Boosted/Busted

Since I last posted about running the Bike on a 12S LiPo setup (44.4 V Nom.) I did some work on the setup- I sewed up a bag for the four 6s packs with padding, and took it on my work commute last Wednesday. Things went well in the morning- cut my biking time in half- with average speed of 19+ mph, and only 3.6 AH used on the 10AH pack. I avoided flat tires and the extremely unlikely but terrifying scenario of a battery fire on BART.

I charged the pack at work using the Hyperion balance charger and on a 24VDC supply I was able to borrow from work. All was well.

Then, just before leaving work for the BART station- disaster! As a coworker was riding it around the parking lot it began making horrible scraping noises- it was clear that something in the front motor pulley had let go causing the pulley to scrape against the belt guard plate- something that had never happened before. There should be absolutely no movement in that pulley- I decided that I would investigate later- removed the belt and pedaled back to BART. What a disappointment!

On Saturday I finally got a chance to autopsy the motor shaft/shaft adapter situation- after blasting it with a propane torch to loosen the bearing locker I filled it with last time I put it together I was able to pull the shaft adapter apart- surprise!

The motor shaft sheared off right where it necks down to 8mm.- but also down into the cross-drill. I figure the side loading on the long shaft adapter, combined with the slight play inside it, must have caused the shaft to fatigue and break. The fact this happened after doubling the motor power input can’t have hurt either.

I decided to pull the rotor out to investigate how difficult shaft replacement would be.

 

It looks like the shaft is swedged into the rotor bell- the way that occurs to me to replace it would be boring a hole in the bottom of the rotor, with a bolt circle, then making up a shaft with a flange that fits the bored hole and bolt circle. It’d be nice to upgrade to a standard 1/2″ or 5/8″ shaft, but the bearings are 26 mm OD metric and there’s not really much meat on the bearing pockets to enlarge them. It’s possible to get smaller profile 26mm OD bearings with much larger inner dimensions, I’m not sure really if that’s an acceptable tradeoff- I’ll have to get some advise about that.

So, it comes down a question- is it worth it to rebuild?

In the interest of having a working ebike, and out of curiosity I did a little re-configure on the bike and installed the front-hub 9 Continent direct-drive motor I purchased from ebikes.ca a little while back-

Taking it on the “standard test-ride circuit” revealed some interesting aspects of the performance, useful to compare with the old Hi-Kol/Poly-V drive. Power and torque were quite similar, the cycle analyst recorded peak amperage of 10A lower with the 9C motor. The top no-load speed is about 37mph, wheras it was likely north of 40 with the Hi-Kol, my estimate, since the speed sensor is on the front. In my short testing I believe the efficiency of the 9C motor is better, but not by too much. Of course that depends entirely also on how you use the throttle. The 9C motor is quieter, but still makes a fair amount of noise.

Most notable is the difference in handling. I’m just going to say my initial assesment- heavy direct-drive hub motors suck when used in the front fork of a bike. The unsprung weight is distinctly noticeable and the handling is plagued by “torque steer” like effects. I’m concerned for the dropouts with the copious torque the motor is generating as well. I’d like to set a lower amperage limit, or get a smaller controller on it. I think that’d make it handle less dangerously as well.

If you keep it generally modest and smooth on the throttle it behaves well enough though, and the torquey acceleration is pretty fun, definitely no worse than the old setup, with the suspected increased efficiency as well it’s a pretty good first impression.

I’m still not a real fan of the hub motor- there’s just something wrong seeming to me about putting a heavy motor in the wheel hub, particularly on a suspended bike- but I’ll ride this for a little while as I consider where to go next- I don’t expect it to convince me to invest more in hub motors, but it has already made me reconsider whether the poly-V drive is the best option for efficiency and flexibility.

Posted on August 21, 2011 at 10:23 pm by Henry · Permalink
In: Electric Poly-V Bike

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  1. Written by Jerome Demers
    on August 25, 2011 at 7:26 pm
    Reply · Permalink

    Oh man! I had the exact same thing happen to me with my brushless electric scooter! The shaft broke right in front of some mechanical engineer!

    That happen two weeks ago!

    Keep it up!

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