broken fork

[xlzeo]

hello everyone,
i bought few weeks ago the front hub motor MP3 ( magic pie 3) from golden motors, and installed it on my commencal super 4.2, on the beginning it worked smooth but after few days while am riding the fork where got broke with no special reason
it got broke when i was trying to start ride from a stop while i passed cross-walk


any ideas how to solve it?
thanks

 

[dogman dan]

That's a pretty small picture. But it would appear to be an shock fork with aluminum alloy lowers, and dropouts.

It's well known that you must do a perfect install of a front motor on alloy shocks. Otherwise, the motor will put out enough force to snap the dropouts. Many say just don't do it at all. I just say you better do it perfectly, which is hard if it's your first time.


You need special C washers and two torque arms. Grin Cyclery is the source for both.

If you look in the review sections, you will find threads that show proper use of the washers and torque arms.

http://endless-sphere.com/forums/viewtopic.php?f=4&t=15163&hilit=Ebikes+ca+c+washers

http://endless-sphere.com/forums/viewtopic.php?f=4&t=15167&p=225525&hilit=Ebikes+CA+torque+arms#p225525

A steel fork would make it easier, but you might still need the C washers, and at least one torque arm is highly recomended even for steel forks.

It may be possible to just grind down the washers you have, to fit better into the cup on the forks. Getting the nut pressure perfectly even on the fork cup is step one.

 

[TylerDurden]

The thumbnail image is a link to a big pic.

The alloy suspension-fork dropouts have cracked off... Fork is trashed.

Alloy dropouts must always be reinforced with torque arms or similar.

The fork can be replaced... but very often, the motor wires get shorted and other electronic components are damaged.

Bummer, dude. That was a decent fork.

 

[hjns]

This would be a typical case of wanting to learn TIG welding, and just weld on new dropouts....

 

[miuan]

Rock shox sells spare lowers for Reba forks, ask your LBS. It's very likely you will also be able to fit lowers from Tora or Recon forks since they all have 32mm OD uppers. I have a Tora with bent steerer and good lowers sitting in my garage. If you pay me EUR 45 plus shipping from Slovakia, they are yours

hjns: the lowers are cast magnesium, welding would probably burn them off right away.

 

[Ypedal]

This would be a typical case of wanting to learn TIG welding, and just weld on new dropouts....

Nooooo...

 

[Erikjan]

Just for my info how much amps and volts did you run the motor when fork broke.

 

[bobale]

In my opinion, Reba is totally wrong choice for front wheel drive e-bike. RS Reba is a racing fork, and therefore it's light. They've used lightweight materials wherever they could, and as a result you've got light but fragile and flexible fork. Exactly what you don't want on e-bike .

 

[miuan]

I've used a Tora amd M29, the dropouts are good enough for 40-50A phase amps with good double torque arms.

 

[dogman dan]

Alloy forks and front hubs are tricky shit. The ones I'm using right now fit the motor, but lock up when the motor is powered. They're cheapos with 80mm travel. So if I see a pothole coming, I have to roll off the throttle to get the shocks to work. Other forks had other issues, from just plain won't fit to binding when the nuts were tightened. That one required lots of fiddling with shim washers to get it working.

 

[hjns]

cast magnesium? Cool, that would give some nice colored sparks when combined with Argon and high current....

OK, I admit welding would be a bad bad idea...

 

[amigafan2003]

You must have been putting some wattage through those forks.

I've been running on a carbon fiber fork with alloy dropouts for 3k miles - although I'm only pulling 650watts peak and have a torque arm fitted on the non-cable side.

 

[dogman dan]

All he had to do to crack em, was not use a C washer, and tighten the nut to 10 ft pounds. Puts the micro crack in the forks right then, before you ever put power to the motor. Improper fit is much more likely to be the cause than motor power less than 3000w.

It helps a lot that they still send the wrong size washers for a bike, every time. C washers should come with every kit by now, but noooooo.

 

[amigafan2003]

All he had to do to crack em, was not use a C washer, and tighten the nut to 10 ft pounds. Puts the micro crack in the forks right then, before you ever put power to the motor. Improper fit is much more likely to be the cause than motor power less than 3000w.

It helps a lot that they still send the wrong size washers for a bike, every time. C washers should come with every kit by now, but noooooo.

That's true - my forks are properly packed out with c washers (from here) and even then I had to shave a mm off the lawyer lip on the fork.

 

[Chalo]

All he had to do to crack em, was not use a C washer, and tighten the nut to 10 ft pounds. Puts the micro crack in the forks right then, before you ever put power to the motor. Improper fit is much more likely to be the cause than motor power less than 3000w.

I remember my disappointment when putting a nutted axle wheel into an RST suspension fork and splitting the tip when I tightened up the first nut. The flange on the nut was just a wee tad larger in diameter than the recess in the fork tip.

It was not the first time I cursed lawyer lips, but it was the most memorable.

Chalo

 

[Ypedal]

FYI .. to xlzeo .. i broke your link.. pls post the image here .. that site hosting the picture is a spam flavoured black hole..

 

[Chalo]

FYI .. to xlzeo .. i broke your link.. pls post the image here .. that site hosting the picture is a spam flavoured black hole..

I got the (huge) photo to come up; it just took a while to start loading and a while longer to fill in.

It is a pretty detestable site, but the photo did appear eventually.

 

[Hillhater]

Alloy forks and front hubs are tricky shit. The ones I'm using right now fit the motor, but lock up when the motor is powered. They're cheapos with 80mm travel. So if I see a pothole coming, I have to roll off the throttle to get the shocks to work. ..

so what happens when you put even more load on them with the brakes ??

 

[Ypedal]

Brakes dont create a twisting action at the axle, in a pulsating fashion like.. outrunner hubs do.

The axle on a regular bicycle hub floats on bearings and is perfectly round, made to fit the dropouts perfectly by design... 10mm flats axles extend past the center point of a normal axle compounding the problem..

hub motors on those kinds of forks are just a bad idea. ... I"ve done it.. but it's still not a good idea. :lol:

 

[D-Man]

Mines still ok.

 

[d8veh]

I have a BPM motor, which I ran at 40amps for a while (now at 30amps) on Reba forks. It's been OK for about a year so far. Like the others say, It's OK as long as you do it right, which means: correctly centred axle (better with 12mm than 14mm though); correctly fitting washers; suitable re-inforcement (torque-arms); and correctly torqued axle nuts. Now that I think about it, if MP motor has 14mm axle, there won't be much of the drop-out left to reinforce after it's been filed enough to get the axle in. BPM is a much better motor for front wheel drive because of the 12mm axle.

 

[dogman dan]

Braking seems to work fine on those particular forks. Rim brakes, so the load is different, not loaded at the axle bending the fork. Perhaps the fork is made to work when flexed one way, but not the opposite direction. They are cheapie forks, RST capa. Just barely better than zoom steel forks. I was going to replace them with something better, but then those forks went onto my dirtbike.

With a 14 mm axle, you have to file the dropout deep enough to get the center of the axle centered in the dropout.