Gear Ratio Calculator

[duivendyk]

There might be a fly in the ointment when it comes to the pedal drive.I assume it would be used for getting started and also in a limp-home mode.Let look at the latter.If possible you would like around 12mph or so in mid gear at 70-80 pedal rpm.That means rear wheel rpm of 210, and at 80 pedal rpm an overall ratio of 210/80=2.6.this ratio comprises the jackshaft to rear ratio of 10/19,so the pedal to jackshaft ratio becomes 2.6x19/10=4.95.,is this feasible?,how small a freewheel can be had? 10t?? for a 10 to 48 drive.
You need to think this over.One way would be to put more reduction in the drive from engine to the jackshaft,lets say a 3:1 reduction instead of around 2,(running it at 2/3 of the original speed and reducing the reduction from the jackshaft to the hub by 3/2 that is a 15 t sprocket instead of 10t.So if we shoot for a pedal rate of say 75rpm or an overall reduction of 210/75=2.8 which means a 18/15x 2.8=3.36. pedal to jack shaft.This is a lot better than close to 5 in the previous lineup.
If you are riding in the limp-home modethe engine is still being driven from the jackshaft and only the clutch isolates it,so from that perspective increasing the reduction engine to jackshaft means more drag from the disengaged clutch through the drive,so there is something of a trade-off here as I see it if we up the ratio
In any case,there are aspects of this design that need to be thought through,I think.Look before you leap is a good adage

 

[BrettMavriK]

Allrighty,
Here's my final choice and I'm stickin' to it and ordering tommorrow.
Somebody stop me if If I'm off. Gonna start with all the gears from the engine to the rear sprocket (which is 20t, not 19t I found out) The two sprockets I'm ordering for my 5/8" jackshaft built for 35 chain on one side / 41 chain on the other, will be highlighted in red.
Here Goes:
20/82 (4.1:1)
10/18 (1.8:1)
11/20 (1.82:1)

This should put me at 13.42:1 in 2nd gear (1:1) 26mph @ 6000 rpm
with 3rd gear to take me to 35-37
I was told 6000rpm was cruising rpm..........sound right?

'BrettMavriK

 

[BrettMavriK]

This bike isn't really all that practical without a motor anyways.....
its a chopper. Limp home means call a friend and load it in the van. =-)'
I would like the ability to pedal if it ever died however. I'd undo the socket screw and pull the sprocket & chain off the motor side of the jack shaft if that was the case.

I was thinking of just having the Jesse James 42t sprocket on the freewheel and then running a 24t on the crank pedal. If I could get 10-15 mph with leg power I'd be happy.

So I want 60 pedal revolutions per minute. From the top (or Feet)
24t
42t
11t
20t

So this gives me..................AAAAAAAAUUUUUUUGGGGHHHHH!!!!!!!!!!!!!

'BrettMavraaaaaaaaaaaauuuuuuuuuuuuuuuuuuggggggggghhhhhhhhh!!!!!!!!!

 

[duivendyk]

The engine gearing is not all THAT critical,just take the plunge.Right brain thinking is OK it gets you off&running but the left side keeps you honest. It's been nagging me that you'll have a hard time pedal starting this thing unless you gear the pedal input way down,if that is your intent, fine.Normally in the HT type engines are started I think by pedaling away to get up some momentum of bike+cyclist (300-350lbs),and letting out the clutch.To quite an extent you are relying on the built-up stored energy to provide the starting power.With your setup you don't have this kinetic energy available because of the freewheel at the hub!.You have to rely on solely on muscle power alone,applied through the torque reducing gearing to do this.If the gearing is too large you will not be able to apply enough torque to the engine.I calculated that your 64 cc engine would require at least 50 + kg.cm of torque to overcome the compression (45 inch.pounds).With a 17 cm (6.7") crank and 125 lbs applied force we have available 6.7x 125 = 835 inch.pounds at the pedals.That means the reduction has to be less than 835/45which is somewhere around 18.5.This means that the gearing from the pedals to the jackshaft becomes 18.5/(4.1x1.9=2.4.To be on the safe side take 2:1.This implies that the pedal rate is about the same as the rear wheel rpm 2x10/20=1.With a pedaling rate of 100 you will be crawling along at 0.060x100= 6mph.But your pedal input is not constant you can generate effort for about 90 degrees out of a half rev but the engine drag is constant ,you'll have trouble to keep going since as I pointed out forward momentum is not available to help you in the dead zones.I think you need to rethink this or use a pull-start.Sorry my left brain is such a spoil-sport.

 

[BrettMavriK]

Cool, but I have 3 speeds in the internal hub.
I have a question.
What type of chain sprocket is the 10 tooth counter shaft sprocket that is on the HT engine? Is it a 35? McMaster.com has a 36t #35 sprocket thats a little over 4 inches in diameter.

I've come up with a gear set that will allow me to pedal as well as be in the powerband, seeing both the pedals and the motor run from the last jack shaft sprocket:

A 36t on the left jackshaft,
a 22t on the right
a 16t freewheel for the cranks
a 44t at the cranks
then of course a 20t at the rear hub,
and a 10t on the countershaft.

Motor-34mph @ 5500rpm in 3rd gear
Pedal-15mph @ 60 ppm in 3rd gear

'BrettMavriK

 

[duivendyk]

Having gears in the hubs will help you of course getting more forward speed,with the 1.33 ratio there,but is no help in gettin the engine started.If not for that everything would be fine.With your 44/16 ratio crank to jack shaft you have an overall ratio of 44/16x19/10x4.1=21.4,implying over 100lbs downward force at the crank to get the starting torque.The 50kg.cm torque value I came up with to crank the thing is on the iffy side,I ignored inertial effects(getting the rotating masses flywheel etc up to speed) but also ignored the compression rebound effect on the piston.So who knows what it really takes ?,it's a ball park figure.If it is too hard to crank,you can still fall back on a smaller chainwheel

 

[BrettMavriK]

My rear tire has 5 speeds. I've nailed down the gears on all points that serve the motor as well as the pedals. I've got the perfect powerband gear for every rpm of the motor and every ppm. (pedals per minute)

'BrettMavriK

Having gears in the hubs will help you of course getting more forward speed,with the 1.33 ratio there,but is no help in gettin the engine started.If not for that everything would be fine.With your 44/16 ratio crank to jack shaft you have an overall ratio of 44/16x19/10x4.1=21.4,implying over 100lbs downward force at the crank to get the starting torque.The 50kg.cm torque value I came up with to crank the thing is on the iffy side,I ignored inertial effects(getting the rotating masses flywheel etc up to speed) but also ignored the compression rebound effect on the piston.So who knows what it really takes ?,it's a ball park figure.If it is too hard to crank,you can still fall back on a smaller chainwheel

 

[ZnsaneRyder]

The gear ratio calculator is awesome, especially for those using jackshafts or multiple gear reductions!

 

[BrettMavriK]

worker fer me... =-]'

'BrettMavriK

The gear ratio calculator is awesome, especially for those using jackshafts or multiple gear reductions!

 

[chrisr1995]

How do I pick the correct rear sprocket..please read.

Is this correct?..The little gear that is next to my clutch on my Gen-II 50cc Star-Fire is 20t. The clutch disk has 83t. That's giving me the 4.15:1 in the first field that you've been talking about. Then the chain attaches to the motor on a 10t sprocket, and the rear sprocket is 44t. That gives me 4.40:1 in the second field and a total drive ratio of 18.26:1 The bike I'm going to use has a 20 x 2.125" rim. I measured from the ground to the top of the tire and got 20". At 5000 RPM I get 16.3 MPH. Am I doing this correctly??? What I'm looking for is to be able to cruise on flat ground at about 22-26 MPH and spin the motor at 4500 or so for long periods of time. I don't want to over rev the motor so it will last. If I input a 27t rear sprocket at 4500 RPM, I get 23.9 MPH. That would be perfect, but how do you think the bike will run?....as far as it chugging when starting off/low speed? I'm a light weight at only 140 lb's and the bike is an old Schwinn Sting-Ray, so the motor isn't trying to move a bunch of mass here. Any suggestions on RPM's and gear choice etc...would be welcomed PLEASE!!