Front Fork for Hub Motor

Joined
Aug 13, 2017
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Hi Folks,

Hope everyone is well. Trying to revive this topic. I'm currently building a 2WD e-bike but having a hard time finding a front fork that will fit. Hoping that anyone experienced in building a 2WD can help.

A couple of problems I see:

1. Currently using a QS205 50H motor w/ 150 mm dropout
2. What kind of forks should I be using? I'm thinking fat bike suspension forks (a few of them such as RockShox Bluto or DNM USD-6 fat bike forks have 150 mm dropout)
3. Even then, both of those suspension forks above have thru-axle setups (which makes it impossible to insert the hub motor axle flats)

Any suggestions is much appreciated!!!

Thanks so much!
 
If you are really trying to use a QS205 50H in a front suspension fork ( :shock: ) I suggest you find a motorcycle fork that you can machine so you can bolt the motor onto that.
 
Alan B said:
Motor torque generally doesn't mix well with suspension forks.


Alan, when you use the front brake very hard, the fork receive alot more torque than what a hub motor can produce.. so when you use hub motor the only diff is the direction of that torque...

The tire grip is also a major limit for the front motor max torque.. i used 3T MXUS on 24"x3 wheel and was not able to set more than 200a phase otherwise the tire slip easy...

Guys dont mix kW and torque !.. torque is the only factor that describe the force a fork will take and torque depend on the motor winding and the phase amp that's it

you can have more torque on a 1000W controller than on a 10kW controller it all depend on witch speed you run at at witch you apply that torque..

Power is torque x speed

Max Power is: at up to how fast you can sustain the max torque your system can do.



Doc
 
The big difference between braking torque and motor torque is the duty cycle. Braking doesn't last long, motor torque can last till the battery dies. During strong braking the suspension fork has a lot of force to move it. The torque affects how well the forks move, increasing fork friction a lot. Many people have reported that their front suspension forks don't work very well with a motor mounted there. They stop sliding, for the most part, when there is motor torque. With braking the effect is temporary, with a motor it can be most of the time.

I procured steel non-suspension forks for my 2WD build rather than put in a lot of work to try and make the alloy parts of the suspension forks handle the motor torque (which can be downright dangerous if not done properly). $100 for nice steel forks, done. No special torque arms to figure out, easy. But putting such a powerful motor in the front as this OP is planning is a much different project.

By the way, 2WD rocks with midsize motors. It really makes a difference. But even with a standard hubmotor in the rear the traction available to the front motor is reduced. So the front motor doesn't need to be very powerful, or it will break traction and that is undesirable. I find that reducing power to the front wheel is required. So a big motor would probably be wasted there.
 
In line with what Alan said maybe you should look at the light weight front wheel 20 mm axle motor from ebikes.ca?
Will not demand machining parts, or look for costly mx forks that also adds weight.

Have a look; http://www.ebikes.ca/shop/electric-bicycle-parts/motors/grin-all-axle-hub-build.html
 
Guys,

Thanks so much for all the advice. Very helpful perspectives & suggestions.

My brother is actually moving away (can't drive powerful e-bikes where he's going) - hence, I suddenly have spare QS205 50H, Adaptto controller, and battery pack. Since we have similar setups (controller, motor, battery pack) - I thought might as well try to make mine 2WD...:)

My nervousness is really on the torque arms & forks - I'm just not sure they're strong enough. After seeing the DNM USD-6 in person, at a shop nearby, I'm now certain it has enough clearance. There's plenty of work that needs to be done though, to fit the axles (and most importantly the torque arms.) I'm at the conclusion that I just need to custom machine the torque arms.

Anyone ever tried rigging a battery pack in front? I'm thinking of setting up an extension bar in front of the handlebar & top part of the fork to hold a battery bag - set up parallel to the one inside the frame...though a smaller one...not sure if that will partly solve the front wheel traction...

Thanks again!
 
There is one here that sells custom made triple crown clamps for mounting your battery pack on the front fork. Don't have a link sry.
 
Nearly every forum has a post claiming that front DD motors are a bummer and should be avoided. Especially on suspension, kind of a wase given the weight....yet is see so many here in the land of who gives a shit what the experts spout. I have my grocery bike with a 1000W front motor and dual torque arms (grin) and it's a great city bike. A pal, DWI fella, mounted his 750W on Electra front suspension fork and I convinced him to use two Grin torque arms. 4th season of daily use, 8 months a year. No sign of any cracking or failure. SO my take is that there are exceptions. In a flat street environment the front drive can be a fine bike. I especially like mine in the winter. Add a bit for front weight and run studded tires. Front wheel drive in snow and ice is vastly superior. As to the spin out in sand, city streets are well kept here. Another non issue. I think the experts protest to much. Front drives can be fine ebikes.
 
Doctorbass said:
Alan B said:
Motor torque generally doesn't mix well with suspension forks.

Alan, when you use the front brake very hard, the fork receive alot more torque than what a hub motor can produce.. so when you use hub motor the only diff is the direction of that torque...
[...]
Guys dont mix kW and torque !..

Don't confuse other forces with torque, either. With a disc brake, the hub has a torque applied to it, but the fork doesn't. At all. However, a hub motor applies a reaction torque to the fork tips at a very small radius. That's the concern. It's way more likely to destroy the forks and crash the rider than braking, even if braking results in greater acceleration on the bike and more energy being moved around.
 
tomjasz said:
Nearly every forum has a post claiming that front DD motors are a bummer and should be avoided. Especially on suspension, kind of a wase given the weight....yet is see so many here in the land of who gives a shit what the experts spout. I have my grocery bike with a 1000W front motor and dual torque arms (grin) and it's a great city bike. A pal, DWI fella, mounted his 750W on Electra front suspension fork and I convinced him to use two Grin torque arms. 4th season of daily use, 8 months a year. No sign of any cracking or failure. SO my take is that there are exceptions. In a flat street environment the front drive can be a fine bike. I especially like mine in the winter. Add a bit for front weight and run studded tires. Front wheel drive in snow and ice is vastly superior. As to the spin out in sand, city streets are well kept here. Another non issue. I think the experts protest to much. Front drives can be fine ebikes.


A single drive front mounted motor would for me be the worst thing to ride in winter conditions, because it would be all new for me. I would easily choose a tailhappy rear wheel drive for winter driving over a front wheel drive. But I guess that also is related to what one is accustomed to. But for me front wheel wash out is the hardest to correct. If tail wash out I can control it with the throttle, usually by adding more throttle while counter steering.

How well does you bike run say steep uphills during winter, wouldn't weight automatic transfer to the rear wheel uphill? Weight transfer to the front wheel by body movement while riding uphill is a lot harder then the other way around. Without ever trying a front wheel bike in winter conditions I would think the advantages from moving your bodyweight aft ie at take off in slippery conditions would make a rear wheel drive get more traction?

I agree for many people front wheel drive will be perfectly fine. Even with kw motor like you run. OP is thinking of using a 25 lbs QS 205 up front. That motor got serious torque and in my head I can just picture all kinds of wrong twisting motions going on in the lower fork legs as power is applied. Then there is also the matter of all that extra weight up front. But if he manage to find a solution that works I am sure a dual drive twin Qs 205 will be insane fun to ride.
 
macribs said:
How well does you bike run say steep uphills during winter, wouldn't weight automatic transfer to the rear wheel uphill? Weight transfer to the front wheel by body movement while riding uphill is a lot harder then the other way around. Without ever trying a front wheel bike in winter conditions I would think the advantages from moving your bodyweight aft ie at take off in slippery conditions would make a rear wheel drive get more traction?

I agree for many people front wheel drive will be perfectly fine. Even with kw motor like you run. OP is thinking of using a 25 lbs QS 205 up front. That motor got serious torque and in my head I can just picture all kinds of wrong twisting motions going on in the lower fork legs as power is applied. Then there is also the matter of all that extra weight up front. But if he manage to find a solution that works I am sure a dual drive twin Qs 205 will be insane fun to ride.

None here. That's perhaps unusual for many, but most of the small cities here have no hills. Making the front wheel drive so very easy to build, maybe all those years on ice and front wheel drive have me trained. I do add 20lbs of front weight. Or maybe I just don't know any better. On the flats, the negatives seem to dissolve, for me and my buddies.


I do have 3 mid drives left in service and actually the simple front DD and MAC 10T are my favored rides. 25-28MPH
 
Folks,

Found this on another forum in ES and thought I'd share it. All credit belongs to kdee122 user, based out of Taiwan

This is a great solution - but only works on upside down fork (preferably downhill.) What I gather from what this user had done:

1. Only works on upside down (preferably downhill) fork
2. Because you can easily slide the tubes out of the crown (to mount the flat axles)

Best one I've seen so far, so I'm going to try a variation of this solution and keep post.

I now wonder if we can take out the dropout from the tube altogether, and just CNC machine a new one that fits the flat axles...hmmm...
 

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Hi ES

20170906_1551081-e1504707566268.jpg


I use 2wd since 2009 and broke quite a lot of forks before these findings :

The only cheap option for a 9c fh205 motor or equivalent under 1500w is the Marzocchi dirt jumper III. Doctorbass tested and documented multi crown / more expensive options that seem compatible with an MXUS 3k level of torque.

As i'm currently using a 2 x MXUS 3k 3t configuration, I moved to a Derbi senda fork ("light" motorcycle), it's a Showa upside down fork, and modified it to lock the hub inside

20170906_143658.jpg

I found that this type of axle system is "easy" to modify :
Baton_de_fourche-1.jpg
. I've put serious torque arms INSIDE of the "axle holder part" (?!! sorry for my french lack of vocabulary), brownish on the picture.

+ modified the hub to attach a 275mm motorcycle rotor. I'm very happy with this move as the top speed is now 120 km/h : I can still brake.

for a 150mm hub I suggest you find a bigger motorcycle fork or make your own aluminium crowns
 
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