Why are Trunnions Swapped?
by John
Upham and Colin Ward
Background
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! |
 |
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
webmaster@ukmotorsport.com