Ask The Pro - Rod Ratio Reality

Got a question for answers? Send it to mcmail@sorc.com

By Tim Carrithers, Photography by Suzuki/Yoshimura R&D

Q In some of my internal-combustion engine design studies, I have come across the term "Con Rod Ratio." Although poorly explained in my textbooks, I believe it refers to the length of a connecting rod relative to the length of the stroke.

I can see a short con rod causing sharp crankshaft-to-rod angles and resulting in high side loads/forces on the pistons, wristpins, bores, etc. I can also see long con rods causing "tall" cylinders, along with the increased reciprocating mass. Can you give me some more insight?
Alex Zinkosk
Via E-mail


A Having come up through the ranks as a Superbike mechanic, team manager and now president of Yoshimura R&D America, Don Sakakura has more pure motorcycle racing insight in his fingernail clippings than most humans amass in a lifetime. On top of that, he's a really nice guy and suffers dumb questions from people like us with seamless professionalism. Here is his mercifully concise explanation of a topic that's complicated enough to blow your head clean off if approached in full, unabridged state.

"Rod ratio equals rod length (bore center to center) divided by stroke. Common rod ratios utilized in today's multi-cylinder motorcycle engines range from 1.5:1 to 2.1:1. The connecting rod ratio can impact several characteristics of engine performance.

Low rod ratio (short rod) characteristics include: 1) increased piston speeds (after TDC) for improved volumetric efficiency; 2) increased port velocity for more efficient fuel atomization and improved power output; 3) packaging benefits; 4) increased mechanical friction, which can increase piston/cylinder wear and running temperatures.

High rod ratio (longer rod) characteristics include: 1) extended dwell time at TDC for improved compression and charge burn; 2) reduced mechanical friction for a reduction in piston/cylinder wear and operating temperatures; 3) packaging considerations."

Buzz of the Ninja
I have a 2008 Kawasaki Ninja 650R with an annoying vibration in the fairing. Is there a solution to lessen or eliminate this noise? It is particularly apparent at idle. When I have asked about this, no one seems to be aware of solutions short of adding rubber gaskets or foam in the seams.
Pam Pogue
Boise, ID

According to our friends at Kawasaki, this sort of thing happens occasionally, but not enough to qualify as a chronic problem. Your dealer should be able to sort the whole thing out with minimal drama. But if you're in a DIY state of mind, check the dampers you should find at the various mounting points. Replace any that have worn out or gone missing in action and everything should be fine. The good news is they're relatively cheap, ranging from a couple bucks to a little over $10 for the most expensive one. Part numbers start with 92161, followed by various four-digit suffixes. There's a nifty diagram at www.kawasaki.com. Look under the three "Cowling" headings to find the dampers.

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