Forks for fwd e-bike

Electrically assisted bikes, trikes, etc.
brynpoeth
Posts: 6960
Joined: 30 Nov 2013, 11:26am

Forks for fwd e-bike

Postby brynpoeth » 28 Nov 2017, 6:41pm

I have ridden a bike with a loose headset, it bucked like a wild horse

When an e-bike has front wheel drive the force has to be transmitted to the rest of the machine

How strong do the forks need to be? Does the drive cut in suddenly? Would rear drive generally be preferable?
Cycling? Of course, but it is far better on a Gillott.. Alternative facts welcome

hemo
Posts: 245
Joined: 16 Nov 2017, 5:40pm
Location: West Sussex

Re: Forks for fwd e-bike

Postby hemo » 28 Nov 2017, 9:34pm

I have a powerful Bafang BPM front hub and have used in 3 std alI forks, 700C rigid & 700c sus and currently on a 26" rockshock. As long as the headset is tight and no play then you should have no issues.
Pas feeds power in a bit gentler then a throttle if used, both options provide no issues barring a bit of wheel slip now and then in slippy conditions.

Geoff.D
Posts: 1943
Joined: 12 Mar 2010, 9:20pm

Re: Forks for fwd e-bike

Postby Geoff.D » 29 Nov 2017, 9:18am

I glad that brynpoeth has raised this question.

I'm currently trying to squeeze a 36v 240w motor into standard (60's/70's) roadster forks. And I asked a similar question in the technical section. There were some answers that clearly felt that standard forks would not be strong enough to cope, especially with the increased torque under acceleration from rest. Also, there were answers suggesting that widening (cold setting) the fork legs was not a good idea.

I note that (some) retro-fitted motors to the Brompton require the forks to be spread to 106mm (from 90mm) and 2mm to be filed from the drop out slot width. It would seem that both these mods still leave the forks strong enough. I assume that the manufacturer of the aftermarket kit, who also offer a fitting service, have the confidence not to be drawn into legal proceedings.

I had thought that my questions were engineering ones and therefore placed them on the Technical board. But, noting hemo's response, I could ask them here, too. With regard to strength and suitability, has anyone any experience of (or thoughts on) -
1. Cold setting a pair of 700c front forks by an extra 9mm
2. Taking 1mm off each side of the drop out slot?

hemo
Posts: 245
Joined: 16 Nov 2017, 5:40pm
Location: West Sussex

Re: Forks for fwd e-bike

Postby hemo » 29 Nov 2017, 9:45pm

Generally most front forks D/O's are for 9mm axles. Most hub motors have a 12mm x 10mm axle so filing about 1mm is usually always required, also the nuts usually have to sit in the D/O's. Some forks have a small recess for the nut or QR to tighten into so often the seat of the D/O's need filing 2 or 3 mm deeper as well.
In general as long as there is plenty of meat to the D/O area there shouldn't be an issue, just make sure you leave no square seats.
Most hub axles have A/R/W's (Anti rotation washers) with a right angle tab this needs to sit tightly in the D/O out so the need to file them deeper becomes clear.
Aluminium may be a bit tricky spreading but steel should prove to be no problem, as with all things a little at a time gently as you go. 3 X 2 deal and folding wedges may be the way to go and leave over night, though expect some spring back when pressure released so may need spreading 5 -10mm more then required to allow for spring back. Xiongda do a 2 speed winch like rear hub that is 146mm wide these are often fitted in to std rear 135mm frames after a bit of spreading initially to 155mm.