Crankin' It - Mid-Mounted Crank Drive
For years now, I have been considering a mid-mounted freewheel crank drive.
Running a geared reduction motor through the 7 speed freewheel sprocket, or freehub cassette.
Most any bike could be converted, adding less than 10lbs of motor hardware. (Battery additional.)
Typically, this involves using a geared reduction motor connected through the front chainring.
A big improvement over most any type eBike drive!
With a 11-34T 7spd freewheel you can have the equivalent of a 7 speed motorcycle, with +300% the torque at low speed.
Great low speed torque and high speed assist ... right to 30mph(?) ... from the same motor!
An important option is a 2 chainring freewheel crank.
This allows motor power without pedals being turned ... much safer!
There is the variable of matching motor to pedal speed.
The size of the 2 chainrings can be varied, and / or voltage adjusted, to match your rhythm.
The next stage option is a freewheel sprocket, instead of a fixed sprocket on the motor, this allows easy pedaling when motor not engaged.
My project motor is the Unite MY1018z gear reduction motor.
I've poked-prodded-tested and tried this model for ~20,000 miles.
As a demonstration,
I graphed the speed and torque available from running the motor through the 7 speed sprockets.
MY1018z motor @24V w/9T sprocket
At 24V Low gear (34T) will give you great power-acceleration till 7mph, great for the worst hills.
Then, there are a variety of optimal hill climbing, or acceleration, choices till about 16mph.
Of note is that a 34-13T would limit maximum speed to the "legal" 20mph.
I just finished purchasing the individual components and will continue to post build reports.
I'll be watching this one, sooner or later I figured you'd build a mid-drive and I'm curious to see how you go about it. I don't always agree with you but I can certainly give you credit for being very inventive on your builds and sometimes you think of good ways of doing things that I would never come up with myself.
Previous graph is for the MY1018z 24V motor with the OEM 9T sprocket.
In order to operate pedals without using the motor the use of a freewheel sprocket on the motor is required.
The smallest compatible freewheel sprocket I've found is a 13T.
Of course, this will vary the motor applied speed noticeably.
MY1018z motor @24V w/13T sprocket
Sadly, this loses much of the low speed torque advantages of a multi-speed mid-drive, might be OK on a 20"er?
On the other hand...
In lowest gear as I am accelerating from a red light, I hit 10mph at my max pedal speed.
So I would be shifting right at maximum torque of motor and pedal assist, maybe ... not so bad.
Personally, I could be happy with a 9T sprocket with motor always engaged and pedal assist.
Optimally shifting at a more casual 60-70 pedal rpm.
A larger chainring from the motor can offset some of this loss.
With an intermediate jackshaft, most any gear ratio can be accomplished.
Stay tuned for more developments ...
Crank Drive Build #1 - Stage 2
Huffy dual-suspension mountain bike
Dual chainring freewheel crank (40T/40T will change after testing)
33.3V 25.92Ah 9s12p homemade recycled Lipo battery
Unite MY1018z 24V (450w) gear reduction motor w/9T sprocket (email@example.comV)
MY1018z motor @ 33.3V though DNP 34-11T 7spd Freewheel
I think this to be nearly ideal for my 1st trial
90rpm pedal speed looks to match optimal shift points at full throttle
Partial throttle can move optimal shift points towards a more casual 60rpm.
Low gear torque is excellent, and top motor only speed looks to be ~26-27mph
Lowering seating profile or moderate pedal assist should make 30mph possible.
Important, IMO, is the survivability of the drive train!
Shifting under throttle will produce a (un)healthy "clunk" ...
an impact on all drive train components: motor > chain > chainring > chain > 7spd freewheel > hub
This "damage" can be moderated by not shifting while under throttle, release throttle before shifting then gradually re-engage.
This works but kinda degrades performance ... substantially!
So, I propose the addition of a heavily sprung idler roller.
Place behind the chainring, pushing chain down, roller should be ~ 50% sprung under full throttle.
Instead of a shift inducing a straight line immediate impact the "impact" will be spread over the unsprung distance the idler provides.
"Impact" might be reduced 90%(?), providing a great increase in durability-reliability!
http://endless-sphere.com/forums/dow...513881d090c530 - $4.74
Surprising to most, there is minimal abuse or wear on the Bottom Bracket!
With motor positioned in front of the BB, motor to BB abuse is evenly offset by BB to wheel abuse.
The exception being the crank freewheel bearing which must endure a constant twisting, worse under heavy throttle.
This "twisting" abuse could be largely eliminated.
The use of an intermediate jack shaft positioned directly behind the BB, (if there is room), inside the wheel chain path, would even up the stress on the freewheel crank bearing.
But, of course, doing this would apply greater stress on the BB bearing ...
Crank Drive - Parts-n-Such
SunRace 13-34T MegaRange - $16.99 shipped
DNP Epoch 7spd 30-32 to 11T Freewheel - $34
Upgrades EZip w/44T Front Sprocket to near 30mph pedal assist (90 crank rpm)
More sizes - 11- 28,30,32,34T - $32
Quality ACS Freewheel for freewheel crank! $16.50
BCD not specified ... might bolt directly to certain chainrings?
Also available _ more components, from Staton-inc.com
Freewheel Cranks - Choices ... >$32
450w 24v, gear reduction motor, standard 1/2" x 1/8" chain (MY1018Z) - $95 + shipping
The "silent" motor (well ... not real loud)
XYD-16 motor w/mounting bracket - $108.89 - Will try to find working link
Have you seen the Bafang mid-drive 350/500/750 models? Extremely compact package and really stealth. The 750Watt model is an absolute BEAST. Adapts to any bike with a 68mm BB. Takes about 10 minutes to install. Add a battery and away you go.
ElectricBike.com - 10 mid-drives info & reviews!
More Mid-Drive Components
GNGeBike.com - Try to combine shipping and negotiate shipping price
Bonus: Very Interesting!
Got 1, then got a spare.
These are some unknown items with original source unknown, so don't ask me.
Got a double chainring, then a discount on a single.
All 3 chainrings are 40T, ... OK for initial trials.
They have an offset so that there is a nice spacing, but with a single, flipped over, chain is positioned directly over the center of the freewheel. This should eliminate the "twisting" stress common with the doubles and greatly extend usable life.
I believe I will try the single ...
Positioning the motor sprocket in front of and below the chainring, I will stretch, or lengthen the chain around a single path, ensuring a ~120 degree motor sprocket contact.
Freewheels are the Pheasant brand ... the equivalent of what we would call a turkey ... but I have an ACS Crossfire for when it fails.
Single Chainring Advantages - Disadvantages
Eliminates the twisting (off-center) stress-wear on the crank freewheel = greatly increased reliability-usable life
Reduced cost! "Standard" crank freewheel with single centered chainring will survive longer or as long as super duty freewheel with multiple chainrings
Negates the possibility of shifting between multiple front chainrings
Eliminates the variance of chainring sizes to match motor to pedal speed.
Doesn't look as complex = doesn't impress the simple minded as easily
Perhaps not obvious ... It is still possible to modify motor and pedal speeds
Motor speed can be varied either by:
Changing the size of the motor sprocket
The Currie motors, MY1018 & XYD-16, have 8T,9T,11T,13T available (mod-able 15T also found)
Adapter is available for 13T-14T-15T small freewheels
Additional adapter is available for the "standard" 16T - 22T freewheels
Changing the voltage of the motor
At 24V, gear reduced (sprocket) speed is ~400 rpm
22.2V = 370 rpm
24V = 400 rpm
25.9V = 432 rpm
30V = 500 rpm
33.3V = 555 rpm
37V = 617 rpm
40.7V = 678 rpm
44.4V = 740 rpm
48.1V = 800 rpm
Then ... rear sprocket can be adjusted
14-34T to 11-28T to 21-26T 7-speed freewheels (or cassettes) available
Now you adjust your pedal ratio by changing the single chainring ... 32T - 62T+.
This has no effect on the motor speed ratio! ...
Motor speed is now determined by motor sprocket to rear wheel sprockets ... not by chain ring.
Chainring is effectively an idler sprocket for the motor contribution, varying the size has no relation to motor speed.
I do not see any reason to use dual chainrings of equal size ... ?
I have 2 - dual 40T and 1 - single 40T.
Advantage of chain contacting 20T rather than 10T?
I can't justify that as any reasonable advantage ...
So, I will likely post up, at least, 2 40T chainrings with "standard" 5 hole - for ACS CrossFire etc. - Appear new - steel.
Or swap for compatible 44T 48T ... other?
Shimano made a freewheel front crank back in the 70s.
Panasonic makes now now.
24T does not offer the recommended "offset"
24T, 30T, 36T, 44T, 48T - sickbikeparts.com
I'm surprised at the lack of a 40T chainring, 36 to 44T is a pretty drastic jump, limiting fine tuning motor to pedal speed.
It should be reasonably simple to build an adapter from the common 5 hole freewheel to the 130 or 144 BCD chainrings.
The adapter would supply the desired offset to any flat chainring.
This would enable a large selection of readily available chainrings ... up to 62T+?
Larger chainrings might create increased "twisting" wear on dual chainring setups but single setups should suffer no additional stress.
Bargain I couldn't pass up!
A Schwinn Front Freewheel (FF) crankset
Circa ... late 70's, this item was touted as "a solution ... in such of a problem!"
Interesting to me, the freewheel seems to possibly run, at least partially, against the BB bearing race.
Dependent on actual construction, this could bypass some of the problems with modern freewheel chainring crank systems.
Just had to get it, cost less than $25 including shipping.
I will post up evaluation after arrival.
Better picture shows freewheel bearings to be easy to keep lubricated and adjustable.
Durability looks to be a big improvement over "modern" methods!
Schwinn freewheel crank arrived.
A 37 year old item in excellent condition! (1977)
No noticeable wear on the chromed teeth and no traces of rust near the bearings.
Freewheel is nice and tight and smooth ... best of all it is adjustable.
It can be tightened up after it wears a bit.
I do believe that a single chainring is preferable.
But this looks to have good potential as a double ... if I ever go that way.
Now just gotta find the "proper" bike to transform.
My Schwinn Searcher 700C, (rescue bike), looks to be the prime candidate.
14-34T 7speed looks appropriate for the 52T chainring @ 33.3V.
Steel frame gives good motor mount options.
Will probably build a cheap Mountain bike conversion with cheapo single chainring as my first testbed ... tho.
Just found a 46T Schwinn-Shimano FF 46T Single freewheel crank.
Perfect for my pedal assist at my preferred 33.3V.
Newest addition seems advantageous over the dual chainring. Chainring is perfectly centered between the freewheel bearings, eliminating the "twisting" that destroys the dual chainring variant.
There is a speckling of rust on the chrome, but teeth and bearings appear to be in excellent condition.
Mid drive ebikes are incredibly efficient and fun to ride. I have been enjoying mine for a full year now. It is set up with a 2 stage reduction on a 67kv motor. I have wrecked a few drivetrains shifting under power. I am very curious how your spring loaded idler will help compensate for that.
Nice project, good luck with the rest of your build.
Hello DrkAngel! It won't let me message you so I will try this post. I bought a used 2009 Ezip. You said some 2009's can be over volted. How do I determine if mine can do that? I have an extra case , do you ever build li-pos for for other people for an Ezip? Also I have been trying to register over at Ebike forums and it won't except my email address. I have tried 3 times and sent the problem to the administrator but have not heard anything back. Any idea? Thanks Sextontom
If the throttle leds light up, it supports ...
42V, full charged 36V, has not damaged any eZips I've tested.
But ... make sure you get Pos & Neg correct ... reversing will likely damage controller.
Red = Pos
Black = Negative
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