4wd Braking Pt 2

"Future next gen S2000 owner" (future-next-gen-s2000-owner) 01/24/2018 at 12:30 • Filed to: 4wd vs 2wd

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Team O’Neill and why 4wd braking is better is snow and ice. More after the jump.

Their reasoning is due to a transfer case locking the front and rear wheel speeds together. Since the front and rears axles are now locked together, the front tires can’t lock up unless the back tires do as well.

Essentially the front brakes don’t work as effectively due to higher driveline forces (due to the front and rear being locked). The front brakes now have to deal with the force of the engine.

This reduced braking force translates into a reduction in angular acceleration(deceleration if you prefer, which is still an acceleration just in the opposite direction) at the front wheels. The reduced angular acceleration is the result to less torque applied to the wheels - higher velocities on the disc reduce the coefficient of friction, ie. less force for a given brake system pressure. This applied torque is lower than the torque applied form the surface onto the road. Hence the wheels don’t lock up. Locked up wheels extend braking distances on snow and ice.

My explanation is terrible but that is the concept.

I’ll try less words. Since the front wheels are now locked in to the engine and rear axle the front brakes aren’t as effective. This is a benefit on snow/ice since they aren’t strong enough anymore to lock the tires at high speeds.

functionoverfashion > Future next gen S2000 owner 01/24/2018 at 12:47

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Dirt fish and why 4wd braking is better is snow and ice. More after the jump.

*Team O’Neil?

Future next gen S2000 owner > functionoverfashion 01/24/2018 at 12:49

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Yep. I’ll fix it. I immediately just assume rally school at dirt fish. I blame roadkill.

crowmolly > Future next gen S2000 owner 01/24/2018 at 12:51

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I wonder if it has anything to do with brake balance and dive as well.

Future next gen S2000 owner > crowmolly 01/24/2018 at 12:54

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I don’t know. I wonder how not having a transfer case affects their results.

functionoverfashion > Future next gen S2000 owner 01/24/2018 at 12:57

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haha, no worries - I mean, it’s one or the other. Not a big industry...

Smallbear wants a modern Syclone, local Maple Leafs spammer > Future next gen S2000 owner 01/24/2018 at 13:11

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All I know is, it’s true. Regardless of everyone on here that likes to say 4wd doesn’t help stop or turn but only accelerate... yeah it helps with both those things.

HammerheadFistpunch > Future next gen S2000 owner 01/24/2018 at 13:16

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Yup. The FP did this last night and I came to the same conclusion just before this second video came out. Have experienced this exact same thing in my car.

TheTurbochargedSquirrel > Future next gen S2000 owner 01/24/2018 at 13:19

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I still want to know what the difference is for a car with a center differential that doesn’t lock the front and rear to the same speeds. They should find a FWD Impreza and compare it to an AWD one. Also what difference does RWD vs FWD make?

wafflesnfalafel > Future next gen S2000 owner 01/24/2018 at 13:42

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I still don’t get it... makes no sense to my puny little brain. Ok, I get why antilock stop distances may be longer, but why would having 4 wheels connected to the drive line make any difference at all when the forces are basically completely controlled by the brakes. Why are either any different than braking if you are in neutral? Bizarre...

benjrblant > HammerheadFistpunch 01/24/2018 at 13:42

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So CDL in icy conditions = not a bad idea

HammerheadFistpunch > benjrblant 01/24/2018 at 13:43

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trouble is the CDL takes away abs, which while decreases stopping distances in a straight line slightly, also takes away the ability to turn and brake. The GX still has abs in CDL lock which is nice for snow (not so great in sand)

Future next gen S2000 owner > wafflesnfalafel 01/24/2018 at 14:19

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Rotational mass and additional forces are greater with 4wd, essentially. The test would yield different results in neutral I think.

TorqueToYield > Future next gen S2000 owner 01/24/2018 at 15:13

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Most modern cars have brake force distribution. I wonder how that changes results...

https://en.wikipedia.org/wiki/Electronic_brakeforce_distribution

brianbrannon > Future next gen S2000 owner 01/24/2018 at 20:14

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Pretty sure all cars are using diagonal master cylinders since the 80's.  

Dusty Ventures > functionoverfashion 01/24/2018 at 21:01

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There’s at least two others in the States, but they’re kinda small by comparison (though the one in Texas is definitely on the way up)

You can tell a Finn but you can't tell him much > wafflesnfalafel 01/25/2018 at 15:29

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When the hubs and transfer case are locked there is a lot more rotating mass that the front brakes need to stop. In 2wd, every bit of kinetic energy that the front brakes dissipate is due to the force the road is putting on the tire. When in 4x4 the front brakes dissipate the road force plus the kinetic energy of the the driveshafts, axles, transfer case and every other bit of spinning driveline mass. The brakes can dissipate kinetic energy at a certain rate regardless of what mode the vehicle is in.

To make a super oversimplified example, we’ll say that the front brakes dissipate 10 units of energy when you stomp on the brake pedal. If the road is slippery there may only be enough traction for the tires to transfer 5 units of energy to the brakes, so your brakes will lock up and it will take a long time to stop. If putting the vehicle in 4x4 adds 5 units of drivetrain energy that the front brakes need to dissipate, now you will be right on the verge of lockup. Your front brakes will be dissipating 5 units of driveline energy plus the 5 units of energy that the road can transfer to the tires so you will stop faster since your front tires aren’t locking up.