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· Royal Floor Sweeper
4,338 Posts
Discussion Starter · #1 ·
I was talking to Eviljack and the subject of wider tires came up.
From my over a decade dealing with small sports cars, I have learned that TIRE design/composition is what determines grip, not really width, unless you have a high power engine.

I thought it would be helpful to post some support for that idea:
Mike Guillory wrote:
> > From: Dave Aley <[email protected]>
> >
> > Perhaps Mike Guillory could pipe in here and enlighten me as to how the
> > contact patch REALLY works. Am I completely off-base in my assertion
> > that a wider tire has a larger contact patch, Mike?
> *** I will, only reluctantly. I'm not an engineer - closer to a
> physicist/chemist/mathematician (retired)! If you *neglect* other forces,
> like tire wall rigidity, for a given weight rider+bike at a given tire
> pressure, upright and still, the contact patches (square inches) of
> different width tires *must* be the same. Physics gives it no other
> choice. The wider tire may have a wider contact patch but, if so, it will
> be shorter from front to back. However, there are other supporting forces,
> like sidewall rigidity, but for a 350 pound force (per wheel) I would think
> that to be pretty small.

I think there's an error in your assumptions here, Mike.

Strictly speaking, it's not the pressure inside the tire that supports
the bike. This pressure is completely constrined by the tire, so that
even when your tires are not in contact with the ground they contain 35
psi (or whatever you've inflated them to).

Furthermore, consider the area of the contact patch. Andrew said he got
a contact patch area of 2.4" for each tire; let's assume (lacking better
data) that the contact patch is round. (This assumption follows
logically from the assumption that the tire width does not have any
effect on the contact patch width; if that's the case, then the 11" or
so radius of the tire does not give you more length than the 7" or so
radius of the tire profile). The area of the contact patch is then 4.5
square inches. Now, Andrew on his bike probably puts a load of around
300 lbs on each tire; 300 lbs/4.5 sq. in = 67 psi inflation pressure to
get this width of contact patch. Since that's obviously more pressure
than Andrew is running in his tires, we have to throw out the assumption
that hydraulic theory can explain the width of the contact patch.

When you put the tire on the road and put 300 lbs of motorcycle and
rider on it, the pressure inside of each tire will rise somewhat.
However, it's not a dramatic rise, considering the small change in
volume. (Note that I'm talking cold pressures here, not the pressure
increase you get when you get the tire hot.) Therefore, we can assume
that it's neither the pressure (35 psi) nor the pressure rise (I'd
expect no more than a couple of PSI) that supports the weight of the
motorcycle. Rather, it's the rigidity of the tire that is imparted by
the fact that it's pressurized that supports the bike. This rigidity is
a function of the tire's elastic properties, construction, and probably
some other variables.

Now, this does not prove that a wide tire has a wider contact patch than
a narrow one. Nor does it prove the converse. However, I think it's an
important distinction to make.

> So, why do racers use larger (wider) tires? More total surface to absorb
> wear? Under hard acceleration, is the wider contact patch more important
> to give grip and prevent rear wheel break-away? Does a wider profile cause
> less of an upset of suspension geometry at high lean angles? Perhaps all
> of these, and more. But, for us mere mortal riders, I think we should,
> correctly, assume that the size of our tire's contact patch, under most
> conditions, is independent on the size or width of the tire. Any small
> differences will be greatly overshadowed by differences in air pressure we
> may choose. Go from 36psig to 24psig and increase your effective contact
> patch by about 50%!

So if I go from 36 psig to 1 psig I'll have a contact patch area! ;-)

One possible reason that racers use wider tires is heat. The heat is
generated by deformation of the tire; even assuming an equal contact
patch width, a narrow tire has to deform more than a wide tire. The
wide tire also has more mass to heat, and more surface area to dissipate
heat. This would help keep the wider tire cooler, allowing it to be run
at lower pressure and therefore increasing the width of the contact
patch. :)

Dave Aley | mailto:[email protected] DoD#454
Maintenance Mechanical Engineer | ASME #6363402 TLCA #8419
Lee Ranch Coal Company |
Grants, New Mexico, USA | '87 VFR700FII '71 FJ40
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· Royal Floor Sweeper
4,338 Posts
Discussion Starter · #2 ·

Handling 102

Cornering Speed
1) Tire's Grip

Most obviously, the selection of tires is decisive to cornering grip. Car engineers have nothing to do with the friction of the tires, which is determined by the compound and texture. However, they can choose the most suitable tires for their cars.

In the past decade, increasing tire's diameter and width is a common trend shared by all car makers. Do you still remember the Lamborghini Countach employed 15-inch tires ? Today's most exotic Ferrari, Porsche and Viper have 18 to 19-inch rubbers! Larger diameter accompany with larger width increase the contact patch area (that is, the area of the tire contacts with the ground), thus result in more grip. However, this also result in poorer wet road grip because the pressure acting upon the contact patch (that is, the car's weight divided by contact patch area) is reduced thus the tire becomes easier to "float" on the water. Therefore the texture also need to be improved for better water clearance.

Low profile tires are also fashionable in these days. Since the thickness becomes thinner, it is more resistant to side wall deflection under substantial cornering force. However, this is not much related to grip.

It must be mentioned that wide tires are not always good. Especially are front tires, the wider they are, the more resistance generates when they are steered. This create a heavy and insensitive steering feel, also more tire roar and wear. If you want to modify your car by using wider tires, always consider the drawback first. In my opinion, most well-sorted production sports cars have already equipped with the most suitable tires.

(There are other interesting stuff in that link, inc. suspension design, Weight Transfer due to lateral force, Weight Transfer due to body roll (why reducing body roll alters handling), and more in related pages.)

· Royal Floor Sweeper
4,338 Posts
Discussion Starter · #3 ·
I'm not finding the on-line link I had in mind.
Though, there is a great article in the Miata Performance book that gives a VERY good and interesting explanation of the science of why it doesn't do much except change the tire patch shape.

· Registered
2,209 Posts
Ok so wider may not mean more traction. It definatly means more stability though. That I can vouch for with real life experiance.
Also I am a little more curious about this tire inflation thing. I never really though about it before but does the weight of the car actually change tire pressure? Does it make a difference as to if you fill your tires when the they are off the ground or when they are planted on the ground?

· Registered
7 Posts
Hi All, I'm new to the forum but when I saw this thread I felt I had something to share.
Having built and raced both motorcycles and cars, I know a little bit about tires.

The problem with the argument made is it doesn't apply to a car. Motorcycle tire have either a round or "V" shaped profile while our cars obviously have a flat profile. On a motorcycle, a wider tire gives you a larger contact patch when you're leaned over. The idea being that when you're upright, you want the least amount of contact patch because 1 - more tire on the ground means more friction and thus drag, and 2 - it's easier to lean over at speed when you roll off of a small contact patch.

Now in drag racing, to increase traction you not only go wider but taller to increase the contact patch. A larger diameter accompanied with a soft sidewall and lower tire pressure will put more rubber on the ground. The problem with this for the street is you have no sidewall stability for turning. The rim turns but the tire wants to keep going straight. Can you say understeer? This is the reason for short stiff sidewalls for road racing.

So, assuming that you have a good tire, wider does mean more grip.

A little handling tip - you can get better handling by adding 2-4 lbs of pressure to your tires. This stiffens the sidewall which helps keep the tire from rolling over in a turn, and also helps to keep the contact patch from distorting. Too much pressure will do the exact opposite.

I've learned quite a bit about my xB on the site and I hope I've made a positive contribution to the thread.

· Registered
294 Posts
Someone actually kinda gave me alittle greif on the subject of over inflation. I read the tires and they are flat on the front all the way across so over inflation is not always a bad deal. I do not run them anymore than what is on the sidewall tho.Also I am running 2-4 more psi in the fronts than the rears.
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