chriskmurray

I have one of the older Arch 26in rims from Stan's. I am wanting to run some 1.3in wide slicks at around 90 psi at times. The rims claim a max pressure of 55psi for 1in wide tires. I was assuming that meant for tubeless set ups, I will be running tubes with these so I was curious and emailed Stan's about it. This is what they said, I am still a little unsure of how big of a deal it is and was wondering if anyone has done similar and could post their experiences. What confuses me is the part about wearing out the rim early yet them also saying it should not catastrophically fail. To me, when rims wear out there is always a risk of catastrophic failure.

Me: I think I remember hearing that even though you rate rims like your Arch to 55psi with skinny tires that you can actually go to very high pressures (like 90psi for a 1.3 slick rated to that) as long as you are using tubes. Is that true? I have a set of really skinny 26in slicks I would like to use at times but the recommended pressure pressures are 85 psi.

Thanks

Stan's: When using a tube you can inflate to the max pressure listed on the tire. The rim is not designed for this and it can lessen the lifespan of the rim.

Me: So I can but I shouldn't is what you are saying? How much risk is there of catastrophic failure?

Stan's: It is not our recommendation to do this. The rim will wear out faster if you do this. It should not. catastrophically fail

dsbrantjr

Coming from an electronics and aerospace background I am always surprised and a little bit confused by folks on this board, who are apparently not mechanical engineers, second-guessing manufacturer's specifications and trying to justify exceeding published maximum limits. In my business maximum ratings are absolute limits, not starting points to see how far you can push things.

In the present case I cannot see how if the manufacturer says 55psi maximum that somehow 90psi should somehow be OK, tube or not. Before trying to overstress such a component I would think very hard about how much energy is stored in an inflated wheel and tire and the consequences of its sudden release, not to mention what would occur if you were on the bike moving at high speed at the time.

Do you really want to ask a bunch of strangers on the Internet if it's OK when the manufacturer says it is not?

FBinNY 

The "hoop tension" of a cylinder (think pf a tire as a long cylinder bent into a circle) is proportional to the pressure times the width. (length, or wheel diameter don't factor. , so A 2" wide tire at 50psi, exerts the same force as a 1" tire at 100psi. Since the rim is what holds the tire together at the bottom, that force is pulling the flanges outward.

There's plenty of margin of safety on a new rim, because the manufacturer has to allow for thinning of the flanges with brake wear. However, if you use rims with tire pressure/width combinations above the design strength, then the rim will wear to critical thinness sooner,. Not an issue, except that you need to allow for it, and not take them all the way to the end of the wear indicators (if any).

IME, rim flanges don't suddenly blow out catastrophically. There's usually some warning in the form of circular stress cracks visible on the outside near the inside of the brake track. You might also feel some pulsing on braking because the rim is bending outward unevenly so the rim width will vary.

I hope this gives you some guidance, the rest is a matter of using some common sense.

chriskmurray

I am trying to figure out what I was told by the company really means, not second guessing the manufacturers . "When using a tube you can inflate to the max pressure listed on the tire. The rim is not designed for this and it can lessen the lifespan of the rim." They say I can inflate the tire to the full 90psi just that it will shorten the life of the rim. I would get that if it was a rim brake wheel but it is a disc only wheel. I have been told before that the max pressures were because non tubeless tires set up tubeless do not have a strong enough bead to handle higher pressures.

FBinNY, thanks for putting an actual formula to what I was thinking was the case.

well biked 

That email advice is indeed confusing and not good. If you have technical questions, I would give them a call at 607-562-2877 during regular business hours on the east coast.

mike6024

I think this is your best answer here, which is basically don't do it:

Is this the rim? disc only, low sides, no brake surface?

Rim Features-
Every other rim in the world has a bead hook. This rim
does not.
This new rim design will eliminate the bead hook and
move the sidewall of the rim down by 2-3mm.
By eliminating the bead hook and lowering the sidewall
by 2-3mm we reduce the weight of the rim. Some of this
material can be used in the center part of the rim
making it stronger and stiffer.
The curve of the rim sidewall is the same shape as the
bead of the tire so they fit together like a ball and
socket joint. This design will help reduce the strain on
the tire bead while creating a tighter seal.
By lowering the sidewall of the rim we also reduce the
mechanical leverage the tire has on the rim. This will
make the rim stiffer and last longer by eliminating
flexing that can cause cracking.
With these short sidewalls the tire should not hit the
rim and pinch flat as often as standard rims, either
with or without tubes. This design will allow you to run
lower pressures and use 4mm more of the tire than ever
before. The tire will have larger air volume giving you
better traction than any rim in the world Can be used with or without tubes.
Color: Black Size: 26/32H Mfr#:TR07

It sounds like with too high pressure the bead would pop off to me. If the sides are low what is to stop that? The tube would not help prevent that.

Yes you could figure the "hoop stress" in pounds of force per inch along the bead would be : pressure times the radius = ( 90 psi ) (1.3 inches) (0.50) = 58.5 pounds per inch. So around the whole circumference would be 26 inch x 3.14 x 58.5 pounds per inch = 4600 pounds ???!!!!

Those of us with Mech Engr background would not want to attempt to do analysis on this or second guess the manufacturer. Not enough information, not even close, and it is complex and dependent on many factors. How does a rim manufacturer know when a particular tire will pop off the bead when the tube type tire comes from another manufacturer?

chriskmurray

The thing is they did not actually tell me not to do it, they actually said I COULD go to the recommended pressure just that it would shorten the lifespan of the rim which is where I got confused. The statement above about shorter rims would actually make me think the sidewalls are stronger, therefore should handle higher pressures just fine.

I will likely give them a call in the morning to see if they can clarify what I was told in email because it seems like there is a lot of grey area in what they told me.

mike6024

Yes, I agree with you. Seems to make no sense why it would shorten rim life. It is not like the high pressure is causing more rubbing or wear on the rim?

Rim apparently has shorter sides so the bead does not overlap as much so more likely to pop off under high pressure, unless I am not understanding what I think I am reading?

I might carefully test it. Put on some safety glasses in case of a blowout. Put it on. Pump to 90 and see if it pops off. Go higher, 100 psi. If it does not pop off maybe it is safe?

But really don't risk it. My sense is it is not safe. I would be curious if you test it if it pops off. But I really don't think you should do that. But if you do be very careful. Like the first poster to your question said, there is a lot of force contained in compressed air even in a small tire. Be careful! And a failure going down a hill on a curve could change your life for the worse so don't chance it.

chriskmurray

They told me a catastrophic failure should not happen, just shortening the life of the rim which really confuses me with this being a disc only rim. If it was used with rim brakes I would completely understand.

I have actually went up to 80psi after their email and did my normal commute but I want more solid answers before taking it on any real rides with these tires.

FBinNY 

If you want to pressure test a rim, possibly destructively, you shouldn't use air.

Pressure vessels are always tested hydrostatically to reduce or eliminate destructive forces. Water isn't compressible so if the vessel is first filled with water, then pressurized, the vessel can burst, but there won't be explosively the way it is with compressed air. Filling a tire with water isn't easy, but it is possible. Otherwise use a long pump hose and inflate the tire to burst pressure behind a wall.

mike6024

I don't really understand what is going on here so should have made no comment in the first place. Just curious because it seemed interesting. Higher pressure puts more force all the way around the rim in the radial inward direction which should be no problem. With a clincher bead tire it also puts more outward force at the rim flange, as previously noted:
.
FBinNY : "IME, rim flanges don't suddenly blow out catastrophically."

But these have no rim brake surface, and very low profile "rim flange" if you want to call it that.

So, if you've mounted the tire and put 80 psi in it how does it look? Is the bead seated well? No evidence of the bead riding high, looking to pop off?

Is the overall diameter of this rim smaller than a comparable regular 26 inch clincher rim meant for tubed tires? I am just curious. Have zero experience with tubeless tires and tubeless rims.

chriskmurray

It actually looks perfectly seated, I just wanted confirmation before doing any serious riding with these tires.

FBinNY, as entertaining as that could be, I will just make a phone call to Stan's in the morning and try to get them to clarify what was told to me in the email.

dabac

You're not.

Once the tire is seated properly, the bead will always sit right under the hook, regardless of how high the side of the rim is. Only exception is a slightly tight tire, which may, until it has stretched a bit, somewhere along the circumference sit a little lower than the hook. It'll be hanging off the bead as opposed to resting against the hook.

The difference beween newer, disc-specific rims and the old stuff is that the distance between the hook and the bottom of the rim is tailored to be the same as the expected thickness of the bead, making the bead contact the rim on three sides(top, out, bottom) as opposed to only top and side.

The clue to (part of) the question is in "By eliminating the bead hook and lowering the sidewall by 2-3mm we reduce the weight of the rim. Some of this material can be used in the center part of the rim making it stronger and stiffer."

I'm guessing they haven't only made what's left of the sidewall lower, they've also made it thinner(together with the corner sidewall-wall.) Earlier, sidewall design had to compensate for wear, so it had to be excessively strong to start with.
Now, when wear is no longer an issue, sidewall strength can be tailored to the expected forces from the tire, and "nothing" more.

Agreed that shorten the life is a bit cryptic. But if there are pressure spikes in the tire as the wheel pass over lumps and bumps, maybe running higher-than-rated pressures may eventually induce fatigue failures.

dabac

No. OD is pretty much the same, as the bead is expected to fetch up against the same diameter. But a light weight, disc-specific rim can have a few mm bigger ID, on account of having lower sidewalls. Usually not by much though, as you still need a dip in the middle to have room enough to mount the tire.

HillRider

That's a most important point. Without a hook bead to mate to a tire designed to require one, higher pressure is likely to allow the tire bead to pop off the rim at even modest pressures. The old straight wall rims of the past would not retain a tire even at 90 psi. Try it and see but don't be surprised if the tire won't stay put.

chriskmurray

The curious this is that the guy from Stan's actually told me I could inflate them to the rec pressure, just that it would shorten the life of the rim. The above statement is a big misleading, there is a hook for the bead, not in the traditional sense but these are not like the old straight wall rims at all. The bead on tires almost seems to fit into more of a socket than a hook on these rims because even after flatting you have to pop the tire free from the bead of the rim much more than most.

Either way, I just tried to call but I think they are out to lunch so will try again later.

chriskmurray

The word from Stan's was that I do not have to worry about the sidewalls of the rims blowing out like one would see on a rim brake rim that is too worn. He said the issue with time will be the spoke bed will see that fatigue from the higher pressures so the rim may split down the center of the rim in time.

I asked him "So I am fine to run them at higher pressures, just don't do it all the time?" And he responded with "pretty much"

Considering these tires will only be on when I want to do longer road rides which is maybe once a month or so I think I will be just fine.

FBinNY 

I think you were just getting boiler plate disclaimer info, to give them an excuse if anything happened. I've never seen or heard of a rim splitting down the middle as a pressure related failure. If anything, higher pressure would lessen the load at the spoke holes by compressing the rim and slightly lowering spoke tension.

Based on that feel free to use whatever you need as long as the tire/rim fit is good enough to prevent blowoff.

chriskmurray

The whole thing had that sort of feel because of the vagueness in their responses. Even the people I talked to on the phone were a little vague like they had no problem with it but they could not come out and say it outright. I will ride without worry, like I said, I will not have these tires on all the time anyways.