Subaru Workshop Manuals
Hatchback 4WD F4-1781cc 1.8L (1983)
Transmission and Drivetrain > Continuously Variable Transmission/Transaxle, CVT > Component Information > Technical Service Bulletins > Technical Service Bulletins for Continuously Variable Transmission/Transaxle: > ATRATB8930 > Oct > 89 > A/T - Math Part II > Page 1643
EXAMPLE: 34 divided by 20 = 1.7 (See figure)
Occasionally, the ring gear is used as the input, and the Sun gear as the output.
The formula for this is: The Sun gear divided by the ring gear = Ratio.
EXAMPLE: 20 divided by 34 = .59
Notice that the output is overdriven.
A transmission using this method must use another planetary gear set to reduce the output. The Mercedes W3A-040 is a good example of this
To get more than one gear forward and a reverse, requires multiple, or compound planetary gear sets.
Two of the most common of these are the SIMPSON GEAR SET, used in transmissions like the THM 350, the Ford C-4, and the TF 6 & 8, and the
Figuring out all the ratios for these transmissions is a little tricky, so I'll give you the formulas, and let you figure out how these formulas were derived.
THE SIMPSON GEAR SET:
For this example I'll use a THM 200, which has 74 TEETH ON THE FRONT RING GEAR, 42 TEETH ON THE FRONT SUN GEAR, 30 TEETH ON
The formula for 1ST GEAR is: rear ring divided by rear Sun x front Sun plus front Sun + front ring divided by front ring.
EXAMPLE: On the THM 200, it would be:
62 divided by 30 x 42 + 42 + 74 divided by 74 = 2.74:1
SECOND GEAR is easy: Front Sun + front ring divided by front ring.
EXAMPLE: 42 + 74 divided by 74 = 1.57:1
THIRD GEAR is Direct Drive, or 1:1
REVERSE is rear ring divided by rear Sun
EXAMPLE: 62 divided by 30 = 2.06
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