Gearing a Bafang BBSHD Top Speed and Off Road

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Bullfrog   10 W

10 W
Posts: 75
Joined: Apr 08 2017 10:37am

Gearing a Bafang BBSHD Top Speed and Off Road

Post by Bullfrog » May 19 2018 9:48am

Hopefully this will help you figure out what gearing works best for you. Below is my experience with gearing a Bafang BBSHD.

I am running a BBSHD on a 14s6p battery (58.8v fully charged) with a rear tire that has an outside diameter (OD) = 725mm (28.5 inches). I have a 28 tooth front chainring and two cogs in the back (took my freewheel apart and rearranged things to get my chainline nice and straight). When riding off road, I use a 23 tooth rear cog and if I want to get somewhere quicker I use the 17 tooth rear cog. The 23 has a top speed of about 17.5 mph and works well in the woods. Faster than 17.5 mph and trees start to jump out in front of me like those trees in the Wizard of Oz that could actually move.

Below is more info on the 17 tooth rear sprocket and why it is the best gear for top speed...for me. Everything in this post is with me not pedaling at all...kick in one PP (person power) and things could change significantly.

I have done a lot of experimenting with gearing and small cogs in the rear to see how fast I could go on flat ground with no wind and without drawing more than 1,000 watts once I reached full speed. The 28 front and 17 rear with the 725mm diameter tire was the fastest set up I could get (remember less than 1,000 watts) and my top speed was about 25 mph...a lot had to do with whether I sat up straight up or leaned over because even at 25 mph, aerodynamic drag is a big factor when you are only working with 1,000 watts which is just over one horsepower. Downhill the speed picked up quickly and uphill it decreased...duh.

Yes I could go a little faster with different gearing but the more wattage (actually amperage) you push thru the motor, the more it will overheat and potentially damage something. I feel comfortable running my BBSHD at 1,000 watts continuously so for me, the top speed of my BBSHD is 25 mph. If you are narrower than me or shorter than me, (all adds up to frontal area and aerodynamic drag :D) then you might be able to go a little faster but even when I was laying horizontal and running an 11 tooth rear sprocket, my top speed was less than 30 mph on flat ground and I was pushing approximately 1,500 watts thru the motor. It takes four times as much power to go 30 mph as it does to go 15 mph. So when you start going faster the power required goes up very quickly.

Not what the BBSHD fans want to hear but my recommendation for a drive when you are going to be putting in miles on pavement is a Geared Hub Motor like the MAC or a Hub Motor like the Crystalyte (many good choices for hub motors...they are very simple). Why...because you don't transmit any power thru your gears or chain (assuming you don't pedal). It is a simpler set up and will have much less wear and tear on your Bike's drivetrain. Less wear and tear means more time riding and having fun instead of working on your bike to fix something that wore out or broke.

If you ever intend to leave the pavement...the BBSHD would be my recommendation :D ...because you can use the bike's gears so the motor is spinning at a more efficient rpm when you are riding slower and less waste heat is produced.

If nothing else, try different combinations and see what works for you.

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