mounting shocks on control arms

[gudge]

doing a wheelbase extension on my gq shorty, got everything done except shock mounts. had the idea of using the original chassi end mounts and mounting the bottom mounts to the top of the new longer control arms , these are very heavy wall tube. has anyone done or seen anything like this before? and can anyone see any major issues? in theory it should be fine yeh?

[lukethedork]

The shock will attempt to twist the control arm, resulting in increased wear to the control arm bushes.

It is late and I am having difficulty finding the right words to describe what I mean.
But the twisting of the lower control arm will result in what I can only describe as the equivalent of 'play' in the lower shock mount. Your shock will not perform optimally.

The further towards the chassis you mount the shocks, the more leverage is applied to them. This means you must have a stiffer valved shock.

Luke.
PS. I read back what I wrote and I sound like a retard.... =)

[brooksy]

Is common onTrophy trucks & similar desert vehicles.
Don't think it would be legal onroad though due to Transport dept being narrow minded & It will transfer extra load on the bushes though which you will need to consider.
I would also try to spread the load along the arm as much as possible to stiffen the load point if required.




brooksy

[gudge]

Thanks fellas, the leverage on the shocks was considered, but these are very long arms, say 1100mm and the shock would only be 200mm from it's original spot on the diff. So if my math is right about 18%

Luke, i think i understand the twist on the arms, ur saying at full compression the shock force will be applied torwards the outside of the arm, not directly through it? I hadn't thought about this and it would put more stress on your bushes, question is, could they take it?

Legalities arnt an issue, it's already registered and will only ever be driven in winch comps

The other thought I had was the orientation of the bushes on the arm end. In the pic it has them opposite to what they would be on the diff, But the original direction won't bind as much? Due to the longer arms there will be more of an arc so not as much bind, did I say that right?

Thanks guys, keep the comments coming

Gudge

[lukethedork]

Not exactly Grudge.

On vehicles with 'radius-arm' type suspension. The seperation of the two axle mounts increases the torsional rigidity of the control arm possibly by more than 1000% over that of a control arm with a single mount on the axle and single chassis mount.

I am a little more fresh today to explain what I originally meant.
If using Heim joints / Johnny joints or even bushes with a single axle and single chassis mount, then the torsional rigidity of the control arm will be VERY low.
Put a Pipe wrench onto your control arm and try to twist it, you will see it moves with relative ease.

So onto my point. With the axle and chassis both locked in a static position. Simulate a force acting on the control arm similar to that of what the shock will be applying. You will see that the control arm rotates.
This will in almost all cases change the mounting distance between the upper shock mount (most likely chassis) and lower shock mount (on the control arm).
A change in shock mount distance without a change in axle/chassis distance will result in poor shock performance.

I hope I make sense today.
Luke.

*EDIT* removed reference to trophy trucks.

[uninformed]

I havent seen TT's with Radius arms???

the general consensis when mounting shocks to trailing arms on TT and the like is to have the mount below the center line of the TA. this will stop the roll or twist that has been discribed. Check out Pirate4x4 there are tons of pics and threads with these. Note the loads on the TA are now alot higher and have to be designed/built to handle it. I have also seen buggies built with the shock mounted very close to the axle end, say within 100mm, and they have only mounted it to the top with double shear tabs. But things to considerer are: the types of ends/bushes used on each end ot the TA and the material/design/construction of the TA.

A simple way to make a new one would be to get some new tube put a bend in it with the apex where you want the shock, box it in and have the bend DOWN so you can now mount the shock lower than centerline. The downside is the arm now has less clearence.

cheers,
the internet jockey

[bru21]

Look through my members thread, and see the shock mounts are below center. Remember loading a bend will make it fail without decent bracing.