G: Gearbox (Twin) Gearing on a Twin

[Albervin]

Now I am fortunate enough to have two twins in the shed I have been thinking about gearing/final drive. I have always had a 48T rear sprocket on my B Rapide and I like it. It gives a useable 1st gear around town, roundabouts and hairpins plus a nice 70 mph cruising. My Shadow has the intermediate (high) 1 st gear and the 48T rear sprocket helps around town but still gives a comfortable 80-90 mph cruising, At the moment I am not sure if it is 7.3:1 or 8:1 compression. Both bikes have their "rough" spots; the Rapide at 75mph and the Shadow at 70 mph. My question is; should I play with the gearbox sprocket or go 5 speed?? Big difference in cost and in reality, anything over 75 mph in Australia is going to lose my licence. However, I hit a big headwind on the last ride and top gear @70 mph gave a mpg of 38 mpg on the Rapide. If I had held 3rd I would have had high revs and low throttle with probably same result. The Shadow is definitely stri=onger than the Rapide but in Australia it is a matter of mpg with the great distances between fuel stations. Any input is welcomed.

 

[vibrac]

I liked the high first gear of the five speed that I had (gone on a racer now) I always felt the standard lower first was a pain from pulling away, in a normal non rocket ship get away, no sooner you are than away, then you are trying for second, most other bikes you have enough time to catch a breath before a change to second.

 

[Vincent Brake]

if you go 5 speed surtees you can also opt for long first gear. only thing |I meet is the difficult to find nutral position, (i do it by hand)

i ride mine 4 gear also with high first one, and then front 22- rear 47 sprocket on a 520 O ring chain.
i use about 4 L per 100 Km when arround town, but at 130KM/H it likes some more.

all with very smooth Mikunies on the rich side, (like chrome pipes)

 

[rogerphilip]

Now I am fortunate enough to have two twins in the shed I have been thinking about gearing/final drive. I have always had a 48T rear sprocket on my B Rapide and I like it. It gives a useable 1st gear around town, roundabouts and hairpins plus a nice 70 mph cruising. My Shadow has the intermediate (high) 1 st gear and the 48T rear sprocket helps around town but still gives a comfortable 80-90 mph cruising, At the moment I am not sure if it is 7.3:1 or 8:1 compression. Both bikes have their "rough" spots; the Rapide at 75mph and the Shadow at 70 mph. My question is; should I play with the gearbox sprocket or go 5 speed?? Big difference in cost and in reality, anything over 75 mph in Australia is going to lose my licence. However, I hit a big headwind on the last ride and top gear @70 mph gave a mpg of 38 mpg on the Rapide. If I had held 3rd I would have had high revs and low throttle with probably same result. The Shadow is definitely stri=onger than the Rapide but in Australia it is a matter of mpg with the great distances between fuel stations. Any input is welcomed.

Hi Albervin, I have constructed an Excel based interactive gearing spreadsheet but cannot upload it because the site does not support the .xls extension

 

[Robert Watson]

I run 22/48 on the Woolly (rapide) I tried 22/46 but with 2 up and luggage was not best pleased starting out. It has a std ratio Surtees 5 speed. I geared the A in the same general area and even though not much two up riding It works well. If I ran mostly solo I think I would run 22/47 on a post war twin. Keith Sides always rode solo with the 5 speed and 22/46 and really liked it. He also had the thing running on BIG DelOrtos and managed to get incredible mileage. I think mileage is a matter of how close to the wind you want to run with rich/lean and there is really one way to confirm all that these days and that is with O2 sensors or a tailpipe sniffer... Reading plugs on modern fuel is almost impossible. I just keep leaning out until I squeak a piston and then go back 1/2 a setting NOT.

I like the 5 speed because you know what kind of terrain we travel here, The climb out of Kelowna going South on the Coquihalla Hwy is 26 km to the summit. A five speed has a gear for every hill!

PS The last Surtees 5 speed I got had to come a circuitous route as Power Sports will not sell to "America", not even to Canada either, I didn't ask about Mexico, but suspect they also got included in that generality. That is not the first UK supplier who has stopped shipping here, Lindsley for magneto things comes to mind. Great to have friends in the Club.

Also rogerphilip, you should be able to just copy and paste your table into your post, although it might need a bit of formatting.

 

[rogerphilip]

Also rogerphilip, you should be able to just copy and paste your table into your post, although it might need a bit of formatting."

I would have thought so but still can't get the file to upload

 

[Robert Watson]

So I'll try one. I copied the content of the sheet. not the file

Model Engine Clutch Trans Rear O/A ratio
A Single 30 56 19 46 4.519298
32 53 22 48 3.613636
32 53 22 46 3.463068
32 53 21 46 3.627976
32 53 22 46 3.463068

A Twin 32 56 22 45 3.579545
alternate 32 56 21 45 3.75
3/8" pitch 32 56 21 46 3.833333
32 56 22 46 3.659091
33 56 22 46 3.548209
34 56 22 46 3.44385
34 57 22 46 3.505348
34 57 22 48 3.657754
35 56 22 48 3.490909
32 53 21 48 3.785714

Post war Single 23 40 18 48 4.637681
1/2" pitch
Post war Twin 35 56 21 46 3.504762
alternate 35 56 22 48 3.490909

It did take 5 mins of formatting but worked OK

 

[rogerphilip]

So I'll try one. I copied the content of the sheet. not the file
It did take 5 mins of formatting but worked OK

Hi Robert
My file is an interactive Excel spreadsheet loaded with formulae that automatically calculate speeds for all gears relative to the users desired rev ranges and sprocket combinations.
But have tried upload and copy/paste and still no luck

 

[davidd]

If you are using Windows you can always do a screen capture and one way is to use their snipping tool. You put what you want in a box and it is saved as a .GIF in your picture file and the Forum likes GIF's. I had to cut the top of this because it would not all fit on my computer screen, but it is the Excel program I use. I made this one for David Tompkin's who was racing at Barber recently.

David shifted at 5,500, on advice to the dyno guy, but after the race he was hungry for more speed. I suggested that he has to shift at 6000 if he wants more speed. The chart shows how important red line is to racing. I shift at 7000. If you compare the speeds at 6000 and 7000 there is a huge difference in speed. I suggested this gearing for 6000 because I like to gear for just around 100 mph on the straight. I would have selected a lower gearing, that is, 103-4 at 7000 for my own use as the fastest the racer was clocked was 105 mph at Daytona with a radar gun. But, when you realize that the difference a 1000 rpm can give you, anywhere between 6 and 16 mph more , it is hard to give that up.

David is running an AMC box right now. He is swapping it out this winter for a BAR box. So, the ratios are Norton. The primary drive ratio is Newby.

I have a few different templates and can do a 6 speed also. If anyone wants the Excel document PM your email to me.

David