What spoke length for a Puma motor in a 26" rim?

[fechter]

The title says it. I'm trying to find out exactly what the proper spoke length is for a Puma motor in a 26" rim.

I think the BMC hub motors are the same size, but I don't know for sure.

I tried one of those spoke length calculators, but there were too many things I was guessing on to feel comfortable with the answers. I was hoping someone had actually done a wheel and would know the right length.

 

[snowranger]

I don't think all 26" rims are the same. You can bring the rim and motor into a bike store and they will tell you. Box Dog Bikes in the City will provide spokes cut to size. I think it's 50 cents per spoke including nipples.

 

[dirtdad]

You are right, different 26" rims may require different spoke lengths.

To measure the numbers you need to input to a spoke length calculator, here is what to do.

1) Spoke Hole Diameter. Put two spokes in spoke holes on opposite sides of the hub. Hold the hub so the spokes hang vertically over a ruler and measure the distance between the tips of the spokes. For the BMC this is 160mm. For most ebike motors this is the same on both sides.

2) Flange to center distances. This is pretty easy too. Every hub motor I have seen is dishless. So measure the distance between the spoke flanges and divide by 2. The flange to center is the same on both sides for a dishless wheel. For the BMC this is 22mm.

3) For the rim, you will need to know its ERD, or Effective Rim Diameter. 26" rims do not all have the same ERD. ERD measures the distance between where the nipples seat on the rim, not where the tire bead sits on the rim. For most rims, you can find the manufacturer's published ERD somewhere on the web. If you are stuck without the ERD, you can measure it yourself. To see it explained in great detail, see http://www.sheldonbrown.com/rinard/spocalc.htm By the way, the late great Sheldon Brown was a bike guru beyond all others. Most technical bike questions are already answered on his website.

I like spocalc. Unlike most calculators you can add a custom hub, like a hub motor, and keep it as part of your spreadsheet for the next time you use it. Every time I build with a new hub or rim I add it to my list. But be careful note the number of crosses since they are all listed together. I am so used to doing 3X lacing that I have forgotten to look a 1x and radial results.

 

[docnjoj]

Just used spokecalc and its accurate to 1mm! Good enuff for me!
otherDoc

 

[dirtdad]

That is what I use and reference above. But you have to know what number to use with it. There are no electric hub motors in its database that I am aware of last I downloaded it.

 

[docnjoj]

There actualy are a couple, like the Sparc! U just need to plug in the correct diameter and split the flange to flange value as U described. The rim values (ERD) are probably better measured that count on theirs. My Sun Rhinolyte was dead on using "The master, Sheldons" values. This is for a respoke of a P2A, so of course I had to measure!
otherDoc

 

[JeffD]

Hi All,

I don't have a Puma but when I checked my 408 Crystalyte Rear wheel I found 2 really loose spokes. Upon further inspection it turned out they were actually broken at the threads. I bought my setup from the very reputable Ebikes.ca and was told by them I needed 192mm length. I ordered them but they were way too long. :x I was a bit surprized they had this wrong.

To keep it my tale short - I took the 4 spokes (got a couple extra) to a LBS and they cut them to 188mm and rolled me some more threads on them all for $5.00.

Now I'm finally up and rolling again after riding public transit for 3 weeks.

I learned that it isn't very easy to figure out what spoke size you need. It will likely cost you more time than money to replace them.

 

[vanilla ice]

Ditto JeffD on a 409 in a singlewall 26. 188. Or 192 and grind.

 

[JeffD]

My rims are double-walled but even so grinding down the spokes to fit would not have helped me as I ran out of threads.

 

[docnjoj]

Unfortunately the threads are the key issue. They make extended nipples for too short spokes, but long ones are a problem. U can cut off the excess, but probably cant tighten them enuff! Sheldon says 2mm either way should be OK! I hope so! I would definitely measure rim ERD as accurately as possible!
otherDoc

 

[fechter]

I was hoping someone had actually built a wheel with a Puma motor and could tell me what the length was. Assume a double wall rim like a Sun Rhinolyte 26". My Puma is at the LBS right now so I can't measure it.

 

[vanilla ice]

On the 192's I kept turning the nipple when it reached the end of the threads even tho it was a 2 or 3mm off. I don't think the spokes will pull out of there?

 

[dirtdad]

On the 192's I kept turning the nipple when it reached the end of the threads even tho it was a 2 or 3mm off. I don't think the spokes will pull out of there?

I am not sure what you are describing. You mean the nipples sort of tapped the spokes a little farther? Generally not a good thing if that is what you mean. Spokes are usually steel, nipples are usually brass or aluminum alloy, the steel usually wins that battle.