GStwin.com GS500 Message Forum

Having read the notes from our European friends Javier & Marc on going in the opposite direction to the prevailing North American crowd (see Excellent mileage with 17 tooth sprocket. More impressions. (http://www.gstwins.com/forum/viewtopic.php?t=20606)), I decided that I'd like to try a 17-Tooth Front Sprocket as well.

My gripe with the stock 16-Tooth is that I have to upshift into 4th to equal the pace of traffic.  There's rarely a situation where 3rd will do.  Though I'm not against rapid upshifts, I started longing for some change that would give me a tad more distance to my gears.  A taller sprocket seemed the easy answer.

So I placed an order with BikeBandit and within a week took posession of an AFAM 17-Tooth Front Sprocket.  With me distracted by home renovations, I finally got a sunday when paint was drying to make the swap.  Unfortunately Mother Nature didn't allow for me to try it out:

(http://img.photobucket.com/albums/v11/redshift/Sprocket%20Change/RainingCatsandDogs.jpg)

But at least it was dry in the garage.  I set to work removing the Front Sprocket cover as per my previous foray (document in the FAQ section under Sprocket Cover Cleanout & Clutch Adjustment (http://www.gstwins.com/forum/viewtopic.php?t=20468)).  With the front sprocket explosed:

(http://img.photobucket.com/albums/v11/redshift/Sprocket%20Change/16TSprocket.jpg)

I took aim at releasing some slack to the chain.  Made careful measurements of the swing arm and after removing the cotter pin, releasing the axle nut, I moved the tension adjusting nuts out 'til the axle was as far forward as she'd go.

The Circlip that holds the sprocket to the spline was the next problem.  Two nice little holes in the clip, but I didn't have anything to get in there and pull them apart.  I'm sure there's a special tool for it, but after trips to both Lowes and Home Depot came up empty with some special Circlip removal plier, I found the next best thing -- a Cotter-Pin Removal Tool:

(http://img.photobucket.com/albums/v11/redshift/Sprocket%20Change/Cotter-PinRemovalTool.jpg)

Got two.  These worked nicely, giving me leverage to spread the Circlip enough to pull it over the shaft lip and away it went -- literally.  It shot off like a bullet.  I heard it hit the floor with a clink and by the stars, I found it without much trouble -- this was gonna be my lucky day after all.

To my surprise, the 16T slid off the splined shaft with remarkable ease, so the gear-puller was not necessary.  After a quick clean-up, I compared it to the 17T from the package:

(http://img.photobucket.com/albums/v11/redshift/Sprocket%20Change/16Tvs17T.jpg)

Even after 9100 miles on it, the 16T looked to be in great shape.  Comparing the two, the 17T had all the right atttributes -- inboard shoulder, spline size exactly the same, both made of steel with the 17T being lighter largely from it's swiss cheese design.

Put the chain on the 17T, then fit the sprocket on the spline.  The Circlip went on a lot easier than it took to come off.  

(http://img.photobucket.com/albums/v11/redshift/Sprocket%20Change/17TMounted.jpg)

Reversing the process, the chain was tightened by using the swing-arm chain tensioning screws.  Made sure to have a little more than a 1/2" of play in the chain, with the same distance from the swing-arm pivot to the rear axle for each side.

Put the front sprocket cover on and the job was done.  Now for the rain to go away to try this improvement.  That would be for another day.

Okay, that was three weeks ago.  Now for the analysis write-up.  There's two ways to look at this change -- by the numbers or by riding feel.  Since I'm an engineer, I pretty well have to look at both sides.

The Analytical Side
Replacing the 16-Tooth front sprocket has caused a fundamental change in the bike.  As you would expect, the 17-Tooth sprocket has lowered the gear ratios (turns the rear wheel makes versus the transmission half-shaft ).  The stock 16T fron sprocket turns 39/16  or2.437:1 whereas the 17T has reduced the turns of the transmition to 39/17 or 2.294:1, a change of 5.86%.  The effective lengthening each gear has made a big difference in how I choose the gear to ride in at city speeds.

Around me are 30, 35 and 45 mph streets.  With the 16T, I would be in 4th gear to cruise at 35 mph, the engine turning at 3900 rpm.  But with the 17T on, I ride the same speed in 3rd gear, with the engine turning at 4400 rpm.  The bike feels more natural, settled and dare it be said; friskier.

When the 16T was on at that speed, 3rd would have the engine turning at 4900, just entering the buzz zone (4900-6200 rpm for me) and making the mirrors annoy to use.  It felt awkward, and I naturally shifted up a gear.

In general, the sprocket change has reduced my engine speed by 400 rpm at normal riding speeds -- for all the gears.  

The Subjective Side
I believe that I'd more or less mastered the art of the "Smooth Upshift" over the past three seasons that I've have my GS500.  But after the 17-Tooth front sprocket went on, I seemed to have developed an even smoother technique.

There's definitely more grunting out of the engine to get going from a stop.  Clutch/throttle action needs a bit more care, but it's easily mastered.  Once rolling, shifting is the same.  It has become even more important to remember and downshift to 1st at a stop -- where the 16T would let me get away with a start in 2nd, the 17T is not so kind and an embarassing stall is much more likely.  (Forget launching from 3rd.)

I noticed a subtle change in my shifting technique.  I'm probably using a wider engine speed range between shifts than I used to.  I used to upshift from 1st to 4th fairly quickly to maintain traffic pace (at the usual 35 mph).  Now I settle for 3rd, but shift at slightly higher engine speeds than I used to.  Not sure why yet -- maybe it's a combination of sound, speed and performance.

One change I'm glad of is that the 400 rpm drop per gear means that I have clearer rear view.  My mirrors don't blur as I can purr below the 4900 buzz zone.  The relatively flat torque curve means I can motor along at 50 mph at about 4400 rpm, when I would gave been getting vibes with the 16T.  In 6th, at 60 mph, rear view is relatively clear with the engine at a sedate 4600.

Fuel Mileage
This evaluation needs more than the 500 miles I've put on the sprocket so far.  Most has been in town; short trips to work and back, and the data shows it's a wash -- can't tell if it's better or not.  It's even possible that mileage is a bit less as I think I'm often running the engine faster in a lower gear rather than lugging in the next higher one.  I seem to like it more that way.

On what little highway or urban riding I've done, there's no conclusive data yet. It was windy, colder and didn't use enough gas to tell.  But one fact of physics is true -- the engine is turning about 400 fewer rpm's at the same speed with the 17T.

Overall Impressions
I'm not going back.  My style of riding runs on the touring side.  I don't care to wheelie, and I like my tires to last as long as possible.  If I want performance, I driop down a gear and the bike still scoots.  Shifting feels smoother to me, and the engine runs slower for the same pace.  It's like I now have overdrive, going 70 mph at about 5200 rpm -- half way to redline.

Would I consider an 18T?  (Yes, there is one for the GS500.)  Nope.  Not likely.  I suspect the clutch & engine would not take too kindly and I'd be concerned the perky spirit of the GS500 would be sapped away.  Besides, it would come dangerously close to the shift push rod -- not sure it would fit!!

It's too bad the season is almost over here in Michigan, but I'm liking my GS500 more with the 17-Tooth front sprocket.  If you do a reasonable amount of distance riding, I'd recommend adding a tooth.

Roy...

Quote from: RedShiftThe bike feels more natural, settled and dare it be said; friskier.

That probably has more to do with it being amenable to how you like to ride.  Gear ratios are all a tradeoff of putting down power vs. top speed, but if a shorter ratio it lands you in a range you don't like and you downsift to lug territory, it does you no good.  Put another way, it just sounds like a better match for your riding conditions and style.  Schweet.

I think you're right.  :thumb:

I was in the same boat abot two years ago when I first went from the stock 130/70 rear tire to a 130/90 which effectiavly drops 1000 rpms for each gear.   for me it was easier to find tires in taller sizes than a new sprocket.    instead of going to a 18 tooth sprocket you may want to try going to a 80 series rear tire.

if i'm going from a 15t to a 17t sprocket..do i need to change my chain?...
my chain is brand new and it's at the shortest settling on the adjuster...

I'd put about 1000 miles on the chain first to get it streched out and then you should be fine.

Of course it depends on you way of driving.

For those who "race" the bike, a 15 tooth sprocket is better.
No very high speed, but blowing mind accelerations can be reached.

For those who "cruise", the 17 tooth is better: cruising at 5K/6K
is better than cruising at 7K/8K: better bike feeling, few gas
consumption, few oil consumption, better engine live, ...

For those who like doing all, keep the 16 tooth sprocket.

Quote from: Frostif i'm going from a 15t to a 17t sprocket..do i need to change my chain?...
my chain is brand new and it's at the shortest settling on the adjuster...

I found the 17T shortened the chain by slightly more than one mark on the swing arm.

thanks...

Nice write up...  Yeah, I think the difference of 16T to 15T is the same as 16T to 17T...  Just in the other direction.

Slightly different RPMs and top speed...  Hardly different MPG...  Moves the shift pattern around a little.

I like the 15T, but I was happy with the 16T...  I was thinking about a 17T, but after riding the 15T and moving MUCH closer to work, I wouldn't do a 17T.  If I moved back out to BFE (15 or more freeway miles per direction) I would probably get a 17T...  But, I'm nearly all city driving with a few situations where very fast acceleration is a good thing.

Excellent explanation, Roy! :thumb:  :thumb:  :thumb:

Thant's pretty uch my experience as well.  I may add on the mileage side, that I had a slight improvement (1 km/liter or about 2 mpg).  I have filled the tank three times since I changed the sprocket, and I like the way it feels.
For the kind of roads I take to work (curvy mountanous backroads), averaging 70-80 km/h (43-50 mph) it makes it easy riding.
I believe that the best mileage improvement would apply on highway type trips, on moderate speeds (65-75 mph).

Even without the hoped mileage increment, I am not turning back to 16.

(my 2 cents worth)

what was the part number for your sprocket and approximate cost?

I paid 12 euro (about 15 USD) for it. It comes from a third party company in Italy... no part number that can help you...

By the way, have you seen the stock dyno run posted by  dbarile?

http://www.gstwins.com/forum/viewtopic.php?t=21789

it shows that the best engine efficiency is when runs between 4000 and 5000 rpm... with a peak at 4500.
Then, by lowering the rpm's in the engine towards 4500 (instead of 5000 or more), not only it consumes less because of lower engine speed, but also becouse it sis more efficient!
:thumb:

I do not know whether that curve would be very representative, but it is the only one I've seen so far on a stock bike!

Quote from: coll0412what was the part number for your sprocket and approximate cost?

There's a variety of sources. materials, sizes and prices.

The best price I found was over at the Sprocket Specialists, where their GS500E front sprockets were running dirt cheap -- $13.  Something seemed a bit too good, so I didn't buy -- can't explain why as others have been happy with their products.

I have an account with BikeBandit and decided to buy from them.  They have an assortment of suppliers, but my research seemed to endorse the AFAM's for the GS500E.  Twice the price of the Sprocket Specialists at $27, but made of chromoly steel, with lightning holes. Believe the Sprocket Specialists were just carbon steel.  I decided to go higher-tech.

JT makes sprockets for the GS500E as well, for about the price of the Sprocket Specialists ($13.44), but not taller than stock.  Same went for Sunstar, who offered stock or shorter for about $20

Roy...

Quote from: Frostif i'm going from a 15t to a 17t sprocket..do i need to change my chain?...
my chain is brand new and it's at the shortest settling on the adjuster...

I've been considering the same swap.  I used to commute in-town most of the time, now I'm putting about 2,000 miles/month on the highway.  I cruise at 80-85mph, that puts me somewhere over 7,000 rpm.  I found with a small windscreen and fixing my bike so it would run with the petcock in the ON position I improved my mpg from 43-45 to 48-50.  I just wonder how much a 17T would drop the rpms and increase the mpg.

Quote from: xtalman

Quote from: Frostif i'm going from a 15t to a 17t sprocket..do i need to change my chain?...
my chain is brand new and it's at the shortest settling on the adjuster...

...  I cruise at 80-85mph, that puts me somewhere over 7,000 rpm.  ... I just wonder how much a 17T would drop the rpms and increase the mpg.

I created a spreadsheet that recorded RPM versus indicated Speed with the Stock 16T and new 17T sprocket.  For highway speeds, here's my data:

Gear........ Speed...... 16T RPM... 17T RPM
. 5th......... 70............ 6500....... 6100
................ 75............ 6900....... 6500
................ 80............ 7400....... 7000

. 6th........ 70............ 5600........ 5300
............... 75............ 6100........ 5700
............... 80............ 6500........ 6200

Fuel mileage numbers from me will gave to wait.  Most of my travel has been in-town.  But has Javier has suggested, the most it should add is a couple more MPG.

Your problem is wind resistance.  Since resistance increases porportional to the cube of the speed, you're not going to get great mileage whatever sprocket you pick.  Best mileage I've had is a constant 50-60 mph.

The conversion is simple:
multiply the rpms by the teeth in your current sprocket, and divide them by the teeth in the new sprocket.
This result is INDEPENDENT of the gear you're on.

In your case you get rpm = x 15 / 17.
Here is a simple table:
15T.......17T
-----------------
4000......3529
4500......3971
5000......4412
5500......4853
6000......5294
6500......5735
7000......6176
7500......6618
8000......7059

You see, you'll get the same speed at 7060 rpm as you do now revving it up to 8000.

I agree with Javier.  The math holds up to what I experienced in reality.  Here's my table for City/Urban riding:

Gear ....... Speed ..... 16T RPM .. 17T RPM .. Est 17T RPM
3rd ......... 30 ........... 4000 ........ 3800 ....... 3765
............... 35........... 4900 ......... 4400 ....... 4612
............... 40........... 5500 ......... 5100 ....... 5176
............... 45........... 6000 ......... 5600 ....... 5647
............... 50........... 6700 ......... 6500 ....... 6306
............... 55........... 7300 ......... 6900 ....... 6871

4th ......... 35 ........... 3900 ........ 3500 ....... 3671
............... 40 ........... 4300 ........ 4000 ....... 4047
............... 45 ........... 4700 ........ 4600 ....... 4424
............... 50 ........... 5300 ........ 5100 ....... 4988
............... 55 ........... 6000 ........ 5700 ....... 5647
............... 60 ........... 6600 ........ 6100 ....... 6212

5th ......... 40 ........... 3700 ........ 3500 ....... 3482
............... 45 ........... 4100 ........ 3900 ....... 3859
............... 50 ........... 4600 ........ 4400 ....... 4329
............... 55 ........... 5000 ........ 4800 ....... 4706
............... 60 ........... 5600 ........ 5200 ....... 5271
............... 65 ........... 6000 ........ 5600 ....... 5647

As you can see, theoretical and measured engine speeds agree fairly well.  Where there's a >150 RPM, I chalk it up to my measurement error -- remember I'm going that speed, looking at the tach, and watching where I'm going.  ;)

Hope this data is as useful to someone as it was to me to get.  :thumb:

OK, you talked me into it.  Eastside tossed a Vortex 17T in with the bike.  After a beeyotch of a time getting the axle nut to move, everything else was straightforward.  I didn't disconnect the clutch cable; just let it dangle to the side.

(http://www.roadstergal.info/10_30_05/1.jpg)

Although I know it's present, I haven't felt much of a difference in the off-line performance.  My speed is now limited by breakin to something like 66 instead of 60, which is the difference between annoying and terminally frustrating.  The gears do seem to be better set up for city toolin'.

Quote from: RoadstergalOK, you talked me into it.  Eastside tossed a Vortex 17T in with the bike.  After a beeyotch of a time getting the axle nut to move, everything else was straightforward.  I didn't disconnect the clutch cable; just let it dangle to the side.

Welcome to the 17T Club.  :thumb: The axle nut is always a bugger to get loose -- leverage is your friend.

How did you get the Circlip off?  I had to improvise with a pair of industrial-sized dental implements.  My tool bag was missing anything special -- curious if there's something that I'm missing that works better than what I've got.

Quote from: RoadstergalAlthough I know it's present, I haven't felt much of a difference in the off-line performance.  My speed is now limited by breakin to something like 66 instead of 60, which is the difference between annoying and terminally frustrating.  The gears do seem to be better set up for city toolin'.

After 600 miles, I'm in full agreement with your assessment.  You get about 5-6 miles per hour for the same RPM, and that's the difference I needed to balance out riding in town with the big boys.  

The spirit of the GS500 is still there -- just spread out a bit more than with the 16T.  That, and I'm getting about a 3 MPG boost over the 16T.  

I have a circlip tool... and I couldn't find it.   :x I spread the two 'arms' apart with a small screwdriver (which also moved the circlip arm out a hair), put a small Allen in one hole (which now held it snugly since it went all of the way through), and pushed on the other with the screwdriver.  *ping!*