Suspension fork suitable for hub motor?

Electrically assisted bikes, trikes, etc.
rotavator
Posts: 143
Joined: 6 Jun 2016, 9:50pm
Location: North Wales

Suspension fork suitable for hub motor?

Postby rotavator » 28 Jan 2018, 1:02pm

I am thinking about putting a hub motor (may be the 250W one from cycotricity) on the suspension fork of my MTB but I have seen various warnings on the web about this but does anyone on this forum have any experience or knowledge to share?

The current suspension fork has Al alloy drop-outs and and I have seen a warning that hub motors should not be used with Al forks. There may be two reasons for this: 1. the fork buckles under the torque and/or 2. the motor spins out of the drop-outs; both of which could lead to a nasty accident. If this is a significant issue then the obvious solution would be to replace the suspension fork with a steel one e.g. Surly Troll or Ogre.

I have also noted that Cyclotricity do not give any warranty for their motors spinning out of the drop-outs so is this a common problem? Another blogger reckons that front hub motors should really be fitted with a torque arm rather than just the small "teeth" that engage with the drop-out. If so, can you point me towards one please.

hemo
Posts: 250
Joined: 16 Nov 2017, 5:40pm
Location: West Sussex

Re: Suspension fork suitable for hub motor?

Postby hemo » 29 Jan 2018, 12:19am

Any ali fork requires a Torque Arm (T/A) if not two, thin drop outs will eventually stress and fail from the axle torque even with T/A's.

My Suntour Nex 700c fork with 350w BPM has been fault less in over 2k miles with T/A's fitted the D/O area is quite meaty and thick. With the Nex fork I was able to fabricate two small T/A's and tap a fixing directly in to the flat D/O area.

Reba 26" forks with same hub and a T/A fitted failed with only appprox 500 miles use, the Reba D/O is weak and thin and has a lot less meat around the D/O compared to the Nex. Only the T/A stopped the wheel from departing form the bike.

The T/A fits over the axle tightly then need anchoring to the fork securely and then the T/A is retained by the axle nut.

12mm T/A is the one you need, the hard bit is finding some where to anchor the T/A too, one answer is to make an F shape bracket that is secured by the disc brake lugs and then secure T/A.
https://www.ebay.co.uk/itm/Torque-Arm-f ... NGXCSPZWHA

In pic 3 on post 2 of this thread is an example of a neat diy T/A fitting using the above T/S and an F bracket.
http://www.pedelecs.co.uk/forum/threads ... 965/page-2

rotavator
Posts: 143
Joined: 6 Jun 2016, 9:50pm
Location: North Wales

Re: Suspension fork suitable for hub motor?

Postby rotavator » 29 Jan 2018, 1:49pm

Thanks for the reply hemo.

It seems to me that the "belt and braces" method would be to replace my suspension fork with a Surly Troll fork because it is made from steel and has plenty of eyelets for attaching the torque arms to. Do you agree?

hemo
Posts: 250
Joined: 16 Nov 2017, 5:40pm
Location: West Sussex

Re: Suspension fork suitable for hub motor?

Postby hemo » 31 Jan 2018, 10:19am

With steel forks and a 250w hub motor all you need to fit is the anti rotation washer inboard as steel is much stronger then aluminium cheese dropouts and secure the nuts as normal, though fitting a T/A is optional on steel but very necessary with Aluminium. The fact there is a threaded eyelet available makes fitting any T/A a simpler affair though means you do need to have an allen key and also the 17mm nut spanner in your kit in case of punctures if using tubes are used.
Depending how deep the dropouts are you may have to deepen the profile by about 2.5mm to accept the 12 x 10 hub motor axle fully and eke out the 9mm fitting if it is tight, a round file or a dremel tool can be used, as per all D/O the seat must have a radius rather then a square corner to prevent stress fracturing occurring.

Greenbuilder
Posts: 88
Joined: 17 Nov 2012, 6:24pm

Re: Suspension fork suitable for hub motor?

Postby Greenbuilder » 8 Feb 2018, 1:50pm

Hi Rotavator,
I would advise against using alloy suspension forks with a front hub motor. I tried it a few years back on some rockshox and the anti-rotation washers tore the drop outs apart resulting in an over the bars crash, at low speed fortunately.
I have fitted quite a few front hub motors to mtb's since then, always onto steel fork retro-type mtb's, mostly with a torque arm bolted to mudguard eyelet. Torque arms can be had off various eBay suppliers.
I do have a used Surly 26" suspension corrected steel fork if you need one.
cheers
Tony

rotavator
Posts: 143
Joined: 6 Jun 2016, 9:50pm
Location: North Wales

Re: Suspension fork suitable for hub motor?

Postby rotavator » 8 Feb 2018, 5:22pm

Hi Greenbuilder,

Your Surly fork could be just the job. Which is it and is it in good condition? Axle to fork crown distance should be about 450 mm to fit my bike. Also what is the length of the steerer?

Where abouts are you and how much do you want for it?

Greenbuilder
Posts: 88
Joined: 17 Nov 2012, 6:24pm

Re: Suspension fork suitable for hub motor?

Postby Greenbuilder » 10 Feb 2018, 11:13am

Hi Rotavator
the fork was a steel Surly Instigator I think, with disc mounts; unfortunately it sold yesterday on another mtb forum.
I have another one, quite similar on an mtb/hybrid hub gear bike currently for sale on this forum, if no one goes for the bike soon I will split it and give you first dibs on the forks. I usually buy them from Triton or Winstanley bikes.
cheers
Tony