Is my frame in danger ?

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CrawlingWhisper   10 mW

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Is my frame in danger ?

Post by CrawlingWhisper » Feb 19 2018 3:41pm


I finally finished my first E-Bike Build. Maybe I'll post pictures later on. While everything is running, I have one concern. For my build I used this Bike, with an aluminium frame (the largest size option): ... 40066.html

I run a Mac 10T hub motor on 52V 30Amp (1560W) and use one GrinTech Rev4 torque Arm:

Is it OK to run the build with only one torque arm ? If not, is it OK to do so on 52V 24A (that's what I originally planed) ? Will there be any negative long term effects on the frame ?
I don't know if it has any major impact but here in cologne it's about 0°C (32°F) at the moment.


Voltron   1 MW

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Re: Is my frame in danger ?

Post by Voltron » Feb 19 2018 9:51pm

At those power levels it should be fine, esp if you're not using regen. Keep those axle nuts tight of course....

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molybdenum   100 W

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Re: Is my frame in danger ?

Post by molybdenum » Feb 19 2018 10:50pm

I'd like to see a pic of the installation...

I don't mean to make you paranoid, but I thought mine was secure with the nuts torqued to 50 nm and 2 TAs hose clamped to the frame. My wimpy 10A 48V motor popped out of the dropoutswhile accelerating from a stop sign; fortunately, the hose clamps kept the hall/phase wires from spinning in the dropouts, I avoided a crash and damage was minimal .

I cut myself a custom hardened steel TA to fit my dropouts and used 3M DP420 metal bond epoxy to cold weld it to the aluminum frame for peace of mind. The cassette side has the hose clamp TA for extra security.
2012 Kona Dew Deluxe, Q128C and 9spd cassette, S12S sinewave controller, 48V Panasonic battery

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amberwolf   100 GW

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Re: Is my frame in danger ?

Post by amberwolf » Feb 20 2018 12:15am

In my experience, at the same lower power levels, a geared hub is more likely than a DD hub to be able to pry dropouts apart, regardless of frame material.

I expect its' from:

--the torque multiplication of the gearing in the hub

--the narrower axle flats most of the geared hubs seem to have vs the DD hubs (which, given the previous point, seems odd).

The only spinout I've had was a geared hub in a steel fork (though I've had multiple motors try to do it due to insufficient torque arms, that's teh only one that managed it).

(not counting bench experiments)

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dogman dan   100 GW

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Re: Is my frame in danger ?

Post by dogman dan » Feb 20 2018 8:29am

As others have implied, you could easily install even two torque arms, and then have the motor spit out and ruin the frame.

Its all in the details of the install. Step one for me is always to file the dropout deeper, so the center of the new motor axle is in the same place as the center of the old 10mm axle. 1mm deeper for a 12 mm axle, 2 for a 14 mm axle.

This is very important especially, if the drops are shallow and vertical, as they are on most alloy frames.

Next, you need to be sure the TA, any washers, and the nut sit very flat on the frame. Anything causing a tilt in the TA or washers will tend to make the frame crack, or at least spit it out of the notch.

Lastly, the TA needs its nut tightened very very tight, if its a two piece arm. And the clamp needs to be a good one. Some china made ones suck, others are ok. I like to seek out vintage US made hose clamps at estate sales.

Do it right, and then one arm is plenty.

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teklektik   10 GW

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Filing Dropouts - Fitting Torque Arms

Post by teklektik » Feb 20 2018 9:40am

CrawlingWhisper wrote:
Feb 19 2018 3:41pm
I run a Mac 10T hub motor on 52V 30Amp (1560W) and use one GrinTech Rev4 torque Arm:
As mentioned above, a pic of the mount would be good. With a solid mount for the TA it should be okay - but you need to realize that any failure in that single TA is going to have catastrophic frame consequences. Two TAs may not always be strictly required, but 'Sunny Day' design is inherently more risky.

Regardless of remarks above, the Mac has a standard 14mm shaft with flats for a 10mm dropout - the flats are no smaller nor does the shaft make the motor more risky than other conventional motors, DD or geared.

The matter of filing the dropouts is related to your question, but carries with it some dangers if done badly. The first problem is simply one of not filing the dropouts to equal depth which will cock the wheel to one side. To sidestep this issue, the overall fit can be improved but not brought entirely to the ideal by simply rounding out the corners of the bottom of the dropout so the original 10mm radius matches the 14mm radius of the axle. This will get the axle seated with a 2mm displacement without actually filing it deeper - so - a much lower risk of cocking the wheel if you are not handy with tools.

The second and more important issue is to avoid filing sharp corners into the dropout at the bottom - of particular importance with aluminum frames. This causes something called stress risers to appear at these corners when the dropout is loaded making the aluminum more prone to failure at those points. By filing the corners with a rounded contour, the stress is not focused at a single point. It's not necessary that the bottom of the dropout be 100% accurate fit to the shaft as long as there is broad contact at the bottom to bear the weight of the bike. This allows you to slightly over-file the corner a bit to get the desired rounded shape. I find that chainsaw files are a good choice for this task - unlike most rat-tail files, they have a small and constant diameter the whole length. Available at Home Depot, hardware stores, etc.

Here's the improved seating just from reshaping the dropout without actually making it deeper:

Same depth dropouts - just reshaped to improve axle seating
FilingDropouts.png (7.25 KiB) Viewed 498 times

Another consideration with aluminum frames is the size of the axle shoulders where the flats end. On Macs, this is quite small. You really want to put a washer or axle spacer inside the dropout so that tightening the axle nut doesn't sink the axle shoulder into the aluminum. This will both loosen the nut and spread the dropout. Dropouts often have a cast circular recess on the inside of the dropout. Make sure any washer you place there fits fully within that recess and does not bridge it or sit cockeyed. Commercially available axle spacers will fit nicely. SAE washers from the hardware store with a small outer diameter can be had by buying one size too small and enlarging the opening a bit.
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