BSSHD sizing (68 vs 73)

[JayCee]

I just received a BSSHD kit yesterday and I'm a little puzzled regarding proper install. I ordered the "standard" 68-73mm kit, and was sent a motor stamped '73mm' by Luna, but my bike has a 68mm bottom bracket. Both 68mm and 73mm motors are identical, but I'm wondering if I should be shimming the motor assembly toward the drive side to properly center the crank arms w/in my frame. Any insight on this?

Edit: I just looked at my downloaded installation manual - the dimension given in the measured drawing across the bracket is listed as 68mm. The other measurements shown indicate that the crank axle is centered, so it appears that the 68-73 std motor is designed for 68mm. Perhaps 73mm is more of the oddball that results in a slightly shifted crankset.

 

[Alan B]

BBSHD comes in 68mm and 73mm sizes. They are slightly different. The one I got was marked 73mm and fit the 73mm Diamondback Overdrive Sport 27.5 perfectly. See my RidgeRunner thread (link in my sig) for more info.

 

[spinningmagnets]

I just put a 120mm kit on a 100mm bottom bracket. I used the ring-shims from Luna, and they worked fine. I now think it's not a bad thing to get a wider kit, and then some shims. This would allow you to lock in the left-to-right location of the spindle closer to where you want it. You can still center it if you want with two of the thin shims (one on each side). Or, you could use the shims all on one side move the drive a little to one side or the other.

Currently available in 1mm, 2mm, and 4mm thick

http://lunacycle.com/parts/bafang-parts/bbshd-parts/spacers-for-bbshd-bottom-bracket-fitting/

 

[JayCee]

BBSHD comes in 68mm and 73mm sizes. They are slightly different. The one I got was marked 73mm and fit the 73mm Diamondback Overdrive Sport 27.5 perfectly. See my RidgeRunner thread (link in my sig) for more info.

That appears to be a common misconception. According to the bafang manual, there are 4 motor sizes: 68mm, 100mm, 110mm, 120mm. I guess they made the 68mm BB threads longer so it can be used on a 73mm bracket on the BBSHD. I figured that it would be correctly centered with a 73mm bracket, since it is longer than the old one, but the measured drawing tells a different story. Still, I'd like to get verification from somebody that knows for sure.

Although I've read that 68mm and 73mm use different motors, as per Eric Luna, there are not two motor widths, just one (standard) that covers both - the difference in the kits is the "shim kit" that is included. The next size up covers up to 100mm. I thought that the included shim kits were the big ones that shift the whole motor assembly around, but given I didn't get any of those, I guess it refers to the shims/washers that take up the extra space between the left side bracket and the motor when installing into a 73mm BB.

 

[JayCee]

I just put a 120mm kit on a 100mm bottom bracket. I used the ring-shims from Luna, and they worked fine. I now think it's not a bad thing to get a wider kit, and then some shims. This would allow you to lock in the left-to-right location of the spindle closer to where you want it. You can still center it if you want with two of the thinnest shims (one on each side). Or, you could use one 5mm/6mm shim to move the drive a little to one side or the other.

It seems that 95% of the users out there are putting these things on fat bikes. I definitely see the allure - almost went that way myself, until I priced tires . I thought I was going to get some shims to locate the motor but didn't. I'm just trying to figure out if I should use a shim to center the motor (spindle), or if it's already centered as is. It looks centered for a 68mm bracket in the manual. Of course, I don't have any shims, so if I want or need to move the assembly, it's going to add up to a significant delay while I source them.

 

[JayCee]

I stand corrected - I just checked out the measured drawing on Bafang's website, and it is different from the one that Luna is including in their downloadable manual. The website shows 6 different widths: 68, 73, 90, 100, 110, 120. Luna apparently does not carry them all, and sells just the 73mm version to cover 68-73. Oddly, the drawing shows the 73mm spindle offset 5mm toward the drive side, which seems to indicate that it would be properly centered on a 68mm bracket and not a 73mm. 73mm motor has a 175mm spindle with the drive side crank 90mm from C/L, or 85mm to the other side. In a 68mm BB, the spindle ends would both be 87.5mm from C/L. This indicates that in a 73mm frame using a 73mm motor, the cranks are offset by 5mm. I'm confused.

 

[JayCee]

I stand corrected - I just checked out the measured drawing on Bafang's website, and it is different from the one that Luna is including in their downloadable manual. The website shows 6 different widths: 68, 73, 90, 100, 110, 120. Luna apparently does not carry them all, and sells just the 73mm version to cover 68-73. Oddly, the drawing shows the 73mm spindle offset 5mm toward the drive side, which seems to indicate that it would be properly centered on a 68mm bracket and not a 73mm. 73mm motor has a 175mm spindle with the drive side crank 90mm from C/L, or 85mm to the other side. In a 68mm BB, the spindle ends would both be 87.5mm from C/L. This indicates that in a 73mm frame using a 73mm motor, the cranks are offset by 5mm. I'm confused.

OK - after a bit of correspondence with Luna, I retract the above statement. Reminds me of the old joke, "I'm never wrong. Well, I was once, and that was when I thought I was wrong, but I was actually right."

Apparently, there is ONLY a 73mm version of the motor, which is sold as the 68-73mm solution. There are motor/bracket spacers that are different for each, but that is it. It might explain the drawing on the web site, actually, since the spindle measurement is the same, but the spacings to C/L are not - apparently, C/L is that of the bottom bracket, and not that of the motor unit. Oddly, it DOES appear that the motor is designed to be centered on a 68mm bottom bracket, and the crank arms will be offset to the drive side (by 2.5mm) on a 73mm bracket. Eeeeentresting.

 

[Alan B]

Good to get some clarity on this.

So on the 68mm, they supply either a spacer ring or a different side plate to take up the 5mm difference? Otherwise the side plate would not sit square on the 68mm bottom bracket tube. It does sit properly square on a 73mm bottom bracket tube.

 

[weldinpatt]

Don’t look at centralised to the detriment of chainline. I got a 68mm when I measure the centre to the BBSHD, 42T chainring gives me a chainline of 47.5mm, absolutely Bob on.
It matches within a mm the rear sprocket on my IGH. I don’t know for sure if the motor is central I just pushed it in fully and the measurement was ideal, proving my chainline didn’t just look good. The left hand side looked after its self and neither washers nor spacers were necessary.