Why does a car whine in reverse gear?

    The answer to an age-old question that troubles motorists around the world.

    Whether you drove a car with a manual transmission or stood near one when it was reversing, you've probably heard a whining / whirring / winding noise coming from it. If you are into motorsports, you might recognize this unusual sound. It occurs mostly whilst reversing, but some cars produce this noise in the first gear as well. So what is it? Why do some cars sound so peculiar when going backwards? Well, here is the answer.

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    It's a gearbox thingy

    Since this distinct sound occurs only in some gears, you might have guessed that it comes from transmission. If you did, you are right! But what is exactly happening? To answer that, I need to quickly explain how does a manual gearbox work.

    As a word gearbox implies that a transmission (that's how Americans call it) is a box full of gears. A main job of the gearbox is to engage proper gears so that a car can send power from its engine to its wheels efficiently. Forward gears usually have a helical structure, which means their teeth are a bit angled (see a picture below). This allows them to operate quietly and smoothly, as they can spread the load on the teeth evenly thanks to the angulation.

    However, there's no rose without a thorn. Helical gears are costly and, what is probably even more important, it is hard to slide them in and out of each other. As a result, they are not a viable option for a reverse gear, which has to be treated separately from other gears due to its unique function.

    Cutaway of a manual gearbox. As you can see, there are sets of helical gears and one set of spur gears (on the right). By: PORSCHE-Museum, Stuttgart, Germany, November 2006 CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1423773

    Cutaway of a manual gearbox. As you can see, there are sets of helical gears and one set of spur gears (on the right). By: PORSCHE-Museum, Stuttgart, Germany, November 2006 CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1423773

    Introducing a spur gear

    Whereas forward gears usually utilize just 2 gears per gear (that's a lot of gears, I know), reverse gear has to spin in the other direction, thus it needs an additional gear. This additional gear, often referred to as an idler gear, is used to reverse the rotation. To do so, it has to slide into two other gears, which is hard to do with helical gears. To solve this issue, manufacturers started using a set of spur gears for the reverse.

    Spur gear is basically what you think of when you hear a world "gear" (you can see one on the image above). It has straight teeth and it can slide in and out of other spur gear with ease. Thus, it is ideal for reverse gear, as it can be easily engaged and disengaged. However, it has its flaws. Teeth of two spur gears collide when coming into contact rather than gently sliding into one another as helical gears do. As a result, they produce a sound. As the gears spin, there are a lot of collisions happening, which results in the whining noise that is so distinguishable. The faster they are spinning, the higher the frequency of the sound and the louder the noise.

    A simplified model of a manual transmission. Reverse uses 3 gears, whereas forward gears use 2 gears each

    A simplified model of a manual transmission. Reverse uses 3 gears, whereas forward gears use 2 gears each

    But it's not all about the noise

    Spur gears have two main advantages over helical gears, one of which makes the car manufacturer's accountants happy, whereas the other has made them popular in motorsports. First of all, spur gears are cheaper to manufacture, as they can be cut out much more easily than helical gears. This difference might not be much, but if you multiply it by millions of manual transmissions being made every year, it turns out that the possible savings are more than marginal.

    Secondly, spur gears do not produce a force called axial load when coming into contact, unlike helical gears. Without going into details, when a car has a lot of power, this load has to be compensated, which adds weight and complexity, which isn't ideal in a competitive world. Using spur gears for forward gears solves this issue altogether. And the noise? Who cares, it's a race car!

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    Quick conclusion

    In essence, whining you can hear while reversing a manual car results from straight teeth of spur gears in a transmission colliding with one another. This ingenious solution to a problem of reversing a car might be noisy, but it minimizes the complexity and cost of building a car. And you don't use the reverse gear that often anyway.

    If you would like to learn more about this, I recommend you read this article and that one as well, as both of them helped to write this piece. You can also check out my article on reverse racing. In any case, I hope that now you understand this topic at least a little bit better. Cheers!

    Writing a more explanatory article was a new challenge for me. Still, I hope you enjoyed it. If you did, you can leave a like and become a follower. Till next time!

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    Comments (8)

    • Thank you. Great article and great cutaway of manual transmission.

        9 days ago
    • Honestly I’m not fussed about the noise, but that question has been bothering me for a while

        9 days ago
    • Straight-cut gears; and/or smallness of the gear itself.

      Nor does reverse have a synchromesh system

        9 days ago
    • Synchro mesh gear box it’s called there’s 2 other types found mainly on old cars but I forget what their called

        9 days ago
    • Straight-cut gears do not have the tendency to force each other along their shafts, away from one another, and create end forces. This is good for the lowest gear in the gearbox because it multiplies the stresses on the components the most. It's easier to make as well. Interestingly, on rally cars and racing cars, the gears are all straight-cut (which is a bit noisy in every gear except direct drive through the 'box) because the energy is driven directly between gears without side forces using energy up by way of friction. All gears have a funny rounded profile, which geometrically makes them ROLL against each other and not slide, which is another ingredient that a straight-cut gear enjoys better utilisation of than a helically cut, conventional gear. Helical-cur gears (normal spiraly cut gears) are quieter because they spread their gear contact period across the same time period while their neighbouring cogs are engaged and dampen out the pulsations caused by straight-cut gears, the whining noise.

        9 days ago

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