Does tyre pressure make so much difference

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Samuel D
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Re: Does tyre pressure make so much difference

Post by Samuel D »

I see it now, thanks.

Is that nonetheless some sort of U-shaped trend line? Statistics are not my thing. To what do you attribute the high standard deviation?

Also, how much did you and your bicycle weigh?

I think drum tests do a good job of measuring or at least ranking tyres by rolling resistance. Unfortunately they don’t tell us about suspension losses in the cyclist.
Scunnered
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Re: Does tyre pressure make so much difference

Post by Scunnered »

Samuel D wrote:I see it now, thanks.

Is that nonetheless some sort of U-shaped trend line?

It is, but it is not statistically significant. Basically, any line/curve that fits within 1 standard deviation (the vertical bars) could be considered valid.

Samuel D wrote: To what do you attribute the high standard deviation?

Environmental factors: small puffs of wind and small variations in slope, both unavoidable outdoors.

Samuel D wrote: Also, how much did you and your bicycle weigh?

86kg and I also took account of the rotational inertia of the wheels.

Samuel D wrote:I think drum tests do a good job of measuring or at least ranking tyres by rolling resistance. Unfortunately they don’t tell us about suspension losses in the cyclist.

That is exactly the issue I was trying to quantify. The results suggest that with a reasonably supple tyre, rolling resistance in the tyre largely offsets suspension losses in the cyclist, for reasonable tyre pressures and normal road surfaces.
Therefore, a hard tyre sacrifices comfort for little, if any, gain.
Brucey
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Re: Does tyre pressure make so much difference

Post by Brucey »

Samuel D wrote:
I think drum tests do a good job of measuring or at least ranking tyres by rolling resistance. Unfortunately they don’t tell us about suspension losses in the cyclist.


Yup I agree. But the suspension losses (mostly above a certain pressure) and the tyre deflection losses (mostly below a certain pressure) are dependant on a lot of things which might vary quite a lot from one situation to another, and even over which you have little control.

For example whilst freewheeling on a bumpy road just raising one's weight out of the saddle slightly and taking weight off the handlebars allows the bike to ride the bumps better and the net rolling resistance is usually decreased.

It isn't obvious but even if the tyres are fairly soft then the amount by which they deflect (and thus the rolling resistance) may also be decreased thusly.

In extremis the bike + rider + tyres are a set of distributed masses that are connected to one another via elements that are both springy and have some damping. These connections vary from rider to rider and even for the same rider on the same bike from one moment to another.

However to a first approximation the rear tyre sees enough load and is coupled stiffly enough that roller measurements are likely to be representative, where by contrast the front wheel may (unless you are leaning heavily into the handlebars or carrying a front load) tend more to ride over the bumps and not work the tyre so hard.

Thus a piece of practical advice; there is much to be said for choosing a rear tyre on the basis of roller data, but it is probably less relevant to a front tyre. Far better that the front tyre is supple and pleasant-feeling than it is low Crr per se (it is likely that the rear tyre contributes about 2/3rds of the drag on most bikes with matched tyres). This is also coincident with the likely puncture resistance; most punctures happen in rear tyres so that is where you should spend the money on a good tyre. Similarly the front tyre can be built lighter and this will only make the bike nicer to ride, by letting the fork work properly.

To summarise the desirable attributes of each tyre;

Front; Supple, good grip, predictable steering. (less important that Crr is measured to be low, and/or that puncture resistance is especially good). Doesn't have to be an expensive tyre.

Rear; Low measured Crr, hard-wearing, good puncture resistance. {less important that the tyre feels super-supple, or that it grips as well as the front}

So you could argue that (say) a thinking man's set of 32mm touring tyres (best value result) might be something like

- A vittoria zaffiro pro in at the front.
- A marathon greenguard at the rear.

Certainly if you were to swap them round, I think you would notice the difference!

cheers
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reohn2
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Re: Does tyre pressure make so much difference

Post by reohn2 »

531colin wrote:Does anybody need 65psi in a two inch tyre?.......

On the tandem yes :wink:
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Scunnered
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Re: Does tyre pressure make so much difference

Post by Scunnered »

Brucey wrote:To summarise the desirable attributes of each tyre;

Front; Supple, good grip, predictable steering. (less important that Crr is measured to be low, and/or that puncture resistance is especially good). Doesn't have to be an expensive tyre.

Rear; Low measured Crr, hard-wearing, good puncture resistance. {less important that the tyre feels super-supple, or that it grips as well as the front}

So you could argue that (say) a thinking man's set of 32mm touring tyres (best value result) might be something like

- A vittoria zaffiro pro in at the front.
- A marathon greenguard at the rear.

Certainly if you were to swap them round, I think you would notice the difference!

cheers

That's similar to my own line of thought.

- Front: Specialized Roubaix Pro (32mm)
- Rear: Marathon Supreme (35mm)

Do I qualify as a "thinking man" :lol: :lol: :lol:
Brucey
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Re: Does tyre pressure make so much difference

Post by Brucey »

'great minds think alike'

and, er, what is the other one now....? :shock:

:lol:

cheers
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rfryer
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Re: Does tyre pressure make so much difference

Post by rfryer »

Scunnered wrote:First result: it is almost impossible to measure rolling resistance accurately outdoors - note the standard deviation bars on the graphic below.
Second result: tyre pressure made no significant, measureable difference.

I find the second result surprising! I've recently been having a go at riding on rollers, and was surprised at the dramatic effect that tyre pressure has on the effort required.

Any thoughts about why this scenario (rollers) would be out of whack with real-world experience?
Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: Does tyre pressure make so much difference

Post by Brucey »

on rollers there are two important things to realise

1) the Crr is a lot higher than normal, because the tyre is deformed a lot more than normal (unless the rollers are a very large diameter indeed)

2) the tyre drag is the main source of resistance; this contrasts with real riding on the roads where the main resistance is aero drag.

All this means that you will easily notice a small (<10%) change in Crr on the rollers, but on the road it would be very difficult to pull that out of the 'noise' that is generated by other things like the air, any changes in wind speed, or variations in the roughness of the road surface.

For several reasons the Crr you get on typical rollers doesn't tally very well with the Crr you will get on a real road.

BTW if you ride enough on the rollers you will find that tyres can suffer (and be fit to drop to bits) without the tread wearing that much.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
mig
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Re: Does tyre pressure make so much difference

Post by mig »

Brucey wrote:
Samuel D wrote:
I think drum tests do a good job of measuring or at least ranking tyres by rolling resistance. Unfortunately they don’t tell us about suspension losses in the cyclist.


Yup I agree. But the suspension losses (mostly above a certain pressure) and the tyre deflection losses (mostly below a certain pressure) are dependant on a lot of things which might vary quite a lot from one situation to another, and even over which you have little control.

For example whilst freewheeling on a bumpy road just raising one's weight out of the saddle slightly and taking weight off the handlebars allows the bike to ride the bumps better and the net rolling resistance is usually decreased.

It isn't obvious but even if the tyres are fairly soft then the amount by which they deflect (and thus the rolling resistance) may also be decreased thusly.

In extremis the bike + rider + tyres are a set of distributed masses that are connected to one another via elements that are both springy and have some damping. These connections vary from rider to rider and even for the same rider on the same bike from one moment to another.

However to a first approximation the rear tyre sees enough load and is coupled stiffly enough that roller measurements are likely to be representative, where by contrast the front wheel may (unless you are leaning heavily into the handlebars or carrying a front load) tend more to ride over the bumps and not work the tyre so hard.

Thus a piece of practical advice; there is much to be said for choosing a rear tyre on the basis of roller data, but it is probably less relevant to a front tyre. Far better that the front tyre is supple and pleasant-feeling than it is low Crr per se (it is likely that the rear tyre contributes about 2/3rds of the drag on most bikes with matched tyres). This is also coincident with the likely puncture resistance; most punctures happen in rear tyres so that is where you should spend the money on a good tyre. Similarly the front tyre can be built lighter and this will only make the bike nicer to ride, by letting the fork work properly.

To summarise the desirable attributes of each tyre;

Front; Supple, good grip, predictable steering. (less important that Crr is measured to be low, and/or that puncture resistance is especially good). Doesn't have to be an expensive tyre.

Rear; Low measured Crr, hard-wearing, good puncture resistance. {less important that the tyre feels super-supple, or that it grips as well as the front}

So you could argue that (say) a thinking man's set of 32mm touring tyres (best value result) might be something like

- A vittoria zaffiro pro in at the front.
- A marathon greenguard at the rear.

Certainly if you were to swap them round, I think you would notice the difference!

cheers


do you bother to run different tyres front and back on a lightweight summer machine?
Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: Does tyre pressure make so much difference

Post by Brucey »

mig wrote:
do you bother to run different tyres front and back on a lightweight summer machine?


if I'm riding on sunny days only, not usually. If they made lighter and faster tyres that were suitable for front only fitment I might, and obviously if I want a bit more net durability there are any number of slightly tougher tyres that you could fit on the rear.

cheers
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MikeDee
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Re: Does tyre pressure make so much difference

Post by MikeDee »

Here is an online tire pressure calculator http://psicalculator.com based on research by Frank Berto. There are also Excel spreadsheets out there too. Here's an article by Jan Heine on the subject. https://janheine.wordpress.com/2016/03/ ... take-home/ Works for me. I was running my 700x47 tires with way too much pressure. I dropped the pressure down per these recommendations and love the ride and didn't notice an increase in rolling resistance.
reohn2
Posts: 45158
Joined: 26 Jun 2009, 8:21pm

Re: Does tyre pressure make so much difference

Post by reohn2 »

MikeDee wrote:Here is an online tire pressure calculator http://psicalculator.com based on research by Frank Berto. There are also Excel spreadsheets out there too. Here's an article by Jan Heine on the subject. https://janheine.wordpress.com/2016/03/ ... take-home/ Works for me. I was running my 700x47 tires with way too much pressure. I dropped the pressure down per these recommendations and love the ride and didn't notice an increase in rolling resistance.

I've found the same.
But it's not an exact science,PSI's can differ as supple tyres need a little more air in them than stiff carcassed equivalent sized tyres.If riding smmooottthhhh tarmac(little chance in the UK)a little more air can mean a faster ride.
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bretonbikes
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Re: Does tyre pressure make so much difference

Post by bretonbikes »

I think 'can of worms sums up this subject a lot. I do a lot of riding in groups and the difference in rolling resistant is hugely varied depending on surface. On a rough surface a very hard tyre seems to run noticeably slower as the bumps (or even course gravel) are putting energy into the whole bike frame rather than just being smoother by a softer (or indeed wider) tyre - likewise they suffer on soft surfaces like cyclepaths. On fabulous tarmac (we all know the feeling of going over a road joint onto silky tarmac and clicking up two gears) - the opposite is the case - with narrower harder tyres gaining an advantage.

So there's no definitive answer, merely compromises, but my experience is that most cyclists ride with tyres harder than optimum.

I've written a fairly long and opinionated article about it all here which has links to the various calculators out there...

http://www.bretonbikes.com/homepage/cyc ... y-tyres-be
38 years of cycletouring, 33 years of running cycling holidays, 8 years of running a campsite for cyclists - there's a pattern here...
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