From: mikesteg@syclone.detroit.sgi.com (Mike Stegbauer)
Newsgroups: rec.autos.driving
Subject: Re: Can I install wider tyres than what specified by the auto-maker?
Date: 26 Jan 93 21:30:20 GMT
Reply-To: mikesteg@syclone.detroit.sgi.com (Mike Stegbauer)
Organization: sgi

In article <1k26mlINN7l6@ariel.ucs.unimelb.EDU.AU>, met@ariel.ucs.unimelb.EDU.AU (Meteorology General) writes:
|> >$|> >If your going from a 195-60-15 to a 205-60-15, the 205 will be taller. 
|> >$|> >This will affect your speedo.  You might try looking at a 205-55-15.  They
|> 
|> I think we are missing the point here guys. The subject line says WIDER tyres.
|> The problem with putting wider tyres on a car is that the set up will then be
|> askew. You would really then need to stiffen the suspension as well. Reason;
|> a car is set up so for a standard set of recommended tyres the suspension is
|> of such a "hardness" that the car "should" slide before it rolls. i.e the 
|> suspension should be set so that the body roll will induce only so much "bite"
|> that before the point of "roll-over" is reached the tyres will give and will
|> begin to slide. If you put wider tyres, and hence get more grip, and the 
|> suspension has not been stiffened, then it is possible that the "bite" will
|> continue past the point of no return, and the car will hence roll before it
|> slides. Think about it. Finding that optimum point is what it is all about.

Ok, let's think about it.  Friction (grip) only depends on two things, weight and the 
coefficient of friction for the surfaces, NOT the area of contact.  This is because, as
area increases, the weight per unit area decreases proportionally, and the terms cancel.
This being the case, wider tires will not increase grip, only better tires, or more weight
on top on them.
This is closely related to why FWD cars drive well in snow, there is more weight over the drive
wheels to push them into, and through the snow.

Please note, I agree with you about changing tires upsetting the designed characteristics of
the car (for the worse), but not this particular one.


|> Testimony to this was Carlos Sainz in the 1991 (I think) Rallye Aust, when 
|> he finished the loose gravel stages and switched to slicks for a short tarmac
|> special stage. Unfortunately the Toyota mechanics didnt change the suspension
|> settings (for whatever reason) that had been set very "soft" for the marble-like
|> West Australian gravel, and Sainz went out, pushed it hard (as is
|> his want) and promptly flipped the car onto its roof almost (ALMOST) putting
|> himself out of the rallye. The rest is history. So the moral to the tail as I
|> see it is more grip, stiffer suspension, or the exercise is pretty much 
|> useless.

I'm sure this was related to suspension, but more likely that the car, when set for
the soft stuff, 'bounced' on it's suspense, and at one point came down hard enough to 
momentarily increase the effective mass of the car, and therefore 'grip', and therefore SSCCCRRRREEECHHHH.  The change in the effective mass from 'bouncing' also makes the car
difficult to drive, because of this constant change in grip.  That is why they change the suspension.


|> 
|> Andrew B. Watkins                    |    `To strive, to seek, to find and
|> awatkins@mullara.met.unimelb.edu.au  |     not to yield'  
|> awatkins@buster.met.unimelb.edu.au   |                  - Lord Tennyson 
Newsgroups: rec.autos.driving