Poll: Has your front hub motor spun out of your forks?

[Blaze]

Yeah, that's why I use a wrench as a torque arm. It fits the flat spots on the axle and holds it very well. With many motors, the fork dropouts are enough to hold the flat spots, but my motor has hill-climbing torque, and it opened up the dropouts, which were the only things holding the axle. After looking at these photos, my drawings should make a lot more sense (this axle is for a rear hub motor, so the axle is a lot longer, but the flat spot looks the same)...

 

[Blaze]

...and here is what my perfect torque arm would look like (here is your template, wheels). I'm not sure if the hole I drew was to scale, but that's the basic idea. If the axle hole is so big it gets close to the edges of the brake arm, then you would need to start from scratch with a wider piece of sheet steel.

 

[gone_fishin]

thanks blaze, i already pretty much understand an important aspect of front e-hubs

sure don't seem like much to hold onto under torque, nor does there seem to be enuff room to really do a solid job of it. couldn't you make a stronger torque-arm if you worked on the outside of the dropout instead? i assume that's a lot harder than average bicycle axle-steel? do the flats themselves ever get twisted or worn?

 

[srdavo]

Blaze,
Was there any permanent damage to your fork dropouts when they spread?
Dave

 

[gone_fishin]

i was just looking at my fork...what if you "layered" 2 more dropouts on the outside, then bolted all 3 together using the upper hole? would that be strong enough?

 

[Blaze]

thanks blaze, i already pretty much understand an important aspect of front e-hubs

sure don't seem like much to hold onto under torque, nor does there seem to be enuff room to really do a solid job of it. couldn't you make a stronger torque-arm if you worked on the outside of the dropout instead? i assume that's a lot harder than average bicycle axle-steel? do the flats themselves ever get twisted or worn?

It won't matter if the dropout is on the inside or outside of the forks (but the inside is easier to work with because the fork tube can stick out a lot). The strength will be determined by the strength of the steel and the shape of the design. The dropouts don't work well because the steel is not hard enough and it is a horseshoe-shaped opening that the axle goes into. With enough force, the dropout openings will be spread apart because there is nothing holding the open end together. With the design I drew on Wheels' brake arm, there is a lot more structural integrity from having an actual hole instead of a horseshoe shaped opening. You can't really bend the hole wider. In order for the axle to spin in that setup, you would actually have to tear the steal. Both the dropouts and the brake arms are made of pretty much the same type of steel. It just goes to show how design can add as much strength as materials.

The hub axles are no harder than regular bicycle wheel axles. They don't have to be. It's 2 or 3 times thicker, and I'm pretty sure they are both made of carbon steel, anyway. That's about as hard as it gets without getting into exotic alloys.

Being made of carbon steel, the only damage I have done to them is a slight flattening of the ends when I hammered the hell out of them while re-seating the bearings. It made the nut go on a little tight at first, but no major changes. The odds of ever damaging the flats on the axle are slim at best.

 

[Blaze]

Blaze,
Was there any permanent damage to your fork dropouts when they spread?
Dave

Not really. They are just a little wider now. It doesn't really matter anymore, because I am not using the dropouts to keep the axle from spinning. All of that force is transferred to a torque arm now, so the only thing the dropouts have to do is hold the wheel on like a regular bicycle wheel. That doesn't take much strength. As long as the torque arm keeps the axle from spinning, the dropouts really don't carry much load at all. The only load they carry is the forward force from acceleration, and a backward force from braking, if you have front caliper brakes.

 

[Blaze]

i was just looking at my fork...what if you "layered" 2 more dropouts on the outside, then bolted all 3 together using the upper hole? would that be strong enough?

Bolting them together probably won't work. It might work if you heavily welded the layered dropouts together. Emphasis is on might. I wouldn't try it because it will cost you a fork to try it, and there is not a high enough likelyhood of success.

The reason I don't think bolting them together will work is because of simple leverage. With the bolt so close to the axle, you aren't getting enough leverage to decrease the amount of force on the bolt. With nearly full axle torque on the bolt, it will probably shear.

The reason torque arms work so well is because of their length. Think about removing a bolt that is stuck on. With a short wrench, you can't put enough force on it. Slide a pipe over the wrench, so it's like having a 2 foot long wrench. The bolt that was previously impossible to turn can now be moved with hardly any force at all. This is what torque arms so. The further they move the torque forces away from the center of the torque, the less effort it takes to hold the axle. By moving it far enough away, the entire axle can be held by just one small, thin clamp. If you try to hold the axle at or near the center of torque, there is so much power it will destroy heavy gauge steel. Pretty cool, really.

 

[Wheels]

You are really clear on this Blaze, is vibration damping an issue?

 

[Blaze]

No, vibration is not an issue at all on the electric bikes. That's one thing I absolutely love about them. It makes them so comfortable to ride.

The clamp you have would be perfect, though. The rubber covering will keep your forks from getting scratched. That is the exact same style of clamp I would use.

You would want the clamp to be mounted with the bolt tab pointing towards the rear. The clamp has the most strength holding something trying to pull away from it. If you look at a rear brake, which I use as an example a lot here, you will see the same setup. The brake arm is pushed down when you stop, so the clamp's bolt tab is poitning down. On the electric hub, the torque arm is pushed back when you accelerate.