[Closed] 26er vs 29er (bear with me!) pressure sciencey question

no idea at all.

here's the thing about 29ers (in my humble opinion):

they feel to me like i've found a near-perfect tyre pressure, when all i usually do is pump them right up without much thought.

Depends whether squashing the tyre actually changes the volume of air inside it. If it does then the larger initial volume of the 29er would mean that reduction in volume would result in less of an increase in pressure, so it would bottom out a bit earlier for the same force. But I'm not sure it does. If it just changes the shape of the volume of air in the tyre (but not the actual volume) then there would be no difference.

force = pressure x area, the 29er would have a larger contact patch and therefore would provide a larger upward force for a given tyre pressure than the 26er, so would bottom out later.

I think.

I assume IRL there's some degree of volume decrease/pressure increase.

I also assume the 29er would have a slightly larger contact patch but only due to higher weight of wheel/tyre.

force = pressure x area, the 29er would have a larger contact patch and therefore would provide a larger upward force for a given tyre pressure than the 26er, so would bottom out later.

But using the same equation; if the tyre pressure is the same and the downward force (i.e. weight of bike + rider) is the same then the area of the contact patch is the same. The only way 29ers have larger contact areas (according to that equation) is because they (or the people riding them) are heavier 🙂

I know the answer but I'm not telling you 🙂

BTW contact patch areas for a given weight and pressure are the same regardless of tyre or wheel size

I am not a scientist either.

my guess is both tyres would bottom out at the same time as they are the same height (sidewall). the 29er has slightly more air in it but its the same pressure so its all how those extra few molecules behave...

do i win a texas instrument t30-stat?

Answer will be determined by the force required, not the same as change in pressure or volume.

The smaller wheel will have lost a greater proportion of its volume when bottoming out so my guess is 29er will bottom out first.

Same height of tyrewall? Surely there'd be in proportion and therefore a percentage of the width (as per most tyres) - and therefore taller.

The 26er bottoms out first.

Yes, same contact area and same pressure mean same force. But the shape of the area, or more precisely how the area varies with tyre deflection, differs with 26 and 29. As the rock/root/pavement moves closer to the rim, the 29 having a more gentle curve will extend the contact patch lengthways thus bringing more area and more force into play for the same deflection. The 26 will have to squish down more to get the same contact area so bottoms out first.

This works in practice. At the same pressure I pinch flat 26 much more often than 29.

Edit: this assumes change in tyre volume is negligible.

And in less precise terms, a 2.0 29er feels a lot better than a 2.2 26er if we're talking about tradeoff between grip/trail chatter and risk of pinching. (There is, of course, other stuff to consider in the righteous holy war of wheel size - what's best overall depends on the individual).

Sideshow has it.

29er will bottom out later because its larger diameter can allow a longer contact path before bottoming out. A longer contact patch is necessary as the width of the contact patch is limited by the width of the tyre.

In case you are interested for a nominal 2" tyre the maximum contact length for a bottomed out tyre is 349mm for a 26" wheel and 367mm for a 29" wheel

I can see why that argument might be right, but I can't see how mine is wrong 🙁

If I understand you right Mr Cynic you are saying the pressure will ramp up faster in a 26er as it loses a greater proportion of overall volume during deflection; therefore the pressure will ramp up faster and thus the force as well, preventing a bottom out.

You're right that this effect will certainly be present, but experimental evidence (that we don't pinch flat 29ers so easily) suggests it's outweighed by the long contact patch effect. Of course that could be down to the decreased angle of attack as well, so it's not conclusive. Still, the increase in pressure is small on impact (and zero in the case of fat bikes with holes in the rim - the opposite to a suspension fork where you squash the volume down to nearly nothing). If you really wanted to satisfy yourself of this (and maybe prove us wrong!) the best way is to do the sums and see. Maybe I'll have a go if I'm sufficiently bored at some point 🙂

What sideshow said. A 29er has more volume so you'd need fatter tyres on a 26er to get the same volume and contact patch.

I think if the tyre is the same radius, the contact patch increases at relatively the samr rate as the tyre is compressed.

if you say the

rim width is Rr
tyre diameter is Rt
contact width is L
and sinkage is Ys

then K2 is is the coefficient representing the contact width to the sinkage of the tyre by the simple formula K2 = L/2Ys

then K2 is proportional to Rr/2Rt.

doesn't work if theres tread on the tyres though.

I can't see pressure build up being much of an issue between the 2. The 29 er has more volume but the contact patch is longer. I recon by the time the rim touches the ground both will have had the same fractional volume change

So the 29 er need more force to get the rim to the ground

Both have the same width contact patch

The 29 er has a longer contact patch

When the rim touches the ground the 29 er covers more area and therefore needs more force

Surely the question is built on a false premise, in that there's no reason why you would be setting up 26 and 29 at the same pressure?

I remain unconvinced - I'm not saying I am right but I've still not seen anything to disprove my argument. "Less pinch flats" is meaningless.

And because I've not disproved anyone else's doesn't meant they are right either!

Bigrich that is far too simplistic - L will change (I didn't even bother to analyse it further once I saw that)

ninfan - we just want to understand the maths here.

L will change (I didn't even bother to analyse it further once I saw that)

a coefficient is not a constant chief.

it's a non-linear relationship.

http://digital.library.unt.edu/ark:/67531/metadc13166/

@cynic-al, it's nothing to do with proportion of volume lost.

You've got X lb (bike+rider+landing etc.) applying a force downwards.

You've got a counter-force acting up on the tyre which is a function of the tyre pressure (lb) and the contact patch (sq. in.), so for a given pressure (psi), as the tyre deforms downwards and the contact patch increases, so does the counter-force. For example 30 psi x 6 sq. in. = 180lbs, enough to support a bike and rider weighing 180lbs.

As richmtb has calculated, a 29er tyre has more room for contact patch expansion. Suppose the above rider lands a drop such that they effectively weigh 360lbs. If a 26er tyre can only expand to 11 sq. in. then its maximum counter-force will be 330lbs and it will bottom out.

If a 29er tyre on the other hand can expand to 12.5 sq. in. it can support 375lbs and will not bottom out.

Rough numbers. The difference isn't that stark, but hopefully illustrates the point.

Of course, if we're talking about pinch flats, they're most likely to happen when you hit a sharp edge, so the opportunity for contact patch expansion along the length of the tyre is pretty limited in either case. As sideshow says, any difference in rate of pinch flats could be down to the different angle of attack when hitting obstacles.

Because contact area is exponential 2 and volume is exponential 3 you could deduce that contact area is the dominant factor.

Hence the 29er would support a greater mass without the rim contacting the ground.

However this is on perfectly flat terrain, a rare thing if you're cycling somewhere decent.