I picked up a Mongoose Dolomite the other day and thought I'd power it with a BBSHD. The first task was to measure the bike clearances to see if the BBHD would fit and, if so, whether to use a 100 or 120mm.
Mongoose Dolomite with 120mm BBSHD
The bike has a 100mm BB shell width with an ID of 33.6mm, a 190mm pedal axle, 190mm rear dropouts, 4 inch tires, and a 14 to 28 7 speed freewheel . The chain line is 82mm. I used a section of 1 inch sched 40 pvc pipe with a slot cut for a rule to measure from the bottom bracket (see pics below). I used the measurements from the Bafang installation manual and those from the forum thread, Chainring Offset Guide, for the drawing. Again, see below.
I determined from the measurements that either the 100 or 120 BBSHD would work, but the 120 results in more symmetrical peddle offsets. Essentially, the drive side of the motor remains fixed in relation to the bike with either, but the non drive side would be shortened by 20mm when using the 100mm model. This would result in 103-87 right and left axle lengths as opposed to the 103-107 lengths with the 120mm model (See the drawing). Both installations would require non drive side spacers.
An unknown is the crank arm clearance. If someone can provide the Bafang crank arm offset, I would appreciate it. As can be seen from the drawing, there's not much clearance for crank arms at 170mm. I doubt that the standard Bafang crank arm would clear on the non drive side using 100mm installation and I'm not sure about the clearance using the 120mm install. One way to determine the offset would be to put the arms back to back at the top and then measure the distance between them at the pedal attachment point. Or, place the top of the arm back side down on a flat surface and measure the rise of the inside of the arm at the pedal end.
The drawing suggests the 46t chain ring would sit considerably inboard of the bike's design chain line of 82mm. A smaller chain ring is probably a better choice for gearing and would move the chain line outboard. For example, using the Luna 40t would move the chain line out by 10 mm to 75mm which is closer to the original and would probably allow usage of all the freewheel cogs.
I believe the Bafang measurements are applied correctly, but I would appreciate people double checking them. The bike measurements, I think, are within a couple mm, but please check before doing a similar install. There seems to be some inconsistencies between frames as built.
I welcome your comments and questions and I am anxiously waiting for someone to give me the Bafang crank arm offset measurement. I will continue this thread as the build progresses.
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