Exceeding max tyre pressure
 

Hi everyone.

I recently got myself in a fairly stupid situation: i've purchased 2 kits of Kenda Alluvium 700x35c to fit on my gravel and not-so-cyclocross bikes.
Now, by itself nothing wrong with this, but there are two problems:

1. I'm about 125kg/280lbs heavy. Bike is another 10-12kgs depending on how much water i have.
2. Max rated pressure on Kenda Alluvium of any size is 50psi. I didn't check it before buying and it's a non-returnable purchase (they both are also already fitted and with sealant inside anyway).

I run them tubeless, on 17mm and 19mm internal width rims (rims are rated much higher, i used to be riding 75 tubed on 33mm tires before, constant pinch flats below 60). I mostly ride a mix of category 2 gravel and paved roads to get there.

While riding at 50psi seems to be possible (i did couple of rides and nothing bad happened so far), the tires do look like they could use some more air. As in i'm pretty uncomfortable with how much they deform even under static load.
Now, all tyre pressure calculators suggest around 55-60psi. I tried them at 60psi, and it seems like a sweet spot for my weight: good contact patch, good rolling. But it's 10psi or 20% above tyre limit.

I contacted Kenda and their reply was pretty much 'welp bad luck dude, we already have your money nice knowing you'

So, questions I have:
1. Is it OK to go 5-10PSI (that would be +10%-20% above max tyre pressure) above max?
2. Should i be riding on 50psi instead and not bother with tires looking half-flat?

Thanks.

1) Anybody who wants to avoid any liability is going to refrain from claiming you're 100% safe going over the max tire pressure, so you're unlikely to get reassurance there. That being said, there are many people who push that envelope without ever suffering any consequences, and I'm sure there is some wiggle room built in just for liability reasons. The question is how much?

2) I would try 50psi. As someone who used to run 140-150 psi in my race tires back in the day, I understand that it "feels" wrong to have a tire that squishy, but if you're not getting pinch flats or damaging the rim it should be fine. I had to gradually work my way down a few psi at a time over the course of a year because it kept feeling like I was riding on a flat tire.

I won't "officially" recommend bla bla.

Since that tire comes in a 45mm version, I'd have no qualms adding 10 psi to a 35mm version. (assuming the construction is the same)
You might do a couple/few less on the front for awhile until you feel confident the rear won't blow.

Can you please elaborate, why having 45mm of same construction (they are same from what i gather) means that 35mm can safely hold more pressure? Does that have something to do with the fact that 45 has more sidewall and if it can hold 50psi with more sidewall surface area then 35 should have no problems holding a bit more with less?

One consideration running tubeless on marginal pressure is that if I get a flat and it seals losing 5-10psi, i'd might end up with a pressure too low. Another one is that while there are no pinch flats, tubeless valve does stick a bit above rim level, so (i understand it's a fairly low chance) hitting the valve on bump due to tyre being fairly squished might end up in unexpected fun.

Maybe I'm extrapolating too much, but it's "alleged common knowledge" that using a wider tire on a rime at the same pressure exerts greater forces on the RIM. I just presumed....

OTOH, the more sidewall area, the greater the chance of a flaw.

Boilers are tested at four times the maximum allowable working pressure. I doubt a few pounds over in a bike tire will end in catastrophe.

Well, it's not really common knowledge - it's physics, we have pressure measured per unit of surface, more surface = more force exerted. Question is, where do we expect a problem.
Rims can hold over 50psi, they are rated up to 90 and on one of them i've been running 35mm at 75psi for years. I did actually run pretty much same rim with 33mm at 90psi too briefly.

I doubt the tyre bead will de-hook either, as you've said, 45 with same pressure holds fine too, and there are substantially more area surface on 45 one.

Biggest concern really is sidewalls blowing out, but there are less sidewalls on 35, so should be stronger overall.

You paid for the tires, and haven already proven that they can tolerate 60psi, which meets your needs.

So, you have only 3 choices. Eat the loss, since there's no recourse. Ride them under inflated (for your needs) and have a miserable time.

OR

GO AHEAD AND RIDE THEM at 60psi, with the worst case being they don't hold up, bringing you back to option 1.

Apples and oranges all over the place. My Paselas rated at 105 don't like to stay on the rim at that pressure on my hookless rims. 95 is what I want anyway, but 105 made me walk a bike 5 miles back to my car one day. The OPs mileage may vary.

Did they come off the rim…or burst?

Came off the rim. Not a catastrophe but it does suck.

Then my analogy isn’t as apples and oranges as you suggest.

Possibilities:
1. You ride at max recommended pressure, but the tires deform while riding and you wear a crack in the sidewall for early failure, or you get more damage than you otherwise would from a pothole or curb jump.
2. You ride at higher than max recommended pressure, and the tires handle it fine, with somewhat increased chances of a blowout and possible bad consequences.

I think your objective ought to be finding the Goldilocks pressure, which would be the minimum amount above #1 that gives you an acceptable amount of tire flex. I propose you increase by 5psi for a dozen rides or so and keep good notes about how they look and feel. Then repeat with another 5psi and re-evaluate. 50psi is a pretty low max pressure rating, I'd feel safe with at least another 15, but go slowly with the changes.

My experience compared to what the OP is wanting to know is an apples and oranges comparison.

True. When I think of going over max tire pressures, I think of bursting, not releasing from the rim.

Hookless rims aren't usually rated for over 65psi, above 65 the risk of blowoff goes up a lot. You need to look at what the rim is actually rated for and don't go over that either.

This might just be my perception, but if I get a puncture that loses 5-10psi, I whip out the pump and add 5-10psi while being thankful that the tubeless system allows me to do that without having to take the wheel off and replace a tube.

I would also think most flats you'd get on a tubeless tire would deflate slowly enough for you to come to a stop before the bump becomes an issue (if it even does). That's including a gash on the tire that spews sealant like a fire hose.

I wouldn't go over maximum when it comes to tubeless, my experience with running too higher pressures resulted in a couple of blow offs and ultimately rim failure(I was about 5psi over what the rim was rated for) the tire was rated up to 70 I was running close to 60 which proved too much for the rear rim on our tandem if you want to run higher pressure chuck a tube in.

He said he was running them tubeless.

I’ve been told you can still “pinch flat” a tubeless tire. Not sure if it means the same thing, or if they’re mistaken, but the point remains.

I don't understand how you have a pinch flat being tubeless

https://ebikepursuits.com/what-is-a-...oogle_vignette

I don't either, so feel free to disregard that part of my post.

Originally Posted by smd4
"Boilers are tested at four times the maximum allowable working pressure....."
Bull!
As one who was a journeyman Boilermaker, having completed my apprenticeship at Puget Sound Naval Shipyard, and had a qualification (Test Director) to hydrostatically test them- (meaning my name went on the bottom of the test forum)
Every 5 years a boiler is required to have a strength test at 150%. NOTHING higher.
If it was relatively minor repair or maintenance, then 125%.

https://cimg7.ibsrv.net/gimg/bikefor...712aac66e2.jpg

Physics isn't some hidden away magic. It's common knowledge, available to anyone.