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#203786 - 02/23/06 8:45 am crankshaft weight for racing 650 triumph  
Joined: Aug 2005
Posts: 168
spook Offline
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spook  Offline
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Joined: Aug 2005
Posts: 168
Melbourne australia.
How far does one reduce the crank weight on a racing 650 p/u motor before problems occur and what are they?

#203787 - 02/24/06 8:05 am Re: crankshaft weight for racing 650 triumph  
Joined: Apr 2005
Posts: 2,332
Mark Parker Offline
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Mark Parker  Offline
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Joined: Apr 2005
Posts: 2,332
Bega NSW Australia
You can take off a substantial amount if you re-phase the crank to 90 deg, on a std A65 crank we cut and re-phased we threw away the flywheel and replaced it with a narrow center section with counter weights bolted on. This can work quite well because the pistons are no longer stopping at TDC and BDC together, instead when one stops the other is still moving contributing some flywheel effect. I think the dynamic load on the crank is also reduced, we had no problem reving it to 9000.


mark
#203788 - 02/24/06 8:17 am Re: crankshaft weight for racing 650 triumph  
Joined: Sep 2005
Posts: 228
Melbourne Metisse Offline
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Melbourne Metisse  Offline
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Joined: Sep 2005
Posts: 228
Melbourne, Australia
To quote from the Stan Shenton book "Triumph Tuning "...
"There is no need to lighten the flywheel on 500 or 650 Triumphs. In fact the heavier the wheel the better. After 650 engine number DU24875, a 2Żlb heavier crankshaft assembly was fitted. This can be used in earlier engines and the balance afctor remains at 85%"
They won 24 hour races with their engines so they knew what they were doing!

#203789 - 02/27/06 5:30 pm Re: crankshaft weight for racing 650 triumph  
Joined: Aug 2005
Posts: 189
bsadb Offline
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bsadb  Offline
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Joined: Aug 2005
Posts: 189
easley south carolina
Look at the Triumph owners club link from the mainn page of this web site and you will find My hero Kevin Cameron's artical. You will be looking for weights to weld on your flywheel.


Always use the Golden Rule.
#203790 - 02/27/06 9:37 pm Re: crankshaft weight for racing 650 triumph  
Joined: Aug 2005
Posts: 189
bsadb Offline
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bsadb  Offline
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Posts: 189
easley south carolina
Some of my freinds at the local dragstrip have discovered adding weight to their mini dragster briggs motors have inmpoved speed and time on the strip.
Someone told me the Nurish racing cranks for Nortons is much heaver than the original?


Always use the Golden Rule.
#203791 - 02/28/06 1:03 am Re: crankshaft weight for racing 650 triumph  
Joined: Aug 2005
Posts: 189
bsadb Offline
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bsadb  Offline
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Joined: Aug 2005
Posts: 189
easley south carolina
I wish someone with a Nurish cranked Norton would give their two cents on this topic. Ive been Thinking about getting a heavy wheel for my a70.
I tend to think the extra weight helps stablize
the crank from "floping around like a wet noodle"
as my friend Teb Hubbard says.


Always use the Golden Rule.
#203792 - 02/28/06 1:20 am Re: crankshaft weight for racing 650 triumph  
Joined: Aug 2001
Posts: 1,851
dave - NV Online content
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dave - NV  Online Content
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Joined: Aug 2001
Posts: 1,851
Elko, Nevada USA
I and others have had long discussions regard the optimum flywheel weight of a Gold Star crank.
Lotsa variables here if you give it some thought...
A single cylinder engine fires and supplies power to the crank once every 720║ of rotation. And the Tri verical twins re this discussion each 360║ of crank rotation there's a power stroke. It's a proven fact the flywheel rotation rpm, @ degrees/sec changes with each and every power stroke.
During the compression cycle the flywheel speed slows under load and then during the power stroke the flywheel is accelerated.
BTW, this is the reason why a "cush drive", on the crank is important to protect the drive train during these changes in rotational speed. The cush is absorbing these changes each 360║ or 720║ of crank rotation. Make sense?

Anyhow .. what we don't know is how much flywheel weight the mfg. added just to make the engine appear to be smooth especially at the lower rpms most road bikes are ridden. As has been mentioned, there is a optimum amount of flywheel inertia required for optimum power carrying the engine efficiently through it's cycles at the rpms we are concerned with.

Interestingly Phil Pearson the Gold Star guru has determined the OEM GS flywheel was a bit too light for optimum performance and his pressed up cro-mo cranks many of us use are a bit heavier then OEM. But then his latest/greatest short stoke 9k rev 350 cranks have lighter 'oval' flywheels. hmmm.

I have long thought that larger diameter but lighter weight wheels in a GS would give the inertia required for good performance. The GS cases have room inside for the larger diameter wheels and merely adding a 'oil scraper' would make them work. hmmm.

Now what's really goofy is to have a GS crank on the bench alongside a modern Yam YZ450 crank. Whoa, what's the deal here? Obviously the Yam's bigger bore and shorter stroke engine design is different, but difference in the size of the wheels is huge and the Yam makes max power only about 1k revs higher than a GS. hmmm.

The common crank modifications to the YZ450 cranks when they are stroked building a 502 cc engine by Halycon is to add #2 1/2 weight with welded on plates and 'heavy metal' inserts. We also use the heavier ignition flywheels. Sadly there's no more room in the cases for more flywheel diameter or width which is the common curse of all these 'modern' designed for MX 450-500 engines.
When 'built' that 60-65 hp makes it fast, but with a touchy throttle.
But then go racing and 'everyone has one'. aarrgh.
hmmm.....


dave - NV
#203793 - 02/28/06 4:48 pm Re: crankshaft weight for racing 650 triumph  
Joined: Aug 2001
Posts: 1,081
Phatt Bob Offline
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Phatt Bob  Offline
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Joined: Aug 2001
Posts: 1,081
Cheshire UK
For my kind of racing, I always use full weight flywheels. Only allowance I make is for skimming the flywheels so they don't batter the tops of the cams when the crank flexes, as it must do.

Vibration is minimal so balance factor is an irrelevance when the motor is under load for 440 yards.

The improved time on the drag strip will invariably be made up by a better launch due to the higher inertia of the flywheel, this is where races are won or lost.

My opinion only, probably not shared by others.


Phatt Bob
'95 Daytona 1200
'98 Daytona 1200 dragbike
ex-850 T140 Caff Racer, 850 Triton, Morgo T120, Starfire and Pretend Daytona 500 owner

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