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Cruising to work the other day on the freeway and looked down at my gauges and noticed that cruising at 70mph is 6,000rpm. Got me thinking, with basic math that means my little 650 is spinning at 100 TIMES PER SECOND!! That kinda speed baffles the thought process. You would think that at that speed things would just disentergrate! Makes me wonder how I could stay on the freeway for hours like that without the pistons exploding or melting in the bores. Does this blur of spinning parts make anybody else think like that?

Yeah, at first it can be a disconcerting thought. My car does 2500 at 70! But most of the bikes I've been on are around the 5000 for 60-70mph and I've done some decidedly extended trips with them. Haven't had a serious problem yet.

Still, have you considered changing the ratio?

I know my bike can take it, but cruising the freeway at 75 or 80, it sounds like a swarm of hornets.
The speed limit in America was 55 mph when my '76 was new. I think just about all of the KZs were sold during the dark years of the 'double nickle'. Some even have a speedo that only goes to 80. At 55, the tach is indicating something like 3600 rpms.

I thought about change the gear ratio with a different front sprocket, but I like snappy acceleration with the stock setup, so I'll just live with the hornets.

I dropped 4 teeth on the rear and it lowered the RPM's down to just over 5K at 70MPH. I know it doesn't sound like much, but it makes a HUGE difference!
I suppose I lose a little snap on take off, but I really don't notice much difference at all.
YMMV!

I just got my twin running right and the other day I got her up to 70mph @5000rpm. Something I noticed is that @ around that speed/the sound went from a high whinney shreik to a nice low growl. I was only @ 70 for a short while so I'm gonna have to check that again.

triple it for an F1 race car

all the newer 600s have atleast a 16k redline nowadays it seems. i think the o6 r6 redline was 18k

the R6 thing was a big yamaha farce. they got egg on their face over that one. bunch of the magazines dyno'd them and they could not get much pat 16,250 IIRC
here ya go
www.motorcycle-usa.com/Article_Page.aspx?ArticleID=2988&Page=1

Post edited by: flht1997, at: 2007/06/20 17:49

Bluemeanie wrote:

Cruising to work the other day on the freeway and looked down at my gauges and noticed that cruising at 70mph is 6,000rpm. ....

Hmmm...would that mean 140 mph at 12K redline?
Rumor has it that optimal gearing is when the bike will just bearly pull redline in top gear.
Rumor also has it that these bike were engineered, designed and intended to spin when operated -- don't lug 'em -- they love spinning.

Since the day I bought it (new 04/77) my '77 KZ650C1 has run (in top gear) at 4500 RPM = 60 MPH; 6000 RPM = 80 MPH; 9000 RPM (redline) = 120 MPH. I'm using OEM stock sprockets. Ed

I went the other way, up 4 teeth in the back to make it accelerate better. I have not been on the highway yet but I reguarly cruise at 55 in 3rd or 4th gear depending on my mood. I never use 5th for that speed. I'm expecting 70 to be no problem in 5th at a reasonable RPM.

My stock gearing is 16/42. Years ago I went with a 15T front sprocket (reborn650 runs a 14T!) and bike was much snappier but was at about 6500 @ 70, plus 1st gear wound up so fast you had to really pay attention. So when I broke the piggy bank for Z1, I went back to stock and bike runs much better now. But in hindsite I should have gone with a 17T front. Not a real big deal as I try to avoid the dreaded boring highway ride as much as possible. Still.... 100 times a second!!?? Still boggles my mind