Why are Trunnions Swapped?

by John Upham and Colin Ward


The front suspension layout of the John Thurner designed Vixen (round tube) chassis includes the familiar (if rather primitive) front upright(2)+trunnion combination. The front upright is sourced from the Triumph 12/50 drum brake Herald and the trunnions are sourced from a variety of Triumph sources including the Herald, Spitfire and Vitesse. The uprights and trunnions are both handed and must be matched. Normally, the left-hand upright/trunnion pair is fitted to the left-hand side of the Triumphs (looking forwards from the inside of the car). John Thurner designed his new chassis so that the "left-hand" combination was fitted to the right-hand side of the Vixen/Tuscan chassis. This was not an accident!
Front Upright

The Reason

Some Formula One cars of the 1950's featured a very similar front suspension layout to the Vixen with double unequal length wishbones and an upright+trunnion type combination. Study the Coopers of the 1950's and 1960's for examples of this set-up. As you know, when the trunnion is rotated on the upright (i.e. during steering) the distance between the top link (1/2) and the trunnion body will either increase or decreasing depending on the direction of rotation.

In a quest for more performance from the front suspension it was arranged that the distance would increase for the suspension on the outside of a corner and decrease on the inside of the corner. In order to acheive this on the TVR the trunnions are swapped whereas the Triumph designers didn't really worry about this. Probably, this is fine tuning that for most people would never be noticed. For competition use with a very stiff suspension set-up and lower profile tyres then it is more worth doing. If you wish the car to be arranged as John Thurner wished it then swap sides.

John Upham and Colin Ward