Aluminum Frame + Torque Arm + Rear Motor = Safe?

[DragonPhyre]

First bike went well. Sorry I didn't post pictures, but it was a pretty standard first built--which means it was riddled with problems and I had to purchase things several times over to finally get it working. The thing that is holding be back at this point was the crappy bike that I purchased. So I bought a better one off of CL and I am going to move it over to the new one. I forgot to check the frame before I bought it and I found out it was aluminum. My heart dropped as I have heard nothing but don't ever use aluminum frame bikes they will break the dropouts and you will be in a bad way...

So I searched, and I saw lots of people using them. They just used a torque arm (or two) to get them to not spin in the dropouts... I have a torque arm that came with my eBikeKit and would it be enough? Or do I need to get some made up from a local welding shop using Doc's templates?

 

[wesnewell]

Just because a bike has an aluminum frame doesn't mean the forks, swing arm, and dropouts are aluminum. Many aluminum frame bikes have steel dropouts. Use a magnet to check if you don't know.

 

[DragonPhyre]

Just because a bike has an aluminum frame doesn't mean the forks, swing arm, and dropouts are aluminum. Many aluminum frame bikes have steel dropouts. Use a magnet to check if you don't know.

Ah, that I did not know. I will check tomorrow for sure. I have to get it up and working as a bike first--new chain, new cables, new pads, some steel wool to get rid of rust on the chrome, maybe new tires and tubes... all the general stuff. I'll check them for steel though. Thanks!

 

[dogman dan]

I can't remember every members build, so remind us what sort of wattage you plan on running.

It won't take that much to safely run 1500w on an alloy frame dropout. Some kind of torque arm. Could be something trick, like Dr Bass's glue on devices or just a basic front torque arm with a few notches or whatever modification it needs to fit, or something you hacked out of a chunk of steel plate yourself. Grin cyclery now sells a universal rear torque arm btw.

Above 1500w, then you might need something much better. Again Two of The docs torque plates, something you made yourself from nice thick steel, and generally two of them. I've been getting away with just one pinch dropout torque device myself, but keep wondering when I'll twist off the whole axle.

Here's an example of a cheapie torque arm I made fit my longtails aluminum rear dropouts. Thin and cheezy, but two of them is adequate for the 1000w motor, and I won't hesitate to bump it up to 1500 if I want to. Had to notch it some to fit the funny dropout geometry on the bike.

 

[MadRhino]

Apart of some extra complications to build, a good alu frame rides better than steel and its lighter weight makes for better performance. I said "a good Alu frame" because building on a cheap frame, you are much safer with steel. Some FS alu frames have a steel swingarm, that makes for easier fit of the motor, but then you lose the stiffness quality of a full hardened Alu DH racing frame.

I ride Alu frames very hard in the mountain trails, and I consider them safer than steel because of their better handling. Yet my bikes are dangerous, not because of their frame material, but because their geometry and power to weight ratio makes them capable of doing a backflip at any moment if one is not careful with them throttles.

 

[DragonPhyre]

well I inspected the bicycle a bit more closely and there is no steel in the rear dropouts. There is an aluminum insert that looks like it is there to hold on the rear derailuer, and plenty of room to mount an arm. but the other issues on the bike abounds... The front fork dosen't support a disc brake, the rear wheel is bent which I did not notice until I got home, the chain is moving but rusty as can be... other little things.

Thanks for the advice on arms though. Good to know that at least.

 

[dogman dan]

20 buck bike then, my price for a bike with 1 part on it I want.

 

[jimincolorado]

well I inspected the bicycle a bit more closely and there is no steel in the rear dropouts. There is an aluminum insert that looks like it is there to hold on the rear derailuer, and plenty of room to mount an arm. but the other issues on the bike abounds... The front fork dosen't support a disc brake, the rear wheel is bent which I did not notice until I got home, the chain is moving but rusty as can be... other little things.

Thanks for the advice on arms though. Good to know that at least.

What you want to avoid with aluminum (or any other thin metal) is what's known as a stress riser. Look it up. If you mounted a torque arm directly to the chain stay and the metal from the arm dug into the aluminum, that would be your stress riser and it would make that section vulnerable to a crack and then failure, which is usually rather sudden with aluminum. to avoid this, you might look into wrapping the chainstay/torque arm interface with some sort of hard rubber wrapping or a strip of carbon fiber.