Tyre pressure

tandrews8664

Had my first puncture of this year today, and on the worst day of the year so far!!!

This has more to do with my tyres than their pressure tho, as this was the first time out in the rain on ultra race rather than gator skins, the latter have never punctured.

But how much pressure should I have in the tyres? I'm running them at max 120psi but recommended is 110. Which is better in the winter and rain? Max or recommended pressure?

Mark Alexander

Do what it says on the rim. Unless you're a tester and then get as much pressure on it before it blows. This can be a juggling act by the way.

John Stevenson

A bit less pressure in the wet will improve handling, and make very little difference to puncture-resistance.

Unfortunately punctures become a lot more likely in the wet because water acts as a pretty good cutting lube, helping glass shards etc slice through the tyre.

Secteur

If the manufacturer reccommends 110, then why have you pumped them to 120? Do you know something the manufacturer doesnt?

bluesparx

I would always stick to recommended pressure. If you pump it to the max on a cold morning it could well go over it on a hot day or during a hard ride. Although it wont immediately blow above the max, it is there for a reason.

nochekmate

Fair bit of hedge-cutting going on at the moment, which never helps the puncture count!

tandrews8664

I have it at 120 because I am only around 12.5 stone and am under the impression that recommended pressure will be on the lower side to compensate for heavier loads, and therefore have it at 120 to reduce rolling resistance

I think I'll reduce them to recommended pressure tho

sungod

tandrews8664 wrote:

I have it at 120 because I am only around 12.5 stone and am under the impression that recommended pressure will be on the lower side to compensate for heavier loads, and therefore have it at 120 to reduce rolling resistance

I think I'll reduce them to recommended pressure tho

that sounds like you're saying you increased pressure because you are lighter

at around 80kg, 90-100psi front and 100-110psi rear will be fine

the low end of the range is better in the wet as the tyre has more chance to conform to the surface and give more grip

unless you are on a smooooooth track, higher pressure will result in the tyres bouncing/skipping on all the surface irregularities, in effect, rolling resistance actually increases

John Stevenson

Tyre manufacturers are typically very conservative with their recommended pressures. They're usually around 50 percent of the blow-off pressure, depending on the tightness of the tyre/rim combo.

That means that it's extraordinarily unlikely you'll blow a tyre off just because it's a hot day. If your tyres start at 120psi in the morning and the temperature is 10C, but it gets up to 35C during the day, the pressure will only rise to 130psi, because pressure rises with the absolute temperature (283K and 308K in this case) and not the temperature in Celsius. Boyle's law and Charles' Law are in action here for those who remember secondary school physics.

You'll dump a lot more heat into the rims and tyres braking on long descents, but this is nevertheless very unusual and requires Alpine or Rocky Mountain descents.

It's different for tandems, however, because the increased weight and higher speeds mean there's a lot more energy to dump into the rims as heat. This is why tandems often have a rear drum as a 'drag' brake and why very few hydraulic disc brakes are recommended for tandems - the heat would boil the hydraulic fluid.

The reduction in rolling resistance between 110 and 120 psi is pretty small. You get into diminishing returns when you go much above the recommended pressure. Worth it for a time trial, but not, IMO, worth the degradation in ride quality and handling that comes with over-inflated tyres in general riding. You'll probably get more benefit by dropping your stem and so reducing your frontal area!

pilot_pete

Have you seen this chart? http://www.ctc.org.uk/DesktopDefault.aspx?TabID=3802

With your weight of 80kg, the pressure chart recommends a minimum of 105psi rear and 75psi front (assuming combined weight of bike and rider + bottles/food and kit of 90kg) if using 23mm tyres.

If you are going to ride on less than smooth roads these recommendations go up to 120psi rear and 80psi front.

I am 82kg and I use between 100-110psi in my rear and 80-90psi front. I have Continental GP4000s 23 mm tyres and max inflation printed on the sidewall is 120psi. I have noticed the ride is much harsher if I put 120psi in the rear and 100psi in the front. The front definitely grip better with the reduced pressure.

In the wet I drop my pressures to 95-100psi rear and 75-80psi front. These makes a noticeable difference to handling/ grip/ ride and doesn't seem to lead to loads of punctures. (oh no, that's done it....I'd better order a box of tubes now!!!)

PP

rick_chasey

6 bar!

oldwelshman

90 psi for wet and 100 to 120 max in dry.
I do not see a point in higher than this and have yet to see any evidence of less rolling resistance for tyres with more than 120psi.

John Stevenson

oldwelshman

Take a look at this graph:



Discussion here

9 kg/cm^2 is 127psi. You can see that RR continues to decline with pressure over 120psi. This is as expected.

The point is that it doesn't decline much, and that for non-TTing applications the reduction is offset by worse ride and handling.