Torque Arm missing - is this a problem?

Have been following ES forums for some time and based on recommendations here I ordered the Smart Pie kit which delivers max output of 400 W. My problem is, not being very mechanical I had the kit fitted by a local cycle shop. A torque arm was included with the kit but when the bike was returned I noticed it hadn't been fitted. Have been riding the bike now for a few weeks with no problems and in fact very pleased with the performance.

Would very much appreciate any advise from forum experts on risks associated with absent torque arm - what is its function? Any feedback greatly appreciated!

Front or rear wheel mounted kit in steel or aluminum dropouts? Photo's?

A torque arms function is to resist the torque or twisting forces generated at the dropouts through the axle. A bicycle dropout, especially front aluminum ones, aren't made with withstand the rather large amount of torque generated by an electric hub motor. The torque arm reinforces the dropouts so the dropout doesn't crack or splay. Aluminum dropouts will likely, rather suddenly, crack or split and the wheel might simply fall off, you'd probably crash in this situation. Steel dropouts might splay or spread in which the axle will turn in the dropouts, likely destroying the wires along with the dropouts.

400w isn't an insane amount of power, but how it effects things is filled with variables. The safe bet is to reinforce the dropouts with torque arms, I certainly would. I'd say if you were using a front kit on aluminum forks to not ride the bike again until you at least installed torque arms, but maybe even consider replacing the forks the steel ones. If you were using something like a steel down hill mountain bike it might be fine to do without the torque arms on a 400w direct drive hub motor, but I'd still probably reinforce dropouts, this becomes especially true if you are using regenerative braking as the axle may end up rocking back and forth, accelerating the rate that things crack or spread.

In the upper right corner there is a search box, just beneath it there is a 'Using google search?" check box, be sure to do searches both with and without this check box checked as the results will be considerably different but both potentially interesting. Before starting threads I recommend always doing 10 minutes minimum of searching on a subject, I promise you your question isn't a new one. A lot of questions come up repeatedly on this forum so I am hoping that by saying this enough the hijacking/derailing as well as redundant threads slows down a bit.

I would say to use it, as it could "possibly" spread the axle slot without it, and it will "not" fail in that manner with it. No brainer if you look at it that way.
And, the fact that the kit came with one makes me ask...... Has your bike store ever installed an ebike kit before? That seems like a liability gamble.
OR....Maybe they thought it was an optional part for ebikes with coaster brakes........

I see a continues current of 18A for the Smart Pie. So it depends on your voltage how much Watt it produces. And that is the nominal rated wattage not the peak. That could be double that amount. So torque arms are recommended.

I've got steel forks and a torque arm and all my teeth except for my wisdom teeth which got extracted by the dentist 40 years ago.

For front motor wheels steel forks and 2 torque arms. My wife's set up since I prefer her with teeth and a face. She likes it that way too. Small gear motors can produce a lot of startup torque.
otherDoc

Idiots should break their own bones, not those of their clients :|

A bike shop typically knows zero about electric installations, and if it's a front hubbie then you really shouldn't ride it until you get proper torque arms installed. Bicycles aren't designed to take ANY twisting force at the axle. It's not about power, but about torque, and even a low power hubbie can put literally thousands of pounds of force on the dropout due to the small radius of just 5mm.

The failure mode is sometimes the axle simply spins, breaking the wiring harness and killing the controller and sometimes the motor. The other type of failure is the axle spinning and climbing right out of the dropout, leaving you without that wheel on the bike and instant unavoidable faceplant if on the front.

With hubmotors all ebikes must have either properly installed torque arms to spread the spinning force of the axle to the bike frame further from the axle, OR the dropouts of the bike must be specifically designed to handle those forces. No regular bicycle is designed that way, since the bike wheels transfer no twisting force at the axle. The exceptions are coaster brakes and internally geared hubs, and those come with built-in torque arms that secure to the frame 2-3" from the axle.

There are quality torque arms available from ES members in the For Sale section of the forum. It's really not an issue to take lightly or procrastinate about.

Many thanks Guys for the advise and feedback.....obviously I'm facing a serious hazard in operating without the torque arm. If I'd known it was that important would never have ridden the bike!

Just to clarify it's a rear hub install and frame is alloy.....

As someone commented, the average cycle shop seems to know zilch about ebikes and reckon this is the case here, more ignorance than malice. Intend to get this sorted over the weekend. Seriously thanks again for timely warnings on the risks involved here.

Maybe they should rename the torque arm to "Skin, Teeth, and Face Saver" :wink:

I've got over 15K miles on my rear motor without torque arms. However, that's in steel dropouts. I don't use bikes with aluminum dropouts. If I did, I'd use 2 steel torque arms/plates.

I’ve ridden 10,000’s miles on aluminum frame 2kW rear DD motors using nothing more than “tight nuts” and single TA for last resort safety measure.

This situation is 400W? As long as the nuts are kept tight it’s probably never gonna be an issue. But if I were the folks performing the install I’d use at least one.

Can OP post a picture of the installation? That might provide a better understanding if this work is totally unacceptable or simply “not recommended” category.

handtight said:

Many thanks Guys for the advise and feedback.....obviously I'm facing a serious hazard in operating without the torque arm. If I'd known it was that important would never have ridden the bike!

Just to clarify it's a rear hub install and frame is alloy.....

Click to expand...

You might be interested to read the axle spin-out tests that Justin_le performed several years back.

https://endless-sphere.com/forums/viewtopic.php?f=2&t=14195&hilit=forks+testing+steel+aluminum+torque&start=100#p221474

The takeaway I got from those tests are that torque arms are not as important as properly tightened axle nuts, and torque arms that use thinner than 3/16-inch steel are pretty much useless. I've been riding a 1kW powered crystalyte 408 motor for years on a good CrMo steel fork without torque arms. When you say your frame is "alloy" that doesn't tell us much. Is it a steel alloy or an aluminum alloy? With your relatively low powered rear kit, a steel frame alone should be safe if you keep the axle nuts properly tightened. Torque arms are a good backup, though, in the event that an axle nut should come loose.

jimw1960 said:

Torque arms are a good backup, though, in the event that an axle nut should come loose.

Click to expand...

A torque arm is not a backup, it is mandatory for almost any frame, alloy especially. If it is fed high power, a motor will spin in most torque arms available, and much more solid torque plates need to be made. A good build doesn't rely on axle nuts, even riding safe without any.

Justin made those tests a long time ago, with a low power system and a steel frame. No matter how tight the axle nuts, they are not making the frame dropouts any stronger, only adding the risk of damage to the axle thread. Sooner or later the true resistance of the dropout will be tested by motor torque, and dropout failure is the most common mistake in building a first ebike.

Nuts and screws often come loose on anything that travels over roadways. It's a good idea to go over the bike and tighten everything that needs it once in a while. :wink:

MadRhino said:

Justin made those tests a long time ago, with a low power system and a steel frame. No matter how tight the axle nuts, they are not making the frame dropouts any stronger, only adding the risk of damage to the axle thread. Sooner or later the true resistance of the dropout will be tested by motor torque, and dropout failure is the most common mistake in building a first ebike.

Click to expand...

You might look at Justin's thread more closely. The dropouts and forks he tested were all stressed to the point of failure using a manual lever system, not a "low power system." He also tested different aluminum alloys. He also showed the effect of different torques on axle nuts, which of course do not make the dropout stronger, but do prevent the axle for being able to move to the point where it is stressing the dropout. Anyway, I know you run some pretty high power setups and agree torque arms are necessary for those. On a relatively low power system like mine on a CrMo steel fork, I feel perfectly safe without them and have several thousands of miles of use without so much as an axle nut coming loose.

jimw1960 said:

...I feel perfectly safe without them

Click to expand...

I feel pretty safe taking a 6 ft drop at 40 Mph
Yet I wouldn't go telling anyone it is something safe to do

Your torque may be low, or your fork dropouts very strong, but you can't presume of someone else's bike. The fact is that most who don't install torque arms, do experiment failure sooner or later.