Triumphrat

Diego · 06-10-2007

sweatmachine wrote:
>>>"don't you have it backwards there Diego?
>>>Long Stroke = more torque lower
>>>Short Stroke = more hp higher"

Jimbonnie wrote:
> Actually, it's neither and another common fallacy.

Hmm, I go away for a couple of days and come back to find I've put my foot in it. The irony is, I didn't even mean to address the stroke-versus-bore thing at all. What I really wanted to address was tradeoffs in using different numbers of cylinders to achieve the same displacement--a very different matter, so I'm not sure why I even couched it in those terms.

Only excuse I can come up with is being groggy after my siesta that day. Must've inadvertently taken a "torque nap" instad of a "power nap."

Thanks for clearing up the misconception from both ends of the spectrum there, Jim! Good posts.

ohiorider · 06-10-2007

Excuse the following rant. it isn't nearly so academic as previous explanations of long and short stroke. I was going all the way back to earlier posts about specific crank degrees. By the time I'd typed it (and got off the phone with a long-lost friend) it had become essentially obsolete.
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Alright - let me add my .02 worth. The angle of the HD cylinders is 45 degrees. Both rods are attached to a single crank pin using a knife and fork arrangement (one rod sits smack in the middle of the other fork's two main bearing arrangement).

It's a twin, so each time the crank rotates one time, there will be one power stroke. Same as with the Bonneville. But the power strokes are not evenly spaced, as with our 360 degree crank Bonnevilles, but are irregular (remember ..potato/potato .... there's a reason HD has this sound).

I think it is calculated like this:

360 degrees - 45 degrees = 315 degrees
720 degrees - 315 degrees = 405 degrees

It's all happening within two revolutions (720 degrees).

So what this all means is that one cylinder fires, then the next fires 315 degrees later, the next 405 degrees later, the next 315 degrees later .............etc, etc, etc. Potato, Potato.

Here's a better explanation on Wikipedia.

Wiki HD firing Order

Bob

[ This message was edited by: ohiorider on 2007-06-10 18:41 ]

lindsayt · 06-10-2007

Sure, but neither the conrod length nor the offset on the crank from the center of rotation to the center of the big-end bearing has to be related to stroke. Neither is necesarily dictated by other primary engine design factors either.

MA0458 · 06-10-2007

GUYS! GET OUTA HERE AND GO RIDE YOUR BIKES!!!! It's summertime! :-D

lindsayt · 06-10-2007

I'd love to! Unfortunately, I've been in the house with a sick toddler all day :cry:

shortys · 06-10-2007

If you are truly curious visit here: http://www.calsci.com/motorcycleinfo/Horsepower.html

worldspeaker · 06-11-2007

When I originally posted this question I had no idea there would be so much information coming from it. For all the info coming from people describing themselves as "backyard bumpkins" the backyard they are referring to must be of M.I.T. This is great info from a lot of very informed people. I have always considered myself mechanically intelligent but wow, the people on this forum are way out of my league. Thank you all for giving me the opportunity to read and learn. Knowing how engines work is one thing and explaining them is another, thank you Jimbonnie and the rest of you for making it interesting and understandable.

This little question has tought me so much about engines that I had not thought about. It also explains a lot of what I see going on in the motorcycle world. If I am extrapolating correctly from the info I am reading I would gather that companies making oversquare V twins are focusing on torque more than Hp. This would mean these engines could run out of breath sooner than their displacements would normally dictate. This means the companies would make larger and larger v twins to allow them to run easier at highway speeds. This would also mean that the frame of the bike would have to be longer to accomodate the engine. Engines that have a similar displacement but use more cylinders would have shorter strokes and would breathe easier at higher revs (larger bore and larger valves) and would create more horsepower. This would explain why v twin manufacturers are in a displacement race and why smaller multiple cylinder engines often have a lot more power than larger two cylinder engines. Hopefully that actually made sense. It seems to fit in my brain but whether I have stated it clearly is another matter. That is my simplified extrapolation of what I have been learning. Hopefully I am getting the info correct in my head.

Anyway once again I want to thank everyone for all your help in explaining this question to me.

Dave