Rear hub motor mountain bike conversion – freewheel question.

[StuartsProjects]

I recently fitted a Swytch kit to my Brompton, its OK for short commuting type trips, but in the area where I live (Cardiff) the roads are so bad the ride is a bit harsh to be fun for long. The extra stiffness of the front wheel cannot help.

I have an old style Mountain bike, its a 26” wheel custom Titanium Rock Lobster, with mostly Shimano Deore XT stuff, Hope mini Hydraulics and Marzocchi front shocks. The ride is a lot more comfortable on the roads than the Brompton. I thought to do an ebike conversion on it and have a Bafang 250W rear hub motor kit due to arrive in a couple of days.

I can see that the PAS is not straightforward on the Hollowtech cranks, I have ordered one of the PAS disks with the large hole, so I hope to avoid having to replace the cranks.

Now to a question;

The gears are Shimano 9 speed cassette, the Bafang rear hub motor needs a 6/7 speed freewheel, and although its no great expense to just fit a 7 speed freewheel and a new 7 speed shifter, it would be useful if I could keep with the Shimano 9 speed setup.

A 9 speed freewheel, with one cog removed, should be about the same height as a 7 speed, so it might fit, the rear drop out is 135mm.

Are there any 9 speed freewheels out there that you could slim down in this way ?

 

[vfr400]

The problem is the width of the freewheels with more than 7 speeds. To get the rim central in the frame, you have to off-set it from the centre of the spoke flanges. A small offset has no downside, but the difference in spoke tension between the two sides rises exponentially as the off-set increases, until the rim is directly over a motor spoke flange where the tension would be infinite on one side and zero on the other.

In other words, when you use an 8 speed freewheel, the off-set makes a sizeable difference in the spoke tension.

It can depend on your frame. Some frames have a built in off-set to allow for a wide freewheel.

If you have enough thread on the brake side axle, you can add spacers to compensate, but that makes the distance between the drop-outs bigger, thenyou have to stretch your frame a bit to get the motor in. It also causes alignment issues between the disc and the caliper, so you have to add spacers there to sort it out.

 

[StuartsProjects]

The problem is the width of the freewheels with more than 7 speeds. To get the rim central in the frame, you have to off-set it from the centre of the spoke flanges.

Thanks you for the reply.

I had found some info on the DNP freewheels, apparently the 7 speed has a stack height of 36mm. The 9 speed has a stack height of 40mm, hence my thought that a 9 speed freewheel minus one cog (if it were possible) would have very close to the same stack height as a 7 speed that the motor and wheel should be sized for.

A similar thought occured to someone in a thread on here, see message #13 below;

9-speed-freewheel