Custom Two Stage Bottom Bracket - Is it Feasible?

turbo1889

100 W
Joined
Feb 27, 2012
Messages
240
Location
Montana
Okay, yes I am a new guy. I have been lurking for a long time though and just signed up so that I could post this thread.

There have been several threads here which have discussed bottom bracket drive systems which use the bottom bracket as the jack-shaft to combine the power of the motor and the human pedal power. An excellent well known example is the Stoker Monkey set-up which uses a tandem crank on the left side that the motor is attached to by a second chain. And thus the bottom bracket itself serves as the jack-shaft although the motor spins the pedals as well and there is no obvious way to get around that if you are going to use the bottom bracket as the jack shaft with the motor chain and sprocket on the left side and the main drive chain back to the rear wheel is left "as is" on the right hand side.

After reading some of the past threads that discuss this arrangement I had a light-bulb go off in my head that others I think have hinted at but not put completely together. The idea comes from those counter-rotating propeller driven airplanes which have two propellers on the front one right behind the other that rotate opposite directions (to have zero or minimal torque load on the air frame due to the counter rotation). The important thing is that how they do that is by having a hollow drive shaft for the rear propeller with a smaller solid drive shaft going through the middle of it for the front propeller. Could not the same thing be done with the bottom bracket cartridge?

The pedals turning a solid shaft that is inside a hollow tube that forms a second shaft around the main shaft with the sprockets attached to the outer tube shaft and the inner shaft with the cranks and pedals attached to the outer shaft via. a freewheel crank assembly? Using approximate dimensions that are not completely to scale but close enough to be relevant I threw together the following diagram in my CAD program as a simple 2-D cut away and then printed it as a 16-color image that was just black lines on a white background and then used color fill in MS-Paint to color code the components:

Color Coding:
----- Dark Grey = Inner and Outer Drive Shafts
----- Solid Black = Needle Bearings
----- Light Grey = Bolts & Nuts
----- Bright Green = Threaded Cartridge Cup Ends
----- Dark Green = Bottom Bracket Tube of Bike
----- Gold-ish = Solid Chain Rings and Chain Ring Spacer Plates
----- Dark-Red/Brown = Standard Right Hand Freewheel Unit Threaded on Right Hand Threads to Right Crank Arm
----- Bright Red = Opposite Direction Left Hand Freewheel Unit Threaded on Left Hand Threads to Left Side of Outer Drive Shaft
----- Yellow = Crank Arms




A custom bottom bracket set-up like that would allow the bottom bracket to serve as the jack-shaft (the outer hollow drive shaft) and transfer motor input from the left hand side chain ring to the front drive chain rings while allowing the inner drive shaft connected to the cranks and pedals to free-wheel while at the same time also allowing the inner drive shaft to drive the out drive shaft while the chain ring on the left side attached to the motor freewheels. Thus both crank pedals and the motor may independently drive the bike all through the front bottom bracket.

I think there is just barely enough room inside a standard 1-3/8” threaded bottom bracket cartridge to fit it all in provided needle bearings are used. As I drew it the outside diameter of the cartridge cups (Bright Green) are 1-3/8”. The needle bearings (Solid Black) are 1/8” diameter needles/rollers. The outer hollow drive shaft (Dark Grey) is a hollow tube that is always has at least 1/8” wall thickness. The inner drive shaft (Dark Grey) is mainly 1/2” diameter but thins down to just a little more then 3/8” diameter in the bearing races on either side, obviously minimal so it would have to be made of high end hardened steel (think drill rod stock). The right hand freewheel (Dark-Red/Brown) threads on the inside of the right hand side crank are standard dimension. The left hand freewheel (Bright Red) is a smaller non-standard diameter threads but it has to be that way in order to be able to assemble the unit.

I actually think it would be stronger then a standard free-wheel crank set-up since the chain rings at least on the right hand side are fully supported and bolted solidly to the outer drive shaft which passes completely through the bottom bracket with double bearing support.



So what do you guys think? We got a light bulb that is going to burn a while here or is it a flash in the pan dud of an idea?
 
Welcome turbo!

I'd say so. :)

This was my first sketch of the same comcept: http://www.endless-sphere.com/forums/viewtopic.php?f=28&t=35846

file.php


It would be good to see someone try out this idea. I won't get around to developing mine for a while.....
 
Curiously, someone's misunderstanding WRT left-hand driven bottom bracket drives was the subject for my first posts on the Sphere :) http://www.endless-sphere.com/forums/viewtopic.php?f=2&t=536
 
I've toyed with a similar concept myself:
http://endless-sphere.com/forums/viewtopic.php?f=28&t=22830

Only what I cooked up has the motor inside a big bottom bracket "box" assembly. Not very practical for retrofitting a standard bike, but it actually makes a bit of sense for someone building a full suspension folding bike driven through the cranks (something I want to do some day). The "box" then only needs (1) mount points for rear suspension pivots, (2) mount point for fold hinge, (3) seat post mount).
 
Just had a thought why are we not using one way bearings, with the right combination/number we could get a really powerful drive.

looks promising, see link..

http://www.alliedbearings.com/mfg_prod/formsprag/csk_clutch.pdf

Neo
 
Torque for weight, freewheels win. Torque at the bottom bracket can be 2 or 3 times as great as at the rear wheel.... Steiber-Formsprag sprag clutches are mucho expensive..... The Chinese ones have been failing regularly on the Elation drive.
 
that's cool but that's going to need a few more complex processes, i'm going to go through a few designs, see how small i can get this with the largest size sprag bearings
 
I hope you're not relying on the torque ratings in that brochure you linked to........ They seem to have taken the Nm ratings as pound feet ratings and then re-converted them into Nm :mrgreen:

Ref: http://www.stieber.de/pdf/stieber/CSK-product-en.pdf

I suppose it's just possible they could be using a different rating system and it's a coincidence.....
 
Neovin said:
any idea what torque rating a human can put through the pedals?

As an outside limit, it would be your weight (in kg) times g times the pedal arm... Pulling representative numbers, (100 kg)*(9.81)*(.17 m) = 167 N-m. That would be a peak number. You'd have to be on a recumbent to produce more than that.

The world record for continuous bicycle endurance works out to something like 400 W continuous. Assuming 60 RPM at the crank, that would be about 64 N-m sustained.

For you or me, 200 W is probably generous, which would be ~32 N-m sustained at 60 RPM.
 
I'd say 200Nm would be an absolute maximum to work to.

The ratings on those CSK clutches are reliant on them being press-fitted into a rigid housing, too..

I've used a Japanese CSK 35 on the back wheel of my Moulton (the driven motor pulley) for 4 years without any problems. It is a 20" wheel, though :)

I think a Stieber CSK35 or CSK 40 would be around $150 to $170

VXB have the Chinese ones: http://www.vxb.com/page/bearings/PROD/Kit12575
 
Miles said:
Curiously, someone's misunderstanding WRT left-hand driven bottom bracket drives was the subject for my first posts on the Sphere :) http://www.endless-sphere.com/forums/viewtopic.php?f=2&t=536


We are all extremely glad that happened my friend. You are such a fantastic asset here, and I still don't know that I've seen another build that matches the elegance and commuting perfection of your beautiful machine my friend.
 
I just found something that could be useful on ebay,

http://www.ebay.co.uk/itm/CPI-Formula-50cc-Starter-Sprag-Clutch-Complete-/270856673460?pt=UK_Motorcycle_Parts&hash=item3f1050d8b4#ht_1271wt_942

it looks like it could work well for me, its the same chain size and ready built, I have seen others for cars and larger bikes i think i'm on to something here
 
i'm thinking if we could use the smaller bearing looking type to seperate the main drive from the pedals and the starter clutch to seperate the motor from the main drive, a left hand motor driven crank may be done. Ooh think of the gears

i wish i had a mountain bike so i could do some measuring up, could you guys give me the internal diameter, thread size and width of a standard bottom bracket.

How wide do you think we could comfortably go on the pedals.
 
Neovin said:
could you guys give me the internal diameter, thread size and width of a standard bottom bracket.
1.37" x 24 tpi. Most common shell width is 68mm.

Neovin said:
How wide do you think we could comfortably go on the pedals.
You need cranks with minimal offset to compensate for the extra axle length.

http://www.spadout.com/a/q-factor-the-space-between-our-feet/
 
Thank you for all the comments and the two links posted to previous threads along these lines (apparently the search function is pretty worthless since I had not seen them before).

I’m not rejecting the use of one-way needle bearing clutch assemblies but it appears that they require rather large sizes to handle the peak torque loads produced by pedaling and do not exist as ready built components built specifically and “ready to use” for bicycle applications.

Where on the other hand freewheel assemblies that can be used on the crank side are available in both right and left hand assemblies. Right hand side being the most commonly known (Sick and Cyclone parts plus others) but there are left hand side units available for Staton-Inc that would serve well on the left hand side of a double stage bottom bracket assembly for motor input:

http://www.staton-inc.com/store/pro...HAND_thread_freewheel_sprag_clutch-967-0.html
http://www.staton-inc.com/store/pro...FT_Hand_threads_Freewheel_Sprocket-695-0.html
http://www.staton-inc.com/store/pro...FT_Hand_threads_Freewheel_Sprocket-756-0.html
http://www.staton-inc.com/store/products/25_80_Tooth_LEFT_Hand_threads_Freewheel_Sprocket-687-0.html
http://www.staton-inc.com/store/products/60_Tooth_35_LEFT_Hand_threads_Freewheel_Sprocket-679-0.html

Granted most of them use other chain sizes then common bicycle chain sizes but there is no reason not to use a different chain size on the left hand side motor chain loop. It may even be an advantage allowing greater choices of industrial type sprockets to put on the motor output shaft and also allows a simple sprocket on the motor since the freewheel is built into the left side crank sprocket. All the sprockets in question are intended for use on the left side of the rear wheel hub for use in Staton-Inc gas motor kits but they should work just as well on the left side of a double stage bottom bracket assembly especially since some of the more powerful motors used in the Staton-Inc kits are putting slightly over 2 horsepower through them so they should be more then strong enough for all but the most over-powered e-bike builds.

The information Miles has provided with his thread (nice 3D solids drawing by the way) has been very helpful. Especially on one key point. That being that there is really no need for a second inner bearing race between the inner and outer shafts and only a tight slip fit is needed. This is due to the fact that there is only significant stress loads on the inner spindle shaft when the rider is pedaling and when they are doing so the inner and outer spindle shafts are rotating together and the bearings on the other spindle shaft carry the load. Unless of course the rider is doing light slow “fake” pedaling which isn’t putting in any power into the drive system and just letting the motor do the work while making it look like they are pedaling in which case there isn’t very much load on the spindle anyway. About the only time there would be significant load on the inner spindle while it was stationary and the outer spindle was rotating around it under load from the motor is if the rider where not pedaling and was standing up on the pedals instead of sitting down on the seat.

So one can simplify the assembly significantly by just making the inner spindle shaft a tight slip fit with a little grease in the inside of the outer spindle shaft. Yes it would wear over time more then if it had an inner race of needle bearings but that wear would be reduced according to how much the operator pedaled and standing up on the pedals while the motor was running would be a “don’t do that” item. In other words, allowing the pedals to freewheel instead of being constantly driven by the motor like the stocker-monkey set-up would be mainly a safety and convenience feature and the operator would still be encouraged to pedal to reduce the wear over time on the custom double stage bottom bracket assembly. Not a bad trade off for the simplification in the design it allows especially for the conventional smaller size bottom bracket sizes.

I did actually dig through my pile of old bike frames and components stash along with taking some measurements off of my up and running bikes and not all bottom brackets are as small as diameter as the somewhat standardized 1.37x24tpi cartridge assemblies. My bike full suspension mountain bike has a much larger bottom bracket diameter that is using a heavy 30mm diameter spindle. I didn’t pull it apart to get internal bearing race diameters but there is definitely a lot more room inside its bottom bracket and it appears to use press in industrial type sealed ball bearing race assemblies one on each side instead of screw in cartridge cups. In addition the frames designed to use the old single piece Ashtabula type cranks have bottom brackets that are larger and more roomy then the standard 1.37x24tpi cartridge bottom brackets as well so there are certainly larger size bottom brackets out there that would make things even easier.

My ultimate goal would be to put this kind of custom double stage bottom bracket assembly into a Yuba Mundo long-tail cargo bike using a sprocket drilled with six holes to match the disk brake mounting pattern on the left side of a hub motor mounted in the rear triangle to provide electric assist power through the bottom bracket just like a stoker-monkey only without having driven pedals. Now that I know it is possible I guess the next step is to save up and buy the Yuba and then pull apart its bottom bracket assembly and figure out what type it uses and take exact measurements and then go from there.

Here is an updated diagram (same color coding):

 
If you could persuade these guys to make one of their frames with a BB30 Standard bottom bracket.....
http://www.designlogicbikes.com/

Ref: http://www.endless-sphere.com/forums/viewtopic.php?f=31&t=36596
 
I have taken a look at the option of using of the various 25mm diameter spindle screw in external bearing cups that use 6805-2RS size ball bearing (25x37x7) with a 25mm outside diameter hollow outer spindle that has a 4mm wall thickness with a 17mm solid square taper end inner spindle running inside of it but thought that was less then ideal due to the expansion of the tread width of the pedals (Q-factor) by the thickness of the external bearing cups which is already going to be expanded by the space on the left side for the left hand thread freewheel chain ring on that side and the custom spider assembly on the right side so thought it would be best not to aggravate that issue further if possible through the use of an external bearing assembly.

Is the spindle diameter for the Shimano Octalink bottom bracket cartridges which as you mention do use internal needle bearings also 25mm?
 
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