After yesterday’s post on the round engine I was thinking about how different it was from the engines we have in our bikes today. I reread this month’s article in Cycle World by Kevin Cameron where he talks a lot about the g forces on pistons in today’s engines at high rpm. The round engine has almost none of these forces since the speed may vary but the acceleration and deceleration is very low compared to the pistons in a normal engine as they fly one direction and then instantly reverse direction at the top and bottom of each stroke. He notes that the force on a piston in one of today’s 600cc sportbikes at 15,000 rpm is about 6500 g’s! You have to wonder how these things actually hold together without exploding between our knees.
I remember going to a drag race a couple of years ago and watching the nitro Harleys. Great entertainment as they carry their front wheel for what seems to be about 500 feet or more during brutal acceleration. Given a chance to ride one of those, I think I’d pass. The rear cylinder is pointed directly at the rider’s chest as he lies down on the bike and there have been instances of those pistons failing to stop on their upward journey which leads to the riders wearing the equivalent of a bulletproof vest to help protect them should their bike get the same idea. The internal forces on those engines is unbelievable but our streetbikes aren’t much different. As Kevin explains, the piston crown deforms like a trampoline at the top of the stroke because the connecting rod holds the center and the edges keep moving. Metal flex at 15 grand, makes a guy think about what happens when he winds the throttle up.
Being a motorhead, I’ve always been able to visualize what was happening in an engine as it ran. I listen for unusual noises and if I hear one, I try to picture what part could be causing the commotion and what it might be doing. As engine performance gets more extreme, there’s a whole new set of movements to visualize. Pistons deforming on each stroke, now that’s something I haven’t thought about.