Fork dropout style for front hub motor

Hi, hope this is the right area to post this. I am researching my first hub motor build. I want to put this on a bike with a Gates belt drive. Seems the easiest solution, then, is a front hub motor. However after a lot of reading here I see the hub motor situation can be very dangerous with an aluminum front fork, and as you might imagine most bikes with belt drives have the more "advanced" aluminum front fork over the nice steel one I want.

So I've decided to build my "dream" commuter bike I will buy a new steel front fork.

Well there seems to be a lot of dropout styles out there. Is there any consensus as to the type of dropout that would be best for the front hub motor?

Here are a few examples:

Soma straight-blade - dropout is angled back towards bike


Surly Disk Trucker - dropouts seem a lot straighter?


This DJ fork seems to have drops angled the other way!


Any thoughts on this? No matter what I plan to install it properly with two good torque arms.

Aquakitty said:

Hi, hope this is the right area to post this. I am researching my first hub motor build. I want to put this on a bike with a Gates belt drive. Seems the easiest solution, then, is a front hub motor. However after a lot of reading here I see the hub motor situation can be very dangerous with an aluminum front fork, and as you might imagine most bikes with belt drives have the more "advanced" aluminum front fork over the nice steel one I want.

So I've decided to build my "dream" commuter bike I will buy a new steel front fork.

Well there seems to be a lot of dropout styles out there. Is there any consensus as to the type of dropout that would be best for the front hub motor?

Here are a few examples:

Soma straight-blade - dropout is angled back towards bike

Surly Disk Trucker - dropouts seem a lot straighter?

This DJ fork seems to have drops angled the other way!

Any thoughts on this? No matter what I plan to install it properly with two good torque arms.

Click to expand...

PM me as I have a different Surly fork that is 1 1/8” I can let go cheap

In reality aluminum dropouts tend to be pretty thick. Steel dropouts are often as thin as possible. Difference in strength? You be the judge.

flat tire said:

In reality aluminum dropouts tend to be pretty thick. Steel dropouts are often as thin as possible. Difference in strength? You be the judge.

Click to expand...

Well I'm reading on here these horror stories of "critical failure" in aluminum, cracking aluminum, teeth on road face paste etc. I was originally just going to use the stock fork, but I don't know. I can't separate the reality from the people who are overly paranoid :lol:

Stuff happens with both kinds.

IF you have properly installed tight-fitting torque arms for the torque level the motor puts out, and are not using regen braking, there's not much to worry about.

If you use regen, it will rock the axle back and forth unless the torque arms and dropouts are a *perfect* fit (which you can't really get; teh closest you can get is using clamping dropouts or clamping plates for a torque arm) *and* the material has zero flex or bending under those levels of torque, on the extremely tiny area that contacts the axle flats.

If the torque arms are not a tight fit, or are not properly instaleld for the direction the torque will apply against them, the rocking can occur even without regen (but is much more severe with it).


Whether any of this ever causes you a problem generally depends on usage and "luck" (really, quality of materials and machining of all the parts involved, for the most part).

The only failures of dropouts I've ever had were *always* because the axle could twist, for one reason or another.

If it can't twist, it can't break anything.




As for the angle of the dropouts relative to the fork, that's more of a match-to-your-torque-arm's-angle thing. Whatever makes them both at the exact same angle and still allow teh torque arm to be installed correctly and compltely fixed in place, is best.


There is probably something about the angle of the dropout relative to the fork leg / tip, and teh shape of the dropout plate, that would be stronger for these purposes, for one vs another, but to figure that out you'd porbably have to set it all up in an FEA program to be sure; most likley it diesn't matter enough to be worth the time doing that.



The only aluminum dropouts I ever broke were in an offground test of a hubmotor, bike upside down, because I didn't have any axle nuts on the thing, and even a small bit of throttle was enough to snap the tips off one side of the dropouts. :/

The only aluminum fork I ever broke while riding was also my only collision with a car (not counting cars sideswiping me on plain bicycles a long time back), and it broke at the crown, no hubmotor needed (or installed), when SB Cruiser impacted a car's front bumper at about walking speed. I could still use the fork if I could replace the crown and steerer (which also bent).

I've spread open steel dropouts (even thick ones) that look like any of those shown, without a proper torque arm (just one of those little slotted plates held on with a hose clamp),

So...don't dpeend on the dropouts, they're not designed for torque no matter which ones you get.

Depend on properly mounted torque plates, clamping dropouts, or torque arms (or more than one of these).



If you had a really low power (250w) motor, it's possible you wouldn't even need to worry about torque arms, if the axle nuts were always correctly tightened--but if they get loose, the arms would be teh only thing helping the dropouts from spreading or breaking.

Wouldn't a through-axle be most secure?

I know PITA when a flat needs fixing, but. . .

Recall reading good stuff about Grin's system, maybe the "All-Axle" ungeared version?

It'll depend on which motor your using, but the Mac doesn't seem to have enough clearance inside a surly disc trucker because the hub is very vertical/wide on the non disc side, and the trucker curves inwards. I'm going to be installing a Mac into a surly troll, which looks a bit more vertical to allow the Mac to sneak in there. Worth checking the hub drawings.

Thank you, this info is really helpful. I was going with a basic direct drive front hub motor, 470 rpm, stated 30-40nm of torque.

My first ebike I was clueless and just threw a 1000w front hub on a dragster.
After a few weeks of riding my forks unexpectedly folded on me and I ate the road while trying to do my best Jackie Chan roll.

Now im rolling with cheap and nasty Zoom forks - only about $200 AUD.
They look flashier then they really are with the triple crown style and most importantly are very strong.
https://www.ebay.com.au/itm/ZOOM-68...139&pg=2367844&_trksid=p2367844.c100625.m3682

Also snapped a frame when riding on my next bike , tricked out with 3000w fat rear hub it was too much for the bikes welds.
The ebike game puts a lot of strain on your hardware. I was lucky the guys at Jube customs who initially sold my fat cruiser had just made a stronger identical frame with additional welding. Still on that frame years later.
The forks that originally came with my cruiser bike were good too - no suspension , just big silver poles triple crown. They looked good and were strong as a house , did get a bit bumpy with no front suspension. Link below is my bike pre electrified...
https://jubecustoms.com.au/collections/bikes/products/street-cruiser-all-black

My first conversion was with a Surly front fork plus Grin torque arm and is still going strong four years later (albeit on my daughter's bike). If I were you, I would procure a Surly steel fork, at least one Grin torque arm and don't use regen.

How drastic would the required measures be for regen?

Use case is long very heavy cargo / tandem in hilly country, more so for reliable drag braking on long descents as for the free extra range.

Already have a 60's steel frame from a Columbia tandem

A pair of Grin torque arms of course

through-axle?

not sure if suspension's called for, tires will be fat for rough desert and forest tracks, slow speeds and no "fun" riding styles, pure utility

added welding? no concern about looks. . .

ZeroCoolFool said:

My first ebike I was clueless and just threw a 1000w front hub on a dragster.
After a few weeks of riding my forks unexpectedly folded on me and I ate the road while trying to do my best Jackie Chan roll.

Now im rolling with cheap and nasty Zoom forks - only about $200 AUD.
They look flashier then they really are with the triple crown style and most importantly are very strong.
https://www.ebay.com.au/itm/ZOOM-68...139&pg=2367844&_trksid=p2367844.c100625.m3682

Also snapped a frame when riding on my next bike , tricked out with 3000w fat rear hub it was too much for the bikes welds.
The ebike game puts a lot of strain on your hardware. I was lucky the guys at Jube customs who initially sold my fat cruiser had just made a stronger identical frame with additional welding. Still on that frame years later.
The forks that originally came with my cruiser bike were good too - no suspension , just big silver poles triple crown. They looked good and were strong as a house , did get a bit bumpy with no front suspension. Link below is my bike pre electrified...
https://jubecustoms.com.au/collections/bikes/products/street-cruiser-all-black

Click to expand...

Were you using proper torque arms with your original setup?

I'm def not putting suspension on mine I kind of want this to be a "svelte" road bike, and I hate suspension on road rides. So, for me it's either the stock fork or buy a close matching steel fork. This bike is 100% commuter / road bike, I have a mountain bike I use for off-road.

I built a cruiser with a 1200w (ebikeling) front hub motor. It's not very svelte with a 12 -15 pound (or whatever) motor. If I were you, a small geared motor would be preferable (probably compatible with your aluminum fork too) unless you plan to go 30 - 35 mph (at 52V).