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Old 03-21-14, 11:10 AM   #1
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Blown Rim

Well, got our Santana out for its first ride this year and found that the rear 40 Fir rim has blown. Rim has about 6,000 miles on it and has been in use with a heavy team/bike weight of 440# with Conti 28c tires pumped to 130#. Tandem has had a rear disc for all but a 1,000 miles or so, so it's unlikely that sidewall wear caused it.

Luckily, we have a spare 40 rim that I can lace on to it but our total team/bike weight is now 350# so it may be time for new wheels.

Suggestions?


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Old 03-21-14, 11:40 AM   #2
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That sounds like a lot of pressure in a 28 mm tire. Maybe you can run at a less-extreme pressure now that the team weight is down. I blew out a rear rim once at a weak spot (near the valve hole) when I insisted on overinflating my 25 mm tires on my half-bike. Lucky for me, Velocity replaced it under warranty.

Congratulations on the huge weight loss. Perhaps you were just riding a 120 pound bike that you have replaced with a 30 pounder?
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Old 03-21-14, 12:43 PM   #3
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Looks like it happened right at where the rim is welded. I'd argue it's a manufacturing defect.

One other observation, though, from the nicks on the rim in the top of the picture, it looks like that rim has been on the pavement at some point, perhaps riding out a flat. The rim manufacturer might point to that.
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Old 03-21-14, 12:51 PM   #4
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I'm not worried about warranty. I've got a spare rim to lace on to the hub. And, yes, the rim has been ridden after a blowout on a steep descent.

As for pressure, since my wife and my weight loss, I was going to run 115-120 which I was pumping when I found the rim was blown.
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Old 03-21-14, 02:59 PM   #5
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Probably a combination of high pressure exposing a defect or damage at the joint. Whatever force made the nicks on the rim may have weakened the weld as well. If it were me I would lace up the spare rim ride it. We have put 10,000-15000 miles on rear wheels with 28s at 130 psi on Velocity Dyads and rim brakes with no failure. We don't brake in the rain a lot and team weight is about 290.

If you want an excuse for new wheels then yes you need new wheels. Better yet losing that team weight I think you deserve new wheels.

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Old 03-21-14, 04:10 PM   #6
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Have blown/cracked a few rims since we started riding in tandem in 1975.
We are a rather light team (under 250 lbs) and most rear rims have lasted into the +/- 20,000 mile range.
Have had some front rims go 50,000 miles.
Others experience may vary . . .
Pedal on!
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Old 03-21-14, 10:06 PM   #7
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"Suggestions?" Velocity Dyad
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Old 03-22-14, 07:40 AM   #8
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Thanks a lot for the info. I'm going to lace up the spare rim and then start shopping for a set of wheels. It's a plan.
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Old 03-22-14, 01:13 PM   #9
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Looks like a combination of rim wear and overinflation. That's definitely rim wear and the way the whole rim seems to be starting to sag outward is not good. Our 300# team touring with full luggage runs 115 lbs. on 28mm tires. Rim manufacturers inflation pressure limits approximately coincide with those recommended by tire manufacturers. Running a disc on an old worn rim means that you can't feel the rim pulsing in the rim brake lever, which is how I tell when it's time to retire a rim.
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Old 03-22-14, 01:54 PM   #10
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Originally Posted by Trsnrtr View Post
Thanks a lot for the info. I'm going to lace up the spare rim and then start shopping for a set of wheels. It's a plan.
In shopping for wheels I would suggest that you consider wheels that are:

1. 11 speed compatible
2. Tubeless compatible
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Old 03-22-14, 06:25 PM   #11
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Hi,

6K miles, congratulations on wearing something out. Great job and you should celebrate, with new wheels.

I always take great pride in wearing out a tire, a chain, cassette or wheels.
We blew out a Rolf rear wheel on the 2nd to the last day of Cycle Oregon this year and I feel we got our miles and years out of that set of wheels.

As long as you did not crash and you can afford to replace it, you have achieved.

Upgrade and keep riding.
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Old 03-22-14, 08:25 PM   #12
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Originally Posted by Trsnrtr View Post
Well, got our Santana out for its first ride this year and found that the rear 40 Fir rim has blown. Rim has about 6,000 miles on it and has been in use with a heavy team/bike weight of 440# with Conti 28c tires pumped to 130#. Tandem has had a rear disc for all but a 1,000 miles or so, so it's unlikely that sidewall wear caused it.

Luckily, we have a spare 40 rim that I can lace on to it but our total team/bike weight is now 350# so it may be time for new wheels.

Suggestions?


image by trsnrtr, on Flickr
Fir rim's are not high on my list on great rims. I had picked up an 93 Santana Rio with Fir rims a few months back and noticed the seam in the rim, they are not welded and most likely pinned where you see the split. You got great advice, replace the rim. What is your replacement rim as 1 spoke eyelet has pulled from my Fir rim?

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Old 03-23-14, 05:06 AM   #13
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Rims do fail, many reasons why and several ways to fail. While I don't like stating it, rims (not wheel assemblies) are pretty much a wear item or expendable. They see a lot of stresses. Some are high quality materials, some lesser quality materials. Regardless, under heavier teams, ourselves included, they don't last forever.

If the hubs are good quality, buy two new rims and relace both ends. If the hubs are not higher end, go for new wheels. If you need or do go with new boutique wheels, shop carefully, there are a few horror stories or two one this website. Myself, I would have no looking back to build some DT or Chris King hubbed wheels, with a rim of our choice, and would consider possibly a custom drilled carbon fibre rim as part of the process.

PK

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Old 03-23-14, 06:04 AM   #14
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IMHO, the rim is not failing at the joint, but rather the edge is separated from the rim just below the joint. The sidewall of the rim looks to be well worn, almost concave in appearance. If so, the thickness of the wall is significantly worn down to the point where the edge is separated from the wall at the worn outer edge of the side. This is a result of a very aggressive brake pad in combination of a machined braking surface (?) which reduces wall thickness to achieve the flat surface. IOW, worn out! Lot of assumptions in this analysis but if they are all right, then the analysis could be too!
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Old 03-23-14, 12:48 PM   #15
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^^^ Chicken and egg scenario. Can't say if the weld popped first or if was the rim-edge.

The rim does look quite worn for having only 6000 miles on it (OP). Plus, a team of 440# probably should have been using a bigger tire (ie: 32mm) at lower pressure than 130psi. I wasn't able to find the specs for Santana's FIR wheels, so no idea what the rim's max psi rating is.
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Old 03-24-14, 01:01 AM   #16
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"Suggestions?" Velocity Dyad
Certainly a tandem workhorse, but not immune to circumferential cracks. We discovered one nearly all the way around on our rear wheel two months ago, somewhere between 4,000-5,000 miles. We run Marathon Supremes (28x50) at about 50 psi supporting a ready to ride combined wight of bike, gear and riders of about 340 lbs. Velocity US quickly offered us a choice of replacements. Chose the Atlas (a bit wider). Velocity also sent us a second if and when we wanted to change out the front. While we did use a supplemental V brake a bit when the Dyad was new, an Avid BB7 did 99% of that wheel's rear braking.
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Old 03-24-14, 08:48 AM   #17
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^^^ Chicken and egg scenario. Can't say if the weld popped first or if was the rim-edge.

The rim does look quite worn for having only 6000 miles on it (OP). Plus, a team of 440# probably should have been using a bigger tire (ie: 32mm) at lower pressure than 130psi. I wasn't able to find the specs for Santana's FIR wheels, so no idea what the rim's max psi rating is.
Only about 1,000 miles on the rim with a rim brake. The tandem has had a disc since the first 1,000.
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Old 03-24-14, 11:05 AM   #18
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Riding on the rim after a blowout down a steep descent throughout in a complete new set of variables! If you hit a sharp object, pot hole, anything that could stress the rim you could have damaged the rim to the point that a stress riser was created that was just waiting to cause a crack. At your former weight and the pressure that you were running and the fact that it was ridden with a flat tire is enough to indicate that a failure was imminent.
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Old 03-24-14, 11:50 AM   #19
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Certainly a tandem workhorse, but not immune to circumferential cracks. We discovered one nearly all the way around on our rear wheel two months ago, somewhere between 4,000-5,000 miles. We run Marathon Supremes (28x50) at about 50 psi supporting a ready to ride combined wight of bike, gear and riders of about 340 lbs. Velocity US quickly offered us a choice of replacements. Chose the Atlas (a bit wider). Velocity also sent us a second if and when we wanted to change out the front. While we did use a supplemental V brake a bit when the Dyad was new, an Avid BB7 did 99% of that wheel's rear braking.
I don't know how your rims failed but we run 40mm tires and this has caused me to look into the added stresses on the rim wall from wide tires. Wider tires put much more outward force on the rim bead than narrow ones and therefore can cause rim cracking. In this case the cracks will run in an arc around the rim. The height of the well between the rim walls can be crucial because a deeper well provide less support to the lip of the brake track. In our case switching to a rim with shorter interior walls solved the problem even thought they were lighter rims from the same manufacturer.
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Old 03-24-14, 01:33 PM   #20
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A few points. I find it hard to swallow that 130psi as compared to, say, "just" 120psi would cause that problem. Anything engineered properly, even a lightweight-by-design bike component, has a bigger safety factor than that. I've run 145psi on 23mm clinchers on a solo bike and never seen that sort of thing.

That it is near the "weld" is insignificant unless the joint wasn't aligned well at manufacture, in which case that would be a manufacturing defect. (Jobst Brandt's book The Bicycle Wheel says that the joint needn't be welded at all, but should be pinned for alignment. He says that there can be half a ton or more of force holding the wheel together, and describes the experiment of cutting a rim into many sections then lacing up a wheel which could not be pulled apart.) The rupture on that wheel does not appear to start right at the joint anyway.

In most cases a wheel will fatigue from load by the spokes pulling through or the inner surface of the rim developing longitudinal cracks. (We had a Matrix rim on the rear of our tandem start cracking at a spoke nipple.)

So without knowing anything more about the rim's history I'd say it is either a defect or the result of lots of braking, which suggests lots of use of course.
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Old 03-24-14, 08:39 PM   #21
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Wider tires put much more outward force on the rim bead than narrow .
Why is this true? Force = pressure x area. Wider tires generally run at lower pressure.
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Old 03-25-14, 10:11 AM   #22
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Why is this true? Force = pressure x area. Wider tires generally run at lower pressure.
Maybe I was not clear that other variables are being held constant. At the same pressure, and the same rim, wider tires put more outward pressure on the rim bead than narrower tires.

The angle at which the tire wall attaches to the rim also effects the outward pressure on the rim. A narrow tire attaches at a nearly vertical angle while a fat tire is angled away from the rim or in an extreme case nearly 90 degrees to the rim wall. While fatter tires are usually run at lower pressures the problem is that the increase in stress due to the change in angle rises at a faster rate than the reduction of stress due to the reduction in pressure. Below is a quote from another thread that explains it in more detail:

Quote:
Influence of tire pressure and width on the rim stress

The tour Forum discussion about the effects of different tire widths
and Luftdrücke was on the load on the rim. This particular case was
about whether a MTB rim as the a 50mm tire with max. Designed 4bar, is
a 25mm tire to cope with 8 bar pressure can be bent without them too.

Since I has not left in peace, I once counted something: The bending
stress on the rim sidewalls consist primarily of two components, the
direct internal pressure acting on the flanks, and the horizontal
portion of the circumferential force of the tire. The latter one has
to think about this: The tire section is simplified seen a ring. When
this pressure is evenly under tension. Because of the "ring" but the
lower part is open and there is held together only available via the
rim, the rim must take this tension there. Interesting for the bent-up
in the case only the horizontal component of the voltage at the
junction of tire rim. This is the exit angle of the sidewall of the
rim depends. All horizontal forces are not neglected because they
hardly relevant for the question have. The calculation is based only
on one edge of the rim on the other hand, the burden of course
identical. Exposure to peripheral force: The exit angle α the sidewall
can be calculated geometrically: with b = B = inside rim width tire
width

The tangential force is calculated according to the boiler formula as
follows: p = tire pressure (1 bar corresponds to 0.1 N/mm2) D =
Outside diameter of the rim horizontal component FU_X obtained cos α
available via:
Calculation of the resulting force in the middle of the edge: the area
of the inside of the rim edge A is: h = height inner rim flank This
results in the resultant force Fp_R:

Calculation of the moment on the rim: at the foot of the Cross, the
forces lead to a bending moment to the outside: It all looks a mess
out in a formula, you get this:
h = internal height rim flank
p = tire pressure (1 bar corresponds to 0.1 N/mm2)
B = Tyre width
D = Outside diameter of the rim
b = internal width of the rim
means of this formula, you can now compare what you nicely with the appropriate
tires and air pressure of its rim so exacts. For ease, I've also
created a small Excel document to calculate.

Last edited by waynesulak; 03-25-14 at 10:16 AM.
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Old 03-26-14, 06:20 PM   #23
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hi,

6k miles, congratulations on wearing something out. Great job and you should celebrate, with new wheels.

I always take great pride in wearing out a tire, a chain, cassette or wheels.
We blew out a rolf rear wheel on the 2nd to the last day of cycle oregon this year and i feel we got our miles and years out of that set of wheels.

As long as you did not crash and you can afford to replace it, you have achieved.

Upgrade and keep riding.
what he said!
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