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Discussion Starter · #4 ·
briscoelab said:
Your perception of what "feels fast" and what is actually lower rolling resistance on rough terrain are two completely different things :)
And none of those things address the question at hand. Is it harder to power forward a softer tire?
 

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The definition of rolling resistance would lead you to the conclusion that it is not harder to pedal a softer tire over rough terrain (less power for the same speed). Sorry that's such a hard connection to make.
 

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bing! said:
And none of those things address the question at hand. Is it harder to power forward a softer tire?
Did you just change over to talking about durometers of the rubber?

A tire isn't "softer" at a lower PSI, it just isn't as full.

You can debate the feel of it, but you cannot argue with numbers.

One of my favorite parts is how tires don't tend to spin-out as much climbing in the wet/rocky/rooty areas when running super low PSI. Always faster to not have to get off your bike and walk.
 

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Discussion Starter · #7 · (Edited)
briscoelab said:
The definition of rolling resistance would lead you to the conclusion that it is not harder to pedal a softer tire over rough terrain (less power for the same speed). Sorry that's such a hard connection to make.
What "feels fast" has nothing to do with my question. Youre smugness is amusing but is not productive. The rest of this response is not addressed to you.

A characteristic of a deformable material such that the energy of deformation is greater than the energy of recovery. The rubber compound in a tire exhibits hysteresis. As the tire rotates under the weight of the vehicle, it experiences repeated cycles of deformation and recovery, and it dissipates the hysteresis energy loss as heat. Hysteresis is the main cause of energy loss associated with rolling resistance.

-- National Academy of Sciences[3]

The Schwalbe study begins with a comparison of 2.1 tires at 57 psi vs. 2.4 at 21. It moves on to compare the same tire at 57 and 21 psi. As with anything, there must be a curve. Both pressure samples seem too far apart with the high pressure value selected looking unrealistic. Id be very interested to know where that curve is as in my experience, the difference between effort exerted at 35 psi and 28 psi is apparent.

I am a bit puzzled why only two air pressures, and why those two, were selected. That is like taking the race times of an 8 and a 65 year old among 25 differently aged racers as the average race time for cyclists in a given sample. (disclaimer: basic analogy, could be better).
 

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Discussion Starter · #8 ·
CharacterZero said:
Did you just change over to talking about durometers of the rubber?

A tire isn't "softer" at a lower PSI, it just isn't as full.

You can debate the feel of it, but you cannot argue with numbers.

One of my favorite parts is how tires don't tend to spin-out as much climbing in the wet/rocky/rooty areas when running super low PSI. Always faster to not have to get off your bike and walk.
Besides playing with words, to which I am sure you understand what I mean, you seem to be honest in your answer. Thank you.

Let me refine the question as below - The Schwalbe study begins with a comparison of 2.1 tires at 57 psi vs. 2.4 at 21. It moves on to compare the same tire at 57 and 21 psi. As with anything, there must be a curve. Both pressure samples seem too far apart. Id be very interested to know where that curve is as in my experience, the difference between effort exerted at 35 psi and 28 psi is apparent and favors the slightly higher pressure.

I am not denying the Schwalbe results. However, I am sure you will accept that lower tire pressure will only give productivity up to a certain point, after which it will trail off to lesser efficiency. Given my own experience, in my environs, which is more specific than 57 to 21 psi, I am questioning if 28 is more efficient that 35.

Not trying to be controversial. Just a questioning mind. That other guy seems to take the study as gospel :D :D :D :D :D :D
 

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bing! said:
Besides playing with words, to which I am sure you understand what I mean, you seem to be honest in your answer. Thank you.

Let me refine the question as below - The Schwalbe study begins with a comparison of 2.1 tires at 57 psi vs. 2.4 at 21. It moves on to compare the same tire at 57 and 21 psi. As with anything, there must be a curve. Both pressure samples seem too far apart. Id be very interested to know where that curve is as in my experience, the difference between effort exerted at 35 psi and 28 psi is apparent and favors a slightly higher pressure.

I am not denying the Schwalbe results. However, I am sure you will accept that lower pressure will only give productivity to a certain point. Given my own experience, which is more specific than 57 to 21 psi, I am questioning if 28 is more efficient that 35.

Not trying to controversial. Just a questioning mind. That other guy seems to take a study as gospel :D :D :D :D :D :D
Did you time your laps or a section that you feel is most relevant to your riding?
Are your legs more sore after riding at 28 psi?
 

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Discussion Starter · #10 · (Edited)
CharacterZero said:
Did you time your laps or a section that you feel is most relevant to your riding?
Are your legs more sore after riding at 28 psi?
Yes, they are definitely more sore, and I did a blind test on my son :D I let air out off his tires from 32 to 28, and he was complaining the whole ride that there was something wrong with his bike :)

I am not sure of my observations. They are just that, observations. I might explore it further, just looking for an honest discussion at this point so I can make a more informed choice.
 

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bing! said:
Ive read the study. How do you reconcile those results with how much harder it is to pedal 28 psi tires with 35 psi tires. Or is it my imagination? Thanks?
Tire pressure is always going to be a trade-off between rolling resistance, grip and pinch flat or rim damage resistance. Add to that the rolling resistance at different pressures will depend on how smooth the trail surface is.
The wider the tire the more you can lower the pressure, gain grip, minimise rolling resistance and gain pinch flat resistance at the expense of weight.

Maybe for where you are testing the tires the right pressure is 35 psi but it might be worth checking that with some qualitative measurements. Tire pressure is a tuning tool, there's no perfect number that you should always set it to, some trails will need higher pressure just to keep from dinging rims or getting pinch flats, some will need lower pressure to keep the tires from skittering in turns.

I changed from 700x23c @120psi to 700x25c @95 psi on my road bike a couple of years ago. Initially they felt slow (though with noticeably better grip and comfort) but my average speed on rides never changed so there can't be much difference in rolling resistance, it was just the softer feel of the lower pressure tire that felt slow because I was used to the rock hard tires = fast feel.

For mountain bike tires I generally start at whatever pressure will get all the knobs but the edge knobs touching the ground when I'm riding straight on flat ground then adjust up or down from there in 5 psi increments depending on how they feel. It's amazing how much difference a change as small as 2-3 psi will make.
 

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Discussion Starter · #12 ·
Surestick Malone said:
Tire pressure is always going to be a trade-off between rolling resistance, grip and pinch flat or rim damage resistance. Add to that the rolling resistance at different pressures will depend on how smooth the trail surface is.
The wider the tire the more you can lower the pressure, gain grip, minimise rolling resistance and gain pinch flat resistance at the expense of weight.

......... It's amazing how much difference a change as small as 2-3 psi will make.
I was thinking it................but not as well :) Great stuff.

Given that I spend 3/4 of my riding time going up and only 1/4 down, I think I will be fine tuning my pressure accordingly. Not to mention that Im unable to climb some stuff at 35 psi :madman:

Thanks.
 

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bing! said:
What "feels fast" has nothing to do with my question. Youre smugness is amusing but is not productive. The rest of this response is not addressed to you.

A characteristic of a deformable material such that the energy of deformation is greater than the energy of recovery. The rubber compound in a tire exhibits hysteresis. As the tire rotates under the weight of the vehicle, it experiences repeated cycles of deformation and recovery, and it dissipates the hysteresis energy loss as heat. Hysteresis is the main cause of energy loss associated with rolling resistance.

-- National Academy of Sciences[3]

The Schwalbe study begins with a comparison of 2.1 tires at 57 psi vs. 2.4 at 21. It moves on to compare the same tire at 57 and 21 psi. As with anything, there must be a curve. Both pressure samples seem too far apart with the high pressure value selected looking unrealistic. Id be very interested to know where that curve is as in my experience, the difference between effort exerted at 35 psi and 28 psi is apparent.

I am a bit puzzled why only two air pressures, and why those two, were selected. That is like taking the race times of an 8 and a 65 year old among 25 differently aged racers as the average race time for cyclists in a given sample. (disclaimer: basic analogy, could be better).
If you are conducting a study you chose variables that are enough different to show significant results. You need to run a large number of tests with each variable to have consistent data. Adding variables vastly increases the time and cost of the test.

After you have established that there is a performance difference, then you can start narrowing the gaps in setup, if you have funding.

I have had a plan for real-world tire performance. I would love to use 3-4 different pressures, but realistically I would not have time to do enough runs in a day with 4-5 setups (including controls) to record enough data points to have valid results.

And my butt and legs tell me that most tires roll better at 28psi than 35 on trails, though the lower pressure does feel harder to pedal on pavement.
 

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bing! said:
The Schwalbe study begins with a comparison of 2.1 tires at 57 psi vs. 2.4 at 21. It moves on to compare the same tire at 57 and 21 psi. As with anything, there must be a curve. Both pressure samples seem too far apart. Id be very interested to know where that curve is as in my experience, the difference between effort exerted at 35 psi and 28 psi is apparent and favors the slightly higher pressure.
so people can see what you're talking about here's a link. :thumbsup:
There are four points going from 21 to 57 psi, not just two. The curve is on page 7, and there are other presentations of the results as well.
bing! said:
I am not denying the Schwalbe results. However, I am sure you will accept that lower tire pressure will only give productivity up to a certain point, after which it will trail off to lesser efficiency. Given my own experience, in my environs, which is more specific than 57 to 21 psi, I am questioning if 28 is more efficient that 35.
If by "lesser efficiency" you mean pinch flats, then yes. :D
21 psi is lower than most people can ride w/out getting pinch flats, and 57 psi is much higher than most people ride, so they covered the whole operating range (more-or-less) and the results are pretty clear. According to the study, 28psi is more efficient than 35 psi on rough surfaces. The rougher the surface, the greater the effect. On the road, the opposite is true. This transition happens somewhere between "meadow" and "road." If you're riding smooth hardpack, you very well might be on the other side of the transition. The roughness of the surface is key.
bing! said:
Not trying to be controversial. Just a questioning mind. That other guy seems to take the study as gospel :D :D :D :D :D :D
questioning minds are good! studies are not gospel, they have to be taken for what they are, with all the facts and limitations they have. :thumbsup: :D
 

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meltingfeather said:
so people can see what you're talking about here's a link. :thumbsup:
There are four points going from 21 to 57 psi, not just two. The curve is on page 7, and their are other presentations of the results as well.

If by "lesser efficiency" you mean pinch flats, then yes. :D
21 psi is lower than most people can ride w/out getting pinch flats, and 57 psi is much higher than most people ride, so they covered the whole operating range (more-or-less) and the results are pretty clear. According to the study, 28psi is more efficient than 35 psi on rough surfaces. The rougher the surface, the greater the effect. On the road, the opposite is true. This transition happens somewhere between "meadow" and "road." If you're riding smooth hardpack, you very well might be on the other side of the transition. The roughness of the surface is key.

questioning minds are good! studies are not gospel, they have to be taken for what they are, with all the facts and limitations they have. :thumbsup: :D
Totally agree with you MF.
 

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Discussion Starter · #16 · (Edited)
shiggy said:
After you have established that there is a performance difference, then you can start narrowing the gaps in setup, if you have funding.

I have had a plan for real-world tire performance. I would love to use 3-4 different pressures, but realistically I would not have time to do enough runs in a day with 4-5 setups (including controls) to record enough data points to have valid results.

And my butt and legs tell me that most tires roll better at 28psi than 35 on trails, though the lower pressure does feel harder to pedal on pavement.
IMHO, a larger sample than just two wouldve have been more conclusive. In a post above, it was actually shown that there was inded 4 samples, instead of the 2 that I thought.

I looked into the SRM training system used to measure results in the the Schwalbe test and it seems to be dependent on rider input. The controlled properties of a human rider is debatable (re: ability to produce the exactly same amount of power repeatedly). THe SRM system is better suited for.....training. FYI, the SRM system is 950 dollars for the head unit only.

Ideally, I think an MTB wheel travelling on a large self powered offroad surface drum wouldve been more conclusive. However, I'm sure, nobody is going to bother.

Appreciate the input. Thanks.
 

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Discussion Starter · #17 · (Edited)
meltingfeather said:
There are four points going from 21 to 57 psi, not just two. The curve is on page 7, and there are other presentations of the results as well.

questioning minds are good! studies are not gospel, they have to be taken for what they are, with all the facts and limitations they have. :thumbsup: :D
You are correct regarding the samples.



The gravel results suggest that MTB tires exhibit no hysteresis (increased rolling resistance due to tire folding) effect in off road conditions. That is surprising. With those results, and momentarily dismissing pinch flats, it would be interesting to see how efficient a tire could be made by further reducing air pressure.

Thanks for the correction.

EDIT: There is a significant error in that chart. According to the study, the highest rolling resistance is in the MEADOW tests. If you look at the chart, the gravel shows the highest rolling resistance. If we are to assume that the meadow result is actually that of gravel, the difference between 1.5 bar to 4 bar is less than 4 watts. Almost negligible between 1.5 and 2 bar, and probably less than the standard deviation for the study considering the human input. I wonder what 1 watt feels like?
 

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bing! said:
Ideally, I think a self propelled cart on MTB wheels travelling on a rail would be the way to go. Or an MTB wheel travelling on a large offroad surface drum. However, I'm sure, nobody is going to bother.
Assuming you want to go as fast as possible for a given effort the best way to measure would be to ride the same trail with a power meter or heart rate monitor and different tire pressures. Try and keep similar power outputs or heart rates for the different pressures and see which gives you the best time.
A drum won't tell you anything about behaviour under braking and turning at different pressures and those can have as much of an effect on speed as rolling resistance.

I'm in Quebec so Northern East-coast roots & rocks. I'd bet that for my local trails I'm faster going to the lowest pressure I can without pinch flatting because it means I can carry more speed through turns and brake harder and later.

bing! said:


The gravel results suggest that MTB tires exhibit no hysteresis (increased rolling resistance due to tire folding) effect in off road conditions. That is surprising. With those results, and momentarily dismissing pinch flats, it would be interesting to see how efficient a tire could be made by further reducing air pressure.
My understanding is that while you still lose more energy to hysteresis at lower pressures you save more energy by deforming around bumps instead of bouncing off them. That's why the rolling resistance on the road decreases a bit with pressure and increase on gravel
 

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Discussion Starter · #20 ·
Surestick Malone said:
My understanding is that while you still lose more energy to hysteresis at lower pressures you save more energy by deforming around bumps instead of bouncing off them. That's why the rolling resistance on the road decreases a bit with pressure and increase on gravel
Good point.

Ergo, by looking at the charts, it is highly possible that hysteresis is a factor that exerts an influence of less than +/- 3 watts. Terrain resistance(?) must supersede the tires rolling resistance at that point.

Thanks.
 
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