9

While reading this question it came to mind:

Why did manufacturers start building transmissions without a 1:1 ratio?

In the older transmission you'd see the 1:1 ratio all the time, either as the final drive ratio or the one just before the overdrive. Is there a reason for this, either technically, mechanically, or financially? Seems to me it would cost more to create your transmission as such because a 1:1 (to me) would entail less gearing*. Power in = Power out ... help me to understand?

*Note: By gearing here, I probably should have said engineering.

  • I'm not sure I get your question. Are you asking about final drive or are you asking about the highest gear in a the transmission? – DucatiKiller Jan 15 '16 at 22:36
11

Gearbox designers should avoid integer ratios like the plague.

For mechanical reasons.

Integer ratios will accelerate wear and tear because it increases the frequency with which tooth A on the driver gear will meet tooth B on the driven gear.

This frequency is known as the hunting tooth frequency, which I've explained in more detail here.


Regarding 1:1

1:1 is a special case known as direct-drive. The input shaft is locked with the output shaft, so the above argument doesn't really apply.

I believe Wikipedia sheds some reasonable light on this matter:

In an era when different models of car with different wheel sizes could be accommodated by simply changing the final drive ratio, it made sense for all transmissions to use direct drive as the highest gear. As noted earlier, however, this would cause the engine to operate at too high an RPM for efficient cruising. Although adding the cruising gear to the main gearbox was possible, it was generally simpler to add a separate two-gear overdrive system to the existing gearbox. This not only meant that it could be tuned for different vehicles, but had the additional advantage that it could be offered as an option, which was easy to add.

I interpret that to mean there are a few motivating factors at play (denoted in brackets):

  • 1:1 isn't sufficient as a final gear ratio because the resultant engine RPM is too high for the cruising speeds of modern vehicles (mechanical)

  • It is simpler to design the gearbox to have two overdrive gears than have direct drive with one overdrive gear (cost, manufacturing, engineering)

  • A design with two overdrive gears is easier to tune for different vehicles, making the same gearbox design more universal (cost)

  • 1
    But in 1:1 the input shaft is locked with the output shaft, there is no gear involved – Move More Comments Link To Top Jan 15 '16 at 17:37
  • @Movemorecommentslinktotop I see what you're saying, you're referring to direct-drive. – Zaid Jan 15 '16 at 17:57
  • @Movemorecommentslinktotop have a look at the edit – Zaid Jan 15 '16 at 18:11
  • Note that direct-drive isn't useful in many (most?) transaxles because the differential is offset from the crankshaft's axial line. Some type of gearing mechanism is needed to move the torque from one shaft to another. – MichaelS Jan 15 '16 at 21:33
  • @MichaelS - While you're right, that is pretty easily done with either a chain drive or gearing ... would be like what is used for a cam shaft. – Pᴀᴜʟsᴛᴇʀ2 Jan 15 '16 at 21:44

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