Is it safe to fit a motor to carbon forks?

Pedalo

Esteemed Pedelecer
Sep 10, 2009
443
1
I'm really tempted by the idea of purchasing a bike such as this:
Flight 04 - Ridgeback

the plan being to fit a Bafang front hub motor. But then I notice it has got carbon forks. Websites such as this make me nervous about carbon forks:

Busted Carbon

Should I steer clear of the ridgeback flight bikes and choose something else?
 

eTim

Esteemed Pedelecer
Nov 19, 2009
607
2
Andover, Hants.
I'm really tempted by the idea of purchasing a bike such as this:
Flight 04 - Ridgeback

the plan being to fit a Bafang front hub motor. But then I notice it has got carbon forks. Websites such as this make me nervous about carbon forks:

Busted Carbon

Should I steer clear of the ridgeback flight bikes and choose something else?
My carbon framed, carbon forked SuperSix seems to run sweetly enough so I can't see a reason why not. But who knows? There are many techniques used to 'lay' carbon to provide strength in different ways, it might be worth doing some research into the forks you are looking at to see how they are manufactured.

The SuperSix is a really comfy road bike even with the front hub motor.
 

overlander

Esteemed Pedelecer
Apr 22, 2009
433
4
Don't think its an issue as the top end wisper is carbon fibre. I know not exactly the same but the Trek FX+ has carbon forks, well the 2010 model did but they removed them for 2011. I believe this was a cost rather than mechanical issues. So if the extra weight and speed is your issue i would say there is no problem. As for torque from the motor, no idea i'm afraid.
 

flecc

Member
Oct 25, 2006
46,476
23,295
The top end Wisper is actually a composite frame, alloy thin wall overlaid with carbon fibre to give combined strength.

I'd be a bit wary about a motor in carbon only forks, considering the sudden failures there have been on carbon only frames and forks even unpowered.
.
 

Kalkhoff USA

Finding my (electric) wheels
Sep 15, 2010
6
0
Probably not a good idea.

A quality carbon fork is as strong or stronger than its equivalent in metal, but there's nothing gradual about the failure mode. Metal will bend or crack before separating completely, but carbon will just come apart when pushed too far.

The other important thing to consider is that the fork isn't designed for the torque and load that a hub motor would apply to it. Actually, being designed for the torque from a disc brake it's actually built stronger for torque in the opposite direction from a hub motor.

There does seem to be an opportunity for someone to manufacture a hub-motor specific carbon fork, but it may be that there's not enough demand yet to make it worth it.

Check with the manufacturer (of the bike) for a definitive answer.
 

Scimitar

Esteemed Pedelecer
Jul 31, 2010
1,770
38
Ireland
I'm really tempted by the idea of purchasing a bike such as this:
Flight 04 - Ridgeback

the plan being to fit a Bafang front hub motor. But then I notice it has got carbon forks. Websites such as this make me nervous about carbon forks:

Busted Carbon

Should I steer clear of the ridgeback flight bikes and choose something else?
Personally, I'd avoid putting a hub motor on carbon forks, I just don't trust them in that application.
 

Pedalo

Esteemed Pedelecer
Sep 10, 2009
443
1
O.K. Thanks for the fedback. I think the advice is clear. I would like a fairly lightweight bike but not at the cost of reliability and the risk of catastrophic failure would make me nervous about riding it.

I'll choose something different.
 

Lloyd

Pedelecer
Jan 22, 2010
166
0
Thing with carbon is most people assume that people go with it for weight advantages, and this is often not the case. Carbon is used particularly in forks because with carbon, unlike many metals, you can change the characteristic of the tube. This is done by altering the type of weave (the thickness of the strands) and the lay-up pattern (the angles of intersection if using multi-dirctional or the angle of laying the uni-directional fibres). By doing this you can make a carbon tube strong, yet stiff in one direction and more compliant in another, depending on the circumstance. The aluminium equivalent will be light and stiff, but with no compliance at all. I personally wouldn't use a hub motor on a carbon fork, but if weight is your reasoning for considering it, you would be far better off looking elsewhere on your bike to save weight. You could fit some Easton forks, but again with light weight comes a sacrifice, and personally I would not want to sacrifice on the area that holds my motor!

Another thing to bare in mind about carbon is that, like everything else, there is cheap carbon and there is expensive carbon. Cheap frames are made in one piece, with no ability to inspect the internals after the curing process. This leads to high failure rates and unreliability. Alarmingly many manufacturers still use this method! Once built it is incredibly difficult to tell the difference.

The expensive stuff is made in usually 2 or 3 seperate moulds, where the tubesets are inspected before finally assembly, another mould and more curing. This means your resin percentage (amount of glue left in the frame) is particularly lower, hence lighter and stronger. Having seen the inside of the tubes before assembly also means any imperfect ones are binned before final assembly, not "discovered" by the end user when it snaps!

I can feel myself rambling now so will stop typing and eat my lunch :D
 

Streethawk

Esteemed Pedelecer
Jan 12, 2011
634
15
I think you can be pretty sure that if the fork sports a disc brake mount it can handle the torque of a 250w motor easily. Braking torque from high speed is many times higher.

I'd not try it on bladed carbon road bike forks though.
 

overlander

Esteemed Pedelecer
Apr 22, 2009
433
4
I think you can be pretty sure that if the fork sports a disc brake mount it can handle the torque of a 250w motor easily. Braking torque from high speed is many times higher.

I'd not try it on bladed carbon road bike forks though.
But the doubt is the torque from the motor is in a different direction from the disc. As previously stated it depends on the weave of the fibre what direction flexes. But would personally think that a torque rotated 180 degrees would be ok but better to be safe than sorry.
 

Streethawk

Esteemed Pedelecer
Jan 12, 2011
634
15
But the doubt is the torque from the motor is in a different direction from the disc. As previously stated it depends on the weave of the fibre what direction flexes. But would personally think that a torque rotated 180 degrees would be ok but better to be safe than sorry.
Agreed, different direction, but the same axis, so it should be ok. I wouldnt do it either, but my reason would be that such light weight is just not required when you have a motor. Light-ish short travel suspension forks would be the best choice. If money allows and you're using 26" wheels i'd fit a Fox F80 fork.