Torque Arms on Hub Motor Bikes

June 29, 2012
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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.

 

 

 

Here is an example of another popular torque arm made and sold by Dr Bass here. This is a unique design and requires that the torque arm be glued with strong epoxy to the drop out. This is a very slick solution.

 

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

The Ebikes.ca torque arm and the parts you will need for this job.

 

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.

 

Here is the hub motor with its axle fitted onto the drop out.

 

the axle with C-washer installed

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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.

 

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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.

 

Get that baby as tight as possible.

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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.

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Written by Eric, June 2012

 

Eric has been involved in the electric bike industry since 2002 when he started a 6000 square foot brick and mortar Electric Bike store in downtown San Francisco. He is a true believer that small electric vehicles can change the way we operate and the way we think.

21 Comments

  1. Often time the dropout eyelet is steel an the fork blades is aluminum. What then?

    • That’s better than all aluminum and depending on the thickness of the steel you may or may not need a torque arm.

  2. i dont get what this actually does and what the point of all the tail parts are, it would make more sense to me if the arm was welded to the fork or clamped on properly with something more substantial than a cable tie, as like that it looks to me about as useful as an extra washer, i think i would me more confident in something that was 1 piece and attached to the disc brake mounts like the one above with white forks

    • It reduces the amount of torque applied directly to the drop-out by routing it to a stronger section of the fork. As for welding, you can’t weld to the aluminum without compromising the integrity of the fork. If you want one-piece arms they make those as well.

      • I have just ordered a set of these (in UK on ebay) for my Kona Ute (alloy frame) where the dropouts are very shallow (thus the desire to fit TWO, one either side of the rear hub motor (1000w) … I am expecting to be able to secure the torque bracket with only one of the two drop links as the Ute is bristling with unused 5mm threaded holes.

        This is a non-commercial board so I have anonymised the graphic – contact me direct for details of the supplier.

        • The photo below show the small dropout on the rear of the 2014 Kona Ute, the paint damage resulted from the motor pulling the shaft out of the dropout… there are plenty of places to fasten a torque arm to (disc brake caliper adaptors x2 and another mudguard(?) retaining stud)

          Might fabricate a DIY plate between the 14mm diameter motor shaft and the upper 5mm threaded mount in a suitable 6mm stainless plate (If I can work out a way to drill the stuff!)

  3. Complete noob here. Is it not possible to do what downhill bikes do and design a hub motor/wheel assembly compatible with a 20mm thru axle front fork?

  4. whats the problem doing this with a front suspension fork? ie the bottom of the fork is steel so why the recommendation not to add front motor to suspension fork?

  5. Suspension fork with disc brakes should be designed to resist forces during braking. Why is it not possible to use it for driving as well?

    • Well, the disc brake calipers are anchored where torque arms should be. And another thing is unsprung mass, which, i guess, adds a lot to unstability on off road 🙂

  6. I purchased a front wheel. I wheel came off when I was riding it (I could have got killed). the kit I purchased didn’t come with a torque arm, does that mean I still need a torque arm? is the fault with the kit or improper installation or not having a torque arm?

    • Could be all of the above. Torque arms are design to prevent spin outs where the axle actually spins out of the drop outs (and all of a sudden you are trying to “roll” on forks ends instead of a wheel). Most kits do not come with torque arms, but they are available and you could always fab some yourself to fit.

    • I snapped the dropouts off my forks with a 600 watt ebike made in china motor. It was a scary experience. I reccomend steel forks, but doing a change out from aluminum to steel on an aluminum frame can be tough because You must source a steel fork that fits your frame, and the ones I scrounged up didnt fit. A new one is like100 dollars. If you go to an all steel frame, it makes it all so heavy. I dont like the look of the hose clamp attachment; is it safe?

  7. First off, the photos and instructions depicting strapped torque arms are not a good way to mount them and should be avoided. If the fork is going to fail, the strap will fail, period. All torque arms should be bolted to the axle (obviously) AND bolted to the forks (even if that means welding tabs or drilling). This is one reason cromo steel forks are a MUST for bigger front hub motors, you can drill and weld them without compromising the forks. The other reason is strength and the gradual fail rate of steel (as mentioned in the article). You should Never run a 500w+ Hub motor on Aluminum forks even if the the drop outs are steel, the entire fork system is only as strong as the weakest part, and your life could depend on it. So why not do it right?

    • Can you show this clearly on youtube, give a link? I have a EZ-1 recumbent bike with a small fork in front which it came with.

  8. Nonsense, almost all.

    • Very helpful.

  9. looking for info on schwinn stingray electric chopper bike controller and hub motors 2005 any help email mistik702@gmail.com

  10. “… on the other side you may have to remove the plug housings to get the torque arm onto the axle”.

    About that lol. You wouldn’t happen to have any tutorials on safely removing and reattching the afore mentioned plug/housings in order to install a torque arm would you? Because at the moment that’s exactly what my bike needs.

  11. I have a 1000W Magic Pie 5 Rear motor on my aluminium bike. I’m planning on installing two torque arms.
    My bike is aluminium, but because it is the rear wheel, should I still be worried it should brake?

  12. I have some doubts about filing down the dropout to get the axle centered
    “In some cases, step one is to file the dropout slightly deeper.”
    I did exactly that (without having read this article beforehand), with the filed result looking almost exactly as shown here. I also got washers that fit the cup (shaped as seen in the photo of the article). Still after only 300km my 250W engine broke the dropout. This luckily happened when accelerating from a stand, so no harm done. The fork I was using was a suspension aluminum fork.
    I was guessing that my filing was the reason, since it changed the round shape at the end of the dropout into one with corners, which reduces the resistance to stress.
    On the other I understand that the washers are needed to press down on an as big area as possible to distribute the torque. So maybe the grove-shaped cups are not really suitable for fitting.
    My next try will be with a rigid steel fork.

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