4wd – does it offer more grip?

[monthefish]

BigLepton said:

monthefish said:

However, I’m also aware that a tyre has a finite amount of grip which I believe is shared between forward/backward motion (i.e. acceleration/braking) and directional changes, so from that respect a 4wd should have less grip (as some of the tyres’ available grip is being used to drive the car forwards).

Errr, no. A 2WD car shares its lateral grip and braking grip between all four wheels, but two of the wheels get all the torque, reducing the amount of the other two forces they can handle, dependent on throttle opening. A 4WD car shares all three forces (lateral grip, braking and torque) between all four wheels which means you should be able at any given point of a corner to apply more throttle without the driven end giving up gripping before the none driven end.

What I was meaning was that each tyres only has so many 'daves' of grip available at any given time, so if some daves are being used for propulsion, there will be less daves available for lateral grip.

Also, how can a tyre be trnsmitting braking and torque at once?

[Mr E]

mechsympathy said:

Which is why awd cars need to be driven differently. You get on the power far earlier and harder than you would in a fwd/rwd car.

Unfortunately, I struggle to find the balls to get anywhere near the limits most of the time...

[mechsympathy]

Mr E said:

mechsympathy said:

Which is why awd cars need to be driven differently. You get on the power far earlier and harder than you would in a fwd/rwd car.

Unfortunately, I struggle to find the balls to get anywhere near the limits most of the time...

That's why ultimately I found my Impreza a bit dull.

[Speed_Demon]

Grip is no higher but cornering speeds can be higher with 4WD systems like yaw control n all that, but thats a whole other kettle of fish.

[OJ]

Lots of good points raised. Here's how I see it...

The 'Daves' of grip method wasn't too far out. The tyre can produce a finite amount of grip be it lateral or longitudinal.

With a 4WD system, the longitudinal acceleration is split between the front and rear axles

With a RWD system, it's only applied to the rear axle.

Assuming a normal cornering condition (i.e. not a powerslide or 4 wheel drift) As you apply the power in corner in a well balanced car with a 4WD system, the car will transfer its lateral grip force evenly to traction force. As the front of the car is playing the biggest part in changing direction, the car will have a tendency to gradually understeer. Note all of the power can be easily applied.

In a well balanced rear wheel drive vehicle with a similar amount of power, the front wheels are left to do the cornering work, and the lateral grip is more quickly used up by the traction force of the rear wheels as power is applied. This will give it a tendancy to oversteer. Despite the fact that the front wheels are left to do most of the cornering work, the rears are overwhelmed more quickly and become the limiting factor.

The crux of it is that no, comparitively well balanced 4WD cars in a normal situation don't have more grip than RWD cars, but they do have more traction and they behave very differently.

In low grip or high power situations where lateral force is easily overwhelmed, RWD cars become more difficult to apply power, and thus rear end grip becomes a limiting factor. In low power situations where traction isn't an issue 4WD creates understeer earlier and thus typically front end grip becomes a limiting factor

4WD is especially useful in extremely low grip situations (such as rallying) as traction is effectively used to corner the car

ETA - Seeing the comment on yaw control, this CAN provide more grip as it uses the heavily loaded outside rear wheel to effectively steer the car, although in certain situations this can be done with RWD

Edited by OJ on Tuesday 6th January 17:23

[pimpin gimp]

randomman said:

I'm well aware I could be totally wrong on this one but bear with me.

The way I always thought of it was a tyre provides a certain amount of grip (based on size, temp, tread, style etc) and that grip is used in any of 4 directions (forward, backward, left, right)

Now lets say a tyre gives 50 daves of grip. In a front wheel drive car if you used 50 daves accelerating and 50 daves turning you would be fine (as the front wheels total 100 daves). But if you used 100 daves accelerating and 100 daves turning you skid.

Now in a RWD car you can use 100 daves turning the front wheels and 100 daves accelerating the back (the back also has to do some turning but I've ignored this for simplicities sake)

So a 4wd car, would depened on the ratio between front and back (haldex, permanent, locked diff etc) and things get a lot more complicated, but while the front tyres can still only offer 50 daves each in a turn, when you accelerate its divided by 4 instead of 2, so if you needed 100 daves for acceleration that would still leave 25 daves for each tyre to turn.

I know this could be completely wrong, but it was fun to type and my boss laughed at the description

Edited by randomman on Tuesday 6th January 16:22

Right or wrong, you used Daves as a unit and thus I shall forever believe you.

[Austin3000]

matt uk said:

4wd – does it offer more grip?

So answer to the question, would a fair summary be, 'no' on the way into the corner (off the throttle) but 'yes' on the way out (on the throttle)?

And surely they offer exactly the same grip (or is it traction?!?!) when braking as regardless of where the drive is (FWD, 4WD or even RWD) the wheels would be trying to do the same thing...stop!

So on an icy road a 4wd is no more better at stopping than a 2wd...in fact the added weight (of say a FWD Golf VS a Golf with 4 motion fitted) may even make it worse!

[BigLepton]

monthefish said:

BigLepton said:

monthefish said:

However, I’m also aware that a tyre has a finite amount of grip which I believe is shared between forward/backward motion (i.e. acceleration/braking) and directional changes, so from that respect a 4wd should have less grip (as some of the tyres’ available grip is being used to drive the car forwards).

Errr, no. A 2WD car shares its lateral grip and braking grip between all four wheels, but two of the wheels get all the torque, reducing the amount of the other two forces they can handle, dependent on throttle opening. A 4WD car shares all three forces (lateral grip, braking and torque) between all four wheels which means you should be able at any given point of a corner to apply more throttle without the driven end giving up gripping before the none driven end.

What I was meaning was that each tyres only has so many 'daves' of grip available at any given time, so if some daves are being used for propulsion, there will be less daves available for lateral grip.

Also, how can a tyre be trnsmitting braking and torque at once?

4WD spreads your torque and lateral grip evenly between the 'daves' of all four tyres, 2WD means two of your tyres are handling all the torque that was spread between all four with 4WD. That means the two undriven wheels have an excess of lateral daves that the driven wheels would dearly like to share as torque daves as they are having to handle twice the torque per wheel as the 4WD car, but they can't share the torque which means the driven 2WD axle will run out of daves before the undriven 2WD axle does, meaning under or oversteer if it's FWD or RWD where the 4WD would still grip due to more even torque distribution.

A tyre cannot transmit braking and torque at once, which is why I didn't say it could.

[Dr Jekyll]

So if I'm deciding between two cars, identical except that one is 4WD and one FWD, what would be the benefits of the 4WD? Would it handle noticeably differently at a sensible-man-on-public-road-but-in-a-hurry pace?

If the choice is between 4WD and RWD, then what?

[matt uk]

Dr Jekyll said:

So if I'm deciding between two cars, identical except that one is 4WD and one FWD, what would be the benefits of the 4WD? Would it handle noticeably differently at a sensible-man-on-public-road-but-in-a-hurry pace?

If the choice is between 4WD and RWD, then what?

When I used to have access to an Audi S4, on a cold wet, rainy night when in a hurry home on the back roads at 7/10ths, no car before or since has made swift progress feels so easy and safe - a very sure-footed car in a way that the FWD version would not have been.

[5lab]

randomman said:

I'm well aware I could be totally wrong on this one but bear with me.

The way I always thought of it was a tyre provides a certain amount of grip (based on size, temp, tread, style etc) and that grip is used in any of 4 directions (forward, backward, left, right)

Now lets say a tyre gives 50 daves of grip. In a front wheel drive car if you used 50 daves accelerating and 50 daves turning you would be fine (as the front wheels total 100 daves). But if you used 100 daves accelerating and 100 daves turning you skid.

Now in a RWD car you can use 100 daves turning the front wheels and 100 daves accelerating the back (the back also has to do some turning but I've ignored this for simplicities sake)

So a 4wd car, would depened on the ratio between front and back (haldex, permanent, locked diff etc) and things get a lot more complicated, but while the front tyres can still only offer 50 daves each in a turn, when you accelerate its divided by 4 instead of 2, so if you needed 100 daves for acceleration that would still leave 25 daves for each tyre to turn.

I know this could be completely wrong, but it was fun to type and my boss laughed at the description

Edited by randomman on Tuesday 6th January 16:22

from what I know, which might also be wrong, the rear tyres are just as important to a turn (lets say whilst not accellerating or braking) as the fronts are. Ie the cornering force is 50:50. yes the fronts are pointing in the direction of the turn, but the rears are just as important. if this wasnt the case, fwd cars could get away with tiny rear tyres, as the acceleration, most of the braking, and the cornering is all done at the front.

in fact, whilst accelerating hard, most of the weight of the car is pushed rearwards, which is why the front of the car rises. so if you were accelerating hard on a corner, the rear wheels would be taking most of the lateral force, but would also have more grip available (as the weight is over them), and the fronts take less. if you brake mid corner, the fronts are more important, and have more grip.

the reason rwd is considered better is the same reason your front brakes are bigger than your rear. under braking the weighs is forwards, so more force is needed there. under accelleration the opposite is true.

[Quattro 3.0]

randomman said:

I'm well aware I could be totally wrong on this one but bear with me.

The way I always thought of it was a tyre provides a certain amount of grip (based on size, temp, tread, style etc) and that grip is used in any of 4 directions (forward, backward, left, right)

Now lets say a tyre gives 50 daves of grip. In a front wheel drive car if you used 50 daves accelerating and 50 daves turning you would be fine (as the front wheels total 100 daves). But if you used 100 daves accelerating and 100 daves turning you skid.

Now in a RWD car you can use 100 daves turning the front wheels and 100 daves accelerating the back (the back also has to do some turning but I've ignored this for simplicities sake)

So a 4wd car, would depened on the ratio between front and back (haldex, permanent, locked diff etc) and things get a lot more complicated, but while the front tyres can still only offer 50 daves each in a turn, when you accelerate its divided by 4 instead of 2, so if you needed 100 daves for acceleration that would still leave 25 daves for each tyre to turn.

I know this could be completely wrong, but it was fun to type and my boss laughed at the description

Edited by randomman on Tuesday 6th January 16:22

This is absolutely correct. Grip has to be apportioned between Traction/Braking(the fore and aft force) and Turning (the side force.
Your measure of "Daves" is spot on - a powerful two wheel drive car will overdo the Daves by trying to accelerate and turn at the same time, while the tyres at the unpowered end still have unused "Daves".By sharing the power out it releases more "Daves" for the powered end to turn and grip. This effectively adds available sideways grip to the very end that would have broken away and skidded.

[tumble dryer]

Seriously impressive thread revival!

(Bet it gets shouty.)

[Balmoral]

Someone explained it very well on a similar such thread yonks ago.

Park a car on a tilt-able plinth, then tilt the plinth until the car slides off. It's not going to make any difference to the grip whether it's rwd/fwd or awd, they will all slide off at the same time.