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Author Topic: Bullet Crank weight.  (Read 4195 times)

chinoy

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Bullet Crank weight.
« on: January 01, 2010, 10:16:18 AM »
Why is the crank so heavy on these bikes ?
What would happen if it was to be put on a diet ?
Anybody have any idea about the new UCE crank weight ?

Anybody know what the crank weight was on the BSA Sunbeam.
Or similar British 500cc single ?

What would happen if we reduced the weight by 60% and got it dynamically balanced.
I know RE tried to reduce the weigh in some of the newer model's and all the owners where standing around machine shops puting the weight back on.


hocko

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Re: Bullet Crank weight.
« Reply #1 on: January 01, 2010, 11:05:59 AM »
I recently read an article in an English Classic bike magazine on this very subject. The upshot is that the cranks have been engineered by the designers/engineers to suit the purpose and that no advantage is to be gained by lightening the crank other than a slight increase in acceleration figures with a loss of reliability and a dramatic increase in vibration despite balancing. I think I would just leave it as nature intended. I must admit though that I have no experience in the field at all, just what I have read. I hope youget a few more far more knowledgeable replies than mine.

Cheers  :)

Chasfield

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Re: Bullet Crank weight.
« Reply #2 on: January 01, 2010, 11:41:12 AM »
With a single you are only making a power stroke every two turns of the crank. You need to store the energy made in a heavy flywheel to tide the engine over until the next power pulse.

Without the fly wheel effect the crank would experience marked decelerations and accelerations on the compression and power strokes, which would hammer the transmission.

2001 500 Bullet Deluxe

ace.cafe

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Re: Bullet Crank weight.
« Reply #3 on: January 01, 2010, 01:30:06 PM »
The flywheels of the old Bullet are approximately 22 pounds.

The comments by others above all touch on various aspects of why the heavy flywheel is advantageous in our engine.

Lightening the flywheel does help to improve acceleration and deceleration.
However, what is almost never discussed is that the rotation of the crankshaft is not anywhere remotely resembling a "smooth rotation". The crank actually rotates in a series of "hitches" that speeds it up and slows it down as it goes around.
It gets a big push of speed on the power stroke, and then it gets slowed down with some resistance by the pumping loss of pushing the exhaust out the valve, and then it sees resistance which slows it as it draws intake mixture in, and then it sees an even harder resistance as it compresses the mixture, and then it gets whipped back into a high speed push as the power stroke delivers to it again.
To say nothing of the effects of reciprocation motion that are happening to it.

The crankshaft actually operates more reliably and better in function when it's heavier.
A single has no help from other cylinders to keep it rotating, nor to help smooth out the rotation. It needs the weight.
Racers do reduce the weight of the flywheels for acceleration and deceleration, and it's good for that, but not good for street use.
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Chasfield

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Re: Bullet Crank weight.
« Reply #4 on: January 01, 2010, 03:43:50 PM »
You could plot a graph of minimum self-sustaining engine speed -  idle speed - against flywheel weight. As the flywheel was lightened this minimum speed would climb. There would always be a point at which the piston just bounced off the next compression stroke, through lack of crank momentum to carry it through.

If you seriously lightened a Bullet crank assy. it would give you crisp pick up on the throttle but the motor wouldn't stay lit below 3000 rpm.
2001 500 Bullet Deluxe

chinoy

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Re: Bullet Crank weight.
« Reply #5 on: January 06, 2010, 06:38:00 AM »
SOunds a lot like what I was told when I asked about reducing the weight on other engines we work with.
We still went ahead and reduced the weight.

Crank weight seems to effect
a. Mileage (You store more energy with a heavy crank and get better mileage).
b. Idle RPM. With a heavy crank you can get low idle. With a light crank idle is a little higher but not by much i.e. 800 vs 1000 RPM.

The reason I asked about the sunbeam is because I dont believe the Bullet is the only 500cc single on the planet. But it sure as hell has the heaviest crank on the planet.

Id like to know what a Sunbeam set up for Track would run as its crank weight.

Vibration's ?
They are there anyway. I dont see it getting any worse or better. It will just happen at another RPM when you play with the crank weight.

Id really like to know if you have crank weights for any modern or old 500



motocamp

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Re: Bullet Crank weight.
« Reply #6 on: January 08, 2010, 03:20:16 AM »
What you do depend's on you end goal for the bike, in the end lightning the flywheel would comprimise the way the bike run's and feels.

The idle rpm will be affected,how much depend's on how much stuff you remove.The acceleration does improve but on a stock bike only a little.

By the way Chinnoy ,do you know someone who does dynamic balancing down south?Can you PM me with the info .I thank you in advance.

chinoy

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Re: Bullet Crank weight.
« Reply #7 on: January 09, 2010, 11:24:28 AM »
An 8Kg RD Crank typically has 2 Kgs removed.
A RZ Crank has 2Kgs removed and a Ignition Flywheel 1 KG lighter installed.

You have anyway compromised with the original design when you start fitting a modern magneto that puts out 300 watts onto the end of the crank.