Sprocket change

I get approx 7000rpm when cruising at 80mph on my kz650. I like to lower the rpm for comfort and willing to give up some performance. Has anyone done that and what is the recommendation(s). Thanks.

On my 750, I lowered the stock ratio by 16% for that reason. I went from the stock 38T rear to a 32T. I rand it for many years and it ran fine, but I am not a hot rodder.

It really depends on a lot of things like how much the rider weighs, how good the torque is on the engine, if you are trying to pull a windshield or fairing (which drags against the wind).

As a blind guess, I would wager you could drop by 10 - 12% and still have decent performance.

Some sprocket related websites:

1. www.gearingcommander.com (helps figure out chain speeds and ratio's)
2. www.sprocketspecialists.com
3. www.pmfronline.com

Stock gearing for the 77 650 is 16/42 (2.62 final drive ratio) - changing this to 16/38 (2.37 final drive ratio) should lower your 60 mph rpms some 6-800 without any noticable loss of accelaration. I've been running a 16/38 combo on my 650 for some 4 years and am very pleased with it although I still occasionally find myself trying to shift into the non-existant 6th.

On my 550 I went up one tooth on the front sprocket (they are cheaper than the rear ) and it cut RPM's at cruising speed in 6th gear by about 400 RPM. I didn't really notice and difference leaving from a dead stop.

Thanks for all the good input. Now some home work and then decision time. I took the Futura fairing off(1/4) since my boys prefer the original naked classic look.

three teeth off on the back is the same as going up one on the front [roughly]

On my 85 GPz550, I've used a 15,16, and 17 front, 38 and 42 rear, and a 120/80-18, 130/80-18, and a 130/70-18 rear and nothing but the stock 100/90-18 front tire. My bike originally came with the 16/38(stock), and when I aquired it, someone had a 15 on the front. I jumped to put the largest front on(17), and rode it like that for many miles. A difference of about 1000 rpm's lower than with the 15 on, just by changing 2 teeth on the front sprocket. The engine likes the higher rpms of the 15/38, or is it I like it, so I'm now running that combo? 15/42 for the track bike.

Find out what sprockets are available for your bike, and play with those gear charts. The only issue I had with what those charts say is some ask for chain pitch(why???), and then there's the gear reduction ratio. Loudhvx and I have been bak and forth on this one, and after testing with the shift light(it was new) the factory numbers were off, even given the different rear tire, and sprocket differences. There's a cool gear chart one using excel I have on my work computer and remember, and I'll attach the link later on today. Looks like this:

Entering all the values in pink gives you everything else. The link to it is at the top of the picture. Everything after "http..."

Speed is read from the front wheel, and rpm's are read off the engine, and involve the rear wheel size, so if it's not stock, you'll need to figure that in as well. Remember the speed won't change with the gearing, or tire size change, just the rpm's at that speed will change.

The book, Automath has this to figure tire diameter, and it's been tatoo'ed in my brain for many years, Given a 130/80-18 tire here's how it works:
[(130 x 80)/2540] x 2 + 18 = 26.19 wheras 2540 is a mathematical constant.

You're tach could be off if it's electric. Mine is, and tested it with a Dyna Shift Light. Reading about 7100 rpm off at the shift light's 8000 rpm.

Eaglecrash2 - something is wrong if you're reading 7000RPM at 80MPH. A stock '77 KZ650 should be turning 6000RPM at 80MPH. Assuming your tires haven't been changed to a size much different than stock I suggest one of the following is true: The tachometer is wrong; the speedometer is wrong; the clutch is slipping; someone has already changed the sprockets to something other than stock ratio (stock is 16/42 teeth). I say this because at a 7000rpm/80mph ratio the engine would need to turn at 10,500 rpm at 120mph when in fact a stock '77KZ650 turns 9,000rpm at 120mph. Before you start changing sprockets you should find out what's causing the discrepancy. Ed

Thanks folks. I didn't have a chance to ride the bike but will find out what 650ed pointed out. I did have the rear tire/rim combo changed back when it was new with a wider chromed wired rim so as to mount a wider tire but the overall tire diameter is the same as the front. I do not know what the original rear tire diameter is. Maybe that's my issue with the rpm. But I'll ride it again to pin point the readings.

I was looking at the test that Cycle did for the original '76 KZ650. The specs said 16/42 combo, and the 'tester' reported 4600-4700 RPM at 60 mph. That would make it 6,133-6,266 rpm at 80 mph.

Thanks. Yes, i rode in to work on my bike this morning and these are the readings: 60mph= 5,000rpm, 70mph= 6,000rpm, 80mph= 6,500rpm. Given % errors on the gage it's close to factory(?). If I can drop 500rpm it'll be just nice. The other thing I need to find out is if the speedometer is accurate. It looks like changing out the rear sprocket is probably an easier method.
I get a little front shimming at 80mph. How do I get rid of that? Yes, they're wiire wheels. Thanks.