From: bqueiser@magnus.acs.ohio-state.edu (Brian J Queiser)
Subject: Re: Can I install wider tyres than what specified by the auto-maker?
Organization: The Ohio State University
Date: Thu, 28 Jan 1993 17:43:48 GMT


*** Warning:  This post is long.  It's my $0.02 about tires and width and
    all that.  You could learn something and/or help me out if I am
    off base on anything.  Hit 'n' now if you`re not interested.

met@ariel.ucs.unimelb.EDU.AU (Meteorology General) writes:
>mikesteg@syclone.detroit.sgi.com (Mike Stegbauer) writes:
>>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.

>>I think we are missing the point here guys. The subject line says WIDER
>>tyres.  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.

>Sorry Mike, but I have to disagree with you here. The formula surely (yes,
>yes; dont call me Shirly) is not that simple. (Take it from someone who did
>a few years of physics - nothing is that simple.) If you want to use an
>analogy; then why do F1 cars, and all racing cars for that matter, have
>wider tyres? Yes the tyres are "stickier", but then the width enables
>more "stickiness" to be applied to the road (Well, you know what I mean -
>bit of a hand-waving argument I know). The case of the narrow tyres in
>snow is a completely different matter - you're not getting your grip from
>the tyre sticking to the surface, but rather from the increased force per
>unit area "pushing" the tyre into the snow and creating its own rut/groove
>in which the tyre travels. Sort of like making your own railway track
>as you go along.

First things first.  If that guy is running 195/60R15s, then putting on
205/60R15s is going to mean about a 2% difference in circumference of the
tire.  Considering that the speedo is lucky to have about a +/- 2% error
and the roughly 0.5" of tread that you can wear off a tire's diameter, that
2% difference in circumference isn't going to mean squat.  In fact, it could
make his speedo more accurate.  Who knows, and who cares?  Stick a pitot
static tube on your hood if you really wanna know your speed.  :^)

Second, there is no such thing as an exact tire science.  The definition
of friction that is given above is correct.  That is, with a given mass
and constant coefficents of friction of the surfaces in contact, friction
force will not be dependent on contact area.

That's the basics.  It gets very fuzzy after that.  But I have studied this
a little, so I think I can impart some more information; please correct me
if I am wrong.

I like to think of friction and traction as two closely related, but 
different things.  Generally, people think of them as the same thing.  
But what about rolling friction?  What about snow and rain and ice?  
What about temperature?  Just why are those Indy tires so wide?

Well, to start with, remember that the coefficient of friction increases
with temperature.  This helps explain why Indy drivers snake around during
the pace laps.  Also remember that as the tire temp increases, so does
the pressure inside (PV=nRT and all that).  These temp increases are
enough that they significantly affect handling and performance.

Street tires have grooves.  The grooves are cut in very deliberate
patterns.  Longitudinal grooves collect and channel water to prevent
hydroplaning.  Lateral grooves help cut into snow and mud; they also
channel water to the longitudinal grooves.  There are also cuts in
modern tires.  Cuts help grip ice.  This is easy enough to understand,
but why, exactly, is friction increased because of these grooves and cuts?
Well, this is what I mean when I say "traction."  Water is a lubricant
between the road and the tire, so you want it out of the way; you remove
the lubricant (water) and you are almost back to the surface contact
you had on a dry road.  "Almost" because you can't possibly remove *all*
the water.  Snow, mud, and ice also have lower coefficients of friction.
But what if you can create your own asperities in the mud and snow and
grip the tiny asperities in the ice?  That's exactly what the lateral
grooves and the tiny cuts do to the snow/mud and the ice respectively.
Effectively, the coefficient of friction is increased; thus, more
"traction."  Also, what helps in the snow is a skinny tire.  The smaller
contact area does two things:  first, it cuts through snow to get to
the road; second, it puts more force per unit area on the snow it is
riding on, therefore compacting it tighter and creating stiffer asperities
with its lateral grooves.  Wide tires help in the mud and sand.  The
reason is opposite the logic behind skinny tires for the snow:  you
don't want to cut into the mud/sand and get bogged down.  Therefore,
you can clearly see that width does matter to traction, but not exactly
as described by the "rut/groove" explanation above.

Rolling friction is that force that best describes the type of friction
we are talking about.  This is the real meat of tire design today.
Coulomb defined rolling friction a long time ago and the SAE still uses
his original equation.  Basically, you can reduce/increase rolling friction
a few ways.  Tire compound matters.  Width matters.  Pressure matters.
Aspect ratio matters.  Speed matters.  Some of these relationships are
intuitive, like the tire compound factor.  A soft rubber will intuitively
have a higher coefficient of friction with the road surface since it can
deform around the asperities in the road surface more easily, making it
more difficult to "scrub" the surface.  The pressure factor is somewhat
intuitive since higher tire pressures decrease rolling friction.  What
this does is make the tire more "solid," effectively more rigid, less
pliable to the road surface.  A tire with increased pressure will
effectively "roll over" asperities more easily, in other words.  Decreasing
the pressure allows the tire to form around the asperities.  What is
counterintuitive is the width and the aspect ratio factors.  Matter of
factly, increased width and lower aspect ratios *decrease* rolling friction.
I wish I could fully explain why, but the truth is, very few people
actually could.  Finally, the faster you go, the less rolling friction
you'll encounter.

So.  What are the conclusions here?

 1.)  Increasing tire width, keeping all other factors constant,
      will not increase traction on a dry road.  It will reduce
      rolling friction, possibly increasing mileage (although
      doubtfully significant).

 2.)  "Stickier" rubber compound most effectively increases traction on
      a dry road.  Additional thrust may be applied to the road asperities
      in the direction opposite the direction of travel, however,
      rolling friction is increased since the asperities in front
      of the tire are more difficult to get over.

 3.)  Wider tires will reduce traction in the snow and rain.

Okay. So now what about Indy tires and the like?

 1.)  Racing tires have rubber compounds that have increased
      coefficients of friction (they're "stickier").  This gives them
      more traction as described in 2.) above.

 2.)  Racing tires are wide to increase stability and handling
      performance as well as reduce rolling friction.

      * Remember, tires don't have forces applied to them only
        in the direction of travel.  Racing involves a lot of
        forceful maneuvers that dictate wider tires for stability
        and control.  This not entirely removed from discussions
        about friction, but it is definently another stroy.

 3.)  Dragsters have so little pressure in their tires that they
      literally balloon when spun hard.  This gives them more
      traction by way of pavement pounding.  :^)

 4.)  Wider tires distribute the thrust loads more, reducing the
      wear on the compound itself.  For instance, given a soft rubber
      compound on skinny tire (A) and wide tire (B).  I install
      each set of tires on my two identical Lola-Fords.  I floor
      it.  Car (A) melts and shreds the tires in an instant.
      I give them a treadwear rating of zero.  Car (B), however,
      grips, and I am off to a day at the races.  

 5.)  Wider tires give you the opportunity to use "stickier" rubber.
      (See 4.))

Finally, how does this apply to the real world?

Tire companies are producing "low rolling resistance" tires these days.
They have special rubber compounds and require higher internal pressures.
Above about 40mph, these tires can really save gas, that's why they are
generlly OEM equipment on economy and family cars.  Every 0.5mpg is
worth millions of dollars by CAFE standards.

Well, I hope this helps.  Again, I welcome corrections and/or additions
because this is sort of a technical hobby for me.  I know it was long,
but I get tired of seeing them same subject come up and inadequate,
incomplete, or incorrect answers given.

Have a good one.

Regards,

Brian

bqueiser@magnus.acs.ohio-state.edu
------------------------------------------------------------------------
I am the engineer, I can choose K.
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Department of Engineering Mechanics
Ohio State University
Columbus, OH