My first folding electric bike had 24 inch wheels, and only six gears, but it did a good job. It was so good, I bought another one like it when I saw it on Ebay, for a good price; that gave me an extra bike, an extra battery and an extra charger. Sadly, that extra bike broke one day; the frame just cracked. Oh well. Spares.
The original 24 inch wheel folder carried on working, but the pedals were getting wonkier and wonkier; the ball bearings on the bottom bracket were obviously shot. The bike shop said it would be impossible to fix, because you can't get the parts. That's because the rear wheel bracket was very wide, to accommodate the motor, the gears and a drum brake (most unusual). So, I thought, if even Dees of Amersham can't fix it, that's that.
I kept it, because it still did actually work, although with a very wobbly pedal action. I wasn;t actually using it because of that.
I had another look at it the other day, and after huge effort, I got the bottom bracket dismantled; it needed a couple of hours of pounding with a hammer to get it apart. Then I went on Ebay to see if I could find parts. The axle was 160 mm, and the longest sealed unit I could find was only 131mm, but I bought it anyway, it's only £12. And I bought a replacement ball bearing set, because the existing axle seemed to me to be only slightly worn, so maybe I could fix it that way. Again, only £7. Bike parts are cheap!
Today, the sealed unit arrived, and I installed it. It turned out to be long enough, although I did have to bend an arm at the bottom of the bike to give the pedals enough room to turn. Then I adjusted the gears, because it wasn't getting down to bottom gear, and took it for a spin. It works fine! So now I have another bike I can use while I'm waiting for the replacement wheel from China.