Radically High Gear Ratio on My KZ650 CSR

There have been many discussions about changing sprockets to lower the RPM for highway cruising. Since I am a "gear head" from way back, I have made an extreme change in my gearing. My goal was to cruise at much lower RPMs. I have little interest in jackrabbit starts. In fact, full throttle in 1st gear would raise the front wheel off the ground. This makes me nervous. Now these engines will rev to high heaven, but I learned a long time ago that high RPMs suck up gas very quickly. For most of my riding, I kept the RPMs down and shifted early. My gas mileage was excellent - about 53 mpg. But after getting into 5th gear, you have no other shifts to make, so the engine just revs higher and higher the faster you go. Thought you'all might like to hear the results.

The motorcycle is an '82 KZ650 CSR. The bike uses the big 630 chain. The stock sprockets are 13 front and 33 rear. This gives a 2.54 ratio (33/13). In 5th gear, the bike ran approximately the following RPMs: 40 mph = 3,250; 50 = 4,000; 60 = 4,850; 70 = 5,650; 80 = 6,500.

I discovered that the front sprocket might be increased by only about 1 tooth without hitting the chain guard. The guard is very close to the clutch pushrod, so it cannot be substantially modified. So I looked to the rear sprocket. My engine has a slight harmonic vibration at about 4,400 to 4,700 RPM. This occurred between 54 and 58 mph. I wanted to move this up to around 70 mph. After doing the math, I decided to leave the front sprocket at 13, but change the rear sprocket from the stock 33 tooth to.....(drum roll please)......a 26 tooth. This brings the sprocket ratio down to 2.00 (26/13). The bike now runs the following RPMs: 40 = 2,550; 50 = 3,200; 60 = 3,800; 70 = 4,450; & 80 = 5,100.

Here are the positive results:
The bike has an entirely new personality. At highway speeds, the engine is very quiet, smooth and economical. At 65 mph I have reduced my RPM by over 1,100. Yes, the acceleration in 5th is considerably less, but still very adequate. Besides, what the hey, downshift to 4th or even 3rd and boom - you are rocketing past everything in sight. The gas mileage is now in the high 50s with my best being 64 mpg. As I move up through the gears, the shifting is spread out upwardly. It is easier to shift and coordinate engine speed with bike speed because things happen more slowly. I do not mean that acceleration is slower, but that you are not popping through the gears so rapidly. By keeping the tach in the power band, the acceleration is ever bit as good as before. The difference is that you may be in 2nd gear as opposed to 3rd gear before for a particular speed.

The drawback to this gearing, as I knew it would be, is in starting off in first gear. It is somewhat slow. You have to engage the clutch a little longer than before. But after you are rolling about 5 to 8 mph, from there on up is just music. The bike would not do so well if used to climb a very steep driveway for example. I live in Houston, TX and the roads are quite flat so this gearing suits me well.

I will be glad to answer any questions. I will also do a followup after logging more mileage.

Thanks for the info Bill.

I added this post to a thread on the 650.ifo/forums site, in a post discussing Gas Milage: MPG

del

I, too, am a gear nut, and came across this chart a few years back.

www.cunnington1.freeserve.co.uk/gearing_v12.xls

Plug in your numbers, and there you go. Change the sprockets on the spreadsheet, before buying new sprockets.

Just remember going too small in the front can cause excessive wear on the chain.

I believe you are on the right track to better gas mileage and more comfortable high speed cruising. On my KZ750H LTD, I moved up one tooth size on the front sprocket, and the results were noticable. (One tooth up on the front equals almost 3 teeth down on the back sprocket.) However, I have hesitated to go further, since my San Francisco and Berkeley rides require that I frequently start uphill at red lights and stop signs. What I currently run is a good middle ground. If I were going to do a long trip, I would drop the rear sprocket by a few teeth.

ALSO: Someone on this board tried putting the SIX SPEED transmission internals from a KZ550 into a KZ650 or KZ750. This would be nice, if it really works. I don't remember how it all turned out. If I get a chance before the weekend is out, I will search for the threads and post a link.

Post edited by: apeman, at: 2007/10/05 14:25

When I converted to 530 chain and sprockets, I went from 15/41 to 17/41. The bike is now much easier to launch hard without lifting the front end or spinning the rear wheel.

apeman wrote:

ALSO: Someone on this board tried putting the SIX SPEED transmission internals from a KZ550 into a KZ650 or KZ750. This would be nice, if it really works. I don't remember how it all turned out. If I get a chance before the weekend is out, I will search for the threads and post a link.<br><br>Post edited by: apeman, at: 2007/10/05 14:25

The six speed will fit in the 750's (I know for a fact), and should work in the 650's that do not have a kickstart. It's a fairly easy conversion (have to split the cases), but you may have to shim the ends of one or more of the transmission shafts.

Oh great, now there are going to be more people looking for a cheap 550 trans, so I may never find another one

It only gives you just a little bit more on top and bottom, mostly it gives you closer gear ratios, which comes in handy for those with aggressive cams and narrow powerbands.

Hi!, just found out that the rear rim on my recently aquired 79' KZ650 is a 16"!! and current gearing is 13-33. wish to have the same ratio as my other 650 w/18rear with gearing at 17-42. Please help!:ohmy:

I have an '83 KZ650 CSR, and tried to order different sprockets, maxing out with 16 & 32 at a dealer. (16 fits with minor filing on the case just in front of the sprocket). Where did you get a 26 for the rear?

I've got a KZ440 LTD with a six speed in the back yard. Do you think its tranny will fit in my KZ650 CSR?

Hello Spot:
Front sprocket: I bought a 16 tooth from Z1. They told me it probably would not fit, and they were right. I was willing to grind, cut, weld, the chain guard, but even if there were no chain guard, and I ground the case supports, it would have hit the clutch pushrod. I was not willing to tamper with this. Your bike must have a different design, or are we talking numbers with a 630 chain? This is a big chain with 3/4" links. So I just left the stock 13 tooth in place. Anybody want to buy my new 16 T sprocket?

Rear Sprocket: I ordered the 26 tooth from Rebel Gears. It bolted right on. Made from aluminum. With the stock sprocket being 33, I dropped 7 teeth. Warning: The stock chain had 84 links. The smaller sprocket needs an 80 link chain. I shortened mine, but I would recommend that "normal" people buy a new one.

My knowledge of the 6 speed transmission is practically nil. I would think that the ratios of the lowest and highest gears need to be spread out somewhat, otherwise all you do is get a close ratio pattern. The perfect set-up would be a 6 speed with first gear low enough to take off briskly, and high gear high enough to cruise at highway speed with low RPM. The sprocket ratios would need to be compatible.

I saw many automotive gear changes made in the mid 1980s. Earlier models would rev perhaps over 3,000 RPM at 70 MPH, but later cars had lower first gear ratios, and an overdrive high gear and higher differential ratios, bringing the RPM down to about 2,000 at 70. I believe that these bikes were designed just before changes of this sort became popular. Of course, motorcycles are designed more for riding pleasure and not to run 200,000 miles without an overhaul. I have had several BMW cars with over 250,000 miles, still ran perfectly and the engines would consume a quart of oil in about 3,000 miles.

Sorry to get so carried away.

Spot wrote:

I've got a KZ440 LTD with a six speed in the back yard. Do you think its tranny will fit in my KZ650 CSR?

The chances are somewhere between zero and none, the 440 is a twin and the 650 is a four, totally different engine design.

If your 650 does not have a kickstarter, then a 550 six speed should fit.