Correct Spoke Tensions Bafang 500W Rear Hub Motor?

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andychrist   100 µW

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Correct Spoke Tensions Bafang 500W Rear Hub Motor?

Post by andychrist » Jan 02 2021 4:07pm

So just put a little over 100 miles on my dual 26” 500W Bafang hub motor e-cumbent cargo bike. Since breaking in, the rear wheel is making clicking sounds when the bike is heavily loaded, seems some of the spokes might be a bit loose.

Bough a tension meter and a new spoke key with slots large enough for the heavy gauge on the Bafang, but can’t find any reference as to what the correct values should be for the drive and disc sides of that wheel. Anybody here know or can point me in the right direction?

TIA

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Chalo   100 GW

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Re: Correct Spoke Tensions Bafang 500W Rear Hub Motor?

Post by Chalo » Jan 02 2021 4:33pm

The correct spoke tension is dictated not by the spokes, but by the rim. Manufacturer recommendations vary from about 90 kgf to 140 kgf. 100 kgf is considered a safe spoke tension for a rim without a specified tension limit.

The bad news, in your case, is that the spoke gauge you have is incorrect for bicycle rims, and you'll suffer chronic spoke loosening unless you glue the nipples in place (in which case you'll suffer other problems). Chinese hub motor manufacturers have opted to appease the moron market by furnishing spokes that don't work with the rims they use.

Bigger isn't better for spokes. The strength of the wheel comes from the rim. A spoke's job is to have the correct amount of elasticity for the rim it's used with, so it can continue to provide support as the rim flexes under load. The heavier you load a given rim, the thinner the spokes must be to accomplish this. Thicker spokes must be turned up much tighter, which means they only work with thick heavy rims like those on motorcycles. Bicycle rims buckle and crack under the tensions that allow thick spokes to work correctly.

In the long run, you'll be much better off if you cut your losses and relace the wheels with 14-15ga butted spokes and sturdy high-quality rims. Short of that, you can apply boiled linseed oil or some other mild thread locker to your spoke threads in an attempt to keep your spokes from unscrewing. It won't keep them from going slack when the wheel is loaded; it won't allow them to provide support to the rim as intended. But it may keep the wheel from self-disassembling.

If you relace with thinner spokes, your will probably need to use small washers (2-2.5mm) under the spoke heads to keep them from burrowing into the hub's oversized holes.
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Re: Correct Spoke Tensions Bafang 500W Rear Hub Motor?

Post by amberwolf » Jan 02 2021 10:54pm

Chalo wrote:
Jan 02 2021 4:33pm
But it may keep the wheel from self-disassembling.
That also depends on the rim. If it's like several OEM rims I've had with oversized spokes, the original spoke tension created stress cracks in the rim around the nipple holes, which enlarged over time. So setting a tension on the spokes and then securing the nipples so they can't unscrew still wont' make the tension hold, as the rim deforms around the cracked holes.

None of mine were allowed to continue expanding those cracks (to perhaps eventually meet and split the rim), once I found the problems, but until I replaced the spokes and rims I did have to repeatedly retension the spokes which further cracked the rims which made me have to retension the spokes which further cracked the rims...ad nauseum. Sometiems the spokes would break at the elbows (usually on the smaller wheels with shorter spokes, after stuff had loosened without me noticing, when I first started using OEM hubmotor wheels before I started just building my own).

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Re: Correct Spoke Tensions Bafang 500W Rear Hub Motor?

Post by andychrist » Jul 25 2021 7:16am

So I used the meter to determine the median tension on the drive and disc side spokes, then adjusted the outliers with the 14 gauge wrench so that they all conformed. No problems in the 1000+ miles I’ve put on the bike since then.

Thanks guys!

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Chalo   100 GW

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Re: Correct Spoke Tensions Bafang 500W Rear Hub Motor?

Post by Chalo » Jul 25 2021 3:34pm

andychrist wrote:
Jul 25 2021 7:16am
So I used the meter to determine the median tension on the drive and disc side spokes, then adjusted the outliers with the 14 gauge wrench so that they all conformed. No problems in the 1000+ miles I’ve put on the bike since then.
Did the wheel stay reasonably true?
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andychrist   100 µW

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Re: Correct Spoke Tensions Bafang 500W Rear Hub Motor?

Post by andychrist » Jul 25 2021 5:33pm

Yes, the rims Bafang uses on their hubs are incredibly strong and seem to stay true independent of spoke tension.

Recently purchased a Bafang 20” front hub, the rim on that is an excellent deep V — very different from the one they use on their 26” model, which is more like a Velocity Dyad or Cliffhanger (relatively shallow but wide.) Was lucky enough to successfully mount a Schwalbe Super Moto X both to that and the Halo Combat rear wheel on the latest RANS Stratus LE I’m building up. Be a while before I finish it though.

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Chalo   100 GW

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Re: Correct Spoke Tensions Bafang 500W Rear Hub Motor?

Post by Chalo » Jul 25 2021 5:42pm

andychrist wrote:
Jul 25 2021 5:33pm
Yes, the rims Bafang uses on their hubs are incredibly strong and seem to stay true independent of spoke tension.
That's not how wheels work.

I recommend putting your wheel in a truing stand sometime, and checking the dish while you're at it. If the tire isn't at risk of rubbing on anything, then there's no hurry, but the next time you have the wheel out (especially when you have the tire off) would be a great opportunity to give it a closer look.
Last edited by Chalo on Jul 25 2021 5:46pm, edited 1 time in total.
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Re: Correct Spoke Tensions Bafang 500W Rear Hub Motor?

Post by andychrist » Jul 25 2021 5:44pm

Do you happen to own any Bafang hubs?

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Chalo   100 GW

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Re: Correct Spoke Tensions Bafang 500W Rear Hub Motor?

Post by Chalo » Jul 25 2021 5:50pm

andychrist wrote:
Jul 25 2021 5:44pm
Do you happen to own any Bafang hubs?
I have at this point built up, uh, eight? Bafang Jump bike hubs, about half of which I relaced to different sized rims. I have three more lined up for conversion, and a couple of customers who bought them but are still waiting for parts to arrive.

I weigh about 375 pounds, and I build pedicabs that carry 2000+ pounds, so I'm kind of a specialist in extra heavy duty wheels.

There's nothing about a heavy duty rim (and Bafang's would not qualify in my context) that keeps it from going out of true when you meddle with spoke tension.
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Re: Correct Spoke Tensions Bafang 500W Rear Hub Motor?

Post by andychrist » Jul 25 2021 6:17pm

Well the wheel stayed true and the rim hasn’t developed any cracks so far. Followed a couple good instructional videos on ebike wheels from Grin Tech and Park Tools which were very helpful.

I only weigh 175lbs and my cargo bikes never haul more than 125lbs (for a grand total ~400lbs) so perhaps my situation is less demanding than yours. Though I do climb mountains with them.

Incidentally, before motorizing a my RANS Stratus XP XXL, was schlepping an unassisted XP XL with Velocity Aeroheat rims. The drive wheel was constantly going out of true and once, unloaded, three spokes came loose on my way to the LBS. (Only ever had them service it, never worked on that wheel myself other than to change tires.)

Stratus LE XL motorizing now received a TSDZ2 OSF 52/42 in addition to the Bafang front hub. Will be interesting to see how the Halo Combat holds up, was the strongest 26” rear wheel I could find in stock anywhere with QR axles compatible with my trailer’s Burley hitch.

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Chalo   100 GW

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Re: Correct Spoke Tensions Bafang 500W Rear Hub Motor?

Post by Chalo » Jul 25 2021 6:49pm

andychrist wrote:
Jul 25 2021 6:17pm
Well the wheel stayed true and the rim hasn’t developed any cracks so far.
That's good.

Spokes usually loosen because they aren't right enough to begin with, so doing them back up sometimes returns the wheel to a more or less true condition. Using a stiff rim (which e-bike rims usually are, even when they aren't very nice) helps distribute stresses around larger sections of the wheel and helps prevent spokes unscrewing.

Where tightening all spokes to the same tension causes trouble is when your rim has already gotten bent somewhat, and differently tensioned spokes are holding it straight.
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andychrist   100 µW

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Re: Correct Spoke Tensions Bafang 500W Rear Hub Motor?

Post by andychrist » Jul 25 2021 7:25pm

Of course the wheel was new so the rim wasn’t yet bent anywhere, problem just careless factory assembly where they didn’t bother to balance spoke tension to begin with.

Thanks Chalo.

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