As the title says, I know that different shifts mean different gear sizes bein used, but I don’t understand why it makes you unable to start moving the car. I have been able to start a couple of cars on the 2nd shift as an experiment and I understand that I could damage the car and I do it just once for testing purposes but I don’t understand why I cannot do so on other shifts. To clarify, I mean start as in start moving the car and not just turning the car on. Thanks
In: Engineering
Of course you can. It’s extremely useful on ice or snow. I was driving a car that even recommended way to start was the second gear as the first one was not synced and manual suggested to use the first gear only under extreme load or starting uphill.
Ever rode a bicycle uphill in too high of a gear? Same thing.
Your engine idles at a certain speed, let’s say 750rpm, for various reasons. If the engine goes much below that, it will stop spinning, called a stall.
When you start engaging the clutch from a stop, your wheels are not moving, so the gears aren’t either. What that means is that the clutch must slip against the engine flywheel until they are both moving at the same RPM. This slipping is what brings the clutch (and by extension, the gears, wheels, and the rest of the car) up to speed.
In first gear, idle RPM may only be about 5kph / 3mph because it’s a very ‘easy’ gear. It doesn’t take a whole lot of energy to accelerate the whole car up to that speed, so the clutch doesn’t have to do a whole lot of slipping.
In your highest gear, maybe 6th gear, idle RPM may be more like 80kph / 50mph. That takes a *lot* more energy for the car to get going, so the whole time you’re doing that the clutch is slipping. This slipping generates heat from friction and wears away clutch material so it’s very Not Good for your car.
Instead, it is Much Better to get up to speed one gear at a time, slipping the clutch for the first gear (because, again, the engine can’t go any slower than 750rpm) but then adjusting RPM during each subsequent gear change so that theres minimum slipping.
It’s a bit like pushing your car up a ramp. To get to the same height, you could either use a long, flat ramp, or a short, steep one. The flat ramp is like a low gear – you use a small force over a long time/distance. The steep ramp is like a high gear – it’s a shorter time/distance, but you need a much larger force to even move at all.
Get on your bike and put it in the highest gear (largest gear at pedals to smallest gear at wheel). Now stop pedaling. Now try to pedal again and (if you do get the bike moving) try to accelerate quickly. That is why.
The speed required to keep your car above idle speed at a high gear like 5th gear would be around 20mph. So essentially your engine would drop to so low of an RPM that it would stall. It’s possible to get around this by riding your clutch. But that would be putting excessive wear on an expensive friction based wear item.
Well, you definitely can. It will be slow and it will probably stink up your car because of the clutch abuse it takes. But if you’ve got enough experience in a manual, it’s not that hard.
I have deliberately driven off in seconds or even third when I’m on snow and ice. Low torque can be your friend.
The gears refer to gear ratios, giving high mechanical advantage.
Low gears give a high mechanical advantage, so force from the engine is multiplied by a larger number, and the high force turns the wheel fewer times.
High gears do the opposite, force from the engine gets multiplied by a lower number, so there isna smaller force turning the wheels more times.
High gears are more efficient at high speeds because the engine doesn’t need to spin extremely fast to keep up with highways speeds. This is why your RPMs drop when you shift to a higher gear, the engine doesn’t need to spin as fast to maintain that high speed.
Low gears mean the engine doesn’t need to spin extremely slowly to keep up with slow wheels. Stopped is very slow speed, and it takes more force to start the wheels turning, but the engine can’t supply that force because it’s being reduced by the transmission.
Are you familiar with levers?
Put two bits of wood on the ground, one short, one really really long. Lift the short one from one end. Now do the same with the long one. The short one is easier right? The long lever has all that extra weight to lift and you’re so far from the axis of rotation. But as the lever gets more upright, it becomes a little easier right?
Gears are like round levers. The bigger the gear, the more torque (angular force) it takes to move them. When you start a car in high gear, you are trying to lift the long lever with an engine designed to only lift the small one from zero. You physically don’t have enough torque to move the higher gears. However at high speed, you don’t need as much torque because you’ve only got to “lift” the difference in distance between the levers, not the full from zero.
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