Can I Safely Run A Front Hub On Aluminum Fork IF...

[The Journey Guy]

Hello all. I have read several posts where it says that it's unsafe to put a front hub motor on a suspension fork, because they are generally made of aluminum and are brittle.

I have a Trek bike that I very much like, which has that kind of fork, and I'd like to run a 9C 2806 on it.

Would it be safe if I used that fork anyways, on the condition that I use a torque arm on both sides of the fork? Would those help to strengthen the fork, and make things safe then?

Thanks for any input!!

 

[OneWayTraffic]

There have been cases of Al forks breaking, even with torque arms. I saw a guy who broke his going over a curb. On the other hand some people do run Al sucessfully, especially at lower powers. I'd say maybe. Better if you can look for a suspension fork with steel drops and castings. How much is your face worth to you?

 

[Doctorbass]

I would personally NEVER suggest to put a hub motor on an aluminum front fork.

Too many good reason to never try that can be found over the E-S.. search by yourself.

Aluminum dropout and hub motor = NO

Doc

 

[dogman dan]

Just bear in mind what flying over the handlebars can do to you. My adventure involved a water bottle stuck in the front wheel instead of a motor. But the results may still be the same. I broke both collarbones, which only cost $15,000 to fix. The rest of the damage, involving the shoulder muscles and ligaments is still in the healing process a year and a half later. For a year, if I picked up something really heavy, my shoulders would pop out of thier sockets. Ouch! I'm up to being able to work 20 hours a week now and have adjusted to the new, much lower income.

It's your body, but do this. Pay one hell of a lot of attention to getting the fit of the washers perfect. Torque arms will not cure a big washer in a small recess. You may need to make your own custom washers to fit the cup from the quick release hubs that is usually small.

If you use a torque arm or large washer to span a gap, it is likey to stress the alloy in a way the designers of the fork never intended, which causes a micro crack as your tourqe down the nut. Then one day, riding along at say 30 mph, the dropouts fall apart at the crack. The torque arm keeps the wheel on the bike, but it still jams into the fork crooked, and off you go over the handlebars at 30 mph. Have fun!

Many people are "safely" running motors on alloy forks. It reminds me of alcoholics who say, "I drive drunk with no seatbelts for 40 years with no accidents". Doesn't make it safe that they are lucky.

 

[liveforphysics]

Hello all. I have read several posts where it says that it's unsafe to put a front hub motor on a suspension fork, because they are generally made of aluminum and are brittle.

I have a Trek bike that I very much like, which has that kind of fork, and I'd like to run a 9C 2806 on it.

Would it be safe if I used that fork anyways, on the condition that I use a torque arm on both sides of the fork? Would those help to strengthen the fork, and make things safe then?

Thanks for any input!!

I would personally never recomend a front hub motor for any type of fork material, and absolutely not with aluminum.

 

[The Stig]

...it has stainless steel inserts?

I've been thinking about doing a 2wd full suspension bike and so I guess I'll need to find some suspension fork with good dropouts. Do you guys think stainless steel inserts would cut it?

http://bikemag.com/gear/suspension/042505_ronin_magura_suspension_fork/

Attention to detail: the Ronin even has stainless steel inserts in its fork dropouts to prevent corrosion of the mag lowers.

 

[liveforphysics]

They could snap the magnesium fork leg right above the insert, which often seems to be a failure point.

 

[OneWayTraffic]

Hello all. I have read several posts where it says that it's unsafe to put a front hub motor on a suspension fork, because they are generally made of aluminum and are brittle.

I have a Trek bike that I very much like, which has that kind of fork, and I'd like to run a 9C 2806 on it.

Would it be safe if I used that fork anyways, on the condition that I use a torque arm on both sides of the fork? Would those help to strengthen the fork, and make things safe then?

Thanks for any input!!

I would personally never recomend a front hub motor for any type of fork material, and absolutely not with aluminum.

I don't know. There has to be a certain size and power of hub where a front fork is perfectly safe. Many of the premade bikes are front hubs with 250W motors, and I've never heard of one of those breaking, aluminium forks or not.

But a 2806 is quite a bit more powerful.

 

[The Stig]

They could snap the magnesium fork leg right above the insert, which often seems to be a failure point.

So then, LFP, I imagine you have a good idea of what exactly causes these failures, which is what I'm curious to know.
I know you wouldn't put a front hub in any fork but steel forks+torque arms seem to handle hubbies just fine...

Anyone know any suspension forks out there that might work?

 

[OneWayTraffic]

Here's a good example of what I'm talking about. For those who don't go the the UK pedelecs site very often, Wisper is one of the better brands and they do a lot of R&D and stand by their products. They wouldn't put this out without being certain it's ok.

http://wisperbikes.com/

I couldn't get a link to the page, but go to their Wisper bikes menu and look at the 906. Not the 905. Rear Alfine hub, front 250W Japanese hub motor nice accessories, and a $3000 USD price tag. It may be expensive, but there's some quality parts there. They see fit to put a RST magnesium suspension fork on it. No doubt the fit of the motor to the fork is perfect.

 

[liveforphysics]

They could snap the magnesium fork leg right above the insert, which often seems to be a failure point.

So then, LFP, I imagine you have a good idea of what exactly causes these failures, which is what I'm curious to know.
I know you wouldn't put a front hub in any fork but steel forks+torque arms seem to handle hubbies just fine...

Anyone know any suspension forks out there that might work?

It's fatigue from stress and work hardening. Every time dropouts made of aluminum or magnesium get flexed from torque loading, it fatigues the metal. As the number of cycles accumulate, the material continues to develop a weakened point. If the weakening grows beyond the level of forces being applied to it, then you end up taking a face plant. Hopefully not at a time with close traffic to drive over your freshly and unexpectedly face-planted body in the street.

If front drop-out were designed for torque loading rather than simple upward and backward compressive loading, they would be a much different design. For setups with minimal torque, it may not be too great of a concern. Keep in mind power is not a good indicator of torque, as even a little "350w" geared hub could be torque the axle harder than a big 1500w hub in the right situation.

 

[The Mighty Volt]

QUESTION:

WHY DO MANUFACTURERS BOTHER MAKING, AND WHY DO CYCLISTS BOTHER BUYING, FRONT HUBS???

Sorry to shout....but its bugging me now.

If frontal hub motors are such a bad idea, why use them at all?

 

[dogman dan]

The majority of my ebikes have been front hubs. On my city bikes I think the fronties work better, on a bike for MTB trails, I want rear drive and a good set of suspension forks. Different tools for different jobs. Like using a screwdriver for screws instead of the hammer.

My front hub bikes ALL have very strong steel forks.

 

[The Stig]

Make sense liveforphysics. And yes power doesn't mean torque, but current is pretty much proportional to torque for a certain hub so I guess it's about where you should set your current according to that hub.

Front hubbies are fine in steel forks, which have better fracture toughness.

Is there agreement on what exactly the main faillure point is?

 

[dogman dan]

Not sure what you mean by the failure point, but on aluminum or steel dropouts, I think 99% of problems are caused by the fit of quick release dropouts being too small for the large hubmotor washers and nuts. I believe Justin of Ebikes.ca agrees with this, from comments he has posted. He now stocks a special c washer to improve the fit of motor hubs on quick release dropouts.

On alloy dropouts that dont fit, the large washer spans an air space rather than fitting into the cup on the QR forks, often called lawyer lips. This little gap causes a spreading force on the dropout, that when the nut is tightened tends to crack the dropout.

On steel forks that don't fit, the same spreading force usually bends the dropout open into a V shape instead of a U shape.

In either case, this can cause a loosening nut, broken dropout, or an axle that tends to want to creep out of the dropout.

In my opinion, the torque of the motor has little to do with hub spinout. But if you do get a loose nut, then the torque of the motor can easily crack the alloy dropout, but on steel the result is more likely just a bent dropout. On steel you still ruin wires going into the motor when it spins out, but at least the wheel doesn't leave the bike in most cases.

It is true though, that once you do get a case of loose nuts, a powerfull motor does a lot more damage a lot faster. Torque arms do help minimize that damage, but my preference is to rely on good fit and proper installation rather than torque arms.

 

[dogman dan]

Responding to Stig's earlyier question. I have never seen a suspension for with steel dropouts that was not a peice of junk with 40 mm travel. I can live with that on my commuter since I don't need to jump curbs on the routes I ride.

There is a much reposted pic of a fork modification where a steel tube had a good steel dropout welded to it. This was then put over the tubes of an alloy fork, converting it to a steel dropout fork. This, so far, is the best thing I've seen yet. You're going to have to just invent it and fabricate it yourself to get a solution.

 

[The Journey Guy]

Thank you all for your input and opinions!

I have given this careful consideration after reading and thinking about these comments, and have come to the conclusion that I will either have to change out the fork, or get another Trek that comes with a steel fork. I was under the impression that those torque arms would help a lot, but guess I was wrong on that too.

That's why I value this forum. I learn much each day, and that in itself raises more questions that I had before...

Anyways, thanks again, all!

 

[OneWayTraffic]

I'm going to jump in here and ask about this particular bike and its fork.

http://www.avantibikes.com/adventure/discovery8-c.aspx?bid=254

Aluminum frame, Nexus hub, all the commuting stuff already on it.

The fork is a RST 790c.

According to the RST website that fork has a steel casting, stanchion and steerer and an Aluminum crown.

What do you think? I'd be putting an Ezee hub on it. Either this or buying one of the premade electric bikes in NZ.

I'll be going back to NZ next month, and the bike will be one of the first things I buy.

 

[The Journey Guy]

Say, I just had a thought... What about using a carbon fiber fork? Would that be as strong as using a steel one?

 

[OneWayTraffic]

The problem with carbon fiber is that it can be laid up to provide strength exactly where it is needed. The other problem is that, when it fails, it just snaps. No bending or flexing.

It would be strong enough, were it designed to hold a front hub, but they are not. I'd say it's even worse than aluminum.

 

[The Stig]

What I mean't by "failure point" is at what point does the fork usually fail.

Is there agreement on what exactly the main faillure point is?

More specifically, is this ever the case:

They could snap the magnesium fork leg right above the insert, which often seems to be a failure point.

Here is dogmans theory which seems to make sense:

...I think 99% of problems are caused by the fit of quick release dropouts being too small for the large hubmotor washers and nuts. I believe Justin of Ebikes.ca agrees with this, from comments he has posted. He now stocks a special c washer to improve the fit of motor hubs on quick release dropouts.

On alloy dropouts that dont fit, the large washer spans an air space rather than fitting into the cup on the QR forks, often called lawyer lips. This little gap causes a spreading force on the dropout, that when the nut is tightened tends to crack the dropout.

On steel forks that don't fit, the same spreading force usually bends the dropout open into a V shape instead of a U shape.

In either case, this can cause a loosening nut, broken dropout, or an axle that tends to want to creep out of the dropout.

In my opinion, the torque of the motor has little to do with hub spinout. But if you do get a loose nut, then the torque of the motor can easily crack the alloy dropout, but on steel the result is more likely just a bent dropout. On steel you still ruin wires going into the motor when it spins out, but at least the wheel doesn't leave the bike in most cases.

It is true though, that once you do get a case of loose nuts, a powerfull motor does a lot more damage a lot faster. Torque arms do help minimize that damage, but my preference is to rely on good fit and proper installation rather than torque arms.

And if this theory is true then these dropouts would probably work by resisting being bent out into a V shape instead of U a shape:

Attention to detail: the Ronin even has stainless steel inserts in its fork dropouts to prevent corrosion of the mag lowers.

 

[Crash Machine]

I ran my aluminum suspension forks(RSTs in fact) for 1500 miles with dual torque arms. I got tired of worrying about it. I never felt safe. My motor is only 500 watts. Like Dogman said the fit is what made me nervous. Aluminum dropouts are also thicker which doesn't even look right. The only suspension forks I could find are the cheap ones(coil spring) Suntour SR2000 that have steel lowers I believe. I haven't personally used them but did consider it. I finally switched to steel forks and I feel better and don't really miss the suspension forks much. I am just commuting and not going off road.
I know I've posted this before but it's such a classic example of a disaster waiting to happen. Notice how the washer sits funny in the dropout due to the lawyers lips. Also a torque arm didn't fit really well so I had them on on the front which was weird, but worked for a while. I was probably a very lucky...Versus steel forks...

 

[mrzed]

Seconded that carbon fibre is the very worst fork material to mount a motor on.

I also have no issues running a front hub motor, but then I also have a proper fit on a strong steel fork and only put max 650 watts into it. Considering upgrading to about 1000 watts for my next battery, and I'd be fine with that. I think front motors are fine if:

- mounted properly on solid steel forks
- used on roads or smooth trails
- used with low to mid-power motors

I've been thinking a lot about what a nice setup would be for front suspension on an ebike, because if I move to 48v, I'll be running about 40kph top. and would feel better about having something to soak up a bump if my reactions were a little slow. Something like 2"/5cm travel would be enough.

Got me wondering if anyone has tried a hub motor in a mert lawill leader suspension fork. These were a truly different design and I can't remember if they had steel dropouts, but they seem like they just might work. very beefy design adapted from motorcross.

 

[DerekG]

I ran my aluminum suspension forks(RSTs in fact) for 1500 miles with dual torque arms. I got tired of worrying about it. I never felt safe. My motor is only 500 watts. Like Dogman said the fit is what made me nervous. Aluminum dropouts are also thicker which doesn't even look right. The only suspension forks I could find are the cheap ones(coil spring) Suntour SR2000 that have steel lowers I believe. I haven't personally used them but did consider it. I finally switched to steel forks and I feel better and don't really miss the suspension forks much. I am just commuting and not going off road.
I know I've posted this before but it's such a classic example of a disaster waiting to happen. Notice how the washer sits funny in the dropout due to the lawyers lips. Also a torque arm didn't fit really well so I had them on on the front which was weird, but worked for a while. I was probably a very lucky...Versus steel forks...

wow... now that is scary

You rode like that for 1500 miles?

 

[dogman dan]

As pedantic as I am about the "dont use alloy forks" I'd be tempted by the ones with the steel insert, but only if I could make it fit perfect. Usually modifying the washers is a better thing than modifying the fork. With the insert, the washer would never contact the alloy, and if the fit is good, you'd never have a spinout.

The bike in the link earlier in one way traffics post is an example similar to what I have on my commuter, a cheap steel suspension fork with not much travel.
I find them ok for smoothing out heat cracks in the pavement, but a speed bump will bottom em out easy. It also depends on the balance of the bike, if you have a lot of weight in back, the front wheel starts floating anyway.