Originally Posted by rnemovr
like i say in every winter thread. those with 4wd remember you have extra to get you going but same 4 as everyone else stopping
This is actually not true. It seems like it would be, but it's not, and I'll explain why.
In virtually all vehicles, the front brakes are much more powerful than the rears. This is because when stopping, all vehicles tend to "nose dive" to some extent or another, putting less weight and less traction on the rear tires and far more on the front. To keep them from locking up every time you step on the brakes, the rears are designed to be MUCH less powerful than the front brakes. You can see this especially on cars with 4-wheel disk brakes - the rear pads are TINY compared to the fronts. I've seen some that were about the size of a large postage stamp!
When braking lightly, the rear brakes may not engage at all, contributing nothing to the total braking power. In very low traction environments, it is entirely possible to lockup the front tires while the rear brakes are barely engaged. You can see this in RWD vehicles especially - I see trucks every winter, stopped at a light, but with a rear wheel slowly spinning on the ice.
Read that again: 2wd vehicles in ice MAY NOT have effective 4-wheel braking.
In a *part-time 4wd* vehicle (NOT full-time; NOT AWD), the front and rear driveshafts are locked together. They cannot spin separately. The front brakes on a 4wd vehicle apply equal braking force to all 4 wheels, through the driveshafts.
I can prove it: Put a vehicle in part-time 4wd on a lift, get all 4 wheels spinning, and apply only the front brakes. All 4 wheels will stop. The rear wheels cannot spin independently of the front.
The same is NOT true of AWD and full-time 4wd - these modes allow the front and rear driveshafts to spin at different rates, so braking force is not transferred through the drivetrain.
So, everyone, operate your vehicles prudently, and you 2wd guys, make sure your rear brakes - especially drum brakes - are adjusted so you can get as close to slow-speed 4-wheel braking as possible. You RWD guys with automatic transmissions, you can greatly increase your braking power by slipping into neutral so your front wheel traction doesn't have to brake both the vehicle and the torque converter.