Smaller sprocket in rear for better top end speed

[cafeguy]

Hi guys, would like to take my 81 xs650 up the coast of California, but its got the stock sprockets in the front and rear i believe, and when i go on the free way at around 70-75mph its revving somewhat high, around 35-4k rpm, would like to be a lot lower if im going to riding long distance, mikesxs says stock is 34 tooth, any of you guys ever tried a rear sprocket with less teeth? how was it on the free way?

 

[peanut]

have a look in the tech section and do a search on gearing . There are a lot of threads on this subject . There is also an excellent gear chart with which you can enter various gear and wheel combinations to see what effect they have on rpm speed acceleration tractability etc

your options are basically
1. fit an overdrive 5th gear (requires spliting the crankcases)
2. fit an 18T front sprocket
3. fit a smaller rear sprocket ie 30T ,31T,32T
4. fit both 18T front and smaller 30T-32T rear sprocket (might need timing adjustment etc )
5. fit larger diameter rear wheel ( possibly in conjunction with sprockets or overdrive 5th
6. dampen vibrations, tune bike , balance recipricating parts etc
7. fit a rephased engine
can't think of any more at the moment

 

[xjwmx]

somewhat high, around 35-4k rpm,

I use stock 17 front and 34 rear and have ridden all over the U.S. with no problems due to that. Last time was a 1700 mile interstate mad dash. 50,000 mi on the motor. If I'm below 3k I feel like it's lugging. But if you are interested in literally top end speed you might be able to do better. I would not go to a different front sprocket unless I truly had to.

 

[azman857]

I went from a 34T rear to a 32T and lost some accelleration, but not not enough to lose any "fun factor. Rider's weight will make a difference too. I'm no lightweight. I'm installing a 5th gear OD so we'll see how that affects things...

 

[DIRTY DOG]

I run 17-31 and runs and rides great at 70-75 mph,but I'm going to build a big cc rephase

 

[650Skull]

So many factors come into this, not the least is what type of bike you have.

How about some information so the answers can relate to you and your bike.

 

[5twins]

Keep in mind here, you're only dealing with a 50HP bike. Gear it too high and it just plain won't pull it in top gear. A couple or 3 teeth down on the rear is about all you really want to go. I went down only one tooth and quite like the results. Tried two down, didn't like that as much. This is on an 18" rear wheel. Two down on a 16" is about the same as one down on an 18".

 

[pamcopete]

Here is an Excell spreadsheet that will yield RPM vs MPH for any combination of gears, sprockets and wheels.

RPM vs MPH

The yellow cells are pull downs for you to enter your values. The chart uses rolling distance instead of rear wheel circumference or diameter as there is no practical way to calculate RPM vs MPH using measurements of the rear wheel due to variations in tire diameter, tread pattern, tire pressure, time of day and phase of the moon, so the simplest thing to do is to mark the rear wheel and the floor (ground) and then roll the bike forward for one revolution of the rear wheel. The distance that the bike rolls forward for one revolution of the rear wheel is the rolling distance.

Yamaha did not change the rear sprocket when they changed the rear wheel to a 16" from the original 18" resulting in higher RPM's for the Special than the standard. I think they did this to get better acceleration for showroom purposes, but if you change the rear sprocket on a Special from 34 to 33 teeth you will end up with almost the same RPM's vs MPH as the standard. That's what I and many others have done.

 

[peanut]

So many factors come into this, not the least is what type of bike you have.

How about some information so the answers can relate to you and your bike.

normally I'd agree with that statement but in this case the op has stated that he has an 81 model with a standard 34T rear sprocket and he specifically is looking to reduce the revs at 70-75Mph on the freeway on a proposed trip up the Coast of California

A quick reference to Pete's gear chart will give the OP the Rpm's for all speeds 0-100 but at the end of the day a comfortable ride at any speed is very subjective.
Edit
ha ha I spent 10 minutes searching for a link to your gear chart Pete not realising that you posted just before mine lol

 

[650Skull]

in this case the op has stated that he has an 81 model

81 model what........Cafe like the handle says, Bobber, brat, if it is an H then the rear wheel may be an 18", if it is a bobber/chopper then with the less weight it may pull with a 18/31 combo from scratch where a standard bike struggles.

Yea lots of information to consider if an informed answer is wanted

I agree with XJ an 17/34 combo on a well tuned Special will run at 120kph/75mph all day and really 4000 revs is cruising on these. The euro was 17/33 with an 18" rear wheell and i think that would be optimin

 

[Tomterrific]

A 17-31 will get what you want for cruising with traffic on the freeway. The engine should be in perfect tune and carb balanced. Fill the bars with shot or sand and the vibration will melt away. I personally like the stock 17-34 with my '77D standard model but I don't do any more than 50 miles on the freeway to get to a nice road. If I decided to travel 100+ miles on the freeway I'd be more inclined to gear my bike higher so she's more relaxed at speed.

Tom

 

[peanut]

81 model what........Cafe like the handle says, Bobber, brat, if it is an H then the rear wheel may be an 18", if it is a bobber/chopper then with the less weight it may pull with a 18/31 combo from scratch where a standard bike struggles.

Yea lots of information to consider if an informed answer is wanted

I agree with XJ an 17/34 combo on a well tuned Special will run at 120kph/75mph all day and really 4000 revs is cruising on these. The euro was 17/33 with an 18" rear wheell and i think that would be optimin

I don't wish to turn this thread into an argument but why would anyone recommend changing the rear sprocket to 31T and the Front sprocket to 18 at the same time ? that is pointless.
Sometimes I think you just argue for the sake of it

It would make better sense to change the rear sprocket to a 30T and keep the front sprocket Std 17T

You'd get exactly the same rpm reduction and not have the expense and extra work involved in changing the front sprocket as well ! if you changed both sprockets you'd probably have to add the cost of a new chain as well which makes the whole job far more expensive than it needs to be.

It costs $20 or less for a 30T rear sprocket and takes less than 15 minutes to change it. You can be testing the new gear ratio on the freeway within half an hour and its cost you shirt buttons.

The OP has already stated that he is not comfortable with 4000+rpm at 75Mph on the freeway for extended periods so why tell him that you are comfortable with that rpm level ? how is that constructive?

bluddy ell I'm in a stinking mood this morning .....just had my monthly phone bill and its $95!
swmbo's nightly phone call to her Mother is costing me $2.78 a night !

sorry for the rant everyone ...normal service will be resumed later when I've calmed down

 

[peanut]

just a quick beer mat calculation using Pete's chart and extrapolated figures

Assuming standard 16" rear wheel then these figures are approximate but will give you an idea of the order of rpm reduction you could achieve at 70mph and 75mph

Stock 17T x 34T
70mph = 4804rpm
75mph = 5147rpm

17T x 30T
70mph = 4060rpm
75mph = 4351rpm

Changing both sprockets to 18T x 32T as suggested by Skull would give you almost exactly the same reduction

hope that is helpful

 

[nj1639]

The only fly in the ointment is that the sprockets should be of compatible wear, throw the chain into the mix also. One way to achieve this is to replace all for new...........

 

[650Skull]

81 model what........Cafe like the handle says, Bobber, brat, if it is an H then the rear wheel may be an 18", if it is a bobber/chopper then with the less weight it may pull with a 18/31 combo from scratch where a standard bike struggles.

Yea lots of information to consider if an informed answer is wanted

I agree with XJ an 17/34 combo on a well tuned Special will run at 120kph/75mph all day and really 4000 revs is cruising on these. The euro was 17/33 with an 18" rear wheell and i think that would be optimin

Peanut. Highlighted in red was my suggestion if the standard 17/34, (which i have found to be ok) was not what was wanted

 

[5twins]

I think Yamaha nailed it with the 17-33 on the Euro model (with an 18" rear wheel). I feel it's perfectly suited to this motor's power output.

 

[Tomterrific]

I might add, both 17 and 31 are prime numbers. This should be best for chain and sprocket wear because of the random tooth/link match up.

That is all good until some engineer (like mrriggs) notes the 31 tooth sprocket may require two links removed from the chain and then 17 is a mutiple of 102. :^) Thanks for keeping me honest.

Tom

 

[mrriggs]

I might add, both 17 and 31 are prime numbers. This should be best for chain and sprocket wear because of the random tooth/link match up.

Tom

As long as the link count is not a multiple of 17 or 31.

Here is a handy calculator for figuring out chain length and contact percentage.

http://www.gofastforless.com/junk/SprocketChainCalculator.xls

 

[cafeguy]

"It costs $20 or less for a 30T rear sprocket and takes less than 15 minutes to change it. You can be testing the new gear ratio on the freeway within half an hour and its cost you shirt buttons."

--Where can i get a 30t for 20$

 

[peanut]

Peanut. Highlighted in red was my suggestion if the standard 17/34, (which i have found to be ok) was not what was wanted

I am not sure what gear ratio an 18" wheel with standard sprockets would give but its a very expensive way of achieving the same result that a cheap sprocket change would give.
Would be nice to have a spare 18" wheel but I would imagine you'd need a longer chain to get sufficient clearance ?