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I know how to drive and also that you have to be in 1st (lower the better) gear to move the vehicle from a full stop, but why is that?

If higher gears mean faster wheel rotation why doesn't the vehicle move from a full stop in other higher gears?

I'm not a mechanic or vehicle enthusiast. I am just curious. sorry if the question is trivial.

Thanks.

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It is possible to get a vehicle to pull away in a high gear, indeed it is possible to get a sufficiently powerful vehicle to pull away on a flat or downhill surface in top gear. However, the way you have to do this is to open the throttle quite wide, pile on the revs and then be very slow with the clutch, effectively slipping it until the vehicle is moving at a speed when the clutch can be fully released.

This is typically accompanied by lots of noise, some smoke, the very distinctive smell of burning ferodo from the clutch-disc and then chugging and a near stall when the clutch pedal is fully released.

Starting a vehicle in motion this way produces a lot of heat for the cooling system to deal with, a lot of wear on the clutch and associated components such as the transmission, uses a lot of fuel and requires quite a large gap in traffic.

One example of this would be the 1994 Barcelona Gran Prix where Schumacher managed to secure second place on the podium with a gearbox fault which rendered his race car stuck permanently in 5th gear for most of the race (including a pit-stop where the car set off from stationary).

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  • upvoted because I have started in 2nd gear, without smoke, chugging or damage. If one is careful, one can indeed operate from second gear, but it's certainly not recommended practice. – fred_dot_u Feb 15 '19 at 14:36
  • @fred_dot_u only second gear? Modern cars are rubbish. There was a famous two-part challenge in 1911 between Rolls Royce and Napier - a race at the Brooklands track, and an observed drive from London to Edinburgh in 4th (top) gear only for the entire trip. R-R won both challenges measured on speed and fuel consumption. – alephzero Feb 15 '19 at 15:46
  • I've personally had an incident where a broken gear linkage left me stuck in 3rd (in a 4-speed car) and I was able to drive home slowly in light traffic using only 3rd. – Steve Matthews Feb 15 '19 at 16:53
  • @alephzero, it was a '51 jeep with a flat head six! – fred_dot_u Feb 15 '19 at 23:15
  • @alephzero this depends on the car, obviously. A modern Bentley can probably start in top gear (difficult to verify though with only automatic gearbox options), an Austin Seven can't. A 1911 RR has a much shorter top gear than a modern car, and a really large, low-revving engine that makes it well suited to such antics. – Hobbes Feb 16 '19 at 14:56
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Simply that the engine doesn't have enough torque in higher gears.

In low gears the drive train has lots of torque but a low speed. In high gears the drive train has low torque but higher speed.

High torque is needed for pulling away. The same as on a pedal bicycle.

While some of the other answers are correct about being able to pull away from stationary in other grears, there are a few things to consider -

Clutches are designed to handle a specific torque. The clutch will be chosen for the car to be as small and light as possible but able to handle the usual torque requirements for the car. Pulling away in a higher than normal gear could exceed the design specifications.

1st gear on a car will be chosen to allow the pulling away while doing a hill start. 1st gear will be able to start a car moving on a much steeper hill than 2nd gear could.

Attempting to pull away in top gear will probably require more torte than the clutch is able to transmit to overcome the car’s inertia, so may well be impossible.

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  • Sorry, had to downvote for two reasons. Firstly, the answer is entirely incorrect and directly contradicts Newton's 2nd law. Secondly, I had 9001 reputation and wanted to step back to a nice round number. – juhist Feb 17 '19 at 9:40
  • @juhist Suit your self. I think you are missing the point of the question. If the engine had the torque required to pull away in 5th without causing damage then I think cars would not be designed the way they are. Don't worry I wont down vote your question and spoil your round number. – HandyHowie Feb 17 '19 at 10:37
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The gearbox in a car does 2 things:

  • it changes the rotation speed of the wheels: first gear often steps down by ~4:1, so 4 revolutions of the engine are needed for 1 revolution of the output shaft. Top gear is ~1:1 (output shaft spins at the same speed as the engine). (1)
  • this speed change has the consequence that the torque (i.e. the force that rotates the shaft) is also changed. In first gear, the output shaft torque is 4x the engine torque. In top gear, it's 1:1.

So in top gear, the force driving the wheels is much smaller than in first gear, often too small to reliably move off in top gear.

To complicate things more: an engine has a lowest possible speed (the idle speed), often ~1000 rpm. So when you move off from a full stop, there is a big difference between the engine speed and the speed of the wheels (0 rpm). To bridge this difference, we use a clutch, which allows us to gradually apply power to the wheels until the speeds are matched.

If you try to move off from a standstill in top gear, this difference is much larger (4x larger), so you have to slip the clutch for a long time until you can match speeds.

1: yes, for the purposes of this question I'm going to ignore overdrive etc.

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It does.

I have successfully started moving in 2nd or 3rd gear on a manual trasmission vehicle. 2nd gear is reasonably easy to do, you just need to release the clutch later. 3rd gear will smoke-test your clutch.

The reason you don't do this often is that the engine has a lowest possible RPM at which it produces useful torque.

I disagree with the upvoted answer that the engine doesn't have enough torque. Newton's 2nd law says F = ma, i.e. force is mass times acceleration. If you can accelerate in 5th gear from 100 km/h to 120 km/h (ok, it's slow but you can do it), you can accelerate in 5th gear from 0 km/h to 20 km/h using the same force/torque in the same time.

The problem is that the lowest possible RPM at which the engine produces useful torque is limited to something like 1000 RPM. So, accelerating in 5th gear from 0 km/h to 20 km/h slowly would totally kill your clutch. You'd have to use the clutch the entire time of the slow acceleration.

If you are driving on totally flat and smooth ice without studded tires, it actually might make sense to start on another gear than the 1st gear. The torque of the 1st gear is just excessive for the situation.

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  • While accelerating from 0 to 20 may sound possible even if done very slowly, the inertia of the car it will probably require more torque to be transmitted by the clutch than it is designed to do. It will just slip and heat up. – HandyHowie Feb 17 '19 at 12:18

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