Topic: Is it just me or is this sprocket/chain crap fricken rocket science !!!!!!

[rustrocket92]

Heres the deal I have several bikes that I am using to build my project ( SEE OPERATION BIRTH KONTROL ) The frame is painted , engine is ready to go back in, wiring harness is good to go, tank and tail are painted. So we are almost ready to complete the assembly. Now here is where the trouble starts. I am using the chrome swing arm and 16" wheel and tire from my 1973 cb750k it has a 45 tooth sprocket. But I am using the frame and motor from my 1976 cb750k which had a 48 tooth sprocket and 18 on front.

So should i just remove the 14 tooth sprocket from the 73 engine and install it on the 1976 motor ?

Or should i buy a different sprocket for the front to run with the 16" wheel/tire and the 45 tooth sprocket ?

[MCRider]

Heres the deal I have several bikes that I am using to build my project ( SEE OPERATION BIRTH KONTROL ) The frame is painted , engine is ready to go back in, wiring harness is good to go, tank and tail are painted. So we are almost ready to complete the assembly. Now here is where the trouble starts. I am using the chrome swing arm and 16" wheel and tire from my 1973 cb750k it has a 45 tooth sprocket. But I am using the frame and motor from my 1976 cb750k which had a 48 tooth sprocket and 18 on front.

So should i just remove the 14 tooth sprocket from the 73 engine and install it on the 1976 motor ?

Or should i buy a different sprocket for the front to run with the 16" wheel/tire and the 45 tooth sprocket ?

Do you really have a 14t front sprocket? That's mighty small for a 530 chain.

The later engines with the bigger 630 chains had 14t sprockets i think.

Generally, you'd run a 16 with the 45. A 17 is OK, but taller. A 16/45 is roughly equiv to a 17/48. A 18/48 is standard on the mid year bikes.

Unless you have a low profile tire, the 16" harley style tire is only slightly shjorter than a stock tire. Not affecting the gearing much.

[rustrocket92]

So do think i should just get a 16 or 17 tooth front sprocket and call it a day ?

[MCRider]

So do think I should just get a 16 or 17 tooth front sprocket and call it a day ?

Using what you have, 18/48. 

Buying something, I've always liked the 17/48. With your wheel, that would be like a 16.5/48. Good for around town get up, still good for highway.

Throw that 14 away.   

That's just me...

[HedNut]

If everything lines up...i'd run the 73' 14t front with the 76' 48t rear....make her Stand up!! ahah I like torque, and light to light acceleration myself....
  
   Of course at the expense of higher rpm's at highway speeds...and faster chain wear (although they make pretty fantastic chains these days)

But theres always a spin off...It's pretty much your personal prefferance as long as you understand the gains/losses from going each way...
More top speed (at lower engine rpm and less torque to the ground)= drop teeth on the rear, or add teeth to the front
More Power/Torque (higher RPM, lower top speed)= add rear teeth, drop front teeth.

Here's a link to a nifty motorcycle gearing calculator I found....if you have all your specs it'll spit out the goods:

http://www.bikerenews.com/Stories_Archives/GearRpmSpeedCalculator.htm

[MCRider]

All true enough...

Strictly speaking the power and torque are constants of the engine,  they do not change.

The gearing changes the ground speed at which they occur.

Picky picky... 

[tlbranth]

Small sprockets are very hard on chains. I'd chuck the 14t. I prefer high gearing so I'd go with the largest front sprocket that would work.

[HedNut]

Ahaha....constant to the engine, agreed.....but not to the rear wheel, which definately changes torque output according to gearing.      Power...more of a constant...you got me there.

[MCRider]

Ahaha....constant to the engine, agreed.....but not to the rear wheel, which definately changes torque output according to gearing.      Power...more of a constant...you got me there.

I'm not saying your wrong on this, as I truly don't know for sure. But my sense of the issue is this:

You have a bike on the Dyno and it puts out 50lbft of torque at 5200rpm peak. Now, you change the sprockets. Does the engine now put out a different amount of torque?

I can't see that. What I see is that the same torque peak simply occurs at a different ground speed. And the engine will travel through its powerband quicker between shifts, assuming the gearing was lowered. It will affect all sorts of things in the feel of the bike and how one rides it, but the torque output hasn't changed.  To do that, you would have to do something internal, like change the cam or displacement, etc.

So i would need a wickipedia style reference or have an engineer chime in to educate me otherwise.

All in the pursuit of knowledge.

[rustrocket92]

So if i run the 16 inch rear wheel and tire with the 45 tooth rear sprocket i should get a 16 or 17 tooth front sprocket. Am i understanding this correctly ?

[MCRider]

So if i run the 16 inch rear wheel and tire with the 45 tooth rear sprocket i should get a 16 or 17 tooth front sprocket. Am i understanding this correctly ?

Well we haven't actually discussed what you want from it, from a riding point of view. I'm assuming you're like me and that doesn't appeal to some people. 

The 16 with a fat tire is about the same as an 18 with a standard tire, so it doesn't factor in much.

I'd use the 48 (with a 17) over the 45 as the chain will wear less onthe bigger sprocket.

But if for some reason you want the 45, then you'll need the 16 for a pleasant all round ride, per my standards.

Put some math behind it: 48/17=2.82   45/16=2.81  Virtually the same ratio.

[rustrocket92]

Maybe i will just get both the 17 tooth and the 16 tooth and a chain. Then try them both out.

[MCRider]

Maybe i will just get both the 17 tooth and the 16 tooth and a chain. Then try them both out.

That'll work!  Buy your chain with enough links for the longest combo the 17/48. i can't remember if its 100 or 102 links. Or something else.

[HedNut]

Just some math info:

Example:

CB750 K8 Gear ratios:

Primary:1.986
1st Gear: 2.5
Final Drive: 2.714 (assuming 14t front/38t back sprocket)
Max Engine Torque: 48 ft.lbs @ 7000RPM...which is 576 in.lbs
assume a rear tire with an overall diameter of 25 inches

...therefore assuming no efficiency losses (there are always some)
Rear wheel torque @ the sprocket is:
(Max Engine Torque X Primary Ratio X 1st Gear Ratio X Final Drive Ratio)/Rear Tire Radius= Rear wheel torque @ pavement (in.lbs)

So......the maximum output of this bike in first gear..to the Pavement (assuming NO losses):

With a 38t rear sprocket: 620lbs of force @ 519 RPM wheel speed (approx. 38 MPH)
With a 44t rear sprocket: 718lbs of force @ 449 RPM wheel speed (approx. 33 MPH)

(Correction courtesy of Kenolds!  (I had 620/718 in.lbs...when it is actually 620/718 lbs of force) Thanks Kenolds!)

again... the engine torque output stays the same....you ride the bike the same...but you got more Balls... get light to light quicker because it has more power to accelerate the bike and rider faster ...of course...at a loss of top speed respectivley.

It's all about your likes and what you want outa the bike... To go faster faster....or to cruize and enjoy longevity

[MCRider]

I had the feeling I was up against an engineer, that's why I was trying to tread softly. 

So, I appreciate the explanation, just what I was looking for!

I learned something today.     

[rustrocket92]

Thanks for all the input everyone!!