carbon forks and hub motors?

[Bikes4two]

Now I know there's been many a discussion on whether it is wise to fit a front hub motor to a bike with carbon forks, but here's the thing....

• I met up with a cycling buddy who I've not seen for many a year and they now ride electric and has three bikes with the Cytronex C1 kit (link) a front hub system.
• They were riding a nice full carbon frame and I queried the hub motor on carbon forks etc and asked if Cytronex had been ok with that and yes, they were happy with the fit (and my buddy lives near the Cytronex shop in Winchester and had visited there a number of times, so face-to-face dealings, not via some anonymous web order form).
• The bike has been ridden a few thousand miles of the 5 years of conversion and all is fine. Oh, and yes, there's no Torque Washer either.
• Searching the Cytronex and indeed the Swytch Bike website, there is no mention of NOT fitting to carbon.
• However, the Woosh site does explicitly say hub motors are not for carbon forks.
• Might whether to fit a motor to carbon forks depend on the torque of the motor as typically the Cytronex motor (shengyi I think) and the Swytch (AKM 100SX) are around 40nm?
• So clearly views differ and I'd be pleased to hear from anyone who has fitted a hub motor to carbon forks and their experience.
• And if anyone knows of riders who have/have not had problems or you have direct knowledge of their experiences (and please, not someone else's view gleaned from an internet forum or other anecdotal source as there's enough of that around already!), I'd be pleased to hear your views and of course those of @Woosh .

Many thanks.

 

[saneagle]

Now I know there's been many a discussion on whether it is wise to fit a front hub motor to a bike with carbon forks, but here's the thing....

• I met up with a cycling buddy who I've not seen for many a year and they now ride electric and has three bikes with the Cytronex C1 kit (link) a front hub system.
• They were riding a nice full carbon frame and I queried the hub motor on carbon forks etc and asked if Cytronex had been ok with that and yes, they were happy with the fit (and my buddy lives near the Cytronex shop in Winchester and had visited there a number of times, so face-to-face dealings, not via some anonymous web order form).
• The bike has been ridden a few thousand miles of the 5 years of conversion and all is fine. Oh, and yes, there's no Torque Washer either.
• Searching the Cytronex and indeed the Swytch Bike website, there is no mention of NOT fitting to carbon.
• However, the Woosh site does explicitly say hub motors are not for carbon forks.
• Might whether to fit a motor to carbon forks depend on the torque of the motor as typically the Cytronex motor (shengyi I think) and the Swytch (AKM 100SX) are around 40nm?
• So clearly views differ and I'd be pleased to hear from anyone who has fitted a hub motor to carbon forks and their experience.
• And if anyone knows of riders who have/have not had problems or you have direct knowledge of their experiences (and please, not someone else's view gleaned from an internet forum or other anecdotal source as there's enough of that around already!), I'd be pleased to hear your views and of course those of @Woosh .

Many thanks.

Every bike is different. You can't use general rules. Some carbon forks have metal inserts for the drop-outs, and some have thin structure and no strength to resist torque. Front motors are pretty bad on most bikes. It's best to avoid if you can.

 

[Az.]

A lot will also depend on how you ride your bike and where.
I have two e-bikes with front hub motors. Both with low powered motors and both survived without torque arms just fine. One aluminum low end suspension fork and one steel rigid.

Having that said I consider it to be poor and risky design. Every pothole, kerb, rock, branch is a risk of wheel flying out. I think it is not worth to check how far you can push the boundaries. Health and life of yours and your partner are worth more than few £saving. I agree with Seneagle - avoid if you can.

 

[matthewslack]

It will be fine until it isn't.

The dropouts of a non-electric bike are not designed to resist torque. Their ability to do so is qualitatively a happy accident, and quantitatively dependent on material properties.

Every application of torque forces the edge of the axle flat against the dropout where a groove or notch may form, from which a crack may start, and in addition the torque tries to open up the dropout slot, which it was not designed to resist.

Some peace of mind might come from regular removal of wheel, cleaning, and careful inspection, but carbon has a reputation for sudden failure.

Maybe wear a chin protecting helmet just in case!

 

[saneagle]

Maybe wear a chin protecting helmet just in case!

And a face guard!

It's not just the safety aspect. The gyroscopic effect of the motor can have a profound effect on the steering and handling of the bike, especially on lightweight bikes. Steering geometry seems to play a significant factor in that too. Some bikes are a lot worse than others.

 

[Bikes4two]

Some interesting and informed comments - thank you. I perfectly understand them, even though my understanding of mechanical forces in structures such as bike frames, forks, drop-outs and so on, is rather basic, but one of the comments above made me think about braking forces on forks too:

Every application of torque forces the edge of the axle flat against the dropout where a groove or notch may form, from which a crack may start, and in addition the torque tries to open up the dropout slot, which it was not designed to resist.

So please bear with me - every day's a school day, especially at my age as what I learnt at school is starting to fade anyway :

Take say a 90Kg rider travelling down hill at 20+ mph and then suddenly braking hard. The brakes are attempting to stop the rotation of the wheel and the reterding force is applied to the rolling mass (of rider plus bike) through the dropouts then fork then frame, and carbon frame or not, you'd not expect the fork to fail would you.

Are these braking forces, although in opposite rotational directions, much greater than that from a 40nm (and that's at max power) hub motor?

 

[chris_n]

The braking forces are carried by the brake caliper, whether rim or disc, not the dropout.

 

[Woosh]

The problem with carbon fork is not so much braking force as when the front wheel hits a pothole at speed. The shock generates several Gs, possibly 3Gs to 5Gs. A normal wheel on a carbon bike weighs less than 1kg whereas an XF07 front motor wheel with tyre and innertube can be as much as 3kgs to 3.5kgs. At 5Gs, that's 15kgs and that force can crack or even shatter the legs. We cannot ignore the risk.

 

[matthewslack]

Some interesting and informed comments - thank you. I perfectly understand them, even though my understanding of mechanical forces in structures such as bike frames, forks, drop-outs and so on, is rather basic, but one of the comments above made me think about braking forces on forks too:


So please bear with me - every day's a school day, especially at my age as what I learnt at school is starting to fade anyway :

Take say a 90Kg rider travelling down hill at 20+ mph and then suddenly braking hard. The brakes are attempting to stop the rotation of the wheel and the reterding force is applied to the rolling mass (of rider plus bike) through the dropouts then fork then frame, and carbon frame or not, you'd not expect the fork to fail would you.

Are these braking forces, although in opposite rotational directions, much greater than that from a 40nm (and that's at max power) hub motor?

There are two forces to be considered, and they will have been formally analysed during design.

1. At the headstock.
2. Equal and opposite reaction to force developed between brake pads and disc.

Both of these act at far greater distance from the axle centre, and unlike the motor, neither apply torque to the axle itself, and so do not create torque between axle and dropout.

Torque is a turning force, and can be thought of as a force acting at a radius. For a given torque value, the smaller the radius, the larger the force.

Take a 40Nm motor with a 12mm diameter/10mm across the flats axle. The radius is 6mm, so the force is 40,000Nmm / 6mm = 6,667N. This is split between both ends of the axle, and both sides at each end, assuming all are in equal contact with their dropout slots, so 1,667N or in everyday units 170kg of force acting on each of those edge contact points.

That's if your wheel nuts are loose of course - with tight nuts, a proportion of the torque is transmitted by the wheel nuts and washers to the surfaces of the dropouts instead, but still at rather small radius. These are not small forces.

Compare with a typical 160mm brake disc, and for comparison the same 40Nm torque. Obviously braking forces can be higher than this.

The disc contact surface is about 10mm wide, so the mean diameter at which the force acts is about 150mm, or 75mm radius. That is 12.5 times 6mm, so the reaction force is 12.5 times smaller, or only 8% of the force that the dropouts need to resist from the motor: a mere 13.6kg, divided between the two caliper mounting bolts, so only 6.8kg each.

That is not a perfect analysis, because I did not take into account the position of the caliper mounting bolts, but you get the idea.

Edit: The dropouts do have to match that braking force, but not as a torque, so there will be the same 13.6kg force between the forks and the axle to transfer the braking force to the wheel and then the road.

The forces up at the headstock bearings have to take into account the height between axle and headstock, and the distance between the upper and lower races. I'll leave that one for some else to look at.

 

[Bikes4two]

Some very interesting points raised here and I'm very greatful for the thought folks have put into their answers .

.. and this comment has enforced my views on how little I understand these things ....

The braking forces are carried by the brake caliper, whether rim or disc, not the dropout.

.... durgh! I hadn't apperciated that particular point especially as it's so obvious .

The next time I'm in Winchester I'll pop into Cytronex and raise this with them to see what they have to say.

Mind you, there's my buddy who has been riding for 5 years without incident. There are two other Cytronex riders in my local group but I don't now what their rides are, but I'll enquire.

 

[chris_n]

Edit: The dropouts do have to match that braking force, but not as a torque, so there will be the same 13.6kg force between the forks and the axle to transfer the braking force to the wheel and then the road.

The dropouts do not see those forces it is effectively the other way around. The force they see is the force involved in the retardation of the mass of the bicycle and the rider against the contact point of the tyre. This can of course be supplemented by the forces involved in hitting a pothole etc with the front wheel while maximum breaking is applied. The forces at the headstock are much higher due to the length of the lever in such a scenario which is why many (most? not completely up to speed with current designs) carbon forks still have a steel steerer tube and steerer tubes in general have been getting larger over the last 15 years or so.

 

[chris_n]

Some very interesting points raised here and I'm very greatful for the thought folks have put into their answers .

.. and this comment has enforced my views on how little I understand these things ....

.... durgh! I hadn't apperciated that particular point especially as it's so obvious .

The next time I'm in Winchester I'll pop into Cytronex and raise this with them to see what they have to say.

Mind you, there's my buddy who has been riding for 5 years without incident. There are two other Cytronex riders in my local group but I don't now what their rides are, but I'll enquire.

I was about to pop out when I posted that but would add that in my mind subsequent posts and posts made while I was typing have covered what I would have said had I had more time. First as @mathewslack said "it will be alright until it isn't ", this is one of the biggest problems with carbon in that you will see no deformation before it fails. The same is true for cast aluminium so MTB type forks for me would be a no, conventional shaped forks in welded aluminium would be much better.
Secondly as mentioned by @Woosh the mass of a motor is significantly higher than a conventional hub so the forces that could generate are much higher.
A third point is that not all carbon fibre is equal, a well designed component with a carbon layup to suit it's purpose that has the correct resin and is autoclave (think F1 components) has significantly better chance of working than something mass produced with epoxy resin.

 

[saneagle]

Some interesting and informed comments - thank you. I perfectly understand them, even though my understanding of mechanical forces in structures such as bike frames, forks, drop-outs and so on, is rather basic, but one of the comments above made me think about braking forces on forks too:


So please bear with me - every day's a school day, especially at my age as what I learnt at school is starting to fade anyway :

Take say a 90Kg rider travelling down hill at 20+ mph and then suddenly braking hard. The brakes are attempting to stop the rotation of the wheel and the reterding force is applied to the rolling mass (of rider plus bike) through the dropouts then fork then frame, and carbon frame or not, you'd not expect the fork to fail would you.

Are these braking forces, although in opposite rotational directions, much greater than that from a 40nm (and that's at max power) hub motor?

Rim brakes apply the torque to the fork crown or high up the forks where the legs are strong. If the forks are designed for disc brakes and therefore have the caliper mounting points, the legs will be made much stronger to deal with the torque low down on the fork. That means that disc brake carbon forks have a better chance of dealing with a hub-motor as long as the drop-outs are strong enough.

 

[chris_n]

Rim brakes apply the torque to the fork crown or high up the forks where the legs are strong. If the forks are designed for disc brakes and therefore have the caliper mounting points, the legs will be made much stronger to deal with the torque low down on the fork. That means that disc brake carbon forks have a better chance of dealing with a hub-motor as long as the drop-outs are strong enough.

Now if you can find a carbon fork designed for a drum brake you would be on to a winner

 

[Bonzo Banana]

The Swytch kits (and maybe Cytronex) are very weak ebike systems providing light assistance only and using small weak hub motors. They typically have low capacity battery packs and low current controllers. I personally wouldn't use carbon forks at all myself being a heavy rider but I can see the case for carbon fibre forks being OK with very weak hub motor setups. There has been a lot of recalls in the past for carbon fibre forks and they seemed to be more about the carbon fibre forks that were carbon fibre blades glued/bonded to an aluminium steerer where the bonding would fail over time. Full carbon fibre forks despite being lighter didn't have that particular issue. So not all carbon fibre forks are the same. It could be full carbon fibre would cope better than glued CF blade forks. You might say that a Decathlon road bike with bonded CF forks with a weight limit of only 100kg total including the bike weight itself were a greater risk than a Halfords Carrera road bike with full carbon fibre forks and a weight capacity of 120kg for the rider alone like the Vanquish model. Some road bikes and CF forks are more overbuilt than others and this would likely be indicated by their weight rating and guarantee period.

 

[Nealh]

If one really wants a front motor then Surly Disc trucker forks imv are the perfect partner for a hub, my Bafang G370 has been faultless and fitted perfectly in to the drop outs.
Make sure the forks are nice sturdy ones and steel , at over £125 Surly DT's are cheap but they are very good.
Forks should be rigid and not suspension loaded.

 

[Bikes4two]

A useful insight Bonzo - thankyou.

The Swytch kits (and maybe Cytronex) are very weak ebike systems providing light assistance only and using small weak hub motors. They typically have low capacity battery packs and low current controllers.

You make a fair point about 'weak' motors and 'low' current. I've been in contact with two cycling buddies now, both have the Cytronex on carbon folks and both are still able riders just needing a bit of assistance on some hilly bits (and one of those riders has another Cytronex on their tandem too).

Each have done a fair number of 100km rides and still had battery charge left, and given that the C1 battery is just 180Wh (or 198Wh for the newer version) then this tells you that (a) they don't use much assist and (b) when they do, it's probably only a small amount and I'm guessing, that the motor isn't exactly a 'newton metre' monster.

 

[saneagle]

A useful insight Bonzo - thankyou.



You make a fair point about 'weak' motors and 'low' current. I've been in contact with two cycling buddies now, both have the Cytronex on carbon folks and both are still able riders just needing a bit of assistance on some hilly bits (and one of those riders has another Cytronex on their tandem too).

Each have done a fair number of 100km rides and still had battery charge left, and given that the C1 battery is just 180Wh (or 198Wh for the newer version) then this tells you that (a) they don't use much assist and (b) when they do, it's probably only a small amount and I'm guessing, that the motor isn't exactly a 'newton metre' monster.

I would have said that an ebike system that can get 100km from 180wh is more hindrance than help, especially as only about 120w will be converted into propulsion energy. The added mass will cause you to use more than 180wh extra over 100km. For each kg of mass you add, you have to pedal 1% harder up any incline. Cytronex is about 3.5 kg, so that would be pedalling 3.5% harder along any road that isn't horizontal or downhill, or it'll slow you down by 3.5%.

 

[matthewslack]

There is a very particular niche requirement which I and probably many others have, and for which there is not yet a perfect solution.

I can no longer manage the small proportion of serious uphill riding without assistance. I just wouldn't ride at all in my hilly neck of the woods without assistance, but I could perfectly well manage without it for about 75% of the time. I would have to travel lighter and my average speed would drop, but it would be fine.

So I can understand the very low average Wh per mile from riders riding that way.

 

[Bikes4two]

I would have said that an ebike system that can get 100km from 180wh is more hindrance than help, .......

I understand where you're coming from of course, but these riders and others like them, aren't worried about the added weight and are happy to trade that off to get the assistance when they need it on the hilly parts.

I've stripped back one of my TSDZ2 bikes (allow frame, carbon forks, and just a tool bag and mudguards) and use an 8Ah battery and am able to comfortably ride on the flat with little or no assistance because the bike is light and I'm not carting around some heffer-lump of a 17Ah battery, but the power is there when I get to the hills: 50-60 miles for me on the 8Ah battery.

If I had the inclination to spend £1k on an ebike conversion, I think I'd rather like the Cytronex and would I put it on my carbon Trek Madone? Quite possibly.