Highway gearing

79 KZ 650 B3

Stock gearing was 16 front / 42 rear = 2.63:1

Mine had 16 front / 41 rear = 2.56:1

My goal for my bike is economy, not power, so I wanted to swap the gearing. The chain broke, so it gave me a perfect excuse.

My daily commute is 16 miles, almost all highway, so I decided to go fairly radical.

I went to 18 front / 40 rear = 2.22:1

I've been able to ride it for two days, so far. Not sure yet how I feel about it.

The rpms are down significantly; close to 1000 rpm at any given speed.

Before, at 50 or above, I always felt myself wanting to shift into a higher gear. Now, it cruises comfortably, but, I find myself having to downshift a lot. While the bike has never had a lot of torque, Once I got into 5th gear, that's where it stayed as long as I was on the highway, even in hilly sections. Now, hills cause dramatic drops in speed, and I have to downshift on medium ones, or my speed falls below 45mph.

Same with low speed, around-town cruising. Before, on a flat street, I had no problem loafing along at 35mph in 5th. Now, there's now way.

It's also a lot harder to get going. I have to use a lot more clutch, and it frequently bogs. That will probably resolve somewhat over time, though, as I get more experience with the new setup.

I don't have any mileage numbers yet, but will post as soon as I get them.

I'm thinking I may have gone a bit too far. I'll drive it for awhile to be sure before I make any decisions.

Some other combinations I'm considering:

Easiest: front 17 / rear 40 = 2.35:1

Front 18 / rear 41 = 2.28:1

Front 18 / rear 42 = 2.33:1 (essentiall the same as 17 front / rear 40 combination, and changing the front sprocket would be simpler and cheaper).

I'm also really interested in the KZ550 6 speed conversion I've seen discussed. It took forever, but we finally got the carbs, etc. straightened out and it runs great, but the #3 cyliner is down a bit on compression and it's blowing a bit of blue smoke, so a rebuild is definitely in it's future. My plan is to keep my eyes open for a cheap complete engine to have rebuilt so I can just swap them out.

I did a search, but can't find the threads on the 6 speed swap, except one that says it will work in a non-kickstart model, so that's the core I'd definitely be looking for.

Anybody have any thoughts?

In most vehicles where I have paid attention to fuel mileage I've noticed that throttle opening has a greater effect than does rpm. Trying to pull too much gear may net you worse mileage , even though you are using less rpm .

hannah1 wrote:

In most vehicles where I have paid attention to fuel mileage I've noticed that throttle opening has a greater effect than does rpm. Trying to pull too much gear may net you worse mileage , even though you are using less rpm .

In part, I don't disagree. I've noticed when it lugs down, my first impulse is to pour on more throttle, rather than downshift, which would clearly use more gas.

On the other hand, it takes less throttle to cruse at 4000 rpm than it does to cruise at 5000 rpm.

The trick is to find gearing that allows you to lower that cruising rpm, while not forcing you to use more throttle due to lost torque multiplication.

Yep, you definately went extreme on changing the gearing IMHO. :pinch:
How the heck did you stuff an 18T front sprocket on there without it rubbing the "U" shaped chain guard on the inner trans cover? :huh:

IIRC, going one tooth larger on the front sprocket is equal to going two teeth smaller on the rear.
So basically you've added six teeth to the front sprocket (or subtracted six teeth from the rear), which is going to make your bike perform like a slug at lower RPM's as you've already noticed.

FWIW, I went one tooth larger (16T to a 17T) on the front sprocket for my '78 KZ650 and it lowered my freeway cruising RPM's by almost 500, yet my bike still accelerates plenty fast enough to avoid the all to common errant cage drivers.:S

Post edited by: RetroRiceRocketRider, at: 2008/03/02 19:29

I run 16/38 and love it.

My $.02 on the 6 speed transmission is that it shuffles the ratios around while going through the gears but you wind up in about the same place in top gear as the 5 speed(not much difference).

YMMV.

The 550 six speed trans has a bit lower 1st gear and a bit higher top gear, but the rest of the ratios are spaced to make it a close ratio gearbox.

But you need a 81 or later 650 bottom end (or a 750) to install it, it won't fit in a 650 with a kickstarter (Mark Wing tried it).

I'm really interested in how you squeezed that 18 tooth sprocket in there also.

Did you remove the metal guard in front of the sprocket?

Post edited by: steell, at: 2008/03/03 09:32

RetroRiceRocketRider wrote:

Yep, you definately went extreme on changing the gearing IMHO. :pinch:
How the heck did you stuff an 18T front sprocket on there without it rubbing the "U" shaped chain guard on the inner trans cover? :huh:

IIRC, going one tooth larger on the front sprocket is equal to going two teeth smaller on the rear.
So basically you've added six teeth to the front sprocket (or subtracted six teeth from the rear), which is going to make your bike perform like a slug at lower RPM's as you've already noticed.

FWIW, I went one tooth larger (16T to a 17T) on the front sprocket for my '78 KZ650 and it lowered my freeway cruising RPM's by almost 500, yet my bike still accelerates plenty fast enough to avoid the all to common errant cage drivers.:S<br><br>Post edited by: RetroRiceRocketRider, at: 2008/03/02 19:29

If you think I went extreme, what about this guy?

kzrider.com/component/option,com_joomlab...w/id,174468/catid,5/

He has a 630 chain, so the tooth numbers are different, but the ratios are the same.

He started with 13 front / 33 rear = 2.54

Compare that to my stock 530's 16 front / 42 rear = 2.62.

He went with 13 front / 26 rear = 2.00:1!

He loves it, but, I don't get it. I'm thinking my 2.22:1 is probably too much, I can't see how you'd ever get it moving with 2.00:1

As far as getting the 18 tooth front sprocket to fit, my buddy jccwall did it, I'll get him to post how he did it.

are any body knows;when u rebuilt an engine, what is the measure openining mouth of piston rings?:blush:

How the heck did you stuff an 18T front sprocket on there without it rubbing the "U" shaped chain guard on the inner trans cover?


removed the guard.

jim

steell wrote:

The 550 six speed trans has a bit lower 1st gear and a bit higher top gear, but the rest of the ratios are spaced to make it a close ratio gearbox.

But you need a 81 or later 650 bottom end (or a 750) to install it, it won't fit in a 650 with a kickstarter (Mark Wing tried it).

I'm only going to swap to the 6 speed when I do a complete engine rebuild. I've found one for 50 bucks; I'll pick it up and put it on the shelf till I do the rebuild. I don't think it would make sense to do it otherwise, but, since the cases are already cracked to do the bottom end, I'm thinking it's not that much more work to swap the gearset out.

My plan has already been to pick up a core to rebuild so I can do it with minimal downtime, so I'll just keep my eyes open for an 81+ non-kickstart engine.

As far as the gear ratios, here is what is posted in the specifications section:

KZ550
1st: 2.57:1
2nd: 1.78:1
3rd: 1.38:1
4th: 1.13:1
5th: 0.96:1
6th: 0.85:1

KZ650
1st: 2.33:1
2nd: 1.63:1
3rd: 1.27:1
4th: 1.04:1
5th: 0.89:1

While the KZ550's 6th gear ratio of 0.85:1 and the KZ650's 5th gear ratio of 0.89:1 are essentially the same, I believe you would notice a difference between the KZ650's 1st gear ratio of 2.33:1 and the KZ550's 1st gear ratio of 2.57:1. Even if you didn't, though, I believe the closer gear spacing would be a big help when running gearing like I am.

To do the 550 trans swap you need the following:

Both shafts with the gears
Shift drum
3 shift forks
The star shaped cam on the right side
(on the right end of shift drum)
The trans cover (Neutral light switch is in the wrong position on GPz750, probably same as others).
And you may have to shim the end of one trans shaft (output, IIRC).

It's been a couple of years since I did it, but I think that's all.

Well, I switched the 18t front sprocket for a 17t. It's made a big difference. I've put about 200 miles on it, and it's much easier to get going, cruises fine on a flat street in town in 5th, and seems to do well on the highway, too.

I have the gauges off of it to send off and have rebuilt, so I don't have specifics on rpms/speeds, but this seems to be a much better gearing solution.