Vehicle Handling Dynamics

[legion]

It snowed on Tuesday. We still have a ¼" of snow on the street after plowing so it is easy to break your drive wheels free if you are so inclined.

I was driving my F150 to work this morning, and I did not have the 260 lbs. of sand in the back I normally do when it is snowy out. I was coming up to the 90º turn at the end of my street. Since the truck has a tendency to understeer in snowy conditions, I usually give it a little throttle (the tiniest bit) around this corner to break the rear wheels free and bring the back end around faster. (I'm not going faster than 15 mph during this maneuver.)

When I do have sand in my truck, the handling is very neutral, much like (but obviously no where as good as) my 951.

So this morning I approach the corner, give throttle, and rather than bring the back end around neatly, I grossly oversteer and end up doing a 540º. As there are parked cars on either side of the cul-de-sac that leads into the 90º turn, I give neither throttle nor brake (steering is useless) and just let my truck rotate until it comes to a stop facing the wrong way.

I was really surprised that taking 260 lbs. out of the back of a 6000 lb. truck affected the handling so drastically.

Can any of you vehicle handling junkies explain to me why?

[notfarnow]

I've noticed the same thing in RWD vehicles... but especially in trucks. Without that weight for traction, the rear never "grabs"... so when it breaks loose it's GONE.

I generally dislike driving pickups in the snow... not enough weight in the back. Even my little suzuki is useless in 2wd. In 4wd it will go anywhere but it's an understeering DOG. If it starts to "push" in a turn, I sometimes snap it out of 4x4 and give it a shot of gas to get the rear to swing out a bit.

I've been told 944s are EXCELLENT in the snow... very nicely balanced and good traction w/ decent snows. Not great in the deep stuff though. That true, or do you keep yours dry?

[legion]

I keep mine dry in the winter. I find no joy in scraping snow off of three vehicles in the winter instead of two. I drop liability/collision coverage from November - March and pay all of $10 for comprehensive during that five-month period. There are several 944 owners in my local chapter of PCA that drive their cars year round and I've never heard a complaint from them.

I'm seriously considering getting rid of my F150 and getting a Chevy 1500 with 4WD.

[oldE]

An unladen 2wd p/u has probably only about 33% of the vehicle weight over the drive wheels. My F150 2wd has a cap and about 13 4x4x8 posts for traction, but when those are not in, snow can really make you go slow and easy.
The computer on the F150s carries a fast idle condition (for warm-up purposes, I suppose) for what seems to me to be a heck of a long time. Fast idle means the truck will creep if you take your foot off the brake. When you entered the corner, there was obviously enough touque to break traction under those conditions and, once traction was lost, the wheels were free to spin on the snow, removing ANY chance you had to 'catch' the rear end. It may have done this even if the idle was normal.
Back in the days of atomatic chokes, I was taught to put in the clutch in a manual transmission vehicle, or shift to neutral with an automatic, if there was going to be too much torque fed to the rear wheels during a manouver (at parking lot speeds).

Les

[Dantilla]

Scoop a bunch of snow into the bed. More weight over the rear, and it removes itself in the spring.

[Bill Verburg]

An unladen p/u has a very heavy front weight bias. This causes an inherent understeer(the front end sticks better than the rear). When the steering wheel is turned the natural tendancy will be to continue going straight. If you had a full load in the back the reverse would be true, inherent oversteer(the back has less grip than the front and will want to swap ends)

Weight in the back does 2 things
1) reduces inherent understeer
2) increases polar moment which causes thing to happen more slowly, any handling issues are easier to catch.

[Nostril Cheese]

This has me thinking..

If you wanted to increase oversteer on a FWD car, you would simply install a larger rear sway bar, right?

[Bill Verburg]

Originally posted by Nostril Cheese
This has me thinking..

If you wanted to increase oversteer on a FWD car, you would simply install a larger rear sway bar, right?

right

[svandamme]

Originally posted by notfarnow

I've been told 944s are EXCELLENT in the snow... very nicely balanced and good traction w/ decent snows. Not great in the deep stuff though. That true, or do you keep yours dry?

skinny tires.

the skinnier the better

any 944 with 205+ front and 215+ back will be just as usefull as any other car FWD or RWD with same tire size, eg not much

but get a 924 or 944 down to 185 all round, and then it's a different game...even better if you can get a RWD brake thing setup for your right hand...

[Superman]

Yup. The narrower the better, in snow. Funny thing, since on almost any other surface the opposite is true....the wider the better. But in snow, and probably mud also, your best tire is as large a radius as you can get, and as narrow a track. If you could mount mountain bike tires on a pickup, that would be ideal.

[JSDSKI]

Entered corner and added throttle on snow - so, broke rear tires loose just as front end was moving towards apex and rear end away from apex. That set up the car's slip angle. Then added heavy counter steer - broke front end loose and set up a tighter spin around the bigger mass in the front of vehicle. The snow just made the event easier (slower speed) to start. Lack of weight in rear - changed rear grip - easier to break traction with throttle.

My 2 cents....

[speedracing944]

Traction, the amount of force which can be applied before slippage, is a function between mass and coefficient of friction. At the same cofficient of friction the tire which has more mass pushing down will have more grip than the one with less mass.

Speedy