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I watched this video regarding double clutching, https://www.youtube.com/watch?v=tescWzTlGQ0, and while I understood the general principle, I’m confused as to why the different gears would start to spin at the same rpm when in neutral.

When you’re in gear, the gears are all spinning at different rpms, but when you’re in neutral why/how would the gears now all spin at the same rpm?

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  • Welcome to Motor Vehicle Maintenance & Repair!
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Aug 1, 2023 at 9:02
  • IMHO, this question is on-topic. It is not about driving technique, but about how do older manual transmissions work.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Aug 1, 2023 at 9:03
  • Virtually every car, pickup truck and suv equipped with a manual xmission have synchronized gears, eliminating the need to double clutch. It was mentioned within the first minute of the video. Unless the presenter describes the physical differences between non synchro xmissions (very old, possibly pre 60s cars and many existing over the road trucks hauling cargo, it's easy to be confused if you never tried shifting a non synchro xmission where gears will grind ($$$). Most truckers are experts without double clutching by matching engine rpm to each gear to slip into gear without grinding.
    – F Dryer
    Commented Aug 1, 2023 at 14:04

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You missed a very important part of it. The gears don't just start spinning at the same speed. In the video, he talks about "blipping the throttle" while in neutral, clutch engaged (pedal out). When you do this, the engine, through the input side of the transmission provides the means for the next gear to become synchronized to the dog on the output shaft, which allows the shift change without grinding.

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  • Hi, thanks for responding!I think my confusion is from the fact that the synchros are needed because the gears spin at different speeds despite them all being connected to the input shaft. Does this mean that when you’re in neutral and revving the engine, the gears are all spinning at the same rpm as the input shaft. I’m kind of assuming that when in neutral the gears spin at the same rpm as the input shaft because I don’t really understand how someone would be able to precisely match the rpm of the gears when shifting in a car without synchros if the gears spun at different rates.
    – Allan Paul
    Commented Aug 1, 2023 at 17:47
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    @AllanPaul no, the gears do not all spin at the same speed in neutral. That's the point: the input is not connected to the output in neutral and they may spin at different speeds or not at all. It depends which side of the gearbox you are talking about, and whether the clutch is engaged or not, and whether the vehicle is moving. If the vehicle is moving and in neutral, when you engage the clutch, you can match the speed of the gearbox input (governed by engine rpm) to the output (governed by road speed). That's the idea of blipping in neutral.
    – Weather Vane
    Commented Aug 1, 2023 at 19:35
  • @WeatherVane but if the gears are not spinning at the same speed, how is the driver able to determine what rpm they should rev the engine in order to minimize the work on the synchros?
    – Allan Paul
    Commented Aug 1, 2023 at 22:51
  • @AllanPaul - Remember, the transmission you pose which requires double clutching, doesn't have synchros. Synchros in a newer transmission are what causes the gears to match speeds with or without driver intervention. In your transmission, it comes down to touch and feel of the driver. Once you get used to what you're doing, it becomes muscle memory.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Aug 1, 2023 at 22:56
  • @AllanPaul by experience. When changing down (say 4 to 3) you need more engine revs for that road speed. When changing up, you need less, so with good timing you don't need to blip the throttle in that case, just let the revs drop.
    – Weather Vane
    Commented Aug 1, 2023 at 23:12

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