This Australian Streamliner Bike Build “page” (40+ pages and counting), and Jon’s liner build, are both massively cool! I have learned a lot to apply in my bike liner build. Thanks to all.
The overdrive/underdrive unit adaptation is quite ingenious. Jon, good on ya for being willing to share and for taking the time to do so.
The downside is that I have a couple questions…no good deed goes unpunished!
Laycock and Gear Vendor are two different manufacturers, as far as I can learn on the web. Right? I have seen Gear Vendor under/overdrive units mentioned in Hot Rod Magazine, and have been meaning to get edumacated about them. What are the tradeoffs? Cost is one I am sure; the Gear Vendor costs roundabout 2700 USD new.
I am curious about the advantages or tradeoffs of using (1) an underdrive unit versus (2) using an overdrive unit with a shorter (numerically larger) final drive to obtain a similar top gear overall ratio. I ran some numbers (below) to shed some light, but there may be other considerations that I am not seeing.
I may be exercising my gift for belaboring the obvious again, but here goes anyway. The tables below are based on Hayabusa OEM gearbox ratios. The percentage changes in the 6-speed stock box ratios are 26, 21, 16, 12, and 8 percent.
UNDERDRIVE. I calculated overall ratios using a 38/20 = 1.9 final drive ratio, which might be in the ballpark for a stock motor. With a 1.27 underdrive, the twelve available ratios listed in order of ratio are shown in the table.
Gear Drive Driven Gear Overall Change Change Change
Gear Gear Ratio Ratio re 1 up re 2 up re 3 up
1U 13 34 3.322 10.073
1 13 34 2.615 7.932 0.21
2U 16 31 2.461 7.462 0.06 0.26
3U 19 29 1.938 5.879 0.21 0.26 0.42
2 16 31 1.938 5.876 0.00 0.21 0.26
4U 21 27 1.633 4.952 0.16 0.16 0.34
3 19 29 1.526 4.629 0.07 0.21 0.21
5U 22 25 1.443 4.377 0.05 0.12 0.26
6U 23 24 1.325 4.019 0.08 0.13 0.19
4 21 27 1.286 3.899 0.03 0.11 0.16
5 22 25 1.136 3.446 0.12 0.14 0.21
6 23 24 1.043 3.165 0.08 0.19 0.21
I think I would try using 1U and then go to direct and up through the gears 1-6. The percentage changes in ratio would be 21, 26, 21, 16, 12, and 8 percent. Note that the underdrive first gear gives a pretty reasonable ratio step (21 %) to the next gear (first gear direct), actually a bit less than the 1st to 2nd step (26 %) of the stock box. Nice! The result is effectively a 7-speed transmission chain that should help at the starting line. The Gear Vendors website raves about gear-splitting, but I don’t see advantage in doing too much of that in this application.
OVERDRIVE. Using a 1.27 overdrive and a final drive ratio of 48/20 = 2.400 to obtain about the same overall drive ratio in top gear, the twelve available ratios are:
Gear Drive Driven Gear Overall Change Change Change
Gear Gear Ratio Ratio re 1 up re 2 up re 3 up
1 13 34 2.615 10.019
1O 13 34 2.059 7.889 0.21
2 16 31 1.938 7.422 0.06 0.26
3 19 29 1.526 5.847 0.21 0.26 0.42
2O 16 31 1.526 5.844 0.00 0.21 0.26
4 21 27 1.286 4.925 0.16 0.16 0.34
3O 19 29 1.202 4.604 0.07 0.21 0.21
5 22 25 1.136 4.353 0.05 0.12 0.26
6 23 24 1.043 3.997 0.08 0.13 0.19
4O 21 27 1.012 3.878 0.03 0.11 0.16
5O 22 25 0.895 3.428 0.12 0.14 0.21
6O 23 24 0.822 3.147 0.08 0.19 0.21
Here I think I would run through the first 5 gears without overdrive, then simultaneously shift back down from 5th into 4th and kick in the overdrive. This would be easier to see if I could highlight the chosen ratios. The percentage changes in ratios in this 8-speed scheme are 26, 21, 16, 12, 11, 12, and 8. The result is an 8-speed tranny with quite close ratios for the top four gears. This is useful to handle the high part of the drag curve, especially if the power band has been narrowed. The overall 1st gear ratio is about the same as the underdrive case.
The stock bike has final drive ratio = 40/17 = 2.353 and first gear overall ratio = 9.822, so the first ratio in either the underdrive or overdrive setup, at just over 10, is actually a little better for getting started. Which we will both need since our rigs weigh quite a bit more than the stock bike. An over/underdrive ratio a bit larger than 1.27 would be useful, but you can only have what is available.
Am I on the right track here? I suspect that you have probably ran some similar numbers. Comments? Is that snoring I hear? Did I do it again?
Larry