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Motors in front forks

chris Peachey2019-04-11 14:17:34

Thorn's response in the current journal is the only one any manufacturer can give regarding something they have not manufactured and/or tested.

Front forks are not useually designed for retro-fit motors and serious injury can occur if the forks are not up to the job. (read Trevor Jarvis' article in a previous journal.)

My advice would always be to consult an experienced frame builder regarding the need for new forks suitable for a motor.

It's always worth considering a new machine as the number of E-Tandems being manufactured is increasing.

 

 

Terry Barnaby2019-04-11 14:48:51

To back this up, there has been at least one, possibly two front fork failures in the West Country Tandem group due to an electric motor on the front forks. The one I know about occurred after a year or so of use ...

John Blackthorn2019-10-23 23:49:39

Does anyone know the mode of failure in the forks mentioned by Terry?

It seems seriously unlikely that the forks bent forward as the motor pulled the tandem forward, what seems more likely is that the throat of the axel fittings at the bottom of the forks opened up due to the torque applied by the flats on the axel. Any info would be appreciated.

Martyn Aldis2019-10-25 10:09:23

I don't agree that fork blade failure is seriously unlikely over a period of use. It's not a question of the forks bending on first use it is likely to be fatigue failure over repeated application and reversal of loading.

The untapered unicrown steel forks on our year 2000 Dawes Discovery Twin failed without any help from a motor.

I had noticed that all the flex in the fork blade on bumpy roads was close below the point were it bends inwards towards the steerer tube. That is where it failed, fracturing about 80% of the way round.  Luckily we had fitted an XT front hub with a robust steel axle and the Tubus Tara front rack may have also helped keep the almost severed fork blade in place. I only found the bone chilling damage when I was cleaning the bike after we got home. I did hear a sharp click when we went over a small pothole about 8km from home so it seems that was when the blade fractured.

The replacement fork from Dawes is intended for a disk brake. It is much heavier and gives a harsher ride than the original or the tapered Reynolds "R" forks on our Thorn Adventure. If I felt inclined to try a front hub motor on a tandem, it would be with a tandem disk brake fork but not with a disk brake fitted.

Terry Barnaby2019-10-25 11:56:24

As far as I remember, (and memory fails sometimes!), on one of those tandems the fork broke where the forks crown is connected to the steering tube through the headset. So the forks and wheel fell away. Luckily the people were not going too fast at the time. It was surmised by people that the failure occurred due to the repeated forwards and backwards force on the lower end of the fork blades due to the electric motor where the blades are only designed to take the braking force of the V brakes. Of course it could have been a fault in the fork blades to begin with, it would be hard to find this out, but having two forks fail in our region with motors on them is, at least,  suspicious.

John Saunders2019-10-25 14:31:15

We too are looking to add a motor on a front hub, as prevoius entries say no one will commit to whether the carbon forks will take the torque, most are telling us " dont risk it ". We are taking our Tandem to get the opinion of Cytronex next week ( the supplier of the motor kit ) who have one installed on their Cannondale with carbon forks, but I am not expecting a full blown confirmation that it will work.

Achim Gloger2019-10-25 15:12:24

The carbon layup of any carbon fork was never designed to take the very different stresses a front hub motor exerts on a fork. I would walk, no, run away from anyone saying it's ok to use a "standard" carbon fork in such an application. Ask for evidence of any long term stress testing supporting such claims. 
 

Anyway, it's your teeth, your face, your head that will hit the deck first. Your stoker probably won't be too happy either...
 

if you absolutely can't manage without electric assistance, go for a rear hub motor. 

John Blackthorn2019-10-31 22:17:48

Thanks everyone, our tandem is Chinese but it has had a fork replacement before we bought it, I fitted a front drive motor but the flat bits of the fork blades were already worn and slightly splayed, almost allowing the axel to rotate, obviously after some testing the axel did rotate and I've fixed that, there's some evidence that this tandem had previously been fitted with a front motor but after two failures the previous owners replaced it with something more suitable.

However the fact that I have fixed that problem is neither here nor there, the problem is evidently at the junction of the forks and the steerer tube.

Martyns advise about twin disk forks makes a whole lot of sense, it's also good to know that they might not break quickly and thorough inspection before and after each use might save a lot of pain and discomfort.

Again, thanks.

John Saunders2019-11-23 12:45:53

I have contacted Cyntronex and they will not commit on the forks , I have approached some steel forks suppliers who also will not commit! I have also contacted Santana ( manufacturer ) in USA who have no test reults and also will not commit.

I see from the journal David Fisher was also looking at the same supplier and wonder his outcome.

 

 

Andrew Brooke2019-12-04 18:32:36

Due to ageing I decided to fit electric assistance to our Longstaff tandem trike. As I was doubtful about the effectiveness I initially went for the cheapest solution i.e. a front hub motor. At first we were delighted with the rejuvenating hill climbing help, but after a while we started to get apparent problems with brake adjustment which I was unable to sort out. Eventually when making yet another attempt one of the fork legs fell off! For some time one fork leg had been taking all the strain while the other had fractured where a dynamo mounting plate was acting as a stress raiser. After Longstaff had repaired the fork (without the dynamo plate) we fitted a bottom bracket motor which was both easier to fit and better in operation.

John Saunders2019-12-04 19:45:30

I am about to put a front wheel motor on our Tandem, may I ask which bottom bracket system you went for and how difficult. Did you use the rear bracket ? What about all the wiring aspect and cables ?

Andrew Brooke2019-12-04 19:55:43

I have used Bafang BBS and Tongshen TSDZ2.

 Both fit the bottom bracket without the eccentric but the Bafang has more clearance. The Tongshen may not clear under bracket cable runs. This is one reason I would favour the Bafang.

If you feel competent in removing the old bottom bracket you will find fitting the motor kit easy. 

In my opinion you can happily lose the double/triple chainring because the motor power compensates, but the Tongshen can take a second ring as  supplied while the Bafang would need an adapter (available but maybe not from UK).

The chain run is pushed outward and far from ideal and you may need a chain keeper device.

If your timing chain is on the left with a square taper crank fixing it should fit OK. If the timing chain is straight through you can still fit the original crank as a single ring on top of the kit chainring. You may need to fit a wider front bottom bracket to keep the timing chain aligned.

The supplied cabling is long enough for normal tandems.

I prefer to use double sided velcro rather than cable ties to secure the wiring because it makes it easier to adjust without wasting cable ties.

250 watt labels are available on eBay to confirm your motor is legal.

Sorry if I have over complicated things. It is actually quite straightforward.