Front motor on shock fork - bad idea?

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donorcycle   100 mW

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Front motor on shock fork - bad idea?

Post by donorcycle » Apr 24 2011 4:45pm

Hello,

I have been reading on this forum that mounting a front motor on a shock fork is not the best practice. However, for the sake of convenience and expense, I am still interested in mounting a low power motor in front. The lower Half of the shock fork is STEEL and I would use two torque arms (any recommendations there would be appreciated). Would it be safe to mount a 500W geared motor with 48V battery, or would that be pushing it? If so, what would be the max for safety. I am more concerned with uphill power. I would like to achieve a top speed <25mph

This is the fork:

http://www.srsuntour-cycling.com/index. ... &tnid=1394

[img]http://www.srsuntour-cycling.com/gateTo ... 62&convert[-resize]=400x600[/img]

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dogman dan   100 GW

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Re: Front motor on shock fork - bad idea?

Post by dogman dan » Apr 24 2011 5:20pm

I've ridden thousands of miles on similar forks with 36v and 48v motors, and never needed a torque arm. Just the tabbed torque washers, and a good tight nut. Those type of pinched steel tube dropouts tend to be pretty strong at the dropout compared to " normal forks".

Once you venture into controllers above 1000w, then you need the torque arms.

But if you install the motor improperly, with poor fit of the nuts and washers, then about 50 watts is all it takes to spin the axle, cut the wires, blow the controller when the wires short.

Bear in mind that I'm a crazy person that loves front hubs on the street. One of my bikes had 4000 watts on a front hub.

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spinningmagnets   100 GW

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Re: Front motor on shock fork - bad idea?

Post by spinningmagnets » Apr 24 2011 5:32pm

This style of drop-out is sometimes called "pinched" steel tubes. Get two of the C-washers from ebikes.ca, two torque arms of your choice, and the 500W hub using 48V should be fine. http://endless-sphere.com/forums/viewto ... =4&t=15163
Image

The big problem with front forks is with the fairly common cast aluminum or the rare magnesium, plus using regen makes a bad situation worse. The drop-out doesnt bend, it snaps off. Best case scenario: the axle spins and your wires are pulled out by the roots,...worst case: you go flying forward at 25-ish MPH and cushion your fall with your face.

If you don't have a preference, I'd recommend the ampedbikes.com torque-arms for adjustability, and the ebikes.ca TA's are good too.
Image
Last edited by spinningmagnets on Apr 24 2011 5:37pm, edited 2 times in total.

donorcycle   100 mW

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Re: Front motor on shock fork - bad idea?

Post by donorcycle » Apr 24 2011 5:34pm

Thank you very much for the reply Dogman. You are a nut. I'm still interested in torque arms since they are cheap and easy to install and I may end up putting over 1000 watts into it. Maybe I am just superstitious, but torque arms would be something from keeping the motor from possibly flying off if the nuts get loose (I might at least get fair warning). It is my understanding that the hole in the torque arm (or washer) needs to be a snug fit with the axle and the size varies with motor manufacturers. I plan on getting a Bafang BPM (higher torque) or a one of Cellman's Pumas (if he gets back to me). Given that, anyone know of any good torque arms given my fork? Thanks

thank you spinningmags , I was writing this while you replied.

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auraslip   1.21 GW

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Re: Front motor on shock fork - bad idea?

Post by auraslip » Apr 25 2011 12:36am

The best option is to make your own.

I'm not really sure if it's worth the time though. Dogman, how well do those steel suspension forks work for you? General consensus is that sub $200 suspension forks do little. Better to get a nice fork and build a strong torque arm, or use a stock torque arm but stay at stock power.

Check out the ebike.ca torque arm.
Maybe I am just superstitious, but torque arms would be something from keeping the motor from possibly flying off if the nuts get loose (I might at least get fair warning).
It's not superstition. It absolutely can and will happen. Using a front motor of any power without a torque arm is....... bad. It might last 2,000 miles before the nuts work loose, and by then you'll be complacent and not used to checking them or watching for "axle wiggle." Absolutely use a torque arm. Check the nuts for tightness regularly.

Image

That reminds me, I should go check mine now!
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amberwolf   100 GW

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Re: Front motor on shock fork - bad idea?

Post by amberwolf » Apr 25 2011 1:57am

dogman wrote:But if you install the motor improperly, with poor fit of the nuts and washers, then about 50 watts is all it takes to spin the axle, cut the wires, blow the controller when the wires short.
Yes. :oops: See my DayGlo Avenger thread for this and the results (luckily I didn't blow the controller but I had to take the motor apart and rewire it).
http://www.endless-sphere.com/forums/vi ... 78#p352278
(pics may still be down due to forum problem)
http://www.endless-sphere.com/forums/vi ... 53#p355453
(pics mostly intact)

10mm wrench for torque arm:
Still, sometimes it's not enough:
http://www.endless-sphere.com/forums/vi ... 40#p374540
so a different type may be needed (used both after this):
http://www.endless-sphere.com/forums/vi ... 37#p375237

So now I am running a Fusin geared motor at up to 1200W with dual torque-arms (such as they are) on a crappy RST-Omni pinched-steel style fork, on DayGLo Avenger.

On CrazyBike2, I'm running the 9C DD motor at up to...um...something like 3.5KW peak at the DeathRace, and normally on the street at 2 to 2.5KW peaks. ;) It's on the perhaps the crappiest "suspension" fork I own that actually still moves up and down :lol: but again is the pinched-steel type fork, like the RST-Omni but much crappier. On the street it was only a very short standard torque arm that came with the 9C originally, but I added a 10mm wrench on the other side for the DR, and it's still on there now, just in case.
http://endless-sphere.com/forums/viewto ... 82#p390482

So...if theyr'e steel dropouts, and you use torque arms, you can put a lot of power up there if you want.

If I had my druthers, I'd rather build John-in-CR-type clamping-dropouts for my forks; I just havent' gotten round to that yet. Time and all, ya know? ;)


THe thing is, those custom-made clamping dropouts like those linked above can be made as tubes to go over existing forks including the really nice (but usually alloy and therefore bad for high-power front hubs) DH forks and the like; so that most any "reasonable" power level can be safely run in the front fork. Say, under 5KW. :lol:

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Re: Front motor on shock fork - bad idea?

Post by rguy56 » Apr 25 2011 2:36am

It looks like it would be fine for a front motor. I've been running 36v in a front suspension fork for a few years with no issues, without a torque arm. I'm about to change the tire on it, and while it's off, I'll add a torque arm, based on Justin's research. I have some Judys on mine, and I think the lower body is a softer alloy.

The forward flexion of the fork system during acceleration, and rearward during braking are far far less of a factor than the twisting of the axle in those dropouts. I consider them not a factor at all.

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Re: Front motor on shock fork - bad idea?

Post by amberwolf » Apr 25 2011 2:57am

Oh, and I forgot to note that the fork you linked to is just about exactly like the one I used in the DeathRace:
http://endless-sphere.com/forums/downlo ... p?id=57390
whcih has dropouts that are pinched steel l
Image
ike my RST Omni and whatnot.
http://endless-sphere.com/forums/downlo ... p?id=57446
I think it'll be ok, but still I'd use torque arms no matter what. I just don't trust wheels to stay on without them. ;)

I also forgot to note my previous "race" configuration, which used a non-suspension solid fork of about teh same design as the one Auraslip's picture shows with bent dropouts, had severe problems with yanking the wheel almost out of the dropouts at most accelerations of 60V NiMH (probably about 40-50A), causing me to decide to not actually race because I was afraid it'd happen while I was at 30MPH+ and launch me across the track. :(

And that was WITH a torque arm, but one that did not secure well enough to the fork leg. I can't find the pic of it right now. :(

THis is the Suntour M2000 fork on CB2 during Heat B before the race (which I didn't make it into).

http://members.ii.net/~aussiejester/ind ... 2_1024.jpg

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dogman dan   100 GW

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Re: Front motor on shock fork - bad idea?

Post by dogman dan » Apr 25 2011 6:22am

I didn't really mean to recomend no torque arms. Yes, use them, they can save your teeth, and your electronics. I just wanted to make the point that the pinched forks are really strong, Much stronger than the bent dropouts in the pic above.

The cheapie forks don't do much, but they do enough to lessen the " bent wheel because my bike hauls ass now" syndrome.

Even really good suspension forks when used with motors can have problems. Often the forward pull on the forks makes them stick. So if you see a chuckhole coming, you must let off throttle to have suspension when you roll over the bump.

My favorite torque arms are the ones from Ebikes-ca. Get the c washers too, for dropouts with a deep cup. On the pinched tube ones, you can just hammer the lawyer lips down on that type.

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