Questions about choosing sprocket sizes/final drive ratios

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Rick
Posts: 340
Joined: Thu Nov 11, 2010 1:12 am
Location: Northern Plains, USA

Re: Questions about choosing sprocket sizes/final drive rati

Postby Rick » Sun Mar 20, 2011 4:02 pm

Yes, I meant roundcases, not GTs- if not, that's what I should have meant.
Rick

DewCatTea-Bob
Posts: 2897
Joined: Sun Nov 01, 2009 10:53 am
Location: Near SE side of Lake Michigan

Re: Questions about choosing sprocket sizes/final drive rati

Postby DewCatTea-Bob » Sun Mar 20, 2011 6:41 pm

By: Rick...
" I don't have technical info on a RT450, but using the narrowcase 250 specs will demonstrate what I do.
So, if I put the RT450 12/50 sprockets on a bike with the 250 clutch and transmission ratios, 8000rpm at the crankshaft would give me 57.5mph, probably not what you want. "

____ I've previously pointed-out that the 350/450s have a 2.11:1 -(57/27t) primary-ratio, (not "2.5" like the 250s).
So correcting for that, (2.5/2.11 = 1.185), the "57.5" (x 1.185) then becomes actually over 68-MPH !
__ To go-forth a step further, we can compare the 50/12 ratio with the stock 450Mk3D 32/12 ratio - (4.17/2.67 = 1.56) to get the expected speed (1.56 x 68) of just over 106-MPH. _ (However a stock 450 can't hardly push to 8000-RPM above 100-MPH.)
And also, we can do the same with the suggested 50/18 ratio (1.5 x 68) to get 102-MPH (@ 8k).
(All providing that that source is actually correct, of course.)


" The Avon website gives a AM26 Roadrider 90/90/18" tire diameter as 619mm.
Calculating the rear wheel revolutions/mile and the crankshaft/transmission ration gives the 'fixed' values, "

____ Actually, they're not really exactly "fixed", as the radius of tires is less (than spec) while ridden-on under 40-MPH and the radius increases (with what's referred to as 'tire growth') during speeds above 60-MPH.
So depending on actual tire-inflation, the rear-wheel's working-radius can certainly vary up to 8%, from standing-loaded to top-speed.
Thus for instance if 5th-gear happens to require 3000-RPM @ 30-MPH, then at 90-MPH it could be possible for the engine-revs to be as low as just 8300-RPM (instead of the expected 9000), due to the "tire-growth" factor.
(So anyone who has ever noted any such speedometer & tachometer (seemingly)- disharmony, ya can now realize a good reason for it.)


Dukaddy-DUKEs,
-Bob

SIDE-NOTE: - The previous four posts in-a-row are all concerning the 'L-twins' without any relevant mention of Duc.Singles as well...
That's not really any big-issue at all (in MY opinion) since those posts were kept fairly short ! _ This raised-point has only been noted here just to help keep us aware that such non-Duc.Single topic-matter is not allowed to 'take-root' here at THIS particular DUCATI-w.site !
(I leave it up to Jim [in this case], to decide whether or not to delete them.)
That's all !
DCT-B
PLEASE NOTE... If this-post is not-yet signed-off with '-Bob', then I'm still in the process of completing it,, and if not also included with 'DCT' near bottom as well, then I may edit this post's wording at a later time. - Dct.Bob

jbcollier
Posts: 86
Joined: Sun Oct 10, 2010 2:30 am

Re: Questions about choosing sprocket sizes/final drive rati

Postby jbcollier » Tue Mar 22, 2011 1:35 pm

I have a configurable excel program for Ducati single gearing. Tire size, sprocket size, change up/down points, etc are all factored in. Ping me off line and I'll send you a copy.

jbcollieratshawdotca


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