Are front hub motors inherently unsafe?

[Pete]

Apologies if this has already been covered but I happened across this in a thread on Endless Sphere

How frequent is a front wheel falling off of a regular bike?...pretty damn rare, however, digging through the posts around here it's pretty common with front motors. Problems are so common, that I'm sure once e-bikes gain significant use in the US, that product liability problems will quickly outlaw front hub motors all together. IMHO in the meantime, vendors that sell them are being irresponsible, because they know full well the level of increased risk, but choose to ignore it out of both their greed and their customers' laziness to do the job correctly.

Sorry for the seemingly too strong opinion, but no one can produce a valid argument in support of the safety benefits of front hub motors over rear hubs unless you consider luck and laziness valid arguments.

Anyone have a view on this? Is it really only relevant to the US where higher power motors are used?
Pete

 

[flecc]

I think that's a US powerful motor problem, and it's certainly not a problem with the complete e-bikes we have on the market in the UK. I suspect a high proportion of US e-bikes are kit motor conversions where it's the creator to blame for an inadequate installation.

There is a separate issue with disc brakes which have caused many wheel dropouts due to the braking force against rear calipers forcing an inadequately tightened wheel out of the dropouts. Since some front motors also have disc brakes that could get them the blame in some of these incidents.
.

 

[Pete]

Thanks Flecc. I am zeroing in on which bike to choose and did't want to rule out front hub types unnecessarily.

I suspect a high proportion of US e-bikes are kit motor conversions where it's the creator to blame for an inadequate installation.

I think you are right about this, the thread was really about bikes made from kits.

 

[emissions-free]

I would say the risk is massively reduced of the motor power damaging the forks, causing the motor to spin in the forks or drop out, if you are using a UK legal 250W motor. However I think there is a risk of damaging an alloy fork if proper care is not taken. I haven't personally seen this but from doing some searching on some forums the way in which this damage can occur is rather compelling IMO. I think people should be a lot more careful generally when fitting motors ino front forks.

My personal experience is this. I fitted a pretty beefy hub motor (500w, 36V) into the steel forks of my bike. I put an adjustable spanner on the flats of the axle to act as a torque arm and used special washers shaped to the shaft with lugs that fit between the dropout. I didn't have any jubilee clips to securely clamp it, so I did the bolts up as tight as I thought safe, without stripping anything. Then put a few tie wraps around the adjustable spanner, temperory torque arm.

I then just span up the wheel with it off the floor. Then brought it outside and attempted to pull away. I heard a snap as the tie wraps broke and the wheel jumped straight out of the forks, even with my 90kg bodyweight on it, although granted I would have had maybe 1 foot on the floor.

I have visited battery manufacturers in China that will also supply e bike kits. I asked about what torque arm they use and got a blank look

I've seen expensive bikes by reputable companies that use a fork that is intended for use with quick release hubs, granted it wasn't a motor that is remowned for stump pulling torque (no boby that posts on here, so please, no need to take offence if you think this is a dig). There are suspension forks that are supplied as e bike forks which are beefed up where required. I'm gonna have some soon and will see exactly how they differ.

I'd use a fork that was intended for quick release hubs and do some mods and use it for my own use, but wouldn't be happy to let someone with little to no knowledge of such things loose with it.

 

[Pete]

Interesting to hear some first hand experience of this as I suspect that the forces involved are underestimated by some. Should be a warning heeded as in your case it happened to someone who not only was aware of the possibility but had taken some precautions to mitigate it. Admittedly yours was a powerful motor, but the again, steel forks. As you and Flecc point out, it shouldn't be an issue with complete bikes in this country, but anyone considering a kit should take note.
Pete

 

[Alex728]

There is a separate issue with disc brakes which have caused many wheel dropouts due to the braking force against rear calipers forcing an inadequately tightened wheel out of the dropouts. Since some front motors also have disc brakes that could get them the blame in some of these incidents..

it was a nasty problem with QR wheels on disk brakes, caused a young man in SE England to crash his MTB a few years back and sadly he was permanently disabled..

 

[Tiberius]

Pete,

If you hang out on endless-sphere for a while, you learn which posters to take more notice of. There, I can't put it any gentler than that.

Nevertheless, the motor spin out problem shouldn't be disregarded. On the other hand, its the same bit of physics as using disc brakes and they don't create the same moral panic.

On e-s at the moment there is a good thread started by Justin (of ebikes.ca), where he tries to take measurements and quantify the motor spin out problem.

Personally, I much prefer rear hub to front hub motors, but that's mainly for reasons of weight distribution and handling.

Nick

 

[Pete]

OK, Gotcha

 

[emissions-free]

Pete,

If you hang out on endless-sphere for a while, you learn which posters to take more notice of. There, I can't put it any gentler than that.

Nevertheless, the motor spin out problem shouldn't be disregarded. On the other hand, its the same bit of physics as using disc brakes and they don't create the same moral panic.

On e-s at the moment there is a good thread started by Justin (of ebikes.ca), where he tries to take measurements and quantify the motor spin out problem.

Personally, I much prefer rear hub to front hub motors, but that's mainly for reasons of weight distribution and handling.

Nick

Tiberius,

I fully agree that a front hub motor, even fitted into alloy forks, designed for quick release hubs, fitted by someone with virtually no technical know how can be implemted with virtually zero risk even with a very powerful motor. BUT that risk is increased signicantly if firstly a proper installation is not done for whatever reason and secondly by the actions of the average Joe that gets the bike 2nd hand, 2 years down the line.

He/she uses a set of pliers to remove/refit the bolts (because that's what they've got in the cupboard under the stairs), looses those odd little washers that fell off when he/she had to change the tyre etc etc. You cannot underestimate the ignorance of probably the vast majority of any population when it comes to anything in any way technical. They might call the AA to swap the front wheel on their car, but they'd maybe have a go at changing a bicycle wheel, can't be that hard...

If you even know this forum exists and bothered to read more than the first few posts you saw, you are already in the top few percent of the population when it comes to technical knowledge of e bikes.

Any reputable company has to build margins into their designs to cover such misuse and if they don't and something does go wrong, they could likely be prosecuted and held accountable.