Exceeding Max PSI
 

My tires are marked as 85psi Max. Is there any rule of thumb as to how much higher you can safely go? My floor pump shows them inflated to 85 but my hand held gauge shows 90.

Is there a reason why you need to go over the maximum PSI?

That's the real question.

The supposed benefits of more pressure are highly over rated, so while you can go beyond max rated pressure, the first question you need to answer is why do you want to.

If you're very heavy that might be a good reason, but it's usually better to use wider tires instead. Here's a guide to tire pressures based on axle weight and tire width.

Now, if you do decide to go above rated max, 10-20% is almost always OK, often up 30-40% is OK, but it depends on the quality of the tire, the construction at the bead, and the type of rim. There's also the effects of the added stress on the rim's flange, which is proportional to the tire width X the pressure (see formula for "hoop stress"). On a new rim there should be plenty of reserve, but tires wider or at higher pressure than would be expected for that rim would change the wear limit, and are more likely to split a worn rim sooner.

In the end you have to use good judgement, and maybe cross your fingers.

The rear tire on my Bike Friday is also rated at 85 psi (Primo Comet), but I regularly inflate it to about 110 psi when the bike is loaded with my camping gear. It's never caused any problem. I've heard that tires are supposed to be able to handle up to twice their rated pressure - but that assumes that the rim can handle it without any deformation or undue stress so I wouldn't recommend going that high.

Most manufacturers rate the suggested pressure at or about half of the failure pressure of the tire itself.

So, technically, you can assume the tire will take twice that.

The rim, of course, won't. With 85 being your tire's rated limit you can max out at around, as Prathmann said, 110 psi without worry.

However, you may not want to unless you're very comfortable with higher pressures.

I assume you are concerned that the one gauge that reads 90 psi is right and the 85 psi max has accidentally been exceeded. It's not a problem. Bicycle pump gauges are not precision instruments and you are lucky if they are accurate within 10%. So if the tire is truly at 90 psi you are still well within the ability of the tire and rim to handle it. As noted, the "blow-off" pressure of most tires is at least 2X the rated max so you are safe.

You sound like you have the "two watch problem", i.e.; A man with a watch knows what time it is. A man with two watches is never sure.

I'm trying to find my "Sweet Spot"

Thank you.

Thank you.
Not necessarily, but I love the anecdote.

I recommend Ear Plugs, so, when it blows your ears wont ring.

When some things are repeated often enough, people assume they're true. This is one of those. It sounds logical but nobody has been able to provide me with an industry source for that statement.

I'm thinking that half of the tire/rim interface is the rim. Rims aren't all the same. If the sidewall maximum pressure really = 1/2 blow off pressure, I'd think somebody would have to indicate a standard test rim. Where's cyclecommute when we need him? He's good at finding source data for things like that.

This gets bandied about, and there may be some truth to it, in terms of the tire companies' design standards and internal testing. But it doesn't carry into the real world, because of the variables. 40 years ago fewer people would have believed it, because as most older riders will remember it was very easy to blow a tire off when filling from the air line at a gas station.

Did tires get better? No, but the pressure ratings did, while the compressor operating pressure didn't. So back when tires were rated to 50 or 60 psi, we had no problem blowing them off. Even 75psi tires blew off. But with tires now rated at much closer to compressor operating pressures we don't have as many blowing off, and when we do, we're quick to blame seating rather than excess pressure.

I still have a clear memory of what happened when I overinflated a Michelin 26x1-3/8" tire on a steel rim. The tire didn't blow off. Instead the rim imploded taking on the shape of pac-man. It turns out that the curvature isn't enough to take the radial load, and for whatever reason one area got pushed in enough to be flatter, at which point failure followed.

IMO it's very safe to exceed rated pressure by 10-20%, and probably OK to go as much as 50% higher except maybe with wider tires. But I wouldn't trust a tire above that.

BTW- some of this may carry from the auto tire world, where there's a significant safety margin. Tire mounters routinely use pressures nearing double the typical 24-32psi range to pop newly mounted tires onto the rim seats. What most don't know, it that commonly causes tires to burst, and injure operators. For that reason, many use inflation cages when first inflating newly mounted tires.

My daughter brought home a 1980s 27" 10 speed that she saved from the dump. When pumping the tires, I had to remember that it was a steel rim with no hook, and so I stopped at 80 psi, instead of my usual 100 - 110. So, as already mentioned "it depends".

While I normally agree with you FB, I disagree with this statement.

1) Heavy braking can cause a SIGNIFICANT increase in rim temperature and hence air pressure. Tire pressure could easily increase by 10% with a long descent. If you're already at +20%, now suddenly you're in a danger zone. The increased pressure will also force the inner tube into spoke holes puncturing the tube, I've seen this on both road and mountain bikes. As long as the tube is within its normal operating range, this isn't really a concern.

2) I've seen tires blow off rims at the "rated" pressure. Granted, it was a very high pressure (~130 psi) tire and probably defective.

3) There's no real benefit. The rolling resistance of the tire is optimized at a certain pressure. Running above that will reduce traction and handling with minimal rolling resistance.

Actually we agree. I was stating lower ranges of what's safe than the mythical 100%. My estimates are based on what I've seen people doing in the real world. I stand by them, there's decent margin for error, and little risk at 20% over inflation, even allowing for some increase due to brake heat (less than you imagine, because it takes a long time for air inside a tire to be heated more than a few degrees (if that).

In any case, your 3rd point which I touched on earlier in the thread is spot on. There's little if any point in going to extreme pressures, and in many cases it's counter productive. The solution for the needs of heavier riders (if that's what this is about) isn't more pressure, but more tire section.

The more air is better mentality has even infected the pros. I know of a few pro team mechanics who argue about this daily, only to be over ridden by riders and promoters. Then we see riders crash for no other reason but lost traction.

Good traction is more important than a marginal reduction in rolling resistance and smart riders keep that in mind.

Hi,

FWIW there is some benefit in exceeding tyre ratings for skinny tyres,
depending on weight, and generally hardly any for fatter tyres where
the maximum ratings often correspond to an extremely heavy rider.

As said, fatter tyres are generally the better option for heavy riders.

A stripling of a lass on a bike needs far less pressure than I do.

rgds, sreten.

And note the "sweet spot" depends on general road conditions,
and generally doesn't "feel" as fast as harder, but is faster.