IMPORTANT - Thinking of Fitting Wider Tyres?

[Anthony]

I have shamelessly copied my thread from the Porsche forum
(I didn't want to have to write this out twice!)


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Just recently I've noticed quite a few questions being asked about tyres, and whether wider tyres will give improved handling

Having searched the forum there doesn't seem to be much information on tyres, and the information that is on here is a little misleading at best

Since tyres are an important, and sometimes overlooked subject, I thought I'd set the record straight, and maybe get a few people thinking along the way

Well, I guess that's enough of an intro.....now, let's get onto the good stuff.........


Contact Patch

It seems many people think by fitting wider tyres their cars handling will improve as their car now has a larger 'footprint'
Well, contrary to this popular belief, a wider tyre does not give you a larger footprint

Dont believe me? Well then, let's have a look shall we?

The contact area (or footprint) of a tyre is calculated by dividing the wheel's load by its air pressure
If the load is in 'Lbs', and the tyre pressure in 'Psi', the area will be 'inches squared'


For Example

A stock 996 Turbo weighs about 3,480lbs

If we assume the car has a 60/40 Rear/Front weight distribution, there would be a load of 1,044lbs on each rear wheel
(3,480lbs x 0.6 / 2 = 1,044lbs)

If the tyre pressure was 30psi, we would get a contact patch of 34.8 square inches (1044lb/30psi)
Remember, the contact area is calculated by dividing the wheels load by its air pressure

If the tyre pressure was 40 psi, the contact area would be 26.1 square inches, and if the pressure was 20 psi, the contact area would be 52.2 square inches

Notice how the contact patch decreases as the tyres air pressure increases
This is due to the tyre 'sitting up' as it becomes over inflated. If you've ever seen tread wear on an over inflated tyre you'll know what I mean

As the tyre pressure is decreased, the contact area increases
Note, this isn't strictly true, as after a certain point the tyre will deform from being under inflated


So then, now we know what dictates a tyres contact patch (the load on the wheel and the tyre pressure) what happens if we fit wider tyres?


Well, quite a few things will happen....but we won't get a larger contact area
The wider tyre will give us a wider contact patch, but, it will reduce in length, as the following picture, provided by Steve Jarvis demonstrates






So if you fit wider tyres and keep the air pressure the same, the contact patch will have the same area, but will be a different shape
It will be wider, but shorter


Well then, if this is true, why does the 996 Turbo come width wide tyres to begin with? Why not just fit cheap narrow tyres if they've got the same size footprint? And, if they've all got the same footprint, does that mean they've all got the same levels of grip too?


Let's investigate........


Slip Angle

During cornering, tyres create a sideways force - a 'Slip Angle' - as can be seen in the following video

http://www.youtube.com/watch?v=W8UiE7yvO_M


'Slip Angle' is where the tyre gets pulled sideways by the road surface. The wheel is pointing in one direction, but the tyre is actually pointing in a slightly different direction. There's a twisting motion due to the friction between the tyre and the road surface.
The difference between these two directions is the Slip Angle

At the front of the contact patch the angle is quite small, but as you move along the contact patch the angle increases

Eventually, the sideways force becomes greater than the tyres grip on the road, the car begins to drift and you now have a 'Slide Angle'

The longer the contact patch is, the more gradually the car will begin to drift .......it's going, going, going, going, still going, about to go, gone
But, when the contact area is shortened, by fitting wider wheels, the drift happens much quicker.......going, gone!!

A wider tyre will give a higher overall level of grip when compared to a narrow tyre. This is because there isn't as much difference between the front and back of the contact patch. And the reason there isn't as much difference is because the contact area isn't as long


narrow tyre = longer contact area, less lateral grip, slower break away, gradual tyre deformation
wider tyre = shorter contact area, more lateral grip, faster break away, quick tyre deformation



The other vital thing we need to know?..........


Heat

Think of it like this
Right now your car is sitting perfectly still. Part of your tyre is in contact with the ground and is therefore flat
The area of that patch is, length multiplied by width
A narrower tyre has a longer contact patch, and that's a problem

In order to get the narrow tyres longer contact patch onto the ground its sidewall is going to have to deform quite a bit. When the car is in motion the tyres sidewall is all the time flattening and curving again as the wheel rotates
This constant bending causes the tyre to heat up
Since the narrower tyre has a longer contact patch it needs to deform more than a shorter contact patch would
This greater amount of flexing results in more heat being created

Whilst we need heat in our tyres in order for them to operate at their full potential, too much heat is a bad thing


A narrow tyre will have a greater percentage of its tread in contact with the ground at any time when compared to a wider tyre
Remember, this is because the narrow tyre will have a longer contact patch

Since more of the tyre is in contact with the ground at any one time, it has less time to cool before it comes into contact with the ground and it's tread and sidewall deform again

In order to combat the additional heat that's being generated, tyre manufacturers will make narrower tyres out of harder compounds
These harder compounds have a lower coefficient of friction, and therefore have less grip, especially when cold


Summary

A wider tyre will have a lower slip angle, a softer compound, a higher friction coefficient, will bend and deform less, and will have more time per revolution to cool itself

This is why a wider tyre will, in most cases, result in better performance

[ferny]

There's a lot of info there but a few things make me go ", hmmm... [il]really??[/i]" and result in more questions. I'm curious as to what his background with regards to tyres are and the experienced knowledge to conjecture ration.

[aar0sc]

I suspect that my switch from 155/80/R13 to 165/80/R13 won't make a massive difference though!

[Anthony]

490 wrote:

except of course in snow.  ;D

.......and on triumphs  8)

[Nick Jones]

That is a nice write up.

One thing not addressed though is that it is assumed through-out that the tyre is being presented to the road more or less perpendicular.

On your normal, small chassis triumph, with errrm, dismal, camber control, this is definitely not the case, so in order for the tyre to maintain a full contact patch, the sidewalls must flex.

Obviously this is less of a problem for a tyre with a long, narrow contact patch and tall squashy sidewalls than it is for a wide, low profile tyre with a short, wide contact patch short stiff sidewalls.

Nick