Calculation of front fork stress due to hub motor

Get all your technical information about electric bikes here.
User avatar
spinningmagnets   100 GW

100 GW
Posts: 11878
Joined: Dec 21 2007 10:27pm
Location: Ft Riley, NE Kansas

Re: Calculation of front fork stress due to hub motor

Post by spinningmagnets » Aug 28 2015 12:58pm

I know what I'm about to say is a curve ball, but...this application is one of the few instances where I believe a friction - drive is the best possible solution. Just a thought...

Trackman417   10 kW

10 kW
Posts: 841
Joined: May 05 2011 9:25am
Location: Schenectady, New York

Re: Calculation of front fork stress due to hub motor

Post by Trackman417 » Aug 29 2015 10:58am

dequinox wrote:
d8veh wrote:There's disk brake carbon forks now, which are designed to take a strong torque around the axle. I can't see a problem with them when using say a 250w motor. The torue arm can be anchored to the brake caliper mount.

Having said that, I would say, why put the motor in the front if you can put it in the rear, which has a lot of advantages?
Now this would be a much better way to compare the stress between braking and motor torque. Avandalen, you may want to re-do your math portion using this instead of the rim-brakes as a comparison.
The designers for the bike most likely used a different weave for the CF to help increase the strength due to the different forces from the disc brake. So, comparing stresses on his current fork would not work out so well.

Going back to Hillhaters post with making the torque arms as long as possible. I would make the torque arms so they can attach directly to the bolts of the existing rim brakes. Then, there would be almost the same forces(except in different direction) on the frame from the motor.
Another thing to note, is you aren't mounting a cromotor on your fork, it is only a 300watt(or so) motor. So making the torque arm as long as possible is not necessary. Just make sure you use some hose clamps with some rubber to insulate them from the frame.
K2 Sidewinder
H4065 36 mph
20s2p 10aH lipo battery pack
If it didn't kill you, it just built your character.
You know your bike is powerful when even you, the builder, can't control it.
Customer 00001 at our very own Bigmooses online store http://mtg-technologies.com/zencart/ Great guy, great products.

Triketech   1 kW

1 kW
Posts: 357
Joined: Mar 31 2015 6:50pm

Re: Calculation of front fork stress due to hub motor

Post by Triketech » Sep 02 2015 10:09am

dogman dan wrote:It's not the "strength" that's the problem. It's the way carbon, and alloy fail. All at once.
And without warning.

Failure points are likely to be at the top of the posts which is the worst way to break at speed. The posts themselves should take the cycle flexing OK, its where they join at the top of the bridge that's likely to fail. The caster angle will significantly stress that point through tension loading with power where braking puts it in compression. The cycle stresses make FWD with carbon forks unwise choice.

User avatar
speedmd   100 MW

100 MW
Posts: 2998
Joined: Nov 14 2012 12:16pm
Location: new england

Re: Calculation of front fork stress due to hub motor

Post by speedmd » Sep 02 2015 10:50am

Regardless what frame and fork materials are used, this is a great comparison for using disk or rim brake options and motors. Loads of disk bike frames and forks are seeing signs of early failure on pedal bikes, so this is a good topic to look deeper into the math for anyone adding a motor on the front end. Front end failures suck for certain, and not something you want to entertain while riding. Been there ....:oops:

Carbon is actually a better material in dealing with a higher percentage of its max stress. Agree, failure mode sucks.

Image

To complicate things a bit, fiber orientation and lay up makes huge differences. Not all the same for certain. Quick look at some glass research numbers for comparison.

Image

Post Reply