Smaller vs Larger Chain Sprockets????

Mark Cigainero · Nov 5, 2007

I am restoring a 74 850 Commando. The previous owner replaced the front chain sprocket with a smaller one?? I assume he did this to get more off the line jump or torgue out of the bike. I am in the process of removing the engine and getting ready to paint the frame. Should I put the original larger sprocket back on while I have the primary off??? What are the pros and cons to a smaller vs larger front sprocket??? First gear is a bit tall for my liking and I do not plan to make a racer out of it. How much will the smaller one effect the top speed of the bike???? Thanks again for all the help you guys have sent to me---

norbsa48503 · Nov 5, 2007

A 19 tooth is favored by those who don't go on the expressways. As soon as you have the requirement to do sustained speeds of 80 MPH you need a 20 or 21 tooth front sprocket. A bike that has a 19 will feel nice and light at take off in fist gear as you make the jump to bigger sprockets they feel heaver till you find yourself slipping the clutch to get off from a stop. to work it backwards at your top sustained speed needed you should be geared to keep the motor running 4250 RPM. I use third gear for 55MPH roads and fourth for the 80MPH roads. 20 tooth with a belt drive makes it like a 21 tooth. Each tooth being good for about 500 RPM in fourth gear.

zotz · Nov 6, 2007

According to the Norton tech digest a 20 tooth sprocket will yeild 97 mph @ 6000 rpm and 92 mph for a 19. I went from a 20 to a 19 on my 750 and went right back up a week later. The bike felt buzzier and like it had less engine though it was quicker off the line. I prefer a more relaxed feel while cruising at 70 mph. It all depends on your preferences.

bill · Nov 6, 2007

the first of october I did 2 650 mile days on a 72 750 combat. It has a 21 tooth and could run 70-75 all day it came with a 19 and would I not go back to it, said it was way to buzzy! I even have thought of trying a 22 as I have a new one just laying around.

windy

tuono · Nov 7, 2007

Completed full rebuild in my 8502A with single 34mm mk2 Amal,1.5"bore exhaust and 22 tooth output sprocket. I do not have to slip clutch on getaway and it happily pulls top gear from 1500rpm + easy 60 mpg. I was warned against 22 tooth for some illogical reason. It also gives 70mph cruising at around 3700rpm which is right in the max. torque area so I am more than chuffed. Incedently, noone got past me at Thousand Bike Festival at Mallory Park with this set up and people were trying! :wink:

79x100 · Nov 7, 2007

tuono said:
I was warned against 22 tooth for some illogical reason.

-

The reason is not entirely illogical and it's not because the 22t doesn't make for a pleasant motorcycle.

According to the NOC Service Notes, the first signs of a spate of gearbox failures occurred around the time that UK spec bikes were being supplied as standard with the 22t sprocket and problems were noted as more common on bikes so equipped.

It was felt that the higher axial loads contributed to the problem and there was a recommendation to restrict to 21t.

If a higher gearing is required, a better solution is to speed up the gearbox. Most belt conversions do this.

BrianK · Nov 7, 2007

Interesting. What is the stock size sprocket? (If relevant, mine is 1 73 850 Mk I, US version). Whatever mine is (and the way it's buried on a Commando, I can't see it without contortions), it seems right to me, and I'd want to maintain the same ratio when eventually I convert to a modern o- or x-ring 520. Thanks - BrianK

Ron L · Nov 8, 2007

What is the stock size sprocket?

In the US, 750 Roadster came with a 19-tooth countershaft, 750 Interstates with a 20-tooth.
850 Roadster 20-tooth 850 Interstate 21-tooth. At least that is what I have found and have been told.

Interestingly enough, the MkIII riders manual lists 20T for USA and 22T for the rest of the world with no difference for models.

Oddly the 1973 riders manual lists a 21T countershaft with a 42T rear and for USA a 44T rear for 850 and a 45T for 750!! I have never seen anything other than a 42T rear sprocket, nor have I found a part number!

tuono · Jan 25, 2008

I have run 22t for years - no probs- go figure. Some contributors seem obsessed with what happened 35 years ago. Surely 99.9% of Commandos on the road are past their warranty claims :!:

Ron L · Jan 25, 2008

I have a 19 on my Mk V 750 Roadster. It does very little freeway time, mostly backroad scratching. The acceleration with the slightly higher compression of the Mk V and the 19 tooth sprocket make it a fun ride.

My 850 Interstate has a stock RH10 head and a 2S grind cam. I run a 22 tooth counter shaft sprocket on it. It definitely sacrifices low end grunt, but is very pleasant on the open road. I don't have to slip the clutch excessively to get it rolling. If I am going across town on the freeway this is the bike I'll take.

The 850 cafe racer has a heavily modded engine. High compression, performance cam, big valves, big carbs, etc. It runs a 21 tooth sprocket. There is considerably more low end than the Interstate, but probably equal to the 750 with the 19. However, once the revs hit 3000, it accelerates hard.

The preferred method is always to change gear ratios at the primary if possible, but with the Commando you don't have that option other than going to a belt drive.

Gino Rondelli · Jan 25, 2008

I've got 4 Commando's all run 22T gearbox sprockets, 2 have belt drives so that ups the gearing as well. On my 850 Interstate I use all the time for big journeys (last year 2000 and a 3000 mile tour) with the 22T & Belt it can be a bit of a pig when its pulling away from lights fully loaded, the clutch takes a beating but I'd rather that than the engine going into meltdown. Hours of Motorway / A road cruising above 4K revs males me nervous mechanically hence the 22T. Ok they aren't as sweet for the run down the shops but I think the trade is worth it.

Coco · Jan 25, 2008

bill said:
the first of october I did 2 650 mile days on a 72 750 combat. It has a 21 tooth and could run 70-75 all day it came with a 19 and would I not go back to it, said it was way to buzzy! I even have thought of trying a 22 as I have a new one just laying around.

windy

I have a 21 tooth over here on my MKIII. CNW is rebuilding my motor and they sugegsted a 21 was a good all around sprocket for a MKIII so I went with it.

BrianK · Jan 25, 2008

FWIW, when I recently replaced my chain and sprockets with a new x-ring setup, I was surprised to find a 19T front sprocket on my 1973 850 Mk. I (one of the very early ones - #3000427). Possibly the DPO swapped out sprockets at some point, but I kinda doubt it.

I ordered a 20T thinking I would be doing a straight exchange. Bike has been in surgery since the sprocket change (alternator, wiring harness, valves, etc. - winter projects) so I have yet to see what change it makes. If I had known, though, I would've stuck with the 19T as I liked the bike just fine with it.

Live and learn! - BrianK

BrianK · May 14, 2008

Folks around here will have noticed that I'm somewhat, um, non-sensitive to nuance (especially with a wrench or a BFH - or both - in my hands), but I don't notice a whit of difference between the 19 and 20-tooth sprockets. Course, I did the conversion over the winter, which means it was a few months between rides with each. FWIW.

grandpaul · May 14, 2008

Tach reading vs speedo indication says it all.

It MUST be noticeable, although probably only slightly.

Gino Rondelli · May 16, 2008

Ok to add my bit to the debate, I run a Mk1 850 Interstate with a 22T & belt drive which makes it great for motorways & main roads but a pain in the towns and country lanes, my JPN runs the same setup. My Mk3 Interstate runs a 22T front with original primary chain setup and its fine. I've also got a 750 which had a 19T (always felt like it was revving way to hard) went up to a 20 (virtually no difference) then a 21 (pretty good) then finally a 22. I suspect ideally a 21T & belt drive or 22T with chain primary would be the best setup IF you do long bouts of 70+ touring. I do though struggle to get good MPG with any of them (2 have Mikuni's 2 standard setups) with 45 - 50 MPG 8) being the max

norbsa48503 · May 16, 2008

I ride a bit with a group of guys that are in their fifties and sixties they like side roads and will only go for short blips on the super highways. They all ride with the 19 tooth front sprockets and have all their fun between 35-70 MPH. I have to ride in third gear to stay with this program but I have fun at these speeds too.
The 20 tooth with a belt drive makes the bike run 4250 at 80 MPH and the bike very much likes that speed and will do it all day. The wind noise is a bit much but 80MPH is self defense speed on our X-ways. A run on a dyno has a lot to do with my opinion about needing to be just above 4000 for a happy motor your mileage may vary of course. If your running these big front sprockets to get down to 3000 RPM cruising speed you may want to Dyno your bike than judge for yourself whats best.
It seems to me that getting fourth gear set up perfect at your favorite speed is what it's all about. Use a GPS handle bar unit, speedos lie at these speeds.

BrianK · May 17, 2008

Y'see, that was my point - the way-too-small 19 is pretty darned good at what it does.

I have not been disappointed in the 20T, but if I'd known, I'd have kept the 19T I had.

I got an airhead for the interstates, the Commando is a backroad bike (and the finest one I know, FWIW). But then again, I need a Beezer....

panic · Jun 4, 2008

19 > 20 tooth reduces engine speed at the same MPH by 5%, reduces torque by 5%, increases MPH at the same RPM by 5.26%.
Duh.

Smaller vs Larger Chain Sprockets????

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