Kbarter

Was setting up some Panaracer 38c tires on Hunt Carbon 30 rims and the rim exploded at about 80psi when I was trying to get the bead to set. Reached out to Hunt, noting that the max PSI on the rim was 100, and got this reply.

Hi Kent

Thanks for getting back to me with the information.

There is one point that you make that I must correct you on, in that the tyre size has no impact on the rim's maximum pressure rating, this is incorrect, the tyres size and volume is the biggest factor of all with regards to the pressure that a bicycle rim can handle, hence why you always work to the rim's maximum pressure and not the tyres.
Exponentially as the tyre size and most importantly the tyres volume increases, the pressure that the tyres bead will place on the rim increases significantly, this is why rim manufactures scale the maximum pressure ratings dependant on the tyre size being fitted.

As from the information listed on our site, the max pressure for the Hunt 30 Carbon Gravel Disc rim with a 38c tyre is 40 psi, when fitting the tyre you went to double (and potentially higher) than that of the rims max pressure rating, in addition you also went to 20 psi above the tyres max pressure rating of 60 psi. For future tyre fits on our and other manufactures wheelsets, please work to the max rim / tyre pressure and work to the lower of the two units when fitting your tubeless tyres.

As a company we will always do what we can to assist riders, in this instance we would offer the rider a crash replacement discounted rim to replace the damaged unit, however I can offer you a replacement rim as a 'free of charge - goodwill gesture' that you can get the wheel rebuilt with at your local bike store. At present we we will have replacement rims back into stock for December week 2.

Curious if the volume not psi argument is legit.
Thanks,
Kent

Kbarter

Too much of a noob to post pictures

mstateglfr 

The second quoted verbiage is from Hunt's site, so yeah it mostly does say what the Hunt rep claims(slight difference in numbers).
...at the same time, I ride 43mm tires on my gravel bike and have 0 interest in running them under 45psi. I am 220# and 45psi is right at what I set my tires at because anything lower makes them sometimes feel sluggish or wobbly in random moments of riding.

Also, every tubeless tire I have seated has required a shot of air that well exceeds the psi I will actually use for riding. The burst seats the tire. Apparently these Hunt rims, which are hooked, cant handle a burst to seat the tire?

Kbarter

I made the point to him that I saw those number as safe ride pressures but didn't understand how the rim knows what size tire was on it while inflating.

It was really loud!

spelger

consider your self blessed. you are getting a free replacement for something that they consider to be pilot error.

Kbarter

That is clearly his message. Reason I posted is I don't really buy it and was hoping for someone could clarify one way or the other.

KJ43

Well they are right as far as I can tell. I have a basic understanding of the physics here. There is psi which is the pressure being exerted inside the tire and rim against the walls of the space created. There is also (separately) the force that the bead of the tire is exerting on the rim preventing it from being blown off. The higher the volume of the tire but at the same psi the higher the force that the bead is exerting on the rim becomes. At least that is how I understand things. I'm happy to be corrected by someone more in tune with the math. I'm an engineering major but my discipline is Civil.

Not sure why you aren't buying it. They sound entirely reasonable in their communications to you and you are coming out ahead anyway. Sounds like customer service above and beyond.

I am currently riding 38mm tires (Pathfinder Pro's) on my carbon rims (25mm internal width). I usually ride at between 34 - 42 psi depending on the surface my route will take me on. When seating a tire I might go barely up to 60 psi to make sure its seated (if its stubborn - usually its well seated by about 50 psi) but there is no way I would ever go beyond that.

indyfabz

See! Crabon fibre does asplode!

PeteHski

This is correct. As the tyre size increases, the tension in the tyre sidewall increases for the same tyre pressure.

Pressure = Force / Area. So as you increase the tyre sidewall area, the tensile force also increases at a constant pressure.

This is the same reason why you need less tyre pressure to support your weight with a wider tyre.

Moe Zhoost

I think this issue was less about the pressure and more about how the tires were mounted. If you don't inflate gradually and check that the bead is aligning correctly, the inflating tube can force a bulge in the tire that eventually forces the bead off the rim explosively. And yes this can cause rim damage.

KJ43

Unlikely imo. You are assuming that he used a tube which in my experience is way down the list of optimal ways to seat a bead. We are all assuming though as the OP never specified how he performed this task unless I missed it somewhere.

Polaris OBark

It is also possible that they recognize it is not 100% the fault of the pilot. This way they can do the right thing with out admitting fault/liability.

Rick_D

On my list "Stuff that might happen" this does not make even page 3. Good to know, if disconcerting.

The Park Tool mounting a tubeless tire videos are complicated enough--don't recall them mentioning 'sploding rims.

Koyote

I'd say the OP is doubly-lucky, since he wasn't killed by a flying shard of cf. Those assplosions can be deadly.

PeteHski

Well I guess he only used double the maximum rated pressure for the rim with that tyre width. How would you split the liability?

base2 

Pressure x area. A big tire has more area so the total force exerted on the rim goes up.

Sorry you blew up your rim. Hunt is being more than fair. Charitable, even.

spelger

could even have been this...

79pmooney

Actually, the rim has no clue as to the tire size (or the pressure). All it knows is how large a force is being applied to the rim wall. When that force exceeds what it is made to handle, it fails. The force the tire applies is a product of the pressure and the tire size. (This was an engineering question I may have answered correctly a million years ago but I cannot remember the answer. Is it pressure times sectional area or pressure times circumference? The test question context had nothing to do with bicycle tires. If it had, I'd remember to this day!)

An easy way to get around this issue entirely is to just back up 5 decades. To sewups aka tubulars. Now tire pressure and size have absolutely no effect on the rim sidewall. None. You do have to embrace a 120 year old technology. Accept being totally self-sufficient since no one is going to have spares. Accept that you have to glue your tires on like your life might depend on that glue.

veganbikes

When you go above the pressure rating for the tire as well as the rim, you are playing a dangerous game. They were quite nice in offering a new rim. Kudos to Hunt for really helping their customers.

terrymorse 

The term you are looking for is “hoop stress”. It’s something that was worked out after cylindrical boilers in ships were exploding.

Hoop stress is proportional to the diameter of the cylinder, so doubling the diameter doubles the stress. Or doubling the tire width doubles the stress on the rim.

Polaris OBark

A 40 psi max for a 38mm tire is unrealistically low.

I inflate my Barlow Pass rear tire to about 50psi, consistent with their guidelines.
https://www.renehersecycles.com/tire...re-calculator/

If my rims said 100 psi max, I wouldn't think twice about pumping my tires up to 50 psi. If Hunt Rims really can't handle going over 40 psi for what is considered a thin gravel or standard all-road tire, then they really need to re-think that 100 psi max label on a gravel rim. Nobody rides 26 mm tires on gravel rims.

79pmooney

Hop Hoop stress. Yup, I remember the problem but I wouldn't want to do that proof now!

Polaris OBark

By post 22, we arrived at a beer discussion.

SoSmellyAir

The webpage for my Gravel wheels (Roval Terra CL) says:

• Tire Size: 28mm - 47mm - Max Pressure: 28c-32c: 80psi, 33c – 36c: 65psi, 37c- 47c: 60psi

That is definitely clearer than just a label stating 100 psi. However, when I used a compressor to seat my tubeless tire, I only added air a squirt at a time, and listened for the pops as the beads seat; did OP add air continuously?

Canker

If you weren't already using it soapy water will help get a tire on the bead without going to 100psi.