Yet Another BLT Mid-Drive System

[truly_bent]

Have completed the mid-drive design to a point of display. It's missing a few nuts and bolts (tedium), but otherwise all the necessary bits are in place.

This is based around the BLT 800W motor and uses a kit from Stanton-Inc, which has many of the jackshaft pieces already fabricated. It's designed to hang under under my recumbent and, given the ample space available, allows for a fair degree of design latitude.

- The jackshaft is the widest bit at 9", but could be cut back to about 8.5", depending on chain clearance.
- The mounting plate is a tad over 11" in length.
- The motor can move forward or back, depending on chain length. Overall length could be reduced if motor was fixed in place, but that would necessitate a chain tensioner.
- The tube hangers can be positioned side to side to allow some axial offset.
- I have a square jackshaft housing designed which would be cheaper to fabricate. The one in the pictures would require CNC machining, but is lighter.

Because the chain from the front chainrings runs past the rear of the motor, this design is dependent on motor width. Naturally, if the 650W or 500W motors were chosen the width could be trimmed down. Also, if the plate were mounted at a different angle, on a triangle frame perhaps, the width could be cut down.

There are a couple of things i'd still like to do to the design. A chain tensioner on the motor sprocket side is probably a good idea as well as a tensioner to take up chain slack from the front chainrings (the rear derailleur does that job normally). The latter can be mounted on the bike frame though, so that's less of an issue. I'd also like to add a working chain into the assembly, but that's purely cosmetic.

It's a start. Things are getting busy around here so I probably won't get around to fabbing this up until the cold weather hits.

[Morgen 3Eman]

Please help an ill-informed old guy to understand your drawings..

Where is front/rear?  I understand the use of the jackshaft, but where does the drive go?   Do you drive the rear wheel using the sprockets on the free wheel? Or  to the pedals?  Or some other way?


TTFN,
Dennis

[Bikemad]

Dennis, this might help you visualise TB's finished drive train:



The dual sprockets are fitted on the right, and are usually equipped with a pair of unidirectional clutches to ensure that the motor does not drive the pedals while it is running, and the pedals do not have to drive the motor when it is stopped.

Alan
 

[Morgen 3Eman]

Oh, that makes sense now.  Pretty cool.

Thanks, Alan

TTFN,
Dennis

[truly_bent]

If i'd taken the time to add chains to the assembly that would have been clear... well, clearer anyway.

The chain from the front chainrings runs to the (20 tooth) freewheel on the right side. The motor chain runs to the (36 tooth) freewheel on the left side. Both of these "drive chains" are therefore free to idle while the other drives. The black (20 tooth) gear is fixed to the rotation of the jackshaft and drives back to the rear wheel gear cluster.

The 36 tooth gear was selected to fairly closely match the motor (at 70% of max rpm) to a cadence of 80 rpm on the front chainrings (I don't have those spreadsheet calculations on this computer, but can post in the future if anybody is interested). A wide selection of aluminum alloy gear blanks are available for this purpose, which would be machined to fit the White Industries freewheel.

The benefit of this system, while considerably more complex to build, is that the motor drives to the rear wheel gear cluster, which allows the rpm to be kept closer to the motor's optimum efficiency. This, in turn, provides good torque from a less powerful motor.

Note: There's a spacer missing between the fixed 20 tooth gear and the 20 tooth freewheel. As shown, the freewheel would rub against the fixed gear. 

[truly_bent]

The YAM-DS (Yet Another Mid-Drive System) has undergone some radical design changes.

It became apparent that the earlier configuration was going to have to be very wide to allow the chain from the front chainrings to clear the fat-azzed motor (oops, my bad ). The chain was likely to interfere with the rider's right foot.

So, I went with a more vertical and narrower configuration. This allows the drive chain, to the rear gear cluster, to more easily clear that fat-azzed motor. This could be improved considerably with the addition of an idler gear to lift the chain clear. The bearing cup, on that side, could be dispensed with entirely and the bearing could be made to fit inside the jackshaft housing, which could bring that chain line in even further. It's about 3" from center at this stage.

The jackshaft has been placed ahead of the motor, too. This happens to agree better with my particular configuration. However, the base plate is symmetrical, which allows for alternate setups.

The 4 bolts holding the tube mounts to the base plate are also securing the motor to the rest of the assembly. This has saved 4 bolts and assorted nuts and washers. This has the side benefit of shortening of the whole assembly by a few inches.

There are only 6 pieces to be fabricated from scratch (in blue), with only 4 actual parts (top hanger (x2), bottom hanger (x2), base plate, and jackshaft housing). The one 36 tooth blank (in gold) has to be machined to fit the freewheel. All the rest are "off the shelf" items.

This configuration appeals to my sense of the aesthetic. Even the empty space in the middle could be put to good use. Perhaps a controller would fit in there.

[Morgen 3Eman]

one thing to note if chain clearance to the motor is still a concern:  the drive sprocket on the motor can be reversed to put the collar on the outer end of the motor shaft, allowing the motor to shift over a little more.  The motor will no longer look centered on the chassis, tho...

TTFN,
Dennis

[truly_bent]

Good idea Dennis, but that's not the exact sprocket the BLT 800 comes with. I wasn't sure how the real sprocket was secured when I drew that up, but it's not reversible. My sprocket chain line is at the correct offset from the motor face though.

There is maneuvering room. The motor is free to move sideways to the extent of its own mounting bracket bolt slots. That's a 1/2" to play with (1/4" either way).

And the motor could easily be dropped down to better clear the right side drive chain - space permitting.

[truly_bent]

There's excitement in my lil' cave tonight! Okay, the fish don't seem anymore excited than usual, but i'm chuffed.

Taking that comment about using the interior space for a controller a step further, I hogged out some more metal and voila! Usable space. The new cavity measures 5" vertically to the motor's surface and 2" wide. Just so happens, my Lyen controller is 3.5" by 1.75", so it fits comfortably.

Haven't done a stress analysis on the design yet, but i'm optimistic.
Gotta love these new-fangled technologies, doncha?

[truly_bent]

Just an update.

Had to lower the motor more than expected to allow clearance for the Lyen controller. This I discovered after inserting the controller assembly (thanks http://grabcad.com/). Previously, the controller obstructed the motor's side-to-side movement. (Had to rebuild the controller in SW in order to shorten it to a 12 FET (45A) model. Gave me an intro into SW's Sheet Metal commands )

The downside is having the motor sit 9.5" below the bike frame. I have about 17" between my frame and the road, leaving 7.5" clearance. It might look ugly too, but the idea of housing the controller inside the hanger frame appeals to me, so it'll probably stay as a design feature.

One point in favor is that now there's plenty of room to raise the chain-line over the motor's fat butt. I think it's time to go back and massage the jackshaft housing design. I'm gonna go ahead and dispense with the bearing cups in favor of placing them into the housing itself. Originally, the design stemmed from a Staton-Inc kit, but that was for converting an existing BB.

Jeff

[truly_bent]

Okay. I promise to stop boring you with this soon.

- Got rid of the bearing cups and tucked the bearings inside the jackshaft housing. A sharp eye will notice space enough for 4 bearings, although I only have two shown here. (todo: set screws for bearings)
- Beefed up the jackshaft housing.
- Designed in jackshaft housing spacer to raise jackshaft axis. Lottsa room now for the drive chain-line back to the gear cluster.
- Added a few bolts and washers. Not my fave thing to do.

Right side outer chain line is 3.0" from frame centerline. I would have preferred closer, but all life is a compromise, they say.

I admit, it's very industrial looking, but I'm an industrial kinda guy.
Now I really have to get back to work.

Jeff