|"Thinking Outside the Four-Wheeled Box"
Torque arms are used to prevent axle rotation in hub motors. When a hub motor is powered up running, for all the torque that the motor generates spinning a wheel forwards, there is an equal and opposite torque on the axle causing it to rotate backwards.
In most electric bicycle hub motors, the axle is machined with flats on either side which key into the dropout slot and provide some measure of support against rotation. In many cases this is sufficient. However, in high power systems that generate a lot of torque, or in setups with weak dropouts, the forces present can exceed the material strength and pry the dropout open. When that happens, the axle will spin freely, wrapping and severing off the motor cables and potentially causing the wheel to fall right out of the bike.
To give a sense of the magnitude of these forces, a hub motor with a 12mm axle generating 40 N-m of torque will exert a spreading force of just under 1000lb on each dropout. A torque arm is a separate piece of metal attached to the axle which can take this axle torque and transfer it further up the frame, thus relieving the dropout itself from taking all of the stresses.
There are countless unique styles and geometries of bicycle fork and dropout area, so one of the challenges for us has been to design a device that will accommodate at least the large majority of them without much customization. Our solution to this has been a unique 2-piece approach, which gives the necessary flexibility to fit the wide range of dropout angles and tube locations without compromising the anti-rotation ability.
We developed our first such 'universal' torque arm design in late 2009. In this model, the arm part was attached to the fork with a single hose clamp, and the axle plate could pivot to fit different dropout slot angles.
When the motor is powered, it tends to rotate the plate clockwise, and with the pivot point on the back this causes it to further 'push' the axle up deeper in the dropout. Notice that if the torque arm is mounted backwards, on the front side of the fork, then it would have the opposite effect and axle torque would tend to pop the wheel out of the dropout.
While this design did work, we did run into several instances of failure from the hose clamp in cases where a motor controller had shorted mosfets. The curved hose clamp slot in this design put stress concentration points on the clamp which facilitated the hose clamp tearing in half. As a result we discontinued manufacturing this model in 2010 and do not recommend its usage.
The Rev2 design was engineered to make use of the existing fender eyelet holes that are on most front forks. This can lead to a smaller arm and a tidier installation than using hose clamps around the fork, and we recommend its use whenever there is a suitable eyelet present.
For forks that don't have fender eyelets, or where the fender eyelet holes don't line up, we created up updated variant of the original hose-clamp model with the Rev3 layout. Here, there are small slits for 2 or even 3 hose-clamps to provide extra spinout strength, and the long curved slot in the axle plate allows for for the clamping arm to attach parallel with the fork tube and always intersect correctly.
Rear bicycle dropouts are usually thicker and stronger than the front fork dropouts, so more often than not a rear torque arm isn't necessary. But in cases with shallow vertical dropouts, or with powerful (>1000 watts) motor setups, it can be as important as the front. Our Rev4 design uses multiple hose clamp slots to secure to the arm to a seatstay tube. The axle plate can flip directions to accommodate both horizontal and vertical dropouts, while the hose-clamp arm clears up and over most eyelet holes present on the rear.
All of our torque arms are currently laser cut in the USA and assembled and finished in Canada. We use thick ¼" stainless steel plate for the anti-rotation slot and use industrial stainless steel hardware for all the fasteners as well.
The front Rev2, Rev3, and rear Rev4 torque arms are all available from our Motor Parts store page, and from various dealers around the world.
The images above show the correct installation alignment for the various torque arm models we've designed. There are a few important points:
Frequently Asked Questions
Is a torque arm really necessary for my setup?
The common answer to this question is "yes, you must always have a torque arm installed with any hub motor setup..." The reality is that for many years torque arms were pretty rare in ebike conversions, without issue. But with people pushing the motors to higher power levels, and with the proliferation of weaker alloy suspensions forks, the need for torque arms has grown significantly.
We did a number of dropout and torque arm strength tests in 2009 to better understand the situation, and for those with a scientific bend many of the results were published in this endless-sphere thread.
In a nutshell, axle spinout happens at around 60-90 N-m with most forks. Aluminum dropouts tend to crack and fail catastrophically while steel dropouts will just spread open. The addition of a torque arm can increase this to >150 N-m. The graphs below show some typical axle spinout torque profiles for plate metal and cast aluminum dropouts, with and without the torque arm. Note that these tests are for only one side of the dropout, so the spinout force would double in practice with both left and right dropouts engaged.
What does that mean in practical terms? It corroborates our own experience which is that for hub motor setups running with a standard nominal 36V 20A input power, torque arms are not really needed provide that:
That last point is an important one, because it can easily happen in the course of replacing a flat tire or the likes that the wheel is put back on with only hand-tightened nuts. If the throttle is applied in this state, you'll be guaranteed a spread dropout and spun axle.
For powerful setups (>1000 watts input power), setups with front aluminum dropouts, setups with regen braking, then a torque arm should be used for sure.
Is One Torque Arm Enough, or should I use two?
For the torque arms we make with the thick ¼" axle plate, one is almost always more sufficient, but of course it never hurts to have two. Installing a 2nd torque arm will be tricky if you have a hub where the cable exits the end of the axle, as you'll need to cut the connectors off the wire and then reattach them.
If you are using a 3rd party torque arm that is made from thinner 1/8" (3mm) metal plate, then we'd recommend doubling up and installing 2 arms.
Does it matter if I have 12mm or 14mm axles?
No it does not matter, the torque arm plates are sized to accomodate a 14mm axle, but will fit a 12mm fine as well. With a 12mm axle, there is a small gap at the top or bottom of the slotted hole, but it is of no consequence and does not affect the spinout retention strength.
A 14mm axle has a much wider flat area and provides more spinout protection, both inside a dropout but especially inside a torque plate. However, a 14mm axle on front forks often leads to interference issues with the large diameter 14mm nut and washers, and it's a bad situation if the washers and nut don't stack up flat against the dropout.
Most rear dropout plates are flat and don't have this issue. For these reasons we tend to spec our orders of rear hub motors with 14mm axles, and the front motors with 12mm so that the axle hardware can fit inside the front dropout cavity. The exception is the new Crystalyte HS motors, which are 1/2" (~12.5mm) both front and rear.
Why not have the torque arm built into the motor?
Because that would be smart design, and most of these hub motors come from China, where intelligent design is not a prerequisite for anything.
What about regen?
In a regular ebike where the motor is torque is always in the forwards direction, the axle only has a backwards torque on it. In practice, the axle will rotate until it has completely bottomed out both against the torque arm and the dropout slot. In a system with regenerative braking, the torque on the axle reverses direction, which can allow the axle to rotate a bit the other way until it bottoms out on the forward surfaces. If there is a sufficient amount of play, this back and forth rocking of the axle can eventually cause the axle nuts to loosen or the axle to wiggle itself out of position.
It can't be overstated how important it is in this case that the axle nuts are thoroughly tightened and that the torque arm plate fits tight on the axle to reduce the amount of 'wiggle room'. It is possible as well to use two torque arms, and preload one in the forwards and the other in the reverse direction.
We don't currently have a "regen specific" torque arm design in production, but do have a few on the drawing board.
Can I make my own torque arm?
If you have metalworking tools then yes, absolutely, and with a custom torque arm designed for your particular frame you should be able to make a tidy single piece arm that makes use of existing eyelets or bolt holes. Some people have also used 10mm crescent wrenches as torque arms in a pinch. There are many great examples on this ES thread.
One important point is that you'll want to make the plate that goes over the axle is at least 5mm (3/16") thick. Most early torque arm designs we saw were made from 1/8" (3mm) steel plate, and even with a snug fit on the axle this doesn't provide all that much anti-rotation strength, just 20-30 Nm more torque than having no torque arm at all.
It's possible to have a custom arm made from two metal plates that "clamp" the axle flats, but if you do this be aware of the strength of the bolts holding the two plates together. We've seen a case of the bolt shearing right in half.
I see many sites still selling the original Universal torque arm, what's up with that?
Unfortunately somebody sent a sample of our first universal torque arm design to China in order to have it made cheaply. This happened shortly before we discontinued this model in lieu of the improved Rev2 and Rev3 arm designs. We've since seen this cloned copy proliferate among many China kit suppliers and traders, in ever declining quality, and have with some bemusement had those companies try to sell it right back to us.
If you like our current torque arm designs and want carry them as part of your business, then please contact us. We offer good wholesale volume pricing, and you will be getting a quality-ensured part designed and made in North America by folks who know what they are doing.
copyright © 2012 by Justin Lemire-Elmore, Grin Technologies Ltd.