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What's your correct tyre pressure?

James Spender
6 Jul 2018

The moment you close the valve, your tyre pressures can fluctuate. Cyclist engages full nerd mode to explain all.

Whenever you see a shiny new bike in a shop, we’d bet you can’t resist squeezing the tyre to check the pressure. Anthropologists will tell you this a link to our horse-buying ancestors, for whom checking the condition of the horse’s shoes could make or break a sale.

So for cyclists, tyre pressure is crucial. A handful of psi either way can affect performance. So what tyre pressure should you be running? And when your gauge reads 100psi in the hallway, what does that translate to out on the road?

‘Tyre pressures are vital,’ says Team Sky’s lead mechanic, Gary Blem [interviewed back in February 2014]. ‘You have to take into account rider weight, the type of tyre, the weather conditions and the length of the race.

'Ian Stannard in a rainy Classics race on FMB tyres will be very different from Richie Porte [now with BMC Racing] on a sunny Tour stage on Veloflex.’

Taking the last point first, let’s quickly deal with the question of tyre type. When pro mechanics such as Blem chat tyres, they’ll be talking tubulars, which tend to have latex tubes.

Latex is a much more porous substance than one might imagine and can leak substantial amounts of air over the course of a day.

‘We monitor tyre pressure over training rides to see how much they lose, then adjust them,’ says Blem. ‘Let’s say we use FMB tyres in the Classics.

'These can lose up to 0.7bar [10psi] in a few hours. Then consider that we pump the tyres at 9am at the hotel, and the race starts at 12pm. We have to look at how the tyres will perform from 9am to 4.30pm, so we’ll often overinflate to compensate.’

Such pressure loss in butyl tubes (those common in clincher tyres) is almost negligible as butyl is less porous, he adds.

Yet that’s not to say the pressure of your tyres in the morning will be the pressure at the end of the day.

Formula for success

‘When inflating tyres with air, the tyre pressure should very closely approximate the ideal gas law, PV=nRT,’ says James Shingleton of bf1systems, the company responsible for the tyre pressure sensors on the Bugatti Veyron.

‘Let’s say we assume that n and R are constants [n is the amount of air compressed into the tyre, measured in moles, and R is the ideal gas constant] and that the volume of the tyre [V] does not change [so no stretching or deformation of the tyre].

'This therefore leaves P [pressure] and T [temperature] to change.’

If your 110psi tyres experience a temperature drop from 22°C to 4°C once you leave your house, the tyres will actually be running at 102psi.

Follow this to its natural conclusion and pressure is directly proportional to change in temperature, such that P(final) = P(initial) x T(final)/T(initial), where T is measured in kelvin, ie degrees C + 273, and P is measured in absolute tyre pressure, ie psi + 14.7psi: the pressure of the air at sea level. 

Let’s consider your 110psi tyres are about to experience a temperature drop from 22°C to 4°C once you leave your house.

Ignoring heat build-up from braking or friction from the road, the tyres once acclimatised will actually be running at 102psi. Not an inconsiderable difference.

But should we factor this in? Kevin Drake, Specialized’s tyre development and test engineer, isn’t entirely convinced.

‘No one wants to do the calculations, so having observed real world conditions we use the rule of thumb that a 5°C temperature rise raises pressure by 1psi. So for most riders, temperature change isn’t going to be a problem.’

Weighty issue

The next thing to consider is weight, or specifically the effect of a load on a tyre. 

‘Let’s look at PV=nRT again,’ says Drake. ‘If nRT stays constant, P can only change if V does.’

This relationship between tyre volume and pressure is explained by Boyle’s Law, where P(initial) x V(initial)=P(final) x V(final).

Let’s assume that temperature is constant and the volume of a bike tyre is approximately 1.2l (based on the idea that a tyre is a perfect torus, and the volume of a torus shape is V=2π2Rr2, where r = radius of the tyre’s cross section, and R = radius from the centre of the wheel to the middle of the tyre). 

If we could exact a volume change of, say, 0.1l what might that mean for our 110psi tyre? Rearrange Boyle’s Law and you get the following: P(final)=P(initial) x V(initial)/V(final).

So for our tyre, P2=110 x 1.2/1.1, which equals 120psi. That’s a big change in pressure. Yet it is predicated on a big ‘but’ – the idea that sitting on a bike compresses a tyre to the extent that its volume changes, by 10% in this example.

‘In the case of most properly inflated tyres, the change in volume under load is negligible,’ says Drake. ‘You might see sidewall bulge, but this does not equal volume change but shape change.

'So feel free to inflate your tyres while you’re sitting on your bike.’

But if that’s so, why would a 60kg rider usually run pressures less than a 90kg rider? And back to our original questions, what pressures should we be all be riding?

‘Lower pressures provide a larger contact patch as the tyre deforms under load, so gives more grip,’ says Blem. ‘But if it’s too soft it can increase rolling resistance and you run the risk of impact punctures [pinch flats].

'However, if you overinflate the tyres then traction and comfort are often comprised.’

That means in practical terms a heavier rider will deform a tyre of a given pressure more than a lighter rider, which is why the bulkier rider runs higher pressures.

The sweet spot is the point where traction is good but tyre deformation isn’t making for sluggish handling, and pinch flats aren’t an issue over uneven roads, yet your tyres still offer enough pneumatic cushioning for comfort.

So what is that figure? Ex-Vacansoleil-DCM mechanic Klas Douglas has a rule of thumb…

‘I take approximately 10% of the combined weight in kilos of the rider and their bike – it’s a good benchmark from which to fine-tune.

'For a 70kg rider on a 7kg bike, I’d look at around 7.7bar [112psi], with the front slightly less than the rear to compensate for the weight distribution of the rider. But after that, it’s down to experience.’

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