Why does four wheel drive hurt 0-60 times?

[jon-]

Recently I've been watching old top gears / clarkson dvds and one thing has started to annoy me.

Whenever there's a 4wd vs rwd drag race, they always claim that the 4wd is at a disadvantage due to "bogging down off the line"

Now I'd understand that if we were talking about a Fiat Panda 4x4, but it's usually serious bits of kit with more than enough power to spin up all 4 wheels, meaning 4wd can only be an advantage.



PS, happy new year all!

[anonymous-user]

I agree!

[The real Apache]

Could it be that RWD squats under power and thus gets more grip whereas an AWD will not thus sit spinning the wheels?

[jimbobsimmonds]

My understanding is down to greater transmission losses because the engine has to get all of those drive shafts and differentials up to speed as well...

[AnotherClarkey]

The real Apache said:

Could it be that RWD squats under power and thus gets more grip whereas an AWD will not thus sit spinning the wheels?

Why would a RWD squat more under power than an AWD?

[jon-]

jimbobsimmonds said:

My understanding is down to greater transmission losses because the engine has to get all of those drive shafts and differentials up to speed as well...

I would agree, but only in a lesser powered car. Cars like the RS4 could spin all 4 wheels with ease, so any transmission losses are nullified.

[tr7v8]

Extra weight, too much traction to allow the "safety valve" of spinning wheels. Not many 4 x 4 cars have enough HP to light up all 4 wheels.

[Bonefish Blues]

In my old RB, problem was that it was all very well cranking the revs up, but unless you were prepared to be truly brutal (and it did take true brutality to break traction with P Zero Corsas) the revs would drop, the engine would go off boost, bog down, and all the sound and fury came to naught.

[balls-out]

I suspect that the clutch can't transmit the required torque to spin up all 4 wheels at once on a dry grippy surface

[randomwalk]

I thought awd would have a better 0 - 60 time advantage than rwd and then after 60 the awd would be at a disadvantage due to driveline losses.

[jon-]

balls-out said:

I suspect that the clutch can't transmit the required torque to spin up all 4 wheels at once on a dry grippy surface

Now that's something I'd not thought of.

[balls-out]

The real Apache said:

Could it be that RWD squats under power and thus gets more grip whereas an AWD will not thus sit spinning the wheels?

squatting just means that the weight and therefore grip moves from the front to the rear. if all 4 are driven then it won't matter which end has the grip. for Front wheel drive obviously squat reduces the grip of the driven wheels

[Mr E]

randomwalk said:

I thought awd would have a better 0 - 60 time advantage than rwd and then after 60 the awd would be at a disadvantage due to driveline losses.

For anything with any sort of power, yes. They can be tricky to launch hard sometimes (when you pay the bills, the utterly brutal sidestep of the clutch doesn't really appeal...)

[bull996]

balls-out said:

I suspect that the clutch can't transmit the required torque to spin up all 4 wheels at once on a dry grippy surface

This.

The car might have loads of power, but usually the clutch will just slip if you give it too much revs and dump the clutch.

You end up having to be gentle initially until the clutch is engaged and then you can floor it.

[The real Apache]

AnotherClarkey said:

The real Apache said:

Could it be that RWD squats under power and thus gets more grip whereas an AWD will not thus sit spinning the wheels?

Why would a RWD squat more under power than an AWD?

Ok, it will to an extent but from my experience not so much

[CraigyMc]

It's not just this.

If you look about for any 4x4 drag cars, you will tend to find that they are pretty hard to steer - they do torquesteer, so they tend to drift about all over the place when all the tyres are lit up.

Here's a video from a few years ago - the driver had a hard time keeping the thing in a straight line. http://www.youtube.com/watch?v=mmqhvs3rbS4

[LongLiveTazio]

Yes, 1350bhp Skylines are representative of the Top Gear tests.

Alternatively, they do it so it makes good television and Clarkson and Co can spout truisms that are utter nonsense, in the same way that 911's are supposedly antique pieces of engineering...

http://www.youtube.com/watch?v=yqqk5V0uiYc

[LuS1fer]

Simple rule. If TG say it, it's not true. It might be nearly true but rarely true.

[Frankthered]

bull996 said:

balls-out said:

I suspect that the clutch can't transmit the required torque to spin up all 4 wheels at once on a dry grippy surface

This.

The car might have loads of power, but usually the clutch will just slip if you give it too much revs and dump the clutch.

You end up having to be gentle initially until the clutch is engaged and then you can floor it.

Reminds me of the old Renault 21 Turbos which were available as either FWD or 4WD. Apparently the 4WD versions ate clutches (and were slower 0-60) because they couldn't break traction at the wheels (unlike the FWD version) so the clutch took all the abuse.

OK, in that particular case, the clutch needed to be beefed up to take the power, but even with the powerful 4WD cars that are around now, they could be bogging down a little because the clutch is having to work a bit harder than the comparable 2WD cars.

[chard]

jon- said:

jimbobsimmonds said:

My understanding is down to greater transmission losses because the engine has to get all of those drive shafts and differentials up to speed as well...

I would agree, but only in a lesser powered car. Cars like the RS4 could spin all 4 wheels with ease, so any transmission losses are nullified.

Have you ever driven an RS4? If you tried to spin all 4 wheels you would just wreck the clutch.