new wheel build

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brucelee
Posts: 253
Joined: 19 Feb 2009, 10:39am

new wheel build

Post by brucelee »

Hi, put this in the tech section rather than e-bike cos it's more about wheel building than anything else. I finally bought an ebike conversion kit for my surly LHT, it came with a prebuilt wheel with a really really heavy pinned rim and 13g spokes in a two cross lacing. Thinking I had read somewhere that the wheel had 2mm spokes, I ordered an A719 and some DT swiss competitions of (more or less) the right length. When I went to inspect the wheel more closely, I discovered it's got very thick flanges (4mm). There is also considerable clearance around the spokes that are present. The spoke holes however are heavily countersunk and I believe that the spokes could be inserted and bent correctly at the elbow. What I'm concerned about is the size of the spoke head and the size of the spoke hole. Anyone got any suggestions. What's the likely outcome of building a wheel with too large spoke holes and are there any fiddles ?
On a broader note, it seems to me e-bike wheels are way overbuilt. I've seen numerous reasons - weight, torque, speed. I've been building my own wheels for 15 years now and the build has never been a problem, despite enormous shopping loads and a heavy rider. I sure a rider can put out a lot more torque than a motor can and in a more irregular fashion from and engineering standpoint. My normal cruising speed is higher than the max the motor can achieve. Are they being overcautious because of liability issues ?
Cheers,
Bruce.
Jdsk
Posts: 11971
Joined: 5 Mar 2019, 5:42pm

Re: new wheel build

Post by Jdsk »

brucelee wrote: 24 Nov 2021, 9:18pmThe spoke holes however are heavily countersunk and I believe that the spokes could be inserted and bent correctly at the elbow. What I'm concerned about is the size of the spoke head and the size of the spoke hole. Anyone got any suggestions. What's the likely outcome of building a wheel with too large spoke holes and are there any fiddles ?
Have you already got washers in there?

Jonathan
iandusud
Posts: 1119
Joined: 26 Mar 2018, 1:35pm

Re: new wheel build

Post by iandusud »

brucelee wrote: 24 Nov 2021, 9:18pm On a broader note, it seems to me e-bike wheels are way overbuilt. I've seen numerous reasons - weight, torque, speed. I've been building my own wheels for 15 years now and the build has never been a problem, despite enormous shopping loads and a heavy rider. I sure a rider can put out a lot more torque than a motor can and in a more irregular fashion from and engineering standpoint. My normal cruising speed is higher than the max the motor can achieve. Are they being overcautious because of liability issues ?
Cheers,
Bruce.
I started building wheels professionally over 40 years ago. Mainly bicycle wheels but also motorcycle wheels. I have recently got my hands on two rear hub ebike wheels. One belonging to friend and another which I have fitted to my cargo bike. In both cases I, like you consider them to be over built with regard to the gauge of the spokes. In both cases the spokes were woefully under-tensioned. However they do need to be strongly built and I don't think that a rider can produce anything like the torque from stationary that the hub motor can, certainly not the 48V one I'm using. There is no way that I could accelerate my loaded cargo bike from a standstill as can the motor hub. Your point about cruising speed is valid but that's not where the greatest load is on the spokes. IMO these wheels would be much better being built with single butted spokes so as to allow more tension.
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531colin
Posts: 14085
Joined: 4 Dec 2009, 6:56pm
Location: North Yorkshire

Re: new wheel build

Post by 531colin »

I'm not entirely sure what the question is.

Re spoke gauge and flange hole diameter.
Hubs are generally drilled so that they can accept 13g spokes (about 2.3mm) although 14g (about 1.9mm) is most commonly used.
It appears that it isn't necessary for the spoke hole to fit the spoke diameter closely. (I'm assuming the hole isn't big enough for the spoke head to pull through)
On the other hand, in a big diameter flange there is plenty of room to drill your own, smaller spoke holes if you want to.....I did this years ago when grafting a moped front hub into a tandem wheel, so I got an effective (hub) brake. (I used the same number of spokes!)
(in the old days, Campag hubs were drilled so that you had to force the spokes through; I haven't come across a hub like that for years)

Re flange thickness.
If the flange is too thin, the elbow bend of the spoke isn't supported well, and spoke washers are used to take up the slack.
The elbow bend of the spoke should form a "dent" in the flange where the spoke exits the hole....this dent is present in wheels which have been ridden, but also in wheels which haven't been ridden, if the stress-relieving is adequate.
In this case where the flange may be thicker than the length of the spoke elbow, provided the spoke will go through I would be happy to ride the wheel, but I would stress-relieve vigorously so that the elbow bend is re-formed, and the dent is formed in the flange.

Re 13g spokes
If we assume that the most common failure mode is that spokes fail by fatigue at the elbow, then making the spokes thicker is about the crudest possible way to make them last longer. If the spokes are thicker, then it will take longer for a fatigue crack to propagate all the way through the spoke.....right?
Well, yes its right, but its far better to stress-relieve the wheel properly in the first place, so that the elbow bend is properly set, and it won't flex and fail.
Also 13g plain gauge spokes are pretty inelastic; at the same tension, any other spoke gauge in common use will stretch more, and also will share out any applied load over more spokes. so if you "must" have 13g at the hub, have them thinner everywhere else.
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