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Old 07-10-12, 09:24 PM   #1
Cazzzidy
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Choosing spokes for an unusual high load application

Hello all!

Burning man is around the corner and it's time to fix up the old long-bicycle. I made it out of a hacked up Schwinn cruiser.



Last year the rear wheel taco'd while carrying three people. Super bummer.

So this year I'm going to build it a super tough (but cheap) rear wheel that will never fail. I'm a handy guy and grew up working in a machine shop, so I'm not too worried about how to do it... but I need the forums help to choose the right parts.

I want to use the following:

Hub: Shimano Nexus 3-speed hub (SG 3C41) http://www.amazon.com/Shimano-SG-3C4.../dp/B001GSKVIQ

Rim: Mavic XM117 Disc Rim 2011: http://www.chainreactioncycles.com/M...?ModelID=36245

Both are 36 hole. That's as far as I've got.

Now how do I choose and order the correct spokes? I want the strongest spokes possible. Should I buy double or triple butted? How long? What nipples?

Also - I am considering drilling out the Shimano hub to accomodate larger spokes, if the rim can take it. Is that a crazy idea?

Thanks all,

Cassidy
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Old 07-10-12, 09:33 PM   #2
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There's a difference between spoke strength in absolute terms such as load capacity and spoke strength in terms of building a wheel that lasts.

For pure strength, the rule is fairly straightforward - more steel = more strength. For your Burning Man wheel, I'd consider 13/14g single butted spokes. The added material at the elbow makes up for the strength lost by virtue of the 90° elbow, while 14g is plenty otherwise. If you can't find 13/14g spokes in the right length, then go for plain 14g spokes.

This advice is different than if you asked about a wheel for a cross country tour, where I'd be suggesting 14/16g butted spokes because they build a tougher wheel (but weaker in the sense you need).
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Old 07-10-12, 09:43 PM   #3
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Might be worth looking into rear wheels designed for tandems. I think 40 and 48 spoke count wheels are used on tandems.

The velocity dyad comes in a 48 spoke version.

I'd presume in drilling the hub to take larger spokes you would lose any advantage of stronger spokes by increasing your chance of the hub failing at the flange.
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Old 07-10-12, 09:50 PM   #4
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Spokes are not the primary problem, they're stronger than you think.

1. Get a beefier rim - for all the strength spokes have in all guages - they're limited by the rim they are built into.
2. As already suggested, try to go 48 hole if you can.

Straight guage DT Swiss 14g and just about any brand 13/14g single butted stainless steel spoke is good enough spoke wise...

=8-)
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Old 07-10-12, 10:11 PM   #5
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It's not the parts, it's the skill of the builder. A 36-spoke 26" wheel on a 3-speed hub will be strong enough to carry several people if it's built and tensioned correctly. A good wheelbuilder can make crappy parts last forever. A poor wheelbuilder can take great parts and create a crappy wheel. Sometimes it's tough to tell the difference.

That being said- if you're carrying a bunch of weight on the back of that longtail, I would choose a wider rim, such as an Alex DX32: http://www.universalcycles.com/shopp...&category=1705 just to give the wheel some additional lateral stability.
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Old 07-10-12, 11:47 PM   #6
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Thank you

Thank you everyone for your suggestions.

Jeff - I really appreciate that rim recommendation. The cross section looks wicked strong! I'm going to buy it.

MrRabbit & Omiak - I will need to stay with a 36 spoke rim because I need to use a Shimano Nexus 3 speed hub with coaster. Given the design of the frame, the 3 speed is the only way I can have gears and a brake. Both are important.

FBinNY - Thank you for the spoke recommendation. I understand I need 14g straight spokes or 13/14 single butted spokes. How do I determine what length spokes I need without measuring (as in, before I order...)? And how do I choose correct nipples? I've found a few great guides to wheel building online but they do not address choosing correct parts.

My plan is to order all the parts and build the wheel with my father on a saturday. He used to build motorcycle wheels in the sixties so I think we can blunder through. But if I don't have all the right parts in hand we're kinda screwed...

Thanks so much everyone,

Cassidy
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Old 07-11-12, 12:01 AM   #7
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motor cycle spokes may be more than 12 gage [more in AWG is a lower number]

example: Brompton uses short 12 gage spokes on it's rear wheel, 14 gage for the front.
same spoke count, the loading heavier.. the spokes are thicker..
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Old 07-11-12, 12:07 AM   #8
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Originally Posted by Omiak View Post
Might be worth looking into rear wheels designed for tandems. I think 40 and 48 spoke count wheels are used on tandems.

The velocity dyad comes in a 48 spoke version.

I'd presume in drilling the hub to take larger spokes you would lose any advantage of stronger spokes by increasing your chance of the hub failing at the flange.
This would be the best for what you want.
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Old 07-11-12, 12:44 AM   #9
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Until you break a spoke, the spoke-strength is not a factor in your previous wheel failure. Wheels are inside-out suspension-bridges that work on tension and the higher the tension, the stronger the wheel. Up to the point where you start cracking the spoke-holes at the rim. I suspect that the wheels that failed you last year had way too low of tension and that's the cause of the wheel failure. Take a look at this chart of spoke-tension at various spots on a wheel:


Notice that spoke-tension in dynamic conditions barely increases as the wheel rolls. But at the bottom it loses most of its tension. The hub effectively "hangs" from all the spokes outside of the load-zone at the bottom. The problem is, if you have too low tension, the 3-4 spokes at the bottom lose ALL tension and the rim is then very sensitive to side-loads. Such as the wobbling from pedaling and people moving around. This side load will then cause the unbalanced spoke-tension to pull the wheel into a taco-shape.

1. What you need then is the strongest rim possible, the Alex DX32 rim that Jeff posted is amongst the strongest. I use it on our tandem and my wife and I have bombed down hills at +60mph for the past 3-years without any issues. Even ran over a giant 25-lb possum/raccoon/ROUS of some sort and split it in two. Wheel stayed perfectly straight.

2. The 13/14g spokes will work for you just fine, that's the maximum that'll fit the 3-spd hub. Figure out the spoke-length you need for 3x lacing with this Damon Rinard - Spocalc programme. Remember, the higher the spoke-tension, the more load-capacity the wheel has before the spokes at the bottom lose all tension. You'll want to tension it up the maximum tension recommended by the rim-manufacturer. In this case, around 110-120 kgf all around since you've got no dish (130-140 kgf has been used with this rim for extreme-load conditions). One popular technique in finding that optimum tension without a tensionometer is to get the wheel perfectly true, the go around and tighten the spokes 1/4-turn all around. Keep on going until the rim starts to taco every so slightly. Then back off 1/8th turn and it should be straight again. This is the limit of what that rim can take and the tension is now optimized for maximum load-capacity.

3. Also use a fat tyre, 26x2.125" at pressure optimized for the load. Probably around 50-60psi in rear, and 30-35 in front for 3 people. The larger the tyre and lower the pressure, the better shock-absorption you have and the lower the peak G-forces experienced by the wheel.

Last edited by DannoXYZ; 07-11-12 at 03:22 AM.
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Old 07-11-12, 01:42 AM   #10
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If you have machining ability there are all kinds of funnies one can make. Adapter flanges for instance. Find a rim with a larger number of spoke holes, say a 48H. Cut a ring out of sheet metal, sized to fit over hub flange and protrude enough to allow for a 2nd row of spoke holes outside the hub flange. Drill outer row in whatever configuration you need, then drill inner row to allow adapter flange to be screwed onto current hub. Remember that spoke holes are offset from one flange to the other.

Or use same concept to fit a motorcycle rim + spokes to a bicycle hub.

Or go for a 24H rim, they usually have to be fairly rigid anyhow. Redrill as 48H. Go for a paired spoke pattern, or you'll end up with one nipple trying to utilize the valve hole. It'll still be crowded, and final tensioning probably isn't much fun. But as far as I can see, it should be doable.

Might look for those nipples that can be turned with a kind of hex socket from the tire side of the rim instead of the regular kind.

Or my current personal favourite wheelbuild daydream, the double decker - which is probably a royal PITA to lace. Starting from a 36H hub in your case, and two 36H rims. Redrill hub as 72H. Use original set of spoke holes to lace to one rim, and bonus holes to the other. Just pretend you're using a rim with a whopping offset and the available calculators should be able to handle the setup just fine.

It'd be doable in radial lace, no doubt. But maybe 3X would lead to too much interference.
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Old 07-11-12, 06:33 AM   #11
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Spokes are not the primary problem, they're stronger than you think.

1. Get a beefier rim - for all the strength spokes have in all guages - they're limited by the rim they are built into.
2. As already suggested, try to go 48 hole if you can.

Straight guage DT Swiss 14g and just about any brand 13/14g single butted stainless steel spoke is good enough spoke wise...

=8-)
Absolutely agree. The problem is the rim, and giant spokes won't fix that.
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Old 07-11-12, 07:16 AM   #12
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Try a wide, beefy downhill, 35 mm wide rim. 36 hole should work.
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Old 07-11-12, 07:29 AM   #13
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Wheels don't taco from radial loads, and are incredibly strong radially regardless of the spokes or rim. (My front tubular wheel with a 300gr rim and 14/17g butted spokes once supported the the rear quarter of a car).

The tacoing the OP experienced was probably caused by excessive side loads. The rules for side loading are very different than for radial loads, and involve flexing the wheel to where the rim was so far sideways that it could move to the lower tension S or potato chip shape. This isn't as far as one might expect, and a decent number of new wheelbuilders experience the same effect while stress reliving their new builds by the lay-flat-and push-the-rim-to-the-floor method.

The key in preventing this type of failure is to reduce the horizontal deflection for any given load. And the way that's done is with more steel, either more spokes, and/or thicker spokes. A stiffer rim will help by handling the increased spoke load better, but that's secondary since it doesn't contribute significantly to reducing the axial (sideways) deflection. Also be aware that increased tension doesn't stiffen a wheel since the elastic property is inherent in the spoke and doesn't change with tension.

As I said originally, there's a difference in absolute strength and ability to last a long time measured in miles ridden, and the OPs max load/few miles needs are different from those of the normal cyclist.

To the OP, you can calculate spoke length using any of the spoke calculators available on-line (my favorite). You need to know the "ERD" of the rim, which is usually available on line form the rim maker. You select nipples according to the spokes' thread end. If you're using 14g or 13/14g spokes that means a 14g nipple. Note that you don't want to use a straight 13g spoke and 13g nipples, not because they're bad luck, but because the OD of 13g nipples is larger, and most rims aren't drilled for them.
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Old 07-11-12, 06:28 PM   #14
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Thank you everyone for the informative replies. Yes, it's definetely the lateral load that destroyed the previous wheel. I remember riding through a soft dune, then the wheel became wobbly for five or six rotations before it abruptly folded in half.

I have used the Spocal excel spreadsheet to determine my spoke needs and just placed a big order! Can't wait to get my hands on that big beefy Alex rim!

One final question - I purchased 2.00x12mm brass nipples. What is the correct spoke wrench size for these? Can't find that anywhere...

Thanks all

Cassidy
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Old 07-11-12, 07:26 PM   #15
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Quote:
Originally Posted by Cazzzidy View Post
Thank you everyone for the informative replies. Yes, it's definetely the lateral load that destroyed the previous wheel. I remember riding through a soft dune, then the wheel became wobbly for five or six rotations before it abruptly folded in half.

I have used the Spocal excel spreadsheet to determine my spoke needs and just placed a big order! Can't wait to get my hands on that big beefy Alex rim!

One final question - I purchased 2.00x12mm brass nipples. What is the correct spoke wrench size for these? Can't find that anywhere...

Thanks all

Cassidy
Spoke wrench/nipple fit isn't totally about the thread size. There are small differences in the width across the flats among brands. So 14g spoke wrenches came in 3 basic sizes, with DT being the smallest, and generic Italian nipples the largest. Unfortunaely the tolerance in spoke wrench sizes is about as much as the difference between the various nipples, so I keep spoke wrenches in a variety of sizes, and use whichever fits tightest regardless of whether it's the "right" one.
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