Carbon frame road bike - help required with 250W motor selection.

New to e-bikes, I'm looking to equip my carbon frame road bike with a 36volts, 250W, front hub motor to assist with hill climbing, in some cases fairly steep. No assistance needed on the flat, when there should be minimum resistance from the motor!

Would appreciate advice on what motor parameters to consider, particularly gear ratio (am assuming direct drive wouldn't provide sufficient power).

Will be building a new wheel with the motor, using a 36 spoke, 700C x 23mm rim.

As an electronics developer I'm also planning to design my own controller, preferably using or based on an already proven, bullet proof design that fully protects the motor from over heating (the motor will of course need temperature sensing, probably retrofitted).

Cheers, Ron
Wellington, New Zealand

Bullet proof way to not fry a 250w hub motor up a killer hill is not do it. Otherwise, you could do a temp overheat switch like the Heinsmanns had. Overheating inside the motor opens the switch would turn off the throttle. Saves your motor, leaving you pedaling up the hill, since you chose too weak a motor for going up that hill. It works great to save the motor, not so great if you climb a killer hill all the time.

What you need, is a mid drive motor. Then 350w is plenty, since you can run it through your lowest gear.

Bottom line is, you either have to pedal your ass up those hills anyway, or not do a weak hub motor. I'm not saying 250w won't be enough for you, just saying you won't be climbing up that hill while whistling. You will still need to pedal hard enough to keep moving 12-15 mph, or you will risk frying the motor.


You don't get to choose gear ratio of geared hub motors. Most likely a good hub motor choice would be more to the 500w type, rather than the smallest ones, for a serious hill. (with the exception of the xionga) The smaller motors will do fine though, if your hill is only about 5%, and you only weigh 200 pounds or less.


Oh, and you best not put a motor on a carbon fork. steel fork.

It is my experience that a 250w “rated” system is not going to be much help on hills as noted above. Have you checked into this:

https://www.ebikes.ca/shop/electric-bicycle-kits/bafang-g310-311/front-g311-kit-advanced-pas.html

Geared, light and if run as a 36v/15A will give you a relatively non stressed 350w kicker that will provide 540w for a bit. I know 350w is only 100 more but I find it to be my preferred wattage for climbing on my road bikes when seeking the best wh/mi for range.

Low wattage hub motors set up with a proper torque arm are not that hard on front forks in my experience. A lot of the fear of failure occurring is based on early attempts at overvolted motors improperly mounted and secured.

Kiwironnie,

I have an aluminium road bike that has a carbon front fork, and have been running a 250-350 watt, Rear , hub motor on it for over a year now.

You are going to want more than that wattage when going up hills. I would typically use around 450-650 watts going up hills.

Most of the 250 watt motors are that wattage when using a 36 volt battery, you can get more watts/power from using 48 volt pack or better yet a 52 volt pack.

What I did was use a 42 volt pack ( 12s pack ) when going up hills, then use a 14s ( 52 volt ) pack once up those hills and on the flats.

Forget about a front motor on a Road Bike/ Road Bike front fork.
A rear is what you want.

For real light weight and stealth, look at Kepler's friction drives , located in the non-hub motor section. The only drawbacks to a friction drive is faster wear on the tire, but he does well on his. The Rain/Wet Roads, and going up steep hills.
FZBob has done a good light weight mid drive conversion, DIY conversions like his are going to be the future of inexpensive, light weight mid drives.
You can find his in the non-hub motor section as well.

It would be good to know how long the hills are that you are going up and how long those hills are.

I have just recently stopped using my little 250-350 hub motor , I put that bike on the trainer for the winter , as well as short rides to the store etc.

I am much happier with the higher power Mac Rear Hub motor on my other bike. That might be a little too much for a Carbon Road frame though. The Mac I have on a cheep MTB frame. The Mac goes up hills much better.
It can run on up to 30 - 34 amps instead of the 17 amps that the smaller motor controller uses. Small motors cannot use much amps, 20 max amps for keeping a small hub motor alive.

Set up a motor to run at something like 10kph free speed and deliver 250W nominal, and it will be able to effectively help you climb steep hills (with your effort). That probably means getting the slowest kind of 48V 750W motor and running it at about 16V.

If your local authorities won't acknowledge that a 48V 750W motor running at 16V is actually a 250W motor, then you have another problem to solve.

If I were you, I'd be looking at a 250W crank drive like BBS01, and using low enough gearing for it to do the job.

I was going to mention the Kepler/other friction drives. Very inconspicuous, light, removable and can be built specifically to climb. Also an easy DIY if you can do any design or machining. With a controllable mount you can run regen on the downhill then pedal the flats with zero drag.
A rubber friction surface (I have a thread where electric skateboard motors are discussed) has the potential to work with little tire wear.