Tire Size vs. Contact Patch

[lemansnsx]

Yokohama Tire Co. "'Inch Up' is the process of mounting a lower aspect ratio tire and larger diameter wheel on your car. This creates a larger contact patch and a shorter
sidewall."

AutoSpeed "Myth 1: Wider tyres have a larger contact patch than narrow tyres -
What actually influences the size of the tyre's contact patch? Is it the width of the tyre, or the profile? The simple answer that it is neither of these; the size of the tyre's contact patch is related to: the weight on the wheel, the tyre pressure. For example, say that the weight on the tyre was 900lb, and the tyre pressure was 10 psi. That internal pressure means that each square inch of area can support 10lb, so, in this case, the contact patch will be 90 square inches. If the tyre pressure was 30 psi, the contact area would be 30 square inches, and if the pressure was 90 psi, the contact area would be 10 square inches. This has been found to be almost exactly correct for most tyres (the exceptions being so-called run-flat tyres, or
tyres with extremely stiff sidewalls). For most other tyres, carcass structure will have an effect, but by far the major factor is tyre pressure. So, as you can see, the size of the contact patch of a tyre is not related to the width of the tyre - it is, in fact, proportional to the tyre pressure. What will change with the fitting of a wider tyre is the shape of the contact patch - it will get wider, but shorter longways. Myth 2: A larger contact patch = more grip Okay, most people will come to the conclusion that if you have "more rubber on the road" you will have increased grip. Sorry to say this folks, but to very close to 100% accuracy, the size of the contact patch is irrelevant. The actual grip that a tyre can generate is dictated by the coefficient of friction of the rubber compound used in the tyre. The higher the coefficient, the more grip which can be generated. The relation that is used is called Armonton's Law, and
the equation is: F=µN, where F is the force generated, µ is the coefficient of friction, and N is the weight on the surface considered (in our case, the weight on the tyre).
So, if you increase the weight on the tyre, then the frictional force will increase as
well, in proportion to the increase in weight on the tyre - but the coefficient of
friction will remain the same. The level of grip of the tyre (forgetting about
suspension niceties - we are only discussing tyres here) is totally dictated by the
coefficient of grip of the tyre and the weight acting on it - not the area of the
contact between the tyre and the road."

So, do the tire manufacturers perpetuate the myth that larger tires = better handling?

------------------
Andrew Henderson
The NSX Model List Page
"We have long acknowledged that enthusiasm for things automotive is a sure
sign of emotional instability if not outright dementia" - Brock Yates

 

[Hrant]

Both are correct because if you read the statements thay are discussing different issues.

Yokohama DOES NOT discuss that this improves handling or grip. their statement on face value is correct because it assumes all other things being equal, and perhaps using the same tire brand but a lower profile size, yes you will indeed get more contact.

As for autospeed, well assuming they have a speelcheck (tyre vs tire), their statements are correct also but only to the extent that the lower profile tires are of a different brand or compound, and the buyer/installer is a moron when it comes to tire pressure etc .....

Now, if only the AOH22 came in 225/45/16 and 255/40/17 .......

 

[lemansnsx]

Originally posted by Hrant:
Both are correct because if you read the statements thay are discussing different issues.

Yokohama DOES NOT discuss that this improves handling or grip. their statement on face value is correct because it assumes all other things being equal, and perhaps using the same tire brand but a lower profile size, yes you will indeed get more contact.

As for autospeed, well assuming they have a speelcheck (tyre vs tire), their statements are correct also but only to the extent that the lower profile tires are of a different brand or compound, and the buyer/installer is a moron when it comes to tire pressure etc .....

Now, if only the AOH22 came in 225/45/16 and 255/40/17 .......

I guess I should have posted this in the thread where the discussion started...
My post was intended to continue a discussion in another thread that had drifted way off topic. I was trying to get some more discussion going in response to an oft-repeated assertion that you can't increase the contact patch no matter what size tire you employ.
Also, I took Yokohama's stuff slightly out of context - but they certainly are asserting that wider lower tires improve handling.
Finally, no spell checker is required for AutoSpeed's use of "tyre". This is the normal spelling in England and Australia and maybe other countries. I apologize for not mentioning that "AutoSpeed" is an Australian publication in my original post but I really didn't think that it was germaine.


------------------
Andrew Henderson
The NSX Model List Page
"We have long acknowledged that enthusiasm for things automotive is a sure
sign of emotional instability if not outright dementia" - Brock Yates

 

[sjs]

Lot's of partial, even mostly, truths here, but as usual it's difficult to cover all the if's, and's and but's without exhaustive detail. Which is why the one made a point of limiting his comments to the tire patch.

None of those "facts" should be interpreted to mean that there is nothing to be gained (or potentially lost) by changing tire sizes. Even given the same overall diameter, the same model tire of a larger base size and shorter sidewalls will handle differently. Likewise, a wider but shorter contact patch makes a difference, partly by virtue of it's shape and partly in how it acts on the sidewall.

If you doubt that it makes a difference, try imagining an extremely narrow tire and how it would act under load.

 

[Hrant]

lemansnsx,no need for any apologies, I understand the off topic ..... I was just noting that both assertions can be correct given the premise of their statement and as sjs notes, there are many ifs .....and if the tire is of equal quality, rubber compound, a lower profile will offer better grip but not necessarily better handling ..... this is endless ....

As for England and Australia, I too was raised on the British spelling (colour instaed of color, etc ...) but never came across tyre but then my spelling was not that great either