advice/opinions/comments appreciated.

[larry]

What year is the car?
Did you use the rear disc brake calipers from the new LSD diff?

Lets say your car is a 1967 super ( they don't have proportioning valves)and you used the newer calipers, they have larger pistons and effectively increase rear brake force i.e. causing rear lock up. an adjustable propotioning valve will fix it or go back to the early calipers, but in the long run a prop valve is the answer.

Larry
Alfa Parts Exchange

[borrani]

I would guess your rear control arm bushings are shot, or your rear shocks are too stiff. If you could have someone observe your car when it's doing the shimmy, it would be clear that the rear end is shuddering and dancing around under heavy braking.

If the rear control arm bushes are shot, the diff assembly can really move around. Unshackling the rear bar would have the same affect, as it is not as well located without the bar.

Good luck!

Steve S.
Seattle, WA

[Shaun C]

Quote:

Originally Posted by larry What year is the car? Did you use the rear disc brake calipers from the new LSD diff?

[Enrico]

I think you suffer from axle tramp. When you press the brake pedals and the rear wheel
would become locked you shouldn't experience such massive vibriation. I would say that during braking the axle isn't well located and it hops. Rubber bushings should soak up all and
are there not to send vibrations onto the chassis spine.
Check rubbers and silent blocs of reaction arm. Also the rubbers of the rods located to
the front of the car.
But...when you accelerate violently what happens???

Considering it is a 105 I would suspect bushings in the rear axle.

[Shaun C]

Hey guys

I've been told that under heavy braking, my wheel (the whole axle) can be seen to jiggle around (more like backwards and forwards), so I'm told my trailing arm bushes are shot. So I'll need to look at that ASAP.

Otherwise we had a great day's racing this weekend. I had my biggest off to date though, but thankfully missed other cars, and tyre walls. The pressure switch for the brake lights was defective (replaced the day before) and BROKE. I'm braking into the hairpin making a pass on the inside, and the next second my foot is on the floor and I've shot right through into the kitty litter.

Anyway, I'll post a link to the pics in a new thread.

[borrani]

What did I tell ya? That is great news. You will have that problem fixed fairly easily.
That's a pretty spooky failure of the brake light switch. Never heard of that failure before.

Good luck,

Steve S.

[Shaun C]

Brake switch thing was pretty spooky all right - but I think I see more good luck than bad out of the whole thing.

As for the axle vibration/axle tramp/locking rear wheels, it was difficult to figure out what was happening as I wasn't too sure what it feels like to lock up the rear wheels - in all cars I've driven if I've ever locked up the wheels, they tended to be the front. I did experience some rear wheel lock on the weekend when I felt the back of the car snake like mad under braking - I must have already been losing brake fluid all over the place and drove over it, locking the one wheel.

Thanks for the advice - I'll let you know how it goes when I replace the trailing arm bushes. I guess heavy duty bushes would probably be best (as available from Alfaholics/Classic Alfa in the UK). If anyone advises otherwise pls let me know.

[borrani]

A fellow NWARC'er had your issue, and he had new trailing arm bushes. In his case, his rear shocks were too firm.

Steve S.

[60sRacer]

Is the rear brake proportioning valve from the 2000's alfas good enough, or should he be using one of the racing (adjustable) versions?

Also, if the rear wheel is lifting, a stiffer front bar should help. For all its trouble, it is amazing how well the rigid axle alfa rear end works.

Finally, for track use, you can open up the LSD and restack the clutch discs to increase power transfer. Stock is about 25%, can up it to 60-75%. THere are four discs on each side; two are driven by the axle and two are driven by the diff carrier. Stock config has only one friction surface working; the discs are stacked c-c-a-a (for cdarrier and axle driven). For race you can stack them c-a-c-a and get a dramatic difference in LSD action. They do wear faster, so don't do this on a street car.

Robert