Rider weight vs tire pressure vs tire width



threaded

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Jul 6, 2006
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Now as I understand it: A high pressure tire has lower rolling resistance. If you weight 200 lbs, and have a tire pressure at 100 lbs/inch2 then there is 2 inch2 rubber on the road, and have a tire at 200 lbs/inch2 then there is 1 inch2 on the road, and hence less friction, but a harsher ride. To make the ride softer you lower the tire pressure, but increase rolling resistance.

Yet a thin tire will present an oval shap on the road and a fat tire a more rounded one, to a very fat tire giving an oval in the sideways direction.

I would suspect a round patch on the road would produce the least friction and hence the best rolling resistance. Is this correct?

So, for a particular rider weight and tire pressure there must be an optimum tire width.

And further to this, considering weight distribution of front and back, differing sizes would make sense. I suspect a fatter tire at the front as I find this does tend to give me a better ride.

Anyone know of a formula for this?
 
threaded said:
Now as I understand it: A high pressure tire has lower rolling resistance. If you weight 200 lbs, and have a tire pressure at 100 lbs/inch2 then there is 2 inch2 rubber on the road, and have a tire at 200 lbs/inch2 then there is 1 inch2 on the road, and hence less friction, but a harsher ride. To make the ride softer you lower the tire pressure, but increase rolling resistance.

Yet a thin tire will present an oval shap on the road and a fat tire a more rounded one, to a very fat tire giving an oval in the sideways direction.

I would suspect a round patch on the road would produce the least friction and hence the best rolling resistance. Is this correct?

So, for a particular rider weight and tire pressure there must be an optimum tire width.

And further to this, considering weight distribution of front and back, differing sizes would make sense. I suspect a fatter tire at the front as I find this does tend to give me a better ride.

Anyone know of a formula for this?
Wow. I find if you experiment with a few combinations you can find the one that suits you best. Tyre composition also contributes to ride quality as one with alot of carbon (pro2race) will have a softer ride than one with alot of silica or whatever makes conti gp 4 seasons ride like wood. I'd run 10 psi less in the front if you are between 140-150 lbs. It works for me.
 
Hmm, so how much a tire deforms in each direction should be taken into account too. I would imagine this to be a function of tire shape, but if the tires are multi compound, or have varying thickness of rubber from wall to wall that'll make it a more complex calculation, but not impossible.
 
threaded said:
I would suspect a round patch on the road would produce the least friction and hence the best rolling resistance. Is this correct?
I'm no tire engineer, but if I had to speculate, I'd say that what we call rolling resistance comes from two sources - the force required to flex the sidewalls of the tire as the contact patch rolls through the contact zone, and the static friction of the contact patch as it is peeled away from the pavement. My hunch is that the sidewall flex is the greater power sink of the two. I think that is why higher pressures generally have lower rolling resistance, since the sidewall flexes less.

Regarding the shape of the contact patch, a longer narrower shape would present a narrower cross section to the road that must be peeled away at any instant in time, i.e. the longer dimension is parallel to the direction of travel. So I could see where higher pressure would again correlate with lower friction, since it lengthens & narrows the contact patch.
 
threaded said:
Now as I understand it: A high pressure tire has lower rolling resistance. If you weight 200 lbs, and have a tire pressure at 100 lbs/inch2 then there is 2 inch2 rubber on the road, and have a tire at 200 lbs/inch2 then there is 1 inch2 on the road, and hence less friction, but a harsher ride. To make the ride softer you lower the tire pressure, but increase rolling resistance.

Yet a thin tire will present an oval shap on the road and a fat tire a more rounded one, to a very fat tire giving an oval in the sideways direction.

I would suspect a round patch on the road would produce the least friction and hence the best rolling resistance. Is this correct?

So, for a particular rider weight and tire pressure there must be an optimum tire width.

And further to this, considering weight distribution of front and back, differing sizes would make sense. I suspect a fatter tire at the front as I find this does tend to give me a better ride.

Anyone know of a formula for this?
I don't know of a formula that can handle all the variables.
I think some of the main criteria are presented in the article by Terry Morse:
http://draco.acs.uci.edu/rbfaq/FAQ/8b.14.html
Seeking lower rolling resistance is ok as long as you consider other issues such as surfaces you ride, wear, cost, potential for a pinch flat, ride quality, etc.
 
threaded said:
Now as I understand it: A high pressure tire has lower rolling resistance. If you weight 200 lbs, and have a tire pressure at 100 lbs/inch2 then there is 2 inch2 rubber on the road, and have a tire at 200 lbs/inch2 then there is 1 inch2 on the road, and hence less friction, but a harsher ride. To make the ride softer you lower the tire pressure, but increase rolling resistance.

Yet a thin tire will present an oval shap on the road and a fat tire a more rounded one, to a very fat tire giving an oval in the sideways direction.

I would suspect a round patch on the road would produce the least friction and hence the best rolling resistance. Is this correct?

So, for a particular rider weight and tire pressure there must be an optimum tire width.

And further to this, considering weight distribution of front and back, differing sizes would make sense. I suspect a fatter tire at the front as I find this does tend to give me a better ride.

Anyone know of a formula for this?
I don't have the Michelin graphs anymore but I was surprised how added air pressure, once you got to a Psi well below the maximum, produced very little added reduction in rolling resistance. What it did produce, was a substantial reduction in cornering traction and a greater occurrence of pinch flats.