Rear hub motor on aluminum frame

[leakimc]

Hey all. I’ve recently been reading up about the risks of having a front hub motor on an aluminum frame. There’s some varying information on the internet: people on reddit and here seem to agree that it’s an immediate death sentance, but some other sources such as ebikes.ca or ebikeschool.com say it’s not that bad with a torque arm.

Now, the motor I’d consider would be a 36V/48V with a 15A controller on an aluminum frame and I’m considering putting it at the rear since it’s much more durable. I have some questions about this. Note that the questions are in the context of an aluminum frame specifically.

• How much more durable would a rear hub be? Would say a 48V 15A setup give me any issues even without a torque arm? What about 36V?
• Is it possible to fit a torque arm if the hub motor has a cassette on the other side and a disc brake on the other? Where would you install a rear torque arm?
• How much does the rear cassette and disc brake complicate the installation? What things to consider?
• Can you transfer an existing cassette to a rear hub motor?
• Is it possible to have cassette rear hub motors with a different amout of gears to fit different geared bikes?
• Direct drive vs geared for a rear hub?
• What other benefits does a rear hub motor have compared to a front hub?

Thanks a lot. Feel free to include other helpful information as well, I’m here to learn!

 

[E-HP]

• How much more durable would a rear hub be? Would say a 48V 15A setup give me any issues even without a torque arm? Always use at least one torque arm. What about 36V? Always use at least one torque arm.
• Is it possible to fit a torque arm if the hub motor has a cassette on the other side and a disc brake on the other? Yes. Where would you install a rear torque arm? On either side or both sides.
• How much does the rear cassette and disc brake complicate the installation? It doesn’t. What things to consider? Axle length sufficiently long to add torque arms. Dropout width that matches the motor. Horizontal dropouts can be trickier.
• Can you transfer an existing cassette to a rear hub motor? Yes, if it is a version of hub that accepts a cassette rather than a freewheel.
• Is it possible to have cassette rear hub motors with a different amout of gears to fit different geared bikes? Yes.
• Direct drive vs geared for a rear hub? Yes.
• What other benefits does a rear hub motor have compared to a front Traction. Ability to run more power.

 

[neptronix]

Hey, belated welcome to the forum and cool that you did your research first.

- Rear dropouts are much stronger typically and it's the right place to put the wheel.
- Torque arms work with cassettes just fine, the casette side will either have a 12mm or 14mm flat.. you just need the appropriate torque arm
- Not at all
- If the hub is designed for cassette, it should work!
- Yes, the cassettes are standard so it's just as flexible as a bike
- It depends on your power requirements and terrain, need more details to answer that.
- Traction power is applied where all the weight is, no weird feel while steering, stronger connection to the bike, no worries about cracking a front suspension fork in half while riding ( very dangerous )

ps - beyond 500w peak, use a torque arm. If you're using regen or >1000w of power, you'll want two torque arms.

 

[leakimc]

Hey, belated welcome to the forum and cool that you did your research first.

- Rear dropouts are much stronger typically and it's the right place to put the wheel.
- Torque arms work with cassettes just fine, the casette side will either have a 12mm or 14mm flat.. you just need the appropriate torque arm
- Not at all
- If the hub is designed for cassette, it should work!
- Yes, the cassettes are standard so it's just as flexible as a bike
- It depends on your power requirements and terrain, need more details to answer that.
- Traction power is applied where all the weight is, no weird feel while steering, stronger connection to the bike, no worries about cracking a front suspension fork in half while riding ( very dangerous )

ps - beyond 500w peak, use a torque arm. If you're using regen or >1000w of power, you'll want two torque arms.

Hi and thank you a lot for the informative reply! About the DD or geared: the bike would mainly be ridden on urban areas aka pavement. Some elevation present, would be good if the bike could go up some kinda steep hills in the city. Although, we did a build a 36V 15A geared front hub a couple years ago with my friend, and that goes up steep pavement just fine.

Regarding the power, would you personally go with one or two torque arms on a 48V (54V fully charged I suppose) 15A geared rear hub on an aluminum frame? In that case peak power would be around 800W I think. Excuse the continued emphasis on this factor, I'm trying to build a solid understanding about the durability since it's a big safety concern.

Do you have any recommendations as to how long the rear hub axle should be in order to fit a cassette, two torque arms and a disc brake?

Thanks again!

 

[E-HP]

A picture of your rear dropouts would help in answering both of your questions. In the pic below, the dropouts are shown with dotted lines. The 135 mm dimension is the distance between the bike's rear dropouts. There is a shoulder cut into the axle at that interface. The cassette is inside of that dimension, so the length of the axle won't affect the room necessary for the cassette. The length beyond the frame dropout is where the torque arm(s) would be mounted, so the length of axle beyond the shoulders of the axle needs to be long enough. In the example, the left side has 24.7 mm of extension, and looks like getting a torque arm on that side would be tight (there's barely any axle beyond the axle nut). On the right side, it looks like there's enough.

 

[Gruesome]

As far as I know, it's not so much the power (torque*rpm) that taxes the frame and fork or chain stays, but the torque [in N*m], and even more specifically, the resulting force [in N] at a point of contact or a weak point in a structural element that transmits the torque. Take the torque a hub motor (geared or direct drive) is putting on the rear wheel. That torque [say 50 Nm] divided by the wheel radius [say, 0.333m] gives you the tractive force [here, 150N, or the weight force of 15 kg, or the force required to push a 100kg rider+bike up a 15% grade]. Take that same motor torque, and divide by the much shorter lever arm length of a torque arm [say, 2" or 5cm or 0.05m], and you get a higher force [here, 50 Nm/0.05m=1000N, or the weight force of 100 kg/220lbs]. So, in that example the question would be, is your aluminum chain stay sturdy enough in the area where you attach the torque arm that you could stand on it on one foot?
Or do you need a second torque arm, or even run a separate link from your torque arm to your bottom bracket?

Anyway, that's how I would approach this question.

 

[leakimc]

A picture of your rear dropouts would help in answering both of your questions. In the pic below, the dropouts are shown with dotted lines. The 135 mm dimension is the distance between the bike's rear dropouts. There is a shoulder cut into the axle at that interface. The cassette is inside of that dimension, so the length of the axle won't affect the room necessary for the cassette. The length beyond the frame dropout is where the torque arm(s) would be mounted, so the length of axle beyond the shoulders of the axle needs to be long enough. In the example, the left side has 24.7 mm of extension, and looks like getting a torque arm on that side would be tight (there's barely any axle beyond the axle nut). On the right side, it looks like there's enough.

Right, thanks! Now I know what dimensions to keep in mind. Very helpful. What is usually a sufficient width of a durable torque arm in your opinion? Also if you have any recommendations for specific torque arms, I'd be glad to know! I know Grin has their new V5-7 models for example.

 

[E-HP]

Right, thanks! Now I know what dimensions to keep in mind. Very helpful. What is usually a sufficient width of a durable torque arm in your opinion? Also if you have any recommendations for specific torque arms, I'd be glad to know! I know Grin has their new V5-7 models for example.

The grin torque arms are excellent. Sorry, can’t help on the axle, you never provided any pics. Good luck though.

 

[leakimc]

The grin torque arms are excellent. Sorry, can’t help on the axle, you never provided any pics. Good luck though.

Thanks. Don't have a bike yet so can't send pics. Do you think a Grin V4 would be enough for 48V 15A rear or should I go with the V5-V7 models?

 

[E-HP]

Thanks. Don't have a bike yet so can't send pics. Do you think a Grin V4 would be enough for 48V 15A rear or should I go with the V5-V7 models?

No problem. I can wait for pics once you get it.

 

[99t4]

Do you think a Grin V4 would be enough for 48V 15A rear or should I go with the V5-V7 models?

As long as the V4 fits snugly on the axle shaft, and there are no frame obstructions that prohibit solidly securing the lever to the frame, the V4 should work fine. Hard to guarantee because we can't see the topography.

 

[copperhead89]

I have alum frame fat bike and would like info on torque control. The earlier threads address lower powered applications. I see the Grin v5 and the epoxy style and would like a recommendation. Any preventative measures to keep from damage also appreciated. Thank you

 

[99t4]

The earlier threads address lower powered applications.

Are you sure? Are you referring to this thread?:

torque arm picture thread

Cross-posting these two threads so a search will turn up both: "Found an awesome torque arm" (3 pages) https://endless-sphere.com/forums/viewtopic.php?f=1&t=11570 xxxxxxxxxxxxxxxxxxxx Original thread died. Figured I'd help out and repost what I saved. Here is the album link...

endless-sphere.com

Main concerns with TAs:

• Must fit the axle tightly-- no slop.
• Sufficient thickness (1/4 - 3/8"?) and strong material.
• Installed correctly (taking into account directions of force and leverage).
• Pinch clamp style generally the best.

If you have the skills and fabrication equipment you can DIY TAs custom for your setup. Otherwise, (you asked for recommendations) the Grin Tech TorqArm_V7 would be a good one, as you mentioned you were concerned about your high power hubmotor in an aluminum frame.

Torque Arms - Grin Products - Product Info

For every force there is an equal and opposite force, and for every torque there is an equal and opposite torque. Sadly with import hub motors this opposing torque is often born entirely by bicycle dropouts that are not up to the task, allowing the motor axle to spin out. Grin has pioneered the...

ebikes.ca

 

[neptronix]

Torque arm v7 is solid as hell. No slop in the mechanism due to clamping. Run a pair on a motor like that.

 

[wturber]

Torque arm v7 is solid as hell. No slop in the mechanism due to clamping. Run a pair on a motor like that.

In their product manual Grin says that in every case they tested, the motor axle sheared in two before the torque arm would fail. So yes. A pair of these.

https://ebikes.ca/amfile/file/download/file/260/

I'm a big fan of running two torque arms. I did that with my 1000 watt rear direct drive in an aluminum frame. As the Grin manual points out, the hollow axle is much weaker than the solid side.