optimal gearing for 850 Commando

My '74 Commando came with a 20 tooth countershaft sprocket which I changed out for a 22-tooth a number of years back. Now I am having second thoughts about this conversion. The 22-tooth definitely makes for relaxed interstate cruising at speeds above 65 mph or so but I am finding the gearing a tad tall for the 2 lane twisties. It also requires a significant amount of clutch slipping to get it moving. Since I am spending more time riding the latter compared to the former, I am thinking of going back to a 21-tooth or possibly the 20-tooth.

So my question is this:

What are your opinions about optimal countershaft sprocket size for an 850 Commando that is ridden primarily on secondary roads at speeds from 55-70 or so? My understanding is that US-spec Commandos generally came with 19 or 20-tooth sprockets but that Euro spec models had 21 or 22-tooth sprockets. Any and all opinions much appreciated!!

Tobin

Ride the bike on a level road at 70 mph, and note the revs, Then look at your manual and find out where max torque is, and set your gearing so that the revs at 70 mph are 10 % to 20 % below the max torque figure. A simple ratio of the number of teeth multiplied on the sprocket you already have in use, by the percentage drop or increase in revs you desire will put you in the ball park. You won't get it exactly right - choose the higher ratio. Your gearing should be set so that you bike will aaccelerate away from 70 mph fairly rapidly, yet still give you smooth cruising. Gearing is quite deceptive, you can have the situation where the gear changes don't do very much, but if you drop the size of the rear sprocket by one tooth the bike will sometimes simply accelerate faster. Going up one tooth on the gearbox sprocket is quite a big jump.

My MkIIA started out with a 22T sprocket and I was hardly ever using 4th gear on the local roads (twisty back-roads, up to 70 mph).
I'm a tall heavy guy and ride 2-up quite a lot.
I put a 20T on and I'm totally happy with the gearing now. It's a bog-standard motor which seems happiest between 3000-5500 rpm, it feels relaxed and long-legged like no Triumph could ever be

acotrel said:

Ride the bike on a level road at 70 mph, and note the revs, Then look at your manual and find out where max torque is, and set your gearing so that the revs at 70 mph are 10 % to 20 % below the max torque figure. A simple ratio of the number of teeth multiplied on the sprocket you already have in use, by the percentage drop or increase in revs you desire will put you in the ball park. You won't get it exactly right - choose the higher ratio. Your gearing should be set so that you bike will aaccelerate away from 70 mph fairly rapidly, yet still give you smooth cruising. Gearing is quite deceptive, you can have the situation where the gear changes don't do very much, but if you drop the size of the rear sprocket by one tooth the bike will sometimes simply accelerate faster. Going up one tooth on the gearbox sprocket is quite a big jump.

Click to expand...

Thanks. Problem I have at the moment is that my tach is in need of a rebuild as it is reading at least 1000 rpm too high and fluctuating wildly. So I don't have an accurate estimate of revs at the moment. Just by seat of the pants, the bike is making good power up to about 60 mph in 3rd gear with the 22-tooth sprocket

B+Bogus said:

My MkIIA started out with a 22T sprocket and I was hardly ever using 4th gear on the local roads (twisty back-roads, up to 70 mph).
I'm a tall heavy guy and ride 2-up quite a lot.
I put a 20T on and I'm totally happy with the gearing now. It's a bog-standard motor which seems happiest between 3000-5500 rpm, it feels relaxed and long-legged like no Triumph could ever be

Click to expand...

Exactly what I am finding as well. Never got out of third gear yesterday on a 50 mile back roads jaunt. Bike is making good power and feels reasonably relaxed up to about 60 mph in 3rd gear.

B+Bogus said:

I put a 20T on and I'm totally happy with the gearing now.

Click to expand...

+1. My situation exactly; 22T originally, 20T now and just right for the back roads I normally ride.

My '74 850 came with a 21-tooth front. I swapped it to a 24, then a 23. The 24 was great on the highway, with easy 80-mph cruise and 55 mpg, but I rode it down Highway 1 and it didn't have quite the pep I wanted on the winding stuff. And there was a fair bit of clutch slip necessary off the line, of course.

Now, the 23 gives me 76 mph indicated at 4000 rpm. With my taste in riding I don't think I would go under 21, but that's me, keeping in mind I do like it with the 23!

A little math should give you a good idea. The torque-curve suggestion above is probably good. I know my bike is really smooth between 3000 and 4200, and the 23 gives me smoothness at the speeds I ride most.

From this page, http://jerrydoe.com/nortonCommandoTechnical.html, come the numbers below (edit--I tried to space those out to read better, but it doesn't stick, so go to the link):

Estimated Speeds Using 19-23 Tooth Gearbox Sprockets
Sprocket Size Overall Gear Ratio 6000 rpm 6500 rpm 7000 rpm
19 4.84 92 mph 99 mph 107 mph
20 4.60 97 mph 105 mph 113 mph
21 4.38 102 mph 110 mph 119 mph
22 4.18 106 mph 115 mph 121 mph
23 3.99 112 mph 121 mph 130 mph

Ran 23 on a 73 750 with Mk II Amals , Combat cam slightly ( maybe ) advanced , and a few other tricks ( BSA Adv Unit ).

Getting under way , 1100 rpm was required as about 10 mph minimum with clutch engaged , 800 rpm then minimum or declutch / slide .

So getting out off the toll gates in winter was a pig in the rain , Even at 1100 rpm letting it in briskly it'd wheelspin.If you blanched and backed off
it'd bog / stall if you didnt watch it 110 % .
The best way to avoid agravation ( this was with a 4.00 19 K70 aft , further raiseing the gearing ) was to let it in at 1100 rpm with a leading throttle .
Keeping it above half throttle allowed you to wheelspin controllably till 90 mph and the shift into top .

Dont swing off the motorway onto the soft when your fed up with the trafic and shift into second and dump the clutch with the wind off the ocean though .
Youll get a 50 ft roostertail and FEEL the dark penetrateing glares , thus a glance over your shoulder will assure you theres better places to be at that time .



22 to the 44 rear sprocket some whaine about as its 2:1 so the same tooths meet repeatably . Was Std Gearing for the Dunstall ' 850 ' I believe .

Obviously a 5 spped is required . :idea: Studying gears about 3.65 : 1 on first ( crank to wheel , is about right.maybe. perhaps .

The old trials ratios had low 1st and 2nd , a gap , high ( close ) 3rd & 4th , I S D T type set up . so the extra one would go in the mddle.
Close 4 speed and reasonable top ratio gets first where second ordinarilly is . Therefore FIRST IS MISSING . This is why FIVE gears isnt a bad idea .
Or SIX , get a ideal first , Overdrive top , and decent spaceing between . You dont need to shift a lot ordinarrily , But chose a ratio for the required Speed / rpm range utilised .
olde Bean .

The contraption initially mention cruised at 80 mph as a natural gait , out of town .And got 70 mpg , 50 mpg in town ( not sitting at lights often )

THEREFORE , thats about right for that. ( four speed )

With the standard cam back in , it was a lot less requireing of ones undivided attention and disenclined to punch holes in the landscape as in its previous mode
which was akin to opperateing a destroyer amougst the sunday yachtsman , if you see what I mean .

Spontaeneious evacuation off the right hand lanes occured on occasion by the minnions. Must have Been Pa Norton sitting alongside the big chief upstairs ,as far as Im concerned . ,
Was still people who appreciated spirited opperation of mechanical contrivances back in 80 . I do own the road .I pay my taxes .

My 850 has the stock 20 tooth sprocket and I have no desire to change it. 65 mph is about 4000 rpm and that's where it starts to come on the cam. Riding the 2-lanes at 45 mph is just under 3000 where the iso's are working OK, and the 850 has enough torque to still have throttle response there. I don't have the nerves or reflexes to sustain 80 mph any more, so 20T it will stay.

Long story but in the 20 or so years I've had the 850 MKIII I have only put 125 miles on it. I bought it seized and it had major lower end issues... Any way the 125 miles were put on with a stock rear sprocket and a 21T up front. I do have an 18 in rear wheel, further lowering the gearing. My recollection is that it was revving uncomfortably high on the freeway @ 70 ish MPH.

I recently talked to two local gurus one says a 22T would be great the other says the 850 will easily pull the 23T. I don't know. I felt like I was abusing the thing with the 21T.
Oh yea, the remanufactured cam failed at 125 MI. No I wasn't jumping on the FWY and holding it at a steady speed, or redlining, with a fresh motor. It's been apart a long time, think decades, and it's time to get it back on the road.

Thanks for all the great replies guys! Sounds to me like the consensus opinion is 20T. I think I will try going back to the 20T and if I find it too low, will fit a 21T. Thanks again.

Tobin

I've found that my seeley is quite deceptive if the gearing is too low. When I first started racing it, it was revving too high, so I raised the gearing substantially. It still revved too high, but just went faster. I'm not used to a motor with so much torque, but these days I do everything I can to use it effectively. I have now fitted a TTI six speed CR box in plasce of the four speed CR box I was using. i haven't tried it yet, but it should be really good.

I've found that my seeley is quite deceptive if the gearing is too low. When I first started racing it, it was revving too high, so I raised the gearing substantially. It still revved too high, but just went faster. I'm not used to a motor with so much torque, but these days I do everything I can to use it effectively. I have now fitted a TTI six speed CR box in plasce of the four speed CR box I was using. i haven't tried it yet, but it should be really good.

Click to expand...

This is what I found with Ms Peel- accidental small port head on light narrowed crankshaft - once 2-1 header +long Dunstall mega got unbaffled enough, OMG. Belt + 20T for 21T ish ratio made 2nd good to 90, 3rd ok to 110 but in 4th pulled great right up to 6000 then just topped out. Would lift front as topped out it 1st and 2nd into red zone. First discovered in far over leans btw not bee lines till later, OMG> If not for the over rev event above engine would already by well tested with Drouin and couple more T in the sprocket. !st would be good to over 70 and 2nd to 100+. RGM has various cog ratios, I'd lower 3rd for better pull to cruise in 4th.