Re: Non-Standard Tyre sizes

[Mike Wattam <101714.1343@domain.elided>]

Not a problem, I just seem to spend most of my walking time answering
'Stag' e-mails!  I really shall have to try not to put my tuppence-worth
into EVERY one I see!  It's not as if I'm an accurate typist.

The place to start is rolling radius, or revs per mile (this assumes you
want to maintain gearing).

Then, as you want to retain your current wheels, check the manufacturers
lists to see what is the widest/lowest you can get on the 6" rim.  Note
that as said previously, wide/flat tyres are very fussy about the rim width
as they MUST be kept flat on the road to maintain a good footprint.  You
will very likely find that you cannot go much wider/lower profile than
where you are already at with 70's, possibly a 60 profile.  Bear in mind
the following factors also, in making your choice of tyre.

If you were going to go for new wheels, you might be able to go up to about
17" with 40 or 45 profiles, but you will almost certainly need at least a 
7" wide rim to do this.  However, I would really advise against this as
apart from the bodywork modifications, so much work is going to be
necessary on the suspension to make it viable ........... but perhaps you
want to do this.  Also do not forget that this kind of tyre/wheel
combination is inherently very harsh and normally has suspension
specifically designed to suit it (e.g., Porsche, Lotus, Ferrari ....) so
you will notice a very choppy ride and lots of noise coming back into the
cockpit.

One factor which I have not mentioned so far is wet road road-holding. 
Wide tyres can be superbly grippy in the dry, aided by fancy rubber.  But
in the wet it is a different thing, as inherently the bigger the footprint
on the road, the less is the load per square inch from the weight of the
car.  So, if 'extreme' tyres are fitted, they will very easily stop cutting
through the water to dry road (aquaplaning) even at very low speeds. 
Therefore, most of these ultra-low profile tyres have deep wide water
channels cut in them to alleviate this problem, but it is still a problem
of which you should be aware.  Look at the Goodyear Eagle F1 as the best
case of a tyre in this category.  Add all this to the change of camber
present in the Stag suspension systems, and you could have big problems.

I mention again the vastly increased loads some of these mods will put
through the suspension/chassis, so be careful.  From your remarks I assume
you are using the OZ yellow bushes which seem to be brilliant, but they of
course also absorb less and so pass more shock loading into the chassis.

The last thing is the cost.  The tyres themselves are very expensive
(215-45 x 17" = £180 each!) and you must inform your insurer you are
running on non-standard tyres which will give them a further opportunity to
bleed your bank balance.

For details of tyre types, rim widths and rolling radiuses etc, contact the
tyre manufacturers technical departments and they will send you brochures
and lists of specifications.

Hope the above helps. Now, what was the question?

Happy New Year.

Mike Wattam
Triumph Stag Register