Rear Drive Ratio

mo650chopper · Posted: May 8, 2005, 7:59 pm Post subject: Rear Drive Ratio

I'm going to be changing my front & rear sprockets, chain as my 1980 still has the original. I'm thinking of going with a 18 front & 32 rear which will change my final drive ratio from 2.00 to 1.78, which should make it more comfortable at highway speeds. Now my qestion what is the "Note: Shift Shaft guard must be removed" when going to the 18 instead of the 17? Is it a big Deal? Will I still be able to do wheelies? You know the important stuff;)

jdschooler · Posted: June 25, 2005, 3:52 pm Post subject: Re: Rear Drive Ratio

Instead of changing the front...I changed the rear to a 31 tooth....think I got it from Mike's XS but possibly JC Whitney....works great but does throw the speedo off a bit.

Later,

Jeff

Jack · Full Member Joined: Mar 15, 2005 Posts: 411

There'll be no wheelies with that set up,most likely it'll be a dog with no bark if ya know what I mean Mr. Green

The 18 tooth sprocket will have clearance problems if I'm not mistaken.Probably best if ya went with a 32 or 31 tooth sprocket to reduce the RPMs but still maybe a little sluggesh on take off............Jack

mo650chopper · Posted: June 26, 2005, 7:37 am Post subject: Re: Rear Drive Ratio

I went with a 18 front & 32 rear on mine. I did loose some acceleration but being that I rarely do any city driving, all 55+, It sure makes it alot more comfy at 75 MPH on the Local NASCAR Circuit Track, AKA Interstate 270 in St. Louis. I have managed a couple wheelies as well.

jdschooler · Posted: June 26, 2005, 7:42 am Post subject: Re: Rear Drive Ratio

I seem to have plenty at take off with the 31 I put on the back...but then again my motor is nice and tight too....I guess I am wondering since the speedo is off on mine....I seem to be running 75 when it says I'm doing 60 - 65 and the RPM's are at 4500....how far can you push the 650 at cruising speed? Is 5 to 6 k rpms ok? Seems a little high to me...

mo650chopper · Posted: June 26, 2005, 8:29 am Post subject: Re: Rear Drive Ratio

jdschooler wrote:

Instead of changing the front...I changed the rear to a 31 tooth....think I got it from Mike's XS but possibly JC Whitney....works great but does throw the speedo off a bit.

Later,

Jeff

Since the Speedo drives off the front wheel changing drive ratios with different sprockets would not effect speedo accuracy. If I remember correctly 0n mine 60 - 65MPH is around 4k RPM. I'm sure the XS650 would cruise at a higher RPM than I'm comfortable with but I wouldn't want to cruise at more than 80 MPH a tank full at a time. I need to pay attention next ride but I believe with my setup 80 is around 5k RPM.

jdschooler · Posted: June 26, 2005, 12:04 pm Post subject: Re: Rear Drive Ratio

mo650,

I just looked out the window and U R right the speedo is on the front....this really makes me curious as to why my speedo is off...I have clocked it with other bikes and with Police Radar...any ideas?

And just to confirm you think 5k isn't too much for an hour or so? I hear the 447 motor in a 650 is damn near bullet proof...

Later
Jeff

mo650chopper · Posted: June 27, 2005, 5:05 am Post subject: Re: Rear Drive Ratio

On my ride yesterday I actually paid attention to my speedo, tach and 60 MPH = 3750 RPM. I didn't have any real good spots to crank her up so if I get my ride into work on Wednesday I'll check my 70, 75, & 80 MPH RPM's. I've never check accuracy but at times it seems I'm going faster than what mine reads, maybe from being parked for 17 - 18 years. One thought on accuracy is if your front tire is shorter or taller than the original would alter your reading accordingly.

bent · New Member Joined: Apr 08, 2005 Posts: 2

I am about to trade off the rear sprocket as well on my 83 Special. In Canada they came with a 36 tooth rear, why I don't know. 17 x 32 seems to be the best for me with a combo of city and hiway. Right now it's way too quick through the gears and am always looking for 6th and 7th gear. I'll change to a thinner modern chain and both new sprockets at the same time.

Bent

grizld1 · Posted: July 26, 2005, 11:31 am Post subject: Re: Rear Drive Ratio

If the chain is in good condition and is kept properly adjusted there will be no problem running an 18-tooth countershaft sprocket with the shift shaft guard in place. A chain with too much slack or tight and loose spots on an 18-t countershaft sprocket will for sure get into the guard. But larger sprockets stress the chain a tad less. A healthy XS650 engine will pull nicely with a bit taller gearing; but 18/32 is a change of almost 12% over stock. That may be OK for cruising, but a sport rider will find it almost impossible to stay on the power band with that set-up, and top speed may actually drop off unless some performance work's been done, in which case staying on the power band will become even more of a challenge. Pulling the guard is no big deal, if you aren't meticulous about chain tension. There's a flat tab at the back of it that bolts onto the case. Just unbolt and slide it off. But before you pick gearing, why not go to an 18-t. countershaft sprocket, see how it feels, and then pick your rear sprocket? If your rear sprocket hasn't been flipped and isn't too badly cut up, you can flip it and get the unworn side of the teeth taking the load. If it's pretty bad, just run it briefly for testing, then swap it out.

mo650chopper · Posted: July 26, 2005, 4:22 pm Post subject: Re: Rear Drive Ratio

It's always best to change chain and sprockets together. New Chains will not work well on old sprockets as it will wear the chain quickly and will have many tight loose spots. This was what I was taught in Motorcycle Mechanics School as well as experiences over the last 30 years.

grizld1 · Posted: July 26, 2005, 7:20 pm Post subject: Re: Rear Drive Ratio

Absolutely right! I only suggested the procedure for testing, if and only if your old wheel sprocket hasn't been flipped yet. Even then, I'd run the old unit only long enough to test the results, then swap it out for a new one once a determination on size was made. You won't harm the new chain; only the load-bearing side of the teeth get significant wear. But teeth worn on both sides get weak, so although a flipped sprocket is OK for awhile, it won't have the longevity of the chain you're running it on. I would for sure not run the old chain on an 18-tooth countershaft sprocket, though; lash from old-chain loose spots can get into that shift shaft guard you asked about. I was speaking from experience about the clearance issue. When I run an 18-t (one of my two wheel combinations performs best with it), I have no trouble, and haven't for over 12 years. But I'm a bit on the fanatical side about keeping the slack just right. By the way, I started riding in '65.

threeballtony · Full Member Joined: Apr 13, 2006 Posts: 20

I could use a little instruction in this area. I am in the process of building/ assembling a xs650 bobber. I am going to be running a set of wheels from a early Harley sportster a 19 inch in the front and 16 in the rear I am going to have a rear sprocket made in order to use these wheels in that the smallest H.D. sprocket is a 48 tooth. I am intending to use this bike for intown use with occasional trips not to exceed 150 miles round trip, I have a Road King for the longer runs. I weigh between 265 and 280 , so I assume that the standard 32 tooth sprocket would be to short, but the 48 tooth is too tall , I have no problem changing to an 18 tooth front if necessary, what would yall recommend? thanks Karl

grizld1 · Posted: April 22, 2006, 11:28 pm Post subject: Re: Rear Drive Ratio

The stock XS650 sprockets are 34t on the wheel, 17t on the countershaft. Larger countershaft sprockets raise the gearing, larger wheel sprockets lower it. I'm sure there there'll be a lot of discussion on this, but at your rider weight I'd recommend stock gearing.

Phred · Posted: April 23, 2006, 2:13 am Post subject: Re: Rear Drive Ratio

Threeballtony wrote: "I am going to be running a set of wheels from a early Harley sportster a 19 inch in the front and 16 in the rear I am going to have a rear sprocket made in order to use these wheels in that the smallest H.D. sprocket is a 48 tooth."

I am also going to run a Harley 16". I have a 51-tooth rear sprocked, but the final drive ratio will be 3.00:1-really, REALLY low! Alternate ideas have been to install a jackshaft with 3:2 reduction to bring final drive to 2.0:1, or to use the center of the harley sprocket as a spacer and bolt or weld a 650xs sprocket to it. The 650 sprocket has an "open" center that (I think) will allow bolts to clear the HD hub, otherwise welding the HD sprocket center to the 650 sprocket may work. I'm still deciding which will give the best result-plenty of +'s and -'s to each method. As I know I'm not the first to want to do this, how has it been done by others (no need to reinvent the wheel, after all!)

mo650chopper · Posted: April 23, 2006, 7:28 am Post subject: Re: Rear Drive Ratio

Phred I await your final design, It will open up the abilty to a run a wheel type with allmost limitless designs / variaty. I just wish there was a way to keep 1st through fourth as stock and only alter 5th gear for highway crusin. I loved my 18 - 32 combo on the highway but it made 1st gear too tall. 17 - 32 is better in 1st but not much fun crusin, for any extended period, much over 65.

Phred · Posted: April 23, 2006, 1:46 pm Post subject: Re: Rear Drive Ratio

Sounds like you want a new 5th gear! There may be something that would interchange, but that is WAY beyond my resources. The only way to do this that I can think of (without major engineering) would be to have a new 5th gear set made by a machine shop, and I imagine this would involve mucho $$$$. I hope to boost the bottom/midrange torque of the engine to make up for the higher gearing, and I'm hoping that the bike will lose enough weight to help as well. What rpm are you running with 18:32 at 70-80 mph?

mo650chopper · Posted: April 23, 2006, 9:23 pm Post subject: Re: Rear Drive Ratio

Don't have a tach anymore but I think my 80 MPH was around 5k RPM.

Phred · Posted: April 24, 2006, 6:08 pm Post subject: Re: Rear Drive Ratio

I wonder what that sprocket shop (linked in a different post-not sure which one) would charge for a 32-tooth harley sprocket, or one that hasn't had hub or bolt holes drilled.....

xsjohn · Posted: July 30, 2006, 11:44 pm Post subject: Re: Rear Drive Ratio 18/29

I used 18/30 ratio for years until it came to me that 18/29 would be perfect which it is. 4th gear is a good passing gear and 5th is 3900 at 70mph. 1st 2nd 3rd still take of like a bandit. I only weigh 145 lbs and I suppose if I weighed 200 or more I would still be riding on the 18/30 gears. Since I don't stuff the burgers I will ride on at 18/29 and love it.

You can get 30s on 650 central but I built my own from 1040 plate so they can be hardened and last forever. Don't know the metal that the 650 central sprokets are made from for hardening.

John Underwood

royfisk · Posted: September 10, 2006, 7:40 am Post subject: Re: Rear Drive Ratio

Has anyone tried like a 17/30 or 31? Seems like to my limited riding experiance on these bikes that dropping the rear size is less dramatic then the countershaft sprocket. The 17/29 would probably be to high. I probably only have about 1000 miles on the xs's (my own bike is not done still) All customer bikes and all stock gesring all pull MOST of the bigger hills in vermont in high gear. But everytime I have ever jumped up a countershaft sprocket on other bikes the change was dramatic. The 18 tooth would seem more appropriate in highway and desert riding then in stop and go or mountain riding. Right now first is ok for jogging gears or pulling out stuck suv's. High gear is also to low, always seeking 6 gear. The bikes do not produce enough power to go crazy on gearing to high though. Unless you have a thing for constantly shifting up and down. personally I think betwen 1;85 and 1;90 gearing would be a really nice compromise.

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