[Fot] Fot Digest, Vol 103, Issue 232

[John Davaies]

GT6 suspension

If you have wishbones,, then you are simulating the Rotaflex modification
done in response to criticism of the swing axle design of early cars.
Then Triumph had the Brilliant idea of the swing spring, to make the car
soft in roll, but hard in bump.     The advantage was in lightness, no heavy
cast iron wishbone, and cheapness.    If you have fabricatd (light?)
wishbones (and don't intend to go into production) there is nothing to be
gained by going swing spring.   Indeed, swing spring and variable length
drive shafts (just won't mix!


On this subject, I recently had a correspondence with Mark Ortiz, the
chassis consultant to many Nascar and other US teams.     Racecar
Engineering is an excellent journal, which I recommend!   And Mark is a
regular columnist, "The Consultant".    A recent column dealt with the
Chevrolet Corvair, that gained an awful reputation for poor design and
on-road lethality.   Ortiz discussed the swing axle that the Corvair had,
and shared with other cars.     In the absence of a Correspondence column in
the magazine, I wrote to him:

Mark,
Thank you for your interesting article on the Corvair, (Racecar Engineering,
August 2020, p.51 Corvair Vindication?) and the swing axle design that so
attracted Nader's ire.    As you say, that design and its shortcomings were
well known in the 60s.   You listed several manufacturers, so attracted by
this cheap and simple suspension as to use it in their cars.   But you
omitted the car whose designers evolved the swing axle more successfully and
cheaply than most, Standard Triumph.    They chose the swing axle for their
'small-chassis' series of models, the Herald, Spitfire, GT6 and Vitesse.
Motoring journalists knew as well as designers how to get such an axle to
'jack-up' and did so immediately, giving the cars a poor reputation.
Triumph replied for the more powerful GT6 and Vitesse by adding a lower
wishbone, but the design was heavy and expensive.      As you say in your
article, "what works best for a swing axle is stiff springing in ride. and
soft springing in roll" but you only gave Formula Vee as the example.
Triumph came up with the "swing spring" that did just that, by allowing the
transverse spring to pivot in the centre, to reduce the roll resistance of
later cars by 75% !
 This most successful modification is, I believe , unique, and I feel that
Triumph's ingenuity should have been recognised!

 

Mark has been kind enough to answer me in his latest Chassis Newsletter, a
regular email column that as he says he "sends out to promote my business,
it is not my intention to spam my correspondents, and the newsletter is
intended to be genuinely useful to its recipients."    No doubt you do,
Mark, you're not spamming me, and your article is most useful.  Thank you!
I hope other Triumpheros will be interested:

THE TRIUMPH SWING SPRING

The "swing spring" is one of many ways to skin this cat.  The oldest I know
of is Mercedes' third coil spring. 

For those unfamiliar with it, the swing spring is a transverse leaf spring
similar to the one that the Spitfire/Herald suspension already had, only
rubber mounted.  This allows the spring to swing as the suspension displaces
in roll, but the rubber resists this a bit.  The spring itself is also made
stiffer than the original.  This allows the system to act like the original
system with a camber compensator added, or maybe a bit better, with fewer
parts.  Also, although I have not investigated the patent situation
surrounding this, I would surmise that the swing spring probably would be a
patentable invention. 

The swing spring reduces the elastic roll resistance by about 75%, compared
to (I guess) an identical leaf spring rigidly mounted.  The system still has
a lot of geometric roll resistance, and it still jacks quite a bit.  Indeed,
Triumph could have just mounted the spring so it could swing freely,
eliminating rear elastic roll resistance entirely.  This would make the
system functionally equivalent to a "zero roll" setup on a Formula Vee.  The
system would still produce substantial geometric roll resistance, and
accordingly considerable load transfer and jacking when cornering.  However,
I doubt that it would have been patentable like that. 

Another way to reduce the jacking and limit oversteer is to add front
anti-roll bar stiffness.  The less load transfer the rear has (meaning the
more the front has), the less the rear swing axle system will jack.  

Depending on tires, road surface, and rear toe setting, swing axles can
produce judder on the inside wheel when we reduce rear load transfer.  I
have seen this with Mk. 3 Spitfires, on the street, on street radials.  This
is sort of the Y-axis analog of rear wheel hop in braking with a lot of rear
brake and anti-lift.  Swing axles can also produce judder on the outside
wheel. 

There is no way to make a swing axle suspension as good as more complex
forms of independent suspension.  The Triumph swing spring does not produce
handling equal to the earlier GT 6 suspension.  However, it is somewhat
lighter, definitely cheaper, and a significant improvement over the earlier
swing axle setup.  And it achieves this essentially for free - really no
added parts at all.  So to that extent, it is indeed a clever solution. 

In theory, GM could likewise have used just a swinging leaf spring on the
Corvair - basically just a multi-leaf camber compensator - and dispensed
with the coil springs, although there might be structural reasons that
wouldn't work, at least not without re-engineering the engine and transaxle
mountings.  Those mounts would then be holding the back of the car up, and
there would also be significant bending loads applied to the transaxle and
engine.

-----Original Message-----
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Sent: 20 July 2020 19:00
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Subject: Fot Digest, Vol 103, Issue 232

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