Prog

We are the team of about 300 lbs plus the bike 50 lbs and the trailer 100 lbs. We had a lot of miles on 47 mm Marathon Plus with no problem. Recently I changed tires to tubeless 27.5 Vittoria Mezcal 2.1, inflated to 40 psi. After 150 miles the rear tire has torn from bead.
What could be a reason for that? Is 40 psi not enough? Or these mtb tires are too fragile for tandem? Vittoria doesnt say what is the aloud weight. But Schwalbe claims for similar XC tires, that max. weight pro tire is 100 kg. For Marathon 115 kg.

P.S. The tire was probably overinflated up to 60 psi (52 is aloud) during mounting. But was never ridden in that state.

Carbonfiberboy 

This is the second one of these we've seen on this forum, different tire brand. On that width of tire and weight, I wouldn't run under 95 psi. The friction of the bead of the underinflated tire against the rim simply wore it out and quickly. Oddly, the website doesn't show the recommended pressure range. Is it on the sidewall?

This is the slightly tricky thing about tandem tires: they have to be able to take the pressure. Certainly on a MTB tire there's no pressure on the manufacturer to make a tire that'll take more pressure than an MTB rider would need, which could be as low as about 40 lbs. on the front.

Use road tires. When I look at tire sizes, I want to see a max sidewall pressure so that pressure X tire width = 3000. Thus you would run those Marathons at say 63 lbs. but the 27.5 tires would want 110 lbs. That said, multiply the pressure at which you ran your Marathons successfully X 47 and use that number instead of 3000.

And that said, never run a tire at more than the max pressure shown on the sidewall. If that's too low using the above equation, don't run that tire!

headasunder

Ive had the same issue with running tubeless on our tandem, 3 tyres have failed in a similar manner to yours, the same tyres with tubes installed have been rock solid I havent totally given up on tubeless but they are very sensitive to pressure too much and they will blow off the rims and under inflation leads to sidewall splits at the bead.

Ross200

2.1 inch = 53.34 mm WIDTH. Using Carbon's equation, his recommended pressure will be more like 56 psi, not 110 psi. Certainly more in line with the traditional 1/3 more than half bike pressure.

OP doesn't mention rim type/width which needs to be considered. More than a few rims are blowing apart from wide tires at high pressure.

Rick

I have never gone tubeless. I use Schwalbe tires on my tandem and my touring bike because they seldom have flats and they are the only tires that my tandem didn't destroy before the tread wore out. I wore out several sets of the Marathon Supreme tires on my touring bike and recently switched to the Marathon Mondial.

Prog

Thanks for your answers. I checked the tire pressures on a sidewall, it is 29-58 psi. I've got 40 psi from the sram tire pressure calculator, while using a little bit higher team weight to take into account the different weight distribution. So it seems to be a bad idea on tandem.
The idea with tubeless mtb tires was to get more comfort by the offroad touring. If we use max tire pressure, then probably there is no much difference between the 47 mm Marathons inflated to 70 psi and the 52 mm Mezcals inflated to 58 psi (the latter has also 2.54 smaller diameter)

The rims I use with MTB tires are DT Swiss Hybrid X491. 25 mm inner width. With Marathons I used Mavick A719, 19 mm inner width.

Carbonfiberboy 

Right. I mistook the diameter size for the width and the width for the version increment, like software. Using my equation for pressure also puts you below the rim manufacturer's max stress. Max sidewall pressure is usually more related to rim strength than to some tire explosion possibility.

Rick

How wide a tire and at what pressure. Would this also be rims that are made for disk brakes are weaker than rims made for rim brakes. I have had cracked rims on my tandem over the years but none have blown apart from high pressure.

jccaclimber

RE: Sidewall failures (clarification, RIM sidewall failures). Not the case above, but I've seen it two times.
First was 700C x 23 mm tires on generic road rims with no wear indicator. They were used until worn thin, and the sidewall eventually cracked.
Second was a kid running a (guessing) 20" diameter x 1.5" wide rim with 2" or so BMX tires. The catch is that he was running something like 120 PSI (don't ask). Over the course of a month or three the sidewalls slowly curled outwards. Surprisingly the sidewall failed before the tire unseated. I'll bet they were pointing outward at a 45° angle before that occurred.
Neither of these were recently.

More directly on topic I did recently fail the sidewalls right above the bead on some older road tires I had. Surprised me because when younger I used tires older than me and never had an issue.
In hindsight, it did correspond with running them at a substantially lower pressure than I usually do.

alo

There is a lateral force on tires when turning. With many modern rims getting narrower, there is greater tire roll. Even more so when the pressure is lower. This puts excessive force on the tire, and causes failures.

Soutions:
1 Wider rims.
2. Higher pressure in tires.
3. Stronger tires.

Carbonfiberboy 

IME whatever the width of the tire, the width of the inflated tire in mm * pressure in PSI should not exceed 3000 for optimal rim health. I doubt there's a difference in rim thickness. If anything, I think disc rims would be thicker as they aren't machined. IOW I think manufacturers use the same extrusion for disc and rim brake wheels.

headasunder

Ive had a few sidewall blow outs on my commuter from brake wear, with tubeless the tyre blowing off the rim is more likely than a sidewall distortion from the pressure. 25mm internal is plenty wide enough for the width you were running. I had a 42mm(real life 45mm) blow off the rim at 60psi luckily we were moving slowly up a step grade I usually run around 50psi in 40mm tubeless for a plush ride(unloaded tandem stoker 68kg/ captain70)

jccaclimber

I don't know about tandem specific rims, but I've seen it all over the map for road rims.
The Easton R90 disc brake rim lists as 0.7 mm thinner wall than the rim brake version. Going off the cross sections it looks like they might be machining them thinner, but I don't have one in hand. Other disk-specific ones I'm used to that taper right away vs. keeping a vertical section for the brake track are a separate extrusion, so all bets are off.
What sort of rim (opposed to tire) sidewall related failures have you seen?

Carbonfiberboy 

Interesting. I've never bought a rim for which the factory showed a brake track thickness. OTOH I've never used anything exotic. Failures: I've only seen one failure, one of my rims, which failed because I didn't know to keep track of brake track dish, it being my first winter riding in the PNW rain. Luckily it failed in the car not on the road, though I know folks who've seen their rim bead go rolling away from them. That said, I've never heard of a rim failing at the brake track/bead for any reason other than rim brake wear. One needs a pin mic to keep track of wear. I do know of tandem rims cracking at the spoke holes, though that's never happened to me and has nothing to do with this thread.

Ross200

https://www.bikeforums.net/tandem-cy...placement.html

New rims aren't getting narrower for gravel/bikepacking/mountain bikes-they are getting wider. Disc wheels with high spoke tension and extreme dish don't always lend themselves to lighter rims. When 30-40 mm wide rims weigh less than 20 mm wide road rims, some compromises are made somewhere. The tire pressure needs to be high enough to keep the tire bead seated but not so high that the spoke bed fails-often catastrophically. I think that safe window is even smaller on a tandem now than it was 20 years ago because of the dearth of quality tandem specific components in the market place.

pria.berdasi

Where do you get the constant 3000 from?

Carbonfiberboy 

From observation.. Run the calc on a few tires yourself: size in mm X max pressure on the sidewall. You'll see some variation.