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I saw a bike that recently had this kit installed on it, and was surprised at the radial spokes:

Advertisement for a "Brompton eBike Conversion Kit" which appears to have a handlebar-mounted battery pack and an electric motor front hub.

From having done rebuilds on 20" motor hub wheels, I can appreciate the temptation to look at going radial. Otherwise, even at 1x, correcting the spoke line at the nipple to avoid built-in stresses takes some careful work. The default angles are extreme, the space you have to work in is limited, and it has to be done at just the right point in tensioning to do it well. Getting it physically laced up is also somewhat challenging, which is probably enough in and of itself to make low-quality manufacturers look for alternatives.

I can also appreciate that with the spokes being so much shorter, forcing them to resist torsional loading in a radial pattern is less of an issue than it would be on a full size wheel. But this is a hub motor. Is there any possible way the math here to choose radial is sound? Or was this just a mistake, or a willfully cynical design?

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  • “I can also appreciate that with the spokes being so much shorter, forcing them to resist torsional loading in a radial pattern is less of an issue than it would be on a full size wheel.” Actually I think longer spokes would be better because they are more elastic. You need elastic deformation for wheels with radial spokes to be able to take any torsional force at all.
    – Michael
    May 1 at 17:28
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It's trash. Anything transferring torque from hub to rim may not be radially laced. There must be at least some spokes that are cross-laced. The brake is a rim brake so they're good on that front, but the motorb still unduly stresses the spokes. The torque may be so small and the hub spoke circle so big that it won't cause trouble immediately, but I wouldn't have it. If for no other reason then because it's a stupid design and poor engineering. I hate that. Some feller with CAD and no engineering background just said: "Those radial spokes are cool, let's have those!", and they did it. The quality of engineering in cycling is generally horrendous, but this is even worse, probably electrical engineers designing mechanical parts. The radial lacing may well be the least of its problems. Avoid.

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  • The radial spokes aren't great but it's not a case of "may not" and "must". There are plenty of good-quality off-the-peg rear wheels sold with warranty that use radial spokes, just not on both sides. Engineering is always a balance of performance requirements, durability, safety margins and cost.
    – Emyr
    Jun 10 at 10:25
  • Sorry, but it IS a matter of "may not" and "must". Transfer of torque between hub and rim requires cross-laced spokes. I did not say that radial spokes may not be ON a wheel transferring torque, I said that it may not be radially laced. Exclusively. In fact I'm planning to build myself a wrong-way radial rear wheel sometime. My last set also had half-radials. That's absolutely ok if you have some cross-laced ones to transfer torque. Sure, engineering is a balancing act - but this isn't. It's not engineering. It's stupidity or at least cluelessness. Jun 12 at 14:43
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    @Emyr I can never find it, but there's a really good piece I read at some point about minimum smallest inscribed cross sectional area of the hub shell and how it determines whether a half-radial wheel will be able to run or not in a torque-transferring application without overly fatiguing the spokes. But that is the trick wheels like that are playing. Jun 13 at 14:50
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This is a poor design that can work due to several properties:

  • On an electric bike, only part of the load is supplied by the motor. Most of the load is still actually supplied by the pedals. Thus, this should be not considered a primary motive propulsion device but rather a hill assist device.

  • An electric assist provides continuous torque. In contrast, the cyclist pedaling provides an oscillating torque. So if you ride 50 meters upwards on a 8% grade hill (625 meters linearly), the cranks would probably rotate about 140 times, meaning there's 280 power strokes and thus 280 load cycles. However, an electric assist would create only one load cycle in this hill.

  • The hub flanges are large. The larger the flange, the better the system is able to withstand torque. Optimally the spoke pattern should probably be 1-cross or 2-cross, in which case there is already a large effective "torque arm" to create the torque. On a radial wheel, the effective "torque arm" is created by the hub flange rotation with respect to the wheel rim. Creating such "torque arm" on radial wheels of course affects the tensions of all spokes (tightening them), so I would expect the wheel to fail in a very short amount of time, unless stress relieved. However, stress relieving the spokes could make them withstand the amount of time it takes the buyer of the "electrification kit" to realize the kit might not be such a good idea and purchasing the electric Brompton from official source would be a far better idea.

Thus, the design is marginal. It will probably last few thousand kilometers. It could last few tens of thousands of kilometers if using the lowest assist level and if the weight of the cyclist is low.

I wouldn't buy it.

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  • It also has a lot of spokes for such a small wheel. Would really be interesting to know how marginal the reliability of this design is. It also depends on the motor torque and the quality of the build of course.
    – Michael
    May 1 at 17:30
  • I think a good approximation could be obtained by building a radially spoked rear wheel and seeing how long it lasts. Then multiplying it by 100 (because not every pedal stroke is a load cycle in electric assist hub motors) could tell a reasonable approximation of how long this lasts. I don't know how much radial rear wheels last. Probably someone has tried it. They aren't sold in any decent bicycles.
    – juhist
    May 1 at 17:32
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    I'd like to point out that every rotation of the wheel is a load cycle. Also, even if the small size might reduce the extra tension from torque, it's still huge compared to cross laced wheel.
    – ojs
    May 1 at 17:39
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    In a radially laced drive or hub brake wheel, the drive or brake forces increase tension. May 2 at 22:30
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    Under load of gravity, the spokes at the top of the wheel gain tension and the spokes at the bottom lost tension. Applying motor torque will turn all radial spokes into slightly trailing spokes and increase their tension. The additional angle will increase the amount of metal supporting the spoke head, albeit minimally. The most likely event to cause flange failure would be hitting a bump under power, so the load experienced by the motor suddenly increases. My approach would be to ride it hard while under warranty and hope the design flaw doesn't present in a catastrophic manner.
    – Emyr
    May 7 at 15:30

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