Kit Car Adjustable A Arm Suspension

The A Arm set-up above, is designed for oval track racers. The ball joint mount (top of picture) can be moved both closer to the front or rear of the car and out-wards or in-wards. One of the ball joint mounts is a threaded stud and the other a clevis joint.

Kit Car A Arm Adjustable Ball Joint

I plan on using a similar system of adjust-ability, both on the front and rear suspensions of my Kit Car. I used Free CAD suspension programs such as Kangaloosh and Suspedit, to modify the standard Haynes Roadster suspension set-up. I am making custom front and rear uprights which allowed me to modify roll centres, anti-squat, anti-dive, camber and caster.

Although I have a certain level faith in such programs, I do have this small sceptical gremlin in the back of my brain that wants me to have as much adjust-ability as possible. This is not just because I have chosen to change every suspension parameter, but because this chassis is unique and has been built from scratch in a shed.

I’ve done everything I can, to measure every single dimension in 3D, to an accuracy greater than than published in the original Haynes book and to higher tolerances than most of the rusty wrecks I’ve owned, but that loud mouthed gremlin is still in there.

Problems with Adjustable A Arms

The above a arms use rose and clevis joints, which would produce a very harsh un-cushioned ride on the road. Another problem would be sealing the rose joints from dirt and stone damage. Sure, you can get covers, such as the one below, but for a car with limited suspension movement, a firm ride and lightweight seating; using the car on a regular basis would be very tiresome.

Rose Joint Cover

Three out on the 4 joints, on the A Arm at the top of the page, have rotational adjustment. This necessitates using a clevis (outer joints) or rose joints on the inner joints.

A much more comfortable solution, that also has greater weather resistance and longevity, would be to use polyurethane joints. For the front suspension they can be used on the inner mounts and on the rear suspension both inner and outer. With the welded Haynes Roadster design all the caster adjustment has been taken out of the A Arms. The only adjustment available is via packing washers inside over-sized U shaped mounting Brackets, but this is a very small amount.

U Shaped Suspension Brackets

With all of this in mind, I intend to use a combination of clevis joints and polyurethane mounts. In the very long distant past, I remember from my Hot Rod days; a company long since gone; used to import some rather great looking universal urethane joints. The nearest I have found, is these. They are on some top budget Mustang replacement arms.

Adjustable A Arms Kit Car

Kit Car Adjustable Suspension

The tube sections are knurled. These are known as turn-buckles and have opposite threads in each end. I intend to use a slightly simplified version of what you see above, but I intend to make the parts myself. I will use standard polyurethane joints with TIG welded lugs to accept clevis joints. I am currently looking a some heavy weight heat shrink tubing to cover the nuts and bolts for the IVA exam. For the top uprights’ ball joint, I plan on using screw in Austin Ambassador (the non-shim’d variant) and custom made cups.

Austin Ambassador Ball Joints

Adjusting Camber Angles

By loosening the locking nuts and rotating the tube the length of each arm can be increased on decreased. If both tubes were to be increased by the right amounts, the uprights ball joint can be moved in-wards and out-wards.

Adjustament A Arm Camber


Adjusting Caster Angles

In the shot below the right hand arm has shortened. This has moved the outer upright ball-joint to the right, adjusting the caster angle.

Adjustable A Arm Caster

Caster Angle Adjustment

It is important to realise the difference between Caster and Caster Offset / Trail

The shot below is similar to my intended set-up.

Haynes Roadster Adjustable A Arms

There will be a pull rod from the top upright mount to a bell-crank mounted on the lower chassis rail (just visible) to a coil-over shock.

I seen several other kit cars using this kind of set-up. Take this Nissan 240sx powered McSorley 442 by Jack McCornack.

Kit Car Adjustable Front Suspension