Gearing

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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StephenG80
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Gearing

Post by StephenG80 »

I have a 22 t crankshaft sprocket on my '54 G80s which goes very well, gets up to cruise at 60mph in a spritely manner. I am going to be doing the G3 cam timing mod and G5 inlet rocker arm conversion in the spring so she will have a little more poke and I am thinking of using that extra poke by going from a 16 t to a 19 t sprocket on the gear box output. Thing is that is quite a jump in gearing ie approx 18%, so currently 60mph would be 70mph (if the rpm is maintained of course). Just wondering if others have geared their G80s to this extent? I would expect an overall decrease in acceleration even with the poke from rocker/cam mod, but don't want to change it into a bit of slug with absolutely no g"go", what I am hoping for is a relaxing rpm for long journeys.
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Rob Harknett
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Re: Gearing

Post by Rob Harknett »

I did enough touring in the 50's on a 350 always 2 up with luggage. Never changed anything. Did many 1,000 + 2/3 day trips from Essex, to all over England, Wales and southern Scotland. Speed limits have not changed since those days. Except many roads now have lower speed limits.
SPRIDDLER
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Re: Gearing

Post by SPRIDDLER »

Just killing time whilst waiting for my roast lamb to be ready so here's some thinking aloud......
Stephen G80 wrote:I am thinking of using that extra poke by going from a 16 t to a 19 t sprocket on the gear box output.
Depends upon whether the notional extra poke occurs throughout the rev band.
Unless you use motorways there won't be much opportunity to maintain 70mph (or even 60) for very long. However, if you plan to make substantial use of motorways or cruise at 70 for any length of time I would suggest you have chosen the wrong bike ;) . The mediocre brakes are a consideration too at 70 or motorway speeds
Most of our single cylinder bikes/engines seem to have a 'sweet spot' at normal cruising road speeds of around 55 to 60mph (your new 45 to 50mph engine speed) due to the engine's designed balance factor. If you drop the cruising speed of the engine you may find you're riding for hours with engine revs below this sweet spot and in an unpleasant vibration band.
Thing is that is quite a jump in gearing ie approx 18%, so currently 60mph would be 70mph (if the rpm is maintained of course).
Depending upon luggage etc. when on these 'long journeys' you may find hill starts (I understand there are some quite steep ones in Scotland) and trickling along in traffic will require more clutch slipping than previously. Long gradients and headwinds may require changing down which will be tedious.
If 'long journeys' involve riding with lights on (at night or daytime abroad in some countries) it's worth considering that the 'short' dynamo which has an optimistic output of 42 watts in perfect condition will be running 18% slower. It doesn't necessarily mean that the dynamo's output will be 18% less but the standard miserable lights are 30W main beam and 3W for each Pilot and Tail light (total 36W) so it's something to bear in mind, and wiring etc has to be perfect too. It is generally reckoned that with standard bulbs (total 36W) one needs to be riding at around 40mph for the dynamo to balance the lights' electrical demand. With the new gearing that would require (if my maths is correct) an engine speed equivalent to about 47mph.
Just wondering if others have geared their G80s to this extent?
Since so far no-one on the Forum has offered direct experience it seems you'll have to suck it and see.
'There is a tide in the affairs of men
Which taken at the flood............'
56G80S
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Re: Gearing

Post by 56G80S »

Sprid is spot on. My G80S used to cruise happily at an indicated 65, probably closer to 60 real time, pulling the small camping trailer (and will again when I sort out the jetting on the new carb).

I don't want to go faster than that for the reasons that Sprid mentions. I am nonetheless tempted to refit the exhaust SH cam, change the rocker arms and move to the 350 timing more for improved pull away towing than owt else.

Maybe look at a twin? Not something I've done.

Johnny B
StephenG80
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Re: Gearing

Post by StephenG80 »

Not looking for a different bike - been around them for 35 years and ridden brits, japs, italian and shovels, I was just wondered if anyone had any first hand experience of this particular gearing and with those mods on G80s.

Re the vibrations - since putting in a new primary chain and tensioning it properly she is as smooth as swiss watch at all revs. Anyone with a nasty dose of vibes -start with checking that primary!
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Rob Harknett
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Re: Gearing

Post by Rob Harknett »

Vibes can also occur if the tank rubbers are tightened instead of just nipped up and wired on, There's holes in the correct bolts heads for this. Also loose head bearings cause vibration. What may feel bad, probably is what the bikes were like. My 350 was most happy at 50 ish touring. I never thrashed my bike touring. The 500 G80 gave a bit more power for the family man with a side car. Solo it would give an extra 10 mph cruising and top speed over the 350. I always did my tours by night, from home to Wales etc. They always seems to run better. I think mainly because I drove at a steady speed. Starting with a full tank of petrol, which deadened vibration.
AJSingleDave18
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Re: Gearing

Post by AJSingleDave18 »

I have a 1960 AJS model 18 (500) in standard engine tune. Most of my rides are long, on empty-ish but hilly roads at 50 to 60mph. At 60 I used to find the engine very busy and tiring to ride so I've changed the gearbox sprocket from the standard 16t to 17t. Originally I considered an 18t sprocket but was advised not to.

Calculated engine speeds with a 100/90x19 rear tyre: -
16t = 15.43mph/1000rpm. At 60mph = 3887rpm.
17t = 16.40mph/1000rpm. At 60mph = 3659rpm.
18t = 17.36mph/1000rpm. At 60mph = 3455rpm.

(For a 3.5” x 19 tyre multiply the above by 1.02)

RESULTS = 60mph cruising is now quieter and more pleasant but the minimum top-gear speed has increased from about 27 to 32mph and more gear changes are required for inclines and speed control. When cruising the engine firing pulses (thumping) are slightly more noticeable. Fuel consumption has improved by about 2mpg.

On flat roads with the wind behind I wish I'd fitted the 18t sprocket and when the going is tougher I'm pleased with the 17t choice.

I suspect that with hotter cams the increased power will be at higher engine speeds with a reduction in pulling power at lower speeds so the engine will be happier with lower not higher gearing. Like a modern bike.

I hope this helps.
StephenG80
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Re: Gearing

Post by StephenG80 »

thanks for the additional comments on this. I am en route to NZ at present and hope to visit https://www.transportworld.co.nz/motorcycle-mecca/ :-) - so will have a play when I get back and let folk know how it goes.
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Pharisee
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Re: Gearing

Post by Pharisee »

I had a similar 'quandary' when I came to sort out the transmission for my '54 G3/LS basket case rebuild. The 'standard' sprocket set up is an 18 tooth engine sprocket driving a 40 tooth Burman clutch basket followed by a 16 tooth gearbox sprocket driving a 42 tooth rear wheel/brake drum sprocket. That gave an overall ration of 5.83:1 in top gear.
I had chosen to fit a later AMC gearbox and clutch (because I had them available and the Burman gearbox I had was a mess). The AMC clutch basket, however, has 42 teeth. An 18/42 primary drive and a 16/42 final drive lowered the overall ratio to 6.12:1 which is a 5% lowering of the overall ratio. The 18 tooth engine sprocket was brand new so I wanted to keep that. I couldn't change the clutch or rear wheel sprockets so that just left the gearbox sprocket. Changing the 16 tooth sprocket for a 17 tooth item would bring the ratio back to 5.76:1 which was close to the original ratio but I'd been advised that the 350 was generally considered to be 'under-geared' anyway so in the end, I opted for an 18 tooth gearbox sprocket and a 5.44:1 overall ratio; an increase of just over 6% on the original. Time will tell if I've made a wise choice or done something really stupid!!
I'm from the Fens.... Gimme six.
MikeM.
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Re: Gearing

Post by MikeM. »

Back in my 500 AJ Twingle's heydays it regularly turned out 90mpg and on the return trip from the 2004 International loaded with camping gear and cruising around the 55 mark with a friend on his 250 I averaged 105 to the gallon. The engine was an all bits and pieces single that I had put together back in 1978. The engine sprocket was standard for a 500 single and the gearbox sprocket had 17 teeth. The starting was always a real pig of a job but once running it never missed a beat, that is until the Cornish water piskies got at it on the 2008 Jampot, never been the same since!☹️
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