i am involved in a manic project using a shaft drive bike engine,

I have found out the 1-5 gear ratios for the bike engine in question,

Listed is primary and sceondary reductions,

is primary the reduction from crank RPM to gearbox input rpm then you have the gear ratio which dictates the output shaft rpm, then finally the secondary reduction which will be the bevel gear on the end of the shaft which governs rear wheel rpm?


i am not using the rear wheel only the shaft,

and am confused as to what i need to take into acount when trying to work out the best diff choice for the car.

HELP

as a completly uneducated guess, i would've though that the primary reduction WAS the gearbox and the secondary the equivilent of the diff, in this case the thingy at the back.

i know nothing about bikes but cant see why they would have a reduction before the gear box?

I believe the ratio from crankshaft to clutch/ gearbox to be the Primary ratio.

In my old Villiers 210 karting days , we changed the Primary chain every couple of meetings , this was the chain that drove the clutch/gearbox from the crankshaft.

Hope this makes sense....................

I think i know what you mean dave, in car terms the primary is the crank/gearbox reduction and then you have gears 1-4/5/6, then the secondary is the diff.

eg. Engine RPM 5000, primary ratio 2:1, gearbox input is 2500RPM, 4th gear ratio 2.5:1, propshaft turns at 1000RPM, rear diff is 2.5:1 drive shafts turn at 400RPM

Ratios made up for ease of maths!

Is this correct?

Think i have it straight in me noggin now.

Would still like confirmation though!

Dear Help:

Your math is basically correct, but one of your assumptions may need confirmation.

Depending on the orientation of the bike crank relative to the drive shaft, the secondary reduction could also refer the 90 degree drive required to transmit the power to the drive shaft in some layouts, and not the final drive ratio.

Jim

Jim, thanks for putting the seed of doubt in my head!

If that were the case would it not need to list 3 reductions as you also have the bevel gear at the rear wheel?

Just a thought.:

As has been mentioned, primary reduction is the ratio that the crank turns compared to the clutch. Theres usually a gear on the end of the crank that drives a ring gear on the outer clutch basket, and the difference between these two gears is your primary reduction.
Secondary reduction on a chain driven bike is the difference between the front and rear sprokets (so not relevent to BECs). On a shaft driven bike, Id imagine it would be for the bevel gear in the rear hub, turning the wheel, so again not relevent. I guess it could be a gearing difference in the bevel on the output of the engine and the one on the prop, but thinking about it, you want the wheel to go slower than the engine output (as on chain driven bike with small front sprocket and large rear), and if you were varying this gearing at the engine end, you'd need a small bevel gear on the engine, and a large one on the shaft, which would stick out a mile so not really feasible I wouldnt imagine.

Anyway, arent most shaft driven bikes ones that have the crank facing north/south, so dont usually have this middle bevel gear transfer?

To do the calcs, use one of the excel spreadsheets floating about, and edit it for your own engine / wheel / tyre / diff combo. you can add in whatever you want and play around with different options until you get a satisfactory top speed or cruising rpm.

Chris,

Thanks for clarification.
the V-max morot has a v 4 but the v is not side to side to the crank orientaton is the same as on inline 4's

I have put the figures into the spreadsheet and with the granada diff we have (think its 3.62) it will top out at 120 mph in 5th at 11500.

The engine has V-boost which kicks in at 6000rpm and drinks fuel like a bastard, 70mph crusing would be 6600!

The tag on the diff has a load of numbers and letters on it but no ratio, how can i derive its ratio from this?

If it is a 3.62 i think we need to change it or increase the rolling circumferance.

There is some info floating about within the heads of some people on the Yahoo BEC list (I think) that shows how to read these codes, but probably the easiest way to check diff is literally turn it. Mark a line on the input / output flanges and turn the input flange. However many times you need to turn it to get the outputs to rotate once, is your diff ratio.

[Edited on 9/10/03 by ChrisGamlin]