Hey everybody. Its been a while since ive been on here. The Rallycar is still going strong, and we had our best finish a few weeks ago at the Lake Superior Pro Rally in Houghton MI. We finished 2nd in class, 3rd in 2wd, and 6th overall!
We did have a problem with one of the rear control arms. It seemed to crack across the top of the arm and then around the wheel bearing on both sides. We caught it, welded it and finished the rally, but it looks kinda scary.
through rotor hat
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backside of arm
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top of arm
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Does anyone strengthen the rear arms?
Any ideas? Or are we just putting this on 25 years of street duty, 10 years of road racing and 5 years of rally racing?
Byron as well. There have been a few. It's just down to above-and-beyond use/abuse, or a vastly overengineered part that is finally seeing fatigue.
The common repair (or mod to prevent) is to weld a plate over the top of the arm in the area you had your fix, like on Page 3 of the link above.
Because when you spend a silly amount of money on a silly, trivial thing that will help you not one jot, you are demonstrating that you have a soul and a heart and that you are the sort of person who has no time for Which? magazine. – Jeremy Clarkson
We see this all the time with the ice racer - reinforced or not.
The added side force from sliding doesn't help (along with springs triple the stock rate and shocks to match, giant rear downforce, and snow banks taken at speed), and wasn't likely planned for in the previously mentioned overengineered part.
We do add plates, top and bottom, and big as possible to strengthen the bearing housing and to spread the loads.
It's also a very common check on our inspection before every race. We've caught a couple in the week before a race while checking driveshaft bolts, etc. and had time to remove and repair in time. Early on, Dad had an arm fail and wrap up around itself into a U shape.
Be sure to check those front arms and the rear crossmember too. The failure will just find the next weak spot - trailing arm mounting ears, moustache bar...
We saw it from road racing and of course the odd street curb hit.
This is what I did on mine, FULL plate across the whole top, and, I cut mine out for wheel clearance on the side, but, we put in a very thick plate in the removed section.
as Richard said, loadings are huge in your/our applications, and these parts are getting very tired. We have seen some failures at Specialty over the last while, even from street duty. Rust and corrosion.
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"Racing makes heroin addiction look like a vague wish for something salty" - Peter Egan
Keith Law
1973 2 Door Slalom/hill climb/road race / canyon carver /Giant Killer 510
1971 Vintage 13' BOLER trailer
Hi everyone, haven't had time to check in much lately.
But here are the mods I did to my rear arms. Might be over kill , Don't know but better safe than sorry.
Let me guess. First, those with rear trailing arm failures run the springs in the stock location (no rear coil overs)? Second, they are not the stock spring rates.
It happens when you reach coil bind and the upward force of the wheel is only resisted by the metal on metal force of the chassis through the solid spring (coil on coil). This will accelerate the metal fatigue and you get what you get. Especially on a 40 year old car. Welding re-enforcing plates will help (resets the fatigue cycles), but unless you remove the cause, at some point it will come back.
As the saying goes "you can do it right or you can do it over". Suggestion, measure the open space between each coil in the rear as your car sits on the tire. That is the total distance the spring can compress. Does that seem reasonable? A typical 510 weighs 450 pounds per rear wheel. A spring in the stock location is only 40% of the wheel rate. (Yes, measuring perpendicular from the line through the 2 cross member mounts a 1000 pound spring has an equivalent wheel rate of 400 pounds). A cheap way to see if you enter coil bind is to place plumber's puddy between the coils to see how much it compresses. If it pinches out all the way, you are at coil bind and should consider a stiffer spring or a taller spring or go to coil overs.
on the top side, the plate is formed over the bearing housing, and continues to the spring pocket area.
also, a small gusset or two for the shock mount won't hurt either.
