on average which tyre size can run lower pressures?
I have tried bikes with both and still experimenting however I think the 27.5 x 2.6 can handle 10% less pressure without any problems
Can anybody do the math? thoughts?
Google how to calculate the volume of a torus.
APF
Who measures the width? Never seen a reliable tyre ruler yet.
Rim width'll effect it too. Most tyre companies use rubber rulers because tyre size is essentially just seen as marketing.
27.5 x 2.6
Wheel diameter wont make a difference - You could have a 36in wheel you would still need the same pressure.
If the measurements are accurate, its a 8.3% difference in width (which will vbe squared in a volume calculation) and a 5.5% in diameter which is enough to say the wider tyre has more volume I'd say (someone can feel free to do the entire calculation incorporating rim width etc).
But I don't think the minimum pressure is related only to the tyre volume, you'd want to calculate the decrease in volume when going over a step as the factor
The width and diameter surely wouldn’t make the same difference to the volume though?
If the cirumference is say 2 inch’s more then you are going to have another 2 inches of tube to fill with air
if you make the tube bigger then it wouldn’t affect the entire volume
I would use the not very scientific method of multiplying 27.5 by2.6 gives 71.5 and 29 by 2.4 gives 69.6 therefore 27.5 x 2.6 wins .
Volume* is almost irrelevant.
Pressure determines the size of the contact patch: a high pressure tyre has a smaller contact patch; a low pressure tyre a larger one.
A wider tyre will cope better with a larger contact patch: roughly speaking one of your tyres is 10% wider than the other, therefore with a 10% drop in pressure it'll have the same length (ie fore-aft) of contact patch. If you imagine your wheel as you look at it from the side is a wooden disc, this is like shaving the same depth of wood from the bottom.
With the lower pressure you'll see more deflection in the tyre as you go over bumps, but since the tyre is larger this is to a large extent (but not fully) counteracted by it being able to deflect more before bottoming out.
But there are so many factors which have contributing effects to the static and dynamic effects (wheel diameter, rim width, sidewall stiffness, etc) that trying to Do The Math is essentially doomed: just work with the basic guideline of "wider tyre means lower pressure" and then fine tune it to see what works best with your particular setup.
(* very roughly proportional to tyre width squared times wheel diameter, FWIW)
@ndthornton, if I understood your post right I disagree - bigger diameter can compensate for smaller cross-section, for some purposes anyhow, as the tyre contact patch makes up in length what it loses in width, and see below also.
@cynic-al, I don't see what the change in pressure has to do with it, my understanding is that the carcass tension and the change in angle the tyre makes with the rim around the contact area is what holds the tyre up. The more you force the wheel into the ground, the greater that angle changes, and the greater the distance over which the angle changes.
The pressure you need will go down with increasing volume for a given wheel diameter, I am not sure volume will be an exact indicator to compare between diameters, though it might give a reasonable guide.
With the lower pressure you’ll see more deflection in the tyre as you go over bumps, but since the tyre is larger this is to a large extent (but not fully) counteracted by it being able to deflect more before bottoming out.
And then it depends what you want from a tyre, on the rear something like a minion SS you don't want the side knobs coming into play before you ask them too, run it too soft and there is a chance of that, same with something like HR2.
The best answer is it varies, lower pressure isn't always the answer.
@greyspoke I was only answering the OP's question* - what can run lower pressures, the limiting factor being getting a pinch-flat (I'm aware many think tubeless makes them impossible, I disagree).
*I don't think volume is the best indicator, more likely cross sectional-area (ie width squared-ish)
I don't think wheel diameter can make up for width - if you have a contact patch of the same area but longer, then the rim is closer to the contact patch, making pinch flats more likely.
Rim diameter does have an effect for two reasons. Firstly that to get the same sidewall compression at the centre of the contact patch you’ll have a longer contact patch with a larger wheel; and secondly the shallower angle of attack will allow a slightly lower pressure before pinch flare occur (assuming that cornering squirm isn’t your limiting factor).
This is obvious if you ride a Brompton and then a road bike. Or a BMX and then an XC 29er. Both tyre widths are similar but required pressure is vastly different as is the feel of the tyres.
@cynic-al. the contact patch will be the same area for the same pressure and load. But imagine loading the wheel up until the tyre is almost pinching. If the tyre widths are the same, the contact patch widths will be very similar, but if the wheel diameters are different, the wheel with the larger diameter will have a longer contact patch, ie more force on the ground. So the bigger diameter wheel will feel vertically stiffer for the same tyre width and pressure, will take more force before it pinches*.
Never mind the chief's Bromptons, this effect was noticeable when moving from 26" to 29" wheels, less pressure was needed for the same tyre sections.
*this will be the case for flat gronud loading, the effect may not be the same for impacts with pointy things and sharp edges.
If the tyre widths are the same, the contact patch widths will be very similar, but if the wheel diameters are different, the wheel with the larger diameter will have a longer contact patch, ie more force on the ground.
If the tyre pressure is the same, the contact patch will be the same area on a flat surface. A longer contact patch will be narrower, regardless of the overall tyre width. Therefore, a larger diameter tyre will have less sidewall flex than a smaller diameter tyre for the same pressure. Whether it actually makes any real world difference to pinch flatting is another question because running over ruts and roots will eliminate the longer contact patch, but reduce the angle of attack of the rim against the obstacle.
I meant contact patch area at full squash @hols2, see my post just above yours - I agree that the situation with pointy things is complicated and it is less clear whether wheel diameter makes a difference.
Just to clarify how I think about this, contact patch area x pressure tells you the force between tyre and ground, sidewall-to-rim angle and carcass tension tell you about how the tyre holds the wheel rim up. The latter would be complicated to model mathematically, but probably tyre engineers have done it. Anyone any ideas where to look for some theory on that?