sprocket math help

[Boriqua]

My chain is fairly well shot. Side to side play is huge and I feel some knocking coming through my left peg under certain conditions. I cant get a good adjustment on it because its done. I took a look and the chain has a master link and says EK630 and appears to be a straight up non oring chain. It is about the last thing I need to do to have the bike finished ... for now.

I have been reading up and it would seem the thing to do is go 530 conversion if for no other reason than availability. Cool. I read that the 630 chain is running though a 15t front sprocket and a 41t rear and if I go to a 530 I will want a 16t front and 42t rear to keep the same ratio. Right?

So here is my question. There is no where in my neck of the woods that doesnt require a healthy stint on the highway at 75mph.. On my bonneville I changed my front sprocket so it could do highway speeds more comfortably and it worked out great without sacrificing the fun factor.

While the 750 handles the highway light years better than my triumph I think I would still like to gear it down a touch. At highway I am currently running about 5100ish rpm. I don't want to massively gear it down but what sprocket combo could someone recommend to shave a few RPM's off .... or ... should I just go straight conversion?

While I do enjoy the occasional throttle blip most of my riding solo or with the wife is pretty sedate. I dont need it to loop like a Harley but getting a hair under 5000 at highway speed would be nice but I am certainly open to "leave it the hell alone" advice too.

 

[Brendan W]

When the front sprocket pulls 15 links through the back will turn 15 teeth. This is 15/41 of a wheel rotation.
15/41 does not equal 16/42. The difference is about 4% up so that 5100 would drop 200 rpm.

 

[Boriqua]

I was under the impression that going from the 630 chain to the smaller links of the 530 necessitated the increase in diameter on the sprockets. If a 16/42 530 will drop me 200 rpm then I am golden! Currently I am running stock sprockets with a 630 chain.

 

[Nessism]

15/41 = 2.73

For a 530 conversion you want to move to an 18T front sprocket so that the stock diameter is maintained.

18 x 2.73 = 49T If you want taller gearing (less rpm) then you need to move to less teeth in the back.

 

[bobgroger]

You need more teeth on the front sprocket to keep the same physical diameter so the chain doesn't eat the swingarm. 1 tooth front =~3 teeth rear so if you go to an 18 front you need a 46 or 47 rear to keep the same gearing. 45 would probably get you where you want to be.

 

[Nessism]

You need more teeth on the front sprocket to keep the same physical diameter so the chain doesn't eat the swingarm. 1 tooth front =~3 teeth rear so if you go to an 18 front you need a 46 or 47 rear to keep the same gearing. 45 would probably get you where you want to be.

46 or 47? Geeze man. I posted the math and you just ignored it and guessed.

 

[Buffalo Bill]

46 or 47? Geeze man. I posted the math and you just ignored it and guessed.

Kinda like talking to most people in the world?

 

[Nessism]

Kinda like talking to most people in the world?

I register a dig here but can't decipher your diction. Vortex derived insults should stay in the Vortex.

 

[Boriqua]

Awesome! So how do I determine approx how many RPM I scrub per teeth I lose. 18t front sprocket 530 chain and 49t giving me back a factory ride what can I drop to in the back to scrub 200 rpm. I am going to go out now in the Freezing cold and count teeth to make sure its set up factory. Thank you!
Alex

 

[Boriqua]

Well just back in from checking physically and I have a 40T rear and a 16T front running a EK630 chain. I kept reading that stock was 15x42 but that aint whats on my bike.

So check if I have the math right. I am currently running the 16 front and 40 rear which brings me to 2.5.

If I go 530 and I go 18t front and 45t rear I am exactly where I am right now.

The 16t x 42t 530 conversion brings me to 2.625 which is gearing down from what I am running now and actually going to give me higher rpm at the same MPH. Basically one of the previous owners has already made the gearing taller when the went to 2.5 ... Right?

Here are pix of my rear sprocket

I am thinking the -40 on the long number was the tooth count but the other number on there is 0.45 and I have no idea what that is.

 

[Buffalo Bill]

I register a dig here but can't decipher your diction. Vortex derived insults should stay in the Vortex.

Not an dig at you Ed, it's about how most people just don't understand clearly stated advice.

 

[Sandy]

Well just back in from checking physically and I have a 40T rear and a 16T front running a EK630 chain. I kept reading that stock was 15x42 but that aint whats on my bike.

So check if I have the math right. I am currently running the 16 front and 40 rear which brings me to 2.5.

I am assuming the 2.5 is tire rotation?

If I go 530 and I go 18t front and 45t rear I am exactly where I am right now.

In order to get taller gearing I need the 2.XX to go up so so the 16t x 42t 530 conversion brings me to 2.625 which is taller than what I am running now. How do I figure out how much rpm that might scrub?

Here are pix of my rear sprocket

I am thinking the -40 on the long number was the tooth count but the other number on there is 0.45 and I have no idea what that is.

Your thinking on this is backwards. The 2.5 or the 2.62 is actually the front sprocket rotation in relation to the rear sprocket and thus will actually increase the rpm for a given speed. 2.5 rotations of the front sprocket for 1 rotation of the rear wheel. Given a speed at 4,000rpm the difference would be as follows.

(2.62/2.5)*4,000 = 4,192.

I think what you're looking for is something like 42/18= 2.333 would give you (2.333/2.5)*4,000 = 3,728. The current gearing is already quite a bit taller than stock, just plug in the the stock numbers to see.

 

[Boriqua]

Yea ... Now I am afraid to go any taller than what I have. I had wondered why the bike felt a bit less peppy than I remembered. I equated it with being 40 lbs heavier and having a more vivid imagination as a teenager but its the taller gearing. I certainly remember the bike being torquier. I also have to do a fair feather off the clutch at lights.

Thank you everyone for the math lessons .. outstanding! I really did want to learn how to figure it all out and make a conscious decision.
Alex

 

[Nessism]

Not an dig at you Ed, it's about how most people just don't understand clearly stated advice.

Sorry Bill. I misinterpreted your comment. It's all good!

BTW, just looked in the factory manual and the original sprockets were 42/15 = 2.80 ratio.

18 x 2.80 = 50T rear sprocket.

I wouldn't go smaller than an 18T in front because the diameter will go down compared to your original sprocket and you don't want that.

How much smaller than a 50T in rear is up to you and the gearing you desire.

Good luck.

 

[Boriqua]

Why is the diameter of the front sprocket so important. Most of the chain/Sprocket kits I see in 530 are coming with a 16t front sprocket. I found an 18t on Z1 but am curious as to the why just for my personal knowledge base.

I certainly like the price on this and its quality components
https://www.ebay.com/itm/SunStar-53...ffffac8a|iid:1&_trksid=p2481888.c100675.m4236

I would just need to pick up new lock washers and a rivet tool.

 

[Sandy]

Why is the diameter of the front sprocket so important. Most of the chain/Sprocket kits I see in 530 are coming with a 16t front sprocket. I found an 18t on Z1 but am curious as to the why just for my personal knowledge base.

I certainly like the price on this and its quality components
https://www.ebay.com/itm/SunStar-53...ffffac8a|iid:1&_trksid=p2481888.c100675.m4236

I would just need to pick up new lock washers and a rivet tool.

The only.reason for the diameter being correct is so the top run of.chain doesn't contact the swing arm. If there's lots of clearance it doesn't really matter but better to err on the plus side.

 

[hannibal]

That's an aftermarket JT spocket: http://www.jtsprockets.com/catalogue/sprocket/JTR818. I guess that's pretty clear though...

 

[Brendan W]

Divide the back number by the front and get a number like three or thereabouts.
This tells you that the drive sprocket must complete three revolutions for every wheel revolution.
Wheel revolutions is road speed so the higher your particular back/front number the more revolutions the engine has to do to maintain the same road speed.
Let's say do the math for two possible combinations, a 3 and a 2.8.
In the 2.8 case the engine has only to make 2800 rpm to get the same road speed as 3000rpm using the 3.0 ratio.
If you want to talk fractional or percentage change from one ratio to a new one we can talk about the change from 3.0 to 2.8 and do something like (2.8/3.0)x 100 = 93% approx.
Then you can say that the new 2.8 set up will run 93% of the old tacho speed.
If the proposed new ratio is higher than the old, something like a 3.2 the fractional change is (3.2/3.0)x100 = 107% approx.
In this scenario the motor will have to run 7% faster to maintain the same road speed.
Note in both scenarios the new number goes on top of the old. Neither is meant to be a real situation, it's just to demonstrate the principle.
In general sprocket size is not a free choice, there are things like space and wear rates to consider so unless you have a very good reason, stick close to stock or a known working alternative solution.

 

[GRAND ROUGE]

Just a thought too re 530 conversion. Did so on my GS 1100. 630 originally a 15 /42 set up and 96 links. I had at some point installed a 40T rear sprocket.....suited my highway cruising needs.... I notice the 750 is a 15 /41 (Owners manual Bass Cliff site) but the important number too is 96 links.

I went with a 18 /48 set up...to closely maintain ratio I had with the 15 /40 .. and the number of links is 114. Boriqua, you mentioned earlier a conversion kit and chain is 110 links....Might be too short?

As others have noted......for sure the 18 tooth front sprocket....and choices are yours re the rear....

 

[Boriqua]

Thank you for the info!

I decided to go with a 18/47.
Give me a touch more pep around the neighborhood than what I am running now but sacrifice around 200 rpm to increase at highway.
Although I am still a touch on the fence about going to 48 for the rear.

I decided I am not going to buy a kit because they seem to be almost double the cost of me sourcing the individual components so my shopping list has a 120 link 530 and I figure I will cut it to length.