Friday, January 7, 2022

The Breeze, the Stratus drivetrain (S1E3)

From the start the Stratus was going to have an electric assist installed.  I'm in my fifth year without a car, in a rural area, in the middle of a transportation system that was 99.9% designed for cars.  The electric assist helps me compensate for the poor road design, the distances, and the nonexistent bus and train network.  Also, recumbents aren't as easy to peddle uphill, and there are plenty of hills here.  So I took most of the original drivetrain parts and stored them in a plastic zip bag in case the bike needs to be restored later on.

Because they can be shifted to different gearing ratios, I've found Bottom Bracket motors are more versatile than the hub motors I've used.  I knew the Stratus was going to be fast, but it also had to climb up my dirt road.  Because my state recently enacted the (questionable) 3 class ebike legislation and I wanted to stick with 750 Watts, I decided to use a Bafang BBS02.  It's more powerful than the 500 Watt Tongsheng TSDZ2, and not as heavy as the Bafang BBSHD,

left to right: TSDZ2, BBS02, BBSHD (click on all pictures to enlarge)
Because these are all the same voltage (48V) and about the same motor speed, the size of the motor case is a pretty good indicator of their amps and power capacity.  I find the BBS02 will climb steep roads around 8 to 12 mph, but the TSDZ2 has to be shifted down one or two gears and is slower.  The BBSHD is a good choice for heavily loaded cargo bikes because it's built like a truck motor and can deal with the force and heat much better.

  I'd installed a dozen BBS02 which had been strong and reliable motors, but then the 13th motor had a manufacturing defect:

There were two dry solder joints on the Hall sensor printed circuit board of the 13th BBS02.  This was easy to diagnose because a 08H error code showed up on the display, and after removing the board the dry joints were visible and an ohm meter showed bad connections.  I fixed this by resoldering the joints because the vendor was slow in sending a replacement.  I consider this a significant fault, but otherwise the rest of the motor looked very well designed and solid inside, and I was willing to give the BBS02 another try.

Testing the Hall sensors to be sure it was the solder joint and not the sensors

The sensors themselves could have been bad, so I tested them before reassembling the motor.  With a voltmeter connected between the ground and the output of each sensor, the output should toggle between a high and a low voltage when a magnet (a magnetic screw holder in this picture) is passed by the sensor.  If you are trouble shooting this part and not familiar with doing this, please unplug the Hall board from the circuit and use a separate power supply.  Note:  these appear to be latching Hall sensors, and the magnet has to be flipped to the other side to change the polarity.  I had expected the vendor to send a replacement Hall sensor board so that it could be swapped easily, but they cheaped out and sent one single Hall sensor.  I don't think this would have helped most bicycle shops.  

This 14th motor has been working fine, it's smooth, quiet, and pulls nicely.  I reprogram the BBS02 motors mainly to turn the power down in the first and second assist levels for easy riding.  With these settings, the bikes ride a lot like the Specialized Turbo Como in our ebike library.  I also change a few other settings, such as reducing the starting current to prevent a large starting load from burning out the motor, but I don't do anything complicated.  There is a ton of BBS02 programming information available on the web so I'm not going to repeat it here.  (I use Penoff, Endless Sphere, EM3EV, Karl Gesslein, Lectric Cycles, and Lunacycle suggestions.)

The new rear wheel setup, with a 126mm freehub, 9 speed 12-36T cassette, and a rear derailleur capable of a 36T granny gear.  (The stick is because I hadn't installed the kickstand yet.)

The Stratus has a 27 x 1 1/4" (32-630) rear tire which is one of the larger diameter wheels, and the original gears were a two speed chainring (52/42T) and a six speed freewheel (13/14/17/21/26/32T).  I needed to keep the 52T chainring for speed, but wanted to add a larger granny gear to make up for the single large chainring.  Almost all Shimano rear derailleurs in my price range have a maximum granny gear capacity of 34T.  I had experimented with a short hanger extension on my Marin MTB conversion, but I had to drill and insert a peg to correct it's angle to Shimano specs.  It worked but it wasn't easy or elegant.  The Shimano Deore M592 rear derailleur has a 36T maximum, so I decided to try it without a hanger extension and see if the motor made up for having less than a huge granny gear.  It will climb a 16% grade OK, but it's a couple mph too fast in rough, bumpy spots.  If this setup lasts, I might experiment with a 10 speed setup to get a 40T or 44T low gear, but there are a few considerations with doing that:

-Many people talk about the torque of a mid motor ebike wearing out chains quickly.  I've been using wider 6/7/8 speed chain, and the wear has been so minimal that I decided to try out 9 speeds for this bike.  But 10 speed chain would be thinner still.
 -Six to nine speed parts are somewhat compatible and I can mix and match some parts while building, but I wouldn't be able to use any of my spare parts for a 10 speed.  The cable pull and the parallelogram angle are too different.
-I've been buying quality regular chains for about $14 each, but have found a comparable ebike chain was $42.  Since the Stratus uses 2.1 normal length bike chains (246 links) and The Breeze will be similar, there is some financial motivation to design for a low wear drivetrain that can use a regular chain 

No one ever talks about using larger gears to reduce the stress on a bike chain like the industrial drive people do for machinery.  At first glance having only a 52T chainring seems like a bad idea, but if it's paired with large rear cogs then it's actually beneficial.  Besides less stress on the chain, I use the lower gears more often than with a 44T or 48T chainring which spreads the wear around.  I'm also becoming less and less of a fan of 11T top cogs because I've noticed chains jumping on them more often, so they seem to be not a great idea for a strong mid motor ebike and 12T or 13T high gears are better.  In general I'm leaning towards larger diameter gears, while still trying to keep a very wide gear range on the cassette.

The original Suntour Mountech rear derailleur has an extra spring loaded pivot that is concentric with the upper idler pulley.  This was famous for packing full of dirt, and then jamming the derailleur into the spokes.  It was called "The wheel builders best friend" by one blogger, and was responsible for much of Suntour's financial problems.  It went into the storage bag with the other original parts.

The Stratus rear dropouts have a 125 mm spacing.  I wanted to upgrade to a stronger freehub with more speeds, but all my MTB parts hubs had a 135 mm OLD dimension.  Since the seat and chainstays are very long and slender on the Stratus and the frame alignment measured true,  I didn't want to cold set the stays and take a chance of it becoming off center.  I found a wheel with a 130 mm Shimano FH-RS300 hub, and it even had a 4 mm spacer on the left side!  After removing the spacer and recentering the rim 2 mm over to the right, I had a 126 mm wheel that fit into the dropouts almost nicely with just a light nudge.  Unfortunately the chain did not fit.

The inside of the right rear dropout had to be modified to clear the chain.  The seat stay tube (top) stuck out too far and had to be cut back and then rewelded closed.  The tube had grooves already worn in it by the old chain, so this probably was a problem even when new.

The finished seat stay with decent chain clearance in high gear.  Originally the stay was a tube all the way around.

1/2 inch black poly tubing instead of idler pulleys (shown with the chain on the top sprocket)

There were a couple more small modifications.  One was that I needed to extend the speed sensor cable.  The bike was almost ready to ride and I didn't want to wait for another order, so I cut 28 inches out of a USB cord and spliced it into the cable.  It looked the same, plus it was shielded.  I filled the splices with silicon sealant before putting the last piece of heat shrink tubing over them.  The other mod was the chain stay protector.  I'd planned on making an adapter in front of the rear wheel that could mount both the kickstand and chain idler pulleys.  However because of the 52T chainring, chain rub on the stay was not a problem.  The kickstand was also more stable when located further forward.  So I took a 6 inch piece of black poly 1/2 inch plumbing tubing and cut a slit in one side, and then snapped it on the chainstay just to protect against rubbing when the chain was bouncing around.  It's light, and I've never heard it make any noise.

This bike is a treat to ride.  It takes a little effort to get it up to 25 mph, (there's some weight, and I should probably revisit the motor programming), but then it will stay cruising at 25 to 30 mph with not much trouble.  The one problem I've had with it is getting started with 60 pounds of groceries on the rear rack.  I don't have a hand throttle on it, (there wasn't any room left on the handlebars!), and I've found that if you use the walk assist mode to get going, the motor will not stay running when you start pedaling.  I need to make a start button to turn the motor on until I get balanced and pedaling.  Overall it's exciting to ride, and it's probably the closest I'm going to get to a high performance sports car with 9 speeds with paddle shifters on the back of the steering wheel.

I'll write one more post that will cover the battery and remaining details that make this bike work.