Front hub motor + Rigid carbon fork = dangerous ?

[thunderstorm80]

Hi,
I have the opportunity of sourcing out new but unused rigid carbon forks that came off new road bikes.
I think to use one for a road bike with a DD motor as an assistant motor, but also as a brake on descents.
The motor is from the family of the 9C+.
The tire is wider than a road tire, with dimensions of 700x47 so the pressure would be 60PSI and not 120PSI like a typical road bike.
I find opposing opinions about whether it's safe or not to use a carbon fork (with a torque arm of course).
What is your take on this?
The torque arm would have to be secured to the frame with a metal cable tie which adds it's own stress as well, and I have no experience at all with carbon fibers...

 

[speedmd]

Too many factors to say for certain that it would be suitable. I would select a disc brake fork for starters as they are typically made much stronger to begin with and could handle the torque arm you need. Also, does your frame allow a tapered steerer tube and concerns exist as to how long is the head tube is. If your frame has a short head tube and only allows a small diameter steerer tube you are increasing stresses proportionally. Lots of testing vids out there on carbon mtn bike forks, and they are strong for certain.

 

[thunderstorm80]

Too many factors to say for certain that it would be suitable. I would select a disc brake fork for starters as they are typically made much stronger to begin with and could handle the torque arm you need. Also, does your frame allow a tapered steerer tube and concerns exist as to how long is the head tube is. If your frame has a short head tube and only allows a small diameter steerer tube you are increasing stresses proportionally. Lots of testing vids out there on carbon mtn bike forks, and they are strong for certain.

The frame accepts a regular 1 1/8 inch head-tube. It's not tapered.
I tend to like rim brakes forks more as they are simpler to install and maintain, and since the motor would be doing most of the braking (and regen) work anyway. Also, most hub motors aren't fully plug-and-play when it comes to disc brake mount.
I think that if I consider the weight savings, than a rigid cromoly fork with rim brakes, would not be much heavier than carbon fork but with a disc brake system, but would be significantly stronger across the board. Is it true?
Regarding the frame's headtube length and the overall headtube length (steerer length) - what does it has to do with stresses? The handlebar part only carries a small portion of your weight.
On one of my E-bikes, which has 2x DD motors, I use a frame with quite short headtube, but the fork is very very long (aluminium fork with suspension) - so I can sit upright comfortably. Is it dangerous? I use standard spacers to fill up the gap and press the stem with a standard crown.

 

[speedmd]

Shorter head tube puts the torque resolving bearing mounts much closer together and amplifies the forces proportionally on both the frame and streerer tube. Just something to consider when looking at what may fail. The fork blade tubes and crowns on disc type forks are much sturdier due to forces acting mainly on one blade and near the wheel mount. Early forks were breaking as they were more like the rim brake versions with disc brake bosses. . Carbon has very different failure mode than most metals so it depends what you are doing and what safety factor you wish to live with and what loads you are expecting. BTW, longer forks will shallow the head angle and greatly increase the torque on the steerer and head tube mentioned above. Just some considerations. If your light weight, I would not worry, but some of the guys here? :lol:

 

[Voltron]

It sounds dangerous. Even if the carbon part is fine, the legs are usually bonded to a pair of aluminum dropouts, and the risk of breaking one open from the axle torque is well documented, esp with regen going too.

 

[d8veh]

Why would you want a carbon fork? It's main advantage is light weight, which will be completely negated when you put a massive DD motor in it. Surely, a steel fork would be stronger and cheaper?

 

[thunderstorm80]

Why would you want a carbon fork? It's main advantage is light weight, which will be completely negated when you put a massive DD motor in it. Surely, a steel fork would be stronger and cheaper?

I was interested in a carbon fork because I had opportunity to buy a new one for practically nothing, and since I build a minimalist road E-bike, I thought it can fit well. (This bike would be lifted on a car rack quite often so every weight saving counts)
It seems from your advices that it's better to go with a rigid steel fork instead, and pay the extra 500gr weight for the sake of safety and reliability.

 

[dogman dan]

Dangerous, but doc that screws your collarbones back together will love the billing.

Almost as bad, any fork that puts lightweight as the foremost design goal. Even steel can be lightened up too much to be a good motor fork.

 

[thunderstorm80]

Dangerous, but doc that screws your collarbones back together will love the billing.

Almost as bad, any fork that puts lightweight as the foremost design goal. Even steel can be lightened up too much to be a good motor fork.

Thanks, but I am already convinced carbon-fork is a bad idea...
I will probably get this one:
http://www.jensonusa.com/Road-Forks/Dimension-700C-Hybrid-Fork?cs=Black
It's a rigid steel fork which seems good for the job: It has eyelets for the fender as well as a place to mount torque arms, and rim brake mounts.

 

[speedmd]

I was going to suggest the old school unicrown fork. Good choice as it is nice and thick everywhere. You can find these in many shop junk piles. Fender and or pack eyelets will be handy to mount torque arms, batteries and even controller. You could get away with a strong cyclocross or road disc brake carbon fork, but I would as has been repeated, forget relying on just the drop outs to resolve motor torque. I would not risk that even with the beefy drop forged steel ones on many of the uni forks.

 

[dogman dan]

That type of dropout on steel forks sometimes does not allow as good contact of the oversize nuts on hub motors, and can result in the nut pressure trying to spit the axle back out of the notch.

At the very least, deepen the notch some, so your nuts don't do that, and put at least one good tight torque arm on it.

A better dropout is the larger steel dropout, with the lawyer lip quick release cup. When used with C washers, you get much less tendency to have the axle spit out of the dropout from just the nut pressure on it. The C washer fills the cup, then resists spitting out some. still best to file any dropout deeper for a motor wheel though.

BEST possible thing though, is take that steel fork, and weld a tab to it, which allows you to bolt on a torque arm, vs using the hose clamp or strap. With the TA bolted on, no way that axle can leave the notch. That is what I did to run 4000 w through a front hub motor.