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Old 03-09-11, 11:59 AM   #1
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Wheel Durability: More spokes or Beefier Rim

I'm unhappy with the durability of my rear wheel. My rim got a crack along half of the outer circumference in the middle of the "braking strip". Sure, the obvious answer is to get a beefier rim, but what about going with the same rim with more spokes? Rider weight 250, disc brakes. I need a new hub anyway.....
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Old 03-09-11, 12:08 PM   #2
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How old is the rim? Rim brakes eventually erode the brakeing surface and cause it to fail as you have described. If the brakeing surface feels convex, that's it. If that's your problem, more spokes aren't going to solve it. You either need a beefier rim or you can plan on replacing the same rim more often.
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Old 03-09-11, 12:10 PM   #3
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It would help to know what kind of riding you are doing, I guess the disk brake implies mountain bike but it would be good to be sure. Also what is the wheel you are currently using and how many spokes does it have.

In my opinion more spokes with a quality but not very deep rim is your best combination. The deeper and thus heavier a wheel is the harder it is to accelerate. I prefer 32 to 36 spokes on a quality double wall rim, if built right you should not have any issues with that combination.
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Old 03-09-11, 12:14 PM   #4
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tomayto, tamahto...

You don't say what kind of bike or what brand rim it was so we don't know if it was good or not or how long or what conditions it was ridden in.
A crack in the middle of the braking surface of the rims doesn't scream spoke problem to me. It sounds like either fatigue or abuse or possibly poor quality rim that can't handle the air pressure applied in the tire, especially if the rim was straight when the crack occurred. Also rims will crack eventually, especially hardened rims depending on usage so it might be normal wear that caused it.

If your thinking of replacing you hub anyway and you have to replace your rim I would just be inclined to replace the set and not worry about getting matching rims..
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Old 03-09-11, 12:21 PM   #5
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15 k miles, disc brakes, loaded touring on asphalt and gravel, 145mm tandem hub - dishless.

Hugi tandem hub, 36x14 ga straight spokes, Dyad rim. Wheel was always true.

Thanks for your responses so far...

Last edited by Cyclesafe; 03-09-11 at 12:26 PM. Reason: Thanks.
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Old 03-09-11, 12:48 PM   #6
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Go for both!

250 lb load / touring / tandem / disk brakes

I`d suggest you go for the highest spoke count you can get in as solid a rim as you can afford. And 26in rims if you`re not already using them. Rims designed for electric bicycles are something else to consider. They can take 13 gage spokes so it depends if you can find a rear hub that can too. The extra weight would be less of an issue on a tandem.
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Old 03-09-11, 12:53 PM   #7
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250 lb load / touring / tandem / disk brakes

I`d suggest you go for the highest spoke count you can get in as solid a rim as you can afford. And 26in rims if you`re not already using them. Rims designed for electric bicycles are something else to consider. They can take 13 gage spokes so it depends if you can find a rear hub that can too. The extra weight would be less of an issue on a tandem.
Ah, but not a tandem. Rider+bike+gear = 250 lbs.

http://co-motion.com/index.php/singles/americano
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Old 03-09-11, 01:00 PM   #8
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Well after reading some of your other posts and responses it sounds like you almost made your mind. I mean just for grins and giggles you could try the Mavic A719 rim. Its a little more than the Dyad but same shape and a little beefier (I haven't had any problems with mine but they only have a little over 3000 miles on them and still look new). Otherwise if your dead set on using 700c wheels stick with your plan of replacing them every so often. I don't really think you have a problem with spoke count especially on a dishless rim and as others have stated more spokes isn't going to solve the braking surface cracks. That is more affected by tire pressure and weight. As others have said, 20000m on a loaded touring set isn't so bad.
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Old 03-09-11, 04:18 PM   #9
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LOL Ya got me!

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Originally Posted by Cyclesafe View Post
Ah, but not a tandem. Rider+bike+gear = 250 lbs.

http://co-motion.com/index.php/singles/americano
Tandem equipment on a bicycle built for ONE!

OK - those Mavics suggested are something I`d consider over the Dyads myself, and there are other rims I`ve been looking at too. Check out the Rigida website and their Andra 10/20/30 and Grizzly rims. The Andras are recommended for ebikes and heavy loads but may weigh more than some people would accept on a regular bike. In your case I don`t know.

But really nice bike!!!
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Old 03-09-11, 05:03 PM   #10
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Need to separate some wheat from the chafe on this.

The rim failed at the tire flange, so more or beefier spokes will have zero effect. Then you mention disc brakes, which if you always had them, means that brake wear isn't the cause. Even if you were using calipers I don't know if various rims have thicker tire flanges.

So if you want a rim that won't fail the same way you have to analyze the actual cause of the failure, which I suspect is chronic over-inflation of the tires, possibly combined with too large a section for the rims.

Before you say that you don't over-inflate, consider that the max pressure molded into tires only relates to the tire's strength. But with wired -on (clincher) tires the rim itself is part of the pressure vessel, and the flanges have to hold against the outward pull of the beads. Applying the formula for hoop stress of pressure vessels, we find that that the stress on the flanges is proportional to both the pressure and cross-section diameter of the tire. So at the same pressure a 45mm tire stresses the rim flange almost twice as much as a 23mm tire.

Rim makers are weight conscious so tend to design their rims carefully around the expected tire section and pressure, and not leave much room for error. If the rim is designed around a 95psi 25mm tire, you have to be careful about max pressure of larger tires.
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Old 03-09-11, 05:28 PM   #11
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36 double butted spokes on a Mavic 319 rim. The drive side should be tensioned to 110kg. That is the upper limit that Mavic recommends.
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Old 03-09-11, 05:34 PM   #12
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36 double butted spokes on a Mavic 319 rim. The drive side should be tensioned to 110kg. That is the upper limit that Mavic recommends.
Yes, that would be fine, but pray tell, how does this improve the strenght of the tire flange, which is where the OP's rim failed
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Old 03-09-11, 06:42 PM   #13
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Need to separate some wheat from the chafe on this.

The rim failed at the tire flange, so more or beefier spokes will have zero effect. Then you mention disc brakes, which if you always had them, means that brake wear isn't the cause. Even if you were using calipers I don't know if various rims have thicker tire flanges.

So if you want a rim that won't fail the same way you have to analyze the actual cause of the failure, which I suspect is chronic over-inflation of the tires, possibly combined with too large a section for the rims.

Before you say that you don't over-inflate, consider that the max pressure molded into tires only relates to the tire's strength. But with wired -on (clincher) tires the rim itself is part of the pressure vessel, and the flanges have to hold against the outward pull of the beads. Applying the formula for hoop stress of pressure vessels, we find that that the stress on the flanges is proportional to both the pressure and cross-section diameter of the tire. So at the same pressure a 45mm tire stresses the rim flange almost twice as much as a 23mm tire.

Rim makers are weight conscious so tend to design their rims carefully around the expected tire section and pressure, and not leave much room for error. If the rim is designed around a 95psi 25mm tire, you have to be careful about max pressure of larger tires.
Thanks! I can certainly live with not inflating tires to the maximum pressure. 80% of max is supposed to be more efficient anyway....
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Old 03-09-11, 06:56 PM   #14
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Since you're building fresh, work backward, starting with optimizing the tires cross section. Then selecting the correct rim for that tire, then building correctly using butted spokes of the appropriate gauge.

Using a larger section will allow you to reduce pressure while still maintaining low rolling resistance.
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Old 03-09-11, 09:45 PM   #15
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You've had a Dyad and you've heard recommented Mavic A719, A319. These are all hefty rim-brake-compatible rims, but since you don't have rim brakes on this bike, you might prefer a disc specific rim instead.

A disc rim will have less unsupported flange to tear out and no brake wear indicator to act as a stress riser; disk rims like Velocity P35 or Salsa Gordo are available with super wide flange spacing which further reduces the stress on the flanges when you use 40mm+ tires. Without having to support a braking surface you also get more strength for the same weight.

Generally though I do agree with the suspicion that you might have been overinflating the tires. What size tire and pressure were you using?
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Old 03-09-11, 10:57 PM   #16
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A stiffer rim will give your wheel better lateral/torque resistance strength, but for vertical strength it's mostly all in the spokes... it sounds like you need a bit strong rim.
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Old 03-09-11, 11:05 PM   #17
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Generally though I do agree with the suspicion that you might have been overinflating the tires. What size tire and pressure were you using?
37-622 Marathon XR's and 37-622 Marathon Supremes inflated at 80 lbs and 30-622 Marathon Racers inflated at 100 lbs.

As for a disc specific stronger rim, how about the Velocity Chukker? Same width as the Dyad, but allegedly much stronger.

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Old 03-10-11, 01:07 AM   #18
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Think I`ll just cut the chase and post the specs on what the OP has currently and why I recommended what I did.

The currently problomatic Velocity Dyad rims the OP has have a 24mm outside dimension and weigh 480 grams in a 622 rim size.

It should be reasonable to assume that using the same materials, and similar design constraints - it would be necessary to add more material to achieve a stronger rim. So a stronger rim should unfortunately also weigh more.

The Mavic A719 rims have a 24.5mm outside dimension and weigh 565 grams in a 622 rim size - 17.7% more than the Velocity Dyads.

The Rigida Grizzly TS rims have an outside dimension of 22.7mm and weigh 540 grams in a 622 rim size- 12.5% more than the Velocity Dyads.

The Rigida Andra 10 rims have an outside dimension of 25.8mm and weigh 800 grams in a 622 rim size - 66.6% more than a Velocity Dyad.

The Rigida Andra 20 rims have an outside dimension of 25.8mm and weigh 730 grams in a 622 rim size - 52% more than a Velocity Dyad.

The Rigida Andra 30 rims have an outside dimension of 25mm and weigh 815 grams in a 622 rim size - 69.8% more than a Velocity Dyad.

ALL of the Rigida rims are cited by the manufacturer as suitable for tire sizes with profiles from 28 to 62mm and are recommended for heavy duty applications like ebikes and loaded touring so I personally think its safe to assume that the sidewall strength was engineered accordingly.

Particularly since larger sized tires always have a lower PSI rating anyway.

And basicly thats why I was looking at these myself. The additional weight on a 250lb fully loaded touring package becomes very incidental.
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Old 03-10-11, 01:41 AM   #19
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We have designed many touring frames with 145 mm rear spacing to use tandem hubs which enables us to build 0 dish rear wheels... this should almost become a standard on new geared bikes as it offers the simplest solution when it comes to making a wheel stronger.

We also offer a 165mm hub but that is a completely different beast...

I'd look at the Mavic 719 as a replacement as it is a rim I have built up many many times for touring bikes and to date their failure rate has been zero... and do follow FB's advice on tyre pressure and wider tyres as this is important info to have.

The Salsa Gordo has also proven itself to be a very good to excellent rim.

The reason many wider tyres have lower psi ratings is not because they cannot handle higher psi but that the higher psi can overwhelm a rim's structural capacity... you really do not want your sidewalls cracking and splitting on you as the consequences can be pretty nasty if it happens while you are rolling.
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Old 03-10-11, 11:15 AM   #20
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Thank you for the responses so far. I am becoming convinced that indeed I need a stronger rim, albiet not so apparently strong as some of those Rigidas. The A719 would be a good one, as suggested.

On the GDMBR I was riding behind someone who had a Blunt on his rear wheel explode, so I appreciate catestrophic rim failure. It seemed as if I could actually see the shock wave. Aluminum shrapnel went everywhere, but fortunately neither of us was hit. We could have been talking serious injuries.
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Old 03-10-11, 11:43 AM   #21
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I have been in the shop when rims have blown out... the last one appeared to stem from a manufacturing defect as the rim itself was fairly new and did not seem to have experienced any trauma... the tyre had not even been inflated to minimum pressure when this happened.

We were fortunate no-one got seriously injured as it blew shrapnel all over the shop at pretty high velocity.
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Old 03-10-11, 05:00 PM   #22
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I`m thinking some people on here would be good candidates to buy shares in the Broklyn Bridge!

The idea that low pressure ballon tire put more stress on a rim than a high pressure tire must be some newly discovered thingy that also bends time and space.

PSI is PSI pounds per square inch. And its the same everywhere inside a tire - with or without a tube. Exactly the same as hydraulics. A tire thats inflated to 60PSI puts 60 pounds per square inch on the rim - thats it! A tire thats inflated to 120PSI puts 120 pounds per square inch on the rim. The rim doesn`t magically change size so if the PSI is lower so is the stress on the rim.

The Schwalbe website has some comentaries where they have claimed to be very happy to be able to cut costs by using cheaper materials and less expensive manufacturing methods mixed with new compounds to produce tire that will deliver lower rolling resistance and carry a higher load using lower PSI because of the REDUCED STRESSES on the tire components. Less stress on the tire components is directly related to a reduced PSI and that would also result in less stress on everything else.

The real concern with narrow rims and wide tires is that the ride quality suffers under hard cornering when the bike is leaned over if the tires are underinflated, and if the tire pressure is low enough, it might be possible to unseat the tire bead.

Of course if you want to believe anything else - its a free country. This one is almost as good as the shop owner that has himself convinced that the bearing pre-load screw still impacts the bearing preload after the steering stem has been locked down.


Think I`m going out for ice-cream.
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Old 03-10-11, 05:27 PM   #23
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I`m thinking some people on here would be good candidates to buy shares in the Broklyn Bridge!

The idea that low pressure ballon tire put more stress on a rim than a high pressure tire must be some newly discovered thingy that also bends time and space.

PSI is PSI pounds per square inch. And its the same everywhere inside a tire - with or without a tube. Exactly the same as hydraulics. A tire thats inflated to 60PSI puts 60 pounds per square inch on the rim - thats it! A tire thats inflated to 120PSI puts 120 pounds per square inch on the rim. The rim doesn`t magically change size so if the PSI is lower so is the stress on the rim.

Think I`m going out for ice-cream.
Burton,

No ones trying to sell you the Brooklyn Bridge here (though there have been legitimate efforts recently to privatize it, and if that happens this idiom goes in the trash).

You start out with an incorrect assumption, or reading of my post. No one says a low pressure tire puts higher stress on a rim than a high pressure tire, that's ridiculous. But tire section is a definite factor on rim flange stress in wired on tires.

Bike tires are basically cylindrical pressure vessels bent around to form a donut. The stress on the tire wall is therefore subject to the same rules. But the tire is only part of the vessel, going only about 3/4 of the way around with the rim making up the last section. So just as the tire wall is under radial (hoop) tension, it tugs outward on the rim flange with equal tension.

Here's a nice explanation showing how pressure and radius are used to calculate hoop tension.

So to summarize, at for the same size tire rim stress is proportional to pressure, and at the same pressure rim stress is proportional to tire width.

I hope you enjoyed the ice cream. I think I'll join you.
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Old 03-10-11, 06:27 PM   #24
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I`m thinking some people on here would be good candidates to buy shares in the Broklyn Bridge!

The idea that low pressure ballon tire put more stress on a rim than a high pressure tire must be some newly discovered thingy that also bends time and space.

PSI is PSI pounds per square inch. And its the same everywhere inside a tire - with or without a tube. Exactly the same as hydraulics.
Okay. Let's talk hydraulics.
http://books.google.com/books?id=NBM...page&q&f=false

When you read that, you will discover that a larger pipe must have thicker walls to resist the same PSI as a narrow pipe.


The stress on a rim's bead seat works just like hydraulics.
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Old 03-10-11, 07:01 PM   #25
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Quote:
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The idea that low pressure ballon tire put more stress on a rim than a high pressure tire must be some newly discovered thingy that also bends time and space.

PSI is PSI pounds per square inch. And its the same everywhere inside a tire - with or without a tube. Exactly the same as hydraulics. A tire thats inflated to 60PSI puts 60 pounds per square inch on the rim - thats it! A tire thats inflated to 120PSI puts 120 pounds per square inch on the rim. The rim doesn`t magically change size so if the PSI is lower so is the stress on the rim.
I think that you're overlooking an important factor.

120 psi means 120 pounds of force on each and every square inch of the tire and rim. Now think about how many more square inches a fat balloon tire has than a skinny road racing tire. A good number of those extra square inches are trying to push the rim's bead seat flanges apart. Every additional square inch means there's an additional 120 pounds of force trying to push the rim flanges apart.

Can you say ka-boom?
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