920 gearing...

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I have a hand held GPS that I have mounted between my tach and speedo, so I use that GPS speed indicator to check my speedometer accuracy and then use "math" and the gearing commander calculator to check my tachometer accuracy too. Ultimately, my clocks are reasonably accurate up to about 55-60 mph, then after that they are a little bit "optimistic", than the GPS numbers indicate. For my needs they are good enough.

At any speed on my GPS, I can remember my instrument readings and check the accuracy for that range, but since I'm just riding for pleasure, not high performance, all I need is reasonable accuracy. I can see where determining exact speeds and rpm's more accurately would be much more important for chosing sprocket sizes for race bikes.

I've measured my tire circumference, and I do manipulate the tire input data in the gearing commander table so the tire circumference column is the actual measured circumference of my tire, rather than the automatically computed one... I'm going to have to revisit my calculations and come back with the comparison of them both for curiousity's sake...
 
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When your overall gearing is right, you should just be able to stay on the max of the torque curve as you change up through the box as you accelerate down the straights. If you use a close ratio box, you can use higher overall gearing, because as you change up the steps are closer, so you don't fall so much below the max pulling power. With a standard Commando box, the steps between the gears are designed for much slower use. Most people probably don't often pull 7000 RPM on their road bikes. My motor is always up around at least 5,500 RPM when racing. I change at 7000 RPM and the most the revs drop is to 5,500 RPM. A lot depends on your cam and exhaust system. If your max torque is above 7000 RPM, you are wasting your time, because with a Commando engine, you never get there.
 
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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
If that's correct either my speedo or rev counter or both are reading wrong. As they have both been rebuilt by Ashley Pople I find it hard to believe. Next time I'm out with my mate I'll check it against his speedo.
 

Fast Eddie

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Sat nav is best for a speedo check. Or, if you have an i phone you can get free speedo apps.
 

Fast Eddie

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Nigel, is the engine smooth at 2500-3000 revs?
I don't intend to ride at that rpm but sometimes ride for hours on twisty, hilly 50 mph speed limit country roads that are police patrolled. At 55-60 mph on these roads one can have lots of fun and probably sneak thru police radar unscathed.
My bike with 21 tooth fitted is smooth as glass in top gear at 55 mph and anything above.
Below 55 some vibration shows up and at 50 in top it's quite annoying, so I shift down into third just to get rid of the vibration. Running for miles and miles in third is also annoying when there is plenty of torque to run in top, but the vibes say no.
Your configuration is so different that I expect the smooth running threshold is also different. Does your bike start to vibe below 2900 revs as standard Commandos do?

The Commando is a bit of an oddity as gearing it lower actually makes it smoother at low speeds. All of the other parallel twins shake more with a smaller gearbox sprocket, hence you find overgeared 650BSAs and Dommies that can't pull skin off rice pudding.

Glen
Glen, I thought about your vibration question whilst out on the Trident Hunter today...

Like I said before, it shakes about at tickover somewhat, and ‘shake’ is a better description than ‘vibrate’ I’d say.

It seems to vibrate above that too, but at 2,000 rpm the vibes drop off rapidly and are all but gone by 2,500, apart from some minor vibes through the tank. Then it just smooths out completely (I say ‘seems to’ becasue there’s not a lot of time between tickover and 2,000rpm when pulling away and it’s not a rev range I’d normally spend any time in deliberately).

Vibes begin to make their presence felt again above 6,500, but still not that much, although at those revs its kinda difficult to remain fully sensitive to engine vibes as there’s plenty more to occupy ones concentration.

So it’s basically like I said before, at actual riding revs the bike is astonishingly smooth, even with the ‘nipped up then back off just enough so they can move freely’ isolastic settings.

As a reminder, we‘re talking about an 11:1cr 920, with JS pistons and rods, lightweight Maney crank, statically balanced by Steve to Jim’s spec.
 
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Just a polite query FE.... With that CR do you use any fuel additives? And what ignition system/timing? Thank you..
 

Fast Eddie

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Just a polite query FE.... With that CR do you use any fuel additives? And what ignition system/timing? Thank you..
Well, that is something quite interesting IMHO...

I use Super Unleaded, which ranges from 97 to 99 RON (97 is equivalent to 91 in the USA). I use octane booster for track days, mainly just for ‘safety’ and force of habit.

In normal use I do not use any additives in the fuel at all. And I cannot detect ‘pinking’ at all. Neither can I detect any difference between the 97 or 99 brands. Compare this to my bog standard T140, which is very easy to get to pink if using too much throttle at too low revs, even using 99 octane.

Ign timing is 28 degree. Ign is Tri Spark. Coil is a cNw dual outlet. I use iridium plugs.

IMHO, this is possible because the squish gap is tight, squish enables a higher CR without knock than would otherwise be possible (lots about this on t’internet).

Getting a high CR on a Norton 850 or above, without domed pistons, kinda automatically gives a tight squish.

My pistons have a ‘dish’ machined into the crown. I specified the crown height and dish dimensions to JS and he accommodated. I was aiming for 10.5:1... but got my calculations wrong. DOH!

Fortunately, it‘s fine at 11:1.

And it goes like shit off a shovel !!
 
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Thanks.. Makes me wonder if it's the Norton design, though I've only 'standard' components and tune I've never had fuelling issues wherever I buy, after lay up or any other time. Let's hope we stay that way....
 

Fast Eddie

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Thanks.. Makes me wonder if it's the Norton design, though I've only 'standard' components and tune I've never had fuelling issues wherever I buy, after lay up or any other time. Let's hope we stay that way....
Basically I’d say yes, the Norton combustion chamber design is better in this regard than, for example, a Triumph. The down side to the Norton design is the difficulty in getting big enough valves in there without them tangling with each other.
 
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