Tire size and rolling resitance.
Well I finally descided to bite the bullet and buy a pair of Schwalbe Marathons. I have no local dealers so I looked at the website and phoned their office for some advice. I'm a big guy (about 260) and I wanted something to commute and tour with. I have been going smaller and smaller with my tires and I'm now down to a small pair of 1.50 (26 inch wheels) The guy from Schwalbe insisted that larger tires where actually faster (see their website) and he thought that 1.9 would be faster then I'm riding now. I still can't get my head around the rational that large tires have less rolling resistance then narrower tires. But I ordered a pair of 1.9, (they have a high pressure rating 75 psi). we will see. Anyone have any thoughts on this?
Older I get, Better I was
This one has been beat to death for years but here goes. If you take a 1.25 width tire and a 1.95 width tire pump them both up to 60 psi,they will have the same amount of contact area on the road, what changes is the shape of the contact patch. A narrow tire will have a oblong contact area on the road while the wider tire will have a more circular contact patch. The theory is that a circular contact patch will produce less friction. The theory is probaly based on comparing a bike tire to an automobile tire. A narrow tire is similar in contact patch shape to a non radial tire, a wider tire is similar in contact patch shape to a radial tire, since automobile radial tires have less rolling risistance than non radial automobile tire it is assumed a bicycle tire will perform similar.
In practice I have never been able to tell much difference in rolling resistance. My personal preference is using 26x1.5-1.75(37-45mm) on my touring bike because it gives a more supple ride, soaks up pot holes and handles well on gravel roads . On the tandem I use a 26x1.95(50mm) because it improves the comfort for the stoker and most importantly isn't affected by heat from the brakes on long downhills. On my road bikes I use a narrower tire(700x30)because it has a lower profile and less sidewall flex making the bike more responsive to steering input.
I'm made of earth!
More tire contact with the rode, more friction, more resistance, better control.
Forget the rolling resistance thing. The marathons are great tyres and have reasonably low rolling resistance combined with comfort which is the thing you should really be concerned with while touring. The larger air chamber gives more cushioning than narrower tyres while the contact area, as has been said, is only a fraction of the tyres width.
I've just ordered 4 Schwalbe Marathons: three of the 26 x 1.9 and a 1.75 Slick. And I've just finished some rolling resistance tests using my old tyres because I want to compare them to the Marathons when they arrive. I choose a day when there is no wind, ride part way up a small hill to a fixed starting point, then roll down hill about 200 metres till I stop. I do three runs to make sure the end point is repeatable. It turns out that it is, typically to within 3 metres.
When I lowered the air pressure from 50 psi to 30 psi, the distance I rolled decreased by 25m. ie about 12%. So I assume rolling resistance went up by 12% with the lower pressure.
My next series of test will be with the Marathons, and if I get keen I'll compare 1.75s with 1.9s at the same pressure but I don't think I'll see any difference.
If it is true that at the same pressure a wider tyre rolls easier than a narrow one, then to obtain the same rolling resistance with a narrow tyre means the pressure must be increased. So there comes into play another reason why a wide tyre will have better rolling resistance - and at the same time be more comfortable - the lower pressure in the wider tyres absorbs road shocks which would otherwise cause the rider to bounce up and down. Every time a rider is pushed up because of a bump, the bike slows down. The energy to push the rider up has to come from somewhere, and that somewhere is the forward speed. The effect is the same as when going over a speed bump: your speed slows as you approach the crest, then increases as you go down the other side; but you never fully make up the speed you lost. A wider tyre causes the rider to move up and down less (the tyre acts as a shock absorber and does the moving instead) so less forward energy is lost.
So, if the theory is correct, you can run a wider tyre at a lower pressure to have the same rolling resistance. You'll also get better suspension which of itself improves the rolling resistance even further.
But I suspect the difference between 1.75s and 1.9s would not be noticed in practice, in part because 1.9s are slightly heavier (by 90 grams), needing more energy to accelerate and to lift over hills.
Originally Posted by Guy Burns
Are you serious or just unemployed?
My understanding is indeed, a fatter tyre has a lower RR, all else being equal (esp. pressure).
For the same pressure (and therefore same contact patch area), the material of a skinny tyre has to distort more when it rolls into the contact patch than that of a fat tyre. This material flexing is the cause of rolling resistance.
But often all else can't be kept equal. Fatter tyres can't take higher pressures without a thicker carcass, which increases rolling resistance. Usually high pressure wins.