Shortstroke Build

Junk yard Dawg said:
Is this the crankshaft for the Dawg Chaser ? Were tuning here in Mudville also.

John Ellis
AHRMA 5X
F750
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No,
not the dawg chaser - It would be the dawg catcher.....
 
You got me, question for Jim the wizard ? you running like 61 present , or higher ? Got a 360 crank here were working on. Smaller wheels as you know.

John Ellis
5X
 
Junk yard Dawg said:
You got me, question for Jim the wizard ? you running like 61 present , or higher ? Got a 360 crank here were working on. Smaller wheels as you know.

John Ellis
5X
Click to expand...

Are you referring to 61% balance?
 
Jim, I wish you every success with this project, however I think you are ambitious. There are probably too many things in a commando-based race bike which work against one another. As I've said previously, I believe that the whole package must work in synchronisation. If you win out, you will have proved a lot. It is not only about the short stroke, however what goes with it is extremely important. My own problem with the short stroke 500cc Triton was that I never had the close ratio six-speed gearbox, so in effect I could choose where I was going to be beaten. With a four-speed box - gear it low to suit the tight part of the circuit and run out of puff at the ends of the straights, or gear it high and have nothing around the tight parts of the circuit. Might be OK for Daytona ?
 
Junk yard Dawg said:
Yes Jim, 61 percent or ? Just curious , as there are many opinions out there.

John Ellis
5X
Click to expand...

I am setting this up at well over 61% since the frame used seems to be more comfortable when the motor is shaking fore and aft instead of up and down.
Not applicable at all for your bike. Jim
 
Jim, as a novice on these things, I just wonder about the 1200 rpm, why those revs?
Tommy
 
fiatfan said:
Jim, as a novice on these things, I just wonder about the 1200 rpm, why those revs?
Tommy
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It makes no difference as to the speed as long as it is above the speed of phase reversal and not at a speed that excites resonances in the fixture or bench -or at a speed that makes the test weights fly off.

If the crank weight is different I can vary the speed to avoid resonances.

Phase reversal occurs when the speed is high enough that the crankshaft rotates on its moment center [the center of the mass] instead of the center of the journals. The sensors record the movement of the journals when the crank is rotating on it's moment center.

When balance is achieved the moment center and the journal center will be the same so the journals will not move.
 
I suggest that with the shorter stroke the rod length and piston mass become more critical. Your motor will probably need to peak at more than 10,000 RPM and use more than a 4 speed transmission to make sense in a race bike. Also the exhaust system becomes more critical, it determines how much bottom end torque you have to make the bike rideable. What you are doing is great in theory, however you won't know until you try to use the crank, what will happen in practice. There is nothing worse than a savage motor which requires heaps of revs to get going, and has no bottom end. I spent my childhood road-racing a bike like that and I can still feel the pain and the anxiety.
I really like your crankshaft, it is very exciting.
 
acotrel said:
I suggest that with the shorter stroke the rod length and piston mass become more critical. Your motor will probably need to peak at more than 10,000 RPM and use more than a 4 speed transmission to make sense in a race bike. Also the exhaust system becomes more critical, it determines how much bottom end torque you have to make the bike rideable. What you are doing is great in theory, however you won't know until you try to use the crank, what will happen in practice. There is nothing worse than a savage motor which requires heaps of revs to get going, and has no bottom end. I spent my childhood road-racing a bike like that and I can still feel the pain and the anxiety.
I really like your crankshaft, it is very exciting.
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Not to worry, The engine is being tuned to make power up to somewhere between 8000 and 8500 rpm with a wide powerband -and we have a TTI box to work with.

The reduction of stroke is mostly to reduce the amount of power given up to friction -which in a longstroke motor is a major loss at 8500 rpm.
Another benefit is room for larger valves so we can use a milder cam and still move the required amount of air.
 
comnoz said:
The reduction of stroke is mostly to reduce the amount of power given up to friction -which in a longstroke motor is a major loss at 8500 rpm.
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You may well find that the heads work better at those rpm too - I've not done the harmonic calculations for a Commando yet, but on the Harley big twins they don't start working properly until the motor is doing 7,500 rpm. Which is a shame, as on the stock stroke they run out of piston speed at 6,500 rpm...
 
Paddy_SP said:
comnoz said:
The reduction of stroke is mostly to reduce the amount of power given up to friction -which in a longstroke motor is a major loss at 8500 rpm.
Click to expand...

You may well find that the heads work better at those rpm too - I've not done the harmonic calculations for a Commando yet, but on the Harley big twins they don't start working properly until the motor is doing 7,500 rpm. Which is a shame, as on the stock stroke they run out of piston speed at 6,500 rpm...
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That is true for a 32mm port Norton also if your just looking at port size.
Of course when you look at the small valve on the end of that 32mm port -nothing works.
 
comnoz said:
That is true for a 32mm port Norton also if your just looking at port size.
Of course when you look at the small valve on the end of that 32mm port -nothing works.
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With large pistons in a short stroke motor, plenty of room up there for bigger valves.

If only the valve gear didn't put them soooo close together.
And with not much metal to downdraft the ports either....
 
comnoz said:
Paddy_SP said:
comnoz said:
The reduction of stroke is mostly to reduce the amount of power given up to friction -which in a longstroke motor is a major loss at 8500 rpm.
Click to expand...

You may well find that the heads work better at those rpm too - I've not done the harmonic calculations for a Commando yet, but on the Harley big twins they don't start working properly until the motor is doing 7,500 rpm. Which is a shame, as on the stock stroke they run out of piston speed at 6,500 rpm...
Click to expand...

That is true for a 32mm port Norton also if your just looking at port size.
Of course when you look at the small valve on the end of that 32mm port -nothing works.
Click to expand...

I got around the piston speed issue on my sprint-hillclimb racer by building a short stroke crank for it - now it goes to the best part of 9,000 rpm. :)
 
comnoz said:
Paddy_SP said:
Kvinnhering said:
What stroke did you use Paddy?
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I apologise if I've confused matters - I was referring to my HD sprint-hillclimb racer: http://www.foxonic.com/harley.htm which has a 3.812" stroke using Cosworth DFL pistons.
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Nice, that looks mean and fun.

Reminds me of the Ecosse TI I was involved in without the excess bling....
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Thank you! I've just looked at some pics of the Ecosse TI - what wheelbase does that have? It looks quite long, which is what I worked very hard to avoid - depending on the set-up I choose, mine comes out at about 1410 mm /55.5 inches.
 
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