Are small radially laced motor hub wheels like Swytch trash?

Question

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

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?

Answer 1

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.

Answer 2

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.