Torque arm, front or back?

Mrcheap

New Member
For front hub kits, do torque arms go in front or back of the fork?

ive seen it done both ways and always people comment it’s wrong for both

where should it be installed?

thanks
 

indianajo

Well-Known Member
Front. The motor is trying to turn to the back, so arms in front puts the clamp in tension, where steel is strong. OTOH if you are using worm hose clamps, those aren't strong in tension. IMHO, don't use those, make a bracket out of sheet metal.
Arms in back gets in the way of the fender struts.
 

Dewey

Well-Known Member
Back, according to Grin Tech https://ebikes.ca/product-info/grin-products/torque-arms.html
“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.”
“When the motor is powered...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.”
 

JES2020

Active Member
I'll round out the vote and say I don't think it matters : )
I'm not a user of front wheel motors, but I assume it the same principle for both front and back. That is to keep the wheel from coming out of the dropout. So it would follow if you cannot take the wheel out of the drop out without removing the torque arm, then it's doing it job.
 

opimax

Well-Known Member
in space without gravity or friction or other factors, does an infinite loop have a back or front? so I am guessing the which ever position where the t arm rides on top using gravity to help secure it.

as usual feel free to ignore me :)

PS in space if it fails can we hear you swear ?
 

JES2020

Active Member
in space without gravity or friction or other factors, does an infinite loop have a back or front? so I am guessing the which ever position where the t arm rides on top using gravity to help secure it.

as usual feel free to ignore me :)

PS in space if it fails can we hear you swear ?
Well, I just learned that the issue is most troublesome with aluminum and alloy forks with high powered motors.
Seem the aluminum drop outs can fracture and fail catastrophically with 60-90 N-M while the steal dropouts spread apart allowing the axle to spin in the dropout.
So besides keeping the wheel in the dropout the torque arm keeps the dropout from deforming or fracturing.
Which adds weight to my thought that, it doesn't matter which side of the fork the arm is attached to as long as it keeps the axle from deforming the dropout. Keeping the wheel in the dropout (IE loose nuts), is a side benefit.
Isn't science fun?
 

AHicks

Well-Known Member
Geez, all the torque arm really needs to do is prevent the motor from spinning on it's axle. If you can get it to do something else in addition to that, it's just a bonus.

So, I agree with JES2020. Makes no difference front or rear - or which side it's mounted on - assuming only that there's room to mount it.
 

AHicks

Well-Known Member
My vote would be silly.
-but-
I think the chance of a hub motor with no torque arm tearing up a fork or a drop out is MUCH more likely that a (any!) VW needing a wheelie bar......
 

JES2020

Active Member
My vote would be silly.
-but-
I think the chance of a hub motor with no torque arm tearing up a fork or a drop out is MUCH more likely that a (any!) VW needing a wheelie bar......
LOL, well there are exceptions to every rule. If the VW has a v8 installed in it for instance.
Likewise a (relatively) low torque ebike, stands little to no chance of deforming the drop out unless it falls from the statue of Liberty and lands on the drive wheel. God bless America !
 

JES2020

Active Member
Its a 250 watt on a steel fork . Came that way on my BTN Fat AWD .


To clear up the issue once and for all.
Here is a excerpt from Ebikeschool.com, a well known knowledge base for ebikes, RE torque arms:

"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."
So you can ride relaxed😎
 

AHicks

Well-Known Member
Just 'cuz works too!
If he ever gets some tires that hook up, THEN he may be glad he has them. Until then, they look like ballast from here.