Tyre pressure unimportant

mikesbytes said:

The following study says that tyre pressure isn't that important

ncbi.nlm.nih.govquery.fcgiOpen ↗

Extract;
"Cycling requires power generation to overcome gravity, air resistance, and rolling resistance. When rolling surface and rolling speed are constant for a given tyre, rolling resistance is determined by tyre inflation pressure and the combined weight (CW) of the bicycle and rider. In this study, the oxygen uptake per unit CW (VO2 x CW-1) of seven trained bicycle racers (5 men, 2 women, 24 +/- 2 years) was measured while each cycled up a 4% incline at 19.3 km.h-1 and 75 revolutions.min-1 on a motor-driven treadmill, using randomly-ordered tyre pressures of 552, 690, 827, and 965 kPa. Subjects (55.8-78.4 kg) rode their racing bicycles equipped with the same set of sew-up tyres and wheels. VO2.CW-1 was averaged over the last 3 min of a 5 min ride at each pressure. A repeated measures analysis of variance was performed and significance set at p < 0.05. VO2.CW-1 ranged from 28.1 +/- 0.6 to 28.9 +/- 0.5 ml.kg-1 x min-1 and was not significantly different between tyre pressures. We conclude that differences in rolling resistance caused by varying tyre pressure between 552 and 965 kPa, are too small to be detected physiologically."

I don't care about rr, the tyre pressure is low on my training tyres for comfort. What does it say about that?

Thanks for the heads up, I'll remember that if I ever get tired of broken glass, headwinds and corners and take up riding on a treadmill instead.

mikesbytes said:

The following study says that tyre pressure isn't that important

ncbi.nlm.nih.govquery.fcgiOpen ↗

Extract;
"Cycling requires power generation to overcome gravity, air resistance, and rolling resistance. When rolling surface and rolling speed are constant for a given tyre, rolling resistance is determined by tyre inflation pressure and the combined weight (CW) of the bicycle and rider. In this study, the oxygen uptake per unit CW (VO2 x CW-1) of seven trained bicycle racers (5 men, 2 women, 24 +/- 2 years) was measured while each cycled up a 4% incline at 19.3 km.h-1 and 75 revolutions.min-1 on a motor-driven treadmill, using randomly-ordered tyre pressures of 552, 690, 827, and 965 kPa. Subjects (55.8-78.4 kg) rode their racing bicycles equipped with the same set of sew-up tyres and wheels. VO2.CW-1 was averaged over the last 3 min of a 5 min ride at each pressure. A repeated measures analysis of variance was performed and significance set at p < 0.05. VO2.CW-1 ranged from 28.1 +/- 0.6 to 28.9 +/- 0.5 ml.kg-1 x min-1 and was not significantly different between tyre pressures. We conclude that differences in rolling resistance caused by varying tyre pressure between 552 and 965 kPa, are too small to be detected physiologically."

To make the test valid shouldn't the riders have been required to get off and pump up a tire from flat like they would have had to after getting a pinch flat?

How thick are the tread and sidewalls on a typical sew-up?

I bet the validity of these results varies depending on the thickness and shape of tread, tube, sidewall, etc.

Retro Grouch said:

To make the test valid shouldn't the riders have been required to get off and pump up a tire from flat like they would have had to after getting a pinch flat?

Another reason the test doesn't apply to us, who rides sew-ups anymore? (The sew-ups used in the experiment are resistant to pinch flats)

garage sale GT said:

How thick are the tread and sidewalls on a typical sew-up?

I bet the validity of these results varies depending on the thickness and shape of tread, tube, sidewall, etc.

I'm thinking that rolling resistance is such a tiny part of what holds you back when you're riding a bicycle that it gets lost in the background noise. Compared to the energy required to push your torso through the air everything else is small potatoes.

Sure, maybe, but since they used a radically different tire, the experiment is not germane to those of us who ride clinchers.

I only have an old school 27" road bike but the difference between 75 and 100 psi seems noticeable. At the higher pressure it's almost as if the bike were on ice skates. Maybe, though, it's the fact that many "touring" tires in this size have thick rubber sidewalls which contribute heavily to hysteresis losses.

Plus I am sure the air resistance only becomes more important when you're actually in good enough shape to maintain 20 mph!:o

garage sale GT said:

Sure, maybe, but since they used a radically different tire, the experiment is not germane.

Radically different? How so? There are a lot of people that are riding tubulars. The wheels that I use most of the time are tubulars.

Rolling resistance dominates at very slow speeds but is quickly eclipsed by aero drag. This make sense since aero drag varies with the velocity squared, whereas rolling resistance--which is small, anyway--goes up linearly with velocity plus some smallish heat storage term.

As for there results....well, their procedure seems a bit iffy. The riders were pedaling at a little under 12mph up a 4%, and since they were racers and experienced riders, it is unlikely that their bodies experienced any real physiologic load. Also, they weren't working against a relative wind, either. I don't know for sure, but I suspect that a human's efficiency is not linear with power output.

After re-reading the abstract, there is one glaring mistake they made: each "ride" was only 5 minutes long! Since rolling resistance requires little power to overcome, they should have integrated power use--or in their case, V02--over a longer interval, like an hour.

All right, all right. it is different from SOME of our tires.

Info I have seen seems to show that clinchers have a sharper response to PSI than tubulars because a larger proportion of a tubular's rolling resistance comes from the hysteresis of the rim glue.

garage sale GT said:

All right, all right. it is different from SOME of our tires.

Info I have seen seems to show that clinchers have a sharper response to PSI than tubulars because a larger proportion of a tubular's rolling resistance comes from the hysteresis of the rim glue.

Rolling resistance is inversely linear with pressure, whether the tire is a tubular or a clincher, so with either tire if you double the pressure, you'll halve the rolling resistance.

You say "linear" but describe "proportional". Linear may be roughly correct; proportional is not.

To say the response is inversely linear does not imply the rolling resistance doubles when the pressure drops by half because there can be (and is) a constant term. To put it in layman's terms, the line doesn't pass through the center of the graph. Jobst and Sheldon seem to think there is a significant offset introduced by the use of street-type rim glue, and it doesn't say what kind the OP's experimenters used in the tests.

It also does not imply the slope of the line is the same for both types of tires.

Maybe the OP aimed this thread at users of tubulars only; if so, then I apologize for butting in. If not, then I do wish to point out that his data does not cover the full range of high speed road tires.

alienator said:

Rolling resistance is inversely linear with pressure, whether the tire is a tubular or a clincher, so with either tire if you double the pressure, you'll halve the rolling resistance.

What if you are on bumpy chip and seal roads?

garage sale GT said:

You say "linear" but describe "proportional". Linear may be roughly correct; proportional is not. [/quote}

I didn't say proportional, did I? If I did, I didn't mean it. Linear is correct for how rolling resistance changes with pressure.

garage sale GT said:

To say the response is inversely linear does not imply the rolling resistance doubles when the pressure drops by half because there can be (and is) a constant term. To put it in layman's terms, the line doesn't pass through the center of the graph. Jobst and Sheldon seem to think there is a significant offset introduced by the use of street-type rim glue, and it doesn't say what kind the OP's experimenters used in the tests.

Linear only describes the functionality of the relationship. Of course there can be a DC offset, but that's easily taken care and doesn't change the linear relationship.

garage sale GT said:

It also does not imply the slope of the line is the same for both types of tires.

No, of course the slope probably is different for various tires, but the linearity is still there.

Maybe the OP aimed this thread at users of tubulars only; if so, then I apologize for butting in. If not, then I do wish to point out that his data does not cover the full range of high speed road tires.[/QUOTE]

This is true.

bobbyOCR said:

What if you are on bumpy chip and seal roads?

Good question. The chip and seal will, first, increase rolling resistance just because the tire casing has to deform more to conform to the road surface. But in addition to that, it's energy will be lost as the tire "bounces" over the larger irregularities in the road surface. The bouncing is the reason that higher pressures are not good on the road as they are on a velodrome. Any tire movement that is not in the direction of travel will be a source of energy loss.

Remember, rolling resistance is only a result of energy lost due to tire casing deflection (and the subsequent heat generation), so technically the bouncing does not contribute to rolling resistance.

alienator said:

Remember, rolling resistance is only a result of energy lost due to tire casing deflection (and the subsequent heat generation), so technically the bouncing does not contribute to rolling resistance.

Ah, that clarifies things for me. So, theoretically, a tyre with very high psi will still have a low rolling resistance on bad roads because it doesn't deflect as much, but the effort of pushing 72kg of weight up and over a (albeit very small) bump slows the rider down, while a tyre with lower pressure would conform and be quicker but have a higher RR ??

alienator said:

Remember, rolling resistance is only a result of energy lost due to tire casing deflection (and the subsequent heat generation), so technically the bouncing does not contribute to rolling resistance.

True, we tend to use the term "rolling resistance" to describe the total "road resistance".

bobbyOCR said:

Ah, that clarifies things for me. So, theoretically, a tyre with very high psi will still have a low rolling resistance on bad roads because it doesn't deflect as much, but the effort of pushing 72kg of weight up and over a (albeit very small) bump slows the rider down, while a tyre with lower pressure would conform and be quicker but have a higher RR ??

Exactly.

Here's my point about the tubulars being radically different from the clinchers:

A tubular is kind of like taking a clincher and sewing the beads together in order to enclose the inner tube, roughly speaking. It is then cemented to a rim without flanges. Street type rim cement is pliable and doesn't spring back with the same force that compressed it when rolling.

The rim cement's deformation is not reduced by higher inflation like a tire sidewall or tread is. This means that it will slow you down the same, regardless of whether you have 80 or 140 PSI.

THIS FACTOR IS NOT PRESENT IN A CLINCHER TIRE. So if our experimenters had tested clinchers, it is quite possible a measurable difference would have emerged between high and low PSI.

Of course, tubulars have treads and sidewalls too, albeit high thread count, thin, quality ones, so their response to PSI is somewhat like a clincher, combined with the more or less constant contribution of the rim glue.

So unless the testers used ultra-hard, non-reusable track glue, their setup was radically different from clinchers due to a constant term- the resistance from the rim glue, which does not change due to PSI.

garage sale GT said:

Here's my point about the tubulars being radically different from the clinchers:

A tubular is kind of like taking a clincher and sewing the beads together in order to enclose the inner tube, roughly speaking. It is then cemented to a rim without flanges. Street type rim cement is pliable and doesn't spring back with the same force that compressed it when rolling.

The rim cement's deformation is not reduced by higher inflation like a tire sidewall or tread is. This means that it will slow you down the same, regardless of whether you have 80 or 140 PSI.

THIS FACTOR IS NOT PRESENT IN A CLINCHER TIRE. So if our experimenters had tested clinchers, it is quite possible a measurable difference would have emerged between high and low PSI.

Of course, tubulars have treads and sidewalls too, albeit high thread count, thin, quality ones, so their response to PSI is somewhat like a clincher, combined with the more or less constant contribution of the rim glue.

So unless the testers used ultra-hard, non-reusable track glue, their setup was radically different from clinchers due to a constant term- the resistance from the rim glue, which does not change due to PSI.

I doubt that their results would have been different if they'd used clinchers. Clinchers, as a rule, have less rolling resistance than tubulars. I suspect that if the rolling resistance curve for a clincher crosses the curve of a given tubular at some pressure, I don't think that the difference in slopes will be that great.

As for the tubular glue, while it may not vary with pressure per se, it will change with temperature which itself will vary with pressure.