5

Ultimately - I've been driving two cars - an old 2003 1.6l 4 door type affair, and a newer Vauxall Corsa 1l.

Even on the smallest of inclines (hills) - the old 2003 car will stall if you lift the clutch up slowly without any gas (therefore you always have to be on and off the handbrake on any type of incline, even something really negligible).

The Vauxall can pull up some even quite steep hills by just bringing the clutch to bite, taking your foot off the brake, and slowly bringing the clutch up. No need to use the handbrake, no need for any gas.

Why is this? Are all modern cars like this? Is this a Vauxall specific thing? Having this makes driving so much easier.

  • 1
    More torque? My 'Vette would roll up slight inclines in 5th gear, from a stop, when feathering the clutch with no gas. – 3Dave Dec 15 '16 at 16:12
  • Gear ratios and engine torque. I would suspect the Corsa has a larger ratio (transmission and rear end total) and possibly the newer engine provides better power. To confirm if these are why a complete specification of both cars would need to be known and compared. The granny low gear in my Landcruiser and my 48 Chevy pickup will just about climb a wall if they could get traction. – spicetraders Dec 15 '16 at 16:20
  • I wonder if that's related to something like the "hill start assist" some of them advertise? From the Wikipedia page: "The system engages automatically when a gradient of 3% or more is detected; it then acts to hold the car stationary for two seconds after the brake is released giving the driver time to apply the throttle." – s1ns3nt Dec 15 '16 at 17:27
6

You've exposed a combination of two things: high torque at low engine RPMs + high mechanical advantage => forward motion without requiring additional throttle (where "high" is defined to be "high enough to move the car").

There's another way to investigate the same problem that is likely easier on your clutch. First, get moving in low gear at a low but moving speed pointed up the hill in question (no clutch pedal required). Now, take your foot off the throttle: is the engine capable of moving the car up the hill at idle? If not, you'll slow and the engine will start to stall (that's the time to use the clutch pedal so you don't lug the engine).

Using this technique makes it a little more obvious what's going on:

  1. At idle, your engine is putting out an effective minimum level of torque. This is what's necessary to spin the transmission and, thereby, rotate the wheels to move the vehicle.

  2. Your transmission acts as a multiplicative coefficient on the torque output of the engine. In low gear, this coefficient is quite small (AKA a low gear ratio) so that many revolutions of the engine amount to only a single rotation of the wheels. This gives the engine + transmission combination a mechanical advantage, allowing the vehicle to do more work for less throttle (remember that idle is still not zero throttle).

So, using my 2004 WRX as an example:

  1. At low revs, the turbo is barely moving. As a result, the engine is effectively producing only as much as a base model Impreza from that year (i.e., not much).

  2. However, my first gear is a fairly low gear ratio. This means that I can get my car up a shallow slope into the garage from a dead stop without using the throttle pedal at all.

The end result is that, when I was teaching my son how to drive my car, I didn't have him use the throttle pedal at all for the first day. We were idling around in third gear without difficulty.

  • Do all modern cars generally have more torque then? (i.e. the ability to move up a hill with just the clutch) – PnP Dec 17 '16 at 21:47
  • @PnP, no, it totally depends on the vehicle and the hill. Most driveway is a shallow slope. The driveway a few doors down is quite steep. There are plenty of cars that will climb my driveway easily while they will stall trying to idle up the steep driveway. – Bob Cross Dec 18 '16 at 0:09
4

Part of the reason could be the engine management system : as you increase the load slowly it is increasing the engine power to keep the idle speed within limits. If you increase the load too fast, then it stalls the engine.

0

Some manual vehicles have an "anti-rollback" feature, this may be what you are experiencing. Here's an excerpt from wiki:

In layman’s terms, the modern hill-holder function works by using two sensors, in concert with the brake system on the vehicle. The first sensor measures the forward-facing incline (nose higher than tail) of the vehicle, while the second is a disengaging mechanism. The 1930s-1950s NoRoL used a ball bearing as a check valve in the hydraulic brake line; when the car was on an uphill incline, the ball rolled back and blocked the brake line - when the car was level or facing downhill, the ball rolled away, leaving the line free. The clutch linkage slightly dislodged the ball when the clutch was released, enabling the car to move away from a stop.

  • 1
    Say, this could afford to be expanded a bit... – anonymous2 Dec 15 '16 at 21:28

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