Is FWD weight transfer a myth?

[HertsBiker]

can someone explain this one (again) in simple terms as I can't see the weight transfer being all that significant part when related to the total machine mass. Surely its 5% tops?? if so, why all the fuss about it?
Thanks.

[otolith]

http://www.youtube.com/watch?v=K1wppus-VEM

[dibbers006]

Why don't you try it!

[Plotloss]

Not experienced the joy of lift off oversteer then Carl?

[RobM77]

I think the OP was talking about straight line acceleration weren't they? Lift off oversteer occurs in any car, regardless of the drivetrain.

In a straight line, the weight transfer is most definitely more than 5% in anything with half decent performance. It has a very significant effect on the acceleration of a car from a standing start. When a car accelerates, the rear wheels are compressed down into the tarmac, and the front wheels go light. It follows that a FWD car is going to have a serious disadvantage getting off the line. Think about a motorbike and how easy it is to pull a wheelie. Imagine if a bike were FWD and how hard it would be to accelerate!

Edited by RobM77 on Wednesday 31st October 15:05

[ewenm]

RobM77 said:

I think the OP was talking about straight line acceleration weren't they? Lift off oversteer occurs in any car, regardless of the drivetrain.

In a straight line, the weight transfer is most definitely more than 5% in anything with half decent performance. It has a very significant effect on the acceleration of a car from a standing start. When a car accelerates, the rear wheels are compressed down into the tarmac, and the front wheels go light. It follows that a FWD car is going to have a serious disadvantage getting off the line. Think about a motorbike and how easy it is to pull a wheelie.

Or just watch the start of a BTCC race - the RWD BMWs invariably get a better start than the FWD SEATs.

[Will2425]

HertsBiker said:

can someone explain this one (again) in simple terms as I can't see the weight transfer being all that significant part when related to the total machine mass. Surely its 5% tops?? if so, why all the fuss about it?
Thanks.

if you are talking about weight transfer under acceleration, then as a biker you should surely know the answer. just think about how much weight is on a bikes front wheel when accelerating hard (i.e. none) now imagine that you are trying to use that wheel to propel the bike forwards -- its just going to spin like crazy

now i know bikes accelerate a lot harder than most cars but the same still applies, you are getting significantly less traction just when you need it most, and it gets worse the faster the FWD vehicle is (more weight transfer, plus needing more traction to put the power down)

i'm sure somebody will be along soon with the maths, but its is definately not a myth.

[RobM77]

ewenm said:

RobM77 said:

I think the OP was talking about straight line acceleration weren't they? Lift off oversteer occurs in any car, regardless of the drivetrain.

In a straight line, the weight transfer is most definitely more than 5% in anything with half decent performance. It has a very significant effect on the acceleration of a car from a standing start. When a car accelerates, the rear wheels are compressed down into the tarmac, and the front wheels go light. It follows that a FWD car is going to have a serious disadvantage getting off the line. Think about a motorbike and how easy it is to pull a wheelie.

Or just watch the start of a BTCC race - the RWD BMWs invariably get a better start than the FWD SEATs.

True, but that's with LSDs and slick tyres, so the effect isn't as noticeable as it is on most road cars. To see it in action just stand at a set of traffic lights in rush hour. Virtually any FWD car that tries to get away quickly spins its front wheels. I challenge you to see a RWD car doing the same on the daily commute!

[308mate]

FWD, RWD, 4WD, AWD - all have significant weight transfer to some degree, depending on what you do with the gas or brake. Why wouldnt they?

Its just the results that differ between drive-trains.

In a FWD, you transfer the weight to break the grip and induce a slide when you change the direction of acceleration.

In a RWD, you break the grip to transfer the weight and get your slide on. IF thats what youre trying to achieve.

Actually, that sounds like boocks doesnt it....

[RobM77]

308mate said:

FWD, RWD, 4WD, AWD - all have significant weight transfer to some degree, depending on what you do with the gas or brake. Why wouldnt they?

Its just the results that differ between drive-trains.

the results of weight transfer in a corner are the same regardless of drivetrain. If you back off in a car, the rear goes light and the tendency to oversteer increases, and if you accelerate the front goes light and the tendency to understeer increases.

[joesnow]

RobM77 said:

True, but that's with LSDs and slick tyres

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

Op, it is said that above 250ish bph is the limit for a fwd. Why do you think this is.

[mat205125]

WRT accelerating, look at the pic below.



It's easy to see the effect of weight transfer with this rwd car, and exactly the same forces are trying to shift the weight of the car towards the rear in a fwd car. Of course the wheels will never get that high in fwd as they would lift, apply zero force and acceleration would cease.

[RobCrezz]

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.

[308mate]

RobM77 said:

308mate said:

FWD, RWD, 4WD, AWD - all have significant weight transfer to some degree, depending on what you do with the gas or brake. Why wouldnt they?

Its just the results that differ between drive-trains.

the results of weight transfer in a corner are the same regardless of drivetrain. If you back off in a car, the rear goes light and the tendency to oversteer increases, and if you accelerate the front goes light and the tendency to understeer increases.

Thats true, but the reason I said to different extents, is that the weight distribution of the car differs greatly. To me, a rear wheel drive car feels very different on lift-off, largely from the feel of it thanks to the weight of the diff and drive train at the rear, as apposed to the FWD, which feels different on lift-off to me due to being lighter back there, with the transmission and drive at the front.

Anyone else think theres a distinction here?

[Munter]

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.

Yep. I like to think of it as I actually drive a 1 wheel drive car. Now that wheel might be on the right or left but only one has drive for most of the time. With an LSD I'd have a 2 wheel drive car.

[RobM77]

308mate said:

RobM77 said:

308mate said:

FWD, RWD, 4WD, AWD - all have significant weight transfer to some degree, depending on what you do with the gas or brake. Why wouldnt they?

Its just the results that differ between drive-trains.

the results of weight transfer in a corner are the same regardless of drivetrain. If you back off in a car, the rear goes light and the tendency to oversteer increases, and if you accelerate the front goes light and the tendency to understeer increases.

Thats true, but the reason I said to different extents, is that the weight distribution of the car differs greatly. To me, a rear wheel drive car feels very different on lift-off, largely from the feel of it thanks to the weight of the diff and drive train at the rear, as apposed to the FWD, which feels different on lift-off to me due to being lighter back there, with the transmission and drive at the front.

Anyone else think theres a distinction here?

Sorry, yes, you're right and your original post is correct, it just put across the wrong impression. The effects of lift off are almost all to do with weight transfer and nothing to do with which wheels are driven (some engine braking on the driven wheels is experienced, but it's a minor effect). The weight transfer that you get acts upon the masses in a car, which as we know are distributed differently in FWD and RWD, and when you're holding or collecting the slide you'll also experience the momentum and inertia that these masses carry.

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.

[hcanning]

Mine pitches quite a bit when you accelerate at least "fairly hard", so no it's not a myth

[RobM77]

Munter 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.

Yep. I like to think of it as I actually drive a 1 wheel drive car. Now that wheel might be on the right or left but only one has drive for most of the time. With an LSD I'd have a 2 wheel drive car.

It depends how you work the diff. It's obviously quite possible to get power oversteer without an LSD, and also to fire yourself off the line with both wheels slipping perfectly. The trick is to measure the power that you put in so as not to let one wheel spin away the power. You can feel this quite acutely in my Caterham, as you're sat over the rear axle.

[Mr Whippy]

308mate said:

RobM77 said:

308mate said:

FWD, RWD, 4WD, AWD - all have significant weight transfer to some degree, depending on what you do with the gas or brake. Why wouldnt they?

Its just the results that differ between drive-trains.

the results of weight transfer in a corner are the same regardless of drivetrain. If you back off in a car, the rear goes light and the tendency to oversteer increases, and if you accelerate the front goes light and the tendency to understeer increases.

Thats true, but the reason I said to different extents, is that the weight distribution of the car differs greatly. To me, a rear wheel drive car feels very different on lift-off, largely from the feel of it thanks to the weight of the diff and drive train at the rear, as apposed to the FWD, which feels different on lift-off to me due to being lighter back there, with the transmission and drive at the front.

Anyone else think theres a distinction here?

Lift off in a rwd see's engine braking applied to the rears, which are also unloaded on weight, so you loose more lateral grip potential.


As per general OP topic, weight transfer is a function of CofG height, wheelbase and acceleration. For a fwd then, a low CofG, longer car is better

Dave

Edited by Mr Whippy on Wednesday 31st October 16:15

[mat205125]

308mate said:

RobM77 said:

308mate said:

FWD, RWD, 4WD, AWD - all have significant weight transfer to some degree, depending on what you do with the gas or brake. Why wouldnt they?

Its just the results that differ between drive-trains.

the results of weight transfer in a corner are the same regardless of drivetrain. If you back off in a car, the rear goes light and the tendency to oversteer increases, and if you accelerate the front goes light and the tendency to understeer increases.

Thats true, but the reason I said to different extents, is that the weight distribution of the car differs greatly. To me, a rear wheel drive car feels very different on lift-off, largely from the feel of it thanks to the weight of the diff and drive train at the rear, as apposed to the FWD, which feels different on lift-off to me due to being lighter back there, with the transmission and drive at the front.

Anyone else think theres a distinction here?

More to do with spring rates IMO. A rwd drive will be softer sprung on the rear than a fwd hot hatch. Harder spring and damper rates on the rear can make a car "arsey"

The feel you describe is the engine braking effect of the driven wheels. The fwd cars slowing wheels will pull the nose in to exagerate the rear slide, yet the rwd slowing will apply a pull into the corner to counter the potential for a slide.