RPM Calculation

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MalcW
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RPM Calculation

Post by MalcW »

I have calculated the rpm of my 650 as follows:
Put the bike on the centrestand, remove plugs, put in top gear and rotate the rear wheel forward to find TDC. Mark the rear tyre in line with the bottom of the mudguard. Rotate the wheel forward some more until back at TDC, and mark tyre again. Measure the distance between the two marks in inches. This is the distance travelled in one revolution of the crank. Divide the measurement into 63360, the number of inches in a mile, giving the number of crank revolutions in 1 mile. Since at 60mph you are doing 1 mile per minute, this will be the rpm at 60mph. In my case it worked out at 3520rpm, which I think sounds reasonable. The value an then be scaled for other speeds.
I always thought it sounded a bit revvy at 60, but I feel happier about it now.

Malc
Mick D
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Re: RPM Calculation

Post by Mick D »

Hi

It's a good approximation but doesn't allow for the effective decrease in rear wheel diameter due to tyre deformation, so your figure will be a little low.

Regards Mick
aobp11
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Re: RPM Calculation

Post by aobp11 »

There is an easier way when you know (and "of course you know") the primary and rear chain teeth ratios. Then you further only need the rear wheel circumference which is much larger (-> better precision) then the displacement in one crank revolution The circumference can also easily be measured under normal load though that way you would slightly under estimate the effective rolling circumference and over estimate rpm .
Albert
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Pharisee
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Re: RPM Calculation

Post by Pharisee »

I've copied this from a thread I started in the G3/M16 forum

"My '54 G3 has a non-standard overall gear ratio as I have an AMC gearbox and clutch fitted. The clutch drum has 42 teeth as opposed to the original Burman item that has 40. The engine sprocket has the normal 18 tooth so the primary ratio is slightly lower. To (over) compensate, the gearbox sprocket is 18 tooth instead of the standard 16 tooth. That gives me an overall top gear ratio of 5.444:1 instead of the standard 5.833:1

I've just wrapped a tape measure around the rear tyre and the circumference is approximately 84". Not exactly an accurate rolling circumference but close enough for me to work out that the engine is doing approximately 3,400 r.p.m. at 50 m.p.h.
"
I'm from the Fens.... Gimme six.
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Pharisee
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Re: RPM Calculation

Post by Pharisee »

My M18S has an overall top gear ratio of 4.7:1 (40/21 and 42/17). This would equate to approximately 2,950 r.p.m. at 50 m.p.h.
I'm from the Fens.... Gimme six.
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