Is FWD weight transfer a myth?

[Noger]

Well, there isn't really any weight transfer going on, it is the load that is being transferred. It just looks like the weight is being moved.

As you accelerate, the force wants to twist you round the centre of gravity. This sets up a torque round that CofG. The road then pushes up with an equal and opposite force.

The force is proportional to the height of the CofG (higher things tip over easier) and inversely properional to the wheelbase (wide things are harder to tip).

So a car with a 2m wheelbase. CofG 50cm in height. Weighing 2000kg. Accelerating at .5G (9.8m/s / 2 - about a 0-60 in 5.5 seconds) - generating an accelerative force of 1000kg. And a perfect 50:50 weight distribution.

On the front tyre we have 1000kg (half the weight) - 1000kg (the force) * 50 (height of CoG) / 200 (wheelbase) = 750kg.

On the rear tyre we have 1000kg + 1000kg (the force) * 50 / 200 = 1250kg.

So a 25% increase in "grip" on the rear, 25% decrease in the front. Not good news for the FWD.

Edited by Noger on Wednesday 31st October 16:17

[Mr Whippy]

Well it's significant if you are cornering already, asking the tyre to put more work into doing longitudinal decelleration means less left for lateral grip, and this will see a reduction in lateral grip + the effect of lower load over the rear tyres vs the fronts...

Not sure about the exact mechanics of how a fwd would respond, most of the fwd's I've pushed that hard all have a high rear roll stiffness, so oversteer on lift-off more than push from the front.

Dave

[308mate]

RobM77 said:

The major difference of course comes when you want to try and rescue the lift off oversteer. Common sense says that you need to get the weight transferred back over the rear of the car again, which is easy in a front wheel drive car as you just accelerate. If you reduce the grip at the front by doing this then it helps even more! (the You Tube link above is a great example of this! - FWD cars are almost unspinnable). With rear wheel drive it gets a bit more complicated, as power application can cause the rear wheels to lose grip even further.

In my opinion, lift off over steer in a RWD is a just a plain old up. Its actually difficult to acheive unless youre in low grip conditions like wet or unsealed road. In RWD, if you want to slide, use power, not momentum. Momentum renders you a virtual passenger.

[Mr Whippy]

Noger said:

Well, there isn't really any weight transfer going on, it is the load that is being transferred. It just looks like the weight is being moved.

As you accelerate, the force wants to twist you round the centre of gravity. This sets up a torque round that CofG. The road then pushes up with an equal and opposite force.

The force is proportional to the height of the CofG (higher things tip over easier) and inversely properional to the wheelbase (wide things are harder to tip).

So a car with a 2m wheelbase. CofG 50cm in height. Weighing 2000kg. Accelerating at .5G (9.8m/s / 2 - about a 0-60 in 5.5 seconds) - generating an accelerative force of 1000kg. And a perfect 50:50 weight distribution.

On the front tyre we have 1000kg (half the weight) - 1000kg (the force) * 50 (height of CoG) / 200 (wheelbase) = 750kg.

On the rear tyre we have 1000kg + 1000kg (the force) * 50 / 200 = 1250kg.

So a 25% increase in "grip" on the rear, 25% decrease in the front. Not good news for the FWD.

Edited by Noger on Wednesday 31st October 16:17

Not when you want to go faster still I imagine

So I guess for fwd people who want faster 0-60's, lowering the CofG is the best bet, bar exotic tyres...

Fold them rear seats down, it really DOES work

No honestly, in theory, it does

Dave

[joesnow]

RobCrezz said:

joesnow said:

RobM77 said:

True, but that's with LSDs and slick tyres

lsds don't have any advantage in a straight line, do they?

Course they do, without an LSD they would just spin one wheel. With an LSD you can put down enough power for both as the diff will lock if one wheel starts to lose grip and stop all the power being fed to the wheel that has lost grip.

So a fwd car without an lsd, such as a 60's mini will just smoke one tyre off the line? Not the case.

I thought an lsd transfered drive to the wheel with most traction, ie: the outside wheel of a fwd car accelerating out off a reducing radius corner, saving the inside one with less grip spinning up.

[Munter]

joesnow said:

RobCrezz said:

joesnow said:

RobM77 said:

True, but that's with LSDs and slick tyres

lsds don't have any advantage in a straight line, do they?

Course they do, without an LSD they would just spin one wheel. With an LSD you can put down enough power for both as the diff will lock if one wheel starts to lose grip and stop all the power being fed to the wheel that has lost grip.

So a fwd car without an lsd, such as a 60's mini will just smoke one tyre off the line? Not the case.

I thought an lsd transfered drive to the wheel with most traction, ie: the outside wheel of a fwd car accelerating out off a reducing radius corner, saving the inside one with less grip spinning up.

Nope. An LSD will send some of the power to the inside wheel as well. You can spin up both wheels with an open diff. But it's far easier with an LSD.

I'm sure we caoul all go on but we are off topic.

Edited by Munter on Wednesday 31st October 16:29

[308mate]

joesnow said:

RobCrezz said:

joesnow said:

RobM77 said:

True, but that's with LSDs and slick tyres

lsds don't have any advantage in a straight line, do they?

Course they do, without an LSD they would just spin one wheel. With an LSD you can put down enough power for both as the diff will lock if one wheel starts to lose grip and stop all the power being fed to the wheel that has lost grip.

So a fwd car without an lsd, such as a 60's mini will just smoke one tyre off the line? Not the case.

I thought an lsd transfered drive to the wheel with most traction, ie: the outside wheel of a fwd car accelerating out off a reducing radius corner, saving the inside one with less grip spinning up.

A plain old LSD doesnt transfer power, it just limits slip. Regardless of which wheel is spinning or had grip, it only allows a difference in rotation speeds between the two wheels of a certain amount (eg 15%) before it locks up and provides power to both wheels.

[Will2425]

308mate said:

In my opinion, lift off over steer in a RWD is a just a plain old up. Its actually difficult to acheive unless youre in low grip conditions like wet or unsealed road. In RWD, if you want to slide, use power, not momentum. Momentum renders you a virtual passenger.

a small amount is good in a track car, it lets you scrub off some speed if you enter a corner too fast, and it can be recovered from by giving the car a small amount of gas (to negate the engine braking) and countersteering

but i agree that it is not really ideal in an every day road car

[308mate]

Will2425 said:

308mate said:

In my opinion, lift off over steer in a RWD is a just a plain old up. Its actually difficult to acheive unless youre in low grip conditions like wet or unsealed road. In RWD, if you want to slide, use power, not momentum. Momentum renders you a virtual passenger.

a small amount is good in a track car, it lets you scrub off some speed if you enter a corner too fast, and it can be recovered from by giving the car a small amount of gas (to negate the engine braking) and countersteering

but i agree that it is not really ideal in an every day road car

Ooookay. A small amount of weight transfer yes, but a full lift off slide?

Of course if youre in too hot and you havent mastered a dab of left-foot braking (and I havent...yet), you change the attitude of the car by lifting off.

On that note, my flat mates S plate Vectra has BUCKETLOADS dialled into it. To the point where its dangerous. She loses it 2 or 3 times every winter. The only reason I dont is cause Im expecting it. Are they all like that?
I think it has too much caster in the wheel alignment(?)

Edited by 308mate on Wednesday 31st October 16:39

[RobM77]

308mate said:

RobM77 said:

The major difference of course comes when you want to try and rescue the lift off oversteer. Common sense says that you need to get the weight transferred back over the rear of the car again, which is easy in a front wheel drive car as you just accelerate. If you reduce the grip at the front by doing this then it helps even more! (the You Tube link above is a great example of this! - FWD cars are almost unspinnable). With rear wheel drive it gets a bit more complicated, as power application can cause the rear wheels to lose grip even further.

In my opinion, lift off over steer in a RWD is a just a plain old up. Its actually difficult to acheive unless youre in low grip conditions like wet or unsealed road. In RWD, if you want to slide, use power, not momentum. Momentum renders you a virtual passenger.

That's wrong actually; you can only use power to iniate a slide in a rear drive car at low speed. Once you're up to speed (say, on a race track), it's all about weight transfer - power will only give you understeer. Unless we're talking hairpin bends, almost all of the oversteer you see on a race track will be induced by weight transfer.

Edited by RobM77 on Wednesday 31st October 16:40

[308mate]

RobM77 said:

308mate said:

RobM77 said:

The major difference of course comes when you want to try and rescue the lift off oversteer. Common sense says that you need to get the weight transferred back over the rear of the car again, which is easy in a front wheel drive car as you just accelerate. If you reduce the grip at the front by doing this then it helps even more! (the You Tube link above is a great example of this! - FWD cars are almost unspinnable). With rear wheel drive it gets a bit more complicated, as power application can cause the rear wheels to lose grip even further.

In my opinion, lift off over steer in a RWD is a just a plain old up. Its actually difficult to acheive unless youre in low grip conditions like wet or unsealed road. In RWD, if you want to slide, use power, not momentum. Momentum renders you a virtual passenger.

That's wrong actually; you can only use power to iniate a slide in a rear drive car at low speed. Once you're up to speed (say, on a race track), it's all about weight transfer and power will only give you understeer. Unless we're talking hairpin bends, almost all of the oversteer you see on a race track will be induced by weight transfer.

I agree to a point. But on a race track, its rare that you WANT to slide, no? And if you do, are not modulating the slide and the rear grip with the throttle?

Im talking about sliding for fun.

[HiRich]

Take a nominal car
- Wheelbase 10'
- CoG 2' above the ground
- Static weight split: 60F/40R (front engine bias)
Vertical load on the axle is a key driver for acceleration.
Static vertical load on front axle = Fs = 0.6g
Static vertical load on rear axle = Rs = 0.4g

Now we want to accelerate the car at 0.5g (an easily achievable amount on dry tarmac). Taking O-level moments:
Fa = 0.6g - 1/10(0.5g x 2) = 0.6g - 0.1g = 0.5g
Ra = 0.5g
Despite having the weight well biased to the front, for 0.5g acceleration the RWD version of this car is as fully capable as the FWD version. Take it up to 0.75g acceleration, 0.15g is being transferred to the rear axle and its capability for delivering acceleration is now 22% more than through the front axle. A lot more than 5%.

If the limit of adhesion is a controlling factor in acceleration, then the grippier the surface the greater the benefit for RWD, and the greater the penalty for FWD. Not a significant issue on ice or wet roads, a very big issue on the run up to Copse at 1.00pm on a hot Sunday in July.

[RobM77]

308mate said:

RobM77 said:

308mate said:

RobM77 said:

The major difference of course comes when you want to try and rescue the lift off oversteer. Common sense says that you need to get the weight transferred back over the rear of the car again, which is easy in a front wheel drive car as you just accelerate. If you reduce the grip at the front by doing this then it helps even more! (the You Tube link above is a great example of this! - FWD cars are almost unspinnable). With rear wheel drive it gets a bit more complicated, as power application can cause the rear wheels to lose grip even further.

In my opinion, lift off over steer in a RWD is a just a plain old up. Its actually difficult to acheive unless youre in low grip conditions like wet or unsealed road. In RWD, if you want to slide, use power, not momentum. Momentum renders you a virtual passenger.

That's wrong actually; you can only use power to iniate a slide in a rear drive car at low speed. Once you're up to speed (say, on a race track), it's all about weight transfer and power will only give you understeer. Unless we're talking hairpin bends, almost all of the oversteer you see on a race track will be induced by weight transfer.

I agree to a point. But on a race track, its rare that you WANT to slide, no? And if you do, are not modulating the slide and the rear grip with the throttle?

Actually, the optimum grip for a car is achieved at about 5-10% slip, depending on the tyre and the track surface. So when cornering in a racing car you'll be sliding by this tiny amount all the time. The art of track driving is to use weight transfer to control the car so that it is slipping by this amount front and rear (called neutral steer, or a four wheel drift), so you're getting the most grip possible out of the car. If you don't get it quite right, you'll end up coming out of this ideal state into oversteer or understeer, which to be honest for most of us (even Schumacher) could be several times in any given bend. True, you are modulating the slide with the throttle in a racing car, but it is the weight transfer you're modulating, not power overwhelming the rear tyres.

Edited by RobM77 on Wednesday 31st October 16:51

[308mate]

HiRich said:

Take a nominal car
- Wheelbase 10'
- CoG 2' above the ground
- Static weight split: 60F/40R (front engine bias)
Vertical load on the axle is a key driver for acceleration.
Static vertical load on front axle = Fs = 0.6g
Static vertical load on rear axle = Rs = 0.4g

Now we want to accelerate the car at 0.5g (an easily achievable amount on dry tarmac). Taking O-level moments:
Fa = 0.6g - 1/10(0.5g x 2) = 0.6g - 0.1g = 0.5g
Ra = 0.5g
Despite having the weight well biased to the front, for 0.5g acceleration the RWD version of this car is as fully capable as the FWD version. Take it up to 0.75g acceleration, 0.15g is being transferred to the rear axle and its capability for delivering acceleration is now 22% more than through the front axle. A lot more than 5%.

If the limit of adhesion is a controlling factor in acceleration, then the grippier the surface the greater the benefit for RWD, and the greater the penalty for FWD. Not a significant issue on ice or wet roads, a very big issue on the run up to Copse at 1.00pm on a hot Sunday in July.

Y'see I was just about to say EXACTLY that...

[Will2425]

308mate said:

Will2425 said:

308mate said:

In my opinion, lift off over steer in a RWD is a just a plain old up. Its actually difficult to acheive unless youre in low grip conditions like wet or unsealed road. In RWD, if you want to slide, use power, not momentum. Momentum renders you a virtual passenger.

a small amount is good in a track car, it lets you scrub off some speed if you enter a corner too fast, and it can be recovered from by giving the car a small amount of gas (to negate the engine braking) and countersteering

but i agree that it is not really ideal in an every day road car

Ooookay. A small amount of weight transfer yes, but a full lift off slide?

Of course if youre in too hot and you havent mastered a dab of left-foot braking (and I havent...yet), you change the attitude of the car by lifting off.

On that note, my flat mates S plate Vectra has BUCKETLOADS dialled into it. To the point where its dangerous. She loses it 2 or 3 times every winter. The only reason I dont is cause Im expecting it. Are they all like that?
I think it has too much caster in the wheel alignment(?)

Edited by 308mate on Wednesday 31st October 16:39

you should try driving my volvo then, its almost scary in the wet.

it will oversteer if you give it too much gas
it will oversteer if you lift off
it will oversteer if you are simply going too fast round the corner

in fact i have never managed to make it understeer in the wet (and i have tried!)

and its RWD so you can't punch the throttle to catch it

[RobM77]

Will2425 said:

and its RWD so you can't punch the throttle to catch it

not unless you're holding down the brake with your left foot that is...

[WEREWOLF]

My brain hurts!!!

Think i`ll just chuck it in a bend and see what happens.

[thetrash]

308mate said:

Will2425 said:

308mate said:

In my opinion, lift off over steer in a RWD is a just a plain old up. Its actually difficult to acheive unless youre in low grip conditions like wet or unsealed road. In RWD, if you want to slide, use power, not momentum. Momentum renders you a virtual passenger.

a small amount is good in a track car, it lets you scrub off some speed if you enter a corner too fast, and it can be recovered from by giving the car a small amount of gas (to negate the engine braking) and countersteering

but i agree that it is not really ideal in an every day road car

Ooookay. A small amount of weight transfer yes, but a full lift off slide?

Of course if youre in too hot and you havent mastered a dab of left-foot braking (and I havent...yet), you change the attitude of the car by lifting off.

On that note, my flat mates S plate Vectra has BUCKETLOADS dialled into it. To the point where its dangerous. She loses it 2 or 3 times every winter. The only reason I dont is cause Im expecting it. Are they all like that?
I think it has too much caster in the wheel alignment(?)

Edited by 308mate on Wednesday 31st October 16:39

A bit of toe in on the back wheels is what's needed there.

[RobM77]

thetrash said:

308mate said:

Will2425 said:

308mate said:

In my opinion, lift off over steer in a RWD is a just a plain old up. Its actually difficult to acheive unless youre in low grip conditions like wet or unsealed road. In RWD, if you want to slide, use power, not momentum. Momentum renders you a virtual passenger.

a small amount is good in a track car, it lets you scrub off some speed if you enter a corner too fast, and it can be recovered from by giving the car a small amount of gas (to negate the engine braking) and countersteering

but i agree that it is not really ideal in an every day road car

Ooookay. A small amount of weight transfer yes, but a full lift off slide?

Of course if youre in too hot and you havent mastered a dab of left-foot braking (and I havent...yet), you change the attitude of the car by lifting off.

On that note, my flat mates S plate Vectra has BUCKETLOADS dialled into it. To the point where its dangerous. She loses it 2 or 3 times every winter. The only reason I dont is cause Im expecting it. Are they all like that?
I think it has too much caster in the wheel alignment(?)

Edited by 308mate on Wednesday 31st October 16:39

A bit of toe in on the back wheels is what's needed there.

Vauxhalls are notoriously badly set up. A lady I knew from work a while back spun hers on a roundabout in Basingstoke in the wet. She wasn't even going that fast!

[collateral]

That pic of the RWD wheel standing has me wondering...FWD presses down the rear wheels from a standing start like that as long as the tires don't spin up and the rear spring rate is firm enough?