Mk3 gearing

Hi all,
I notice that Andover offer final drive sprockets up to 25T - up from the 22T standard for my Mk3. I spend a fair amount of my riding at dual carriageway speeds, so I'd value a relaxed cruise over blistering traffic-light grand prix acceleration. The question is - will a Mk3 in reasonably fine fettle, on a 34mm Mikuni, with a reasonably trim 13 stone solo rider pull, say, a 24T gearbox sprocket? Anyone done it?

Cheers, Jon

All I know is 22T is PLENTY tall, cruise 80 mph at 4K RPM, 70 mph at 3500, 60 mph at 3000. And I like tall gearing.

If you are running a 22T I would just stay with that. Knowing fuel costs way more on the other side of the pond would be my consideration. But over-speculations of late by greedy margin traders signal oil price crashes for the next while so just enjoy.

Mine must be on lower gearing than that then - it seems to be doing 70 at 4000rpm. maybe it's on a 20T?

Centerstand , crawl under and start counting with flashlight. Of course bring a roll of paper towels along.

Torontonian said:

Centerstand , crawl under and start counting with flashlight. Of course bring a roll of paper towels along.

Click to expand...

+1

Assuming stock 42T rear (and no PO dicking with the primary drive ratios), then a 19T would return ~4050 rpm at 70 mph. I run 22/42 on the '74 and really like it for freeway work, running anywhere between 55 and 85 mph (legal in certain parts of Idaho) while turning ~2750-4250 rpm. Single 34mm Mikey returns 56-58 mpg all the while, and it still feels like it could pull another gear/overdrive! My money is on a 19T front sprocket...

Nathan

Boxerfan said:

Mine must be on lower gearing than that then - it seems to be doing 70 at 4000rpm. maybe it's on a 20T?

Click to expand...

According to the gear ratio table in the MkIII manual, 22T = 106 MPH @ 6000 RPM, so 70 MPH at 4000 RPM would be correct (17.66 MPH per 1000 RPM) however, can you be sure both instrument readings are accurate?

Thanks chaps, I'll get counting!

L.A.B. said:

Boxerfan said:

Mine must be on lower gearing than that then - it seems to be doing 70 at 4000rpm. maybe it's on a 20T?

Click to expand...

According to the gear ratio table in the MkIII manual, 22T = 106 MPH @ 6000 RPM, so 70 MPH at 4000 RPM would be correct (17.66 MPH per 1000 RPM) however, can you be sure both instrument readings are accurate?

Click to expand...

Foiled again! Thanks for the shot of reality. I should know better than to blurt out without proper verification.

After paying closer attention to the clocks, it looks like the speedo is about 8% optimistic. 'Turns out, the tacho is actually better at giving ground speed than the bike's speedo is, as reported by a friend's GPS-based speedo. Maybe I should make little pencil marks on the tacho...

Nathan

L.A.B. said:

Boxerfan said:

Mine must be on lower gearing than that then - it seems to be doing 70 at 4000rpm. maybe it's on a 20T?

Click to expand...

According to the gear ratio table in the MkIII manual, 22T = 106 MPH @ 6000 RPM, so 70 MPH at 4000 RPM would be correct (17.66 MPH per 1000 RPM) however, can you be sure both instrument readings are accurate?

Click to expand...

I was just away looking at the same thing. the chart stops at 23T, which gives 18.7mph/1000rpm. I guess if I want 4000rpm @ 80mph, it's going to have to be 24 teeth...

I ran a 23 tooth sprocket on my MK1 850 for long enough to wear it out. Mainly for long country runs. It was relaxed and nice to ride. I then went to a 22 tooth and found that to be just about ideal for all round use. I also run a 34 Mikuni.

The problem with gearing up a Commmando is that it increases the minimum speed where smooth running starts. On my bike with 21 tooth cs things start to smoothen out at 50 MPH in fourth and become very smooth from 55 MPH on up. This is handy when running on old highways with low speed limits or in heavy, slowish traffic. If most of your travelling is on an Interstate or Motorways then the higher gearing might be beneficial, otherwise best to keep stock or lower gearing for smooth running.

Glen

I'm running R.G.M. belt enclosed primary kit to spin the gearbox faster to stop all lugging to the gears. 19 tooth sprocket on that is the best cash outlay ever for Crazy. That and the D.T. headsteady.

Nothing like trial and error but I found gearing for 80 @ 5000 was best on Peel with beveled front iso's so nil vibes lugging in 46h down to 30 mph, which if easy on throttle would smoothly pick up to top speed w/o jerking drive or engine. With this tallish gearing 1st gear is a definite launch commitment to expect, which is not noticed on pavement but groin tightening on some tight steep non paved paths. Could take off in 2nd though with a slight bit of clutch slip and hang on too almost 90 mph which was Peels most used gear for sport bikes but could pass normal traffic in 4th smartly and wanted to shift to 5th as so easy to ascend steeps on low throttle it tended to gain speed too easy. Gearing so hi cruise speed is 500 - 1000 rpm or 7-14 mph below torque peak feels right to me, as not sensitive to throttle till ya on purpose crack it then wakes up Commando torque rise response.