920 gearing...

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We do have guys using tyre warmers on historic bikes in Australia. I am much better in the wet than many riders. Comparing with Kenny Roberts is silly stuff. You cannot extrapolate from an historic bike to a modern - they are completely different animals. I found it quite amusing the last few times I raced. I was always up with the leaders - I am too old for that. When I raced back in the 60s and 70s, I always rode a very out-classed machine so usually I got blitzed. I only led All-powers C grade on one occasion. I lowered the gearing on my 500cc Triton, , dropped the tyre pressures and softened the suspension. I led them for almost a lap, but they out-gunned me towards the end of the longest straight. I almost knocked two guys off their Kawasaki 900s, so I did not do that again.
It is always important to notice where you get passed, if your bike is underpowered. I needed 7 gears like the Geoff Duke Gilera had in the 50s.
 
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Classic bike racing is very competitive over here and there are no tyre warmers. 60bhp is a good amount of power in the right hands though.
 

lcrken

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....One thing I've noticed in most of Ken's photos, it looks as though most guys are using about 45 teeth on the rear sprocket. If you do that the bike will accelerate at a certain rate, dropping it to 40 tooth will probably make the bike go faster with no drop in revs.
Commando rear sprocket stock is 42T, and it's fixed to the drum (except for MK3), so that's what you see in many of the photos. You can modify it to take larger sprockets, but not smaller ones. Even with a different rear wheel that allows the use of other sizes, you frequently see something in the same range, just because it works well on the street and on many tracks with a stock primary and the common 19T - 22T range of countershaft sprockets.

Ken
 
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I have wondered why some of your bikes still have drum rear brakes. My rear sprocket could be even smaller than 38 tooth if it needed to be. Every time I have raised the gearing on my Seeley, it has gone faster. I always look for more torque rather than top end. My 500cc Triton was the exact opposite. It accelerated faster the longer you revved it out. On a big circuit, it was ridiculously scary as I reached the ends of the longer straights. But slow coming out of corners. If I lowered the gearing, it was fast around corners but got blitzed at the ends. There was never a happy medium.
I have rarely seen a torque curve off a dyno, but the other day I saw one on Youtube for a 300cc MotoGP bike. Visually there seemed to be almost no peak. But with my 850, it seems to be fastest at 6,500 RPM regardless of the gearing. I think the heavy Commando crank is lovely - once it is spinning high, nothing stops it. But it is useless with a standard gearbox. If you lose revs, you will wait forever.
 
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I might be different to a lot of guys. When I raced regularly, I was severely disadvantaged. My 500cc Triton was 1957 vintage and I raced it in All-powers C Grade in late 60s, early 70s against all comers. So you learn. With a Commando-based bike, you are not really so disadvantaged. At about the time I was racing, Geoff Curley was sponsored in A grade on an 850 Commando. I watched him win an A grade race on it at Winton. If you can do that with an almost standard Commando, they cannot be all bad. Geoff was an excellent rider, but there is not a mechanical bone in his body.
 

acadian

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Commando rear sprocket stock is 42T, and it's fixed to the drum (except for MK3), so that's what you see in many of the photos. You can modify it to take larger sprockets, but not smaller ones. Even with a different rear wheel that allows the use of other sizes, you frequently see something in the same range, just because it works well on the street and on many tracks with a stock primary and the common 19T - 22T range of countershaft sprockets.

Ken
I'm sure it's been done, but not seen it myself, but would a triumph conical hub be an alternative?
 

lcrken

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I'm sure it's been done, but not seen it myself, but would a triumph conical hub be an alternative?
Sure, as well as the non-conical Triumph wheel. People have adapted all sorts of other rear wheels to Commandos, both drum and disk, most with bolt-on sprockets.

Ken
 

Brooking 850

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Hi Ken, I modified my rear standard Commando sprocket /drum brake , and I can now bolt on different size sprockets although still using the standard drum brake. This is on my race bike.
I run a Maney 40 mm belt primary , 5 speed TTI g/box and a 21 tooth g/box sprocket.
The smallest rear sprocket I can fit is a 44 tooth (standard is 42 as stated already).
With this set up , it works well on the tracks I race here in NZ.
Regards Mike
 
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I usually race at Winton Motor Raceway which is a 3 KM circuit with 5 straights and 12 bends. Compared with my 500cc Triton, the gearing of my Seeley 850 is insanely high. What I have found, is that with torque you don't know if you have improved it until you raise the gearing and find you are going faster.. Raising the gearing costs money, so you don't do it so often - but you cannot raise it if you are using a standard gear box - has to be close ratio. The standard gearbox is impossible.
I tooth less on the rear sprocket is a substantial increase in gearing. You use 42, I use 38. - But I also use methanol fuel with a 2 into 1 exhaust system. Methanol does not give you that much boost.
 
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If you want to find out what your Commando will pull - while it is rolling, put it in third and rev the tits off it, then slip the clutch.. Just don't have it pointing at a fence. Nothing stops that crank. I told my mate what I did and thought it would absolutely have to have stalled. He'd been around race bikes for 60 years. My bike took off like a bloody rocket, I could not believe it could do that in third or fourth from about 5 MPH. I did not know what gear it was in and got a bit frustrated. I caught it in time- just.
The last time I raced I did that in first with a close ratio 4 speed Norton box. The bike jumped away from the start perfectly, but I was worried the box might blow up and I could be hit from behind.
 
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Nigel, I think you make a rod for your own back when you stay with British cycle parts. Japanese bits are a dime a dozen. I don't know what your rear hub is, but you might find the hub out of a Mk3 Commando might be good - sprocket on one side and disc on the other. One tooth change in 40 on the rear sprocket is better than one in 21 on the countershaft. And the rear sprocket is much easier to get at. When I raced the 500cc Triton, I used to change the engine sprocket. Between three circuits there was one tooth difference for each, but the gearing was never perfect. With a Commando engine the gearing is less critical, but I would still try to do the changes at the rear sprocket.
With British parts, the dealer always has the last one in captivity and charges accordingly
 
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On my 920 I run a 36x72 primary & 21x43 final which gives 4.09:1. This seems perfect for my type of riding. At 100mph it's doing 6000rpm & will pull redline in top.

Martyn.
 

storm42

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On my 920 I run a 36x72 primary & 21x43 final which gives 4.09:1. This seems perfect for my type of riding. At 100mph it's doing 6000rpm & will pull redline in top.

Martyn.
Gearing Commander give just over 114 mph With those ratios, what size rear tire do you have?
 
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The problem is that with the heavy crank, the motor can pull redline and the bike might still not be going as fast as it can. With most bikes you can lower the gearing to get better acceleration. If you lower the gearing on a Commando the acceleration rate does not seem to change much. The fastest acceleration is when you race change up through a close ratio box, keeping the revs at maximum torque, but with high overall gearing. You can ride your bike just achieving redline, but if you raise the overall gearing, you can still achieve redline at the same rate but go faster. When the heavy crank is spinning high, it has a lot of stored up energy. A light short stroke crank makes the bike more versatile, due to better throttle response, but you cannot use it's inertia so effectively. Commandos have very poor throttle response.. Once you have got the crank spinning high - keep it there (even in corners) .
When I was previously racing the Seeley, I raised the overall gearing a few times in an attempt to knock it's backside in - it still has not staggered or accelerated slower. I've fitted the six speed box with one tooth bigger on the engine sprocket, so the next time I race, the overall gearing will be higher again.
You don't know what the bike will do with higher gearing until you raise it - forget the theories.
 
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When I was a kid, I raced regularly on Winton Raceway over a 12 year period when the circuit was shorter. There is now a short straight which used to be the main straight. I know how fast a bike has to go down there to be competitive with a Z900 Kawasaki. I can tell whether my bike is going faster by the degree of difficulty I have in getting around the 90 degree bend which is now at the end of it, and the time it takes me to get down it. The corner before the straight is a hairpin. Two things have made an improvement - raising the overall gearing, and changing the steering geometry so I get around the hairpin faster. With Winton, if you get that part of the circuit right, you will be fast everywhere.
 
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storm42

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The Gearing Commander site is down just now https://www.gearingcommander.com/
But,
Primary 2:1
Gearbox direct
Secondary 2.0476
Tire circumference 82.938
6000/2=3000/2.0476=1465.1299X82.938=121514.9436/12=10126.2453/3=3375.4151/1760=1.9178X60=115mph
 
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When you use gearing commander, I think the best way to make accurate calculations is to do a little "lay out work" to calculate your rear wheel circumference. You put a chalk mark on your rear tire sidewall, then transfer that mark to the pavement. Sit on the bike and roll forward one revolution until that mark is in contact with the ground again, then make a second mark. The measurement of the distance of those marks are your tire circumference (with your weight and tire compression calculated into the result)

Then you go to gearing commander and use the "custom" line of input variables to get the circumference number in their table to match the measured circumference number from your lay out work. This way, the fixed ratios which predict speeds, rpms, and other desired variables being sought, will be using a more accurate tire circumference than just using the "estimated circumferences" based on the manufacturer's tire size and best guess as to what your tire circumference might be. This method should reflect a more accurate result.
 
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Is top speed important on a road bike ? On a race bike, it only becomes relevant towards the ends of the straights. If your gearing is too low, your bike can run out of puff. So it is important to note where you get passed, if that happens. My feeling is that a Commando motor usually spins up at the same rate in response to more throttle - pretty much regardless of the gearing. Top speed depends more on top end power than it does on torque ?
 

storm42

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It wasn't about top speed, just that the gearing commander gave a speed quite a bit different to what Matchless gets. I was just curious about the accuracy of the clocks.

o0norton0o,
I like to use the commander to get an idea about differences revs different sprockets give on the track bike. I think there are many things that compromise its accuracy for speed though, tire growth and wheel slip at speed could be 2 of them, probably the slight difference between a circumference measurement or calculation wouldn't mean much, maybe.

I just thought a 14 or 15 mph difference was a lot, how much should we trust our clocks?
 
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