Extreme Downhill Kawasaki KX125 mountain bike with Rohloff Speedhub gearbox

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RichPober
May 16, 2009

This used to be a 1991 Kawasaki KX125H until I took out the engine and sold it to someone who had just rebuilt a much newer engine and blew it up on its first race. I shortened the frame, reducing the wheelbase by 9 inches, by cutting through the welds around the headstock and cutting out about 4 inches from the frame's side spar tubes. The top tube was dispensed with and the shortened side spars were re-attached to the top of the headstock, instead of their previous position at the bottom. I maintained the original head angle.

The 1991 H-model lends itself best to this type of conversion because the top of the shock absorber is held in place by separate bolt-on brackets, as opposed to the later bikes which have dispensed with them and have had them integrated into the frame tubes. I then suspended a 14-speed Rohloff gearbox hub with brackets I made from some flat 1/4" bar. I attached the brackets to the bolt holes used to secure the brackets holding the top of the shock absorber.

I used 415 gauge (1/2" x 3/16") chain to make up the three drive chains. I used a Gusset 10mm BMX chainwheel, Aprilla front sprockets on the jack shaft, and a pair of specially made B&C Express sprockets to mount onto the Rohloff gearbox hub and rear wheel hub. All were 3/16" thick except for the standard Rohloff drive sprocket supplied with the hub. I worked out the ratios to match a typical downhill bike's gear range. The jack shaft was necessary to reverse the drive motion of the Rohloff hub and to take the drive from one side of the bike to the other. The bottom bracket was specially fabricated from a long length of turned chrome/moly steel bar and a mild steel outer tube. I used BMX Primo Powerbite 175mm cranks and push-in bottom bracket cups and bearings because this required the least amount of machining. I cut and re-welded the cranks to shorten them down to 160mm.

The frame was powder coated in a zinc-rich base coat and then top coated in Melon Yellow, RAL 1028. I used a seat from a 2004 Yamaha YZ125. I replaced the rear foot-operated 14mm master cylinder with a 14mm hydraulic clutch master cylinder from some Honda big bike mounted on the left handlebar. I made up the 152cm rear brake hose with fittings from Think Automotive.

I rebuilt the front forks and had the rear shock absorber serviced. I replaced swingarm needle roller bearings with sold bronze bearings and installed grease nipples. The suspension system is brilliant now and soaks up everything.

I shod the wheels with the classic MX knobblies, Cheng Shin Tires C-755.

The bike weighs about 50kg and the rolling resistance is massive. On the flat it's like riding an exercise bike and up hill it's a killer. However, downhill it's unstoppable!

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RichPober (2 months ago) Show Hide

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As you're only going up 25cc, the gearing should stay about the same.

Gearing is easy to work out. Just calculate the ratios between the drive sprocket and the driven sprocket and compare them to the standard set-up. If I need to alter the ratio then I increase or decrease the number of teeth on whichever sprocket it's easiest to replace with either an off-the-shelf gearbox sprocket or a custom made/off-the-shelf rear wheel sprocket.

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