Mid-drive motors like the popular Bafang BBSHD and BBS02 have many advantages over hub motors including: 1. Low and middle center of gravity for better balance. 2. Use of the drive train by the motor. On a hub drive the motor only has a single gear while a mid-drive motor alomg with the cyclist gets to use all of the gears so the the motor's RPMs stay up in it’s most efficient range where power is maximized and battery use is more efficient. This allows the bike under motor power to 3. Go fast and climb steep hills much better. 4. Retain the quick release hubs on both wheels to make fixing a flat easy or removing the wheels quick - the way we like it. I remember adding a 19mm wrench to my seat bag toolkit so i could change a flat on the road with a rear hub motor - not ideal.
However despite these great benefits most mid-drive motors require a huge sacrifice over hub motors. These motors have a single gear up front requiring us to sacrifice our front derailleur and 2 or 3 chainrings to a single chainring. This means your 27 speed is now a 9 speed. If you want to go fast with a comfortable cadence you get a large chainring. If you want to climb hills you get a small one. Unfortunately, you can't have both. People with steep hills to climb are putting small chainrings on their BBSHD. Because of the architecture of the BBSHD chainrings smaller than 42 teeth tend to mess up their chain line. It also sacrifices their top speed. If you stay with the 46 tooth chainring that comes on the BBSHD or put an even bigger one on it you can bog the motor down on steep hills even in the lowest gear. Hence, we see the single track and San Francisco crowds putting on 42 tooth and smaller chainrings like the Luna Eclipse or Lekkie all the way down to the 30 tooth Luna Mighty Mini. These are great for climbing but at the expense of speed and chain line below 42t.
The 10 or 11 tooth high gear you have on your cassette is as high as it goes so if you want speed and a higher gear the only lever you have to play with is the front chainring. This you do at the expense of your low gears for climbing. But I did say you could have it all in the title above didn't I? Indeed you can if you upgrade to a wide range cassette. If you have an 11-32t or 11-34t like mine you are in for a hill climbing eye-opener when you move up to an 11-42T wide range cassette. You still have your top speed 11 tooth high gear but now you have a 42 tooth low gear in the rear to grind up the hills with. As for the front chainring no need to go below 42T for the steepest hills so you can keep your chain line intact. In fact I had a hard time finding hills the stock 46T couldn't handle. You will still want to upgrade it just on principle though (and to save weight) as it is a plain, heavy piece of stamped steel, whereas the new chainring upgrades from Luna and Lekkie are a lightweight and attractive upgrade.
So what is entailed in the upgrade? It depends on what you are starting with. If you have a 10 speed cassette already you are in luck. Most wide range cassettes are made for 10 or 11 speeds and you can just add a wide range cassette. Sunrace makes a good one that is 11-42T with 10 or 11 cogs. You may need to make some changes to your derailleur like a Wolf Tooth Components Goatlink or a longer B-screw to enable it clear the 42 tooth cog.
If however, you are like me and have an 8 or 9 speed cassette you will need to upgrade to a 10 speed as that is where the new wide range cassettes start. (Edit: the previous sentence is no longer true as Sunrace now makes wide range cassettes 11-40/11-42 for 8-speed and 9-speed drive trains. Available here.) Fortunately, Shimano and SRAM 8 and 9 speeds have the same cassette body type as the 10 speed so without needing to get a new hub and wheel you can upgrade the cassette. In addition to a new cassette you will also need a new shifter and derailleur. This can all be had for less than you might think. I will give a list of the components I used later. This is also an excuse and an opportunity to get some good performance components in your drive train. While I was at it in addition to a nice drive train upgrade, I took the opportunity to fix up my handlebars using Shimano I-Spec B components. Handlebars can get pretty crowded and awkward for DIY e-bikers. More about that upgrade here.
The bike I started with was a Cannondale Lefty with a 9 speed cassette. After doing a BBSHD conversion on it I loved everything about it except for the unfortunate tradeoff between speed and climbing ability the loss of 2 of my chainrings forced on me. It was while researching my options that I discovered that this limitation could be overcome with a wide range cassette but I would need to upgrade my drive train to a 10 speed. The best 11-42t 10 speed cassette at the best price I could find was made by Sunrace. I got the Sunrace CSMS3 steel cassette.
I recommend the Sunrace CSMS3 rather than the lighter and more expensive CSMX3 aluminum cassette. It is cheaper and made of steel but with the high torque of the BBSHD bearing down on it, it will hold together better. Karl Gesslein found out the hard way that the MX3 aluminum version will not hold together under BBSHD loads and he wrote about it here. In an update to the article and a picture of a trashed MX3 he says, "Forget about the aluminum version of this cassette, only buy the steel version CS-MS3 do NOT buy the CSMX3 which will catastrophically fail under any real power. You have been warned."
After researching more into drive train components I went with the Shimano XT M786 Shadow+ 10 Speed Rear Derailleur with a medium cage. The medium cage coupled with a Goatlink allows this mass produced (read that less expensive) high-end derailleur to clear the massive 42T cog on the Sunrace CSMS3 cassette. Here it is installed on the Lefty with the Wolftooth Goatlink and working flawlessly right out of the box.
I chose an I-Spec B compatible shifter so I could get my handlebars the way I wanted them. The shifter is a Shimano XT M780 10 Speed Trigger Shifter. More on getting the handlebars right here.
This upgrade turned a great mid-drive conversion into what for me is the ultimate e-bike. I love it and if you have struggled with the speed vs climbing power mid-drive motor issue, you will too.
What do you think about mid-drive gear ratios on your e-bike?