Myths about tyres.....

[Super7]

Myth 1:- Wider tyres have a larger contact patch than narrow tyres??

Myth 2:- A larger contact patch = more grip??

 

Does tyre width has direct relation to the amount of grip generated? or is it just a secondary factor?

 

How does the cooling potential in tyres relate to their width/aspect ration and how it affects their grip?

[Genie47]

Wide tires have more or less the same contact patch as narrow tires. Check Chris Longhurst's Tire and Wheel Bible. The main advantage of low profile wide tires have is the resistance to deformation in those hard corners.

[Yttrium]

Strictly speaking... a larger contact patch or surface area does not equate to more grip or friction since friction is only a function of the downward force on the tire (weight) and the coefficient of friction. In practice, I'm not so sure since there may be other factors to consider like thread pattern etc.. but then again this should be taken care of by the coefficient of friction.

Edited June 28, 2004 by Yttrium

[Ghostrider]

Pardon the no brainer...

 

I tot that bigger patch of rubber means more contact over a smaller patch of contact (narrow tyre).

 

This one no need rocket scientist rite ? Use naked eye and common sense nia leh.

 

Myth 2 : Debatable.

 

If a 185 footprint is using happening tyres (say AD07, SO3e etc) vs 205 footprint using comfy rubber (Yoko dB)....I am very sure that the former (smaller contact patch) will have more grip.

 

Another point to note is the profile of the sidewall. Thinner side wall equals to stiffer sidewall and lesser flex on the tyre...improvement in grip. Higher sidewall tend to flex at hard cornering.

 

All things begin equal, same compound of tyre used.....the bigger patch of tyre will have more grip in the dry...but in the wet, a bigger patch of rubber encourages aquapanning.

 

Cooling should not be a issue if you are refering to road use. If you are a hard driver who does jack rabbit starts and stops....your tyre will heat up pretty fast...but will still hold. If that is your concern, look out for the heat range that your tyres will perform in. Using the wrong heat range, you wil see your tyre melting.

[Super7]

I read somewhere that what actually influences the size of the tyre's contact patch is neither the width nor the profile of the tyre, it is the weight on the wheel and the tyre pressure that influence the patch size....... (all other things being equal)

 

like this:-

 

say for example the weight on the tyre is 900lb, and the tyre pressure is 10 psi, ie that the internal pressure means that each sq inch of the area can support 10lb, so, in this case, the contact patch will be 90 sq inches.

 

if the tyre pressure is 30 psi, the contact area would be 30 sq inches, if 90 psi, contact area would be 10 sq inches.

 

Except for run-flat tyres, or tyres with extreme stiff sidewalls, this has been found to be almost exactly correct.

 

For most other tyres, carcass structure will have an effect, but by far the major factor is tyre pressure.

[Super7]

i am also not very sure, just read something recently:-

 

as mentioned in my above reply to Yttrium, the wider the tyre doesnt necessarily give wider or larger contact area.

 

when we use tyres with same thread pattern and sidewall stiffness to compare if larger contact patch = more grip, although most people will come to the conclusion that 'more rubber on the road' = increased grip, it seems actually that, to very close to 100% accuracy, the size of the contact patch is irrelevant to the amount of grip.....

 

the actual grip a tyre can give is dictated by the coefficient of friction of the rubber compound used in the tyre. the higher the coef, the more grip. there is this relation called Amonton's Law:-

 

frictional force (grip) = coef of friction X wieght on the surface (ie on the tyre)....

 

so, if we increase the weight on the tyre, the grip will increase also, the coef of friction is a constant.

 

for the same suspension setup, the level of grip is therefore dictated by the coef of friction of the tyre and the weight acting on it -- not the area of the contact between the tyre and the road..

Edited June 28, 2004 by Super7

[Super7]

what about cooling effect of the tyre itself??

[Acemundo]

what are the determinants of coefficient of friction? threw my jc physics knowledge away for some time liao.

 

the equation is f=ur (u is the coeff and u is the reaction or weight as layman knows)

 

u is constant under a set of conditions which i cannot remember.......water, i assume would affect u. what about surface area or heat? would they affect u?

 

someone pls enlighten.

[Ghostrider]

Well well...learn something new everyday !!

 

Sounds logical to me....now have headache when buying rubber liao

[Tien]

heard from tyre shop guys... grip depends more on the stiffness of tyres. ST115 has good grip becos it has a softer surface. softer surface also means it will wear faster.

[Super7]

not sure about how heat affects the amount of grip, but based on my earlier posts, it does seem that area of contact has little to do with the amount of grip...

[Super7]

if what i read is true, ie tyre grip has little to do with the size of the contact patch rubber >> ie wider tyres do not necessarily give more grip (all other things being equal), then why bother with wide, low profile tyres at all?

 

Well, we must consider heat also.....

 

If we want to get a contact patch of a certain size on the road, we need a certain portion of the tyre to be flat. Taking the contact patch to be basically rectangular, then the area of that patch will be its width X length. For a narrow tyre, the contact patch will be quite long compared with a wide tyre to achieve the same amount of contact patch area.

 

This introduces 2 problems for the tyres:-

 

First, to get that long flat section to give the req'd contact patch, the sidewall of the tyre needs to deform quite a lot. This deformation actually causes the bending and unbending the rubber of the sidewall as, when the tyre rolls forward, it flattens and then the tread curves again.

 

Result:- a lot of heat is generated. The more it needs to deform (as in a narrow tyre compared with a wide one), the greater amount of heat will be generated.

 

The second relates to the length itself:- There will be a greater % of the tyre tread in contact with the road than if the contact patch length were shorter; this reduces the amount that the tread can cool. Also, there is a greater % of sidewall at any given time that is actually under bending stresses, again resulting in less opportunity to cool.

 

.......... to be continued next post...

[Super7]

continued........

 

So, how much extra bending do we really get, and how much is potential tread cooling reduced?

 

Let's look at a theoretical example ----

 

Using a 155-width tyre compared with a 225 tyre of the same circumference (extreme example):

 

asumming -- wheel/tyre unloaded circumference is 60cm, tyre pressure is 30 psi, weight on the wheel is 600 lb, giving an area of contact of 20 sq inch (or 129 sq cm), rectangular contact patch shape:

 

With the wider 225 tyre, the patch will be 5.73cm long, while the 155 tyre will have a 8.32cm long patch.

 

(this is getting abit technical/arimathical, sorry)

 

Now, the circumference of the wheel-tyre combination is 188cm, (dunno how the writer of the original article got this?), so the wider 225 is heating for 3% of its cycle, and cooling 97%, whereas the 155 is heating for 4.5% of the cycle and cooling for 95.5%.

 

So, we can see that the narrower tyre is generating heat 50% longer than the 225, and is not spending so much of its cycle cooling.

 

(let's take a break.... )

 

....... to be continued....

[King]

Uncle 7,

i only know that anything above 215,will aquaplane.

 

that why my posh friend tell me ,in rain,he drive slowly

[Genie47]

Yes. Thin tires are like a knife. Can cut right through water. So the aquaplaning is less. Fat tires are like boogie boards. Can surf on water .

[Yttrium]

That is correct. For a car of a certain weight sitting on different sized tyres, each tire is supporting the same weight. The weight each tire is supporting is equal to the tire pressure X contact patch area. If the tire pressure changes, the contact patch changes accordingly but the weight supported is still the same. And since grip/traction is determined by the normal force (weight) and the COF, the grip doesn't change at all. Therefore, contact patch area does not come into the equation at all.

 

If anything, a bigger contact patch has better heat dissipation (traction, cornering, braking forces distributed over a larger area) which helps maintain the tire compound properties during operation. And of coz.. last but not least.. but perhaps the most IMPORTANT... wider, lower profile tires look better mah!

Edited June 29, 2004 by Yttrium

[King]

according to some wheel agliment expert,

 

big tires and big rim doenst mean can corner well...

[Capricorn]

Hi all bros,

So am I right to conclude that a 14" rim with 185/60 Pirelli's tyre has better grip and handling then a 15" rim with 195/50 or 205/50 Dunlop's tyre. (Actually I am thinking of changing my 14" original Honda sport rim to a 15" or 16" taiwan made rims). It is advisable, taking into consideration of the price of 14" tyre to that of 15" or 16"?

 

Point to note: As you can see changing from a 14" rim to a 15" is nothing special, such that what 1" can do a different? So to 16" is more practicable rite.

Edited June 29, 2004 by Capricorn