I was thinking of boxing in the radius arms on my trials axle. However it occurred to me that it may be easier and stronger to fabricate new arms from tube. Has anyone done this and would rectangular ERW section be adequate or is CDS round tube a better bet?
I would stick to the standard Austin radius arms. Virtually all the VSCC trials cars use them without any obvious problems. If you make the radius arms very heavy and rigid, the next weak point is the front cross-member. Where do you stop. I take the view that if you hit a large rock or tree with a front wheel, it is better to have a bent radius arm than a buckled chassis. A standard radius arm is relatively simple to straighten.
Austin 7 axle parts are quite light but are made of very good metal and are very strong.
23-07-2020, 09:35 AM (This post was last modified: 23-07-2020, 09:36 AM by Steve Jones.)
I agree. Malcolm and I have done countless trials in my trials Chummy and have hit all sorts of things but have never bent a radius arm. I have straightened a bent spare one recently and it was easy to do.
I have fabricated radius arms on two of my cars. The two main reasons for doing this is to get better rigidity under hard braking and to be able to easily adjust caster angle. Can't really see that either of these would be a priority for trials. But here's the radius arm setup on my special which has a Big Seven front axle:
The tubes are 3/4" 16SWG CDS, the single "ball joint" is a 1/2" teflon lined rod end mounted to the cross member. the reduction in axle twist (i.e. movement) under braking was really noticeable when I did this mod. Doing something similar with a Seven front beam would really need the "batwing" bracket to be tied into something other than the big bolt hole to prevent it from rotating. Either a shock absorber mount hole or the other bolt hole if you have a Girling style front axle would do.
The answer is - as everybody comments - it's not worth bothering. I've run over some rough ground on occasions in the past, motoring with slight briskness - and never a problem.
Thanks for all the responses. It’s comforting to know that Austin’s arms are generally up to the job.
However my question was partly because I have two good radius rod ends and no actual rods (although I have some complete rods “on order” so to speak). It should be relatively easy to fabricate rods that are similar to Austin rods but I am unsure what steel grade would be needed.
Would this thread have proceeded differently had the OP posted...
I have two good radius rod ends BUT NO arms remain.
Of what steel, and how exactly would you fabricate new arms so the original parts aren't wasted?
(Or please contact me if you have some spare radius arm parts that I could use!)
Thinking about it, a brake tie rod would be very sensible on my bowed axle as the drop plates lower the axial braking force and increase the moment on the axle.
Two problems I would have are; I’m using an early axle with no second bolt hole and my steering arms face backwards. Of course I could change both but....
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