Gearsets For Commando/AMC Gearbox

lcrken

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Posts on another thread prompted me to post this in a separate thread. It's a compilation of all the data I have on 4, 5, and 6 speed gearsets that can be used in the AMC gearbox. I think I posted this info several years back, but I've updated it lately, mostly to add the TTI info. I hope some of you find it of interest. If you find errors in it, or have additional info to add, please post it here. There are 4 pages, and I didn't think to put page numbers on them, but the order is pretty obvious.

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Ken
 
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Can someone explain the ratios? The numbers don't work out mathematically. For example, 23:18, 23:19, 22:20, 25:17 and 21:19 are all listed as a 1:1 ratio. What am I missing here?
 

lcrken

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In all cases, top gear is 1:1 because the drive is directly through the mainshaft to the sleeve gear, with no reduction through the gear train. The gear pair containing the sleeve gear (output to the sprocket) and its matching layshaft gear provide the drive path in all other gears, so for any given gear the total output ratio is a product of the top gear pair and the mainshaft and layshaft gear pair for that gear. For example, the stock 4-speed has a 23/18 "fourth gear" pair, and the third gear pair is 21/20, so the overall ratio in third gear is 23/18 times 20/21, or 1.22. It's a little confusing, because we call the sleeve gear and it's matching layshaft gear the 4th gear pair (or 5th gear in a 5 speed and 6th gear in a 6-speed), even though the drive doesn't go through both gears, but only the sleeve gear. Not sure where that nomenclature got started, but it's been in use for a long time. The explanation is a lot clearer if you have a picture of the power path through the gearbox, but I don't have on handy at the moment.

My format might be adding to your confusion, by showing the number of teeth in the top gear pair directly above the top gear (1:1) ratio. I guess that could be a bit non-intuitive. But it's a fairly common way of displaying the information, and it never occured to me to do it differently.

Ken
 
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Nice, should have seen that earlier. Might have bought another one for replacing the 5 speed laydown gearbox. What about those from Mick Hemmings?
You could have added one or two speed versus r.p.m. sawtooth diagrams, which transforms all those numbers into something useful for choosing the right one.
Found them very useful when I designed 5-speeders for BMW racing outfits. Easier to discuss with the riders about ratios. Wind resistance at high speeds makes it necessary to have a very small revs drop shifting to top gear (800rpm). On lower gears, you can have wider spacing between gears.
It's much easier to select gear ratios on roadracers than it is for street bikes. There you have to choose low gears for congested traffic. Then useful gearing for overtaking trucks. Finally a fairly tall top gear for decent vibrations and low fuel consumption at cruising speed.
 
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One thing which I do not understand - if you have a close ratio gear set and run low overall gearing - if you raise the overall gearing - do the individual gears effectively move further apart - which you should feel in a larger drop in revs as you change up through the gears ?
I have raised the overall gearing on my Seeley several times and the motor always seems to spin up at the same rate, regardless. I would expect it to spin up slower.
I find the heavy crank deceptive when compared with a light crank. You don't know how much overall gearing your motor will pull, until you raise it.
There must be an optimum overall gearing for each race circuit. When you start selecting individual internal gear ratios to suit particular corners, it must get really silly.
 
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One thing which I do not understand - if you have a close ratio gear set and run low overall gearing - if you raise the overall gearing - do the individual gears effectively move further apart - which you should feel in a larger drop in revs as you change up through the gears ?
I have raised the overall gearing on my Seeley several times and the motor always seems to spin up at the same rate, regardless. I would expect it to spin up slower.
I find the heavy crank deceptive when compared with a light crank. You don't know how much overall gearing your motor will pull, until you raise it.
There must be an optimum overall gearing for each race circuit. When you start selecting individual internal gear ratios to suit particular corners, it must get really silly.
The revs drop will be the same, regardless of overall gearing. But the acceleration will be slower with taller gearing.
So you are right on the first point, which you can easily read on your rev counter. But how fast your engine revs up is only felt in your pants. Much harder to tell. A small overall gearing change will make little difference
The usual practice to determine overall gearing is to have max revs at the place where you start braking at the longest straight. I hate when I have to roll off a hundred meters before my braking point not to overrev.. Or when rev needle is long from the red line at the brake point.
 
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History lesson. When fairings became common in the late fifties, the Manxes and G50s increased top speed about 10 Mph, taller gearing gave problem with having to use clutch slipping to keep the engine in its powerband, especially in the hairpins. So an extra low first was needed. Birth of the first 5-speeeders. Progresses in engine tuning, effectively shortening the powerband, resulted in a need for closer ratios. So 6-speeders arrived.
As gearbox axles has to be as short as possible to minimize bending, space for gears is limited. Fitting more gears results in shorter cog length. Affects both service life and load carrying capability. Mostly overcome by metallurgical advances and improved manufacturing methods. Plus progress in oils.
 
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Gear ratio not only affects acceleration, but also braking.
I have a Daytona 1st gear (longer) in my Commando to reduce the engine brake effect when riding down hill, entering hairpin turns.
 
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On some of the set the third gear is 1.1 and 4th is 1. It doesn't seem like lot of difference. What is that like to ride? Do you feel the difference?
 

lcrken

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On some of the set the third gear is 1.1 and 4th is 1. It doesn't seem like lot of difference. What is that like to ride? Do you feel the difference?
I've never ridden a 4-speed close ratio, but have lots of time, both on track and street, with the 5-speed that has the same 1.1 for 4th and 1 for 5th. It's quite nice on the race track where it (as well as the rest of the close ratio set) really helps to keep the engine in the top part of the power band. It can also be fun on the street depending on riding style and types of roads. I used the Quaife 5-speed with lower first gear that allows use of the kick starter, but still had the 1.1 and 1 top gears, and it was fun. Although it did make for a weaker first gear, and I had a couple of them break before switching to the close ratio set and race track use only. With a 4-speed close ratio, you might find 1st gear a little tall for city riding.

You definitely feel the difference at higher rpm. Shifting from the 1.1 in 3rd (or 4th) at 7000 rpm will drop the revs to 6400 in the direct 4th (or 5th) . That's a very noticeable difference. Not so noticeable at lower rpm.

Now that I'm old and feeble, I think I prefer the stock 4-speed in a Commando for the street, particularly one built for a strong mid-range. The 883 MK3 we built for my grandson is aimed at being a real torque monster, and works great with standard 4-speed. When I ride it I do a whole lot less shifting than on the 5 and 6 speed bikes I normally ride. And that makes for less wear and tear on the rider. On the track, the 5-speed is the ticket.

Ken
 
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When I built the Seeley 850, it was only because it would resemble something authentic. I never really believed in it until I actually tried to race it. I am now running very high gearing, but if I lowered the gearing I don't believe the bike would accelerate faster. Every time I raise the overall gearing a bit more, the corners arrive quicker. I have always believed light crank, short stroke and high revs were the way to go. But the Commando engine is different and possibly better. It loves methanol, even at low comp.
I could not believe how bad the Commando motor could be until I tried to race it with standard ratios in the gearbox - It was useless. Every gear change was horrible - up or down - never smooth enough, and the bike would not accelerate quick enough. OK for a road bike, but not for racing.
You don't know what overall gearing is best until you start raising it. I had trouble getting decent starts with the very high first gear of the 4 speed close box. Until I got really brave and revved the motor to 5. The bike jumped away perfectly, but I would not do it again with the Norton box. Once the Commando crank is spinning high - NOTHING STOPS IT. My bike once pulled 5th gear from a standing start and took off like a rocket - it was slow for about 6 feet. I did not know the change was upside down an revved the motor and slipped the clutch.
Put your bike in 3rd gear and try it - rev to 5 and slip the clutch. - don't point it at a fence as I did.
 
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What do you do when you think you have your race bike geared sensibly and you ride it changing up at near peak revs, and everything happens at a certain rate. Then you raise the overall gearing and everything still happens at the same rate, but the bike goes faster ?
Theoretically the time between gear changes should become longer as you raise the overall gearing - unless the motor is not pulling as hard as it can when the overall gearing is low ?
 
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There are three things - crank inertia - throttle response - torque. They tend to work against each other. The balance between them you choose, usually depends on how you want to use your bike when you race. If you like to scrap, you need good throttle response. If you want to be smooth in high speed bends you need good crank inertia.
 
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When I bought the 6 speed TTI box for my Seeley, I was not really thinking. I have never ridden on a race circuit where I have ever needed to use first gear other than in a clutch start at the beginning of a race. My four speed close box was perfect everywhere except at the start of a race. The standard Commando first gear fits straight into the 4 speed close box without modification. All that would happen would be you'd need to rev higher in first on the first gear-change in a race. With the 6 speed box, I still don't know if first gear will be low enough to get me off the start line really fast. For a road bike in traffic, the wider gap between first and second might be a bit frustrating - but it depends on how you normally ride your bike.
I always got a laugh when my Seeley 850 used to stagger-off in first gear. The Commando engine pulls like a train. But if you rev the tits off it, it is fast enough. I'm just too chicken to do it often.
 
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