A different rear torque arm idea

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CGameProgrammer   10 kW

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A different rear torque arm idea

Post by CGameProgrammer » Apr 12 2008 4:26pm

It occurs to me that the axle nut is the single most important component preventing the axle from spinning in the dropout. The axle nut is threaded onto the axle, and if properly tightened with a locking washer (or other mechanism) compressing it to keep it locked on, it effectively becomes part of the axle itself. And if it's clamped onto a solid flat surface that's part of the bike, then the axle can't spin. From what I've read online, clamping force is extremely good at resisting shearing.

Common wisdom here seems to suggest the torque arm mainly needs to squeeze the two flats of the axle. But it's well known that solid steel dropouts, that do just that, can be ripped apart if the axle nut isn't tight, suggesting they really don't do much at all.

I'm lazy and it seems like the easiest method to make a torque arm, which should be very effective, is to make a steel arm that grabs the swingarm, with the other end wrapping around the axle, but not necessarily the flats. You can have a 14mm round hole there, and if the axle nut is clamped very tightly against it, surely this would be a perfectly good torque arm? I'm considering doing just that, probably with red Loctite (vibration-resistant, requires a heat gun to remove, which I have) for good measure.

We have tons of electrical engineers but are there any mechanical engineers here?

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Re: A different rear torque arm idea

Post by TylerDurden » Apr 12 2008 5:18pm

I'm not a mech engineer.

But I will note that most of the driveshafts I've seen have flats or splines for the pulleys/gears/impellers.

:?
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Re: A different rear torque arm idea

Post by Mark_A_W » Apr 12 2008 5:23pm

CGameProgrammer wrote:
We have tons of electrical engineers but are there any mechanical engineers here?

Yes.


There are lots of IFs in your suggestion. It makes me very nervous. For instance on one side it will want to undo.

I would stick with putting steel around those flats, and mounting the torque arm rigidly.

IMO the original dropouts should be passengers, not bearing any load ideally, or at least no torque (they are NOT designed for a torque load at all).
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Re: A different rear torque arm idea

Post by CGameProgrammer » Apr 12 2008 5:47pm

Well I'd put a torque arm on both sides of course.

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Re: A different rear torque arm idea

Post by lawsonuw » Apr 12 2008 7:11pm

If you're planning to glue a torque arm on, use the right glue. http://www.loctite.com/int_henkel/locti ... 1&layout=2 the 601 and 603 listed on that site are good options. Your local industrial supply retailer will likely have some (or a relative) sitting on a shelf. With a clean and tight joint you can count on ~1000psi bond strength. (rated strength is like 3000psi, but who knows if the joint is perfect?) Something like Mcmaster #2305K16 is an idea, It would be hard to get a grip on the drop-out and axle at the same time though.

Really, ideally the motor shaft would be round going through the drop-out and the axle spacers would be splined to the axle with integrated torque arms. Then the motor+torque arms are essentially universal AND strong as heck.

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Re: A different rear torque arm idea

Post by CGameProgrammer » Apr 12 2008 8:03pm

You misunderstand -- the torque arm, in my case, would be strongly clamped onto the frame with hose clamps. An adhesive would only be used on the axle nut to keep it from coming loose... and possibly to help clamp it onto the torque arm, though a properly tight grip won't have any room for glue anyway.

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Re: A different rear torque arm idea

Post by gogo » Apr 12 2008 9:25pm

Shearing force is very high for metal. Motorcycles use a cotter pin on their axles. I wonder if a cotter pin through the axle and nut would allow one to put a closed end wrench over the nut and then clamp the wrench to the frame. It would be a cheap torque arm solution.
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You could secure the wrench between the cotter pin and frame to ensure it wouldn't come off. The picture above shows a disposable pin, but you could use one like this:
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Re: A different rear torque arm idea

Post by knoxie » Apr 13 2008 5:48am

Hi

The axle will still turn inside the nut though thats the problem, no matter how tight you make it, I have seen this on the Pumas, they will bend 4mm stainless steel no problem, most steel rear dropouts do not need torque arms, I would only bother with them on aluminium frames or manganese forks, there are a number of ways you can make a simple torque arm, my Puma is putting out 1.2KW on the rear of my steel bike, I had been using a torque arm but removed it recently as it was clear that it was not being put under any stress, the dropouts seem fine after a few weeks.

The team hybrid torque arm was pretty simple just a spanner shaped 3mm flat piece of stainless steel with an axle shaped hole in 1 end, someone also posted a nice simple version recently which was basically an open ended spanner shape that sat over the top of the axle and bolted to the rack carrying holes on the rear dropout, however this was on a steel frame and probably didnt need one anyway, Steve Heads little dirt monkeys were throwing out some amazing levels of torque in those 12 inch wheels and he never fitted a torque arm and never had any problems.

IMHO e-bike suppliers should sell them with the kits in a better to have one and not need it kind of idea.

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Re: A different rear torque arm idea

Post by Mark_A_W » Apr 13 2008 6:11am

knoxie wrote:Hi

The axle will still turn inside the nut though thats the problem, no matter how tight you make it, I have seen this on the Pumas, they will bend 4mm stainless steel no problem, most steel rear dropouts do not need torque arms, I would only bother with them on aluminium frames or manganese forks, there are a number of ways you can make a simple torque arm, my Puma is putting out 1.2KW on the rear of my steel bike, I had been using a torque arm but removed it recently as it was clear that it was not being put under any stress, the dropouts seem fine after a few weeks.

The team hybrid torque arm was pretty simple just a spanner shaped 3mm flat piece of stainless steel with an axle shaped hole in 1 end, someone also posted a nice simple version recently which was basically an open ended spanner shape that sat over the top of the axle and bolted to the rack carrying holes on the rear dropout, however this was on a steel frame and probably didnt need one anyway, Steve Heads little dirt monkeys were throwing out some amazing levels of torque in those 12 inch wheels and he never fitted a torque arm and never had any problems.

IMHO e-bike suppliers should sell them with the kits in a better to have one and not need it kind of idea.

Knoxie
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And it makes no difference what diameter wheel you put on the motor, the torque reacted by the dropouts is still the same for a given motor ;) A smaller wheel will accelerate faster though, granted. Stall torque is always the same, you just get away from stall torque faster with a smaller wheel, so you will reduce the load sooner.
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Re: A different rear torque arm idea

Post by Jozzer » Apr 13 2008 9:49am

To do it right steel flats against the axle are needed IMHO. Also, closed ends are much better than open. If your going to go to the troublke of making a torque arm, why cut corners?
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Re: A different rear torque arm idea

Post by Drunkskunk » Apr 15 2008 9:51am

your idea just won't work. there is no path for the force of the axle to be placed on your torque arm if it doesn't grip on the flats. the clamping force of the nuts is only about 30 pounds. more than that and you strip the threads on the axle stud, and that doesn't translate to 30 pounds of shear force against the arm. that depends solely on the friction between the torque arm, the dropout, and the nut, but it will be less than the clamping force.

But none of that matters since the axle isn't putting the rotational force directly on the arm, all you'll get is 10 pounds of resistance against one nut, causing it to loosen, the other won't tighten, since its at a 3:1 disadvantage.

As for hose clamps, .02 MM of stainless steel isn't going to hold a torque arm if 4 mm of steel dropout won't. I remove old hoseclamps off my car by sticking a screwdriver under them and twisting. They pop right apart so easy its not worth taking the time to unscrew them.

On the motorcycle axle nuts and cotter pins, there's no torque being placed on the shaft of a motorcycle. the cotter pin pin is to keep the nut from vibrationg off. its a safer, more secure method of locking a nut than anything we use on a bike, but you can shear those cotter pins off with an extra 10 pounds of force on the wrench, if they are too stuck to pull out before removing the nut.

The only way to make a torque arm work is if the rotational force of the axle is directly inhibited by something that can withstand more than the maximum shaft torque the motor can produce. Most of these motors can produce 75-100 foot pounds of torque under ideal conditions. thats the same as if you took a 6 inch wrench, put it on the axle, and stood on the end of it.
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Re: A different rear torque arm idea

Post by CGameProgrammer » Apr 15 2008 11:45am

Drunkskunk wrote:your idea just won't work. there is no path for the force of the axle to be placed on your torque arm if it doesn't grip on the flats. the clamping force of the nuts is only about 30 pounds. more than that and you strip the threads on the axle stud, and that doesn't translate to 30 pounds of shear force against the arm. that depends solely on the friction between the torque arm, the dropout, and the nut, but it will be less than the clamping force.

But none of that matters since the axle isn't putting the rotational force directly on the arm, all you'll get is 10 pounds of resistance against one nut, causing it to loosen, the other won't tighten, since its at a 3:1 disadvantage.
But the evidence suggests the axle nut is the main factor when determining if the axle spins in the dropout or not.
As for hose clamps, .02 MM of stainless steel isn't going to hold a torque arm if 4 mm of steel dropout won't. I remove old hoseclamps off my car by sticking a screwdriver under them and twisting. They pop right apart so easy its not worth taking the time to unscrew them.
That's not the kind of force they'd be subjected to here. Also 0.02 mm is like paper. I think you mean 0.2 mm at the very least, but even that sounds too thin. Anyway, the way I'll be using them, the force would be parallel to the direction of the screw, but my torque arm is 10" long and the hose clamp goes on the very end, so due to leverage its job will be easy. I can also use multiple hose clamps if I deem it necessary.
The only way to make a torque arm work is if the rotational force of the axle is directly inhibited by something that can withstand more than the maximum shaft torque the motor can produce.
But again, the evidence, as I see it, suggests the shearing friction inhibits the torque more than a steel [open-bottomed] dropout.
Most of these motors can produce 75-100 foot pounds of torque under ideal conditions. thats the same as if you took a 6 inch wrench, put it on the axle, and stood on the end of it.
That's actually a good test for a throwaway bike frame. It shouldn't damage the motor, just the torque arm and dropout if they're inadequate.

In any case, I'm not actually using a 14mm round hole; I drilled out a 10x14mm space that fits the dropout, but naturally it is not an exact fit and there is a few degrees of play.

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Re: A different rear torque arm idea

Post by TylerDurden » Apr 15 2008 1:49pm

CGameProgrammer wrote: It shouldn't damage the motor, just the torque arm and dropout if they're inadequate...
As I understand it, when the axle spins, it rips out the wires, shorting the halls and blowing the FETs & whatnot in the controller.

:(
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Re: A different rear torque arm idea

Post by Link » Apr 15 2008 3:35pm

TylerDurden wrote:
CGameProgrammer wrote: It shouldn't damage the motor, just the torque arm and dropout if they're inadequate...
As I understand it, when the axle spins, it rips out the wires, shorting the halls and blowing the FETs & whatnot in the controller.

:(
I think he means the "stand on the end of a six-inch wrench" thing. Really puts the torque of these things in perspective, though.

And if you keep the wires free while doing that, they shouldn't get pulled on.
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Re: A different rear torque arm idea

Post by CGameProgrammer » Apr 15 2008 3:45pm

Link wrote:I think he means the "stand on the end of a six-inch wrench" thing. Really puts the torque of these things in perspective, though.

And if you keep the wires free while doing that, they shouldn't get pulled on.
Exactly. The wires would be unplugged, but the axle can't even rotate more than 45 degrees or so before your foot slides off the wrench.

Is there any proof of the 75-100 lb-ft claim? My 650cc motorcycle makes a peak 45 lb-ft of torque.

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Re: A different rear torque arm idea

Post by Link » Apr 15 2008 3:59pm

CGameProgrammer wrote:Is there any proof of the 75-100 lb-ft claim? My 650cc motorcycle makes a peak 45 lb-ft of torque.
It's a gas. It will never come close to the torque of an electric. However, that engine is operating at a much higher RPM, so you get more horsepower.

Check out the simulator over at ebikes.ca. A 5305 at 72V with a 50A controller will do just under 90lbs/ft.
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Re: A different rear torque arm idea

Post by CGameProgrammer » Apr 15 2008 4:16pm

OK. It says 66 lb-ft of torque with a 5303, 66V, 35A, and 86 lb-ft with 99V 40A. It's important to note that the "lbs" measurement on that simulator is *not* the same as lb-ft; you have to use N-m (Newton-meters) and then convert to lb-ft. Multiply the N-m by 0.7376.

Of course those figures assume I use maximum throttle from a stop, which I never do. But it would be good planning for the worst-case scenario...

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Re: A different rear torque arm idea

Post by Mark_A_W » Apr 15 2008 4:28pm

"have to use N-m (Newton-meters)"

YES.

"and then convert to lb-ft"


NO!!!! Why ruin a perfectly usable unit by converting to something completely ridiculous???


The world is metric. Catchup.
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Re: A different rear torque arm idea

Post by Miles » Apr 15 2008 5:05pm

The technical world is SI...

Many advantages from using coherent units :mrgreen:

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Re: A different rear torque arm idea

Post by Link » Apr 15 2008 5:21pm

CGameProgrammer wrote:OK. It says 66 lb-ft of torque with a 5303, 66V, 35A, and 86 lb-ft with 99V 40A. It's important to note that the "lbs" measurement on that simulator is *not* the same as lb-ft; you have to use N-m (Newton-meters) and then convert to lb-ft. Multiply the N-m by 0.7376.

Of course those figures assume I use maximum throttle from a stop, which I never do. But it would be good planning for the worst-case scenario...
I was being lazy. I just stuck the thing in a 24" rim and used that. It gives the exact same number, since the wheel is one foot away from the axle.

And I was using the 5305, not 5303, just as a reference for the ~100lbs/ft figure.
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Re: A different rear torque arm idea

Post by monster » Apr 16 2008 3:44am

i think the torque plate is the best solution. you can buy mild steel cutoff's from ebay and then drill them to whatever shape you need. they will keep your axle from moving in the dropouts. this will be enough for most motors but if you have a +1kw motor and plug braking then you will need more. alternate braking and accelerating will eat your dropouts. to stop this you may need to double up on torque plates and add a torque wrench. its better to put the torque stuff on before you realize you need it because after its half eaten your dropouts you will need even more support.

i found a simple wrench could be cable tied to the chain stay to keep it in place (lots of ties!). this was a pretty effective solution and its easy.

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Re: A different rear torque arm idea

Post by Link » Apr 16 2008 4:42am

There's someone on here who has crazy thick torque plates. Something like 3/8" steel. Bet they could take a pounding.
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