I'm building a 750 with a GSX-R front-end. Using a CognitoMoto hub means that the stock speedometer drive doesn't fit. I knew that when I started, but now I'm trying to think through other options to try to use it. One option I'm thinking through is trying to mount it to drive off a different input, for example, the edge of the front brake disc or the
rear sprocket:
I posted in my build thread about this, but I'm trying here in order to hit a wider audience. Especially evanphi, if he's still out there! Here are some numbers I've got turning around in my head. I wonder if anyone would like to think through this with me?
Here are some front wheel numbers:
Speedometer drive ratio is 2240:60, i.e., 2240 rpm @ 60 mph. 2240 rpm is the speed of the output / cable end of the speedometer drive. Thanks to math genius
evanphi, we know that translates into 1391.881 revs/km:
(2240 revs / 1 min * 60 min / 1 hr) * 1 hr / 96.56 km = 1391.88 revs/km
Also thanks to evanphi, we can calculate the revs/km of the wheel (the input to the speedometer drive):
.002055 km / 1 rev * 1 / x = 486.68 revs/km
The ratio of wheel revs/km to speedo drive revs/km (i.e., input : output) is:
486.68 : 1391.88 = 1 : 2.86
That is, every time the wheel turns, the speedo drive turns 2.86 times; or, to say it another way, an input of one rev to the speedo drive yields an output of 2.86 revs. This can be seen by turning the speedo drive by hand and counting how many times the output end turns for every turn of the input drive—just under 3 times.
What if the speedo drive was turned by the edge of the brake disc? The revs/km of the brake disc alone (i.e., if it was detached from the wheel and rolling along the ground on its own) are:
.001005 km / 1 rev * 1 / x = 995.03 revs/km
NB this is theoretical; when attached to the wheel the revs/km of the disc are the same as the wheel (486.68; see diagram above). However, if the speedo drive is to be driven from the disc, its theoretical revs/km must be used.
Here's the big question: what size ‘wheel’ would have to be on the speedo drive to get it turning at effectively 1 : 2.86 to the wheel? I'm thinking there's got to be a way to calculate that (e.g.,
http://woodgears.ca/gear/ratio.html), but I'm not an evanphi, math genius...
I'd love to hear any ideas! Thanks.
