[Closed] Is anyone else fed up...

You need to get out more!

....pulls up a chair......

Is contact patch how much of the tyre touches the ground? If it does then why doesn't longer=larger (assuming same pressure and tyre width)?

I'm going to be up all night worrying about this now 👿

There's a reason tyre pressure is often measured in pounds per square inch...

ah its just you...dont worry about it...but for the record heres the biggest contact patch (production) tyre...and its 26" rim,29" outer and runs at 5-15 psi 😆

[img] [/img]

So does this mean that if I sit on my 29er, and on my daughters Islabike, which is probably only 14er (?), the contact patch will be the same size (given equal tyre widths and pressures)?

I feel a late night, slightly drunk experiment coming on......

[i]...of non-thinkers suggesting that 29ers have a larger contact patch?[/i]

I'd laugh for a long time if anyone suggested that it made any difference.

Sweet David Dimbleby, WTF is THAT????! 😯

Not if you run it at 90psi (until it goes pop obviously) - but then you don't do that do you?

that, my pointy eared friend, would be a pugsly.
goodliness. i want one, but i'm not altogether sure why.
probably for the same reasons i want a big dummy.
what's wrong with me?

the OP is correct of course, for the same reasons that the neds fitting carlo vandango super wide wheels and tyres aren't actually getting any better grip.
if you don't change the weight of the vehicle it makes sod all difference, but there are other factors, such as puncture resistance ( from pinches ) etc. on mtbs that can be factored in too, so in some cases an increase in volume is worthwhile.

OK, then if (as you said) the contact patch is longer, then if the tyre is the same width, the area would be larger, no?

[/sleepy]

just to be clear I've never suggested anything to the contrary, neither to I own a 29er.

i want one

I ALSO WANT ONE.

To ride over the faces of my enemies, laughing as I crush them into the dirt where they belong like the dogs that they truly are.

I assume you all work in it. Please provide excel spreadsheets that the rest of us can ignore and just carry on riding.

Work in what?

IT.

[img] [/img]

Work?

What type of science or engineering is tyre/pressure/width/size? Looks like there is a PhD project in there somewhere.

Well if you do the math and work out area from length x width using the same tyres and rider/bike then you could see if the contact area is increased or decreased.

Then the 2nd experiement would be to setup a jig or controlled rig for grip on different surfaces.

Might be fun if you can get funded and there I was at 17 looking at Material science degrees/Dunlop work experience with motorcycle tyres and thought nahhhh... Dam it would be fun!

What we need is a time machine for us mid life crisis folk! 😆

Are we talking about the contact patch on a perfectly flat conveyor belt (where theory reigns supreme), or are we talking about out in the real world, where the world is rarely perfectly flat?

Lactic - Member
So does this mean that if I sit on my 29er, and on my daughters Islabike, which is probably only 14er (?), the contact patch will be the same size (given equal tyre widths and pressures)?

I feel a late night, slightly drunk experiment coming on......

Yes (also assuming that the Islabike weighed the same as your 29er). Tyre width is irrelevant in terms of the size of the contact patch but will affect the shape.

Been (sadly) thinking about this and I have to disagree.

I can certainly get my head round theory that for a given unladen pressure (eg 40psi) 2 identical width tyres, (26" and 29" dia, x 2" width) would distort under an identical load (say 160lb) until the contact patches are both the same - in this instance, 4sqin.

HOWEVER, if we take into account the dynamics of the distortion process, a 26" tyre would need to distort (squash) more than the 29" in order to achieve the same 4sqin contact patch, therefore there would be a greater percentage change (reduction) in volume of the 26" tyre and corresponding increase in pressure within the 26" tyre which means that the net contact patch required to maintain equilibrium would in fact be smaller.

Ergo, unless the tyre pressure is measures when the bike is laden, for a given tyre pressure, and identical rider conditions, the contact patch on a larger wheel WILL in fact be larger.

the contact patch on a larger wheel WILL in fact be larger

But will that difference be significant or negligible?

i.e. a 185lb man on a bike with tyres inflated to 35psi would have a nominal (total) contact patch of 5.3"sq. Obviously as the air in the tyre is compressed the pressure ramps up and the contact patch is actually a bit smaller. But is the difference between a 26x2.1" tyre and a 29x2.1" tyre going to be 1%? 10%? Will anyone notice? Care?

BLaasadkoaghiuyt98`6t

Longer possibly, but not larger.

does this mean it equates to more lateral grip but less traction?

The answer may depend on other issues such as sidewall compliance under loading.

Tyre pressure measured in a static bike which does not have someone sitting on it may be an oversimplification.

It ain't necessarily that simple.

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Another Angle on 29ers
Imagine a skateboard wheel running over a series of 1" bumps. Since the skateboard wheel has a radius of only an inch, the wheel has to move 1" vertically in just 1" of horizontal motion every time it contacts a bump. This is an abrupt change, or high angle of attack has drastic effects on the rider's motion. With a 20" BMX wheel, that same 1" of vertical motion happens over about 4" of forward motion. This turns what was a radical bump into a mild annoyance. Now ride over the bump on a 26" wheel, and you have almost 6" of horizontal distance to spread out the impact. The bump is hardly noticeable. Not only does the bump disturb your riding less, but it also has less effect on your speed. Bigger wheels maintain your momentum. This effect is somewhat similar to that of a suspension fork, except that it happens on all bumps, regardless of suspension fork tuning. Tire pressure changes the effect somewhat, but with the same results regardless of tire size. Since riding off road is normally on less than smooth surfaces, the effect of a larger wheel is constantly enhancing the ride. If both wheels are larger, the effect works on both ends of the bike. On the Fisher 29ers, you'll notice this effect in two ways. First, the bike will ride smoother and more comfortably. Second, the bike will be slightly faster, especially on broken or rough ground.

Bigger Wheels Have a Longer Contact Patch
The contact patch is the amount of tire you have on the ground at any one time as the wheels goes through its rotation. As The diameter of the wheel increases from 26" to 29", the shape of the contact patch gets longer. The benefits of this are noticeable. For technical climbs, the added length increases traction as the rider can shift their weight more freely without losing pressure on the contact patch. In addition, the increased contact patch length on a 29er benefits the rider when cornering in uneven terrain as the tire will want to stay in contact with an obstacle and not skip over it. Last, in mud, sand or snow, the longer contact patch can help keep the tire on top of the surface as opposed to sinking in and bogging down. Think about snowshoes. They are designed to keep you on top of the snow, where the going is easy.

thank you mr fisher

Oh god...

Between a 26" and 29" it won't be significant.

The volume of a torus is 2(pi^2)R(r^2) where R is the radius to the centreline of the torus (lets assume this to be the radius of the wheel, it's not quite, but it's close enough to get an order of magnitude difference in volume) and r is the radius of the "donut" in this case the tyre/tube.

If r is the same between the 2 tyres (eg both using say a 2" tyre width), and the only variable is R then everything cancels out and the volume ratio of the 2 tyres is purely 29"/26" = 1.115 or approx 11.5% more volume in the 29" tyre.

So a given volumetric distortion % (lets call that X) there will be 0.115X more increase in pressure in the 26" tyre.

I can't be bothered to work out exactly what that will equate to, but if say the tyre volume changes by 5% (a pretty high estimate to be honest) then there will be about +0.5% (5% x 0.115) difference in pressure change between the 2 tyres. So to then use the example above if they both started at 40psi, one would end up at 42psi (40x1.05) and the other would be 42.24psi (40x1.05x1.00575).

For a 160lb rider, this makes the contact patches 3.809sqin and 3.788sqin for the 29er and 26er respectively.

Completely insignificant and unnoticeable I happily admit, but still theoretically slightly greater for the 29er.

(BTW I don't ride a 29er, I'm not an evangelist, just a bored ex-engineer)

tazzymtb

Snowshoes work partly because they are wider and reduce the pressure loading. Which is also why I have fat skis for powder.

Your analogy is false however because the size of the tyre contact patch is mediated by the tyre pressure.

take it up with that gaz fisher chappy, he's the one who started advocating big wheels and that's his info. i couldn't give a monkeys, but I do know that on the same loose gravelly climb with the same rider, same tire design (racing ralph) and same pressures my 29erss climbs better than my 26er ss.

I don't care how it works I just enjoy the extra grip (however it's derived)