03-15-2014, 01:21 AM
(03-14-2014, 07:02 PM)Flynrider_imp Wrote: I was cruising down the freeway last Sunday when I noticed I hadn't bothered to shift into 5th (as usual). I was turning 5K rpm in 4th and that feels normal to me, since that's what the CB750 does in 5th at 75 mph. When I shifted into 5th and the rpm dropped to 4200, I noticed that I actually felt more vibes in the bars than I had at 5K. I thought it was a bit strange, so I shifted back and forth and it definitely seemed shakier at the lower rpm. Maybe that's why my brain so often forgets to shift up from 4th gear.
I used to own a lot of twins with 360 cranks, so vibration was just a normal part of motorcycling. When I took my first ride on an I-4 bike ( a Kaw Z-1) I was stunned. I never knew a motorcycle could be so smooth. In fact after I started it, I had to double check the tach to verify that the engine was running. I don't think I'll ever go back.
BTW, the shakiest bike I've ever ridden was a Yammie XS650 twin that was converted to a "twingle". The cam was modified so that both cylinders would fire simultaneously. Interesting bike, but it couldn't be kick started by anyone under 250 lbs. and the electric start would only work if you hooked a car battery to it. The guy that modified it didn't think of adding a compression release
That's an interesting observation which I concur with. I've found when I rev my bike beyond 5k rpm up toward 7.5k rpm the motor definitely passes out of the 'vibe' range and smooths out again at those higher rpms. You might call me crazy, but I think a freer-breathing exhaust helps a motor run smoother at higher rpms. I have a staintune on my bike, but then again I didn't really rev the bike high when it had the stock exhaust. The reason I say this is that I had a Subaru that got very vibey above 5k with stock exhaust, but once I installed a free-flowing exhaust it was smooth as glass all the way to redline, as if the stock exhaust choked the motor up and caused vibes at higher rpms. In any case, that was my experience with that car.