Dropout Failure Experiments, and a call for Fork Donations

[teklektik]

electricwheels.de wrote:
Depending on the use:
for Pedelecs a standard cable tie will do, PA 6.6, UV stabilised, up to 600 N pull
for e-bikes with some recouperation a stainless steel cable tie, up to 1100 N pull
for e-bikes with heavy recouperation (like braking) I'd use 2 torque arms, mounted in opposite directions.

Keep in mind that the mounting point of the securing band is about 110mm out from axle centre point.

I believe these figures are for straight line tension under the best test circumstances. Given aging, mechanical abrasion/wear, temperature extremes, and derating because of sharp bends, the ties seem like a recipe for trouble. For me as a consumer, the cost/risk ratio just doesn't seem to justify this approach. As a producer there are greater potential consequences and I think you may want to err on side of safety...

You might just replace the hook with a more conventional ring so the faint of heart can slip a bolt through it to a clamp on the stay - the bold can stick a cable tie or two through the bolt hole - everyone gets cake. If you really want to support the cable tie faction, chamfer the edges of the hole to help minimize the abrasion and sharp bend in the tie at that point.

 

[dogman dan]

Hose clamps and cable ties are decorative ways to attach a torque arm.

Here's how you do it right, weld a tab to the frame and then bolt to it with two bolts. Two bolts prevents any swiveling of the arm, which cures the designed in problem of the adjustable pivot on a two piece torque arm.

 

[avandalen]

See here for a new topic about the calculation of front fork stress due to hub motor
http://endless-sphere.com/forums/viewtopic.php?f=2&t=27928&p=403233#p403233

 

[vax]

I found a simple solution to everlasting torque arm problem. I made my own dropouts to my frame (moped). They are machined out of 10mm thick stainless steel plate, welded to the rear swingarm and feature vertical bolt at the end for tightening. So fist I insert a motor, tighten dropouts slightly, then tighten motor nuts and then tighten droputs properly.
Sorry I don't have picture right now, stay tuned...

 

[dogman dan]

Yup. A properly designed frame specificly for a hubmotor would require no additional torque arm.

All this is a result though, of a poor design on the motor axle, and bearings. Heinzmann motors got it right, with thier axle design integrating a torque arm that bears on much longer flat surfaces than our hubmotor axles. As a result, their axle is normal bike diameter, and all the nuts and washers are the right size.

Basicly, all the hubmotor manufacturers are sending us a problem that could be fixed with a better axle design.

 

[Zesty Bonanza]

When the Cannondale Headshok was tested, did you have to file the drop outs so the axle will fit? I have a headshok and it seems to fit only 9mm axles whereas the electric hub/wheel that I bought uses a 10mm axle. I am thinking of filing down the drop out so my wheel can fit in it and I will use both rev 2 and rev 3 torque arms.

 

[Hugues]

Good analysis thanks.

Am I right to say that with regenerative braking the problem could get a little worst as the torque applied would be in the other direction ?

thanks

Hugues

 

[justin_le]

When the Cannondale Headshok was tested, did you have to file the drop outs so the axle will fit?

Correct. If any of the dropouts were less than 10mm then they were filed accordingly until the axle would just fit. This is pretty common with hub motor installations on front forks. Standard front axles are 9mm or 3/8" diameter, and the normal fork opening is 9.8-10.0 mm for a generous fit, but some are done tighter.

Justin

 

[John in CR]

Good analysis thanks.

Am I right to say that with regenerative braking the problem could get a little worst as the torque applied would be in the other direction ?

thanks

Hugues

Yes, the extra issue with regen isn't the magnitude of the force, but the repeated forces trying to spin that axle first one direction on acceleration and the other during regen. Over time it loosens things up, so check the axle nuts even more frequently, and be alert for an audible "tick" when you hit the ebrake, which indicates that axle is starting to move in the dropout.

 

[JoseEJo]

The strength of bicycle forks, their ability to handle the torque of hub motor axles, and the importance or non-importance of torque arms, has been a heavily debated topic in the ebike community. And rightly so, since failures that result in a wheel falling out of a vehicle can be more than a little disconcerting. But I haven't seen much in the way of empirical and analytic testing to show the actual range of dropouts strengths that exist among bicycle forks, and the quantified improvement that a torque arm may provide, so here is an opportunity to change that!

First, a bit of a background.

We've been involved with hub motor ebike conversions since 2004 or so and have seen many installations. The vast majority of those didn't have any torque arm at all, and that includes even taboo arrangements like 5304 motors in aluminum suspension forks, 72V setups with 50A modded controllers etc. When we DID see dropout failures and spinout it almost always fit in one of the following 3 categories:
a) Most commonly, somebody forgot to properly tighten the axle nuts after fixing a flat or what not, and hit the throttle when the nuts were still loose.
b) The motor was installed in a fork with quick release lawyer lips, and the large diameter washer/nut didn't sit flat against the flat recess in the dropout but was instead tightened only against the bottom of the lip. or:
c) With aluminum forks, the nuts were made good and tight, but the axle having a curved rather than a sharp transition at the shoulder ended up "wedging" the dropout opening apart, and caused it to crack even without the throttle ever being applied. See below:



In installations where the motor was fastened with the axle nuts tight and sitting flat against the dropout, problems were rare. But that changed in early 2008 when we started carrying both the eZee and Nine Continent motors. Both of these motors had higher stall torque than the Crystalyte hubs and both of which lacked the extended 10mm keyway that Crystalyte has machined on one side of their axles.

View attachment 1

Just wanted share something that I worked very good to solve the problem of the wedging action of the axles.

I had this problem on a Golden motor smart pie 4, I wasn't able to apply much torque on the nuts because on the freewheel side the axle wedged on the dropouts. So I installed a keyed washer to avoid this. This didn't work either because the washer also began to be wedged. So I screwed a jam nut to act as a stronger washer as it is thinner than a normal nut, the thread is M14x1.5.

With that I have been able to torque the nuts to 70N-m which makes a very solid attached rear wheel.