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Re: Its all about snow tires... (archive)

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Posted by Peter Warton on December 14, 2000 at 13:22:31:

In Reply to: Re: Its all about snow tires... posted by Joe Ording on December 13, 2000 at 23:07:34:


Downshifting puts a lot of decelerative force on the rear tires. This results in a forward weight shift, unloading the rear suspension and reducing the already-taxed traction of the rear tires.
Unless done very carefully and at lower revs, this will result in the back tires spinning. The reason that the brakes are biased to the front of the car is to keep you from locking up the rear brakes.

All true, and worth pointing out. Of course braking also shifts the weight forward, unloading the rear suspension, but you don't have as much decelerative force acting on the rear wheels, due to the brake bias. Anyone who's driving/shifting at high revs (up or down) in snow or ice is asking for trouble, but you'd have to really work to break the rear tires loose in the rain (and pity the clutch in that car), and in the dry it's damn near impossible, unless you're in a high-G corner.

When the rear tires lose traction - either because they are spinning when decelerating or are locked in heavy braking - and the front tires still have traction, the rear end will try to whip around. If you're braking and this happens, you can merely let off the brakes, usually causing enough weight to shift back to the rear to stop the loss of traction. If you downshift into another gear and begin to spin around, it's much harder to catch. I don't suggest even mild downshifting in rain or snow.

Here I dissent... downshifting in rain is perfectly fine (I'm not talking about 6k RPM heal and toe kinds of driving, though that's ok in the rain too, if you must ;-). And in snow it's ok to do if you're careful (low revs as you point out etc.). Two reasons for downshifting in snow/ice: 1) to decelerate gently in a corner, when a strong decelerative force on the front wheels will likely result in understeer (off the road). This has risks, of course, and granted, you ought not to be needing to slow down in a corner if you're driving prudently, but it happens sometimes. 2) Around here, lighting up your brake lights in nasty weather is a signal for other drivers to panic, especially when the driver behind is following too closely (the rule, alas, not the exception). When they're following too closely, I downshift (this is all in automatics for me these days, sadly) sometimes to scrub off speed, slowing down the driver behind me without them freaking out. I use this technique to modulate the distance between me and the, well, idiots in SUVs who think the laws of physics are only for those who don't drive SUVs.


Just use the brakes, and put the car back in gear when you're ready to accelerate again.

Keeping the car in a lower gear to maintain speed while going downhill shouldn't be a problem, however.

Too right!

Peter


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