Cycling UK Forum

Thanks.

It's the "given equal pressures" and "higher pressure would be needed" that I can't follow.

Jonathan

I read it as meaning that in smaller wheels, the contact patch being wider (for constant contact area) then the sidewall has to flex more than in the same section tyre fitted on a larger diameter wheel. But maybe I've misread it.

cheers

With smaller diameter wheels, the contact patch is shorter and so to maintain the same total contact patch area (given that weight of rider and bike = 2 x tyre pressure multiplied by contact patch area) the contact patch is wider which entails more deformation of the tyre sidewalls, hence more flexural/hysteresis losses in the rubber as the tyre rolls, assuming equal tyre pressure and width on the smaller and larger-diameter wheels (and equal bike plus rider weight).

bgnukem wrote:With smaller diameter wheels, the contact patch is shorter and so to maintain the same total contact patch area (given that weight of rider and bike = 2 x tyre pressure multiplied by contact patch area) the contact patch is wider which entails more deformation of the tyre sidewalls, hence more flexural/hysteresis losses in the rubber as the tyre rolls, assuming equal tyre pressure and width on the smaller and larger-diameter wheels (and equal bike plus rider weight).

As a heavy rider you find larger 700c with slimmer tyres have to be run at a very high pressure with no deflection of the sidewalls as hitting a pot hole would end up with going down to the rims so the contact point is very low. In contrast a wider profile tyre you can have some deflection which is what the tyre companies recommend as even when you hit a pot hole you won't be down to the rims. If I need 120 psi on a 700x25 tyre you may find 65psi is fine with a 26"x 1.5" tyres. There is always more contact with larger profile tyres in my experience because you can run them at lower pressures. Some lighter riders ride tubeless mountain bike tyres sub 30psi to give that extreme contact. The difference between 559, 584 and 622 is not huge compared to the profile/width of tyres which could go from 23mm to something like 70mm.

622x23 =11.5x11.5x3.14x622=260k square millimetres
559x70=35x35x3.14x559= 2.15 million square millimetres

So according to my rough possibly wrong calculations you can see how minor increases in wheel size are nothing compared to tyre width and I'm sure fat bikes would show an exponential increase over normal mountain bike tyres. I couldn't see a comparison of contact area sizes of different tyre size but they would reduce as tyre pressures increases.

There is a complication when trying to work out the effective circumference of a tyre. It is (as I look at it) pi x Radius, but the "radius" in question is not that as measured by a tape measure with no weight on the bike. It is the radius from hub centre vertically down to the ground with the weight of the rider on the bike. Which is a smaller radius than you get by measuring in other ways. So a wheel with a big fat tyre at low pressure will have a smaller effective radius than might appear to be the case. I think that can throw out the setting up of cycle computers.

However, the flattened contact area on a wide tyre at low pressure will iron out some road irregularities and thus aid efficiency.

Brucey wrote:I read it as meaning that in smaller wheels, the contact patch being wider (for constant contact area) then the sidewall has to flex more than in the same section tyre fitted on a larger diameter wheel.

bgnukem wrote:With smaller diameter wheels, the contact patch is shorter...

Thanks, both. That feels right to me. But why is the contact patch shorter?

Jonathan

Jdsk wrote:

Brucey wrote:I read it as meaning that in smaller wheels, the contact patch being wider (for constant contact area) then the sidewall has to flex more than in the same section tyre fitted on a larger diameter wheel.

bgnukem wrote:With smaller diameter wheels, the contact patch is shorter...

Thanks, both. That feels right to me. But why is the contact patch shorter?

Jonathan

Because the circle is tighter. It curves up in a tighter arc.

It is. But you need to take compliance into account to make any estimate of the size of the contact patch at equilibrium. So there is an assumption about compliance in there somewhere. (I think that it's correct.)

Jonathan

When thinking about these things I find it useful to come up with exaggerated examples to tease out the principles. Imagine a 28mm wide tyre but on a rim the size of a Ferris wheel. I think it is easy to see how the contact patch would be longer than with a 28" wide tyre on a 20" wheel, both tyres at the same pressure and both with the same weight on them. I know that would be impossible to achieve, but imagine you could do it....

Even Alex Moulton recognised that a small radius wheel has problems on real world surfaces unless you mitigate that with suspension.

Mmmmmmm.... thought experiments.

Me too.

: - )

Jonathan

Brucey wrote: 26" wheels are still the best choice for lugging a really heavy load but they are fast becoming less popular everywhere else. The effect of all this is that the choice of 559 rims and tyres is diminishing at a rapid rate.

cheers

This is probably true but by chance I was in Wilkos in Derby today and had a look at the tyres they had for sale. Obviously Wilko is not a bike shop but I think they are indicative of the sort of place you might end up buying emergency tyres when your preferred Vittorias or Schwalbes explode on that apocryphal tour of far-off places. So, for sale in Wilko Derby, Halloween 2020:

1 type of 700C tyre, a chunky wire-beaded thing about 32-35mm wide and vaguely similar to a Marathon;
1 type of '27.5"' folding knobbly, one only for sale. Interestingly the packaging had no ETRTO information at all, just the 27.5" labelling.
4 different types of 559/26" tyres, one folding knobbly and three rigid, comprising one knobbly, one sort of semi-slick and one slick 'with puncture protection' which was rather hefty and not what I would choose as a premium tyre, but would probably get you happily to your destination.

I think fears about the disappearance of 559 tyres are somewhat over-exaggerated. I accept that there may not be quite the choice in 559s in a few years time than there is now - although I think in general that there has never been so great a choice and specialisation in tyres as there is today - but I find it hard to believe that there will come a time when decent 559 tyres for on- or off-road use are unavailable. Looking at the SJS website shows tyres for sale in all sorts of arcane ETRTO sizes such as 288, 390, 540 and 635 and many more. You might not be able to get precisely the perfect tyre you want in 559 in fifteen years' time, or maybe they'll be the New Thing again all those people who banked on 584s will be losing sleep instead, but there will be something..

And apologies once again for mentioning Moultons, but there have only ever been about five 17 x 1/14 (369) tyres available for Moulton F-frames and AMs since the mid-1960s, and usually only one available tyre at any one time*. None of these tyres have ever been perfect either. But Moultoneers have just grumbled and got on with it. Especially in this internet-enabled, next-day-delivery, ultra-connected world, you are realistically never going to be hamstrung by the inability to get hold of tyres that will fit your bike.


*Admittedly there was a period in the late 1970s and very early 1980s when there were no production 369 tyres (or production Moulton bicycles), and some mileage-heavy Moultoneers were forced to convert to 349 wheels.

what usually happens in less common (on new bikes) rim sizes is that you might end up with a handful of 'quality' tyres to choose from (if you are lucky), and when touring you are best advised to carry a spare tyre. I don't think we will live to see the extinction of 559 tyres but we are already in the time where tyre/rim choice/availability is a factor when choosing which wheel size to use.

BTW a straw poll of tyres available in Wilko can give some interesting results; it might be that what you see is what is left over. I have seen them 'clear the decks' at this time of year and sell all their tyres to make way for new stock. Depending on when you look you might think that they only sell 27" tyres for a pound each...(yes really)

cheers

Brucey wrote:what usually happens in less common (on new bikes) rim sizes is that you might end up with a handful of 'quality' tyres to choose from (if you are lucky), and when touring you are best advised to carry a spare tyre. I don't think we will live to see the extinction of 559 tyres but we are already in the time where tyre/rim choice/availability is a factor when choosing which wheel size to use.

BTW a straw poll of tyres available in Wilko can give some interesting results; it might be that what you see is what is left over. I have seen them 'clear the decks' at this time of year and sell all their tyres to make way for new stock. Depending on when you look you might think that they only sell 27" tyres for a pound each...(yes really)

cheers

yep if passing (at least in normal times) I often pop into wilcos to check out their bike department - still regret not buying some interesting looking cheap chains they had once for a very short period - they seemed to come from nowhere (had never seen before) and then just as quickly shoot back to nowhere. Maybe they have some rather opportunistic buyers.

I ride both 26 and 700c bikes. They are however quite different as the 26 is a Rohloffed Surly Troll and the 770c is a Genesis Tourer.

It is very hard to tell which is most efficient as the bikes are so different but my observations are:

Tyres make a huge difference. I was using Coni winter tyres on my Troll. They are lovely tires and the bike rode well with them. Even though they are winter tyres and obviously not exactly fast, I didn't think they were super slow but I changed them for some Rene Herse Rat Trap Pass for the summer. The difference was amazing. The bike was faster, smoother and much more fun.

On smooth roads the difference is minimal. I think that on gravel or softer surfaces larger wheels start to show an efficiency advantage.

Lots of the differences are in my head. I was happy with my 26 inch wheels. Then the trends changed and I read about how bigger wheels are better and I started to feel that my Troll was slow... Whilst I think 29+ wheels roll very well off road, for on road and grave track touring and commuting the 26 wheels with good tyres are fine. I toy with the idea of changing the Troll but then I think to myself that I am about to dismantle a really good bike and spend a small fortune all for 11mm or so.

The 26inch wheels make the bike really nippy through twisty paths and junctions.

Not sure the above is much help. I'd say go with what ever you gut feeling tells you...