A torque arm is an extra piece of support metal added to a bicycle frame to more securely hold the axle of a powerful hubmotor. But let’s back up and get some more perspective on torque arms in general to learn when they are necessary and why they are so important.
Many people choose to convert a standard pedal bicycle into an electric bicycle to save money over purchasing a retail ebike. This is a great option for a number of reasons and is surprisingly easy to do. Many manufacturers have designed simple ebike conversion kits that can easily bolt onto a standard bicycle to convert it into an electric bicycle. The only problem is that the poor guy that designed your bicycle planned for it to be used with lightweight bike wheels, not giant electric hub motors. But don’t worry, that’s where torque arms come in!
Torque arms are there to help your bicycle’s dropouts (the part of the bike that holds onto the axles of the wheels) resist the torque of an electric hubmotor. You see, normal bicycle wheels don’t apply much torque to the bicycle dropouts. Front wheels actually don’t apply any torque, so the front fork of a bicycle is designed to simply hold the wheel in place, not resist its torque while it powers the bike with the force of multiple professional cyclists.
Rear wheels on standard bicycles traditionally do apply a small amount of torque on the dropouts, but not more than the standard axle bolts clamped against the dropouts can handle.
Torque arms add strength to weaker bicycle frames
drawings courtesy of Grin Technologies (ebikes.ca) and used with permission
When you swap in an electric hub motor though, that’s when torque becomes an issue. Small motors of 250 watts or less are usually fine. Even front forks can handle the low torque of these hubmotors. Once you start getting up to about 500 watts is when problems can occur, especially if we’re talking about front forks and even more so when the material is weaker, as in aluminum forks.
In this case a torque arm is required to resist the torque of the hub motor. Torque arms come in all shapes and sizes. Some are mass produced, one-size-fits-most styles that slide over the axle of the motor and then clamp or bolt into the bicycle frame, offering a firm connection to the bicycle further away than the surface of the axle.
Other torque arms are custom jobs made by guys and gals in their garages, specifically suited to their own bikes and motors. The one thing all these torque arms have in common is that they grip the flat part of the motor axle and connect to the bicycle frame in a sturdy way to help resist that torque from forcing open the dropouts.
Gasp! Do I need a torque arm?!
So when do you need a torque arm? Well, the short answer is that it’s better to be safe than sorry and that a torque arm will always help. Practically speaking though, there are a few factors that will tips the scales in favor or against the need for a torque arm.
First of all, if you are buying a retail, commercially available electric bicycle then you don’t need to worry about a torque arm. The ebike’s designers will have already included one, if necessary, or more likely will have designed the bicycle to be a purpose built ebike with strong enough dropouts to not require a torque arm.
If you are doing your own electric bicycle conversion though, you might need a torque arm depending on the type of bike and power level of the motor.
As mentioned above, front and rear mounted hubmotors have different requirements. A front fork is usually much weaker than the rear dropouts. This means that front hub motors are more likely to require a torque arm than rear motors.
Next, frame material plays a big factor. Steel is a stronger material than aluminum and resists bending easier. That means if you have steel dropouts combined with a rear hubmotor, you’re in a much better position than a front aluminum fork.
Lastly you have to consider the power of the motor. Like we discussed above, 250 watts or less should be fine in nearly any steel or aluminum dropout without torque arms. 500 watts is about the limit you’d want to put in rear aluminum dropouts without a torque arm. If you have a 500 watt motor in the front, especially if you have an aluminum fork, you’ll want to use a torque arm. 750 watts or above should almost always use a torque arm, even in the rear of the bike, even in steel. Generally speaking, 750 watts in rear steel dropouts will probably be fine, but it’s getting near the limit. That’s why we recommend 750 watts or above, using a torque arm.
Torque arm specifics
Three main factors control the effectiveness of torque arms, so you’ll want to pay close attention to these when buying or making your own torque arms. First is the material choice. Look for stainless steel torque arms if possible. These will be even stronger than the mild steel or aluminum that your bike frame is made out of.
Next, thicker is better. Always. You want as much meat gripping that axle as possible. Try to find a nice thick torque arm. I’ve seen thin torque arms simply cut a slit around the axle and still allow it to spin, damaging the bicycle and motor. A quarter inch (0.635 cm) is a good torque arm thickness to aim for. Even thicker is better, of course.
Lastly, the further away the torque arm mounts to the bike, the better. A one inch long torque arm is good, two inches is better, and three inches is better yet. The further from the axle that the torque arm mounts to the frame, the more force it can resist.
Good torque arms won’t be cheap. Don’t expect to spend less than $15-20 a piece for a decent stainless steel torque arm. The good news is that you can find them all over the internet from many reputable sellers including for between $9-$38 from ebikes.ca (one of the best electric bicycle parts vendors in North America) which are also available to order even easier from Amazon, including a front torque arm here and a rear torque arm here, or for $25 from Electric Rider for a beefy Crystalyte torque arm.
If you’re in a pinch or you really want to make your own, a 10 mm spanner wrench makes a surprisingly good torque arm. Just make sure the wrench doesn’t somehow interfere with the axle nuts closing firmly.
When in doubt, use a torque arm
Torque arms are there to save your motor and your bicycle. The cost of not using torque arms when you should have is quite high: often a destroyed motor and/or destroyed bicycle frame. You can avoid this tragic end by making sure you’ve got the right tools for your ebike build, including knowing when to use a torque arm.
Hi,
Speaking of torque… I have a 500 w 36 v motor from China, I bought a conversion-kit. I am pulling a chariot with two small children in it and the steep hills where I live seems to get my motor all heated up. And sometimes, if I stop for some reason in a steep hill, I cant get moving again.
I have read somewhere that I need at least 70 Nm of torque to get my ass up them hills, and probably a stronger (more watts) motor, more instant power.
How to calculate or know how much torque I have to my disposal, and do I need to order another maybe 1000w engine and a new battery to be happy?
Btw, I read your wh/amps/volts- article with great interest, thank you for that. It made it clear to me that Bafang (8FUN) are holding back on the specs and I am in the dark here.
Jan
It’s difficult to measure the actual torque of a motor without putting the bike on a dynomometer, which most people (including me) don’t have access to. For steep hills with a trailer, you’ll probably want 48V and a motor that is meant to go slower speeds so it runs more efficiently. Also make sure you have a battery than can handle it. More AH is better for higher power needs (like big hills).
I have a Hill Topper motor rated at 300W, but its running on 36V not 24V.
The front dropouts are showing signs of fretting and wear after 18 months and about 2000 km.
The axles aren’t long enough to allow a 1/4″ torque arm and still have space for a nut on the outside.
Do I have any other option except get a replacement front fork when it gets too bad?
Are the dropouts actually expanding a bit? If so, then you definitely want to consider some type of torque arm. A hub motor coming loose in the front is more dangerous than the rear because of the potential to go cartwheeling over the handlebars.
A few suggestions:
1) You said a 1/4″ torque arm won’t fit, but will a narrower one? Some companies that sell cheaper torque arms use 3/16″ or 1/8″ material. I always recommend http://www.ebikes.ca for torque arms when possible, but because their torque arms are such great quality, I believe all of them are 1/4″ stainless. If you can find a lower quality, thinner torque arm instead, it will still help hold your motor. The hill topper kits aren’t super high power so you don’t need a crazy strong torque arm. Something is better than nothing.
2) Can you replace one of the washers, either inside or outside of the fork, with a torque arm? I’ve done that multiple times when I haven’t had enough axle length. I just removed a washer (hub motor washers, especially the ones with the tooth, are usually pretty thick) and that gave me enough room. The face of the torque arm worked just like a washer anyways, spreading out the clamping force of the nut.
3) Do you already have a ‘torque washer’ in your kit, which is basically one of those funny washers with the tooth? If not, consider replacing one of your washers with a torque washer. The tooth goes into the dropout and works like a torque arm, giving a surface to resist motion. Torque arms work better because the point where they contact the bike is usually a couple inches away, making a bigger lever arm. But even a torque washer can help, especially on a lower power motor like the hill toppers. Here’s an article that includes info on torque washers, if you aren’t familiar with them: http://www.ebikeschool.com/install-hub-motor-washers/
P.S. I just saw your ebike in the International Ebike Garage (http://www.ebikeschool.com/spot-15/). Very cool budget build! What you call a “POS” I call a great demonstration of building a fully capable ebike for very little investment!