Over on the Fiat Forum website we are having a bit of a debate as to the effect of tyre width on rolling resistance.
In particular, a contributor is unsure as to whether he should fit 14in wheels shod with 165s or 185s. This is to a 1.2 petrol Panda.
I feel that the difference in rolling resistance between the 2 widths of tyre is minimal and that, for aesthetic reasons, the 185s would be better.
I've Googled for some decent research in this area but have only found a study on bicycle tyres where tyre width seems irrelevant to rolling resistance.
I would value the opinion of some of the true engineers and scientists on this forum.
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>>I would value the opinion of some of the true engineers and scientists on this forum.
I should by rights keep my neb out then. Never mind.
Reducto ad adsurdum, imagine a very very wide or a very very narrow tyre and it seems likely that a wider tyre will have more rolling resistance, but in relative terms a 185 is not much diffferent to a 165. The frontal area presented, and hence drag, will probably be as much of a factor, in favour of the narrower tyre.
The wider tyre will also create a higher unsprung weight, and perhaps load the steering components a bit more.
The 165 is not really a narrow tyre, and my instinct is that other things being equal (tyre quality and construction) it would probably be the better choice as well as being slightly cheaper; but there's probably not enough in it to worry about if you think the appearance matters and you prefer the wider one.
E&OE - I haven't done much engineering for quite a while, I didn't really have the Latin for it ;-)
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>>I didn't really have the Latin for it
>>other things being equal
Yes, you missed a good opportunity for a "ceteris paribus" there!
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I've Googled for some decent research in this area but have only found a study on bicycle tyres where tyre width seems irrelevant to rolling resistance.
That bit doesn't surprise me. I suspect it's because bike (and motorbike) tyres have a curved tread surface to allow the bike to lean. This means at any given time, most of the tyre's tread width is actually not coming into contact with the road at all. Contrast this with a car tyre which, in straight and level use at least, has its entire tread width in contact with the tarmac, and you can see how width differences have a bigger effect.
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Rolling resistance is dependant on many things other than width - inflation pressure, tread design, tread compound, temperature, tyre structure etc.
There's a nice PP thingy here: tinyurl.com/apcejc
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Of course, if they fit energy/green type tyres, the 5% odd less friction should offset the increased friction/drag from the 185 width over the 165.
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On a rough term, the tyre material creates a kind of bond with road surface. So, wider tyre usually means more grip.
Few months back I asked this question in this forum.
www.honestjohn.co.uk/forum/post/index.htm?t=70144
Edited by movilogo on 27/02/2009 at 12:04
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Not engineer, merely 'A' level physics [in days when 'A' levels actually......p'raps best not go there].
Small width differences matter very little as there are so many other factors, the main one being the area of the tyre's footprint on the road. So tread density and 'squidginess' , which is influenced by both the hardness of the compound and the tyre pressure, will be more important. Too complex to calculate!
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Just found a rather scary statistic from Michelin - one tank of fuel in every five is used just to overcome the rolling resistance of the tyres.
tinyurl.com/coaj5m
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The contact area of the tyre would remain the same, but its shape would be different. Given that the pressure in the tyre is the same and the vehicle weight unchanged.
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Thankyou to all for your opinions.
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