FWD Vs. RWD Vs. AWD
19 August 2002, 08:49 AM
#1
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after owning FWD (205 GTI, MX-3, Civic hatchback) cars, driving some RWD cars (NSX, Corvette, C-class) and owning and driving AWD cars (owned 3 Impreza GT, and driven a Legacy and Forester),
I can't help feeling AWD is the best choice, and its capable of both safe driving and fun (power drifts etc.).
now recently I've been reading people dismissing the AWD for power loss (although the Impreza feels very powerful for a 220hp car) and that FWD cars like the Focus RS and RWD cars like the M3 can be just as efficient both on dry and wet roads.
now I just can't see how, and if so then why the Cosworth will be an AWD car or the 911 Turbo uses AWD, just stick to FWD or RWD.
Ferdinand Piech once said that just like cars use four wheel breaks its only logical to use all four wheels to drive the car (and break using the engine).
What do you guys think?
I can't help feeling AWD is the best choice, and its capable of both safe driving and fun (power drifts etc.).
now recently I've been reading people dismissing the AWD for power loss (although the Impreza feels very powerful for a 220hp car) and that FWD cars like the Focus RS and RWD cars like the M3 can be just as efficient both on dry and wet roads.
now I just can't see how, and if so then why the Cosworth will be an AWD car or the 911 Turbo uses AWD, just stick to FWD or RWD.
Ferdinand Piech once said that just like cars use four wheel breaks its only logical to use all four wheels to drive the car (and break using the engine).
What do you guys think?
19 August 2002, 09:05 AM
#2
This is a big one.
RWD is arguably the most efficient / performant on dry tarmac in the hands of a good driver. But AWD in the wet / rough roads, etc in the hands of an inexperienced driver is probably a damn site safer / easier in most situations.
The quote about braking with 4 wheels, therefore power 4 wheels is a little off as you probably wouldn't say the same about braking all 4 wheels if you had to steer with the rears instead of the fronts.
cheers
Simon
RWD is arguably the most efficient / performant on dry tarmac in the hands of a good driver. But AWD in the wet / rough roads, etc in the hands of an inexperienced driver is probably a damn site safer / easier in most situations.
The quote about braking with 4 wheels, therefore power 4 wheels is a little off as you probably wouldn't say the same about braking all 4 wheels if you had to steer with the rears instead of the fronts.
cheers
Simon
19 August 2002, 09:14 AM
#3
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What you're saying is FWD is crap cause it also steers,
I tend to agree with you, and every time I drive a FWD in the wet I can't see how anyone will prefer a FWD with its understeer and lack of pull through the twisties over an AWD car.
one more thing,
you say a RWD would be the better bet on dry tarmac and in the hands of a good driver, so why how come Audi where smoking all the rivals in the BTCC (I think) to the point that they had to make Audi add weight??
or how come AWD cars rule the rallies, even on tarmac, and if they wheren't limited to 300hp the advantage would even be bigger.
I tend to agree with you, and every time I drive a FWD in the wet I can't see how anyone will prefer a FWD with its understeer and lack of pull through the twisties over an AWD car.
one more thing,
you say a RWD would be the better bet on dry tarmac and in the hands of a good driver, so why how come Audi where smoking all the rivals in the BTCC (I think) to the point that they had to make Audi add weight??
or how come AWD cars rule the rallies, even on tarmac, and if they wheren't limited to 300hp the advantage would even be bigger.
19 August 2002, 09:26 AM
#4
OK.. this is a huge subject, but it comes down to the way tyres work.
Tyres can produce lateral and longitudinal forces. They can produce a maximum (for a given load, etc) in one direction or the other, not both at the same time. The more lateral grip you use, the less longitudinal grip there is available.
If you are accelerating out of a corner, the weight moves rearward, so you have even less lateral (and longitudinal) grip available. Start taking that away even more by putting power through them and you reduce are soon understeering.
In essence if you cannot put enough power to the rears to make use up all the required lateral forces, there is absolutely no point in putting power to the front as well (as you are just reducing your cornering performance).
Why the Audi kicked but? Lots of other reasons apart from the pure fact they had AWD. Other limitations of the formula made the AWD less of a disadvantage, and they just did a fabulous job of developing the car. There are too many unknowns to be absolutely sure, but you could argue the other way and say "in that case, why didn't EVERYONE go AWD?".
Rallying is completely different. You don't get the opportunity to find the perfect line, you are rarely on the true limit of the car and tyres, merely the limit of the combination of events leading up to you barrelling in to a bend you've never been round before at an appreciable speed, rather than 50 times in the last hour and thousands of times before. You need something that gives you the ability to get on the gas early. Put an AWD car sideways and you can get on the gas almost immediately.
You also need as much traction as you can get, and this is what AWD has in spades. Grip and traction are 2 very different (and sometimes opposing) things though.
FWD is not crap, it can be incredibly good downhill, but it is without doubt the most hampered of the 3.
All the best
Simon
Tyres can produce lateral and longitudinal forces. They can produce a maximum (for a given load, etc) in one direction or the other, not both at the same time. The more lateral grip you use, the less longitudinal grip there is available.
If you are accelerating out of a corner, the weight moves rearward, so you have even less lateral (and longitudinal) grip available. Start taking that away even more by putting power through them and you reduce are soon understeering.
In essence if you cannot put enough power to the rears to make use up all the required lateral forces, there is absolutely no point in putting power to the front as well (as you are just reducing your cornering performance).
Why the Audi kicked but? Lots of other reasons apart from the pure fact they had AWD. Other limitations of the formula made the AWD less of a disadvantage, and they just did a fabulous job of developing the car. There are too many unknowns to be absolutely sure, but you could argue the other way and say "in that case, why didn't EVERYONE go AWD?".
Rallying is completely different. You don't get the opportunity to find the perfect line, you are rarely on the true limit of the car and tyres, merely the limit of the combination of events leading up to you barrelling in to a bend you've never been round before at an appreciable speed, rather than 50 times in the last hour and thousands of times before. You need something that gives you the ability to get on the gas early. Put an AWD car sideways and you can get on the gas almost immediately.
You also need as much traction as you can get, and this is what AWD has in spades. Grip and traction are 2 very different (and sometimes opposing) things though.
FWD is not crap, it can be incredibly good downhill, but it is without doubt the most hampered of the 3.
All the best
Simon
19 August 2002, 09:36 AM
#5
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O.K
AWD can transfer power either to the front or rear (well good AWDs like the Subaru's) so like I said they are the perfect solution,
and I guess real world driving is closer to Ralliing , since you don't get to test the road before, no one promises you the road will be clean (like a track) and the weather can change.
AWD can transfer power either to the front or rear (well good AWDs like the Subaru's) so like I said they are the perfect solution,
and I guess real world driving is closer to Ralliing , since you don't get to test the road before, no one promises you the road will be clean (like a track) and the weather can change.
19 August 2002, 09:49 AM
#7
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Thats my point, AWD cars can benefit from all worlds, understeer, oversteer at your request.
Skyline is a good example (like a 22B I understand) for oversteer AWDs but even my GT can oversteer or 4 wheel drift easily.
unlike FWD cars that can only oversteer with lift off, and even then its not like making the rear come out under power,
and RWD cars can catch you offgaurd.
Skyline is a good example (like a 22B I understand) for oversteer AWDs but even my GT can oversteer or 4 wheel drift easily.
unlike FWD cars that can only oversteer with lift off, and even then its not like making the rear come out under power,
and RWD cars can catch you offgaurd.
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19 August 2002, 09:50 AM
#8
elondan
Like i said, in the hands of an inexperienced driver, where the conditions aren't perfect, AWD is safer / easier.
Unfortunately, a standard scoob will transfer power forwards under acceleration as the diff will send power to the path of least resistance.. which is exactly what you don't want *if we are talking pure performance*.
Cheers
Simon
Like i said, in the hands of an inexperienced driver, where the conditions aren't perfect, AWD is safer / easier.
Unfortunately, a standard scoob will transfer power forwards under acceleration as the diff will send power to the path of least resistance.. which is exactly what you don't want *if we are talking pure performance*.
Cheers
Simon
19 August 2002, 09:54 AM
#9
skyline is very different to a 22B. The skylines diffs are active, meaning the actually decide through logic to put power to a different set of wheels. The 22B simply has the manual ability to set the resistance of the centre diff.
RWD in the hands of a skilled driver is by FAR the more adjustable car as you have independant control over the front and rear of the car.
Don't get me wrong, I love my scoob, and really REALLY love AWD. It is just important sometimes to not get carried away and think they are the savious of the universe. And they REALLY can catch you out if you're not ready for it. Everything is so easy when you're on the gas, but go into a corner too fast and have to lift and they're just as difficult as any other car to hang on to.. the difference is that you are all relaxed as they were so easy until you lifted your foot.
Cheers
Simon
RWD in the hands of a skilled driver is by FAR the more adjustable car as you have independant control over the front and rear of the car.
Don't get me wrong, I love my scoob, and really REALLY love AWD. It is just important sometimes to not get carried away and think they are the savious of the universe. And they REALLY can catch you out if you're not ready for it. Everything is so easy when you're on the gas, but go into a corner too fast and have to lift and they're just as difficult as any other car to hang on to.. the difference is that you are all relaxed as they were so easy until you lifted your foot.
Cheers
Simon
19 August 2002, 09:57 AM
#10
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Simon, do you have any insight in why FWD appears to get better & better in some cars? (thinking Honda ITR & CTR ) Is that purely suspension related ?
PS: Don't tell me it's a hughe subject and comes down to tyres
PS: Don't tell me it's a hughe subject and comes down to tyres
19 August 2002, 09:58 AM
#11
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RWD is arguably the most efficient / performant on dry tarmac in the hands of a good driver.
19 August 2002, 10:04 AM
#12
LOL!
Well..
it's a HUGE subject.. and it's all to do with the tyres
I think it's maily that some manufacturers are starting to understand more about what makes a FWD car better and realise that people actually want that!
You can do a lot of things like, moving the roll couple distribution forward (I know you know what that is theo but ... meaning the front has less roll resistance than the rear), but that can be done to any car...
lowering the centre of gravity at the front, reducing / removing bump-steer, adding rake (rear higher than front), better diffs (bit of a black box to me, that one. But they're definitely better to drive), better tyres (designed with long and lat demand, at the same time, in mind), suspension set-up (particularly dampers - which historicall just came straight off the back of the technology for RWD), steering geometry (possibly more castor / camber).
I think it just comes down to some road car manufacturers doing the job properly now.
I guess in the past, FWD was never the performance car direction, but now (at least for the road) it is far more prominent.
Cheers
Simon
Well..
it's a HUGE subject.. and it's all to do with the tyres
I think it's maily that some manufacturers are starting to understand more about what makes a FWD car better and realise that people actually want that!
You can do a lot of things like, moving the roll couple distribution forward (I know you know what that is theo but ... meaning the front has less roll resistance than the rear), but that can be done to any car...
lowering the centre of gravity at the front, reducing / removing bump-steer, adding rake (rear higher than front), better diffs (bit of a black box to me, that one. But they're definitely better to drive), better tyres (designed with long and lat demand, at the same time, in mind), suspension set-up (particularly dampers - which historicall just came straight off the back of the technology for RWD), steering geometry (possibly more castor / camber).
I think it just comes down to some road car manufacturers doing the job properly now.
I guess in the past, FWD was never the performance car direction, but now (at least for the road) it is far more prominent.
Cheers
Simon
19 August 2002, 10:10 AM
#13
carl
It's not a myth. You just can't get away from physics. The 4WD F1 cars were a complete disaster, and that was in the days when the tyres technology was such that traction played an even bigger part in the entire performance package.
Why aren't all race cars AWD? Why are the cream of performance cars RWD?
AWD is head and shoulders above RWD on rough roads / low grip surfaces, but on clean dry smooth tarmac physics dictates that standard (in other words, non-active) AWD can never be as performant as RWD.
Bare in mind, that the top rally cars are completely active and on tarmac run as RWD cars for a huge percentage of the time. The AWD comes in one when traction is a bigger demand than grip and handling.
Cheers
Simon
It's not a myth. You just can't get away from physics. The 4WD F1 cars were a complete disaster, and that was in the days when the tyres technology was such that traction played an even bigger part in the entire performance package.
Why aren't all race cars AWD? Why are the cream of performance cars RWD?
AWD is head and shoulders above RWD on rough roads / low grip surfaces, but on clean dry smooth tarmac physics dictates that standard (in other words, non-active) AWD can never be as performant as RWD.
Bare in mind, that the top rally cars are completely active and on tarmac run as RWD cars for a huge percentage of the time. The AWD comes in one when traction is a bigger demand than grip and handling.
Cheers
Simon
19 August 2002, 10:10 AM
#14
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No,
The subaru will transfer the power to the rear, they even state this in all their AWD "propoganda".
accelarate hard coming out of stop sign/light while turning and see how the end comes out just like RWD.
put your foot down on the accelerator while cornering, and again see how the rear end comes out (easier in the new STI).
The subaru will transfer the power to the rear, they even state this in all their AWD "propoganda".
accelarate hard coming out of stop sign/light while turning and see how the end comes out just like RWD.
put your foot down on the accelerator while cornering, and again see how the rear end comes out (easier in the new STI).
19 August 2002, 10:14 AM
#15
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but on clean dry smooth tarmac physics dictates that standard (in other words, non-active) AWD can never be as performant as RWD.
Intuitively the physics suggests to me that AWD can never be worse than RWD in any conditions -- it should at least be its equal and mostly better (e.g. more traction on exit from bends).
19 August 2002, 10:18 AM
#16
on each of those three pictures weight is transfered to the front of the car, which means they are either on the brakes or at least off the throttle.
I think the marketing stuff is probably cleverly worded. Accelerating out of a T junction and getting sideways does not mean power is being transfered to the rear, it just means there is enough power still at the rear to break the lateral grip through longitudinal demand. If the road is slippy enough, all 4 wheels will break this which will put you in to a 4 wheel drift.
Cheers
Simon
I think the marketing stuff is probably cleverly worded. Accelerating out of a T junction and getting sideways does not mean power is being transfered to the rear, it just means there is enough power still at the rear to break the lateral grip through longitudinal demand. If the road is slippy enough, all 4 wheels will break this which will put you in to a 4 wheel drift.
Cheers
Simon
19 August 2002, 10:22 AM
#17
carl
as above, the physics are the way tyres work. More traction out of a bend, does not mean higher speed.
Using the longitudinal grip of a tyre reduces the amount of lateral grip available. This means you can accelerate really quickly, but you just might not go round a corner at the same time.
Unless you stick the car sideways, you can get on the power earlier with RWD as you can do this before all the major cornering is done (as you will not be reducing the available grip in the fronts as much as you would in AWD).
Cheers
Simon
as above, the physics are the way tyres work. More traction out of a bend, does not mean higher speed.
Using the longitudinal grip of a tyre reduces the amount of lateral grip available. This means you can accelerate really quickly, but you just might not go round a corner at the same time.
Unless you stick the car sideways, you can get on the power earlier with RWD as you can do this before all the major cornering is done (as you will not be reducing the available grip in the fronts as much as you would in AWD).
Cheers
Simon
19 August 2002, 10:28 AM
#18
I should add...
I've done a fair bit of development driving for tuning active diffs.
The main focus is keeping the car in RWD as much as possible and only moving the power forward when the rears start to slip, or you get really sideways, etc.
Cheers
Simon
I've done a fair bit of development driving for tuning active diffs.
The main focus is keeping the car in RWD as much as possible and only moving the power forward when the rears start to slip, or you get really sideways, etc.
Cheers
Simon
19 August 2002, 10:28 AM
#19
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Using the longitudinal grip of a tyre reduces the amount of lateral grip available.
I understand the 'circle of traction' but cannot see why grip (i.e. friction) along one axis of a tyre is dependent upon grip along an orthogonal axis (not sure if I mean orthogonal or perpendicular
)Is this a static vs. dynamic friction thing?
19 August 2002, 10:30 AM
#20
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The main focus is keeping the car in RWD as much as possible and only moving the power
19 August 2002, 10:36 AM
#21
carl.. I'm not sure where you heard of this second school of thought that they are independant. I can assure you this is not the case.
Remember, tyre dynamics is not just about friction. Friction is in-fact a tiny part (and by far the simplest) of the whole picture.
For instance, actual turning forces are created by slip angles. Slip angles should be called something else as they make people assume that there is some "split" involved. This is not what it means.
Up to the ideal slip angle of a tyre on dry clean tarmac, there is pretty much zero actual slipping happening at the contact patch. Friction is well within it's bounds of ability.
The force is generated by the fact that the trye is twisted and the leading edge of the contact patch is preceded by a bit of tyre that is slight to the left (or right) of it. Meaning the when it hits the ground it will move the tyre a little to the left. As the tyre rolls, it crabs the whole tyre in that direction.
Once the maximum slip angle is exceeded, the tyre can no longer cope with that ammount of twist and the tyre springs back creating loss of grip.
Cheers
Simon
Remember, tyre dynamics is not just about friction. Friction is in-fact a tiny part (and by far the simplest) of the whole picture.
For instance, actual turning forces are created by slip angles. Slip angles should be called something else as they make people assume that there is some "split" involved. This is not what it means.
Up to the ideal slip angle of a tyre on dry clean tarmac, there is pretty much zero actual slipping happening at the contact patch. Friction is well within it's bounds of ability.
The force is generated by the fact that the trye is twisted and the leading edge of the contact patch is preceded by a bit of tyre that is slight to the left (or right) of it. Meaning the when it hits the ground it will move the tyre a little to the left. As the tyre rolls, it crabs the whole tyre in that direction.
Once the maximum slip angle is exceeded, the tyre can no longer cope with that ammount of twist and the tyre springs back creating loss of grip.
Cheers
Simon
19 August 2002, 10:39 AM
#22
the loss of power through the drivetrain thing is very real, but that has nothing to do with handling, only to do with grunt.
Active diffs are designed and developed to improve handling, not power delivery.
Cheers
Simon
Active diffs are designed and developed to improve handling, not power delivery.
Cheers
Simon
19 August 2002, 10:40 AM
#23
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Remember, tyre dynamics is not just about friction. Friction is in-fact a tiny part (and by far the simplest) of the whole picture.
More research needed, I think
19 August 2002, 10:42 AM
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Is the "spring back into shape" something they actually can control/design in a tyre ? My personal experience is that a Toyo had a bit less grip than an SO2 (let's say in the wet), but started "sliding" more gradually, hence I assumed the getting back into shape/going beyond optimal slip angle on the Toyo happened less "sudden".
19 August 2002, 10:49 AM
#26
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In essence if you cannot put enough power to the rears to make use up all the required lateral forces, there is absolutely no point in putting power to the front as well
19 August 2002, 10:50 AM
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I guess real world driving is closer to Ralliing , since you don't get to test the road before, no one promises you the road will be clean (like a track) and the weather can change.
19 August 2002, 10:51 AM
#28
carl.. agreed.. it's bloody complex. I *think* the 2.5 G thing is down to the downforce. I wasn't aware of the figures, but it could well be right. Downforce is great because it isn't actually weight (so therefore doesn't need to be laterally accelerated, but it creates load on the contact patch so therefore increases grip).
Downforce is also crap because I'm useless in downforce cars!!!
Theo.. absolutely.. This one really IS a huge subject though and I don't pretend to userstand it to a low level. I would expect that a stiffer tyrewall (among probably millions of other things) would make the "spring back" more abrupt, and a softer compound would make it less abrubt. Probably tread pattern would have a lot to do with it also if the outer edge of the tyre had stiffer tread blocks than the middle / inner??? who knows - probably nobody. Hence the need for test drivers (hope nobody finds out! )
Cheers
Simon
Downforce is also crap because I'm useless in downforce cars!!!
Theo.. absolutely.. This one really IS a huge subject though and I don't pretend to userstand it to a low level. I would expect that a stiffer tyrewall (among probably millions of other things) would make the "spring back" more abrupt, and a softer compound would make it less abrubt. Probably tread pattern would have a lot to do with it also if the outer edge of the tyre had stiffer tread blocks than the middle / inner??? who knows - probably nobody. Hence the need for test drivers (hope nobody finds out!
)Cheers
Simon