Regearing

[Mickey85]

OK, I've done about as much research as I can on this forum, so time to drop the question...

I have an 81 XS400 that currently is running 16/37 gears with a 16" rear wheel. In 6th gear, it runs about 1:100 (I.e. 60= a hair over 6000 RPM). As I do quite a bit of 55+ mph riding, I'd like to calm that down a bit. xschris said on one of the threads I found that he's running a 16" rear with 17/37, and at 60 was at about 4800 RPM...

So, can I really lose like 1400 RPM by gaining a tooth up front?

I was thinking of going 17X36...would that be too much? I don't really hot rod it around town, and would prefer to keep the RPM fairly low on the highway (i.e.60-70 MPH). I realize it's a 400, but this is going to be my main bike this summer, so I'm gonna need it to keep up...

Mechanically, it's stock, with shorty glasspack mufflers and rejetting.

 

[Durney]

OK, I've done about as much research as I can on this forum, so time to drop the question...

I have an 81 XS400 that currently is running 16/37 gears with a 16" rear wheel. In 6th gear, it runs about 1:100 (I.e. 60= a hair over 6000 RPM). As I do quite a bit of 55+ mph riding, I'd like to calm that down a bit. xschris said on one of the threads I found that he's running a 16" rear with 17/37, and at 60 was at about 4800 RPM...

So, can I really lose like 1400 RPM by gaining a tooth up front?

I was thinking of going 17X18...would that be too much? I don't really hot rod it around town, and would prefer to keep the RPM fairly low on the highway (i.e.60-70 MPH). I realize it's a 400, but this is going to be my main bike this summer, so I'm gonna need it to keep up...

Mechanically, it's stock, with shorty glasspack mufflers and rejetting.

I have a 17 tooth sprocket on my stock '81 xs400, at 5k rpm I am at 56 MPH in 6th. In 5th gear 5k = 50 MPH

 

[xschris]

My 81 and 82 has 17/37 sprockets and a 16" rim with a 120/90-16 tire. Both tachs say 4800 at 60mph. For every tooth you go up on the front the rpm's drop 500. On the rear for every tooth you drop you loose 250 rpm's. On my other two xs400s' that run 16-37 with 18"rim and 110/90-18 tire the rpm's are the same. Maybe a slipping clutch is happening. Also count the teeth on your sprockets to confirm that is what you have.

 

[Mickey85]

I did count the teeth, they're 16/37. It's not a slipping clutch - it'll spin the tire (lazily) when it's on the center stand and you rev it in neutral. I understand that 6000 RPM @ 60 mph is fairly the norm on these, I chalked up the discrepancy to a smaller rear tire, as I'm running a Shinko 244 3.00X16 in the back, which is a bit shorter than stock (these are 100/80-16, stock is 120/90). Hence the 100-150rpm raise in engine speeds. I'm looking to get stock sized tires back on it in the next month or so (street tires) and gear it to 17X36, so theoretically, it would be running 5250 at 60.

 

[MrStubb]

Sounds like you've got the right idea. Your tire may be giving you a few extra r's at speed. I have a 17t front with the 36t rear and have brought my 55 mph r's down about 500. Also don't notice much change off the line. But I didn't ride it much because the sprockets were in such bad shape. I'd suspect that you will find the need to down shift a bit more for passing to get it up into the power band.

 

[xschris]

With your 16-37 sprockets and small rear wheel you will get those high of rpm's.

 

[Mickey85]

Sounds like you've got the right idea. Your tire may be giving you a few extra r's at speed. I have a 17t front with the 36t rear and have brought my 55 mph r's down about 500. Also don't notice much change off the line. But I didn't ride it much because the sprockets were in such bad shape. I'd suspect that you will find the need to down shift a bit more for passing to get it up into the power band.

That's alright, I find after about 35 mph I'm in 6th gear anyway...It'd probably be healthier to stir it around a bit more than that...

 

[xschris]

A 3.00 tire would be 90/90 in metric. Same as the front tire but in a 16"

 

[xschris]

Sounds like you've got the right idea. Your tire may be giving you a few extra r's at speed. I have a 17t front with the 36t rear and have brought my 55 mph r's down about 500. Also don't notice much change off the line. But I didn't ride it much because the sprockets were in such bad shape. I'd suspect that you will find the need to down shift a bit more for passing to get it up into the power band.

Where did you get a 36 rear sprocket? Most I have seen for the sohc xs400 is 39-37.

 

[MrStubb]

BUSTED!!!! I mistakenly said 36t I actually don't remember what I ordered but it was the stock tooth count.

 

[Mickey85]

Are there any places where I could nab a 36 tooth sprocket?

 

[xschris]

I would start with the 17-37 sprockets and a stock 120/90-16 tire first.

 

[Mickey85]

Chris, I'm struggling with numbers here...

With a stock tire size, with the stock 16/37 sprockets, 60mph should be about 6000 RPM, from what I read on here, and from a bit of fudging with my real-world observation...If each front tooth is worth 500 RPM, how are you getting a 1200 RPM decrease from one tooth difference from stock?

 

[16VGTIDave]

Who says your tach or speedo is accurate? I've checked my speedo against my phone and the error with the speedo is wonderfully random. No idea what the tach error is above 2000 RPM, but it is reasonably accurate below 2000. Reasonably... [emoji6]

 

[xschris]

All I know what I get from the setups I have. When I had the stock gearing the rpm's where 5300 at 60mph. I am not sure how much running a 3 size smaller rear tire adds to it but it will affect it. Is your front tire stock size? If it is not that will affect the speed. With four bikes running the same rpm to speed I would have to think they are close to accurate. I have used other stock speedos and tachs with the same readings.

 

[Mickey85]

Alright, finally got the old boy back together. So now we're sporting stock tire sizes (100/90-18 front, 120/90-16 rear), with a 17 tooth sprocket. Didn't have much opportunity to get some speed out of it, but at 60, we're appearing to be at about 5200 RPM. I like the way it rides around town, so we'll see if I get too annoyed with it.

I originally went with the dirt bike tires because I had plans to go on dirt roads/fire trails more than I actually did (my parents live on a dirt road, etc). So now I've switched to Kenda Challengers. It's a whole different bike. Taller, more confident, less wandery. The engine isn't so high strung and obnoxious, and I don't scrape exhaust on every turn. Also I still get freaked when using the side stand because it leans over so much relatively.

 

[xschris]

Actually 90/90-18 is stock for the front. So your speedo will be off a bit.