Chit ya normally don't see, Diff disected.

TURBO200R4 said:

i think that the idea is to get an rpm drop at about 75 to 80 so instead of 3000 it might drop to about 2500 i would guess that the 2.50 diff would accomplish that.
will wait for the math guys.

Click to expand...

Yes that is the idea.

R-III-R Turbo said:

with 80mph being round 3000rpm, changing to the 2.50 ratio would drop revs to 2631rpm @ 80mph

Click to expand...

So that's about a 12% RPM drop, would this be what we are looking for ?

When I built the 1000cc Engine for my 750cc Trident, the gearing was way to low.
I rode down the road, in top gear, ignoring the instruments completely then slowed to a speed that the Engine sounded good.
In this case it happened to be 3000RPM, but was only at about 50MPH.
I did the math to get 3K RPM at 60MPH.
I then machined up a rear wheel sprocket to suit.
The original gearing was 19-50, I dropped the rear to 38, it worked a treat, and yes the Engine did pull this gearing (fully worked Engine)

Paul Bryant said:

So that's about a 12% RPM drop, would this be what we are looking for ?

Click to expand...

if my sums are right then that would give us 9000rpm carpenter heads a theoretical top speed of 273mph

and a miserable 197mph for the 6500rpm slow-pokes

R-III-R Turbo said:

if my sums are right then that would give us 9000rpm carpenter heads a theoretical top speed of 273mph

Click to expand...

Cool, but my teeth start to rattle at anything over 270MPH

R-III-R Turbo said:

with 80mph being round 3000rpm, changing to the 2.50 ratio would drop revs to 2631rpm @ 80mph


done some quick maths & interestingly the final drive ratio appears to kind of take the tranny gearing (when in 5th) out of the equation. think 5th is around 1:1 ratio anyway

take a Metzeler 240/50 R16 rear tyre. 16" is 406mm, plus the 50 aspect ratio on a 240 section means 120mm tyre depth to rim. 406+120+120 = 646mm overall rear wheel diameter.
circumference C = 2 TT R, so in this case C = 2(22/7)323 in mm
=2030mm
=203cm

1 mile is 1,609.34m or 160,934cm

so the rear wheel must rotate (assuming no slip) 792.78 times per 1 mile
since we are talking about 80mph as the example then the rear must rotate 63,422 times in 1 hour so that's 63,422 RPH @ rear wheel
or 1057 RPM @ rear wheel

now - final drive ratio based on 13/37 is 2.85, so engine is running 3000 RPM, divide that by the 2.85 ratio gives 1052 RPM @ rear wheel

5 RPM difference in results going about it in 2 different ways

change the 2.85 to 2.50 and that should work back to 2631 engine RPM @ 80mph

Click to expand...

I'm lazy but found a way to check some of your maths. Does anyone know the maximum tyre width and/or aspect ratio that will fit the stock rim and swingarm setup? Maybe better and cheaper to go aftermarket rims like IMFASTOO has to get a wider choice of rear tires.



240/50-R16 vs 240/55-R16 Tire Comparison - Tire Size Calculator | Tacoma World



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Side by side Overlay


240/50-16 240/55-16 Difference
Diameter inches (mm) 25.45 (646.4) 26.39 (670.4) 0.94 (24) 3.7%
Width inches (mm) 9.45 (240) 9.45 (240) 0 (0) 0%
Circum. inches (mm) 79.95 (2030.73) 82.92 (2106.12) 2.97 (75.4) 3.7%
Sidewall Height inches (mm) 4.72 (120) 5.2 (132) 0.47 (12) 10%
Revolutions per mile (km) 792.5 (492.43) 764.13 (474.81) -28.37 (-17.63) -3.6%
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Speedometer Difference
Speedo Reading Actual Speed
20 mph (32.19 km/h) 20.74 mph (33.38 km/h)
25 mph (40.23 km/h) 25.93 mph (41.73 km/h)
30 mph (48.28 km/h) 31.11 mph (50.07 km/h)
35 mph (56.33 km/h) 36.3 mph (58.42 km/h)
40 mph (64.37 km/h) 41.49 mph (66.76 km/h)
45 mph (72.42 km/h) 46.67 mph (75.11 km/h)
50 mph (80.47 km/h) 51.86 mph (83.46 km/h)
55 mph (88.51 km/h) 57.04 mph (91.8 km/h)
60 mph (96.56 km/h) 62.23 mph (100.15 km/h)
65 mph (104.61 km/h) 67.41 mph (108.49 km/h)
Speedometer forum embed code:
Gear Ratio / RPMs for New Tire (@ 65 mph)
2.35 2.73 2.94 3.07 3.21 3.31 3.42 3.55 3.73 3.91 4.11 4.27 4.56 4.88 5.13 5.29 5.38 5.71 6.17 7.17
1945 2259 2433 2540 2656 2739 2830 2938 3086 3235 3401 3533 3773 4038 4245 4377 4452 4725 5105 5933
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vs 275/45-16 vs 275/50-16 vs 225/55-16 vs 240/55-16 vs 245/50-16 vs 300/50-18 vs 240/40-18 vs 200/50-16

@Paul Bryant - re much earlier comment re rear diff drain plug height preventing full drain - wouldn't it be much easier to use a large syringe (with a bit of plastic tube attached) to drain remaining used diff oil than risk dropping the bike etc by leaning bike over?

Ishrub said:

@Paul Bryant - re much earlier comment re rear diff drain plug height preventing full drain - wouldn't it be much easier to use a large syringe (with a bit of plastic tube attached) to drain remaining used diff oil than risk dropping the bike etc by leaning bike over?

Click to expand...

Indeed it would, it's always the simple things that make life easier
When I first got my Rocket, and not knowing what sort of service history it had, I changed the diff (and all) oils about three times just to purge out any possible crap lurking in there.

R-III-R Turbo said:

and a miserable 197mph for the 6500rpm slow-pokes

Click to expand...

Well, that's out then......

R-III-R Turbo said:

with 80mph being round 3000rpm, changing to the 2.50 ratio would drop revs to 2631rpm @ 80mph


done some quick maths & interestingly the final drive ratio appears to kind of take the tranny gearing (when in 5th) out of the equation. think 5th is around 1:1 ratio anyway

take a Metzeler 240/50 R16 rear tyre. 16" is 406mm, plus the 50 aspect ratio on a 240 section means 120mm tyre depth to rim. 406+120+120 = 646mm overall rear wheel diameter.
circumference C = 2 TT R, so in this case C = 2(22/7)323 in mm
=2030mm
=203cm

1 mile is 1,609.34m or 160,934cm

so the rear wheel must rotate (assuming no slip) 792.78 times per 1 mile
since we are talking about 80mph as the example then the rear must rotate 63,422 times in 1 hour so that's 63,422 RPH @ rear wheel
or 1057 RPM @ rear wheel

now - final drive ratio based on 13/37 is 2.85, so engine is running 3000 RPM, divide that by the 2.85 ratio gives 1052 RPM @ rear wheel

5 RPM difference in results going about it in 2 different ways

change the 2.85 to 2.50 and that should work back to 2631 engine RPM @ 80mph

Click to expand...

ooh!! my head hurts ... my brain stopped spinning somewhere around the word rear wheel!!
and all this for what!! just a few RPM , we are swatting mosquitoes with thor's hammer here.

Ishrub said:

@Paul Bryant - re much earlier comment re rear diff drain plug height preventing full drain - wouldn't it be much easier to use a large syringe (with a bit of plastic tube attached) to drain remaining used diff oil than risk dropping the bike etc by leaning bike over?

Click to expand...

I have a 20" bit of clear tubing which I use to suck the bottom oil out big point here is do not swallow