Lightened Crank
Moderators: oldjapanesebikes, H2RICK, diamondj, Suzsmokeyallan
-
bikingbagga
- On the street
- Posts: 41
- Joined: Sun Mar 08, 2009 10:36 pm
- Location: CASTAIC, CA
Lightened Crank
Can anyone explain what you do to the crank to lighten them
-
tz375
- Moto GP
- Posts: 6204
- Joined: Mon Nov 03, 2008 10:47 am
- Location: Illinois
At the risk of sounding facetious, you machine metal off them.
On the GT500, the trick was to reduce the OD of all 4 flywheel disks and then fit a tubular stuffer that effectively maintained the same primary compression ratio.
There are people who will drastically reduce the mass of a GT flywheel, but I have not read any reports of what they are like to ride. back in the day, Suzuki tried it and reportedly went back to stock flywheels at long fast tracks like Daytona.
I habve had many Honda twin cranks lightened by chamfering flywheels on both sides, but on a two stroke there's a loss of primary compression that may be an issue, though modern thinking is that maybe it's not the problem it used to be thought of.
On the GT500, the trick was to reduce the OD of all 4 flywheel disks and then fit a tubular stuffer that effectively maintained the same primary compression ratio.
There are people who will drastically reduce the mass of a GT flywheel, but I have not read any reports of what they are like to ride. back in the day, Suzuki tried it and reportedly went back to stock flywheels at long fast tracks like Daytona.
I habve had many Honda twin cranks lightened by chamfering flywheels on both sides, but on a two stroke there's a loss of primary compression that may be an issue, though modern thinking is that maybe it's not the problem it used to be thought of.
-
johnakay
- I likes them jubblies
- Posts: 2365
- Joined: Wed Oct 15, 2008 11:19 am
- Country: UK
- Suzuki 2-Strokes: bike
-
tz375
- Moto GP
- Posts: 6204
- Joined: Mon Nov 03, 2008 10:47 am
- Location: Illinois
Ron Grant used to lighten the 500 cranks and there's an article showing all of that on line somewhere.
The guy in England lightens 750 cranks by "pork chopping" them and Kris Bernstein bought one for his race project. Details in his thread.
Back in the day, the theory was that more primary compression made more power, so there was an obsession in the sixties with raising it. Unfortunately the effect was to make more power over a very narrow band, so the bikes of the time needed 14 to 16 gear sets in the trans to keep them on the boil.
With reed motors and advances in pipe and port design, the thinking changed so that a larger crankcase volume is now believed to be beneficial - within reason. The rationale is that if the volume is small, there is not much air/gas mixture available to be transferred up to the combustion chamber, so outright power is limited.
Instead of thinking that the descending piston pushes the mixture up through the transfer ports, it's better to think of the pipe and resonant waves sucking the mixture up. there has to be some pressure differential and the volume downstairs has to be appropriate for the capacity of the motor and state of tune.
At least one racer here has discovered that a lighter crank and no stuffer made for a faster 500 which was also easier to ride. Doing more with less.
The guy in England lightens 750 cranks by "pork chopping" them and Kris Bernstein bought one for his race project. Details in his thread.
Back in the day, the theory was that more primary compression made more power, so there was an obsession in the sixties with raising it. Unfortunately the effect was to make more power over a very narrow band, so the bikes of the time needed 14 to 16 gear sets in the trans to keep them on the boil.
With reed motors and advances in pipe and port design, the thinking changed so that a larger crankcase volume is now believed to be beneficial - within reason. The rationale is that if the volume is small, there is not much air/gas mixture available to be transferred up to the combustion chamber, so outright power is limited.
Instead of thinking that the descending piston pushes the mixture up through the transfer ports, it's better to think of the pipe and resonant waves sucking the mixture up. there has to be some pressure differential and the volume downstairs has to be appropriate for the capacity of the motor and state of tune.
At least one racer here has discovered that a lighter crank and no stuffer made for a faster 500 which was also easier to ride. Doing more with less.
