Fun night in with the Patenoster tonight. I’ve been pondering how to make an internal throttle for quite a while now as it seems either very difficult to find a 7/8th throttle or excessively expensive. I would like one of the Posh made ones from Japan but at around £250 it seems rather over the top.

I starting thinking about how to make one and with a full workshop downstairs, if I can’t make one it seems like a bit of a waste of machinery.

I started thinking about what angle slot I would have to cut in a tube to get the desired horizontal movement of the cable which directly moves the slide in the carb. With a bit of thought, a few beers and banter with Maxwel I think I have come up with a formular which will give me the angle of the spiral slot ill have to cut / mill out to get the right distance to move the slide with any given throttle rotation and handlebar size.

a = angle

d = distance to move carb slide

bd = handle bar diameter

r = rotation of throttle

tan(a) = bd / ((diameter*Math.PI)*(rotationInDegrees/360))

So given all this, if I have 22.2mm (7/8th inch) handle bars, and want to rotate the throttle 120 degrees (1/3rd a turn) and want to move the carb slide 40mm (just a guess, can’t be arsed to go down stairs and measure) then the angle of the slot I can cut out of tube is 30.16 degrees.

the length of the slot can be worked out as follows:

slot length (squared) = bd (squared) + ((diameter*Math.PI)*(rotationInDegrees/360)) (squared)

which gives me a slot length of 46.27mm

Obviously I would make the slot length longer in real life just to be safe but this should take the guess work out of the whole process.

The formular in the end was pretty simple geometry but the initial application of rotational movement to horizontal displacment was racking my brains for a while. I could be massively wrong but have order the bar and will get cutting when it turns up and see how it turns out.