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#858492 09/15/21 12:22 am
Joined: Jan 2006
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I got some close ratio gears from Tony (Hillbilly) a while back for a 4 speed 650 Triumph. I believe what I have is c/r 1st and 3rd pairs. Am I right in thinking this will make for a taller first? And a taller 3rd? When used with std 2nd and 4th?
All the talk of Semper Gumby's C/R woes has me revisiting these parts that have languished in my parts bin.
Could be a fun track bike experiment...
Thanks

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Mike Baker #858515 09/15/21 9:31 am
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A question would be, are they meant to be used with a C/r 2nd? Is std 2nd ok?

C/r 1st will be taller so you change at higher speed so rpm in 2nd is higher when it goes in, closer to your power area. I'm not familiar with the Triumph 4 speed but if 2nd was close to 3rd and there was a big gap to 4th like the std 5speeds it needs closing up. The faster you go the less hp you have to push, if you rev out 3rd and shift into a very tall 4th it can drop you out of the power rpm and it could slow down or at least be very sluggish. If 3rd is higher you change at higher speed and have more rpm up in 4th. And be in stronger power to push for more speed.

Generally ratios with a 4speed C/r Gear box have taller first and a spread that puts the taller gears closer. This is because being at speed uses hp, the faster you go the less you have remaining to accelerate. So it needs to change into a strong power zone which means higher rpm.


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Mike Baker #858518 09/15/21 11:05 am
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Ideally, the 2 sets I have would have been meant to be used with a std gearbox to (1) make for a taller 1st and (2) close the 3-4 gap some. I guess what I'm asking is does anyone know if thats how a triumph cr is designed to work? I have found part #'s and know thats what I have but haven't found documentation of comparitive gear ratios.

Mike Baker #858591 09/15/21 11:31 pm
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Victory library has this on BSA gearboxes. One on Triumphs may be available somewhere. I have a calculator for BSA gears. Possibly you could put the teeth numbers into that and it would compute the ratios speeds and rpm. If I could find it. It may be on a flash drive I'm yet to locate.

http://victorylibrary.com/brit/A65-4.htm


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Mike Baker #859072 09/22/21 2:54 am
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It’s all about lap times. I’m afraid I don’t have any direct experience with 4 speeds to share but I do have a suggestion. I never road raced a 4 speed Triumph, but rode many many dozens of road races with a 5 speed. The short stock first gets off the line really well geared 4.6 or so (20/46 using stock 2:1 primary) and is still reasonably good at 4.3, so I wouldn’t want to change that. Also, I never had a problem with the gap between 4th and 5th, however I was geared, it always seemed like that was the tightest split in the gearbox and never caused a problem. But the real speed limiter in road racing a stock Triumph 5 speed is the big ratio gap between 2d and 3d. In the garden variety road race hairpin, it can make second too short, and 3d too tall, and require a shift where you don’t want one. Charlotte’s Web at Barber, turn one at Miller, International Hairpin at Daytona, and to some extent turns 4 and 5 at Roebling are examples. Early on racing my bike, when it was at a lower state of tune, with less torque, this was a big problem. Fully developed at the end, I had 11-1/2:1 pistons, and twin plugged head, and longer pipes, and it could make acceptable power down low below 5000 in 3d, and I could avoid an extra downshift/up shift, which improved lap times a lot. I looked for a taller 2d gear for the 5 speed but could never locate one. That same big gap between 2d and 3d seems to show up in my street 70 Bonneville 4 speed. That makes me think a taller (close ratio) second would be a really good thing if that gear split is a problem in your 4 speed.

Last edited by linker48x; 09/22/21 3:13 am.
Mike Baker #859079 09/22/21 4:57 am
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I was accused of a 'Bodge' ages ago with those 5 speed boxes, i rode a bike
at a circuit that had a layshaft top with an extra tooth on it, it was bloody excellent.
It closed the box up nicely and made 1st actually usable.
A few blokes here did this, just had a gear hobbed with an extra tooth. Sure it was outside
the specs etc but for race use was fine for a couple of years.
The quaife set was way off, this was much better, do the ratio calcs and you'll see.

Nick


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