Friday 11 December 2009


I had always thought that a backlash had something to do with whips. Specifically, I thought that it was the reverse stroke, when the whipper's arm is being pulled up ready to strike, and there is a danger that the whip might strike an innocent bystander. Here is an illustration of my imaginings.

It fits the usual sense of the term perfectly: the violence that is being directed one way requires that it should suddenly be flicked back from the victim towards those who thought they were safe. Indeed, it looks here as though the chap in the white vest has turned his head to avoid the cat-o'-four-tails that the whipper is irresponsibly allowing to fly out into his blind spot. I can only assume that health and safety regulations were not as advanced in first century Judaea as they are now.

I was thinking about this because I read in the paper today that "the backlash over the golfer's sexual liaisons could eat him alive."

So I cast imagination to the sewer and decided to look it up. Backlash has nothing whatsoever to do with whips. It is, instead a property of cogwheels. No two connecting cogs can ever fit perfectly. If they did the friction would stop them moving at all. Moreover, in most systems a metal cog could heat up slightly and expand locking the device completely. As a result, you have to leave some room between the teeth and this space is called a backlash.

A backlash doesn't do much except when you reverse the system. In the diagram above the bottom wheel is turning anticlockwise. That means that if you start turning it clockwise there will be a moment of stillness before the tooth on the bottom left connects with the upper wheel and starts to move it. Moving diagram here. Video here.

Well, this stymied me. It means that the usual sense of backlash to mean a violent reaction is utterly wrong. The backlash is the empty period, it is the blank at the bottom of the page, it is the gap between songs, it is the please wait while your request is being processed.

But if a golfer were to get caught between the teeth of a very, very big gear system, I have to concede that he could be eaten alive.

Just so any whip enthusiasts don't get too disappointed, "enough room to swing a cat" is probably to do with a cat-o'-nine-tails.

P.S. Anyone interested in how much room is actually needed for feline swinging should check here. It's 10'1" for me, but obviously varies with height/size of mog.


  1. But the definition in the OED is "The jarring reaction or striking back of a wheel or set of connected wheels in a piece of mechanism, when the motion is not uniform or when sudden pressure is applied. back-lashing (in same sense)."

    the first citation:
    "1815 Brit. Pat. 3887, There is a great risk of breaking these wheels from the backlash or returning stroke of the engine."

    It's nothing to do with whips, but it is a violent reaction.

  2. I've added some video links now to make it clearer. When the backlash is complete the teeth connect so a mechanism could be damaged by backlash, just as it could be damaged by any movement. However, every time you put your car into reverse or twiddle the dial on an old radio there is backlash. If you put your car into reverse whilst driving forward at 100mph the backlash would probably result in the gears breaking, but that would be unusual, not to mention irresponsible. (Inky Fool does not condone suicidal automotive experimentation).
    As I say, have a look at the videos. A backlash isn't a reaction, it's simply a reversal of a gear mechanism.
    Think about running up and down with a kite (I know I do) when you turn there is a period when the string goes slack and the kite starts to fall, that's the backlash. When the backlash is complete, the string jerks taut again.

  3. "Backlash" as an engineering term might not be a violent reaction, but the word as defined by the OED means "The jarring reaction..." etc. Now maybe the OED left out the technical engineering definition for some reason, but as I've said before, "non-technical" doesn't mean "incorrect".

  4. I'm not quite certain that you can argue against technical correctness by citing the OED. I fear that the OED's lexicographers may have been utterly innocent of mechanics.

  5. I'm not arguing against technical correctness. I'm just saying that it seems strange to say that "the usual sense of backlash to mean a violent reaction is utterly wrong", when "violent reaction" is clearly one of the meanings of the word.

  6. Birds in their little Nests agree; And 'tis a shameful Sight, When Children of one Family Fall out, and chide, and fight.