It depends of WHICH 'sprocket' you are discussing!
Usually, you will be discussing the DRIVEN sprocket, but you also need to consider the DRIVE sprocket as well.
It's simple, the LARGER the driven sprocket, the more rapid the acceleration and (up to a certain point) the LOWER the potential top speed. The reverse is true for a SMALLER driven sprocket.
The concept is easy to work out..... especially for someone who calls himself a Karting Student!
If the DRIVE sprocket is 10 teeth and the driven sprocket is 80 teeth, then the DRIVE sprocket needs to rotate 8 times to make the DRIVEN sprocket (and thus the wheels) rotate ONCE. If the DRIVEN is 70 teeth, the engine now only has to rotate SEVEN times to turn the rear wheel ONCE!
So:
a 10:70 pair at 10,000 rpm will rotate the wheels 1428 times in a minute
a 10:80 pair at 10,000 rpm will rotate the wheels 1250 times in a minute
a 10:90 pair at 10,000 rpm will rotate the wheels 1111 times in a minute
You will be able to see which is the 'faster' speed for yourself.
The higher the potential top speed, the LOWER the acceleration: it's a trade-off.
BUT.... if you change the DRIVE sprocket, the same sort of thing happens...... but BACKWARDS. This time, the LARGER the DRIVE sprocket, the LESS the acceleration but the HIGHER the top speed!
Taking just one of the above examples:
a 10:80 ratio at 10,000 rpm will rotate the wheels 1250 times in a minute
BUT
an 11:80 ratio at 10,000 rpm will rotate the wheels 1375 times in a minute
Finally, notice that 'fun' in changing BOTH sprockets:
a 10:80 ratio at 10,000 rpm will rotate the wheels 1250 times in a minute
AND
an 11:88 ratio at 10,000 rpm will rotate the wheels 1250 times in a minute....
......both sets of 'pairs' perform IDENTICALLY!
If you divide the DRIVEN sprocket by the DRIVE sprocket, you get the ratio expressed as a REAL 'ratio'
Thus a 10:80 is a ratio of 1:8 (Often called 8:1)
A 10:88 is 8.8:1
but an 11:88 is ALSO 8:1
Here's the quick list for many of the standard sprocket pairs 'sorted' from lower ratio (worst acceleration but highest 'potential' top speed) to higher ratio (best accelerationand but lowest top speed) :-
Drive Driven Ratio....(:1)
10 70 7.00
10 71 7.10
10 72 7.20
10 73 7.30
10 74 7.40
10 75 7.50
10 76 7.60
10 77 7.70
9 70 7.78
10 78 7.80
9 71 7.89
10 79 7.90
9 72 8.00
10 80 8.00
10 81 8.10
9 73 8.11
10 82 8.20
9 74 8.22
10 83 8.30
9 75 8.33
10 84 8.40
9 76 8.44
10 85 8.50
9 77 8.56
10 86 8.60
9 78 8.67
10 87 8.70
9 79 8.78
10 88 8.80
9 80 8.89
10 89 8.90
9 81 9.00
10 90 9.00
9 82 9.11
9 83 9.22
9 84 9.33
9 85 9.44
9 86 9.56
9 87 9.67
9 88 9.78
9 89 9.89
9 90 10.00
Print it and keep it with your tool box so that you can always see which 'pair of gears is the 'next' in the sequence and by how much a 'change' will help/hurt you. For example, note that the change between 9:71 to a 10:79 is ONLY 0.01 in ratio but a change from 10:79 to 9:72 is TEN TIMES as large (at 0.10, difference)
Ian
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Gearing to help the Engine by 'TheKartingStudent' (08 Nov 2009 @ 22:43)
