Spoke length calculaton

[merwe]

There is a couple of threads like this already on this forum, but since it is a topic that deals with a lot of specifics, I hope I will be forgiven for starting a thread like this yet again.

 

I want to replace the Shimano HB-RM65 disc hub on my front wheel with a Hope Pro EVO 2 15mm, since I want to upgrade my shock to a 15mm axle. I measured the ERD of the rim, an Alex Ace 17 that came with the bike, to be approximately 543mm. SInce the current hub and spokes are still on the rim, It was quite a mission getting this. I measured the rim diameter, to be 571mm, subtracted 2 x the height of the rim wall plus the length of the protruding nipple, and added twice the nipple length (12mm). I also subtracte 2mm for I noticed the spokes sit about 1mm deep inside the nipples from the back.

 

I then removed two spokes from either side of the hub, and measered them to be 260mm and 262mm.

 

I measured the shimano hub and got the following dimensions:

 

Right flange ø: 45

Left flange ø: 45

Centre to right flange: 34

Centre to left flange: 22

Spoke hole ø: 2.5

 

I fed this in to the spoke calculator i found here http://lenni.info/edd/ and by specifying 3 crosses and 32 spokes got these results:

 

Left spoke: 263.4mm

Right spoke 264.6mm

 

This is a bit longer than what I measured.

 

I then did the same calculation with the Hope hub that I got from the site database, and got these results:

 

Hope pro 2 evo dimensions:

Right flange ø: 54

Left flange ø: 56

Centre to right flange: 33

Centre to left flange: 20

Spoke hole ø: 2.4

 

Left spoke: 261.6mm

Right spoke 263.2mm

 

The spoke brand is "SLE spoke" from this website: http://www.mrrabbit.net/docs/spokeheads/main.html, for what it is worth.

 

Now for my question, unless I messed up the rim diameter measurement, why does the calculated result differs so much from the spokes actually used, and will these spokes be fine with the hope hub. If I look at only the hope results and the measured spoke lengths, it certainly looks like it.

 

Sorry for this long and winding post, I just want to make sure I present all the data.

[kosmonooit]

Good luck, everyone seems to have their own story on this. Double check / cross reference all sources and measurements is my only advice.

 

I've used this spoke length calculator with some success http://www.wheelpro.co.uk/spokecalc/

 

Generally round off upwards, some add a mm or so.

[merwe]

I suppose I have nothing to loose, i can just buy the hub, try it out and if it does not work buy other spokes and try again.

[Greg29]

Spokes are tricky to get right. If you want expert advice give Johan Bornman from Yellow Saddle a call on 0823086847 (website: www.yellowsaddle.co.za). He is a full time bike builder / mechanic but specialises in wheel building.

[David Marshall]

Good luck, everyone seems to have their own story on this. Double check / cross reference all sources and measurements is my only advice.

 

I've used this spoke length calculator with some success http://www.wheelpro.co.uk/spokecalc/

 

Generally round off upwards, some add a mm or so.

 

I would say reduce the length by 1 to 2.5mm (1 for competition type spokes and up to 2.5 for revolution type spokes). You need to measure the ERD more accurately than you have done above. Measure the length of two spokes from the end of the spoke to the J bend. then put the spokes into opposite holes in the rim, fit a nipple to each of two spokes and screw the nipple to the point where the spoke is flush with the slot in the nipple. Pull the spokes towards each other and measure the distance between them (to the same point on the J bend that you measured the spoke length. The length of the two spokes plus the gap between them will give you the ERD.

 

The spokes on your existing build are a bit short. They should not be recessed in the nipples, or stand proud for that matter.

 

On the surface it looks as if your spokes should work for your proposed hub. Be sure not to mix up drive and non drive side spokes AND make sure that you use inbound and outbound as they were.

 

Check and recheck your measurements- preferably with someone else as any error will only become apparent well into the wheel build.

[Chro Mo]

I use the ubi spoke calculator - generally very accurate

 

http://www.bikeschool.com/tools/spoke-length-calculator

 

as stated above, it is imperative that the erd is measured completely accurately

[Minion]

Now for my question, unless I messed up the rim diameter measurement, why does the calculated result differs so much from the spokes actually used, and will these spokes be fine with the hope hub. If I look at only the hope results and the measured spoke lengths, it certainly looks like it.

The calculated result gives you the distance from the flange hole to the ERD of an unloaded rim. The spokes you measured were untensioned.

 

In reality, once you tension the wheel, the spokes stretch, lengthening them and the rim compresses, reducing the diameter. This is why you generally knock a millimetre or two off the calculated length to get actual cut length.

[merwe]

Thanks for all the helpfull responses so far.

 

The calculated result gives you the distance from the flange hole to the ERD of an unloaded rim. The spokes you measured were untensioned.

 

In reality, once you tension the wheel, the spokes stretch, lengthening them and the rim compresses, reducing the diameter. This is why you generally knock a millimetre or two off the calculated length to get actual cut length.

 

Ok, so the ERD should ideally not be measured on a loaded rim, as I did in this case?

Edited July 16, 2012 by merwe