How Does This AL Front Fork Look for E-Bike?

[alienbogey]

I've read tons of posts on the pros and cons of aluminum front forks with a front hub motor kit. It appears to me that with quality forks, a relatively low power motor, and properly installed torque arms it should work just fine. What do you all think of the potential of the following combination:

My bike is a 2011 Specialized Crosstrail Sport Disc with aluminum frame and front suspension forks and disc brakes. From the Specialized web site, the forks are described as "SF11-NVX-DS-MLO, alloy lower, 28mm stanchion, coil spring, mech. LO. Forged dropouts."

Here are pictures of the dropouts themselves:





As you can see the dropouts are approximately 9mm thick and appear to me to be very well made.

Torque arms will be properly made and installed.

I am considering the "Sparrow" kit in the following comparison chart:

http://www.liontails.com/?page_id=1196

The motor has a listed "peak motor power" of 504 watts.

What is the collective wisdom of the hive mind regarding this potential combination?

 

[dogman dan]

Those suck badly for putting a torque arm on. The torque arm will have to be cut down too much to be strong enough. You need a fork with some room around the dropouts.

First step choosing a shock fork for front hub use, is figuring out if the motor will even fit between the tubes. A great many forks won't have enough space. Then there is another problem, those tiny motors tend to be the widest.

 

[dnmun]

you need a C washer in the dropout recess also because the nut will not fit in the recess and will crack the aluminum of the dropout when you tighten the nut on the axle. the C washer allows the force to be applied down in the bottom of the recess where the quick disconnect skewer seats.

 

[d8veh]

That motor is not very powerful. You don't even need torque arms with that one, although it's always best to put one on for insurance. 504 watts is the input power, which means 12 amps from the controller. That's very weak, and won't give you much help up hills. Why do you want such a weak system?

 

[alienbogey]

Thanks for the replies.

I don't want a weak system per se, but I have no desire to turn my bike into an e-motorcycle, either.

Actually, my main goal is help up hills. Slow speed, high torque.

I have an appointment to meet the guy who sells and installs this system and others (listed in his table). He has demo bikes with the different systems on them, and we'll be discussing what would work best for me.

 

[d8veh]

12 amps isn't going to help you much up hills unless you're very light. You'd be much better off stating your weight, budget and all your requirements (Speed, distance, hills etc) and then people can advise you properly which system would be best for you.

When you just aske about the forks, everybody answers from their own perspective, which is often 500w or 1000w motors drawing four times as much power as the one you proposed, so you get meaningless answers.

Coming back to your question: In Europe there's thousands of Chinese bikes with similar motors to the one you proposed except that they usually run with 14 or 15 amps rather than 12. They all have aluminium forks, and a lot of them are a lot weaker than yours. None of them have torque arms.

The one thing that is very important when you fit a front motor is that the first washer fits inside the dimple around the drop-out. If it doesn't, it'll break the drop-out when you tighten the wheel-nut, so you need eccentric washers,or you need to file the drop-out deeper to re-centralise the axle.

 

[dogman dan]

My main concern with this fork is simply that the dropout is molded in very tight to the fork tube. To put a torque arm on this fork, you will need to use many washers to get the torque arm to clear the tube.

Maybe the axle is long enough, maybe not. Or you could cut away a corner of the torque arm, but it looks to me like you'd be cutting away more of the arm than I'd be comfy with.

Like every front motor on an alloy fork install I've done, you will very likely have to get out a file or grinder, and modify something. C washers to start with, then perhaps a thick washer on top of the c washer, and then a bit of shaving of the torque arm on the part touching the fork tube. You might have to shave some of the washer between the torque arm and the c washer too.

The key thing here is that all washers and torque arms need to sit flat and straight. Anything crooked, and your dropouts will crack as soon as you tighten a nut.

TWO torque arms. one on each side. I recommend the ones from Grin Cyclery.

Two c washers and two torque arms is how I have always done it. Many thousands of miles ridden with no problems. If your axle is 14 mm, you also should file the dropout notch 1mm deeper, so the C washer fits right.

 

[mrzed]

Having run a 1000w setup on a front fork for years I can confirm exactly what Dogman says. The radius on the area above the dropout is all wrong and you should not use that fork. You need a flat area around the entire dropout, the c-washer is totally required too unless you have no cup and no lawyer lips. I ran with an ebikes.ca c-washer and torque arm on magnesium RST shock for about 2 years before finally moving to a rear motor. Properly installed, a 1Kw front is both safe and nice for road riding, but the torque on the motor, even with torque arms, plays hell with cheap suspension, and I wouldn't waste a good shock on a front drive ebike.

I found at about 700w, the shock was more reasonable, but the need for a shock is more based on speed than power. I preferred rigid when I was running 36v NiCad when I first electrified, at about 32 kph but upgrading to 48v LifePO4 had me running 45kph, and that is way too fast to be fun without some suspension unless the road is totally smooth.

I'm still a fan of front motor for sub Kw and sub 40kph setups. I'm also loving having working front suspension.

 

[d8veh]

Having run a 1000w setup on a front fork for years I can confirm exactly what Dogman says. The radius on the area above the dropout is all wrong and you should not use that fork.

It's a 200w system he wants to fit, not 1000w!

 

[Chalo]

Just put a real fork in there and keep the worries at bay. It's a rigid framed bike with not-huge tires; a suspension fork is something of a mismatch anyway.

If you use a rigid fork with a forward-set plate dropout like the DMR Trailblade or Origin 8 Cro-Mo Lite MTN, you'll have easy options for anchoring torque arms.

 

[dogman dan]

I agree, replacing that fork with something else would be best. Once, trying to fit a new fork on my commuter, I went through three forks before finding one that actually fit well.

So choose that fork carefully, if you keep suspension. Again, the really big issue is just going to be whether the body of the motor even fits between the tubes. Look for forks with the dropouts mounted to the inside, rather than the center of the tubes.