AussieSteve
Active Member
I've been playing around calculating gear ratios, speeds in each gear and pedal-starting ratios for my shift kit.
With the right combination, it should be easier to pedal-start while still having a good range of speeds.
I'm basing all calculations below on a max 7400RPM, (my engine pulls to 7400), with a 25.57" wheel and a 2-stroke HT engine with standard primary drive and countershaft sprocket.
The gear cluster is 6-speed: 28,24,21,18,16,14.
Also, I've only done calculations for some of the range of freewheel chainrings available from SBP.
Stock shift-kit ratio for the RHS is 10T>44T/36T, with a top speed in 1st of 23.6mph, (37.8kph) and a top speed in 6th of 47.2mph, (75.5kph).
The pedal-start ratio is 4.4:1 - very hard to turn over with my hi-comp billet head.
For myself, I don't need excessive speed, but want easy starting and a fairly low first gear for take-off, so I'm going for an 11T jackshaft sprocket driving a 36T sprocket, with a 24T sprocket driving the rear gear cluster. (The last setup in the list below.)
This gives a pedal-start ratio of 3.3:1, a top speed in 1st of 21.2mph, (33.9kph) and a top speed in 6th of 42.3mph, (67.7kph).
A 10T jackshaft sprocket would provide lower drive gearing for a lower 1st gear, but would increase the pedal-start ratio to 3.6:1, still 20% easier than the stock 10T/44T, but a little hard for me, I think.
The downside of using a driven RHS sprocket smaller than 44T is that the sprocket/chainring speed increases, possibly shortening the life of the freewheel bearing.
All speed figures below are calculated, not measured, so wind resistance etc will play a big part in top gear speeds.
Also, the higher final drive gearings might be near impossible to pedal-start.
I hope someone finds this info useful - I did.
... Steve
Below are figures for:-
10T>44T/36T, (standard shift kit ratio)
10T>36T/30T
10T>30T/24T
11T>36T/36T
11T>36T/30T
11T>36T/24T
10T>44T/36T, (standard shift kit ratio)
GEAR...SPEED
1........23.6mph / 37.8kph
2........27.5mph / 44kph
3........31.5mph / 50.4kph
4........36.7mph / 58.7kph
5........41.3mph / 66.1kph
6........47.2mph / 75.5kph
Pedal-start ratio = 44:10 (4.4:1) - hard - (standard).
Chainring speed = 241RPM @ 7400RPM. (4 revs/sec)
10T>36T/30T
GEAR...SPEED
1........24mph / 38.4kph
2........28mph / 44.8kph
3........32mph / 51.2kph
4........37.4mph / 59.8kph
5........42.1mph / 67.4kph
6........48.1mph / 77kph
Pedal-start ratio = 36:10 (3.6:1) - medium-hard - (18% easier than standard).
Chainring speed = 295RPM @ 7400RPM. (4.9 revs/sec)
10T>30T/24T
GEAR...SPEED
1........23.1mph / 37kph
2........26.9mph / 43kph
3........30.8mph / 49.3kph
4........35.9mph / 57.4kph
5........40.4mph / 64.6kph
6........46.2mph / 73.9kph
Pedal-start ratio = 30:10 (3:1) - easiest - (32% easier than standard).
Chainring speed = 354RPM @ 7400RPM. (5.9 revs/sec)
11T>36T/36T
GEAR...SPEED
1........31.7mph / 50.7kph
2........37mph / 59.2kph
3........42.3mph / 67.7kph
4........49.4mph / 79kph
5........55.5mph / 88.8kph
6........63.5mph / 101.6kph
Pedal-start ratio = 36:11 (3.3:1) - medium - (25% easier than standard).
Chainring speed = 325RPM @ 7400RPM. (5.4 revs/sec)
11T>36T/30T
GEAR...SPEED
1........26.4mph / 42.2kph
2........30.8mph / 49.3kph
3........35.3mph / 56.5kph
4........41.1mph / 65.8kph
5........46.3mph / 74.1kph
6........52.9mph / 84.6kph
Pedal-start ratio = 36:11 (3.3:1) - medium - (25% easier than standard).
Chainring speed = 325RPM @ 7400RPM. (5.4 revs/sec)
11T>36T/24T
GEAR...SPEED
1........21.2mph / 33.9kph
2........24.7mph / 39.5kph
3........28.2mph / 45.1kph
4........32.9mph / 52.6kph
5........37mph / 59.2kph
6........42.3mph / 67.7kph
Pedal-start ratio = 36:11 (3.3:1) - medium - (25% easier than standard).
Chainring speed = 325RPM @ 7400RPM. (5.4 revs/sec)
I stopped here because this is the best balance I'll get for my intended use.
P.S. I should add that the SBP kit's standard ratio is probably fine for most people, as far as starting the engine goes. When I describe pedal-starting as 'hard', above, that's largely due to the hi-compression billet head and my low body-weight, 50kg, (110lbs).
Tttthat's All Folks.
With the right combination, it should be easier to pedal-start while still having a good range of speeds.
I'm basing all calculations below on a max 7400RPM, (my engine pulls to 7400), with a 25.57" wheel and a 2-stroke HT engine with standard primary drive and countershaft sprocket.
The gear cluster is 6-speed: 28,24,21,18,16,14.
Also, I've only done calculations for some of the range of freewheel chainrings available from SBP.
Stock shift-kit ratio for the RHS is 10T>44T/36T, with a top speed in 1st of 23.6mph, (37.8kph) and a top speed in 6th of 47.2mph, (75.5kph).
The pedal-start ratio is 4.4:1 - very hard to turn over with my hi-comp billet head.
For myself, I don't need excessive speed, but want easy starting and a fairly low first gear for take-off, so I'm going for an 11T jackshaft sprocket driving a 36T sprocket, with a 24T sprocket driving the rear gear cluster. (The last setup in the list below.)
This gives a pedal-start ratio of 3.3:1, a top speed in 1st of 21.2mph, (33.9kph) and a top speed in 6th of 42.3mph, (67.7kph).
A 10T jackshaft sprocket would provide lower drive gearing for a lower 1st gear, but would increase the pedal-start ratio to 3.6:1, still 20% easier than the stock 10T/44T, but a little hard for me, I think.
The downside of using a driven RHS sprocket smaller than 44T is that the sprocket/chainring speed increases, possibly shortening the life of the freewheel bearing.
All speed figures below are calculated, not measured, so wind resistance etc will play a big part in top gear speeds.
Also, the higher final drive gearings might be near impossible to pedal-start.
I hope someone finds this info useful - I did.
... Steve
Below are figures for:-
10T>44T/36T, (standard shift kit ratio)
10T>36T/30T
10T>30T/24T
11T>36T/36T
11T>36T/30T
11T>36T/24T
10T>44T/36T, (standard shift kit ratio)
GEAR...SPEED
1........23.6mph / 37.8kph
2........27.5mph / 44kph
3........31.5mph / 50.4kph
4........36.7mph / 58.7kph
5........41.3mph / 66.1kph
6........47.2mph / 75.5kph
Pedal-start ratio = 44:10 (4.4:1) - hard - (standard).
Chainring speed = 241RPM @ 7400RPM. (4 revs/sec)
10T>36T/30T
GEAR...SPEED
1........24mph / 38.4kph
2........28mph / 44.8kph
3........32mph / 51.2kph
4........37.4mph / 59.8kph
5........42.1mph / 67.4kph
6........48.1mph / 77kph
Pedal-start ratio = 36:10 (3.6:1) - medium-hard - (18% easier than standard).
Chainring speed = 295RPM @ 7400RPM. (4.9 revs/sec)
10T>30T/24T
GEAR...SPEED
1........23.1mph / 37kph
2........26.9mph / 43kph
3........30.8mph / 49.3kph
4........35.9mph / 57.4kph
5........40.4mph / 64.6kph
6........46.2mph / 73.9kph
Pedal-start ratio = 30:10 (3:1) - easiest - (32% easier than standard).
Chainring speed = 354RPM @ 7400RPM. (5.9 revs/sec)
11T>36T/36T
GEAR...SPEED
1........31.7mph / 50.7kph
2........37mph / 59.2kph
3........42.3mph / 67.7kph
4........49.4mph / 79kph
5........55.5mph / 88.8kph
6........63.5mph / 101.6kph
Pedal-start ratio = 36:11 (3.3:1) - medium - (25% easier than standard).
Chainring speed = 325RPM @ 7400RPM. (5.4 revs/sec)
11T>36T/30T
GEAR...SPEED
1........26.4mph / 42.2kph
2........30.8mph / 49.3kph
3........35.3mph / 56.5kph
4........41.1mph / 65.8kph
5........46.3mph / 74.1kph
6........52.9mph / 84.6kph
Pedal-start ratio = 36:11 (3.3:1) - medium - (25% easier than standard).
Chainring speed = 325RPM @ 7400RPM. (5.4 revs/sec)
11T>36T/24T
GEAR...SPEED
1........21.2mph / 33.9kph
2........24.7mph / 39.5kph
3........28.2mph / 45.1kph
4........32.9mph / 52.6kph
5........37mph / 59.2kph
6........42.3mph / 67.7kph
Pedal-start ratio = 36:11 (3.3:1) - medium - (25% easier than standard).
Chainring speed = 325RPM @ 7400RPM. (5.4 revs/sec)
I stopped here because this is the best balance I'll get for my intended use.
P.S. I should add that the SBP kit's standard ratio is probably fine for most people, as far as starting the engine goes. When I describe pedal-starting as 'hard', above, that's largely due to the hi-compression billet head and my low body-weight, 50kg, (110lbs).
Tttthat's All Folks.
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