Moe Zhoost

A friend asked me to look at the rear wheel on his e-bike because he noticed some cracks around the spoke nipples. Sure enough, on almost all of the drive side spokes there were cracks with a few on the non-drive side as well. I measured the tension on the drive side, which averaged 100-110 kgf - nothing that would alarm me. The dish on the wheel is not much, about 5 mm offset. The wheel has a pretty big direct drive hub motor with a spoke circle diameter of 233 mm laced with 36 short 2.6 mm straight gauge spokes laced 1X with all of the spoke heads to the inside of the flanges. The rim is a 26" Weinmann Goliath with no eyelets. 4,500 miles on the wheel. He has contacted the manufacturer and is getting a complete new wheel at a discount so his issue will be resolved soon, but since he wants to replace the damaged rim and have a spare wheel, he asked me for advice about how to choose a stronger rim.

I have a lot of opinions about why, but none that I feel comfortable with:

1. The wheel should be plenty strong with 12 gauge spokes at a lower tension; however after about 6 months of use he felt that they were loosening up so he gave them all a half-turn. It seems that they need a lot of tension to overcome the loosening.
2. Perhaps thinner spokes would be better, though I think the spoke holes on the hub motor are designed with 12g in mind.
3. I like rims with eyelets, though I do hear reasonable arguments about why they are not necessary.
4. Can the torque of this 750W direct drive motor be too much for this rim? Since the torque is applied equally to both flanges why wouldn't the non-drive side show little effect. Some spoke tension is added by the rotation of the hub when accelerating and braking but can this be as much as would be needed to over stress the hub? He tells me that he uses mostly pedal assist and doesn't accelerate quickly.
5. There are no signs that the spokes are pulling through the rim. Could the cracks be a product of cyclical stress?

Here's a picture:

ThermionicScott 

So you were seeing 32-33 on the Park Tool gauge?

FB would have argued it was "too much spoke for the rim." 12g/2.6mm spokes probably don't elongate much, if at all, at 100-110kgf.

But I don't know what the solution is, besides exercising the warranty.

Kimmo

DT Alpines, and hope the hub flange doesn't mind 2.3mm elbows?

Probably struggle to find them in a short enough length, though...

WizardOfBoz

Even with a large hub, 1-cross lacing is pretty far from tangential - a good torque by the e-drive is gonna put a lot more force than 110kgf on the spokes, yes?
I'm guessing the low cross, the very stiff, short spoke, and (probably) a rim that is more made for a bicycle than an e-bike. Although the obsolete Goliath looks a lot like the E-Dyno.

Jon T

Try yojimbosgarage.com. He'll cut/thread spokes to what ever length you need.
Jon

Moe Zhoost

Yes 32-33. The manufacturer is offering him a complete new back wheel for half-price. They claim that they have had no reports of this from other customers. Could be a fib, but it is likely that only 20% of their customers have as many miles and of them, maybe half would notice the cracks.

Yeah, I think that these would be great. They do come in lengths as short as 130mm so that's good. The hub spoke holes are 3.2-3.4 mm, specifically designed for 12g heads so that could be an issue. Washers may be necessary. What he wants to do is just replace the rim without investing in new spokes. His argument is that it would just be used for a spare. It would probably be a better idea to rebuild the hub into a strong wheel and use the new one as a spare (or sell it). There seem to be a lot of reports of spoke breakage on these bikes (RadPower) so there is certainly something going on. This is the hub motor.

This is the hub motor. It's huge. I'd estimate that the spokes leave the hub at 30 degrees from radial so 2-cross is probably doable.

The more I think about, the more convinced I am that 12g spokes are trouble. I'm thinking that a higher quality rim, with smaller diameter spokes (DB if available), and careful attention to proper tensioning is the way to go.

Thanks to all.

WizardOfBoz

Wow, the spoke hole diameter is 233mm.

The spoke theory appeals to me, too. The large hub makes for shorter spokes. Short, very thick spokes would seem to directly translate motor torque to stress at the spoke nipple. I was trying to quickly calculate how torque at the hub translates to torque and compressive forces at the rim. Suffice for now to say that 1 cross translates to more compressive force.

That said, I see a lot of these things laced up 1-cross with short, thick, straight gauge spokes. Why aren't there more rim breakages?

ThermionicScott 

I looked up the Weinmann Goliath, and it looks pretty beefy, but something like a Sun CR18 is probably better since the extra walls are closer to the spoke drillings. Good luck, and I'm interested to hear how the new wheel fares.

mrrabbit 

Switch this person over to rim such as a:

JetSet CH-300A
Kruzer King

Basically double wall 26 x 1.75 with a 2.5mm to 3.0mm thick inner wall.

Most e-Bikes are supplied with cheap rims that simply aren't up to the job.

=8-)

Moe Zhoost

Thanks for the recommendation. I've never heard of the JS or KK rims. I see you carry the JS on your site so you probably have good experience with them. I wasn't able to find the KKs anywhere ... defunct? I was thinking Sun Rhyno Lites because I've had good experience with them (although not on an e-bike). A local shop sells Ryde Andras, which look good as well. My buddy's said that he just wants to replace the rim without changing spokes and plans to move the spokes one-by-one over to the new rim, DIY style. I'd really like to see this new rim laced up 2-cross, which means new spokes. which means that we'll have to measure the ERD (Can't find good data for his current rim), which means that his DIY plan won't work. Oh well. He should be getting his new wheel soon, so we'll see.

Here's my graphic depiction of 2 cross. Look doable? The angle of the spoke exit from the rim looks too much to me. All of his spokes route to the outside of the hub flange, which makes sense with only 32 mm interflange width, so that may be an issue. I admit to being more curious about this wheel than he probably is, so I'll see if I can talk him into tearing it down and trying a few things.



Thanks again, all.