Quote:
Originally Posted by Hotrodfil
Didn't think the Macs specs seemed right...!
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I first noted these specs (6.9:1 and 8.75:1) in "Model A Specifications" on pages 30 thru 33 of the May-June 1988 issue of "The Restorer" (MAFCA), subsequently included in Volume V (pages 10-13) of "How To Restore Your Model A" so they seem to have a life of their own. However, I believe the 1.85:1 and 3.12:1 are more accurate.
Final drive gear ratios for transmissions such as are found in the Model A, are calculated as the ratio of the output gear (diameter or number of teeth) to the input gear, or where there are multiple gear engagements (such as in first and second gear) the product of such ratios and are expressed as the ratio of the number of input shaft revolutions required to give 1 revolution of the output shaft.
Thus, for first gear, where the 16-tooth input shaft drives the 31-tooth gear on the cluster gear, and where the 18-tooth cluster gear then drives the 29-tooth low/reverse sliding gear, the number of input shaft revolutions to give 1 output shaft revolution is given by 31/16 x 29/18 = 3.12 (rounded down). Consequently, first gear is said to be 3.12:1.
Similarly, second gear, where the 16-tooth input shaft drives the 31-tooth gear on the cluster gear, and where the 24-tooth cluster gear then drives the 23-tooth second/high sliding gear, the ratio is given by 31/16 x 23/24 = 1.86 (rounded up), giving the 1.86:1 (sometimes seen as 1.85:1) second gear ratio.
For third gear, the forward portion of the second/high sliding gear internal teeth engage with the after portion of the input shaft external teeth and neither the low/reverse nor the second/high sliding gears otherwise engage with the cluster gear (which keeps spinning due to its permanent engagement with the input shaft). The after portion of the second/high sliding gear remains splined to the output shaft and consequently the output shaft is driven with no gear reduction and the final 1:1 “ratio” is achieved.
Reverse is the product of three ratios, inasmuch as the 16-tooth input shaft drives the 31-tooth gear on the cluster gear, the 15-tooth gear on the cluster gear drives the 18-tooth reverse idler gear, and the 18-tooth reverse idler gear drives the 29-tooth low/reverse sliding gear, giving a 3.75:1 reverse gear ratio.
Probably more than is needed for this discussion…
Model A Transmission.jpg