Single-pin crank

Message

Amdonim · #1 Post by **Amdonim** » Thu Aug 06, 2009 2:34 pm

I've been trying to get my head around the difference between single and offset-dual pin cranks. My basic understanding (which may be completely wrong) is that in a single pin, the con rods connect at the same place on the crankshaft, so therefore, the pistons move at the same time. And I guess the reason for having separated firing times is that the pistons have slightlydifferent tdc times, due to the degree of separation (if this is correct, than I assume the firing order will always equal the degree of separation on the cylinders). If all of this is correct, than wouldn't the power created by the first cylinder be wasted initially while the second was still reaching tdc?

Any corrections? Am I even close?

paul246 · #2 Post by **paul246** » Fri Aug 07, 2009 2:27 am

Since these are 4-stroke engines, the spark plugs fire on every 4th stroke of each cylinder. Therefore, even on the single pin crank the firing order can be set so that the pistons are not working against each other when firing.

Check this site for a good example of each.

http://home.cogeco.ca/~rwbaxter/The%20V-Twin.htm

Amdonim · #3 Post by **Amdonim** » Fri Aug 07, 2009 8:18 am

Oh ya I guess that makes sense. So is the single pin crank just for the sound or something?

Amdonim · #4 Post by **Amdonim** » Fri Aug 07, 2009 8:22 am

Neat website, that also cleared up some fuzziness on pushrods for me. Thanks for the link.

jstark47 · #5 Post by **jstark47** » Fri Aug 07, 2009 8:31 am

Amdonim wrote:So is the single pin crank just for the sound or something?

I think decade and decades ago (like maybe the 1920's) the single pin crank was considered the simpler and more robust engineering solution, hence more reliable, given the metallurgy and techniques of that time.

BuzZz · #6 Post by **BuzZz** » Fri Aug 07, 2009 8:55 am

Everything already mentioned is part of the equation, but the biggest reason for a single crankpin is space. A single pin crankshaft is shorter (also less machining and materials) than a double pin crank. This lets the engine be narrower.

And it is only possible on a V-twin or radial engine.

mydlyfkryzis · #7 Post by **mydlyfkryzis** » Sat Aug 08, 2009 7:27 am

BuzZz wrote:Everything already mentioned is part of the equation, but the biggest reason for a single crankpin is space. A single pin crankshaft is shorter (also less machining and materials) than a double pin crank. This lets the engine be narrower.

And it is only possible on a V-twin or radial engine.

Most V-8 use single crankpins per pair of cylinders, so I guess you might call a V-8 = Four V-twins.

You can do it on any V engine (up to 180 Degrees). Chevy did it on their V-6 (Single pin) but for smoothness, Buick did a dual offset pin on their V6.

Shorter, narrower engines are handy on small engine compartments too.

Here's a video of a V-8 http://vodpod.com/watch/1459961-motor-a ... ate-engine