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I'm thinking of 1st gear (for manual transmission car) as equal to the Smallest Chainring (for mountain bikes).

I have drive a mountain bike, and while in flat/straight road, I don't switch my chainring to the smallest one at all... ONLY when I'm in UPHILL.

I'm usually in 2nd-4th Chainring all the time in flat/straight surface.

Now for manual transmission car, in a flat/straight surface (not uphill):

  1. Can I switch to 2nd gear all the time?

    • Let's say I'm approaching in an intersection (while I'm in gear 2) and I need to stop completely,

  2. Now I'm in an intersection, so I'll make a complete stop.

  3. let's say I only need a complete stop for 15 seconds, so I switch my gear to neutral and wait for the GO signal.

  4. Now, the GO signal turns on,

...If I came from a complete stop, is it A MUST to switch from neutral to 1st gear before switching to 2nd gear (instead of neutral to 2nd gear and skip 1st gear) if I want to accelerate again?

I'm thinking that... for manual transmission, gear 1 is only for uphill.

If I can accelerate in a Mountain bike from complete stop to 2nd chainring  (second one after the smallest), then why not with manual transmission (neutral to 2nd gear)?

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    You can pull away in second - you just slip the clutch more... That’s fine - you get to replace it sooner : it’s your money...
    – Solar Mike
    Mar 27, 2019 at 6:20
  • I had a car that could (just) pull away in 5th. I know, because I tried. Loads of clutch slip. Wouldn't do it again.
    – Rory Alsop
    Mar 27, 2019 at 10:03
  • With a conventional car, you are just putting more wear on the clutch for no good reason. On the other hand, I know some guys who have a 1910-vintage steam powered truck, modified a bit for modern roads and modern tires. That has only two gears, and you never change gear while moving (because there is no clutch at all, a steam engine doesn't need one.) You just decide before you start whether you want to do 0-40mph in first, or 0-80mph in second. (And you can also 80mph in reverse if you want - no reverse gears, the engine just runs backwards).
    – alephzero
    Mar 27, 2019 at 10:25
  • One key here is that your legs don't have a minimum RPM and they can provide a great deal of torque from zero RPM. A car engine doesn't work like that. It will stall below a certain RPM and have very little torque when it it close to that stall speed. All that means slipping the clutch until the car is moving a certain speed.
    – JPhi1618
    Mar 27, 2019 at 15:34

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It's not really a "must" - but it's a very good idea.

I'm thinking of 1st gear (for manual transmission car) as equal to the Smallest Chainring (for mountain bikes).

This is where you're going wrong, in many ways the basic purpose of gears on a bike and a car are the same but there's some important mechanical differences between the way they operate. When you've stopped on a mountain bike your feet aren't pedaling any more, and as you pull away from stationary the "pedaling revs" build up from zero with the wheel revs (obviously in whatever ratio the gear you are in provides)

In a car when the engine is at "idle" it is still turning over hundreds of times a minute and when the transmission is in gear the input shaft of the gearbox will be turning at the same rate, which is then reduced through the magic of gear ratios to give a certain wheel (and hence vehicle) speed but by good old physics that wheel speed isn't going to be zero! Which is why you need to slip the clutch in order to travel at a speed that's lower than what the engine RPM translates to - and of course the higher the gear you are in the greater the wheel speed for a given RPM.

So if you want to pull away in second gear you're going to have to slip the clutch more than you do in first (otherwise you're going to stall), and this means more wear on the clutch.

Doing this once or twice isn't going to be a big deal - and there's circumstances where you might specifically want to do this. Doing it every time however is going to result in significantly quicker rates of wear on your clutch and for no benefit - acceleration is generally reduced more than you are going to save by skipping a gear change (ever seen anyone suggest doing a launch for a 1/4 mile or 0-60 run in second gear?)

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  • The only "good" reasons for starting in 2nd are (1) driving on snow, if you need to limit the engine torque to wheels to avoid wheel spin, and (2) starting downhill, if the road is steep enough so the car will accelerate quickly up to say 10 mph with no engine power at all, and you can just release the clutch a bit late in 2nd gear rather than slipping it.
    – alephzero
    Mar 27, 2019 at 16:08
  • @alephzero snow (or similar low traction surfaces) are pretty much what I had in mind. The downhill one works too - not so much that you need to do it more that it can be done without the usual downsides.
    – motosubatsu
    Mar 27, 2019 at 17:15

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