Aluminum Bike and Steel Fork

[hulk]

My bike is aluminum but has steel fork and I would like to put a 500w Bafang front motor on it. I prefer front motors on my ebikes because I can have AWD when I pedal the rear wheel.

I know this forum is against using front motors on aluminum bikes but if my bike has a steel fork would it be OK?

Here is the bike:

 

[indianajo]

Post & run. Use some other forum.
Yes. I do. 1000 w. Use torque arms.

 

[m@Robertson]

I know this forum is against using front motors on aluminum bikes but if my bike has a steel fork would it be OK?

Short answer is yes, up to a point and provided you take precautions.

As an aside, its not that this forum is against using front motors on aluminum bikes. Its that the entire (experienced) DIY community is against using motors on aluminum forks (or any suspension fork). In particular, the internet is literally littered with zillions of pictures of suspension fork dropouts that have sheared off - regardless of the torque arms used - when asked to hold onto a front motor. This is the reason all of my 2wd bikes have steel front forks, which makes my wrists very sad (and why I use the Kinekt suspension stem wherever I can).

You will see a lot of builders pooh-pooh front motors and cite the better traction you will get from a rear motor. And they're right. But I've used low power front motors in conjunction with using my legs for power and I was very impressed by the bang for the buck you get from a small motor - so long as you want to pedal the bicycle. If you want to throttle it then front motor only is a bad call.

A steel fork can handle the burden so long as you keep the load reasonable. What is reasonable? Well, trying to drag a cargo bike fully loaded is a pretty extreme duty cycle. If there is going to be a problem with your fork you will find it in that scenario.

To make your bike work best

1. Don't do a big powerful motor (i.e. don't push your luck; dropouts can only take so much regardless of steel vs. alloy)
2. Set the controller to slow-start (this also improves safety so when you hit the throttle if the wheel isn't pointed fwd, it doesn't pull the bike sideways out from under you)
3. Use torque arms. In fact, over-use them. This 45 Nm motor with a 25a slow-start controller on this steel fork has a visible torque arm, which is great. But there is a second one on the other side for the sake of overkill (do not concern yourself with the issue of whether the second arm is on 'backwards' or not. Unless you are using a motor that is far too powerful for a front wheel in the first place, torque arm direction will not matter and will be a benefit regardless).
   
   

Just be smart and careful and you'll be fine.

 

[m@Robertson]

One more thing: Pulling on the bike with a front motor will cause higher wear on the front headset bearings and races. Thats just life. A cheap Mongoose bike is not going to have a robust headset and you can expect that to need to be addressed at some point with a headset replacement. A cheap AliExpress headset that has sealed bearings would be plenty and one of the ones I used was something like ten bucks. Also a front fork being pulled upon can loosen up. I solved this issue by pounding in a second star nut into the head tube, and using an extra-long M6 retaining bolt to screw thru both of the star nuts together. Plus I used a metal end cap. Anything not alloy will do. I actually used titanium but if you can find a steel one that'd be ideal.

 

[Catalyzt]

One thing I miss about my front-wheel hub Hiltopper is the cornering from AWD. I found it to be like cornering in a front wheel drive car-- you can lean the bike way over on a hairpin, and as you are exiting, nudge the throttle as you pedal hard (if you can avoid pedal strikes) to snap out of the turn at speeds that are a few MPH faster than normal with good traction, but YMMV, be careful.

My front hub was probably no more than 40nm, likely a bit less, and did have variable throttle. So no 'slow-start,' but I was careful never to nail it from a standing start.