jawnn

How much force can a MTB free hub body take? I want to use a motor do drive it, but the motor can produce 100Nm force (73.75foot pounds). Infact the company that makes the motor says 130Nm, but I don't trust them. How ever I would not like to find-out the hard way.

sch

Reasoning by inference, a pro cyclist can generate 1500 or so watts for 10-30 seconds; alternatively said pro weighs 160# mashing on a 7" pedal would
be at least body weight pushing down, probably more when you add muscle pulls against the handlebar which would be at least 95 foot-lbs transmitted
to the chain. Don't think it likely the freehub would fail under the motor power but I would make sure it is steel and that the cassette cogs are mostly
on carriers.

AnkleWork

Clue: neither Nm nor foot pounds are units of force.

CliffordK

Freehub bodies vary in the number of pawls from 3 to 6. They should all run in some kind of hardened race/sprocket.

@sch is probably right that your motor is probably not generating much more force than the maximum sprinting force of a pro rider. Although, a motor can generate 1000W more or less continuously, whereas the cyclist can't.

Watts is watts, whether it is applied standing in the 11t sprocket, or spinning in the 30T sprocket. The only difference might be the shape of the force peaks and valleys, and perhaps jamming the freehub.

I dislike feeling slack in the pedals, then slamming force into them.

Your motor should give you a more continuous applied force which is good, but could potentially slam full force into the pawls when first engaging which could be destructive.

I believe there are some center mount motor kits which will drive off of a right side disk brake sprocket using 6 bolt, or centerlock.

AnkleWork

Just FYI 1000 watts is produced by:
continuous 80 Nm at 120 rpm
continuous 159 Nm at 60 rpm

And bicycle drivetrains are limited by torque and force, not power.

Any internal combustion engine will have much higher torque variation than a rider.

CliffordK

The OP mentioned "motor", not "engine".

Perhaps the power source should be verified as electric, or gas.

So, yes, you do have a speed factor at the hub, but the 1000W force at the freehub is the same whether it is applied to the 11T or the 50T sprocket. Just different forces on the chain and other parts.

AnkleWork

Haw!

jawnn

this is the best answer.... it shows what kinetic energy can do with momentum.

" neither Nm nor foot pounds are units of force." how should I be refering to force? I am just a backyard engineer.

the motor I am going to use is a 2500 watt to over 3000 w motor I will be restricting it to about 1300 wats. and I hope not to be jaming for burnign rubber. but I do want to drive the left side so I can use a much stronger chain #41 and maybe 60 to 68 sprockets from a 16t max on the motor. ultamatly I want to be able to get by with a smaller battery, other wise I would just go for a hub motor. but I did not realize how hard it wouldl be to engineer the power train.

Andrew R Stewart 

This thread is familiar... didn't we visit this a few months ago?

My questions are about the legal issues. In many US states the E bike laws are evolving and not well understood by lay people. Just wait till the stupid incident happens and insurance companies will let you know what's legal and covered or not. Andy

fietsbob

Applying a lot of force, over and above a meer human capacity, for any service length of time??
Expect replace things more often..

jawnn

ya I am well past this now, I have sever arthritus and had to start the process of motorizing my bike no mater what. I cannot ride my bike with out a motor now. I will be programing the motor down to a reasonable wattage but I still need a certain wattage to get up the hills with cargo.

this is the hub I haveATB Disc Rear Hub

#h49-XX - but I don't think they can tell me how strong it is. so I am just going to have to accellerate slowly.