Stabilising Castor in the Classic Suspension

The classic front suspension simply uses the anti roll bar to control the fore and aft movement of the top link, and hence castor. The later 'uprated' version adds a second arm to effectively create a new upper wishbone.

So what was wrong with the original? On first sight it would appear that the ARB should control the movement pretty well. It's mounted in solid aluminium blocks at the front, and there's only a small amount of compliance at the rubber bushed joint with the top link.

However this is an illusion. With a cycle wing removed and the two stays grasped firmly one in each hand, it's possible to move the top ball joint fore and aft about 10mm as the ARB slides side to side in its mounts. As it slides, the bar flexes at the bend and the distance between the mount and the ball joint changes, altering the castor angle. To make matters worse an increase in angle on one side is accompanied by a reduction on the opposite side...

With equal forces on the two uprights that clearly won't be an issue, however on irregular surfaces or under hard braking such forces are very unlikely to be nicely balanced. The witness marks always seen in the ARB paint finish either side of the mounts demonstrate that the sliding movement does occur when driving.

Retrofitting the uprated suspension is quite expensive and also involves difficulties in cutting holes in the side panels and making good. It would seem however that much of the benefit it provides can be gained by simply preventing the ARB sliding in its mounting blocks. This keeps the control lengths constant and makes the section of the ARB between the mount and the top link function more nearly as part of a 'wishbone'.

This can be achieved quite simply with off the shelf components. I used two 1/2" (double split) clamp collars on the ARB acting against the inboard surfaces of the mounting blocks. These are very cheap, the modification is easily reversible and is totally invisible when the nosecone is fitted. An example from an eBay listing:-

These dimensions seem pretty standard. The outer diameter is then a little too big to snug up directly against the blocks, and spacers are required to bring them further inboard to clear the edges of the chassis tubes. These spacers simply comprise two M12 nylon washers (i.d.=13mm, o.d.=24mm, 2.5mm thick), each cut so they can be twisted onto the bar.

With the clamps in place the movement in the top ball joints is limited to the small amount of compliance in the rubber bushes, but is it a change for the better?

Conclusion

After fitting the collars I searched around a bit more and of course found there's nothing new under the sun... There was a rather confused discussion in 2002 on the L7 club website <here>, where it's noted that similar limit stops using hose clamps were often fitted to racing Caterhams in the 80s, and an image on SimpleSevens.org ( <here> ) showing wire rope clamps doing the same job on an early Lotus 7.

It's very easy to make changes and convince yourself of a dramatic improvement. (The degree of which is proportional the the amount of money spent!) Usually it's difficult to switch back to the original for comparison so any self-delusion therefore tends to persist. In this case however the clamps can be moved in and out of position in a matter of minutes to swap between the two conditions to help decide if there's any real effect.

That said, I do 'feel' that it has genuinely reduced tramlining and any minor 'nervousness' on poor surfaces, and extreme braking no longer leaves me with that 'ever so slightly out of control' feeling...!

It's certainly hasn't been detrimental...