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Triumph TR6 - Suspension Tuning - (a guide)

After reading various posts on the subject ranging from just expensive to the outright dangerous, I thought it may be useful to post this 'guide' for anyone contemplating 'tweeking' their suspension.

(I trust that GJ Thomas will view this as 'for greater good' rather than an infringement of copyright!)

Excerpts from “TUNING MANUAL (Triumph cars) – GJ Thomas – Coventry. 1986.

“One reason for the popularity of dual unequal wishbone suspension systems is the precision with which it compensates for problems like body roll. To make systems stable, 2 extra elements are added:

Slight toe in which corrects the tendency to toe out at speed, and positive castor, which corrects steering geometry for independent suspension action.

As steering is the more precise of operations, dual wishbones are used on the front, with some other rugged torque handling system on the rear.

On road cars, manufacturers reckon that the public are able to cope better with understeer, a situation where the front suspension widens its arc of turning beyond the actual amount dictated by the steering system. For this reason, most production cars are set up to understeer.

To change suspension characteristics, the following rules are useful:
(In practise, these cannot be treated separately because whatever is done to one end, inevitably affects the other).

FRONT

· Spring stiffening decreases understeer then after a certain point makes it worse. It decrease roll too: softening increases roll.
· Shock absorber stiffening gives better control, then after a certain point worsens understeer.
· Roll bar stiffening always makes the car widen its turning arc. On the front, this increases understeer but obviously makes the car roll less. The general impression is greater stability and less rear oversteer. (e.g. On TR6’s it is common for the front wheel to lift, as the loaded suspension side bottoms in a corner. This indicates inadequate spring rate and control. Increasing roll stiffness increase the understeer so the best cue is a harder front spring and a rear roll bar.
· Adding negative camber increase grip to a certain point but gives less stability at speed and greater steering kick back. After a certain point, more camber loses grip.
· Lowering ride height lowers the roll centre so the car rolls less.

REAR

· Spring stiffening decreases oversteer to a point, but can increase bump steer, lift off, oversteer and final oversteer after a certain point. As for the front, harder decreases roll, softer increase roll.
· Stiffening shock absorbers improves control, but after a certain point causes worsening oversteer.
· Adding a roll bar or increasing the roll stiffness increases oversteer and of course decrease understeer.
· A Limited Slip Differential increases understeer, and causes oversteer.
· Increasing camber can increase rear grip to a point. Too much wears tyres quickly and diminishes stability.
· Lowering ride height lowers the roll centre so the car rolls less.
· Wider track diminishes roll, increase stability and so helps the car turn in i.e. steer positively into corners.

Don’t try to eliminate roll completely, you can easily make roll stiffness too high.

The only critical point is that the suspension camber change must always follow roll to give the right wheel loading and camber at the road. Excessive roll of course deprives you of grip because the suspension usually runs out of travel.

As a general guide;
Road cars are setup with softer springs controlled with roll bars. Racecars, to derive maximum grip on smooth tarmac, are setup up with stiff springs and minimal size roll bars.

In practise, setting a car up is a mixture of stiffening spring and roll bar rates and controlling moment properly. Finer adjustments can usually then be made with different bush materials, rose joints and slight changes in spring and damper rates.”
Roger H

Good info, Roger. Have a XXXX for me.

BB
Brent B

This thread was discussed on 06/06/2004

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