Build 6: The off-roader V2 mid-hub Gravity FSX

General Discussion about electric bicycles.
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parajared   10 kW

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Build 6: The off-roader V2 mid-hub Gravity FSX

Post by parajared » Sep 19 2013 10:16pm

My GNG setup bit the dust back in July and I have been wanting to go off-roading again so I am building another off-roader.

For this build I am going to blow about $500.
Bafang BPM from $122
"Force" brand 48t crank from $12
donor bicycle from thrift store to chop tail off of and put motor in dropouts $5
muffler clamps to attach donor bicycle dropouts to bicycle $10
velo-solo gear from $25
Gravity FSX from $329
My trusty Lyen 12 fet sensorless.

Misty Glamour Fun bicycle riders best look away now!
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All done
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Update 11/23/13: Bike was working great until the rear derailer came apart. I was just cruising along down a smooth road and heard a big kerchunk. I hit the brakes and watched a gear from the rear derailer roll past. Not sure what happened, it appears that it just wanted to come apart for some reason.

Update 1/16/14: Got her going again, still not sure why the old derailer decided to come apart, but new derailer is holding fine.

Update 2/12/14: I have been experimenting with a rear rack for the batteries setup and I think I am liking this better than batteries up front; the front suspension doesn't mash in as hard on the jumps. I think batteries up front was ok for my GNG kit because the motor is so light, but the extra weight of my hub setup makes this less viable.

Update 5/19/14: I decided that now that I have a Mobius it would be neat to do a video on my new setup for the off-roader V2. I put the batteries behind the seat instead of up front, and reduced down to 10ah instead of my usual 20ah. I am glad to report that I still am able to get my standard couple of hours of riding in with just 10 aH on board. The lighter weight yields improved maneuverability and all around a better ride and it's nice that it makes it easier to pop wheelies and trials bike bounce your way over stuff but unfortunately it makes it pop wheelies when you are climbing steep grades too. My tack welds on my battery mount (unsurprisingly) didn't last after much riding and broke on me. Fortunately I was able to bungee cord everything back together and get a good ride in.

I am also pleased to report that mid-hub has been a better setup for me than chain modded GNG kit. GNG was better than hub because of it's lighter weight and because it took less effort to just strap the sucker to my bike than having to fabricate a mid hub mount but worse than mid-hub in almost every other way. Both systems have an issue where if you run too much power down them, the chain skips on the sprocket and grinds so you are limited to only about 1500ish watts (input) or so. My biggest gripe with GNG was the noise levels; in an attempt to avoid confrontation I would find myself staying off throttle when around hikers and other trail users, certainly I could just blast by but it was noisy enough it was hard to do that without feeling like a total dick. Mid-hub allows a much appreciated access to all 24 gears opposed to GNG's configuration that only allows only 8 of your gears to be used. Perhaps it was just my build, but I had a tougher time of keeping the reduction chains on the GNG from de-railing typically leading to a bit of cussing every now and then opposed to mid-drive which has had no de-railing issues at all. The wider range of speeds, the reduced noise, and the increased reliability make me believe a mid-hub is a superior system to rc drives and GNG style setups at the expense of weight and ease of initial setup.

Update 7/24/14: 2000 watts works on my system if I stay on larger gears (11t and 13t grind to bits) but it seems that after a quite a number of rambunctious off-road adventures I have ran into an issue with the gears bending inward. My gears (the three on the front, not the 8 on the cassette) have bent so bad that it clutches on to the chain and tries to rip itself apart. I am torn between the decision to reduce my power input in an effort to preserve my equipment or just replace the gears every 5 or 6 months.

Update 5/6/16: I replaced the bent gears and dropped to 1200 watts which wasn't nearly as fun as a couple kilowatts but not half bad either. I swapped from battery mounted under the seat to battery in backpack and ended up liking this much better. I also swapped from Anderson power poles to xt90 anti-spark which solved a problem with of the connectors wearing out. I waterproofed my stuff after watching a youtube video to increase my confidence of biking through streams and enjoyed a number of rides in the 15' season.

I presumably peanut buttered my gears when I was cruising along and my rear tire got wedged between two rocks and I hammered on the throttle but I have yet to crack open the case and take a look.
Last edited by parajared on Sep 16 2019 8:39am, edited 16 times in total.

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waynebergman   10 kW

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Re: Build 6: mid-hub Gravity FSX

Post by waynebergman » Sep 19 2013 10:28pm

Looks good. Keep the updates coming. I hope everything goes smooth for you and you can be back on the trail soon. I like the left side pedal assist design, it has worked well for me.

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Samd   1.21 GW

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Re: Build 6: mid-hub Gravity FSX

Post by Samd » Sep 20 2013 12:59am

Nice work!

I still have a dozen or more leftover laser cut dropout reinforcements in 7mm plate going to waste - I should post you a couple. They were made to precisely fit a BPM axle with no slack for my custom frame projects.

Must get a lot of torque back on the mid-drive dropouts. Or is spinout not an issue?

"Knowledge is knowing that a tomato is a fruit. Wisdom is knowing that a 165mm rear shock can't deliver 220mm of rear swingarm travel." - Oprah Winfrey.

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spinningmagnets   100 GW

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Re: Build 6: mid-hub Gravity FSX

Post by spinningmagnets » Sep 20 2013 5:57am

Or is spinout not an issue?
If the motor has a reduction of 3:1 to the BB, and then the BB has a common 1:3 gear-up to the rear wheel, then...the ratio from motor to rear wheel would be 1:1 and the drop-out torque on the axles would be the same. Typically, though. The benefit of a BB-drive is that the motor has the use of the gears, so when starting from a stop, the bike will be in a lower gear, so the motor can provide adequate acceleration with fewer amps = less stress on the axle/drop-outs. Of course the motor will top out at 10-MPH (as an example), and you will have to upshift.

If you take a hub-motor that is designed to provide 25-MPH when it's in the wheel, and then as a mid-drive you gear everything so that top-gear can provide 30-MPH, then...starting out in top-gear (whether by accident or on purpose), it is possible for a mid-drive like this to actually put more stress on the axle/drop-outs. Of course when you now have gear choices for the motor, that means it would be normal to make good choices.

r3volved   100 kW

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Re: Build 6: mid-hub Gravity FSX

Post by r3volved » Sep 20 2013 11:21am

I like your mounting bracket!
Great idea

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