Replacing Rear Sprocket

[jonathan180iq]

To anyone who has done this,

Are there any surprises that I should know of before I switch out my rear sprocket?
Anything that you came across that isn't mentioned in the service manual?

Thanks,
Jonathan

PS:
I'm getting a 42T rear sprocket to see if it makes up for the deficiencies caused by the 16T front sprocket in the top end. I've also considered a 43T but I think that may be too close to the 15T to worth all the trouble.

[Sarris]

My Suzuki guru says that one tooth on the front is equal to about 4 teeth on the rear, so one rear tooth won't make alot of difference, but may help. You may want to go 2 teeth (if available) to split the diierence.

[jonathan180iq]

I was thinking that too, but the final drive ratios would end up being so close to that of the 15/41 standard combination.

15/41 - 2.73
16/41 - 2.56
16/42 - 2.63
16/43 - 2.69

If a change of .17 made that much a difference in the top end, then moving that back to .10(16/42) should be somewhere in the middle. I think that at a difference of .5(16/43) the variation from the standard 15/41 wouldn't be great enough to justify all of the extra work required with the new rear sprocket install.

[Easy Rider]

I think that at a difference of .5(16/43) the variation from the standard 15/41 wouldn't be great enough to justify all of the extra work required with the new rear sprocket install.

I don't think ANY small change justifies all that work.........unless you are trying to turn it into a mountain bike !! :tdown:

Changing the front makes a noticeable but not drastic change. Anything less is a waste of time, IMHO. ;)

[jonathan180iq]

We'll see.
Worst case scenario, I'm out $54.
Best case scenario, I find the PERFECT gearing combination

[Easy Rider]

We'll see.
Worst case scenario, I'm out $54.
Best case scenario, I find the PERFECT gearing combination

$54 and a LOT of time! :)
You will be adding 2 teeth total, right? One front and one back.
Are you sure the stock chain will still fit ??
If not, it would be a WHOLE different ball game.

[jonathan180iq]

No no. I already have the 16T up front.
I'll be adding two rear teeth over the factory rear sprocket.

I've thought about that. I think there is enough play in the adjustments to account for it. I need an adjustment anyway.

Like I said. We'll just hav to wait and see.

[Orpheus]

I've thought about this before, too(not planning on doing it, but just as a kind of thought experiment). Seems like you'd have the best of both worlds, but I might be mistaken.

If you only changed the rear sprocket to, say, a 43 tooth but left the front as a 15, would that pretty much eliminate the need for first gear (might also make it really "jumpy" on takeoff and give it a little more power uphill)? Also, would the RPMs at the top end run the risk of causing damage to the engine? I really don't know too much about this kind of thing.

[jonathan180iq]

The 15/43, as you suggested, might be over revving the engine. That gear ratio would be somewhere around 2.867, which is almost as much a difference in the high rev direction as the 16T is in the lower rev direction. I think that would rev limit the top end, instead of gear limit it. You're right. ....I'm just kind of rambling.

....

I just finished doing some math and I get the feeling that it might be a "better" overall gearing change to do the 15/40 (2.66) than the common 16T swap (2.56). It would, however, cost more money and wouldn't be as simple to install and you'd only want to do it if you haven't already done the 16T swap.

I think I've decided on the 16/42 (2.625). It's right in the middle.

[Quimrider]

sounds kind of interesting. let us know how the 16/42 works. I've had the back wheel off before to replace a tire. It's really not that big of a deal to take the back wheel off unless you don't have a lift or haven't made wood stands for your bike. You might even be able to get away without unhooking all the rear brake stuff, but it's just a couple xtra nuts to take off anyway. not a big deal. It is convenient how the sprocket literally falls off the wheel once the axle is taken out. I might consider going to the 15/40 if they make a 40t sprocket that fits the GZ250. That 65-70mph brick wall gets annoying on the freeway.

[Easy Rider]

That 65-70mph brick wall gets annoying on the freeway.

If you do a lot of freeway riding above 65, you might have the wrong bike. <Gasp> ;)

[Quimrider]

:??: What! you mean this bike isn't meant to drive 155 continuous miles mostly at wide open throttle? :lol: I actually did that last weekend. Something I wouldn't recommend on a regular basis.

My daily commute involves a 10 mi stint on the freeway. There are no good surface streets to take. Sometimes I take them anyway so I can have a longer ride :) It gets a little white knuckled at times when I can barely keep up with the morons in cages at rush hour. A little xtra headroom would help.

[jonathan180iq]

sounds kind of interesting. let us know how the 16/42 works. I've had the back wheel off before to replace a tire. It's really not that big of a deal to take the back wheel off unless you don't have a lift or haven't made wood stands for your bike. You might even be able to get away without unhooking all the rear brake stuff, but it's just a couple xtra nuts to take off anyway. not a big deal. It is convenient how the sprocket literally falls off the wheel once the axle is taken out. I might consider going to the 15/40 if they make a 40t sprocket that fits the GZ250. That 65-70mph brick wall gets annoying on the freeway.

Sprocket Specialist part # 461-40 for the 40T rear sprocket.

[Quimrider]

Sprocket Specialist part # 461-40 for the 40T rear sprocket.

Thanks for the info. Not worth $53 though. Maybe I'll try it when I wear out the rear sprocket in several thousand miles. Speaking of that, anyone know the ballpark on how long sprockets, chains are good for?

[tazzman]

I'm new to the bike world, but I was thinking, Wouldn't a 16T front and a say 38T rear work better? What would the gear rateing be on that? :??:

[Easy Rider]

I'm new to the bike world, but I was thinking, Wouldn't a 16T front and a say 38T rear work better? What would the gear rateing be on that? :??:

WAY, way too low, numerically.....2.375
You'd have to help push with your feet to get it moving and 5th gear would be useless.

[Quimrider]

38/16=2.375 That would be very tall. The engine probably wouldn't be powerful enough for 5th gear to be usable. The taller you make it the fewer RPM the engine turns to go the same speed in the same gear. It also requires more power from the engine. If you've got an 18 or 21 speed bike think of this as using the biggest front sprocket and the smallest rear sprocket. you will notice it's very hard to pedal especially from a stop.

The reason to go to a 16t front sprocket is that it spreads out the gears by making the final drive ratio slightly taller. By "spreads out the gears" I mean you don't have to shift up thru the gears quite as fast spending more time in each gear. It is many people's opinion (myself included) that the stock gearing is too close. The negative side effect of going to a taller sprocket ratio is that it lowers your top speed. About 5-10mph for the 16t sprocket.

With the factory sprocket ratio of 41/15 which is 2.73 in 5th gear you will usually run out of engine power just before or near redline (8000 RPM) which is 80mph.
The 16t sprocket would yield 2.56. So going with a taller sprocket ratio (ie. less than 2.56) you would run into top speed problems due to lack of engine power and going with a shorter sprocket ratio (ie. greater than 2.73) would put the gears even closer together and your top speed would be limited by over revving the engine.

To calculate the sprocket ratio, you simply divide the rear sprocket by the front sprocket to get the ratio.
41/15= 2.73
41/16= 2.56
42/16= 2.63
43/16= 2.69

Hope this explains things a little more clearly.

[jonathan180iq]

Yep. They're right.

[tazzman]

OK, maby bikes are differant? :oops: I normaly work on cars, and in automotive rear diff. the closer the ratio the faster the top end. Buy the way what is the base HP on this GZ250? :sad: What cuold i do to get more power out of it? Would a 300cc or400cc head (cylinder) fit the 250 crankcase? (ie. a bore and stroke job, shave the head to increase compretion,stuff like that) I like the allready talked about K&N airfilter and muffler swap ideas, but there got to be more that we can do to it, since everyone whants to go faster on a 250? I don't plan on doing much if any of this to my GZ250 (it's fast enough for in town rideing that I do) but since others seam to want to go faster and not just get a bigger bike, I thaught , why not beef it up with more power and taller gears?

[Sarris]

You'll spend more time and effort to build a GZ motor than the bike is worth. There ain't much available to fit the GZ. There are a few mods on this site to bump power, but they aren't going to do a lot. The 16 tooth front sprocket IMHO is the best as the others just make more noise and decrease mpg.

Enjoy the bike for what it is and if you want more, buy a buy a bigger bike. Keep your GZ if you can afford to, as it make a fine addition to any bike stable.

The GZ is an inexpensive, fun to drive, and high gas mileage commuter. It is what it is.

Oh yeah, the motor puts out about 18 hp.

Welcome brother.

:2tup:

[Quimrider]

I agree there's not much worth doing the GZ250. Unfortunately there isn't much between the GZ250 and a 650cc bike. I thought I had read somewhere that it was 20 or 21 hp?

[tazzman]

thats cool, well I was not planing on changing a thing performance wise on this 250 . just wanted to see why every one was wanting to mod a mophead powerd bike? :neener:

I only use my GZ250 in town and top speed limit in town is only 45mph so any mods are not required. :chop:

but thanks for the info. If I whant to ride the open road I'll get a 600 or better. This little bike rides like a dirt bike at high speeds, very sreary.

[Easy Rider]

just wanted to see why every one was wanting to mod a mophead powerd bike? :neener:


That's a good question, really.

I think the main reason is the big HOLE in bike sizes. The last time I traded (down) what I really wanted was an air cooled, light, short, twin (or single) about 350-400 cc. There just aren't any......new or recent.

I have REALLY had to adjust my highway riding style. Came from a 600-4 that would crawl up the trunk of the guy ahead if I dropped a gear and rolled the throttle at 70 mph...........to a bike that won't even DO 70.....well, just barely.

We really NEED something in between.

[Sarris]

If I could only buy a immaculate Honda 350 four cylinder it would make my day. I'm way too a-n-a-l to buy the old POS's that are for sale lately. I would want a prefect one as I always and must buy new.

Stupid, huh?

:blush:

[tazzman]

not stuped at all :tup: , I don't like buying oter peoples problems eather. unless I'm looking for a prodject. lol. thats why I bught a new bike insted of a used one.

[tazzman]

just wanted to see why every one was wanting to mod a mophead powerd bike? :neener:



I think the main reason is the big HOLE in bike sizes. The last time I traded (down) what I really wanted was an air cooled, light, short, twin (or single) about 350-400 cc. There just aren't any......new or recent.


We really NEED something in between.


I know they make a 400cc scooter but not a 400cc bike, stuped ain't it.

[Sarris]

Yeah, and the fag factor for scooters is way too high for me. If you have to step over it and it has a manual transmission, I dont care how small it is, it's a motorcycle.

If you step through it and it's a CVT, it's a scooter and you ain't gonna get me on it.

:)

[Quimrider]

I was looking at some other Suzuki bikes and got to thinking about motor swaps. since many of the engines are very similar. The Suzuki DR350 has 29-30hp and a 6 speed tranny. It even has a 520 chain with 15/41 sprockets like the GZ250. Now if only I could find a free DR350 to attempt the swap.

[jonathan180iq]

It all goes back to the basic argument that, even if you could swap out a bigger engine or something, you'll be putting a lot of time and effort into something that will give very little return.

[Quimrider]

If i could comfortably keep up with 70mph traffic and still keep in the 60mpg range it would be worthwhile to me.

[finallyinthesaddle]

Yeah, and the fag factor for scooters is way too high for me. If you have to step over it and it has a manual transmission, I dont care how small it is, it's a motorcycle.

If you step through it and it's a CVT, it's a scooter and you ain't gonna get me on it.

:)

This is exactly why I have a GZ. Comparable price, yet allows me to keep my dignity and look like I still have a pair. :)

[finallyinthesaddle]

If i could comfortably keep up with 70mph traffic and still keep in the 60mpg range it would be worthwhile to me.

Amen to that. I wish our bike would support a 6th gear. I've only got 630 miles under my riding belt, but I've been driving sticks long enough to know that my bike wants another shift at 55-60.

[Orpheus]

If i could comfortably keep up with 70mph traffic and still keep in the 60mpg range it would be worthwhile to me.

Amen to that. I wish our bike would support a 6th gear. I've only got 630 miles under my riding belt, but I've been driving sticks long enough to know that my bike wants another shift at 55-60.

I usually don't even shift into 5th until around 55mph on straightaways/highway driving :??:
Although I'm usually in 5th at 45mph in city.

[finallyinthesaddle]

I usually don't even shift into 5th until around 55mph on straightaways/highway driving :??:
Although I'm usually in 5th at 45mph in city.

I always have been one of those that likes to keep the RPM's as low as possible. I'm not sure why, but I figure, fewer RPM's = less engine wear + more mpg's.

[Quimrider]

I always have been one of those that likes to keep the RPM's as low as possible. I'm not sure why, but I figure, fewer RPM's = less engine wear + more mpg's.

That might be practical if you're accelerating very slow and don't need to merge with any traffic. Accelerating on the freeway I usually take 4th up to 60 - 65 before shifting into 5th. Some times I take 4th up to 8000 RPM before shifting. I do have the 16t sprocket so that makes a difference.