Gbox sprocket for Aus conditions

There is some where on this forum a calculator that tells you what rev's your engine is doing at what speed. All you need to do is put in the 4 sprocket sizes and the gear ratios and rear tyre size. That's how I proved a 21 tooth drive sprocket coupled to an Avon 100-90 [ 25.5 inches in dia ] (3583 rpm @ 62 mph )tyre was hardly revving much slower [ actually about 350 rpm difference] than a 19 tooth sprocket couple to a TT100 410x19 [ 26.5 inches in dia ]( 3448 rpm @63 mph )

With the 21 tooth drive sprocket the lower gears do get used a bit more but on the open road the bike pulls reasonable y well from 35 mph onwards in top gear.

rpm to speed conversion Commando

RPM =( ES/CS)X(FS/RS)x(3.1416X60XWD)/63360 wd = wheel diam

RPM ENG/SPROCK CLUTCH F/SPROCK R/SPROCK (ES/CS) X (FS/RS) GXH/I WDIAM MPH

TOP 3448.183337 26 57 21 42 0.228070175 3.1416 60 63360 0.002975 26.5 62 850
3RD 4172.301838
2ND 5861.911673
1ST 8827.349343

TOP 850 1734 REVS@ 30 MPH = 50 KPH 850 with 21 tooth & 100.90 rear tyre 25.5" 3rd 2100 2nd 2950 1st 4440
2600 REVS@ 45 MPH = 70 KPH 850 3150 4421 6660
3005 REVS @ 52 MPH = 80 KPH 850 3640 5110 no no
3612 REVS @62.5 MPH =100 KPH 850 4370 6140 NO NO NO


TOP 750 1810 REVS @ 30 MPH = 50 KPH 750 with 19 tooth & 400x19 rear tyre 27" 3rd 2190 2nd 3077 1st 4633 4000 = 67 MPH OR 107 KPH
2715 REVS @ 45 MPH = 70 KPH 3285 4615 6950
3140 REVS @ 52MPH = 80 KPH 3800 5333 NO NO NO
3770 REVS @ 62.5 MPH = 100 KPH 4563 6410 NO NO NO

850 21 tooth and 26.5 tyres [ tt100]
1668 30 2019 2836 4271
2502 45 3028 4254 6407
2892 52 3500 4916 no
3448 63 mph 4172 5861 no no


750 19 tooth plus 25.5 tyre [ 90/100 ]
1916 30 3rd 2318 2nd 3258 1st 4906
2874 45 3478 4886 no
3321 52 4019 5647 no no
3960 62 4792 6733 no no no

750 19 tooth Plus 26.5 [ tt100]
1844 30 2231 3131 4720
2766 45 3347 4702 no
3196 52 3867 5433 no no
3811 62 4611 6478 no no no

Hi Connie73.
To answer your query, just pulled the bike down to check the tooth count on the drive sprocket. 21T gives a tad over 4000rpm for 110kph per gps.

Actually, fitting a reinforced swingarm that has been waiting on the shelf for 2+ years since the engine had to come out as oil analysis was showing steel particles.
Ta

I have a MKIII with a 22 T sprocket. I think it's the best all around gearing. I've tried higher and lower gearing, 22 T seems best for me.

needing said:

since the engine had to come out as oil analysis was showing steel particles.

Click to expand...

And this engine had an oil filter ???

Hi Rohan.
Since you keep asking - yes - but as I have said elsewhere, Combats and shop-bought crankcase breathers return unfiltered oil to the oil tank by design but my breather circuit has now corrected this flaw.
Ta.

We can't just assume this is the same bike/problem as in other threads....

Combats don't have oil filters anyway ?
Weren't they 1st introduced for the 850 models.

If you have metal particles coming out the breather, I'd stand well clear !!

Hi rohan.
Metal particles in oil means 'fix it'.
Anything else you want to tell me about my '72 while I get on with fixing it?
Ta.

How much metal though ?
Fine particles, or large bits ??

I fitted magnetic sump plugs to various things I own(ed), and was amazed at how much magnetic stuff was attracted.
Some of them have done hundreds of thousands of miles, and still going strong...

But, no, no 72 Combats amongst them.
We seem to have diverged somewhat from sprog sizes though.

Hi Connie73.
My rear wheel has 2120mm circumference coupled with the 21T drive sprocket, if that helps.
Ta.

Never mind.

hobot said:

Never mind.

Click to expand...

Not good at this stuff but ...

How about a gearbox sprocket recommendation for Canada????

Stephen Hill

I'm in Oz and use a 21 tooth front sprocket on my Mk3. Our local highway speeds are mostly 100 kph and some 110. My bike still has plenty of acceleration and I can cruise all day at 120 (4000 rpm), around town it will plod along in top at 60 kph but I normally use 3rd because we have 50 kph zones also.
Have been known to cruise along at 100 in third gear for several miles before realizing still have another gear, probably been too busy soaking up the beautiful country side here in the Grampians. Typically on a run my normal shift into top is 100 anyway.
Where are you?
Cheers.

Hi all, went with the 21t, new rear sprocket, new chain, new cush drive rubbers. 1st set of lights, stalled. hahah. Its so much smoother now, more application of the gear shifter is needed in the hilly stuff but overall its a great upgrade. Thanks all for your advice.
Im on the south coast nsw

Connie

vintner said:

I'm in Oz and use a 21 tooth front sprocket on my Mk3. Our local highway speeds are mostly 100 kph and some 110. My bike still has plenty of acceleration and I can cruise all day at 120 (4000 rpm), around town it will plod along in top at 60 kph but I normally use 3rd because we have 50 kph zones also.
Have been known to cruise along at 100 in third gear for several miles before realizing still have another gear, probably been too busy soaking up the beautiful country side here in the Grampians. Typically on a run my normal shift into top is 100 anyway.
Where are you?
Cheers.

Click to expand...

Hi Connie.
Thanks for following up the thread to complete the circle.
Ta.