Catastrophic front fork failure / BMC front wheel drive

[Green Machine]

I have a bmc front wheel drive (running 1200 watts) motor mounted onto a zoom front fork. I use steel torque arms on both sides of the fork.

On 3 thousand miles on this fork I have had 3 forks fail....2 in the last week.

It turns out I should have heeded the advice and not used cheap chinese aluminium suspension forks with a 1000 watt hub motor.

Because of the torque arms, each of the breakages were a non event.

I am having a hard time sourcing a front fork for this bike.

Does anyone know where i can get a steel front fork 1 inch threaded, with disc mounting tabs>

 

[itchynackers]

It doesn't look like your torque arms are doing their job. Can you move the torque arms (at least one) to the inside of the fork? This way it looks like the pressure at the top of the torque arm will hit directly in the middle of the fork, instead of rubbing along the side of the fork. Get those screw clamps nice and tight, then tighten your axle bolts, not the other way around.

 

[TheBeastie]

It turns out I should have heeded the advice and not used cheap chinese aluminium suspension forks>

I know a guy who gets surprisingly offended when people say "cheap Chinese" as he always remarks "everything is made in china", I think the better term is "grey market" or even "Chinese grey market" would do.
I know your not the only one but I think we gotta start somewhere

 

[teklektik]

It doesn't look like your torque arms are doing their job.

That was my thought as well. From the front-on image above, the dropout on the disk brake side appears to have failed first - this is the side with the weak clamping technique.

Although a steel fork would be the best, if you decide to try this again with an aluminum fork, I think something like Justin's TorqArmRev2 to pick up the lower disk brake bolt would serve you better on that side. We don't have a good photo of the non-brake side, but perhaps a more robust clamping technique there might help - perhaps a TorqArmRev3. Together these would certainly be more than enough torque arm for 1000W - here the idea is just to get the best (most immobile) clamping/attachment technique possible.

It looks like you are using AmpedBikes torque arms (I use two on my front BMC installation). Since you have these in hand, you could probably cut one down and drill the two pieces to create a custom clone of the TorqArmRev2. A trick with the AmpedBikes TAs is to flip the inner star to obtain 1/2 tooth offset, sometimes allowing a better angle.

Also, I found the BMC to have a very shallow shoulder on the axle. I picked up some thin stainless washers at the hardware store (Ace) and enlarged the opening with a Dremel sanding drum to get a snug fit on the threaded shaft (14mm washers were a sloppy fit - I think I used 12mm washers). I put one on each side on the inside of the fork to prevent the narrow axle shoulder from sinking into the dropout and spreading it. This effect could definitely stress an aluminum fork simply by tightening the axle nuts.

 

[dnmun]

this is so important. the metal fails because the strain fractures the aluminum. when the nut is tightened.

justin has the Cwashers which are supposed to prevent this from happening too.

i actually wonder if it would be appropriate to make some sorta tool that would cut a flat face on the inside of the dropout. something like a forster bit with a flat cutting face that could be aligned inside the dropouts and each recess would have a surface inside the lawyer lips that was uber flat and orthogonal to the axle for the washer to sit on. maybe even find a washer on the side that touches the aluminum which is softer than aluminum and then a backing washer of steel to tighten the nut against. and make certain the nut face is orthogonal to the threads. all to prevent the fracture of the aluminum under the torque load strain. does the star locking mechanism of the amped bikes torque arm allow any motion or is totally tight?

 

[teklektik]

does the star locking mechanism of the amped bikes torque arm allow any motion or is totally tight?

Very tight.

And since the BMC is a gear motor, there are no regen effects so the torque arm arrangement is always under tension instead of twisting back and forth - a nice simplification from a construction perpective.

 

[iperov]

Good example of wrong installed torque arms.

 

[TheBeastie]

On the subject of the broken forks maybe you should try Doctorbass super torque arms combined with the 4500 psi epoxy glue technique he recommends.
http://endless-sphere.com/forums/viewtopic.php?f=31&t=29129
Never tried it my self but it looks pretty good.

 

[Green Machine]

Yes its true i could do a better job next time of installing the torque arm if i go again with this fork.

The last 2 times it broke was when loaning it to a 260 pound rider.

But right now I would like to find a set of steel forks.....and not keep giving $60 to downtube for forks that who knows where they will fail next.

We had a friend in our group have his suspension fork bust high next to the steering tube landing him in the intensive care unit with a critical head injury.

IT was on a bike built to be an electric bike, with a front wheel motor on a suspension fork.

Since then i have been concerned with possible front end failure.

I would think it would be easy to find a steel 20" fork but am having no luck. 1 inch threads...disc brake tabs. No one knows of one available?

Do you guys think i could use a 26" fork with this bike with the 20" wheel.

Here is a picture so you can see the geometry:

 

[Chalo]

Does anyone know where i can get a steel front fork 1 inch threaded, with disc mounting tabs>

Why do you need disc? 1" forks are way more available with cantilever studs, and good V-brakes are every bit as strong as discs.

If you insist on discs, consider getting a good strong non-disc fork and having a framebuilder add a disc tab to it.

If your frame is set up for the American headset size (32.7mm cup diameter), then it can probably be reamed out to 34.0mm which is the 1-1/8" standard, and that opens up a world of options.

Chalo

 

[dogman dan]

One possibility is the spreading force happend from the inner side of the dropouts. I use an inner washer on my alloy fork front hub to help prevent the skinny 9c axle shoulders from digging into the alloy.

Looks like you had c washers or equivilant in the dropout cups.

It's a lot of toruqe with a big gearmotor, possibly just too much for a less than perfect fit held by a hose clamp. You might simply have had just enough movement just in where the arm clamps on.

My good results have used the ebikes CA two piece torque arm, and for higher power applications I recomend tacking the two pieces with a welder once fitted.

Or, elimiate potential slippage like this if your fork is steel non suspension type.

 

[Drunkskunk]

Finding a 20" steel fork, 1" threaded head tube with disk brake tabs is going to be trouble. A 26" fork will lift the front end up roughly 3 inches, and will affect the bike's usefulness as a folder, but it isn't a bad idea.

Another possibility is just get a ridgid 26" steel fork. the suspension part of those forks sucks anyway, and while a ridgid fork won't give as soft of a ride, it will be far stronger.
The reason for sugesting a 26" for for your 20" tire is to make it long enough to replace your current fork.

 

[Alan B]

Torque Reaction Forces 1A

Folks need to consider the DIRECTION of torque reaction forces. The torque arm converts the torque reaction into two opposing forces on the fork, one at each end of the torque arm. The torque reaction attempts to rotate the torque arm which creates a force at the end of the torque arm and another on the axle. These forces must be considered when choosing the design and application of the torque arm on the fork.

It is important to convert the torque reaction force into an upward force rather than a forward force by appropriate design and application of the torque arm. In the dogman photo above the pivot in the torque arm does this correctly and the torque reaction force on the axle is upward on the left side (from rider perspective). In the failed fork on the left hand side, incorrect torque arm application caused the torque reaction forces to convert to a forward axle force which adds to the thrust reaction force and significantly increases the forward forces on the axle. The dropouts are not designed for these forward axle forces. Note that these forces are larger than the thrust reaction force by the ratio of tire contact radius to torque arm length. So in this case the torque reaction force is about 2.5 times the thrust reaction force in the forward direction on the axle, and these forces add together in breaking the dropouts. Proper design and application of the torque arm would reduce forward force on the axle by about 3.5:1, a substantial reduction in breakout damaging force.

Front wheel motors are risky, especially at high power. Reconsider the options for rear motor drive or decreasing power. You have been very lucky Eric.

 

[neptronix]

The only suspension corrected 1" steerer fork i know if is made by 'dimension', but it doesn't include disc brake tabs.

Those torque arms that do not directly bolt to the fork & have a bunch of flex in them can never prevent this kind of failure. The best they can do is keep the wheel somewhat connected to the fork so that you don't go flying.

Maybe it is time to consider getting another bike and switching to rear drive. Mounting something that powerful to something that is long and flexy ( a front fork ) is not good. I guarantee your life span will be extended

 

[chvidgov.bc.ca]

Here is a Bachetta 20 inch non-suspended steel fork with disk tabs...

http://shop.bacchettabikes.com/parts/steering/steel-fork-w-1-steer.html

 

[Chalo]

Here is a Bachetta 20 inch non-suspended steel fork with disk tabs...

http://shop.bacchettabikes.com/parts/steering/steel-fork-w-1-steer.html

Good find.

Note that they say the 20" version will accept up to a 24" wheel, which means it should be about the same length as the uncompressed suspension fork on the OP's bike.

 

[docnjoj]

Here are some more possible forks for you. This guy has been around for a long time and is quite reputable from my personal experience.
otherDoc

http://www.gaerlan.com/bikeparts/frame/forks/fork.html

 

[Gordo]

Maybe it is time to consider getting another bike and switching to rear drive. Mounting something that powerful to something that is long and flexy ( a front fork ) is not good. I guarantee your life span will be extended

Stacking the odds in your favor instead of against you is a better plan. Doing the same thing twice, while expecting a different result, is not wise. 3 times.......
I figure the cost of one hospital visit is more than that of all the bikes one can imagine, and I have full, free medical. IIRC all the current wisdom boils down to 500W max for front wheel drive AND NO ALUMINUM FORKS, EVER. Why are we here?

 

[Green Machine]

I have an front wheel drive on almost all of my hub powered bikes because I like having an IGH in the back, and like the weight balance of having the motor up front and battery in back.

I own about 10 ebikes and this one has been my favorite and has been really good to me. As I have said I have put thousands of miles on this bike and I would hate to see it go.

That Bachetta looks exactly like what I am looking for. But do you happen to know if it has a threaded steering tube or not? I will call them tomorrow for sure

I need threaded.

All the advice from everyone I greatly appreciate. If all else fails i will order the same fork and try again. The torque arms have done a great job so far of atleast making sure i do not go end over end.

 

[Green Machine]

On another note... I reviewed the new Ezee / Hebb bike lately. You can see the review here: http://www.electricbike.com/hebb/

What suprised me is that Hebb is selling the bike with a new configuration...a front hub motor running 1000 watts.

And Zoom suspension forks...same brand as mine but with beefier aluminum drop outs.

Seems like a lot of product liablility to sell this configuration in the open market. The Zoom suspension forks both on my folder and the ezee perform terribly. You can barely feel like you are riding a suspended bike.

I talked to Bill, the owner of Ezee, and on the lower watt configuration of this bike (350 watts) he had 3 suspension forks break. He says these were all broken by riders for the same Pizza delivery company in NY and he simply stopped selling to them. He thinks they were riding off curves at speed etc.

If i did ride with zoom forks and a front wheel drive motor, i would not go off curves or ride on fire roads . Something I use to do (riding really rough off road at times) on this bike before i had the first fork failure which put the fear of God in me. (it took a lot to break the first forks, and they fatigue failed on level ground at low speed out of the blue).

That is the only time the forks broke on me and i know exactly what happened. The 2 other times they broke they were with a 260 pound teenage rider (19 years old) who borrowed the bike, and returned it both times after less and 20 minutes of riding with busted forks. I was just happy he was ok.

 

[Chalo]

But do you happen to know if it has a threaded steering tube or not? I will call them tomorrow for sure

I need threaded.

When you contact them, if they have a threaded version, make sure that it's long enough, and threaded far down enough to replace what you have. You'll also need to be sure that the expander wedge on your stem doesn't expand inside the threads, so that's another constraint.

If you had a relatively normal bike, I'd recommend using the occasion to switch to threadless, but I assume that your steering mast mounts with a stem quill at the bottom end.

 

[John in CR]

Convert the broken AL forks to having nice thick steel dropouts. I did that on my first hub driven ebike when the AL snapped like yours in the 1st 10ft of the first ride with dual torque arms.

Have dropouts welded to the end of steel tubing that will just fit over the existing broken shock. Weld disc tab to the left one. Make sure it fits and the wheel aligns. Bolt wheel on. Slide assembly off and slather with silicone. Slide on tightly and allow silicone to cure.

These are ugly and overly strong, but still alive 4 years later. Doing it again I would make the dropouts clamping type. Note that the tubing I had was too small, so I slit is up the inside that faced the motor.


Here's a similar one, but with the dropouts horizontal, which offers some alignment flexibility.

 

[Gordo]

Do you have a battery in the "V" of any of your bikes? I was never comfortable on my first 2 builds with the battery on the rear carrier. After only one short ride on a new bike with the battery in the "V", I was so surprised at the improved stability and balance, I moved the battery of my spare bike, from rear to center, the same day. I can certainly understand liking the balance of a front wheel motor with the battery on the rear, but lowering the battery to the center of the frame is a huge improvement. Having a folder makes it more of a challenge, but with 20" wheels, a battery on the rear rack must present some wobbly moments?
I like John's cave man solution to the point of silicon as an adhesive. JB Weld maybe, carbon or glasfibre in Epoxy for sure, but silicon I don't know if I would try that?

I have an front wheel drive on almost all of my hub powered bikes because I like having an IGH in the back, and like the weight balance of having the motor up front and battery in back.

I own about 10 ebikes and this one has been my favorite and has been really good to me. As I have said I have put thousands of miles on this bike and I would hate to see it go.

That Bachetta looks exactly like what I am looking for. But do you happen to know if it has a threaded steering tube or not? I will call them tomorrow for sure

I need threaded.

All the advice from everyone I greatly appreciate. If all else fails i will order the same fork and try again. The torque arms have done a great job so far of atleast making sure i do not go end over end.

 

[shock]

Front wheel drive is so stupid, lol. Such a waste of time and parts, looks and handles poor. The best fix for this is do it right and get a rear drive hub. This is my opinion, so not everyone will agree, but I would rather have no ebike than a front drive version.

 

[auraslip]

Looks like you got some cool pictures and experiences to write about for your site.