Does tyre pressure make so much difference

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Thornyone
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Does tyre pressure make so much difference

Postby Thornyone » 16 Feb 2018, 2:39pm

For quite some time until the other day I have been using a pump with a broken gauge. The tyres were reasonably firm, I thought. For some time too, I have noticed an increasing number of younger cyclists overtaking me (on mountain bikes: I expect most people on racers to be able to pass, because I use 26x2.0 Schwalbe Green Guard). I thought I was just showing my age.
Now that I have just acquired a pump with gauge, I find that my tyres are harder than for a while (65psi is what I used to put in when my previous gauge worked). I seem to be going faster, and managed to overtake someone a third of my age (who was not stationary at the time :mrgreen: ). Can tyre pressure make so much difference?
Last edited by Thornyone on 16 Feb 2018, 3:48pm, edited 1 time in total.

hamster
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Re: Does tyre pressure make so much difference

Postby hamster » 16 Feb 2018, 3:11pm

In short yes - the difference between squashy and firm. Between hard and very hard, no!
http://road.cc/content/feature/180830-h ... t-and-grip

amediasatex
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Re: Does tyre pressure make so much difference

Postby amediasatex » 16 Feb 2018, 3:40pm

Can tyre pressure make so much difference?


Yes, but how much depends on the magnitude of the difference and the type of tyre.
Very flexible supple tyres suffer less from pressure variation than thicker, less supple tyres.

No real numbers below but some hypothetical stuff to illustrate:

A pair of flexible supple tyres @60psi might be say 5Watts more resistance than @80psi, and 8Watts different again @40spi
A pair of stiff, not so supple tyres @60 might be 10Watts more resistance than @80psi, and 20Watts different again @40psi

So regardless of what the actual rolling resistance of the two tyres are, the impact of being outside fo the 'sweet spot' is greater on the less supple tyre.

However... don't underestimate difference in environmental parameters and on the day performance, simply overtaking, or being overtaken by someone else is not an indicator of much other than one of you was trying harder, unless you know all other factors were equal, which they rarely are.

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531colin
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Re: Does tyre pressure make so much difference

Postby 531colin » 16 Feb 2018, 6:08pm

Does anybody need 65psi in a two inch tyre?
I'm currently running 45/55 psi front/rear in 32mm tyres.

tooley92
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Re: Does tyre pressure make so much difference

Postby tooley92 » 16 Feb 2018, 6:15pm

30 years ago I when I was a time trialling racing snake I would have said the narrowest tyre pumped up as hard as possible was best, but now a days old age and rubbish roads its wide tyres and low pressures!

My hands and wrists can certainly tell now if my front tyre is pumped up too much!
Remember folks 'A pessimist is just an optimist with experience!'

Scunnered
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Re: Does tyre pressure make so much difference

Postby Scunnered » 16 Feb 2018, 9:51pm

Thornyone wrote:I find that my tyres are harder than for a while (65psi is what I used to put in when my previous gauge worked). I seem to be going faster...

Harder tyres will transmit more road "buzz" through the frame and that is often mis-interpreted as feeling faster. To know if you are going faster you need to measure your speed and not just compare with random other cyclists to happen to pass.
You will find websites that offer measured values of rolling resistance for different tyres, but that is only half the story.
A hard tyre will transmit vibrations through the frame and this energy will be absorbed by your body tissue, and energy absorbed means you have to pedal harder. A softer tyre with a supple casing will deform more as it rolls which will increase the energy lost in the tyre, but decrease the energy absorbed by your body tissue. As a result, a softer, supple tyre can result in a total rolling resistance which is lower, higher or the same as a harder tyre, it depends on the road surface and how supple the tyre is.
But there is no doubt, a softer supple tyre is a lot more comfortable.

Brucey
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Re: Does tyre pressure make so much difference

Postby Brucey » 16 Feb 2018, 10:17pm

FWIW I have found any Marathon rear tyre to get incredibly draggy if it is run below about 50psi. They are usually sufficiently stiff that you don't feel that the tyre is that soft until you measure the pressure.

With some of these tyres you have a choice between fast and a bit jiggly, or comfy and rather slow.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

rmurphy195
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Re: Does tyre pressure make so much difference

Postby rmurphy195 » 16 Feb 2018, 11:50pm

A couple of weeks after buying my new Brompton, with its Marathonj tyres, I noticed that it fel;t like the brake was on when riding it. I checked everything I could think of - turned the cranks with the chain removed, spun the derailler jockeys round, spun the wheels, checked the brakes - nothing found. As I rode it I noticed tyre noise when I pressed the pedals. So I pumped the tyres up as har as I could with the pump supplied with the bike (which wiith my arms isn't much!) - and the noise disappeared, and the bike seemed to fly!
Brompton, Condor Heritage, creaky joints and thinning white (formerly grey) hair
""You know you're getting old when it's easier to ride a bike than to get on and off it" - quote from observant jogger !

Thornyone
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Re: Does tyre pressure make so much difference

Postby Thornyone » 17 Feb 2018, 8:27am

Brucey wrote:FWIW I have found any Marathon rear tyre to get incredibly draggy if it is run below about 50psi. They are usually sufficiently stiff that you don't feel that the tyre is that soft until you measure the pressure.

With some of these tyres you have a choice between fast and a bit jiggly, or comfy and rather slow.

cheers


I think that the fact that my tyres are Marathon puncture-proofed is significant. Because my pump gauge was kaput, I was relying on feel to assess hardness. They didn’t feel noticeably soft, but may well have been a bit under 50psi. Certainly 65psi seems much harder. Although the ride isn’t generally uncomfortable, I do notice uneveness in concrete section roads more than before. So maybe I’ll experiment with 55psi.

My Thorn originally came with Marathon Extreme, which I liked because I could handle bridleways and tracks, but I now use the puncture resistant Marathons, which don’t have much grip. Unfortunately I got so fed up with punctures (mainly hawthorn, nearly always the rear tyre) using non-puncture proofed, that I don’t want to go back to that sort. And I have tried all sorts of anti-puncture ploys: slime (IMHO only good for hawthorn, and only as long as the wheel is rotating), and kevlar and other tyre inserts.

Thornyone
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Joined: 7 Dec 2017, 11:15am

Re: Does tyre pressure make so much difference

Postby Thornyone » 17 Feb 2018, 8:42am

Thanks for all the info. It does seem that I could drop my pressures somewhat and not suffer. I was possibly running on slightly under 50psi of late, whereas looking back, I think I was using about 60psi when the gauge worked. The problem is that my broken gauge was reading 20lb at rest, so when it reached 65psi on the dial there is no telling what the real value was, though I relied on feel rather than this spurious reading).

The puncture-proofed Marathon tyre with Greenguard strip under the tread is certainly not exactly supple. I have used lighter and nippier tyres in the past and liked the ride but have become so tired of having otherwise-enjoyable rides ruined by punctures that I wouldn’t go back to them. Right now the farmers round here are slaughtering every hawthorn hedge in sight and strewing the debris everywhere, and every yob for miles around must be out smashing glass too :(

Thornyone
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Re: Does tyre pressure make so much difference

Postby Thornyone » 17 Feb 2018, 8:44am

Thornyone wrote:
Brucey wrote:FWIW I have found any Marathon rear tyre to get incredibly draggy if it is run below about 50psi. They are usually sufficiently stiff that you don't feel that the tyre is that soft until you measure the pressure.

With some of these tyres you have a choice between fast and a bit jiggly, or comfy and rather slow.

cheers


I think that the fact that my tyres are Marathon puncture-proofed is significant. Because my pump gauge was kaput, I was relying on feel to assess hardness. They didn’t feel noticeably soft, but may well have been a bit under 50psi. Certainly 65psi seems much harder. Although the ride isn’t generally uncomfortable, I do notice uneveness in concrete section roads more than before. So maybe I’ll experiment with 55psi.

My Thorn originally came with Marathon Extreme, which I liked because I could handle bridleways and tracks, but I now use the puncture resistant Marathons, which don’t have much grip. Unfortunately I got so fed up with punctures (mainly hawthorn, nearly always the rear tyre) using non-puncture proofed, that I don’t want to go back to that sort. And I have tried all sorts of anti-puncture ploys: slime (IMHO only good for hawthorn, and only as long as the wheel is rotating), and kevlar and other tyre belts.

Thornyone
Posts: 363
Joined: 7 Dec 2017, 11:15am

Re: Does tyre pressure make so much difference

Postby Thornyone » 17 Feb 2018, 8:56am

amediasatex wrote:
Can tyre pressure make so much difference?


Yes, but how much depends on the magnitude of the difference and the type of tyre.
Very flexible supple tyres suffer less from pressure variation than thicker, less supple tyres.

No real numbers below but some hypothetical stuff to illustrate:

A pair of flexible supple tyres @60psi might be say 5Watts more resistance than @80psi, and 8Watts different again @40spi
A pair of stiff, not so supple tyres @60 might be 10Watts more resistance than @80psi, and 20Watts different again @40psi

So regardless of what the actual rolling resistance of the two tyres are, the impact of being outside fo the 'sweet spot' is greater on the less supple tyre.

However... don't underestimate difference in environmental parameters and on the day performance, simply overtaking, or being overtaken by someone else is not an indicator of much other than one of you was trying harder, unless you know all other factors were equal, which they rarely are.


My current tyres certainly fit into the “stiff” category, whereas the ones I was using when I felt like a boy racer were rather more supple. Point taken about the importance of other factors too. But one thing is certain re performance: I’m sure it is nothing to do with the fact that I’m probably technically now an OMIL, though I categorically deny having ever been MAMIL :mrgreen: (What actually is middle age anyhow? Never been there, so I wouldn’t know :D ).

Scunnered
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Re: Does tyre pressure make so much difference

Postby Scunnered » 18 Feb 2018, 12:00pm

Last year I measured the rolling resistance of bike + cyclist outdoors on real road surfaces.
Tyres tested were Continental Grand Sport Race

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.

Image

These measurements were made using the "coastdown" method. A Hall sensor and a microcomputer were used to accurately measure and record the time for each revolution of the front wheel while coasting for 30 wheel revolutions, starting at around 10km/h. Measurements were conducted in pairs, once in each direction, as even apparently level road is not perfectly level. The tests were done on windless days and air density was calculated for the air resistance component. The results presented above comprise 35 pairs of measurements split across the indicated tyre pressures.
Last edited by Scunnered on 18 Feb 2018, 2:18pm, edited 2 times in total.

Samuel D
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Re: Does tyre pressure make so much difference

Postby Samuel D » 18 Feb 2018, 1:12pm

Unfortunately I (we?) can’t see your image, Scunnered.

Scunnered
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Re: Does tyre pressure make so much difference

Postby Scunnered » 18 Feb 2018, 1:50pm

Samuel D wrote:Unfortunately I (we?) can’t see your image, Scunnered.

Hopefully you can now