Too much seat time maybe?

[mneblett]

So, imagine if you could view the engine from above while the bike was in motion, and you could see the pistons moving in and out. If there were a point of light on the end of a piston, and you could take a time-lapse photograph of that light, what shape would it be? In other words, plot the motion of the piston on a time/distance graph. Would it be a smooth sinusoidal motion, or something far more spikey?

Definitely a sinusoid. And you are correct about needing the time-lapse photography -- you wouldn't be able to directly "see" it without the aid of a strobe light (the real-world equivalent of the time-lapse photography), as it's kinda tough for the unaided eye to resolve something moving over a nearly 3 inch path at 4 KHz (4,000 rpm)!

 

[marchyman]

Definitely a sinusoid.

You sure? The rod big end would trace a sinusoid (a circle drawn over time), but the piston is constrained to move within the cylinder and is not a fixed distance from the rod big end. I'm not sure that the waveform would be the same.

 

[mneblett]

You sure? The rod big end would trace a sinusoid (a circle drawn over time), but the piston is constrained to move within the cylinder and is not a fixed distance from the rod big end. I'm not sure that the waveform would be the same.

Definitely sure -- the actual displacement of the piston exactly follows the sinusoidal motion of the big-end journal.

You can see this at various points during the cycle, e.g., when the big end journal is at the top (pointing toward the fuel tank), the piston is half-way up the bore and at max speed (1st differential of position) with zero acceleration (2nd differential); when the piston is at TDC (big end at closest point of approach to the cylinder), the 2nd differential is at a maximum (max negative acceleration toward the crank) and zero 1st differential (speed); etc.

 

[breyfogle]

ANDYVH: Piston distance is easy !! It would be a much more interesting exercise to determine how far your rear wheel valve stem has traveled.

 

[crazydrummerdude]

..and the funny thing is; was any work done? Considering 3000rpm, at t = 0 and t = .02, the piston is in the same place.

 

[mneblett]

..and the funny thing is; was any work done? Considering 3000rpm, at t = 0 and t = .02, the piston is in the same place.

Interesting theoretical Q -- if one ignores the F x d that happens between those two points in time.

But in any event, our real interest is at the crank -- plenty 'o work delivered down there.

 

[ANDYVH]

Wow,...when I posted this goofy thread on a personal lark to answer some "what does this do" questions in my head, I never thought it would blossum this way.

And it's not even winter up here yet!!

This doesn't bode well for the coming down time from riding. Good thing I have an ice-bike to work on my low traction riding skills on the frozen stuff.

 

[alanrd]

ANDYVH: Piston distance is easy !! It would be a much more interesting exercise to determine how far your rear wheel valve stem has traveled.

looks like 150,000 miles to me.

 

[crazydrummerdude]

Interesting theoretical Q -- if one ignores the F x d that happens between those two points in time.

But in any event, our real interest is at the crank -- plenty 'o work delivered down there.

Same story with the crank, actually.
Take a picture at t = 0.00 s.
Take a picture at t = 0.02 s.
You wont see any distance traveled.

looks like 150,000 miles to me.

What about the tip of the valve stem cap? It's spinning in a different circumference.

 

[GSA Girl]

Remember that peak piston speed is attained when the line between the piston pin and the center of the crank throwl on the crankshaft is at a 90 degree angle with the line between the crank throw and the center of the main journal. This usually occurs at between 30% and 40% of the down-stroke travel. Just Saying.

 

[dwestly]

looks like 150,000 miles to me.

Ahhh, but the real question is how many rotations has the top of the valve stem cap made? To answer that:

1. Assume an approximate inner wheel diameter of 15" (I just ran out and measured)
2. Top of the valve stem cap is 1 3/8" above inner wheel surface (w/BMW logo valve cap)
3. This reduces valve stem cap top rotational diameter to 12.25"
4. Calculated circumference of valve stem cap top rotation = 38.49"
5. 150,000 miles x 5280 feet=92,000,000 feet, x 12"=9,504,000,000" traveled, /38.49"=246,921,278.25 valve stem cap top revolutions...

 

[ka5ysy]

You guys are in bad need of a road trip !

 

[rocketman]

I think about that all the time myself and wonder why the damn motor doesn't self destruct considering the forces and velocities involved in the piston motion. Just glad my airhead piston stick out to the side!
Now on my Honda V-twin one of said pistons is pointed right at my..... well, you know....

Now That's something to worry about! Ha Ha!

RM

 

[Rudyjo]

When you get around to selling the bike, mention that the engine only has
600,000,000 revolutions on it, thats a very good selling point.