The Snow Driving Uphill In Reverse Debacle: Myth or Fact?

driving
saab
winter
9-5

#1

I live on top of a steep hill, and during the most recent snow storm here I got stuck about 5 times, causing a 3/4 mile trip to take about 3 hours.



I have a front wheel drive car. On the final section of incline, there was only about 1/2 inch of snow, but I could not get up the hill, and the car kept sliding backwards and to the side, nearly putting me off the road. Somehow in the back of my mind I remembered someone saying that if you put a front wheel drive car in reverse, it will shift the center of mass over the front wheels. Sure enough this worked, although just barely. I have directional tires, but even in the reverse direction, the difference was obvious.



A friend of mine says that the mass is not really shifted in this situation, and I’m inclined to agree. When carrying a couch down stairs the weight does not very noticeably shift to the bottom. My best guess is that the phenomenon has to do with the pre-defined weight distribution of the vehicle. Since the engine is behind the front wheels on most FWD cars (except maybe a saab 900) putting the car in reverse allows us to have that center of weight in front of rather than behind the front wheels.



So what does everyone think? What are the physics behind all of this, and did I really benefit from going up in reverse?


#2

"When carrying a couch down stairs the weight does not very noticeably shift to the bottom. "

You must have some very gentle stairs. I notice a major shift in weight. The steeper the slope, the greater the shift. If you had a vertical staircase, 100% of the weight would be on the bottom, right? That’s why backing helps a fwd car, puts more weight on the driven wheels.


#3

Even without talking about suspension loads (in reverse the front suspension is loaded - in drive, the back suspension is loaded) turning the car around and backing up the hill puts the center of gravity closer to the drive wheels, which is a good thing.

But you have to be careful with this technique, as the back of the car is now unloaded suspension-wise, and the CG is farther away from it, which means it’s going to be more willing to step out of line.


#4

why play with the weight thing. Just get real winter tyres and get the traction you need. BTW Traction when you are trying to stop or keep from sliding off the road is far more important than worrying about getting up and hill. If it does not want to go up the hill, it likely is not safe trying to do it.


#5

My vote is for a good set of snow tires.


#6

An old Frenchman, or to be accurate an Aerican of French descent had a Volkswagen Rabbit and he told me that it could back up in snow a lot easier than it could go forward. The old Frenchman, he knew better by golly up there. It worked for you too.

I had this 85 Escort and it was good in slippery conditions with really worn out front tires. I never noticed whether it was better in reverse or not.


#7

The physics of carrying a couch downstairs is different from road vehicle propulsion. Imagine you’re looking at a car’s left side and it is propelled forward. A force below the center of gravity (CG) is applied toward the front of the car and it is torqued counterclockwise. A higher force on the rear axle than the front is necessary to balance this torque. Otherwise, it pops a wheelie like a motorcycle. When going in reverse, the propulsion torques the car counterclockwise. The front axle needs to push up harder than the rear. When you’re driving in reverse on slick stuff, this extra upward push on the front tires helps them with traction.

When carrying a couch down a flight of stairs, you’re applying a force more or less in line with the CG of the couch. No distance b/t force and CG => no torque

Jump to the middle to watch this experiment by Mythbusters


#8

I vote for fact.

The degree of weight transfer depends on how high off the ground the vehicle’s center of mass is and how short the vehicle’s wheelbase is. It’s a lot easier to do a wheelie on a motorcycle if the rider stands on top of the seat than when he’s sitting down in the seat because he has raised the vehicle’s center of mass by doing so. Also, off road bikes with their short wheelbase and high ground clearance are ridiculously easy to wheelie while long low cruiser style bikes are very difficult to wheelie. Top fuel dragsters are long and low for a reason.

When carrying a couch down a flight of stairs, if the couch’s center of mass is higher above the ground than the lifting points, then the weight will shift to the man on the bottom. If the lifting points are on top of the couch and the center of mass is below the lifting points, the opposite will occur.


#9

You’ve already explained the physics. It transfers more of the weight over the drive wheels. Added weight means added traction.

Yup, you benefitted.


#10

TSM, you did it again. This thread will be having its 6th birthday soon.


#11

What is this, a gang initiation activity?
I had a bad day. I think we’ll all live.