Non-Standard Tyre sizes

[Mike Wattam <101714.1343@domain.elided>]

What I want to know is, how come your questions are always so short and the
answers so long?!?!?!?!?

The 'correct' tyre size is 185-14 (later cars had 175-14) high profiles,
for which the suspension was designed.  This is an important issue.

But a lot of people fit low profiles, the most popular being '70' series. 
You can go down to '40' in theory, but to maintain rolling radius (and thus
gearing) you will need a very wide tyre indeed.

There are various problems this brings;
1. wheelarch and suspension clearance
2. finding a wheel that fits the car with the right offset
3. the wide tyres have so much dry grip they can tear out bushes and rusted
mounts
4. the geometry becomes very bad and bump-steer occurs at both ends
5. the extra noise transmitted isn't very pleasant
6. these tyres are very rigid and will transmit every small bump into your
seat

If you can put up with problems 5 and 6, it becomes easier.

The suspension geometry, particularly at the back needs sorting out.  This
type of tyre must continually run perpendicular to the road to be
predictable and wear reasonably well, otherwise you get lots of bump-steer
effects from excessive toe-in/out.  Plus the variation in footprint as the
suspension goes up and down can lead to sudden and unpredictable
break-away.

Ideally the back suspension at least should have a complete redesign to
minimise camber variations and toe-in variation.

If you can get suitable wheels (very low profiles MUST have a wide enough
rim) then there is lots of work to be done on the offset to give adequate
clearance against the suspension, while still running under the wheel arch
- some of the tyre/wheel is certain to stick out of the arch and should
have some cosmetic surgery on the body (otherwise Mr Plod might pick you
up).  

Finally you will need to increase the spring/damper rates due to the
increase in unsprung weight.

All in all I wouldn't do this, but either stick with high profile tyres or
go for the best '70' series tyre I could get to find the best compromise
between grip and NVH, without having to lots of re-engineering.

Happy New Year.

Mike Wattam
Triumph Stag Register