2018 Torque Arm Tests, Splined Interface Design and Tabbed Washers

[justin_le]

I wonder if the flat head taper causes the fastener to stretch and loosen given the back and forth torque creates a wedge lever.

Yeah, I think there's a high chance that the countersink wasn't helping things at all and me and Robbie had talked about this. Unfortunately the design requires a flush mounted screw head since it goes up flat to the inside face of the dropouts; there just isn't the space for a normal socket head or the meat to do a recessed socket head (arm piece is just 1/8" thick).

Look at ur disc brake rotors on ur bike. No taper on the screws

That is totally true, but then the disk brakes only have to experience torque in one direction so disk rotor rotates until the side of the holes bottoms out against each screw and then stays there. If you were to take a bicycle disk rotor and give it backwards and forwards torque of +- 100 Nm like this, then my bet is that the screws on that would loosen up pretty quick as well.

I suppose we've got the machine here to test that! But priority for how will be getting torque arm results.

 

[e-beach]

.... there just isn't the space for a normal socket head or the meat to do a recessed socket head (arm piece is just 1/8" thick)....

What I am seeing is two exposed screws on the bottom of the arm that could be changed to hex bolts. If the arm had it's shape changed (made bigger) so that a third and maybe a forth area were able to fit a hex nut, it could solve the loosening problem.

 

[Chalo]

.... there just isn't the space for a normal socket head or the meat to do a recessed socket head (arm piece is just 1/8" thick)....

What I am seeing is two exposed screws on the bottom of the arm that could be changed to hex bolts.

That won't help. Cap screws on a flat surfaced hole are indeterminate in location, and can allow the torque arm to slew within the holes' clearance. That back-and-forth movement will unscrew them much faster than any potential movement between a conical hole and a conical-headed fastener.

The only advantage of a cap screw instead of a flat headed screw in this application is the longer grip length that would allow a small amount more fastener stretch. In this case, I doubt adding 1/8" of grip length, but at the same time adding rotational slip clearance, would help.

 

[e-beach]

Well,....It may be that the screws just aren't tight enough. Consider the automobile lug nut. Crank them and they stay.

 

[justin_le]

Well,....It may be that the screws just aren't tight enough.

Quite probably. Luckily for us the plate design has 12 threaded hole locations in it, so we were able to rotate the arm by 30 degrees so that the 6 arm holes now line up with the unused 6 threaded plate holes, and this let us tighten them up really good on the first round without the threads already being compromised and stripping like last time. We're just 2 days in, so let's give it another couple weeks and 100,000 or so back and forth cycles to see if it holds up proper! At least this way we haven't had to reset the experiment with a new inner plate spline and can continue the cycle counting on the critical 7075 piece.

-Justin

 

[flathill]

.... there just isn't the space for a normal socket head or the meat to do a recessed socket head (arm piece is just 1/8" thick)....

What I am seeing is two exposed screws on the bottom of the arm that could be changed to hex bolts.

That won't help. Cap screws on a flat surfaced hole are indeterminate in location, and can allow the torque arm to slew within the holes' clearance. That back-and-forth movement will unscrew them much faster than any potential movement between a conical hole and a conical-headed fastener.

The only advantage of a cap screw instead of a flat headed screw in this application is the longer grip length that would allow a small amount more fastener stretch. In this case, I doubt adding 1/8" of grip length, but at the same time adding rotational slip clearance, would help.

I had excellent results with a sprocket on a bike with high Regen with plain button head bolts and a flat interface, but I always using bearing retainer compound (anaerobic adhesive

Must make sure to wipe down surface with acetone and scotch pad. Proper torque is also critical

Probably not a good option for Justin but a good trick for other situations.

 

[Maginot]

It would be interesting to see some reference testing with a standard flatted axle to help calibrate/compare to what we are accustomed to.

+1

Looking forward to a "control" grouping of info on what is normally used. What I would really like to see is the test in a standard MTB frame with a conventional (Girn?) rear torque arm as a reference.

Data Inbound! This is exactly what we've started doing this week while waiting for the next batch of sample Splined axles to come in from MAC motors. In the title of this thread I mentioned tabbed washers, and for those who don't know Bafang has done away with axle flats on most of their newer low power motor. Instead these rely entirely on an inside tabbed washer for anti-rotation torque, which fits over a short section of 12mm axle with 10mm flats.

Bafang Axle with Keyed Washer.jpg

This has some of the advantages that we were after with the splined torque arm. The axle is round 10mm at the dropout allowing it to seat all the way into the dropout for disk caliper alignment, and it uses conventional M10 nuts and washers for fitting inside laywer lips without the need for spacer washers.

However this design is transmitting ALL of the motor torque to the dropout via that tabbed washer, while the point of a normal torque arm is to do the very opposite and ensure that torque is not spreading on the dropouts. On that front it seems like a bad idea. But if you look closely at the design the washer tab acts much further down than an M12 axle and further even than an M14. It's got the same engagement as a 15mm axle that was ground flat.

Can you provide more clarity why the Bafang washer provides advantages? Seems to be that while the tab is further away from the centre of the axle, this advantage is exactly offset by the tab also being further out and producing greater leverage on the dropout arms?

 

[e-beach]

Hello Maginot.

Welcome to ES.

This is an interesting thread to start on, and hopefully we will have an update soon.

 

[justin_le]

Can you provide more clarity why the Bafang washer provides advantages? Seems to be that while the tab is further away from the centre of the axle, this advantage is exactly offset by the tab also being further out and producing greater leverage on the dropout arms?

If the dropouts were actually spreading open by some imaginary pivot point on the motor axis then your argument would be correct, but the way that they actually deform it's not quite like this. An M12 axle with flats on it will literally smear open a 12mm diameter pathway in the 10mm dropouts without a whole lot of 'spreading' on the base of the dropouts angling them open.

With this keyed anti-rotation washer of or with a larger 14mm axle with flats on it, then the dropout really needs to bend open and deform a lot more for a given amount of axle rotation. Hope that makes a bit more sense? Your analysis is correct in that if you took it to the extreme and had like a 100 or 200mm long tab on the washer and extended the dropouts to be that length too but keeping the widths the same, then this additional length wouldn't make any difference.

 

[Chalo]

Well,....It may be that the screws just aren't tight enough. Consider the automobile lug nut. Crank them and they stay.

They all have conical clamping surfaces like a flat head screw, too. It's not coincidence.

 

[e-beach]

If you are saying a clamping surface whether conical or flat is a clamping surface, well, that is what I was thinking. As I read the posts I had the impression that some thought the conical shape of the fastener was contributing to the loosing of the screw. That didn't make sense to me based on the automobile lug nut.

 

[j3tch1u]

it looks like the countersinks are compromising the strength of some of the splines on the TA. maybe use a larger pitch circle diameter with the added benefit of more clearance from hub to add loc-tited locknuts at the end of those screws? might also consider using "chainring style bolts" that bottom out when tightened (making the interface sorta semi-floating).

 

[mat h physics]

Modern day street lights and rigid stoplights, try to avoid 90deg support angles. May get some mechanical advantage by off setting the power cable from perpendicular to the acting force.

While you may not want to hear dramatic redesign, why not route power from the brake side. Better yet hollow the axle, the center material of an axle provides the least modules of inertia, and making it a weight liability in relation to it's structural support. Extrusion dies have the advantage of added grain structure strength. Cost of production is determined by the number made, design for asymbly if you have to make two parts, threads aren't a requirement, just a complexity.
Hydraulic input on one side, discharge on the other, integrate frame as a fluid carrier.

 

[Taswegian]

Will there be a front wheel GMAC, or is it too crowded to fit this kind of torque arm?

 

[matmaxgeds]

Also crossing fingers for a front wheel GMAC!

 

[JShook]

+1 for Front Hub GMAC

My co-worker recently installed the GMAC on his (Tern?) cargo bike. He does not appear to be using more energy for his commute, and he is really enjoying the extra power on the hills and the regen on the braking.

Another option is the GRIN front hub motor, but I wonder if the regen is higher with a geared motor.

 

[nikolay]

Hello everyone,

I am building my first ebike and need advise for the torque arm. It's with Bafang G01 front motor (with the anti rotation tabbed washer) which I will run on 48/22A Chinese controller with "who knows how much" phase amp but to be in the safe side let's assume it can produce up to 55 Nm. The bike is Canyon Urban with relatively thick alu fork. I initially didn't have any doubts to install torque arm and thought about Grin's arm for G311 motors as the most clever design, but found out that these motors are different. On G311 the torque arm is installed on the disk side on the place f the tabbed washer, but with G01 is not possible as there is about 5-6mm lip above the bearing on that side. So I then decided - no issues, will install the torque arm on the other side but then the cable exit on the side plate is just where the torque arm sits. So I have to rotate the torque arm. Then I found in the ebay some 5mm torque arms very similar to grin's design for fender eyelets and decided to experiment with that one as ordering from Canada is not cheap to EU just to try it out.

The fork is with total width 32mm, the dropout is 7.5mm thick where the spindle sits and it' total thickness is 9.5mm. So it has a lot of material compared with the alu fork tested by Justin.

I have a few options to install somehow torque arm:

1. I already drilled a hole on the torque arm just bellow the eylet fender bolt is. In that way I can rotate opposite the torque arm - going upwards, and it's rotation will be blocked by the eylet bolt. However even with preloading there might be few degrees rotation as the hole needs to be a bit bigger that the bolt. However this solution looks as the easiest and I will keep the other side washer.

2. I can rotate the spindle a bit so the cable exit it on the front side of the fork and then will be able to install regular Grin G311 torquearm. However wit this solution I have to remove the other side tabbed washer or make a custom one because the spindle will be rotated incorrectly to fit the original washer.

3. Make a new custom torque arm for G01 for my frame. This is good option although I have to find a someone who can cut the pieces from a proper material which is not very easy...

There is also another option I consider - use only the tabbed washers, secure the nuts as tight as possible and that's it. Do you think with this size fork it is acceptable?

 

[qwerkus]

It's with Bafang G01 front motor (with the anti rotation tabbed washer) which I will run on 48/22A Chinese controller with "who knows how much" phase amp but to be in the safe side let's assume it can produce up to 55 Nm.

Not sure you even need a torque arm for that power level, and if so, since you won't have regen, any torque arm that counters the forward spinning pressure on the dropout will do the trick; won't even have to be super tight because only one side of the TA hole will be pushing against the axle. Easy to machine yourself or use the piece you already have.

Those fancy torque arms you can read about in this thread are meant for much higher power levels. The g010.250d will fry way before you reach those levels. Keep in mind that it has been designed for 250W. 500 would be already pushing it. 54*22=1188 (fully loaded 48v with your controller) ... no way... Get a bpm or something stronger to push over 1Kw.

 

[nikolay]

You are right, I am exaggerating here. This motor is rated at 32 Nm, but since I do not know what phase current this controller provides theoretically it can go beyond the 32Nm. Of course the gears can strip before that, I don't know... I moved to 48V since I needed speed on flats and instead of buying 6FET, I got 9FET controller just to be more reliable with less heat. According to the bike simulator on the flat I can run 500-600W without any issues to carry me with 35-38 kph. If I use 15A max controller (used commonly on 250W setups) which is rated at 7.5A constant battery amps, it will be overheating. Also I can manage startup torque with careful throttle and these controllers have some "slow start" option to limit the phase amps from start.

 

[e-beach]

Well, if your front hub wheel comes out of your forks at any speed, your face can easily be heavily evolved with the pavement. There are plenty of break-up songs in the world of songs, do you know of any that sing about the break-up of your face? That is what can happen if your front wheel hub motor snaps off and you face-plant at speed.

I have a front hub motored bike with over 11,000 miles of use on it. I am working towards 12,000 miles as I write this.

These are my thoughts:

I ran only one torque arm for about 5000 miles, then one day the fork broke on one side at the arch due to uneven pressure forces due to one side having a torque arm. If you want to keep from potentially loosing your teeth, run two torque arms.

My two front hub torque arms not only keep the front hub axle from twisting, they are also securely fastened to the fork so if the dropouts crack, the wheel will stay in place and not fly off the bike. And, the proof of this is that almost 2 years ago, one of my dropouts cracked and the wheel stayed in place. So I decided, as a test matter, to ride around on it for a while to prove my design. (I would not recommend doing this to anyone else.)

The takeaway is this: You need two torque arms on a front hub motor, and if you can figure out how to do it on your bike, also attach them to the legs of your forks.

 

[nikolay]

What is the power of your front setup? Of course having two torque arms is the best in all cases, but I have to create custom arms as the disc side is pretty tricky - there are only like 3mm between the hub a the fork (I need to measure it). Also finding proper shop to machine it for me might be an issue.

 

[Balmorhea]

There is also another option I consider - use only the tabbed washers, secure the nuts as tight as possible and that's it. Do you think with this size [aluminum] fork it is acceptable?

No.

 

[qwerkus]

I sorry about the accidents you had, but I think you guys should read the details about the power levels involved before scaring the shit of out proportion. The g310/311 is a common motor on newer cheap commercial ebikes here in Europe and NONE of them come with a torque arm. Even if OP pushes way past the designed power level, I still think a simple homemade TA would be enough if not overkill.
Modern alloy forks (like OP has) come with a grove on each side where a washer ensure it cannot slip out. Of course with an older steel fork, it's a different story.

 

[Balmorhea]

I sorry about the accidents you had, but I think you guys should read the details about the power levels involved before scaring the shit of out proportion. The g310/311 is a common motor on newer cheap commercial ebikes here in Europe and NONE of them come with a torque arm. Even if OP pushes way past the designed power level, I still think a simple homemade TA would be enough if not overkill.
Modern alloy forks (like OP has) come with a grove on each side where a washer ensure it cannot slip out. Of course with an older steel fork, it's a different story.

Yes, but... this is an aluminum fork we’re talking about. The material is much weaker and much more brittle than steel, even the mild steel that most motor axles are made from. It works great for bike frames, but those aren't pried with steel tools in normal use.

I agree that pretty much any torque arm would be adequate to prevent damage to the fork tips, as long as the torque isn’t transmitted to the open-ended slots of the fork tip.

I think using only tabbed washers would probably be adequate if the fork tips were steel of 5mm thickness or more. But I’d inspect them regularly until I could be sure they were holding up.

 

[ZeroEm]

Have one of grins front torque arms bolted to an eyelet on my mothers schwinn meridian (steel trike) G311. Have max amps set 13 or so 4-500w max. My brother is on his second front hub motor, first was geared no torque arm on alloy front forks (250-300w), it never gave problems. He is on his second front 1500w leafmotor (run with less than 400w) one torque arm on alloy front forks. The main point of the three is all of them run less than 500w and are dependable. Would not run with out torque arm. No torque arm keep it less than 250w, More power more problems. My brothers second bike was one of my projects but was worried that two torque arms on the cheap alloy forks would not last but is good for his power range.