Front Sprocket deliberation

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olympus

olympus

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The std front sprocket for 72 750 is i believe 21 teeth
On mine is currently fitted with 19 teeth
I understand its smaller so will rev higher for a given speed etc.
But How much difference does this actually make
Are you better off with a 19 or 21
If i was doing lots of high speed motorway/freeway operation then yes the 21 would be best ... but it wont be used for that, so am i better off with the 19 teeth??
Your thoughts would be appreciated
 
My original 71 Commando had a 19T sprocket. Since, at the time, I was pretty much into drag racing anybody at every stop light, its was the perfect sprocket. When I bought my current '73 in '06, it had a 23 tooth sprocket - the previous owner did a lot of interstate cruising so I guess that was a good choice but I thought the acceleration was a bit lacking. In '08 I installed the CNW 520 chain kit and a 21T sprocket and I have been very pleased with that combo for general-purpose use. As you noted, it would depend on what you plan to do with the bike but I think the 21 is a good compromise.
 
If you consider 20 teeth as the amount of teeth in a front sprocket, then loosing or adding 1 single tooth changes the ratio by 5%. (1 in 20) changing up or down by 2 teeth changes the ratio by 10%... So whatever speed you are doing at 5,000rpm, if you add 2 teeth to the front sprocket then you'll be doing that same speed at 4,500 rpm. (being that 10% of 5,000rpm is 500rpm )

If you consider the back sprocket to be ~ 40 teeth, then loosing or gaining 4 teeth would be 10% change, so 1 tooth change in 40 is 2.5% change per tooth.

I use a 21 front sprocket myself too.
 
On four of my Commandos I changed out the 19 tooth sprocket for a 21. I tried a 22 tooth on one bike, but found it too "tall", as in the acceleration was a bit lame. Lots of previous discussion on this topic. Some folks make this decision on the basis of acceleration over cruising, some on the basis of vibration, some on where they live: desert dwellers seem to go for the taller gears.

Stephen Hill
 
Keep in mind your location or where you ride the most. If you have a five speed you can get away with more teeth obviously.
 
20 tooth here but I avoid open road long distance rides and prefer to prowl the local canyons where coming off corners while riding with guys and gals on modern bikes is a priority. I confess however that I have a 21 tooth on the shelf which I will try the next time I have the clutch off. Stuck with a four speed, I hit redline too soon when chasing those bikes. And that problem includes those guys on five speed Meridian 750 Triumphs.
 
a bigger sprocket needs more rev to get a good connection to the following gear. this means you need to trash the engine harder for good acceleration, especially with a 4 gear transmission. 21 is good on plain lanscape, not for winding roads in mountains. ive bought a 21 with my 750 and i am happy with it, but i will change to 20 for a good compromise on the next good opportunity, which will hopefully be a worn sprocket.
 
I have an interesting way of checking my bike's speed/rpm/tach/speedo/etc

With any bike, there can be variation in the instrument accuracy. A commando's instruments are primitive but they can approximate readings well enough to be useful if they are calibrated. I run a GPS between my norton instruments to have an accurate speed calculation. I measured my rear wheel circumference and play with the calculator on the website:

https://www.gearingcommander.com/

I enter all the info for a commando and it sets the gear ratios up in the calculator. I adjust the tire size input until the calculated circumference result matches the physical measurement I've made for my rear tire. I add all the optional measurements in the green custom line of calculator that are appropriate, then go down the page to read out the resulting gear, speeds, and rpms.
 
My experience FWIW...
Do not forget to factor in the actual rear wheel diameter. Makes a huge difference.
A 19 tooth with 19" rear wheel and high profile tire as from factory is one thing.
But when I bought my Combat it had original chain primary, 19 tooth sprocket, and 18" WM3 steel rear wheel.
I put on Bridgestone BT45 120/90 soon after to replace some ancient rubber.
It was excellent off the line, but crazy high RPM at highway speed was not to my liking.
I installed a 21 tooth to replace the 19 AND put on a belt drive at the same time.
Much more reasonable at 60-70 mph, but had to slip the clutch a bit to get smooth starts in first gear.
Just replaced the WM3 (2.25") rear wheel with a WM4.5 and the same tire.
Noticeable improvement in acceleration off the line and even had to re-calibrate my eSpeedo for what is now about 1/2" smaller diameter rear due to new rim/tire profile.
4,000 RPM @ 70 MPH seems like a perfect compromise for my riding.
I would consider going to 22 tooth, but never go back down to 19 - racing CB750 at stoplights does not come up too often any more in my world.
 
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olympus & tomspro
19/42 is optional 750 4.84:1 gearing I believe while 21/42 is stock 850 4.38:1 with a four or five speed gearbox.
The Quaiffe five speed has a higher first gear, which suits healthy 850 torque better than a similarly tuned 750 but fourth gear is 4.84:1 like a 750, so you can compare both ratios on the road.
My 21/42 and wheel tyre combo is a wider 18" like tomspro, 4000rpm @ 70mph and for me, the ideal all round gearing from motorways to mountains twisties.
Good acceleration and fuel economy at 70-80mph cruising but a five speed makes it perfect.

Burning off CB750,s Z1,s or Z1000,s not a problem these days as most are show ponies or trailer queens that only get occasional dry weather outings.
Rarely see any Japanese classics in the depths of long distance touring, more likely to spot a Commando or other Brit bike but some get used properly, like a couple of friends riding immaculate, trouble free RD400,s on our Spanish Moto Giro tour in 2013.
 
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With an 850 Commando, high overall gearing with a close ratio box probably give best acceleration, once you get moving. The heavy crank takes a long time to spin up if you rely on the throttle. However if you keep the motor spinning high and race-change up through as close box, the acceleration is very quick. Sometimes the intended use for the bike determines it's functionality ? With a small engine sprocket, the likelihood of spreading the gearbox is greater when you drag race. A large engine sprocket and smaller gearbox sprocket turns the gearbox faster, so the internal loads are less.
 
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FWIW
1975 MKIII with a stock rear sprocket and a 21 tooth front. Roadrider AM 26 100/90-19 rear tire. With a new tire 1000 rpm=17 mph. That translates into 68 mph @ 4000 rpm verified by GPS. That number changes slightly as tire wears out.
I did replace the 2nd gear set with an earlier ratio to offset the one tooth increase on the front sprocket. There was a 20 tooth installed prior to this change. I like this combination much better. Not sure how that plays out on a 750 in terms of power/torque range.
Pete
 
I've run a 22 on my 850 for 32,000 miles, and wouldn't go lower. 240 lb. rider, it works just great in all (street) settings, including mountain switchbacks and superslabs. Any gearing difference is only a very small advantage/disadvantage in each setting.
I truly feel being able to maintain prevailing speed, on a modern highway, is a huge safety factor, so plodding along at 65mph waiting to get hit from behind, versus being able to roll 80 comfortably when traffic is flowing that fast. Although I avoid the slab when enjoying my vintage bikes, sometimes, it makes sense.
 
I believe there is a chart in the owners manual showing speed at RPM based on sprocket size. A good guide to understanding how many RPMs drop for each tooth increase in the front sprocket.
 
The gearbox is a torque multiplier. With a standard gearbox, a commando is too slow for racing. If you use a four speed close ratio cluster, first gear is too high for road use. But with the top three gears of the close cluster and a standard Commando first gear, you'd have a gap to overcome between first and second, but it would not be insurmountable. You just rev a bit harder in first. When I bought a four-speed close cluster, it cost $700. The biggest thing which stops a Commando from accelerating fast is the heavy crank which gives slow response to throttle, so you need to keep it spinning high. The close box keeps it up there.
 
All good advice , I run a 21 on the MK 111 Interstate. My 72 Combat got fitted with a belt drive before I knew that this changes all the geometry and ratios. So off came all my work to fit a different sprocket then primary back together again. One tooth made all the difference.
 
One tooth anywhere makes a big difference, however you often don't know your overall gearing is too low until you raise it. It can be very deceptive. The heavy Norton crank has the tendency to always spin up at the same rate, regardless of the gearing. That is the reason for using a close ratio box when racing. Because the steps are closer, the revs don't drop so much, so you don't depend so much on throttle response. A lighter crank might not necessarily improve the situation, because it affects the torque characteristic. Once a Commando is accelerating, nothing stops it. On a race track there are bumps and head wind changes. The heavy crank handles that better. One of the things which really pleased me when I got my bike going, it that the actual design of the motor is so good in almost standard form. And it really loves methanol fuel. I'm sorry that I never believed in the motor when I built the bike - that something so horrible could be so good.
 
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