Steering box & radius arm relocation.

[JohnD]

I have now stripped the chassis [early 30s LWB] down and starting the rebuild for a single seat special. 

Looking at various photos and other cars the steering box has been lifted by around 6-8" from the chassis and tilted back, my question on this is, is there an exact science to this, or is it more for the comfort of the driver? 

 Iam also looking to move the engine back around 8-10" in the chassis, and so the cross member that the radius arms mount to will no longer be avalable in the centre. I have seen cars where the mouting has been moved outwards and put a kink into the arms, how are these fixed to the crossmember, originally they would have had an element of torsion/friction between the two arms through the single ball fixing but this will be lost, are these now just tightened up on single balls each side, or is it better to use a modern style ball joint or Rose joint? 

 Any other advise on the above that i may have over looked would be welcome.

[AustinWood]

The original fixing uses a bolt and spring with the centre portion of the bolt narrowed. This allows the radius arms to rotate around the ball with minimal friction.

[Stuart Giles]

From a design perspective the steering box is usually moved up relative to the chassis to ensure that the drag link remains more-or-less parallel to the road surface when the car is normally loaded. The greater the angle the drag link makes to the road surface, the greater the 'bump steer' effect will be. When the chassis is dropped a long way, to preserve decent steering geometry, the steering box will need to be raised as the front axle will now be above the chassis. There will be a pause here while lots of users say their drag link goes massively uphill or downhill and they've never noticed any bump steer; to which my reply (from experience) would be you would certainly notice the difference in your car's straight line stability when the bump steer is gone after some redesign work.

To use conventional radius arms, they really need to go back to a single pivot point to work as intended. If there are two joints with any significant distance between them either the radius arms will twist in the axle (pre 1937 axle), or for 1937 onward cars where the radius arm can't rotate, the front axle will function as a massive anti-roll bar, with what is likely to end up as huge oversteer or if the centre of gravity isn't really low, the car wanting to tip over when it's pushed really hard -my experience again.....

[JohnD]

From a design perspective the steering box is usually moved up relative to the chassis to ensure that the drag link remains more-or-less parallel to the road surface when the car is normally loaded. The greater the angle the drag link makes to the road surface, the greater the 'bump steer' effect will be. When the chassis is dropped a long way, to preserve decent steering geometry, the steering box will need to be raised  as the front axle will now be  above the chassis. There will be a pause here while lots of users say their drag link goes massively uphill or downhill and they've never noticed any bump steer; to which my reply (from experience) would be you would certainly notice the difference in your car's straight line stability when the bump steer is gone after some redesign work.

To use conventional radius arms, they really need to go back to a single pivot point to work as intended.  If there are two joints with any significant  distance between them either the radius arms will twist in the axle (pre 1937 axle), or for 1937 onward cars where the radius arm can't rotate, the front axle will function as a massive anti-roll bar, with what is likely to end up as huge oversteer or if the centre of gravity isn't really low, the car wanting to tip over when it's pushed really  hard -my experience again.....

Stuart, thank you for the reply. After reading your reply on the steering box this makes perfect sense and not sure why i didnt see this before! 

Do you or anyone else have any suggestions on how to relocate the radus arms successfuly ?

[Terrytuned]

steering box mount during build (photo 1) finished setup (photo 2)   Terry.

[Paul N-M]

Stuart's advise is spot on.
Depending on your particular layout its not too difficult to come up with a mounting for your steering box. It has to be strong enough not to flex though. Re outside of chassis located torque arms this can be more difficult. You must avoid the anti roll bar effect trap. This can be done if the arms are articulated at both ends. Using twin arms both sides also lets you set castor as well as location.

[Stuart Giles]

Tubular radius arm setup on my special. This is a Big Seven front axle, but something similar wouldn't be to difficult rig up on a Seven beam. The tubing is 3/4" 16 SWG CDS. What's at the other end is in the second photo. The clevis joints allow adjustment of the castor angle.


John, you don't say if you're intending to remove the front crossmember when you move the engine back in the chassis, if you haven't lowered the chassis massively, it will be in the way of the gearbox/crankcase/sump. If removing the front  crossmember turns out to be a necessity, you can just install a crossmember further back in the chassis. This is the tubular crossmember I fitted to my special along with the tube radius arm setup as part of a suspension and steering tidy up 35 years ago. It's 3" diameter ERW tube; about 14 SWG from memory, was in fact previously the tube section of a Vauxhall Cresta propshaft, I suppose if you're going to compete in VSCC, you could say that it came off a Bentley or such..   The centre pivot is a 1/2" Teflon lined rod end. this crossmember is much stiffer and lighter than the factory item.



If your build does preclude any kind of centre pivot, you will need twin radius arms on each side with full articulation as Paul says in the post above. This means twin joints each side one above the other at the axle beam end, and twin joints one above the other at each chassis side. this is the only way to get movement of the axle, preserving the castor angle and dealing with the torque effect on the axle under braking.

[Malcolm Parker]

    On my project I have moved the steering box back about 6" and raised it a similar amount by mounting it on a fabricated steel saddle that is bolted to the chassis rail. The drag link has been lengthened to suit.

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    A clearer photo taken earlier on.

[JohnD]

My intention was to retain the front cross member but just cut down & strengthen the centre section as required. 

I have attached an image of a car i have seen which has modified the girling type arms as i previously discussed, but from your advice this would cause an antiroll bar effect. I liked this approach as it seemed fairly simple and kept the car looking relatively standard looking without adding twin tubular arms.

 If i was to move the cross member back, could i just extend the arms?

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On my project I have moved the steering box back about 6" and raised it a similar amount by mounting it on a fabricated steel saddle that is bolted to the chassis rail. The drag link has been lengthened to suit.

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A clearer photo taken earlier on.

Thanks for posting these photos, it was seeing your bracket that got me thinking on the need to lift the steering box, Have you welded on the bracket or is it bolted?

[Paul N-M]

Hi John,
By all means extend the arms to a central point on the relocated cross member. However you must ensure that there is enough clearance for the arms to move up without hitting the chassis. This could be tight if the chassis in now sitting lower than std. I would suggest boxing the arms in as well especially if they are now going to be longer. Pay attention to how it may effect your castor as well.