Tyre Pressure for Small Wheel Tyres

[simonhill]

Not sure if this is a daft question or not.

Should small wheel tyres have same pressure as you put in larger wheel sizes? Given weight (rider, bike, etc) and tyre width are the same. Brompton.

I can think of reasons both ways, so decided to ask those who (will hopefully) know.

[tatanab]

I'd say no. I run my 700C at 100psi, topping up at about 90. I have a small wheeler (20") with modern rims and Schwalbe tyres. First time out I automatically pumped them up to 100 psi - never again. Any little uneven surface, even just the top dressing irregular, was enough to make it feel as if the fillings were going to be shaken out of my teeth. I'd certainly go lower. I have another "20" small wheeler with old style non hooked rims and suitable tyres, these of necessity run much lower pressures which is fine. But that is on a 1970s small wheeled shopper.

[foxyrider]

Check the tyres, the 25c tyres on my Airnimal generally have @ 90psi. OTOH, kids MTB's with similar size tyres run more like 35/40psi so it's not a given. As a general rule, wider is lower and I would expect some decrease with smaller wheels. (some BMX tyres are rated for 100psi!)

[fausto99]

I'd say no. I run my 700C at 100psi, topping up at about 90. I have a small wheeler (20") with modern rims and Schwalbe tyres. First time out I automatically pumped them up to 100 psi - never again. Any little uneven surface, even just the top dressing irregular, was enough to make it feel as if the fillings were going to be shaken out of my teeth. I'd certainly go lower. I have another "20" small wheeler with old style non hooked rims and suitable tyres, these of necessity run much lower pressures which is fine. But that is on a 1970s small wheeled shopper.

Precisely why Dr Moulton put suspension on his bikes and Raleigh put balloon tyres on the RSW20.

[John1054]

I've used 110psi on my Brompton with Schwalbe Marathon Plus tyres, on the advice of my wheel builder, to avoid "pinch punctures". Works well for me over the last three years. Yes, the ride is lively, but I've quickly got used to that.

[softlips]

Brompton reccomend 100 PSI, I tend to run mine at 80-90, that’s with Marathon Racers.

[Tigerbiten]

My best advice is to play around with the pressure abit.
I've found there's a sweet spot just before the road buzz suddenly gets a lot worse as the pressure slowly increases.
On my bent trike it's around 65-70 psi for 42-406 Supremes on the front and 60-65 psi for a 50-406 BA on the back.
Running them softer doesn't make a lot of difference, while running them harder does.

YMMV .........

[Stradageek]

From experience with small wheeled recumbents and upright folders, I'd definitely say "as low as you can go without risking pinch punctures or experiencing significant drag". Depending on load and tyre width I find 50-70psi is my normal operating range. My larger tyred bikes (for the same tyre width and load) run +20psi on those figures.

I've settled on these figures to ensure a) good grip in all weathers and b) lowest rolling resistance on normal road surfaces. Smaller tyres are at an initial disadvantage on both counts due to lower contact area and the reduced ability to roll over uneven surfaces.

Anyone agree?

Cheers

Stradageek

[Brucey]

IMHO you ought to be aiming to use tyre pressures as high or higher than in a large-wheeled bike with similar width tyres. The reason for this is that the tyre footprint is shorter in a small wheeler, and the tyre works in 'about the same way' (in terms of Crr, pinch flats, spring rate etc) when it is similarly deformed. However this may make the tyre uncomfortable to ride on (intrinsic to small wheels, stiff forks etc), and if it does you might want to lower the pressure and take a hit on the other things.

The whole idea of the Moulton was that you could run the tyres at high pressure and the suspension would take care of any ride quality deficit.

BTW the tyre 'as a spring' has a steep rising rate initially, but as the tyre deflects more and more, with increasing vertical load, I think the spring rate flattens off somewhat, in a way that differs with wheel size. I don't think this is widely appreciated, and indeed how it might influence the way the bike handles; on the one hand small bumps can be made to feel vaguely similar with different width tyres (at different pressures) but some of the other handling traits, like wallow when pedalling/cornering hard or riding out of the saddle, work very differently.

cheers

[foxyrider]

IMHO you ought to be aiming to use tyre pressures as high or higher than in a large-wheeled bike with similar width tyres. The reason for this is that the tyre footprint is shorter in a small wheeler, and the tyre works in 'about the same way' (in terms of Crr, pinch flats, spring rate etc) when it is similarly deformed. However this may make the tyre uncomfortable to ride on (intrinsic to small wheels, stiff forks etc), and if it does you might want to lower the pressure and take a hit on the other things.

The whole idea of the Moulton was that you could run the tyres at high pressure and the suspension would take care of any ride quality deficit.

BTW the tyre 'as a spring' has a steep rising rate initially, but as the tyre deflects more and more, with increasing vertical load, I think the spring rate flattens off somewhat, in a way that differs with wheel size. I don't think this is widely appreciated, and indeed how it might influence the way the bike handles; on the one hand small bumps can be made to feel vaguely similar with different width tyres (at different pressures) but some of the other handling traits, like wallow when pedalling/cornering hard or riding out of the saddle, work very differently.

cheers

All of that contributes to why it's taken me a couple of weeks to regain the confidence in handling the Airnimal after a few months exclusively on big wheelers.

[simonhill]

Thanks for the replies.

I'm surprised at how high people go with their tyres (big or small).

I tend to use the Bicycle Quarterly "tire drop" graph as a guide for my pressures and run much lower than most seem to.

The tyres on my Brompton are Schwalbe Marathon Originals Greenguard, 35s (1.35"). Rated (printed on sidewall) at 65 - 110 psi.

I weigh about 65kgs. Add on 12 kgs for the bike - lets say total of 80 kgs.

I'm guessing at wheel load of 30 kgs for front wheel and 50 for rear.

According to the tire drop graph that would give tyre pressure of (about) 33 front and 55 rear.

Even if I stick in an extra 10 to avoid pinch flats, etc, from what most say, these are pretty low.

Any thoughts?

[rmurphy195]

Thanks for the replies.

I'm surprised at how high people go with their tyres (big or small).

I tend to use the Bicycle Quarterly "tire drop" graph as a guide for my pressures and run much lower than most seem to.

The tyres on my Brompton are Schwalbe Marathon Originals Greenguard, 35s (1.35"). Rated (printed on sidewall) at 65 - 110 psi.

I weigh about 65kgs. Add on 12 kgs for the bike - lets say total of 80 kgs.

I'm guessing at wheel load of 30 kgs for front wheel and 50 for rear.

According to the tire drop graph that would give tyre pressure of (about) 33 front and 55 rear.

Even if I stick in an extra 10 to avoid pinch flats, etc, from what most say, these are pretty low.

Any thoughts?

On my Brommie with Marathons, pressures below about 40psi result in so much drag that it feels like the brakes are on, accompanied by a buzzing "helicopter" noise from the tyres. Pump them up as hard as possible with the Brompton-supplied pump (in my case about 45 or so) and you can feel and hear the difference. 50-60 front and about 10 more on the rear seems to do the trick, though on rougher tracks I let a bit out to improve the ride.

[Tigerbiten]

I'm another who thinks around the 60-70 psi range is a good place to start.
Then up/drop it by ~5 psi and see how that affects the handling/feel.
Ideally if you have a local stretch of rough tarmac/ bad chip and seal then you could do multiple runs and find what works for you.

Luck .......

[Mick F]

It depends on the tyres.

I have a Conti GP on the front, and a Conti SC on the rear.
They have different pressure requirements because the casings are different.
The GP is a faster lighter tyre and needs more pressure than the SC which is a tougher thicker heavier tyre.

I had GP on front and rear some time back but the rear one wore out very quickly so went to SC.
At that time, I had 115psi rear and 100psi front for the GPs.

Now, with a SC on the rear, it has only 100psi in it, making 100psi front and rear.

[igauk]

I was contemplating this just the other day. Running 32mm Panaracer Minits on my Moulton. With an all up weight of about 90kg (say 40 front 50 rear) according to Frank Berto’s tire pressure chart I should be running 50psi front and 65psi rear. I tried these pressures briefly but they felt in danger of pinch flats, especially the front. At the other extreme I dropped an entire Co2 canister in each (no idea of the pressure but there was still gas left in each canister and the tube didn't explode). The ride wasn't appreciably worse but grip certainly was. Now at 70psi front and 80psi rear which is a much better compromise but I think I can go up another 10 psi on each.

I worked out the difference in volume for my smaller tyres as a 'correction' to Berto's chart (which I presume was calculated on 700cc).

406 32mm = 1306.09 cubic centimetres
700 32mm = 2251.89 cubic centimetres

So the 406 tube has 58% of the volume of the 700. So increase Berto's recommended pressures by 42%? That gives a front of 71psi and rear of 92.3psi. Not far off where I am at the front but the rear could go up 10psi. Disclaimer - I'm crap at maths!