mid-drive physical specs?

red

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Hello, I am new to ebikes; got a million questions. I am in the USA. Dollars count here, so I am thinking I might build up a thrift-store bike. I would like a mid-drive set-up with freewheel. I can see the mid-drive will replace the usual pedal crank. After the pedal crank is gone, I will be left with an empty hole in the frame. These frame openings will be different sizes on different bikes. How wide (long) should that frame tube be (between the pedals), where the mid-drive fits? What inside diameter should this frame tube be? By what names do you call these frame dimensions? Will the mid-drive use any threads now in the frame tube, or does it mount with adapters? I would guess that some mid-drive kits will fit more stray bikes than others, so any recommendations? TIA.
 
68-73mm width for a normal bike.
73-100mm width for fat tire bikes.
120mm extra fat bikes.

Now you might not want to do a bbshd install but here is a vid of an instasll of one:


Most times the motors spindle will just slide right into the bicycles bottom bracket, other times there are some things needed but i have not encountered that as of yet.

BBSHD mid drives are pretty bullet proof but they are cadence sensors not torque sensors so they have a ramp up and ramp down,
as where the torque sensor motors dont ramp down as the cadence sensor motors do.

Welcome to our humble home by the way, red :cool:
 
HumanPerson,

Thanks for the specs, that will be helpful. I'm guessing that pedal shaft is thin enough to fit through most frame tubes. The install seems easy enough, but those special tools will be needed. I should get them anyway, so no problem there.

I would hope to have two chainrings at the front, though. This place is high desert; we just had 30+ days of 100 degrees F (38 degrees C) this summer. It would be easy to overheat any motor here.
Thanks again.
 
Good advice from HP. I’ve had one fat bike with 120mm that I had to do some filing on the bafang mid drive shaft housing you get it to fit. You want a very close fit in there anyway. I’d suggest you just use a smaller front sprocket to reduce the work of the motor to keep it simple (dependable). You’ll still have plenty of gear options. I only use 2 or 3 on mine. Never had overheating problem. FL whereby heys warm too.
 
Greg,
Thanks. I got files, and I know how to use 'em. ;) I believe that a second (smaller) chain-wheel can be added to the BBSHD kit, with a front derailleur, if needed. Some have said it won't be necessary, but it seems like a good option to have at the ready.
 
Check to make sure the bottom bracket is whats called BSA, that means it has an I.D. of 1.375" Sometimes there will be a little bit of a casting inside that needs to be filed down in order to slide the bafang through the bottom bracket hole. BSA refers to British Standard, many but not all bikes use this. I always coat the shaft of my Bafang mid drives with a little grease, it helps if you need to remove the motor at a later date. I have used Bafang and Tongshen mid drives, neither use the threads in the bottom brackets. Each use a nut on the end of the pedal shaft that when tightened prevent the motor from rotating. The Bafang BBs02 and HD also have another bracket that fastens to "2" threaded holes on the motor and slips over the pedal shaft before the large nut is applied. You may need to use something like a washer(s) to make the bracket sit level. They are 4mm and stainless is best. The Tongshen motors I've installed also had an additional mount to prevent the motor from rotating. If you go with a Tongshen it's kind of self explanitory.
 
I am currently installing a BBS02 on a full suspension mountain bike for a friend and it is proving to be difficult. Most older styles of bikes are designed that where the bottom bracket is the frame tube goes up at an angle to the the head tube. This makes it easy to mount a Bafang motor and still allows good ground clearance. On my friends bike you have the hole for the bottom bracket and then the frame extends forward a couple inches before arcing up to the head tube. Before starting the bottom tube bracket was BSA type which is whats needed and the measurement was 73 mm. However the frame tube is not the old style round but rather a wider tube almost rectangular with a rounded top. So the 73mm did not stick out enough to fasten it securely. So we ordered a 100mm BBS02. That came with a different set of issues but I think it's handled. For this style of bike, it's a Sisku Polygon a mid drive motor from CYC would have been a better fit. As it is my friend will have to be careful when riding over rocks/logs because of ground clearance issues.
 
Hugh,
All good to know. Hopefully I will know enough, when the time comes. I'm building a reference file. The bike frame here is very conventional, just round tubes going straight to the bottom bracket. Thanks . . .
 
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