Suntour forks

[alsmith]

Is anyone using Suntour forks on a front wheel drive e-bike? I just got an email from them saying they don't recommend any of them (apart from an SR pedelec kit sold direct to manufacturers). If so which model, and what are your experiences?

 

[tony67]

It is generally advised against using front motors on suspension forks. I suppose low power levels could be used fairly safely but any kind of medium to high power would exert forces they aren't designed to withstand. Suntour are not exactly high end either so would be less capable than other more solid units. My own bike has cheap suntour forks and TBH I'd never consider a front motor with them.
Tony

 

[amoras]

hmmm, I wonder why they have a suspension fork named Ebike?
http://www.srsuntour-cycling.com/index.php?screen=sh.detail&intendenduse=E-BIKE&tnid=3287

 

[neptronix]

It's probably designed for 250w

Front suspension forks with hubmotors on them are a total no-no. They are already flexy as is with disc brakes on them.

 

[dogman dan]

Well, at least for experts anyway. Noobs should not put a motor onto an alloy fork. Dig deep enough, and there is a thread in the pix section showing how I do it. I went through 4-5 forks before finding one compatible with the motor and the torque arms. It's not an easy deal, and when you are done the forks don't work right anymore in many cases.

 

[alsmith]

Thanks. I've seen fixed forks that say suspension adjusted- it seems to be the length of the fork when moving from suspension to fixed. What effects will longer forks have, apart from height off the road- if any. Maybe the same feel since they are corrected, whereas uncorrected would change the feel?

 

[induna]

I'm currently running a Bafang QSWX front hub motor on a Trek 820 with an all steel Suntour suspension fork, and it appears to be working fine (the fork actually does something under power). I specifically bought the low-end, steel framed Trek for an e-bike conversion. I'm using a 36V 10Ah Ping battery with a Lyen controller limited to 20 Amps and a CA limited 16 Amps, so the motor doesn't see much over 700 watts very briefly, or 600 or so for somewhat more extended periods. The hub motor is only rated for 250 watts and 24 volts, so I try to go easy on the power to keep it cool. It's tops out at about 25mph at 36 volts, but I keep the speed under 20mph most of the time to increase the range. I'm not really in a hurry anyway.

 

[auraslip]

Thanks. I've seen fixed forks that say suspension adjusted- it seems to be the length of the fork when moving from suspension to fixed. What effects will longer forks have, apart from height off the road- if any. Maybe the same feel since they are corrected, whereas uncorrected would change the feel?

You want the correct length forks. It can really mess with the handling of the bike. Too long and the bike becomes hard to steer at slow speeds. If it's free, go for it. If you have to pay for it, get the right sized fork.

 

[amberwolf]

Is anyone using Suntour forks on a front wheel drive e-bike? I just got an email from them saying they don't recommend any of them (apart from an SR pedelec kit sold direct to manufacturers). If so which model, and what are your experiences?

For Death Race 2011, I used an old set of steel Suntour M2000 forks

on CrazyBike2 with a front 9C at 2KW, with a wrench as the rightside torque arm, and the old ebikekit short torque arm on the left. I had no problems with the forks themselves, although my resulting bike geometry at the time left much to be desired at any decent speed.

Pics of torque arms/etc here:
http://www.endless-sphere.com/forums/viewtopic.php?p=390482#p390482

These forks are the type with pinched-flat steel dropouts. I would not have done it with aluminum dropouts--if I had a fork with those that I really wanted a front hubmotor on, i'd probably cut the dropouts off and make tubes with clamping dropouts to go over the fork lowers, like John in CR did.


What I really recommend is sticking with a rear hubmotor if you have front suspension and actualy want the suspension to work well. I have moved my front hub to the rear (see the latter pages of the CrazyBike2 thread from last month), and now run a normal wheel in the same Suntour fork. Even though it is a very low-end fork, the suspension is incredibly better just without having all the weight in the wheel, allowing the suspension to react in time for bumps and holes and stuff like it's supposed to.

 

[dogman dan]

Agreed. I find only pretty expensive forks function well with a front motor. But then you have to find expensive forks that have space to fit the motor and torque arms. Many many many suspension forks have no clearance for the motor case.

But even so, a fork that binds with motor under power may still work ok if you just roll off the throttle when you hit a speed bump or pothole. That's what I have right now on my commuter, a cheap alloy fork. Throttle on, it's a stiff fork, but if I see a big bump coming, I coast and it soaks up the big bump, saving spokes and rim. No dirt riding, so it works, sort of. 1000w, so with dual torque arms the motor is not breaking the forks. It take no genius to see this fork will die an ugly early death, and stop soaking up bumps even when coasting pretty soon. Likely metal shavings in the fork oil already.

Alsmith, If you replace a suspension fork with a regular fork, you need a suspension corrected, longer one. Same thing works the other way, if you put a very long suspension fork on a bike that didn't have suspension, it will ride funny.

 

[ddk]

Agreed. I find only pretty expensive forks function well with a front motor.

strangely enough, I've so far experienced -0- problems with a front motor attached to this springer suspension fork
It cost $50.00
LOL
However, I'm not recommending anyone to use it, just saying it works in my application:
MXUS geared-front hub motor with a 23A controller @ 36V (nom)


I suspect it works ok because the straight tubes capture the axle to the bent tubes dropouts.

 

[dogman dan]

I haven't fooled with those kinds of forks. But they are steel, which helps. No dampening, so about as good a ride as the pinched tube style 20 buck forks. Crappy springs beats no springs in my book though.

 

[d8veh]

Many of the ready made ebikes sold in the UK are fitted with Suntour forks and don't even have torque arms. Provided everything is fitted correctly, there's no reason not to use them with a 250w motor. Anything more powerful would still be OK, provided torque arms are used and everything is fitted correctly.

The main problem that I've seen is that electric motors have a larger diameter axle than the standard wheel, so, when the axle is inserted, the centre is too low. The result is that washers don't line up with the "lawyers lips", and when the nuts are done up tight, the washers exert a huge sideways force on the drop-out, which breaks it - nothing to do with the torque of the motor. The solutions to this problem is to file the drop-outs a bit deeper to re-centralise the axle or use the asymmetric "C" washers.

If you get the above bit right, you still need torque arms if you go above about 15 amps.

In all of the above I'm talking about light alloy forks (aluminium or magnesium). However, you need to look at the drop-outs to see if they look strong enough. You need a decent thickness and support to the drop-outs. Some are too light, but you have to make a judgement. Some of the forks fitted to ready-made 250w bikes have very weak looking drop-outs, but I've never heard of one breaking without abuse.

I would summarise by saying that you could go up to about 25 or more amps provided you know what you're doing, although there's always a small risk.