scootz
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| posted on 29/4/11 at 05:19 PM |
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Brain Fade... Calculating Rear Sprocket Size!
Can anyone give me a cyber-slap to shock my brain back into action, or alternatively help out with the maths!
I'm trying to work out what rear-sprocket size I'll need for the reverse-trike.
The wheel I'm using is an Avon CR500 (245/55/13) with a circumference of 1787mm
The standard rear wheel for the Aprilia RSV-R Bike has a circumference of 1973mm.
The Aprilia ratio's are:
Cheers in advance!
It's Evolution Baby!
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zilspeed
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| posted on 29/4/11 at 05:39 PM |
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You need the Westgarage gearing calculator.
Another thing.
What top speed are you aiming for ?
What does the engine rev to ?
[Edited on 29/4/11 by zilspeed]
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daviep
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| posted on 29/4/11 at 10:25 PM |
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If you are trying to get the same gearing then you need a 36 tooth sprocket on the back. I think 
Davie
“A truly great library contains something in it to offend everyone.”
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trikerneil
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| posted on 30/4/11 at 05:22 AM |
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My calcs agree with Daviep, 36 tooth rear sprocket for the same gearing as the bike.
However you might consider slightly lower gearing as you will be moving more weight with the Trike.
HTH
Neil
ACE Cafe - Just say No.
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scootz
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| posted on 30/4/11 at 07:42 AM |
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Cheers people!
It's Evolution Baby!
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scootz
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| posted on 30/4/11 at 07:49 AM |
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quote:
Originally posted by zilspeed
Another thing.
What top speed are you aiming for ?
What does the engine rev to ?
The engine maxes out at 11k rpm.
I hadn't really thought about top-speed... I'd think that 100mph would be enough!
[Edited on 30/4/11 by scootz]
It's Evolution Baby!
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MikeRJ
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| posted on 30/4/11 at 08:20 AM |
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You probably don't want to aim for the original RSV gearing; it's very long, requiring plenty of clutch slipping in first. Most owners
drop a tooth or two on the front which turns an already quick bike into a bonkers device that likes to point it's front wheel skywards most of
the time.
Where did you get the rolling circumference numbers from, they both seem a little bit low?
I wrote a gearing calculator for the RSV in Excel which you are welcome to. The
built in rolling circumference calculator uses a fixed 97% of static circumference which seems to tie in with published in gear speeds, but obviously
you can change this to suit.
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zilspeed
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| posted on 30/4/11 at 08:54 AM |
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Personally, I would be gearing it as short as possible.
You're never going to get it up at the mad speeds the bike will do, so why not gear it short so all the gears in the box become available to
you.
Maybe look for a top end of maybe 120mph ?
Going on 10500rpm rather than 11000, a 53 tooth rear sprocket would give 119mph on a 21.68 Inch diameter tyre.
A 60 tooth sprocket would give 105mph. If you really are going to rev it out to 11000, 110mph.
I would have the 60 tooth sprocket as the vehicle is going to be all about point and squirt at speeds under 100mph.
Anything more is irrelevant.
Mikes calculator backs this up with the following intermediates through the gears.
1st 39.93
2nd 55.48
3rd 69.62
4th 82.97
5th 94.28
6th 102.32
40mph in first is still enough for anyone.
[Edited on 30/4/11 by zilspeed]
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Triton
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| posted on 30/4/11 at 09:26 AM |
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Mine is going to be geared like a crosser...mad acceleration but not so mad top speed.
Mainly because I drove the Hotbuggy a few years ago which was fitted with a humble CBR600 motor from the early pig styled things, it had an huge
sprocket and hit the limiter really quickly....what a hoot to drive but a cow to stop as your feet bounce off the damn pedals(has two brake pedals for
this very reason...
My Daughter has taken over production of the damn fine Triton race seats and her contact email is emmatrs@live.co.uk.
www.tritonraceseats.com
www.hairyhedgehog.com
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scootz
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| posted on 30/4/11 at 10:22 AM |
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Ta for the inputs!
This is my favoured combo so far... could be a bit wearing if cruising though.
[Edited on 30/4/11 by scootz]
It's Evolution Baby!
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MikeRJ
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| posted on 30/4/11 at 10:39 AM |
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What's the target weight for your trike? It's going to be mad as a box of frogs with a low geared Mille engine 
If you don't already have them, you can get the Aprilia service manuals for the engine and rest of the bike
here.
[Edited on 30/4/11 by MikeRJ]
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scootz
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| posted on 30/4/11 at 01:48 PM |
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Thanks Mike.
I'd love to get over 500bhp/tonne, so anything under 280kg would be a boost!
It's Evolution Baby!
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Ivan
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| posted on 30/4/11 at 02:05 PM |
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Scootz - although your chosen gearing might well cause it to accelerate like a warm cat don't you think that more than 10 minutes of driving it
at 7000RPM at 70 MPH will become very tedious - never mind all the bikers etc who suggest you change up a gear as they pass you.
I suppose the one big plus is that it is easy enough to change a sprocket if you need to. Wonder if a dérailleur system could be made to work on chain
driven BEC's then you could have overdrive 
[Edited on 30/4/11 by Ivan]
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