Simple question: when tires are marked with minimum and maximum pressures, the maximum clearly has to do with safety on all the recommended rims. But what does the minimum mean, and how seriously should one take it? I have an e-bike commuter, and have mounted Schwalbe Marathon Efficiency 60-584 tires. The pressures on the sidewall are 26-52 psi. 26 psi gives a pretty harsh ride, and Silca’s pressure calculator gives me roughly 19 psi for my usage. How concerned should I be about dropping the pressure 2 psi at a time to see how it feels and corners?
The “minimum” value is ultimately defined by each tire manufacturer and is a relatively high number chosen to reduce the risk of riders damaging their rims in use. There is guidance in ISO standard 5775 that says tires should not deflect by more then 30% of their height when inflated (without specifying how much load the tire should be measured under…), and they do list some minimum values (listed below) based on category, but the printed minimums tend to be much higher than these because it’s in the interest of the tire maker to err on the side of a higher value to avoid liability for damages.
If you’re a heavier rider then the recommended pressures should be given more consideration, but lighter riders can and often should go lower than the “minimum” in practice.
I got curious and went to look at the tires on my bikes - as a 150lb rider my ideal tire pressures in practice range around 60-80% of the marked “minimums.”
(ISO 5775 minimums: 300 kPa (43.5psi) for narrow tires (25 mm section width or less), 200 kPa (29psi) for other sizes in normal highway service, and 150 kPa (21.75) for off-the-road service)
I think the minimum pressure is set by lawyers, based on the maximum rated load.
That’s been a frustration introducing new folks to gravel, especially lighter riders. They have their tires at the minimum shown on the sidewall and then complain about how rough the ride is.
My 38mm Pathfinders are marked 50-80psi. At 160lbs I run in the low 30s on gravel, and a lighter person could easily go lower. 50 would be miserable.
Minimum value also relates to whether the tire will roll off under cornering load. Just be aware and test it out carefully.
On dirt, the ideal pressure is probably not much above that at which the tyre bottoms on the rim; it’s usuallly a good thing if the tyre can deform and maximise the contact patch, and also reduce shocks psssed to the rider.
However, on paved roads a higher pressure is needed to avoid excessive squirming and wretched handling. Trial and error is required, but the various tyre pressure calculators can help find starting points.
If the tyre sidewalls start to come apart and the cords become more visible, the pressure is definitely too low - or the tyres are old and UV or ozone has killed them.
I think your plan of experimenting by 2psi increments is a good one. If you are on a commuter, you likely aren’t ripping corners at the limits of grip, but still good to be cautious on your first ride out with lower pressure.
The two most likely negative consequences as others have mentioned are rim strikes and folding the tire sidewall. You can experiment with both by riding over a representative surface to check for rim strikes (don’t go too fast into it, but make sure you won’t hit the rim all the time on your normal ride) and try to corner with increasing speed to see if the sidewall collapses (it will feel bad and you may hear the tire squirm). If you don’t get either negative consequences under normal riding conditions, it’s probably fine to keep riding like that.
Fun anecdote: I worked at a shop a number of years ago that sold a lot of Salsa bikes. There was a period of maybe a year or two where a couple of the early gravel offerings from them had gravel tires on MTB wheelsets leading to a minimum tire pressure that was above the max pressure listed on the rim. That was fun to explain to customers.
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Thanks for all the replies. Taking it down from 26 psi to 23 psi seems safe, although I’ll probably try it a bit lower in 1 psi increments. 23 is enough that I no longer feel like I’m at a British boarding school being caned for bad behavior.