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A few questions do you get less rolling resistances with more or less pressure? Just switched from 2.3 in front and 2.2 in back to a 2.1 in front and a 2.0 in back. My BIL said I can get the same rolling resistances with the fatter tires by lowering the pressure. That seemed opposite to what I would have thought. What say you?
 

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Fat-tired Roadie
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It depends on what your definition of "is" is.

On a trail, with a rough surface, lower pressure lets your tires conform to the surface better. Your bike won't move up and down as much, and you should experience some improvement. You'll also experience more hysteresis.

On really smooth tarmac, your bike wasn't going to get bounced up and down anyway. So you're experiencing more hysteresis, but without the improvement in efficiency.
 

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It depends on what your definition of "is" is.

On a trail, with a rough surface, lower pressure lets your tires conform to the surface better. Your bike won't move up and down as much, and you should experience some improvement. You'll also experience more hysteresis.

On really smooth tarmac, your bike wasn't going to get bounced up and down anyway. So you're experiencing more hysteresis, but without the improvement in efficiency.
On smooth dirt, as you lower pressure, hysteresis in the soil will increase faster than in the tires. So on softer surfaces, lower pressure should be faster, even if smooth.
 

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Fat-tired Roadie
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^^^
That makes sense to me. I wasn't going to touch the intermediate cases because that's where holy wars happen. :p But I've been doing pretty much minimum pressure myself for years. :D
 

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"Above all, a bicycle with narrow tires is much easier to accelerate because the rotating mass of the wheels is lower and the bicycle is much more agile"

I get by with narrow tires at lower pressure........low 20's
 

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On smooth dirt, as you lower pressure, hysteresis in the soil will increase faster than in the tires. So on softer surfaces, lower pressure should be faster, even if smooth.
Your theory is that less pressure means a larger contact patch, so your tire sinks into the dirt less? Correct?

I guess the overall benefit, would depend on how soft, and how smooth, the surface is, vs the energy loss from greater tire deformation.
 

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Sticky: http://forums.mtbr.com/wheels-tires/tire-pressure-all-around-xc-riding-561602.html It has many pageses of discussion that might be of interest.

Another Sticky under Wheels and Tires that might be of interest: http://forums.mtbr.com/wheels-tires/tire-test-results-german-bike-magazine-419392.html

I'm kind of agnostic over the whole thing. My recorded times are better, even in technical climbs, with smaller, lighter tires run tubeless at medium low pressures than with bigger heavier tires, but I much prefer riding with bigger, heavier tires for the comfort and increased fun over all, especially in the very uneven terrain I ride. I think part of the reduction in energy expended with lower pressure has to do with a more level "axle path". The axle is not going up and over every feature as much. Also, with the wider, shorter patch, the angle of attack at which the leading edge of the tire hits the feature is steeper, so less of a head on impact. I have a NN/RR combo I ride with on my 26er. When I run the 2.25 NN front/2.1 RR rear Vs the 2.4 NN front /2.25 RR rear, my times are consistently faster on long technical climbs. Both NNs are the same versions and both RRs are the same versions.
 

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Your theory is that less pressure means a larger contact patch, so your tire sinks into the dirt less? Correct?

I guess the overall benefit, would depend on how soft, and how smooth, the surface is, vs the energy loss from greater tire deformation.
More about the ability of the tire to conform and rebound (suppleness) to the surface rather than being deflected by it (bouncing).
 

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More about the ability of the tire to conform and rebound (suppleness) to the surface rather than being deflected by it (bouncing).
I get that, but I was responding to the post about less rolling resistance for lower pressure tires even on smooth soft dirt. If its smooth, there is nothing to bounce off of.
 

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Your theory is that less pressure means a larger contact patch, so your tire sinks into the dirt less? Correct?

I guess the overall benefit, would depend on how soft, and how smooth, the surface is, vs the energy loss from greater tire deformation.
Yeah, the pressure on the soil is equal to the pressure on the tire (Every force has an equal and opposite...). Tires bounce back, dirt doesn't. So if you're compressing dirt, you're losing all that energy, and getting none of it back on rebound (trailing half of the contact patch).

There is a limit to this, depending how firm the soil (or rock) is.
 

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Discussion Starter · #13 ·
Sticky: http://forums.mtbr.com/wheels-tires/tire-pressure-all-around-xc-riding-561602.html It has many pageses of discussion that might be of interest.

Another Sticky under Wheels and Tires that might be of interest: http://forums.mtbr.com/wheels-tires/tire-test-results-german-bike-magazine-419392.html

I'm kind of agnostic over the whole thing. My recorded times are better, even in technical climbs, with smaller, lighter tires run tubeless at medium low pressures than with bigger heavier tires, but I much prefer riding with bigger, heavier tires for the comfort and increased fun over all, especially in the very uneven terrain I ride. I think part of the reduction in energy expended with lower pressure has to do with a more level "axle path". The axle is not going up and over every feature as much. Also, with the wider, shorter patch, the angle of attack at which the leading edge of the tire hits the feature is steeper, so less of a head on impact. I have a NN/RR combo I ride with on my 26er. When I run the 2.25 NN front/2.1 RR rear Vs the 2.4 NN front /2.25 RR rear, my times are consistently faster on long technical climbs. Both NNs are the same versions and both RRs are the same versions.
Are you a Michigan State Alum or Mississippi State alum? I graduated Michigan State in 1991.
 
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