Torque arm play? stretched dropouts?

I looking into getting or making torque arms for my bike. What I can not understand is how effective are they from preventing the drop outs from getting damaged or stretched by a small amount.
Most torque arm setups I can see having a bit of play, especially the ones that are not welded on. Some I see are attached with plastic wire loops or rubber pipe clamps. These setups must have play in them.
When the motor applies torque, will it not put this torque first into the dropouts before applying the pressure into the torque arm, as the dropouts are more solid with no play?

Hopefully someone can understand what I am trying to say here. I don't know how tight of a fit my bikes dropouts are to the hub motor but I assume that there is a chance that the bike dropouts will be a tighter fit than the torque arms as the torque arms that are not welded on will have more play in them. The motor will then contact the bikes dropouts before it applies it to the torque arm.

Please let me know your what you think because I may want to put my bike back to normal. I do not want my rear dropouts stretched out as I think that might put some kind of play in the rear wheel and therefore effect the handling of the bike.

yes you are right, the torque arm needs to be super snug on the axle and also firmly fixed to the rear triangle, I also agree hose clamps are not an ideal solution, you can use hose clamps to secure the bracket to stop it moving but you are correct it all needs to be solid to avoid the situation where the drop outs take up the slack before the arm does.

I have steel drops on my main bike so dont use a torque arm as I only push 1.6KW through it, they have been fine for over 5 years, all my ally drops have 1 x custom made arm and supporting bracket, the bracket is wrapped around the rear stay so that the arm pushes up against it, its very firm and there is no slack.

Making your own arms means you can file them to fit exactly the rear axle, as we know there is quite a manufacturing tolerance between motors on the axles.

As a general rule though fitting a torque arm even with a little bit of play is preferable to not having one esp if you run high power and also if you have ally drops, if you ever fit a motor to front suspensions forks (and I dont suggest you do this) then you must have snug fitting arms either side very well secured else you will instantly snap the fork axle eyelets off, seen it done on more than 1 bike.

Like Knoxie says, the biggest issue is the power you are putting through it, and the material of your dropouts. I ran a BMC motor on 36 volts @ 40amps on my wife's bike (chromolly frame, tight nuts) with no torque arm (just a torque washer, ie, a washer with a lip that sat in the dropout gap), and I removed the motor months later and it was as virginal as Britney Spears before she met JT. Seriously unmarked.

But I have run a X5 motor on 20s/84v @ 40amps with dual torque arms (ie on both sides), and I have blown apart the stainless steele torque arms, and destroyed the aluminium drop outs in the process, the whole thing looked like Britney after she met and had kids with K-Fed. Later I changed that bike to a HS3540 on 48v @40amps, and I still managed to fully spin the axle (the Hx axles are notoriously damaging) inside an Ebikes torque arm. In the end I had to drill and then bolt Doc Bass torque arms onto the dropouts to get the bike to work (because I had shattered the drops).

It depends on the motor, the power and the quality/substance of the bike dropouts, but to be honest, if you are going 48V @ 40 amps and above, you are going to need a good torque arm, and even then, yes you will at least chip away at the dropout. You might not turn your bike all Brit, but you will at least get it all Aguilera.....

I guess what I am saying is, I wouldn't chuck an ebike motor into a bike frame (even with torque arms) with anything over 1200W, and not be expecting some chipping and stretching.... I have always been amazed at the force of the torque in the axles when you go over 1200w ish.

I was just reading a thread on different torque arms I look for it.

Operative term is "power through it" and not just driving power but strong regen. Those elements together tend to work things back/forth so that no play whatsoever is acceptable. I avoid regen for this reason and I see very little net return power into my batteries.

Properly tight axle nuts in deep dropouts should be enough for 2kw DD motor, no regen. But, torque arms are excellent insurance and I won't run those levels without at least one.

More than a few torque arms that are just decorations. Including some real good designs, if installed improperly or expected to hold more force than designed for.

When installing for no regen, some play between the arm and the axle is permissible, you just think about which way the force will go, and snug it up for forward power.

But idealy, there is zero slop in a good torque arm. For serious people, nothing beats the clamping type torque controll plate. Then you can go twist your axle in half. 8) Suprising I haven't done it yet.

Make them like mine for ultimate clamping..I did also make a bigger axle too. See this thread to see the rest of the story


http://endless-sphere.com/forums/viewtopic.php?f=3&t=40785&hilit=snapped

Philistine said:

Like Knoxie says, the biggest issue is the power you are putting through it,

Click to expand...

Just to be purely technical, while power does give a fair indication of what it must be able to resist, it is actually torque that matters, hence the name.


As was pointed out above, regen braking is often a fairly big factor when things go wrong with torque arms/dropouts, because you have very high torque in both directions that way, first during accelration and then during braking, which rocks the axle back and forth.

Without regen it's not so bad, but can still twist an axle inside the dropouts over time if it's not perfectly secure and tight.


Clamping dropouts, which move the torque arm/plate to *be* the dropout, are the best way to make a torque arm, but require modification of the frame or fork.


FWIW, I've had hose-clamped torque arms fail (clamp snapped), with only 2KW peaks and regen braking rocking it over time. It also snapped a box wrench on the other end of the same axle.

Using 4KW+ peaks (same motor, different controller) on the very basic clamping dropouts I built out of old BMX thick rear dropouts welded to the existing ones, I have had no wiggle, no problems, and no spinouts, etc. Even when I broke one retainign ring for the nut/bolt that clamps on one side, so it only has one clamping bolt on one dropout, it's still not been a problem, for months now. I could probably leave the axle nuts off and it'd still be fine.

So I tend to go with clamping dropouts as the "ultimate" solution, if using standard hubmotors (modified or not), for anything with serious torque to it or regen plus medium torque. This includes medium powered geared hubs, too, as those are often (perhaps always) higher torque than a similar-power DD hub because of the gear reduction.

Well that's three votes for clamping high torque rigs.

What you are doing, on what bike, regen or not etc, will determine what you really need. For less strenous use, a torque arm or two from good supplier like Grin will suffice, if properly installed. By that I mean "normal" stuff like 48v controllers.

I have not tested and reviewed grins new rear torque arms, but I have several of his front designs. They all have fit the axle quite snugly, which is a key starting point.

Imitations of his desgns are appearing from china, which may not have the snug fit of the ones from Grin. You are quite correct that the quality of the install is everything. It might be desireable to replace a hose clamp with a stronger strap using a bolt or several bolts. Eliminating any play either direction where it attaches to the bike is crucial, and the strap must be stronger if higher torque is anticipated. I recomend fitting, then tack welding the two piece torque arm designs so they cannot slip at the nut as easily. If the nut rotates, the strap can slip down the frame tube. So eliminating that possibility is good.

Here's my bolt on pinch dropouts.

I use heavy regen and run at 84 or 100 volts..My peak power is 14kW, cruise at 4-8kW.

All of my work is not as fancy as in my picture above.

If you wanted to really go mad I suggest you go for a clamping one, but with damping..something like a metalastic coupling.. So the axle is clamped very tight in a clamp that is allowed to rock slightly in some sort of rubber or polymer mount, to help relieve the shock that occurs on sudden accel or deceleration. No idea how you would go about that yet..I have thought about it but not looked closely at the problem yet..Go on be the first..


I have done some rough ones on steel frames too

WIth clamping torque rigs, could I build one using the disk brakes holes near the rear drop out? I do not plan on upgrading to disk brakes anytime soon.

With dogmans picture above, is that climping torque on the other side also? Do you need to do that to both sides of the dropout?

The other side where the rear derailleur is doesn't have any bolt hoes around the area to attach a bracket to.

Also, I was thinking about making a torque arm like in this youtube video.
http://www.youtube.com/watch?v=mVu0t36-8IE

Do you think I am better making something that is clamping or something like in the video? I could do something like he made in the video and design it to be clamping also. Not sure if clamping offers any benefit over what is in the video.

One other question, is aluminum torque arms OK to use, or is aluminum too weak? I have thick peices of solid aluminum that I can cut up.

I just have the clamp on the disk side. Just an axle nut on the other side. The disk mount bolts could be used to attach the plate, but idealy the bolts would be close to the axle. Since this frame was steel, I just drilled holes where I needed them.

My big clamping ones a few posts up are aluminium, no problems to far. But it is a much softer metal so may cause issues in future. On the other hand, when clamping, it is going to distort a bit so may give better clamping.

FWIW, those pics up top never appeared for me. Just an x in a box. I have no idea if the guy asking can see em.

Strange. I did not upload them today,they are old ones already uploaded to the site and other threads months ago.
I can see them

Can someone recommend a good thickness to make torque arms from steel?

Where is a place I can buy steel in this thickness?

5mm is usually enough for high power; 3mm can be fine, too, for lower power.

You don't need really hard steel, but really soft steel may not work well either. I used dropouts off an old BMX, cuz they're nice and thick (4-5mm, IIRC) for my clamping dropouts, amd they are doing fine at 4.5KW startup bursts.

You may find steel that will work from many recycled sources, including old desk chairs--the legs or the angle bracket that holds up the back of the chair--most likley both will be 4-5mm thick (or more) adn good enough steel to use for this.