Monday, December 10, 2012

How To Not Crash In Snow (Manual Transmission)

More than 13 inches of snow have fallen in the Twin Cities in the past 24 hours. As the driver of a manual-transmission automobile and as a seasoned veteran of catastrophe, I assembled this simple guide to the physics of snow driving as a gift to my fellow man. May it teach you the way.

I'll try to start from basics to be thorough, but I will also try to cover the topic rather quickly, without a million words.

There are three pedals on your manual-transmission vehicle.
When pressed, the furthest right pedal increases the speed of your engine.
When pressed, the middle pedal slows, limits, stops, or prevents the rotation of your wheels.
When pressed, the left pedal disengages the connection between engine speed and tire speed. If this pedal is released, the engine and the wheels will attempt to move in unison according to whichever gear is selected, or they will continue to move independently if your stick is in neutral.

To maintain traction, you want the speed at which your wheels are rotating to match the speed at which you are covering ground. If your car is moving more speedily than your wheels are rotating, you are skidding or sliding. If your car is moving more slowly than your wheels are rotating, you are spinning your tires. While it's obvious in either case that you have bad traction, both of these things actually make your traction worse; in other words, they prevent you from regaining traction and control of your vehicle. The best way to maintain or regain traction is by having your wheels rotate at the speed at which you are covering ground.

Rule 1: (If you don't have Anti-lock brakes) Your brake should be used less in the winter. More accurately, your brake should only be used gently and at low speeds in the winter. This sounds like weird advice, but I'll explain why: the brake's only function is to slow or stop the rotation of the tires. Abruptly stopping the rotation of the tires, or even slowing it beyond a certain amount on a slippery surface, will change you from rolling to skidding. You have less traction and less control. Slowly decreasing the rotation speed of your tires will slow you down more quickly than quickly decreasing the rotation speed of your tires. NOTE: If your car has ABS (anti-lock braking system), which started coming standard on cars that are too new for college students to drive, this does not apply, because the ABS is designed exactly to keep your wheels rolling while you are slowing down rather than locking them in place when you slam on the pedal. Pumping the brake on older cars is a way to do the same thing, and I would recommend that in cases where your instinct tells you to slam on the brakes.

Rule 2: The clutch should only be used when shifting gears or when shifting into and out of gear. This is sort of a rule year-round, but it can be broken during the warm months when I like to take it out of gear and coast at times.
If you worry about the clutch less, it maintains the correlation between your engine speed and your wheel speed. If you noticed before, I mentioned that your accelerator controls your engine speed, but then all I've mentioned so far as being important to skidding is your wheel speed. Avoiding needlessly taking it out of gear or coasting down hills, for example, maintains the accelerator pedal's relevance to the problem at hand.

Rule 3: Use your accelerator to decelerate. Or, perhaps, decelerate by un-using your accelerator.
For the wheels and engine to move together, one of them has to be controlling the speed of the other. When you're not causing the engine to run at a certain speed with your foot on the accelerator, the momentum of your car and the rotation speed of your wheels gets transferred back into the engine rotation through the same mechanism (in reverse) that causes the engine to increase your wheel speed. If your car is in gear and moving and you let off the gas pedal, the car will slow down.
Now, letting off the gas pedal slows your wheels down much less abruptly than a tap of the brakes, and therefore avoids losing traction by going into a skid. This is good, but if you actually want to stop anytime soon, you may tell me that it is simply too much less abruptly.

Rule 4/Life Skill: Downshifting
Downshifting is intentionally using the momentum of your car, rather than pressure on the accelerator pedal, to increase the RPM of your engine, with the goal of draining some of your wheels' momentum in order to achieve a skidless deceleration.
Very simply, each gear will make your car travel at a different speed, even if you keep pressure on the accelerator exactly constant. As an example, if I run my engine at 2000 RPM, I will be going about 6 MPH in 1st gear, 16 MPH in 2nd, 26 MPH in 3rd, 36 MPH in 4th, and 46 MPH in 5th. Regardless of the mechanics or details of this, I'm sure you have to be familiar with this occurrence in order to even drive a manual, so I'll stop elaborating on this specific aspect of it.
If your foot is off the accelerator, your engine speed naturally wants to drop to below 1000 RPM. (Mine usually idles at 700-800.) However, you usually want to drive in the range between 1800 and 2500 RPM for maximum efficiency.
So here's an example: I'm travelling at 26 MPH in 3rd gear, with my engine at 2000 RPM. I'm 100 feet or so from a Stop sign. Rather than beginning to brake, I shift downward into second and let off the accelerator. As I release the clutch after my shift, all of a sudden, I'm going 26 MPH in second gear, and my engine, which wants to drop to 700 RPM, is instead forced up to 3200 RPM. Suction is generated in the combustion chambers in the engine (very relevant to the process, but not necessarily important to understand), and the engine speed drops fairly quickly, slowing the wheels along with it because their fates are bound until I hit the clutch again. By the time the engine drops to 1500 RPM, which has only taken a couple of seconds, I'm down to 12 MPH and I'm not skidding. Then I can try my luck at downshifting to 1st or I can just calmly pump my brakes to a smooth stop. Heck, at that point, I could even slam them and probably stop before much of my car slid slightly askew into the intersection.

1. Only shift down one at a time. If you're going 50 in 5th gear and you drop to 3rd and your engine is uncomfortably and unexpectedly forced up to 4000 RPM, that's too much and you will skid.
2. As a general precaution against abruptness, always re-engage the clutch with gentleness and tact. Make sure all parties are comfortable with what you are about to do before dropping the full weight on them.
3. Decelerate before turns, not in the midst of them. Try to gently accelerate out of turns, so that your front wheels are pulling you through the end of the turn and toward your destination. NOTE: Do not do this with a rear-wheel-drive car.
4. Down hills are easier to navigate than up hills. Roll down steep ones in 2nd or 1st gear and with your foot gently holding or pumping the brake, being sure that your wheels are indeed rolling, but only ever so slowly. NOTE: Do not use this strategy for rear-wheel-drive cars. It is not advised to drive on snowy hills at all with RWD.
5. In the unfortunate instance that you may be required to stop on an up-facing hill, use your hand brake in place of your foot brake to avoid the slight roll-back experienced at take-off. Put the shifter in 1st, bring the clutch right up to the point of engagement, coax the accelerator downward with only one toe, and then release the hand brake so that the first movement you make is forward and upward. NOTE: Please don't do this if your car is rear-wheel-drive. Seriously, just drive backwards in the winter if that's your only option.

I hope that these tips improve your road safety. Failing that, I hope that a cute girl offers to help push you out of the snow bank in which you find yourself.

Best wishes (with specifics mentioned immediately above),


P.S. I haven't blogged consistently since 10th grade, and I've felt rusty every time that I've written on here thus far. Oh well.