Hub motors put a lot of twisting torque on a bicycles drop outs, more than any bike was designed for. This is a special concern when running a hub motor on the front, because if your fork snaps, it can have potentially fatal consequences (think face plant onto the concrete). There have been electric bike riders who have died this way so use extreme caution. The more powerful the motor the greater the danger.
If you’re running a 350W Cute geared motor, you probably will not have any problems. If you’re running a BMC 600W that’s amped up to 2000-watts you could have a severe problem, especially if running on an aluminum fork. To be extra cautious, with aluminum forks and powerful motors, you should use two torque arms, one on each side of the fork.
Broken front forks on electric bikes are a common occurrence because of one often-made mistake made by the home bike builder: aluminum front forks are not suitable for hub motors. To make them suitable requires an expert builder who knows what he is doing, and then setting them up properly with steel reinforcement (torque arms). If this is your first-time bike conversion, do not do a front wheel drive unless your forks are non-suspension and made of STEEL! To test to see if your forks are steel put a magnet to the fork and see if it sticks.
Understand that when aluminum fatigues or breaks…it snaps with no warning. Steel on the other hand will fail by bending…not snapping. Steel does not fatigue over time as quickly as aluminum. In E-bike applications where high strength is more important than light weight, common carbon-steel and chromoly steel are a better material in general for frames and forks, compared to aluminum. Also DIY’ers like the fact that they can weld to steel when needed. Heat treated aluminum cannot be welded to. Heat-treated aluminum can be torch-brazed, but that ruins what little strength it has, making it even softer.
Steel bike forks are standard equipment on the cheaper grade of bikes. Also many aluminum frame bikes use steel forks for strength. Ironically most high priced bikes are made from aluminum because of the weight advantage. There are exceptions to this. For example, all Surly bikes are fairly high priced, but are made out of steel. No surprise that Surly frames are one of the favorites for hub motor electric bike conversions.
If you do have aluminum or suspension forks and you decide to do a front wheel drive anyway, it is safe on a low power hub motor (under 500-watts) especially if you use steel torque arms to reinforce the fork. If you go with over 500-watts with an aluminum fork, you must use torque arms and C-washers to be safe…and even then you are still taking a chance (C-washers fill the “lawyer lip” recess to make a solid connection).
Here is an example of a universal torque arm used on a front fork mady by e-bikes.ca and sold for $28. It is made with THICK 3/16-inch stainless steel, and it is well worth the price.
Some electric bike builders like to add steel drop outs onto the rear to be extra safe when running a rear hub motor on an aluminum frame. This is usually not necessary if you are using a mountain bike frame of any decent quality. Also, the type of failure where you snap your rear drop outs its not as big of an issue as snapping in the front since locking the rear wheel will usually not result in a crash, and if it does, it will not be an end-over face-plant.
Although to be prudent, when running an aluminum frame and a powerful hub motor in the rear, rear torque arms are still important:
How to Properly Install a Torque Arm on a Front Hub
Whether your front forks are steel or alloy, nothing is more important that getting the fit of the washers and torque arm correct. In some cases, step one is to file the dropout slightly deeper. To fit the axle correctly, you want the center of the oversize axle to be centered in the dropout. Oversize axles will fit too shallow, and this could lead to the axle working itself out of the dropout.
The next very common problem is that the axle may have a deep cup designed to receive a quick release hub. Some will grind down a washer that came with the kit to fill the cup. The best solution is to use a C-washer.
The last thing to do is span the cup with a washer or torque arm. You want the nut pressure to bear down on the spot the designers intended it to bear on, the bottom of the cup.
After the washer has filled the cup, add the torque arm, and another washer if you have enough threads left for it. Get the bolt holding the two pieces of the torque arm together as tight as possible.
Then put the hose clamps on the fork tube. Use as many as will fit well, in this case only one fits.
Use two torque arms if you are using alloy forks. On the other side, you may have to remove plug housings to get the torque arm onto the axle.
Written by Eric, June 2012