7

I changed a 38-622 size tyre to a different brand 37-622 one. The original tyre stated a pressure range of 40-60 psi, while the new one states 50-85 psi. What are the consequences of going below the recommended lower limit (50)?

The bicycle is used by a light rider, typically with pressures between 40-50 psi for comfort reasons. This bike is mainly used for getting around town, sometimes on gravel roads.


EDIT: I received many useful answers about how to decide whether the pressure is too low. One aspect of the question that was not fully addressed is why there is a different pressure recommendation between two different tyres of (almost) the same size, and whether the type (rather than size) of the tyre really makes a significant difference in how low one may go with the pressure.

10
  • 1
    What model tires are involved here? Or at least, are you on a hybrid bike, a gravel bike, something else? I'm not sure it materially affects the answer, but on my performance gravel bike, I'd typically run my tubeless tires under 40 PSI, maybe a lot lower; they're nominally 38mm tires.
    – Weiwen Ng
    Apr 29 at 13:23
  • @WeiwenNg "Riveside 120" size small hybrid bike from Decathlon with original tyres. The new tyres are Schwalbe Smart Sam 37-622. I made a mistake: the originals are 38-622, not 37-622, but does that make a real difference?
    – Szabolcs
    Apr 29 at 14:45
  • Does this answer your question? What is the significance of tire minimum pressure? Apr 29 at 23:51
  • 2
    Why has no one chimed in with "higher rolling resistance"? True, with gravel roads, the more stable ride would offset the rolling resistance (jumping up and down takes up energy too)...But it is worth knowning about.
    – Aron
    Apr 30 at 4:23
  • 3
    I think (as I said under Michael's answer) that you were probably riding the previous tire successfully with <50ps, which makes all mechanical issues like stiffness against puncture, rolling resistance etc. moot: You were just fine, thank you. But you wonder: Is there a difference in the tire that makes a higher minimum pressure mandatory? (Like, it may be damaged or comes off.) Apr 30 at 22:08
8

As others have answered already, the most common problem with low pressure is pinch flats

I think with thick and inflexible sidewalls it can also damage the sidewalls of a tire over time. However if you have flexible, lightweight tires (e.g. cyclocross/gravel or performance oriented MTB tires) this shouldn’t be a problem.

At very low pressures the tire can “roll off” the rim in tight turns.

I think I’ve written this on StackExchange several times, but my favorite method for gauging minimal tire pressure is to put the front wheel on a sharp corner (e.g. edge of a curb or stair) and push down on the handlebars with your whole body weight. If you can press it down all the way to the rim the pressure is probably too low, unless you ride very carefully.

Of course low pressure increases rolling resistance, but with good, performance oriented tires the increase is surprisingly low.

The Schwalbe Smart Sam is a pretty good and supple tire. 40 psi (2.8bar) is low for a 37mm tire but can still be fine with a lightweight rider (how light are we talking here? 50kg? 60kg?) and bike. But you should be careful when riding over potholes, roots, tram tracks etc.

As a 65kg person I went as low as 2bar on 33mm tires in cyclocross.

5
  • We're talking low 50s and non-aggressive riding. Thanks for the practical advice!
    – Szabolcs
    Apr 29 at 15:02
  • Since you mentioned cyclocross, I'd just add that a CX tire would probably burp well before it rolls off. For others, burping means that the beads lose contact with the sidewall and you lose pressure. It can happen at low pressures in turns or when hitting something.
    – Weiwen Ng
    Apr 30 at 13:21
  • @WeiwenNg: I assume that’s only possible with tubeless?
    – Michael
    Apr 30 at 20:38
  • You're quite right, I forgot to mention that I was thinking about tubeless clinchers. I don't know what would happen to tubed clinchers if you ran them at low enough pressure to burp them, but that could result in a flat.
    – Weiwen Ng
    Apr 30 at 20:47
  • 2
    Apparently the OP has ridden with <50psi and the old tires; that should provide more or less the same resistance against pressure (because the tire as such doesn't add much, as one can easily see when there is no air in it). So the OP's question is probably "is there anything that can damage the new tire if I ride the same pressure I was fine with on the old one?" Or, from a different perspective: What is the 50 psi lower threshold based on? Apr 30 at 22:03
6

There is flexibility in tire pressures. Try the lower pressure, keep an eye on how compressed your tire is. If it feels like bumps are hitting the rim increase the pressure. A quick ride around the block should tell you if the pressure you choose will work. If you bring a pump or other tire airing tool on your ride you can adjust on the road.

40 to 50 PSI on your new tire will probably be fine.

As tire pressure goes lower the chance of pinch flats goes up. At an the extreme end the tire will slip and spin the tube (for tubed tires) possibly cutting the valve stem.

I've run off road mountain bike tires with as little as 10 PSI with no ill effect (they were 2 inch tires - so further to go before pinching).

4

The biggest risk of underflation is pinch flats. Pinch flats occur when the tube gets pinched between two layers of the tire. The design of the tire influences the minimum and maximum pressure. Size, material and intended use come into play along with the average rider weight the engineers used in their calculations. If the rider is well below what would be deemed average then you can reduce the pressure to an extent. I would start at the minimum recommended pressure and if need be drop it a couple of pounds per ride. You want to avoid being so low that pinch flats are an issue or you start riding on the sidewalls.

4

The tyre itself can be damaged when run for a too low pressure. That will be highly dependent on the tyre construction.

With tubeless tyres you can see the consequences when the sealant starts to penetrate the sidewall: White foam around tubeless tires

3

One problem is rim roll-off, which can also be described as squirm.

This is probably not going to happen at 50 PSI, but if your tyre is very low on air, going around a corner can feel awful. The tyre tries to follow its original track while the rim is forcing it to turn, so a low-pressure tyre can be "wrung" between the two.

It's an unpleasant feeling and you have to stop immediately to add air.

2

What are the consequences of going below the recommended lower limit (50)?

Pinch flats. If you ride hard over an obstacle, the low pressure inner tube will develop a pinch flat that causes a "snakebite" puncture on the inner tube.

If you ride tubeless instead, the tire will bottom out and has a risk of rim damage. (Every pinch flat on tube-type tires is also a risk for rim damage.)

I ride 100psi on 28mm tires (though I inflate so rarely that occasionally the pressure can go as low as 75psi due to natural air leakage). If I scale linearly inveresely to 37mm, I get 75psi as a good pressure to use in initial inflation (and reinflation is required at 55psi).

If you inflate below 50psi, you are already below the recommended limit for reinflation (55psi).

I'm a rather heavyweight rider who has a tendency to ride fast across every obstacle, not wanting to lose speed. If you don't ride as hard and your weight is average or lower than average, then I could understand less than 50psi. Otherwise I can't understand such low pressures. Besides, the lower the initial pressure, the lower the pressure will be when you reinflate (and if you reinflate every 2 weeks, the pressure will be very low then.)

2

Apart from the key points users have mentioned above, too low of a PSI will also cause excessive heat on the sidewall due to increased friction as the tyre contracts and expands under load. This will lead to severe damage (namely, sidewall damage which begins first on the inside of the tyre as you'll see chunks of melted and broken off pieces of rubber).

1
1

Efficiency is reduced a lot.

Turning can become less controlled at high speed, as the tyre deforms sideways against the road from being too soft: if you turn the front of the bike, the back will try to keep going forwards, reducing your turning radius at speed.

Low pressure will cause the rim to hit stones and corners, causing a pinch flat.

If you have a super durable and light tyre, the rubber bending in the middle will fatigue the side wall material a bit faster, although it won't pop unless the rubber it's old and fragmenting.

Whatever is safe against pinch flats should be Okay, a bit less than MFR specs is fine, but much below that, it will pinch on a stone in the gravel. The front can be flatter than the back to be he same shape while riding, because the back carries more wieght.

enter image description here

5
  • 3
    A word of caution about rolling resistance: if you go a bit further into the link, you'll find that yes, rolling resistance decreases as you increase pressure up to a point. After that breakpoint, rolling resistance increases very rapidly. Basically, it's a lot less risky to leave the tires a bit underinflated than to overinflate them from a rolling resistance standpoint - you obviously don't want to underinflate so much that you risk pinch flats.
    – Weiwen Ng
    Apr 30 at 13:35
  • The front tire cannot reasonably have a pressure lower than the rear. When encountering an obstacle, you are already braking at full force. It means all of your load is on the front tire, and none of your load is on the rear tire. The only case when all of the load is on the rear tire is when climbing a steep hill seated. You don't get pinch flats climbing a steep hill seated because the speed is so low. So for considering pinch flats, the front needs MORE pressure than the rear!
    – juhist
    Apr 30 at 16:14
  • 1
    @juhist Yet the usual advice is to use less pressure in the front than the rear. It would seem that most people use less pressure in the front than in the rear. Despite that, most people are saying they experience more pinch flats in the rear.
    – Szabolcs
    May 1 at 9:56
  • Braking so hard that all the weight is on the front and then running into a pothole: One doesn't worry about pinch flats.
    – gschenk
    May 1 at 21:50
  • 1
    @juhist I mostly agree with your analysis.. When braking so hard over has to go straight. Squirming, rolling of the tyre, sidewall wear and pinch flats are no concern then. The only thing that matters then: does the tyre hold on the rim and is there enough traction. However, there usually is plenty far below pressures that would matter for lateral stability.
    – gschenk
    May 1 at 21:54

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.