A friend of a friend owns a fast car business. His road cars can reach speeds of over 250 mph. he wanted his $6000 Trek MTB converted, and he didn't want it limited to 15 mph. It's not an easy bike to convert because it has front and rear through axles that prevent a hub-drive conversion, and it has a BB90 bottom bracket so crank drives are tricky, especially as I wouldn't want to risk clamping anything to the carbon fibre frame.
It looked like the best chance was going to be with a BBSHD, but it still wasn't clear whether the thickness of the BB was going to be too much because it's not tubular. Instead, it's a bulbous moulding and has the cables exiting at exactly the worst position:
The overall width of the BB was about 80mm, so it looked like the 100mm BBSHD would be best. We decided to take a chance and ordered the motor from Em3ev along with one of their 50v triangle batteries. While I was waiting for delivery, I dismantled the crank to see what we needed there. The first problem was that on eachside of the BB was an inner web of about 28mm diameter, which would prevent the 33.3mm main shaft going through. I ground those away with my dremmel, which left it looking like this:
The plan was to make a couple of steel top-hat bushes to replace the original pressed in bearings and bring the inner diameter down to 33.3mm to fit the BBSHD. here's one of them that was made to fit exactly in the empty profile of the BB:
The motor arrived and we were able to do a test fit, but it was a nogo. There was insufficient clearance between the motor and the BB (13mm dimension). We wondered whether there was a chance that it might fit if we removed the cables and the little outlet plate from the frame. It just fitted with nothing to spare, so the cables were re-routed down the outside of the frame. It just about scraped some paint of as it went into position.
The next step was to stop it from rotating. the normal bottom bracket clamp plate wasn't going to work because if the motor moved a couple of degrees, it would exert a huge crushing force on the frame. Also, we didn't want to apply any crushing force to the carbon fibre BB. That meant we need a torque arm somewhere. The only convenient anchor point was the suspension pivot bolt, which rotated unfortunately. We solved it bu replacing the titanium pivot bolt with a steel replica that had an additional shoulder that we could use as a pivot point for the torque arm. That means that the torque arm is retained, but not clamped, so that the arm is stationary while the bolt can rotate with the suspension. We needed an additional spacer to go behind the plate on the main motor shaft to hold everything in line. Here's the torque plate:
One further problem was that the battery's bag was too big for the frame, so we cut the back off and stitched on some velcro flaps. It works but we might want to make a complete custom bag later.
Everything worked a treat. there was a minor electrical problem with the motor, which was soon sorted. I was surprised with how user-friendly the motor is. Em3ev or whoever had programmed it with some sort of power ramp. You can feel the power ramping up as the speed increases. It's much less violent that the GNG system that I used on a previous build. The gear sensor also worked very well. Overall weight is 25.1kg, which is very light for a bike with a 19Ah 50v battery. The guy has his bike now and seems to be pleased with it. I don't think hell be getting a bike rack for his car though. This car has 1500 BHP:
Here's the finished bike:
It looked like the best chance was going to be with a BBSHD, but it still wasn't clear whether the thickness of the BB was going to be too much because it's not tubular. Instead, it's a bulbous moulding and has the cables exiting at exactly the worst position:
The overall width of the BB was about 80mm, so it looked like the 100mm BBSHD would be best. We decided to take a chance and ordered the motor from Em3ev along with one of their 50v triangle batteries. While I was waiting for delivery, I dismantled the crank to see what we needed there. The first problem was that on eachside of the BB was an inner web of about 28mm diameter, which would prevent the 33.3mm main shaft going through. I ground those away with my dremmel, which left it looking like this:
The plan was to make a couple of steel top-hat bushes to replace the original pressed in bearings and bring the inner diameter down to 33.3mm to fit the BBSHD. here's one of them that was made to fit exactly in the empty profile of the BB:
The motor arrived and we were able to do a test fit, but it was a nogo. There was insufficient clearance between the motor and the BB (13mm dimension). We wondered whether there was a chance that it might fit if we removed the cables and the little outlet plate from the frame. It just fitted with nothing to spare, so the cables were re-routed down the outside of the frame. It just about scraped some paint of as it went into position.
The next step was to stop it from rotating. the normal bottom bracket clamp plate wasn't going to work because if the motor moved a couple of degrees, it would exert a huge crushing force on the frame. Also, we didn't want to apply any crushing force to the carbon fibre BB. That meant we need a torque arm somewhere. The only convenient anchor point was the suspension pivot bolt, which rotated unfortunately. We solved it bu replacing the titanium pivot bolt with a steel replica that had an additional shoulder that we could use as a pivot point for the torque arm. That means that the torque arm is retained, but not clamped, so that the arm is stationary while the bolt can rotate with the suspension. We needed an additional spacer to go behind the plate on the main motor shaft to hold everything in line. Here's the torque plate:
One further problem was that the battery's bag was too big for the frame, so we cut the back off and stitched on some velcro flaps. It works but we might want to make a complete custom bag later.
Everything worked a treat. there was a minor electrical problem with the motor, which was soon sorted. I was surprised with how user-friendly the motor is. Em3ev or whoever had programmed it with some sort of power ramp. You can feel the power ramping up as the speed increases. It's much less violent that the GNG system that I used on a previous build. The gear sensor also worked very well. Overall weight is 25.1kg, which is very light for a bike with a 19Ah 50v battery. The guy has his bike now and seems to be pleased with it. I don't think hell be getting a bike rack for his car though. This car has 1500 BHP:
Here's the finished bike:
