Need help figuring pulley sizes...

silverbear

silverbear

The Boy Who Never Grew Up
I just posted the following request for help on my 1934 Elgin build thread...
http://mottorbicycling.com/showthread.php?p=414682#post414682

As a quick review, this bike will use a slow revving vintage Villiars 2 stroke engine. It revs out at around 1600 RPMs so I'm not kidding when I say slow revving. The engine crankshaft will use a pulley going to a pulley on a jackshaft. On the pedal side of the bike the jackshaft will turn a chain going to an SBP shift kit free wheel and from there a chain to the internally geared 3 speed Sturmey Archer 3 speed hub.


"This is where I need some help from you gear heads out there. I need to figure out what size pulleys to order for the engine crankshaft and the engine side of the jack shaft. In reverse order here's what I have already. The pedal driven sprocket on the rear wheel is 13 tooth. From there to the freewheel sprocket receiving the jack shaft chain the chain ring is 44 tooth. On the other end of that chain at the jack shaft the sprocket is 9 tooth.

At this time there is a sprocket on the engine side of the jack shaft (17 tooth), but a pulley is going there in it's place. And a pulley will go on the engine crankshaft.

This is way beyond the capabilities of me little brain to cope with. Anyone have an idea of what pulley sizes I need? Keep in mind this is a slow revving engine and I have a three speed internally geared rear hub. I'm not looking for great speed, but low thirties in the top gear would be nice. I don't expect to hit the pulley sizes right on the money and figure it is likely I'll need to change something to make it mo betta later on, but I need a starting point anyway. Many thanks to anyone willing to help me out here.
SB"
 
If you need to calculate more than 3 ratios you can combine two of them.

http://jimsitton.net/ratiocalc/

I like to open 3 or 4 versions of the calculator across my desktop so I can play with ratio changes and compare them.

Depending on the size of your belt you will need to subtract a bit from the diameter to be accurate as the belt driving diameter is less than the pulley.
 
I believe the rule of thumb is to subtract .15" from pulley diameter
the belt sits inside the pulley, rather than riding around the outside
 
Thanks for the input, but when it comes to math, ratios, gears, etc. I'm a dummy. Anyone venture a guess at the pulley sizes for the engine side of the equation? I can tweak it from there by experiencing what happens... oh this is too low or this is way too high. I don't know enough to even ask good questions. I do appreciate the help.
SB
 
silverbear said:
Thanks for the input, but when it comes to math, ratios, gears, etc. I'm a dummy. Anyone venture a guess at the pulley sizes for the engine side of the equation? I can tweak it from there by experiencing what happens... oh this is too low or this is way too high. I don't know enough to even ask good questions. I do appreciate the help.
SB
Click to expand...
Put the gear/ pulley pairs that you are using on separate lines in order and I would be happy to help.
My eyes roll back in my head when I read your first post. :D

Also need real wheel diameter and engine rpm redline.

X teeth on 1st gear / pulley
y teeth on 2nd gear / pulley

w
z
 
The_Aleman said:
Sorry I missed this thread when it was first posted. Hey SB, what inner sprocket (the sprocket that goes to rear wheel) and wheelsize are you planning on using?

With that info it'll be easier to complete the equation. With a 36 inner and 26" wheel I calculate you need approximately a 3:1 pulley reduction for 31.4MPH @ 1600.

Using the gear calc I have this info entered:

1
3
----
9
44 (outer crank ring)
----
36 (inner crank ring)
9.75 (this is 3rd gear on your Sturmey with a 13T. For 1st gear enter 16.25, 2nd gear is 13, 3rd gear is 9.75 effective)
----
26.2
1600

If you open the gear calc, leave the first boxes at 1 and 1, and enter all your gear sizes, you can then fine-tune the first ratio to what you need.

Hope this helps. Let me know if you need any more help!
Click to expand...

Thank you, this is very helpful. So as long as the driven pulley is three times the diameter of the drive pulley I can use whatever sizes I want, is that correct? This is assuming I leave all of the other geared sprockets as they are. The speeds in each of the hub ranges seems good. 1st gear would be 18mph, shifting to second would be 23mph and 3rd would be 31mph. This seems about right since I'll be pulling a light sidecar with the dog.
SB
 
The_Aleman said:
Yep, the object is to get the 3:1 reduction, actual pulley diameter doesn't matter. A 1" on engine and 3" on jackshaft, a 2 and a 6, etc
Basically you'll have similar gears that a Huasheng Q-matic has. I believe most of them are ~2.76:1

The gearing may be a little tall with the 36T inner, a 30T might be a better option if you have one. If you need a 30T, I have one that came with my SBP kit that I'll never use. PM me if you want it. You can always put a 10T on the jackshaft output (or 11 or 12) if you want more top speed. Giving yourself some room to move around might be a good idea!
Click to expand...

With a 30T sprocket that would make 1st gear yield 16.25 mph, 2nd would give 19.25 mph and 3rd would be 26.0. Considering the sidecar, that might be a more realistic speed range. I appreciate the offer of the 30T sprocket and have sent you a PM. Thanks again for taking the time to work with me. That gear calculator is a handy thing!
SB
 
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