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Autocross issue - bad axle hop problem

BigIrish

New member
Joined
Feb 1, 2016
Location
Houston, TX
I've recently got my 240 up and racing. I'm experiencing a bizarre issue that hopefully some of you racers have an idea on. When decelerating from high rpm in 2nd and turning, the inside rear wheel hops violently. If I give it gas or brake it stops, but it upsets the car and freaks me out!

Current setup:
Kaplehenke coilovers all around. 500# springs front, 250# rear. Koni race shocks front, bilsteins rear

28MM front bar, 16MM rear

Ford 5.0 with T-5, stock 3.73 volvo rear with open diff (for now)

Kaplehenke torque rods, IPD adjustable Panhard

17" x 255 autocross tires (RE-71Rs)

Recently replaced original rubber TABs with poly bushings, made no difference. Still have original rubber front trailing arm bushings, which seem fine.

Any ideas? Too much engine braking? Slop in the rear end gears?
 
Recently replaced original rubber TABs with poly bushings, made no difference. Still have original rubber front trailing arm bushings, which seem fine.

Any ideas? Too much engine braking? Slop in the rear end gears?


That could be an issue but I doubt its causing the hop.

What are the specs on the bilsteins?
 
Yes, I guess you are right. Crap. Based on this suspension design, with the shock in front of the axle, I'm thinking you need a very stiff shock. I wish the shocks were behind the axle or directly attached to it.

Koni race single-adjustable stiff enough? I'm not spending $1300 for DAs.
 
I concur...250# is way too stiff for an off-the-shelf HD.

Do your rear brakes lock up first under hard braking too?
 
Yes, I guess you are right. Crap. Based on this suspension design, with the shock in front of the axle, I'm thinking you need a very stiff shock. I wish the shocks were behind the axle or directly attached to it.

Koni race single-adjustable stiff enough? I'm not spending $1300 for DAs.

Ben sells Konis for not a ton of money. Yoshifab also sells GAZ for a bit less if you are willing to wait a couple months for them.
 
Koni's - yes the single adjustables aren't bad at $360 but the double-adjustables are $1300. Not sure the single-adjustable is stiff enough.

Brakes - no actually the fronts lock first. I have Hawk HPS pads on the fronts only, stock on the rears. I was debating going to an 8.8 axle so didn't want to buy Hawk pads for the rear. It's a work in progress.
 
For reference, here's the corner weights for my 242. Set up much like your car with the exception of custom valved HD's



Here's my rear valving







Open diff?
 

Well I guess that settles it!

Canuk - thanks for the info. You're more advanced than me...

Yes, open diff for now. That's the final piece to the puzzle. Just need to find a good time to take the rear apart and not miss too many autocrosses. This is the best time of year in TX.
 
The Bilstein B6 part numbers and rates (afaik):

Standard:
front p/n: 34-181539 rate: 131/83
rear p/n: 24-002967 rate: 225/107

Rally:
front p/n: 34-030837 rate: 300/150
rear p/n: 24-022118/3 rate: 275/200

Heavy Duty:
front p/n: 34-031520 rate: 300/175
rear p/n: 24-002967 rate: 225/107

Cup:
front p/n: 34-030806 rate: 305/206
rear p/n: 33-067230 rate: 225/150

The first one is the HD that generally sold by (web) shops. I run the 33-067230 in the rear on my 245 with lesjofors lowering springs and I'm very happy with them.
 
Yes, and this is exactly the point I'm trying to make. Both HD sets use the same rear shock, but there's a difference of the rate in the front. If you would have looked at the numbers you might have noticed.

Wow - there's my problem right there! I can't believe the HDs are the same in the rear as the standard. Makes no sense to me.

Great info though - thanks!

I ordered some Koni race from Ben, we'll see how they do.
 
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