Timberwoof’s Motorcycle Page
Optimum Shift Points
Honda CB-1

A discussion on rec.motorcycles let me to create a spreadsheet and make some graphs that plot engine torque and power for road speed in all the gears. Start with the torque and power curves for your motorcycle. Here's a graph for mine:

Read the graph to get the numbers at 500-RPM intervals. Enter them in on sheet "1. Data." Then check the graph to see if it looks like the one for your motorcycle.

Yep, that looks pretty close. The Excel version is a bit pointier because I'm only entering data every 500 RPM. For a smoother graph, you could modify the spreadsheet to use data points more closely spaced.

Gather up some basic information about your motorcycle. Here's mine:

Quantity Number  
Gear Ratio I 4.16

These numbers mean engine revolutionns per transmission output shaft revolution.

Enter zero for gears you don't have. For instance, mine's a five-speed but I wrote the spreadsheet for six-speed transmissions as well.

Gear Ratio II 2.91
Gear Ratio III 2.13
Gear Ratio IV 1.74
Gear Ratio V 1.45
Gear Ratio VI 0
Final drive ratio 0.33 This number means wheel revolutions per transmission output shaft revolutions. You could write down <number of drive sprocket teeth> / <number of wheel sprocket teeth> and then in the spreadsheet enter that as =15/45 (or whatever) and it will calculate the ratio for you.
Rear tire size 150/70-HR17 This is just for reference. You enter the numbers in the following cells…
Rear rim diameter (inches) 17  
Rear tire width (mm) 150  
Rear tire profile (%) 70  

Enter these numbers in the spreadsheet. If your motorcycle's transmission has fewer than six gears, then just enter 0 for VI, as I did. A bunch of calculations will fail and the lines for sixth gear won't plot … which is what we want.

The calculations are all done on sheet "2. Calculations." Based on the information you supplied, the yellow cells calculate engine speed in all five gears for road speeds up to an outrageous 200 MPH. The orange cells round that off to 500-rpm to match the data you entered. The pink cells look up the maximum engine torque for any given road speed. The purple cells correct those numbers into theoretical rear-wheel torque by multiplying by the gear ratios and the final drive. (The numbers are theoretical because I ignore losses due to friction.) The blue cells do the same lookup but against maximum power.

The spreadsheet plots two graphs. The first one plots the calculated maximum rear-wheel torque against road speed for every gear. The second plots maximum power against road speed for every gear.

Chart 1:

Chart 2:

And now the point of all this. Using either graph, you can plot the optimum shift points: That is where the torque or power curve for one gear drops below that of the next higher gear. It turns out that either graph yields pretty much the same results. For my motorcycle, the shift points for maximum power are 45, 65, 80, and 100 mph or 7500, 7500, 7000, 7000 RPM.

(Notice that the curves are kind of stairsteppy instead of continuous. That's a side-effect of the VLOOKUP() call I use to get the numbers out of the torque/rpm table. The smaller the intervals in the torque and power table on the data sheet, the less stairsteppy the graph will be. You'll also have to adjust the rounding amount in the calculations sheet.)

The spreadsheet contains data on my motorcycle so you can see how it works. You have to put in your own numbers: only change the cells on sheet "1. Data"; the others are all calculated for you.

I suppose I could come up with a clever way to calculate the shift points, but it's easy enough to just look at the graphs and see where the lines cross.

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