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Author Topic: Performance curve  (Read 1599 times)

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Offline Mdub

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Performance curve
« on: May 31, 2009, 02:36:52 PM »
This has bugged me since I was 10 and Dad came home with his brand new rootbeer/brown 1972 K3 CB750.
Being the book worm nerdy kid, I had to read the service manual cover to cover so I would know where to hold the light when Dad worked on it! I would also learn what to hand to him and when.

The driving performance curve I'm sure has much to be gleened from, but it doesn't make much sense to me.

It has rpm going up the left side, MPH along the bottom and "Driving force" (FT/LBS of torque @ rr wheel?)

I get the diagonal 1st,2nd,3rd etc. lines showing speed/rpm for the range of gears but what about the "level road with rider in prone position"? Are they showing rpm vs speed vs torque in 5th gear for different uphill grade percentages?

And how about that upper left hand "1st" curve? Is that rpm vs torque or speed, or both???
Maybe someone who has sussed it out can comment...
Does anyone know how to read this chart?
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Offline Mdub

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Re: Performance curve
« Reply #1 on: June 01, 2009, 10:48:52 AM »
Ha! 31 views and no comments...
I guess I'm not the only one that can't make sense of it. ???
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Offline mlinder

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Re: Performance curve
« Reply #2 on: June 01, 2009, 10:53:42 AM »
Yeah, that's a nutty graph....
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Offline Achmed

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Re: Performance curve
« Reply #3 on: June 01, 2009, 11:00:30 AM »
There are two sets of gearing curves so I'll bet one is for the left vertical axis and one for the right.
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Offline mlinder

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Re: Performance curve
« Reply #4 on: June 01, 2009, 11:02:24 AM »
Quote from: Achmed on June 01, 2009, 11:00:30 AM
There are two sets of gearing curves so I'll bet one is for the left vertical axis and one for the right.

Who's Mr. SmartyPants now, Achmed?

Wait, what?
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Offline Achmed

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Re: Performance curve
« Reply #5 on: June 01, 2009, 11:06:26 AM »
As far as the percent curves, I can only guess that is the grade of the road. And it doesn't make sense to me that RPM would correlate with those. Just my first inclination, though.
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Offline kenolds

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Re: Performance curve
« Reply #6 on: June 01, 2009, 12:16:28 PM »
I interperet this graph to be the relationship between required power and available power.  The required torque is shown by the curves with the percentages (the bottom most being the "level road with rider in prone position curve).  The percentages represent uphill grades.  The torque curve looking plots with ordinal annotations (1st, 2nd, 3rd, 4th, 5th) are the amount of "thrust" you get in a particular gear with WOT.  The straight lines simply represent the gearing chart for the gearbox, sprocket combo, tire size that the bike was furnished with.
SO...if you wanted to know how fast you could go up a 20% grade in 3rd gear the answer would be about 90Mph revving about 8500 rpm.  When you went to shift up to 4th, the bike would slow down and only be able to pull to about 85MPH at 6,5oo rpm.  I'm not sure how useful this chart really is except to give you an idea of how fast it can go... (or let you know that your top speed up a 60 percent grade is 47Mph at 9,000 rpm in first gear - and you'll have to rev it to 3,000 rpm and slip the clutch to keep it from stalling)

Kenolds
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Offline Bodi

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Re: Performance curve
« Reply #7 on: June 01, 2009, 08:54:27 PM »
I have one of those for a Honda 50 somewhere, but it isn't as complicated (3 speeds and about 4 horsepower after all).
The explanation by kenalds is basically correct. There are two separate charts superimposed, one has engine RPM (left side) vs road speed (bottom) in straight lines and illustrates the bikes overall gearing. The other plots the rear wheel force (right side) - essentially the thrust applied to the bike by the motor - in each gear against road speed. These are the series of torque curves, first gear is short and 5th is pretty long. Overlaid on this is are the curved lines, an approximation of how much rear wheel force is needed to go the road speed at various percentage hill grades (these are not straight lines because wind resistance increases non-linearly with speed).
So this bike could climb a 60% grade in first gear... has anyone here ever seen a 60% grade? And if you have seen it, been insane enough to consider riding up it? (hill climb events aside)
As I recall the Honda 50 was capable of climbing a 50% grade. According to the chart, anyway.
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Offline Achmed

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Re: Performance curve
« Reply #8 on: June 01, 2009, 10:46:06 PM »
60% is pretty extreme even for skiers. Are you serious about the curves in the lines being due to wind resistance? And do you think the RPM axis applies to the grade curves? Just curious.
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Offline crazypj

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Re: Performance curve
« Reply #9 on: June 01, 2009, 11:15:25 PM »
Bodi's explanation is correct as far as I can remember.
Carl Shipman explained it in a 1970 book using graph for a 450.
Cant remember book name off hand though

PJ
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