Tyre pressures
Re: Tyre pressures
I've never been sure whether lower tyre pressure increases or decreases likelihood of penetration by thorns, glass, etc. But I suspect I would find it easier to push a sharpened nail into a fully inflated football than one a bit under pressure. The latter would be able to deform a little instead of being punctured. Does it work that way?
Re: Tyre pressures
I don't think so, a tyre is not like a balloon. I often hear objects pinging off my pumped up tyres, not very scientific I know. I also get more punctures on my touring bike @80 psi than my road bikes @120-130 psi.
Re: Tyre pressures
The preponderance of evidence suggests that lower pressures would make you faster, perfect roads aside. They would certainly make you more comfortable. Most likely you would have fewer punctures too.
Is there any harm in trying for yourself? Just remember that lower pressures feel slower, because we associate vibration with speed.
What 25 mm tyres are you running, by the way?
Is there any harm in trying for yourself? Just remember that lower pressures feel slower, because we associate vibration with speed.
What 25 mm tyres are you running, by the way?
Re: Tyre pressures
I’d run a bit more air in those than in something with a supple sidewall – else the already high rolling resistance gets worse – but I’d still recommend experimenting with lower pressures than those you’re using.
Re: Tyre pressures
I'm confident that relatively higher pressure reduces the risk posed by thorns and the like. I think with an under-inflated tyre the inward deformation as the sharp object pushes into the tyre allows the object to stay centred on it's contact point and is almost funnelled into the tyre as they come together. If the tyre is pumped up harder there is a better chance that the object will skid off the surface before it is able to start digging in.
It's a bit like a mattress. When our kids jump up and down on our orthopaedic mattress they migrate all over the show, whereas on a soft premier inn bed they tend to stay on one spot- apologies to the downstairs neighbours!
At max 70kg I never go above 80psi on 25mm as I find the ride becomes intolerably harsh on my wrists over rough tarmac. Last winter I noticed the 32s on my cross bike were a bit soft and pumped them up to about 50psi and that felt pretty horrid too, I reckon they're probably comfortable at 45. I thought the whole point of pneumatic tyres was to give additional compliance and hence maintain contact with the road so I don't see any benefit in pumping up rock hard- if there's no give in the tyre presumably you have to rely on flex in the frame to soak up the bumps in the road. The article has suggested a potential sweet spot based on what? Presumably for a given load one could calculate the contact patch for a given pressure, and then account for difference in feel between tyres according to sidewall, material differences etc. I'd like to see how this would extrapolate with increasing load to give me a better idea how much air to put in our tandem tyres depending upon who happens to be stoker that day, which could vary by 50kg.
It's a bit like a mattress. When our kids jump up and down on our orthopaedic mattress they migrate all over the show, whereas on a soft premier inn bed they tend to stay on one spot- apologies to the downstairs neighbours!
At max 70kg I never go above 80psi on 25mm as I find the ride becomes intolerably harsh on my wrists over rough tarmac. Last winter I noticed the 32s on my cross bike were a bit soft and pumped them up to about 50psi and that felt pretty horrid too, I reckon they're probably comfortable at 45. I thought the whole point of pneumatic tyres was to give additional compliance and hence maintain contact with the road so I don't see any benefit in pumping up rock hard- if there's no give in the tyre presumably you have to rely on flex in the frame to soak up the bumps in the road. The article has suggested a potential sweet spot based on what? Presumably for a given load one could calculate the contact patch for a given pressure, and then account for difference in feel between tyres according to sidewall, material differences etc. I'd like to see how this would extrapolate with increasing load to give me a better idea how much air to put in our tandem tyres depending upon who happens to be stoker that day, which could vary by 50kg.
Re: Tyre pressures
an idea underpinning https://www.compasscycle.com/wp-content/uploads/2015/03/BQTireDrop.pdf but that isn't clearly expressed within it is that a given tyre will work 'best' when it is deformed by a certain amount, i.e. by a certain proportion of its cross section.
'Best' is a combination of suspension action (comfort and 'suspension losses' over bigger bumps) vs raw Crr value (i.e. against a smooth steel roller) with of other factors such as a little longevity and changes in puncture resistance thrown in.
The cords in nearly all bicycle tyres are set a bias angle; (there have been a few radial tyres but they are very much the exception). This means that the cords which pass through the contact patch make a kind of 'X' shape (in a view from the ground upwards, as it were) in the structure of the tyre. These cords are thought to become slack(er) within the part of the tyre that is squashed flat against the road, and only retain tension at longer range by virtue of the shear strength of the tyre carcass. The losses that occur as the carcass strains in shear contribute significantly to the Crr but if the tyre is run softer than it should be (for a given load) then the amount of the carcass that is strained becomes much larger and the losses may rise again as a consequence.
The area within the contact patch is simply governed by the tyre pressure; if you have a tyre with 50psi in it and a load of 50lbs on the wheel the area of the contact patch is (pretty closely) going to be one square inch. [The presence of tread may alter the local contact pressure, but that is a different matter.] If the tyre is sufficiently hysteric to change the contact patch area with speed, it is almost certainly a very draggy tyre.
Anyway it turns out that bicycle tyres work pretty well if they deform by about 15% or so; if they deform more than this then pinch flats become likely and they are draggy, if they deform less than this then the suspension action becomes rather poor on average surfaces, (although this might still be OK on very smooth surfaces). You can vary the pressure in accordance with the load, and of course wider tyres allow heavier loads and/or lower pressures too.
It is worth bearing in mind that
a) the spring rate of a tyre is not constant with load (i.e. the vertical displacement isn't linear with load) and
b) the way this varies changes with the tyre section (and weakly with the wheel diameter too)
c) because bumps are not 'flat' then there is an absolute effect varying with tyre pressure when running over small objects that are smaller than the contact patch area.
So anyway there is a case for running different width tyres in accordance with the load, which would lead to a narrower tyre on the front wheel than the rear in most cases.
I also think that there is an opening for a 'mixed use' semi-slick-esque tyre in which you run a highish pressure on the road, and run on a smooth centre tread, but when running on loose surfaces, you deflate the tyre slightly and bring additional elements of the tread pattern into play eg for increased traction.
But overall there are choices to be made; comfort and fashion have as much to do with tyre selection as anything else.
cheers
'Best' is a combination of suspension action (comfort and 'suspension losses' over bigger bumps) vs raw Crr value (i.e. against a smooth steel roller) with of other factors such as a little longevity and changes in puncture resistance thrown in.
The cords in nearly all bicycle tyres are set a bias angle; (there have been a few radial tyres but they are very much the exception). This means that the cords which pass through the contact patch make a kind of 'X' shape (in a view from the ground upwards, as it were) in the structure of the tyre. These cords are thought to become slack(er) within the part of the tyre that is squashed flat against the road, and only retain tension at longer range by virtue of the shear strength of the tyre carcass. The losses that occur as the carcass strains in shear contribute significantly to the Crr but if the tyre is run softer than it should be (for a given load) then the amount of the carcass that is strained becomes much larger and the losses may rise again as a consequence.
The area within the contact patch is simply governed by the tyre pressure; if you have a tyre with 50psi in it and a load of 50lbs on the wheel the area of the contact patch is (pretty closely) going to be one square inch. [The presence of tread may alter the local contact pressure, but that is a different matter.] If the tyre is sufficiently hysteric to change the contact patch area with speed, it is almost certainly a very draggy tyre.
Anyway it turns out that bicycle tyres work pretty well if they deform by about 15% or so; if they deform more than this then pinch flats become likely and they are draggy, if they deform less than this then the suspension action becomes rather poor on average surfaces, (although this might still be OK on very smooth surfaces). You can vary the pressure in accordance with the load, and of course wider tyres allow heavier loads and/or lower pressures too.
It is worth bearing in mind that
a) the spring rate of a tyre is not constant with load (i.e. the vertical displacement isn't linear with load) and
b) the way this varies changes with the tyre section (and weakly with the wheel diameter too)
c) because bumps are not 'flat' then there is an absolute effect varying with tyre pressure when running over small objects that are smaller than the contact patch area.
So anyway there is a case for running different width tyres in accordance with the load, which would lead to a narrower tyre on the front wheel than the rear in most cases.
I also think that there is an opening for a 'mixed use' semi-slick-esque tyre in which you run a highish pressure on the road, and run on a smooth centre tread, but when running on loose surfaces, you deflate the tyre slightly and bring additional elements of the tread pattern into play eg for increased traction.
But overall there are choices to be made; comfort and fashion have as much to do with tyre selection as anything else.
cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~