Is a steel bike critical for a 2000W rear hub build?

[kerravon]

Hi all,

A noob here who's experience is limited to a cyclone 48v, 40A mid drive which I've enjoyed for a couple of years. I now find myself lusting after a hub motor setup for no reason other than I want to try something new. I'm thinking of going with the MAC running 48V at 40A peak.

I read a lot about needing a steel frame bike to handle the torque of hub motor in this power range. My problem is that it's proving hard to find a steel frame bike. (I'm travelling in Taiwan, want to build it while I'm here, don't have great chinese language skills, all the shops I visit say they only have aluminium, carbon fibre or VERY expensive custom built steel frames, and I can't find anything good on the second hand market). Even the Costco / Carrefour bikes which used to be steel when I was here previously now all seem to be aluminium.

I'm sure if I kept looking / engaged with a helpful local I could surely find a steel bike in this bike building nation but for now my question is: Can I get away with an aluminium bike at 2000W rear geared hub motor as long as it has good drop outs and I use a torque arm?

Or am I dreaming and need to go back and look harder for a steel frame?

A free set of steak knives to anyone in Taipei who can point me towards a steel frame bike!

Thanks Endless Sphere!

 

[wesnewell]

Lots of aluminum framed FS bikes have steel swing arms. You should be able to tell by looking at the width of the dropout, Aluminum dropouts will be a lot thicker than steel dropouts. Or just take a magnet with you to check. But using torque arms with aluminum dropouts will work too. Although torque plates would be better imo.

 

[dogman dan]

You can use an alloy bike frame for 2000w. You just need adequate torque arms, preferably two. The cheaper full suspension bikes have steel rear ends, but it's far from mandatory that it's steel. I would agree that once you reach 10,000w, the whole bike tends to be steel, or at least the swing arm is custom made steel.

Grin's universal rear TA for example will work great for you, or if you have lots of flat space near the dropouts, you can do this with the typical front torque arms used in back. Or you can hand craft a torque arm to fit your bike with simple tools. The bike in this picture has been running a big powerful 4305 motor, with 48v 40 amps all summer with no problems other than the high power motor eats dirt tires fast.

 

[MadRhino]

You can use an alloy bike frame for 2000w.

You can use an alloy bike frame for 20 Kw and make a better ride than any steel bicycle frame, but you need to buy a big stiff DH racing frame and that is making a build much more expansive.

 

[Alan B]

Aluminum dropouts will be damaged by 2000W of hubmotor torque, but this can be avoided if you use torque arms on both sides. Just make sure the torque from the axle is supported by the torque arms, you cannot depend on the aluminum dropouts themselves to absorb it without deforming.

I have two bikes (the Novara mountain bike and the BikeE in my signature) that use dual torque arms on aluminum framed bicycles and handle the torque fine. The mountain bike has definitely seen 2kw without problems, the gearmotor on the BikeE has not seen that much but the gearmotor makes a lot of torque so is similar in the stress department.

Setup good torque arms and consistently install them, don't test without them even for a minute. Keep those dropouts in good shape.

 

[999zip999]

40amps and hills can turn your mac golden brown or worse. How much time at wot and that long lugging hill. Or just be mindful.

 

[leo99]

If you think about going with a mac, Paul from em3ev is supposed to be doing a new model of the Mac in the beginning of the year so you better wait for it. He is supposed to have upgraded the cooling of the motor.

 

[dogman dan]

Most geared motors best kept closer to 1500w,, or 48v 30 amps, if big hills are involved. But other variables matter, your weight, and the exact length and grade of the hill.

 

[Ykick]

If avoiding regen, steel isn’t that critical for up to 2kW DD hub motor IMO.

Powerful torquey 1500-2000W geared hub motor? I’d start getting more concerned and if either/both dropout shape and general mechanical abilities of the assembler are lacking, all bets are off.

TA’s (torque arms) for many of us should really be a last line of defense. Good thick, deep dropouts and tight nuts will be what keeps the spinout gremlins away.

 

[kerravon]

Thanks everyone for these really helpful inputs, which I'm keeping in mind as I progress this build. I really appreciate the advice.