FAQ - front or rear hub motor?

Hub motors come laced into a standard bicycle wheel, and are spaced to fit in either the front fork(100mm) or the back dropouts(135mm). While both front and back hub motors are pretty much drop in replacements, their are a few differences.


One of my front hubs drilled for ventilation and secured with a custom torque arm

Front hub motors

Being pulled around by a front hub can be a fun little departure from a solely rear wheel drive bike. They also let you keep your bikes rear wheel and rear gear stock. They do however have some difficulties with aluminum forks, or higher powered setups; it's hard to properly contain the torque of the motor in the front fork.

Good:

Rear hubs need to use freewheels; a some what dated technology. It's become hard to source quality freewheels, especially the 11 tooth type that allow you to pedal comfortably at high speeds. A front hub motor allows you to keep your drive train stock, so you won't have to spend $20-$30 for a freewheel.



Front hub motors are pretty much a drop in install that can be done in minutes. They can also be uninstalled even quicker, so it's easy to de-electrify your bike for normal riding.


Bad:

Because of the torque the fork must contain you simply cannot safely install a hub motor into a fork that is not steel. This is problematic because only the cheapest suspension forks are made from steel. Be sure to check with a magnet if you are getting a front hub motor. The magnet should stick to steel. Another option is to reinforce the fork, but this is quite a job!


A reinforced suspension fork

Even with a steel fork you will still need a torque arm to safely contain the power of the front motor. Given enough time, the dropouts holding the motor axle will fail. The best case scenario for this happening is the motor spinning and tearing up the wiring and the fork. The worst case scenario is flying over the handle bars at 30 miles per hour. A torque arm transfers the rotational force from the axle to the much stronger fork.

A universal torque arm

What happens without a torque arm

Rear hub motors

Eventually you'll get power hungry and go with a rear motor; It's easier to safely contain the force of the motor in the stronger rear dropouts, and with enough power it's hard to keep a front hub from losing traction. Rear hubs can be a bit of a pain to install some times though.


Good:

The drop outs in the rear are much stronger, so you won't need a torque arm with a steel frame. Although losing a rear wheel isn't nearly as bad as losing a front wheel, I'd still recommend a torque arm.

You'll get better traction with a rear hub. Front hubs are limited in power by their (lack of) traction.


Try this with a front hub

It's a lot easier to do a "stealth" electric bike with a rear hub!




Bad:

You'll need to buy a freewheel for a rear hub. Your also limited to 6-7 gears in the rear.

Some bikes have clearance issues that require spacers and finagling. It can be a pain to get certain disc brakes to work properly as well. Especially when the hub motor is pressed right up against the brake caliper adjusters!


Clearance issues on an old "thread on disc" style 9c hub by El_steak



It's not a very hard choice to make; If you have a steel fork and you are getting a low powered kit, a front hub is easier and cheaper. If you don't have a steel fork or you think you might be interested in overvolting and other upgrades, then get a rear hub motor!

Nice job. I feel it can be safe to put a front motor on an alloy dropout. But only if done perfectly by an expert. So you noobs forget it, as he says.

I'd like to add, I love front hub on pavement, but I MUST have rear drive for dirt riding. You can ride your front hub on a dirt road, but for a serious trail, rear hub is mandatory. Dual drive is ok, since you still have one hubmotor on the rear wheel.

You are not limited to 7 speeds on a rear hub. My BMC has a 9 speed hub.

-Warren.

The problem I have with any type of front wheel drive is locking the wheel. I have read several horror stories including one death that have resulted in a front motor either jamming or otherwise failing and throwing the rider.

I have thrown a chain into the rear wheel at 30+mph and merely skidded to a stop. No major drama there.

Other than that, I love the idea of front wheel drive for the mere reason of ease of installation and being able to leave the rear pedal gearing alone as you mentioned.

Matt

thanks for the input - this is why one person shouldn't write a FAQ - 9c and clyte are limited to 7 speeds... smaller hubs aren't

my front hub spins like crazy in the dirt

The problem I have with any type of front wheel drive is locking the wheel. I have read several horror stories including one death that have resulted in a front motor either jamming or otherwise failing and throwing the rider.

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that is the most terrifying thing I've ever read - do you have any sources? I'm morbid enough to wanna read about that. i heard someone saying similar stuff about the glue holding the magnets in golden motors stators coming undone and locking the wheel up, but it was from a one time poster and didn't seem legit.

The man who died was named Rhett. I believe Aussiejester knows more than I about the situation. He was going around a corner when his front wheel failed and he hit his head on the curb IIRC.

He was on life support for months before he passed. It was a bad day for us all.

Matt

In our original 2-piece torque arm design that used a single hose clamp to secure to the fork, we had I think 3 customer failures where the metal hose clamp actually tore in half from the forces that were present when they had a controller fault while moving at high speed in a direct drive motor.

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http://endless-sphere.com/forums/viewtopic.php?f=2&t=14195&start=165#p384032

If ebikes were popular and made in the US I wonder how often the makers would get sued?

Scary stuff....

auraslip said:

If ebikes were popular and made in the US I wonder how often the makers would get sued?

Scary stuff....

Click to expand...

It will be often enough that front wheel DD hubbies will quickly be outlawed. The problem is that with a high torque motor, a controller failure can result in full on plug braking. I would never recommend a front DD hub to anyone. That doesn't mean I won't build my own 2wd, but mine won't have a low speed wind motor and I'm fully aware of the risks. I'll use my "torque sleeves" to ensure no failure at the dropouts.

It's a lot easier to do a "stealth" electric bike with a rear hub!




anyone happen to know the specs on this bike?...as far as which wheel/battery/ etc its using?...its kind of along the lines of what i'm looking to build...

Search "Rhett" and you'll find AJ's knowledge about the matter. Front DD didn't fail and cause his demise, as I understand it.

I maintain excellent attachment and wiring standards for my pair +1000W front DD motors. I actually prefer the front motor in snow - easier to use legs as outriggers and pull yourself along through deep stuff. If it loses traction, you're probably going fast enough for the conditions anyway. But, it's not dirt bike like in any way.

There are obvious dangers though - washing out in a lean with too much power and not enough traction. Locking up is obviously something that can be very bad and can happen electrically via controller or wiring failure. Loose magnets, possibly with hotrod stuff but unlikely mild power?

But I also like rear motors - if there's a technical problem best it happen there but also not imperative IMO...

Commuting around on somewhat slick surfaces I might prefer the front motor but after using both for several hundred miles it's all a toss-up for me.

Weird thing is how much harder it is on some seats constantly lifting a rear motor bike using the saddle. One of my cheap seats fell apart from metal fatigue lifting into my apt daily.

anyone happen to know the specs on this bike?...as far as which wheel/battery/ etc its using?...its kind of along the lines of what i'm looking to build...

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It was a review of the ebikekit geared hub motor a year or so back. Using lipo IIRC.

Weird thing is how much harder it is on some seats constantly lifting a rear motor bike using the saddle. One of my cheap seats fell apart from metal fatigue lifting into my apt daily.

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That's true.... and if you grab the seat post you get grease on your hands.

You could build a handle that's like a seatpost-rack stub, just to do the lifting from.

Better seats might do the trick.

Re the front hub locking and causing over the bars. I'm sure it can happen. But other things happen all the time too. My big crash was fumbling a waterbottle into the front wheel, jamming in the forks. For sure, having your front wheel suddenly lock is NOT FUN.

Busted heck out of my shoulders, and a helmet. No head injury smacking my temple very hard into the curb. Did i mention I was wearing a helmet. Am I repeating this too many times? I was wearing my good for nothing, won't help you in a crash cheapie bicycle helmet.

Just my two cents. I don't know if the guy who died was wearing a helmet. I just know it didn't take a front hub for me to nearly die. But the helmet saved my head beautifully.

auraslip said:

The problem I have with any type of front wheel drive is locking the wheel. I have read several horror stories including one death that have resulted in a front motor either jamming or otherwise failing and throwing the rider.

Click to expand...

that is the most terrifying thing I've ever read - do you have any sources? I'm morbid enough to wanna read about that. i heard someone saying similar stuff about the glue holding the magnets in golden motors stators coming undone and locking the wheel up, but it was from a one time poster and didn't seem legit.

Click to expand...

WOuld the problem be as severe on a recumbent? I've slammed into things on my bent, and it is really difficult to pitch over the handlebars. If you do, you are going feetfirst, not headfirst. In my case I'd get a handlebar in the crotch, not any fun but also not something that will render you a vegetable, at least in the brain department.

No, it's not the same on a recumbent - at least not on a trike. I have a tadpole version (two wheels up front and single drive wheel in the rear) and front brakes are primarily used on those for stopping power. Rear brakes is only for fun power slides and a little control or mind slowing down of the vehicle. Motorcycles are a bit different too in that all the braking power is also up in the front and rear is for extra braking power.

Anyway, I wanted to reply to this thread to thank the OP. Thanks so much for all that info and the pictures! Photos help a ton and I'm pretty new at all this stuff. So thanks for any and everyone who contributed to this thread. I'm learning lots.

Hey auraslip I was just wondering if those small ventilation holes made any difference at all in your temperatures, I have always wondered if holes that small make any difference at all.

Good thread. Just what I needed to know.
I'm planning a fast hub motor e-bike conversion for my Performer suspended 26/26 high racer. And was wondering which would you think would make the bike most stable at 40 mph and in crosswinds with a full fairing. Adding the 7.6kg gyro effect of the Crystalyte Hs3540 hub motor to the front or the rear wheel? But then again, the possibility of a front wheel lock up rules that out for me.

Hello.
I was just planning my second E-bike build - a 500w bafang front hub, then i read about the risk for lockups or jamming.. You are all mentioning the lockups in conjunction with direct drive, what about the geared motors - like bafang - are these not as prone to lockups?

And how big really is the risk? is there any known case here, more than just rumors?

There are several rear motors available now that can take a cassette gear cluster, such as the Bafang 8FUN CST.

manneokoko said:

Hello.
I was just planning my second E-bike build - a 500w bafang front hub, then i read about the risk for lockups or jamming.. You are all mentioning the lockups in conjunction with direct drive, what about the geared motors - like bafang - are these not as prone to lockups?

And how big really is the risk? is there any known case here, more than just rumors?

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There's probably a higher chance of being hit by an Asteroid that getting a lock-up with a front geared motor, so I wouldn't worry about that. The main danger is that if you don't fit it correctly, the drop-outs can break. I would fit two torque arms to a 500w geared motor and one to a less powerful one. There's lots of rear motors available now with cassette splines, which means that a rear motor becomes a much more attractive choice. You can get cheap 500W ones from Bafang, Xofo and MXUS. Smaller ones are also readily available and now there's DD motors too like the Falco.

Exactly right. The real danger is installing them improperly, front or rear, and having the axle jump out of or break out the dropout slot of the frame or the fork. If front hub, over the bars is likely if the wheel falls off.

Having the motor lock up is possible, but not a common problem at all. You have a much better chance of going over the bars in many other ways. Bike, motorcycle, scooter, over the bars is always a possibility. One you minimize by the way you ride.

Fit of the washers is the key thing with hubmotors, particularly with front forks that have tabs or a cup shape to them designed to retain a wheel with a loose bolt. In either case, the required smaller c washer is not provided with many of the cheaper priced kits. You can grind down the provided washers to get the same fit, filling in the cup so the next washer lies flat. Installed wrong, just tightening the bolt can ruin your fork.

And, you want to also include two torque arms, if you are using an alloy suspension fork.

http://endless-sphere.com/forums/viewtopic.php?f=4&t=15163&hilit=Ebikes+ca+c+washers



http://endless-sphere.com/forums/viewtopic.php?f=4&t=15167&p=225525&hilit=Ebikes+CA+torque+arms#p225525

I remember reading about the unfortunate biker who died some time ago, but not the cause of death that was mentioned and I thought I'd look further this time. It's not explicitly stated or linked to here but a friend of the gent who died posted about a medical cause on a different forum. I'm not a doctor but I guess there's always the possibility of a link- the trauma starting the coma caused by a low blood sugar level. My respects go to him and his family and friends.

This led to me looking at the forum, and I fairly quickly came across a post which makes me want to thank the moderators here for their work and measured use of editing/censoring.
And also for the fairly low level of contributors whose posts that the vast majority of members would happily do without, and the generally measured and civil responses to these people.

I've been a member of a number of these kinds of boards over the years and in general I think this is probably one of the best for getting and giving help and information without much of the angst that afflicts so many others. Opinions are given, some are opposed most often in a helpful manner, and that differing viewpoint is then more often than not received in a reasonable manner by the OP- although not always fully accepted. Healthy debate is good.

So, many thanks to everyone - moderators and posters - who help to make this such a useful friendly place, with healthy open discussion, and much good personal advice and shared knowledge.

Good idea in adding some strength to the front mounted hubs.

Hi all,
IS it even possible to use a front hub motor and place it to the rear? :lol:

iceeci said:

Hi all,
IS it even possible to use a front hub motor and place it to the rear? :lol:

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Why not, if the axle is long enough and you don't need disc brake nor to pedal.