Normally, we think of four-wheel drive as something that helps you move, not stop. When you’re trying to make the car stop, whether or not you’re in 2WD or 4WD shouldn’t really matter much, right? I mean, that’s what I’d guess, but I can’t say I know for certain. That’s why this video from Team O’Neil Rally School is so good—they actually tested this out.
Essentially, Wyatt there went out in the cold and snow with their Ford F250 snowplowing truck to perform some braking tests: with and without 4WD engaged, and then again, but this time with ABS active. Here’s how it played out:
Wow, I’ll admit, I wasn’t expecting these results: having the truck in 4WD mode did seem to reduce stopping distance, pretty significantly. Why, exactly?
I mean, logically, it really shouldn’t matter; under braking, all wheels should be doing the same amount of not-moving—whether or not those wheels are being driven shouldn’t be relevant.
Perhaps because of engine braking, there’s a moment of less free-wheel before the brakes are pressed? That seems pretty minimal.
I posed the question to our captive automotive engineer, David Tracy, who I convinced to help me by offering him two long, brownish-orange rails of rust-powder to snort. Years of exposure have made him an addict, and now he’ll do anything for that sweet brown.
He suggested that perhaps the truck in 4WD mode stopped in less distance because in 4WD mode the front and rear driveshafts are locked, so if you lock/stop the front wheels, the rears HAVE TO stop at the same time.
Regarding the ABS tests, he added
“The 2wd and 4wd with no ABS both lock the wheels up. A vehicle with locked wheels doesn’t care if it’s in 4wd or 2wd, so perhaps there’s a difference in WHEN all four wheels lock that maybe isn’t apparent in the clip.”
So, the best guess is that perhaps 4WD mode forces all four wheels to stop at the same time, and in 2WD mode, there may be a slight gap where one set of wheels is rolling a bit longer than the other, possibly long enough to account for the additional stopping distance.
This is a just a guess, but based on the limited data available in the clip, I think that’s the best we can do.
For now. There can always be other theories. The power of love? Confidence? Orgone energy? Anything is possible.