Pitch, Wallow and Twitch

Pitch, Wallow and Twitch may sound like a firm of comic lawyers in a Christmas pantomime, but to owners of Triumph Stags and their similarly suspended brethren, they are three gremlins that have afflicted the handling of these cars since the day they were built.

Twenty five years ago, contemporary car design, road conditions and driver expectations were such that these short comings were not the bugbear that they are today.

Even ten years ago when I first got my hands on a Stag, I was prepared to accept that they were all part of the charm (or challenge) of driving a Classic Car, so I concentrated on enjoying its other attributes for the next five or six years. Eventually however, the gulf between the Stag's handling and that of the 90's cars on today's busy roads meant that just to keep pace on anything less than a motorway was 'bloody hard work'. I therefore decided that some serious improvements were needed if me and the Triumph Stag were not to go in different directions (literally). Having an engineering background and a lifelong interest in cars and their technical development, I decided to investigate, identify, and modify the Stag suspension to provide safe and predictable handling, with no loss of comfort, at an affordable price.

My investigations began with a series of controlled (?) manoeuvres, (mainly slaloms) on private roads and car parks, backed up by static deflection tests and finally simulations on a home made 1/18th scale model of the rear suspension assembly. I clearly identified the three distinct characteristics and their likely causes, and then studied the rear suspensions of relevant modern cars in manuals and local breakers yards to see how their designers addressed the problems.

The pitch is predominantly in the diagonal mode (i.e.. corner to corner), and is mainly due to the trail angle of the rear subframes and trailing arms interacting with the strut type front suspension. Modern cars have a much reduced trail angle on the subframes and are invariably fitted with an anti-roll bar as standard.

The wallow is mainly caused by the inability of the standard shock absorbers to control and damp-out the combined effects of the primary and secondary spring modes (i.e. the coil springs and the tyres). More effective shock absorbers, correctly matched to the coil spring rates and lower profile tyres give today's cars their poised handling.

The twitch happens when one of the rear wheels suddenly changes track (normally 1/32" toe-in), usually just as you change gear coming out of a corner. This is caused by friction in the splined drive shaft temporarily holding the out of track wheel in this state until the load is reduced by de-clutching and changing gear, whereupon it promptly tries to re-align itself.  The track variation of one, or sometimes both wheels is due to the sideways deflection (during cornering) of the relatively long trailing arms being restrained only by their closely spaced pivot lugs and soft rubber bushes. Much wider spacing of the pivot lugs to reduce this sideways deflection is favoured by today's suspension designers, together with cleverly angled mounting points and bushes to give a measure of automatic track correction under load (i.e.. passive rear wheel steering).

A further refinement found on more sophisticated high performance cars is the use of dedicated track control arms or links to give precise and predictable behaviour under all conditions.

Armed with all of this knowledge, I began to apply it systematically to the Stag as follows:

  1. The pitch was easily tamed by fitting a readily available and proven anti-roll bar to the rear suspension, once I had figured out where and how to mount it. There was no obvious or easily accessible chassis strong point to fix the bar to, so I opted to clamp it directly to the trailing arms in a position that gave the effect of a 'twist beam axle' (as used on the rear suspension of many front wheel drive cars). I did however contrive to leave enough compliance to retain the benefits of the independent suspension over small bumps and ridges.
  2. The wallow was also easily taken care of by fitting new good quality shock absorbers all round, (adjustable on rear), combined with wider but lower profile tyres ( nothing drastic, just 195 X 65 on std. alloy wheels).
  3. The twitch proved the most difficult to eradicate completely, although the two previous stages of improvement had reduced it noticeably under most driving conditions. Determined to make the Stag handling completely vice-free I then carried out a series of trials with variously positioned tubes and links until I hit upon a simple but effective design of 'track control rods' that gave me just what I was looking for. As a bonus the Stag has also gained a new found precision in straight-line driving (even on bumpy 'B'roads), combined with improved steering response and turn-in on bends, together with the ability to hold a good line through the curves without 'sawing' at the steering wheel.
Well, thats my contribution to the Stag handling saga and the proof is demonstrable by arrangement to anyone who is seriously interested in comparisons with their own modifications and ideas. Maybe someone could even advise me as to the viability of making my modifications generally available, as I can only produce the occasional set for local club members.

 

Yours Triumphantly!

Gordon J. Timms
S.O.C. Warwickshire area.

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