kelvin – Member

I certainly don’t want to be thinking where is my next ride going to be and what tyres should I put on for it ?

This whole thread is about “tyres”, so stop wasting your time here if you don’t want to think about them.

Changing tyres is easier than changing wheelsize, and it’s effective, but it doesn’t sell new bikes.

This thread is about a magazine saying 650 and 29er tyres grip better than 26″ tyres . I don’t really think I’ve strayed off topic . You seem to be trying to turn it into a wheelsize debate .

honourablegeorge – Member

Think your physics are being a bit too basic here. If you were dealing with perfect, circular wheels that didn’t deform in contact with the ground and had identical curved profiles, then yeah. But realistically a fatbike tyre will have a much squarer profile, and a wider contact patch, even if pressure/deformation etc are all equal.

You re getting confused – it really is that simple – there is no factor other than pressure.

I was fairly gobsmacked at the utter twaddle I was reading, I’m no maths expert, but the size of the contact patch on a 650b can’t be large enough over a 26″ to offer any noticeable difference?

So, back to this… same tyre, same width, same tread, same compound ,same rim profile… any “noticeable” difference in contact patch between 26 and 650b versions? Anything more than messing around with pressures would achieve?

cynic-al – Member

You re getting confused – it really is that simple – there is no factor other than pressure.

I’m not remotely confused – a cylinder and a sphere won’t have identical contact patches – length yes, but not width.

I didn’t say identical, I said identical area.

You’d still be wrong, then.

prove it.

This is the internet. I don’t have to prove anything.

Cynic-al is right

Its as simple as this:

Say you and the bike weigh 200lbs

Each tyre* is supporting 100lbs If each tire has 25 psi of pressure it must have a contact patch area of 4 square inches

Shape of the contact patch differs for different wheel and tyre sizes but the area doesn’t as long as the weight the air in the tyres has to support remains fixed.

*the back tyre will have more weight on it on the front but the example still stands

Precisely.

I win.

Physics explained…

So given the simple physics lesson Giant’s claims have a very distinct whiff of shite about them.

Contact patch area for 26″ and 27.5″ has to be the same for the same rider with the same pressure in their tyres.

Contact patch length?

I’m not clever enough to work that out but if the tyres were the same width I can’t see how the contact length would differ by 2 cm (a 33% increase!) when the difference in circumference of the wheels is less than 5%

I’m not clever enough to work that out but if the tyres were the same width I can’t see how the contact length would differ by 2 cm (a 33% increase!) when the difference in circumference of the wheels is less than 5%

there will be a difference, theoretically, in “feel” a short wide contact patch will be different to turn compared to a long thin contact patch. Once you throw in rocks, roots, wheel diameter i doubt you would feel it in the real world.

I’m not clever enough to work that out but if the tyres were the same width I can’t see how the contact length would differ by 2 cm (a 33% increase!) when the difference in circumference of the wheels is less than 5%

Nope, I can’t make the maths work for that either. I can see that the length would increase, but can’t make it jump by that amount no matter what assumptions are made.

When you increase from say a 2.1 to a 2.3 tyre on a 26″ wheel you generally run it at a lower pressure, because of the increased volume?

I might be missing something here, but assuming you’re running the same width tyre on a 26″ and a 29″ wheel wouldn’t you run the tyre on the 29″ wheel at a lower pressure because of the higher volume of the tyre, which in turn would give a larger contact patch?*

* this could be completely wrong.

Nope, I can’t make the maths work for that either. I can see that the length would increase, but can’t make it jump by that amount no matter what assumptions are made.

Fit a really narrow tyre and let all the air out 🙂 I doubt that Giant have made it up, but I bet they’ve made some very unrealistic assumptions.

For the record I have a 26″ bike and a 29er and I’ve never felt this supposed increase in grip. The bike that grips best is the one with the grippier tyres. If they both have the same make/model of tyre fitted then it’s the one with the lower pressure. Beyond that I really can’t feel a difference in grip, but then again I’m not a particularly sensitive soul.