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 Post subject: FAQ: FWD vs RWD vs 4WD
PostPosted: Fri 12 Aug 2005, 7:45 pm 
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This is the best thing I have ever seen on the internet.
http://www.gsr-evo-club.net/forum/viewtopic.php?t=2218

FWD vs RWD vs 4WD By Bill Sherwood.


Okay, I'll have a go at writing something as definitive as I can make it on this subject. Hopefully it'll help! One big problem with doing it that I'll have to generalise a fair bit when it comes to suspension geometry & set-up, driver ability, etc. So what I'll try to do is to go through the generalised stuff first then be a bit more specific to try to give a big picture. The horsepower divisions I've made are my own arbritray ones, others may dissagree. Later on I'll talk briefly about some actual examples that may not agree with what I've said here, and why. And of course all of this in IMHO, so if you have anything to add please do.

One thing that's important to understand before I get started is to understand the difference between road holding and handling; they are quite separate things but closely related. Road holding is simply the G-force that the car makes in a corner, and so it doesn't matter if the car is badly under or oversteering. However, if the car is badly under or oversteering it may well limit the maximum cornering power, or road holding.

Generalised stuff

FWD

Drivetrain losses are about 17% - 22% at an educated guess.
A rough weight distribution of 55/60% front to 45/40% rear.
The complete car will weigh roughly 30kg - 40kg less than a RWD, and about 60kg - 80kg less than a 4WD.

RWD

Drivetrain losses are about 24% - 30% at an educated guess.
A rough weigh distribution of about 48/53% front to 52/47% rear.

4WD

Drivetrain losses are about 35% - 40% odd.
A rough weight distribution of about 48/53% front to 52/47% rear, though this tend to be less important.
Torque split front/rear is a significant factor in handling.



Low power (up to 100hp)

FWD


Straight line
This will be the fastest car as although it has the least traction out of the three it doesn't have enough power to really smoke the tyres badly. It'll also have the most power at the wheels as it has the least losses.


Top speed
Also the highest due to the least losses.


Braking
Not allowing for engine braking assist, FWD will be the worst due to the most forward weight distribution, thus reducing the braking ability of the rear wheels. Allowing for engine braking assist, it'll also be quite good and nearly as good as the 4WD.


Stability
Almost equal with 4WD due to the forward weight distribution. "Flight of the lead-tipped arrow" is the way to describe it, as the car that has the most weight up the front will have the pendulum effect to keep the car going straight. (big generalisation, I know, but it'll do for the moment)


Cornering
In theory the slowest, but in practice not a heck of a lot of difference between all the types. It's tough to describe without any diagrams but when a car - of any type of drive - is cornering on the limit then all four tyres will be slipping at about a 7° (roughly) angle to the road. This is a result of a thing called the 'traction circle', which I won't get into at the moment, but what it means is that in a stable situation with the car very near the limit of the tyres, if you add power in a FWD then the available lateral traction of the front tyres will be exceeded and so the car will start to understeer. The more power you add, regardless of the steering wheel angle, the more the car will understeer.
This effect varies with the speed of the car naturally, with it being greatest at high speed. At very slow speeds the front tyres tend to dominate and still pull the front around anyway. It's also why the front end of a FWD tucks back in when you lift off the throttle. (as well as weight transfer with deceleration)
And again this is a generalisation, not taking into account the modern clever active LSD's, suspension geometry, etc. They have a significant effect on the way a FWD behaves, and for example with the superb active LSD that the Honda Integra Type R has it can alter the torque of the left and right front wheels to give the outside wheel the most torque, hence the more power you use the more it goes to the most heavily loaded wheel, which of course has little chance of spinning and so cause understeer.


Ease of set-up
Probably not a great deal different to the other types, though to get the best it requires careful balance of the front and rear spring rates, and also F/R anti-roll bars. There can be a wide difference between wheel rates front to rear to get a good handling car.


Effect of total weight
The most affected of the three types, as it has the greatest effect of the weight on the front wheels. Lighter is always better.


Cost
By far the cheapest of the lot to produce, and this, along with the inherant stability of the low power FWD make it the main choice of manufacturers. If they want to make a 'sporty' version of the car, all they usually do is add some sort of LSD to the front (viscous, active, etc) and that generally tunes out most of the nasties from them. They also have the most room inside, which naturally the manufacturers and public like.





RWD


Straight line
Second fastest of course, though about equal in the initial jump off the line with the 4WD.


Top speed
Second fastest again.


Braking
They have the most rearward weight distribution, so they will pull up the best as the weight transfers to the front under heavy braking. If the driver doesn't match engine revs when downchanging though, the rear will lose grip when the clutch is let out and so the rearward weight will work against the car making the corner facing in the right direction.


Stability
Worst of the three due to the majority of the mass being rearwards. (on some cars) A good driver actually turns this into an advantage, as it allows better control of the car when on the limit in corners, and also for direction changes between corners.


Cornering
Probably a little better than FWD due to the slip angle on the rears increasing at the limit when adding more power, thus making the car oversteer a little. In theory, a touch of oversteer is the fastest way through a corner but it may not always be the case in practice, as the cars (all types) that tend to oversteer also tend to have that amount of oversteer vary with speed.


Ease of set-up
A lot of variable again, but there is usually a wide variation that can be used quite successfully. The variation between front and rear wheel rates is usually a lot less than FWD, and perhaps much the same as 4WD's. This is one reason why RWD's also tend to wear down the front and rear tyres more evenly than a FWD.


Effect of total weight
Fairly insensitive, to a point. Lighter is always better.


Cost
Second most expensive to produce. Depending on the rear suspension, they may have a fair bit of boot space taken up (like the AE-86) to allow the diff housing the move around. There's also a tailshaft tunnel.





4WD


Straight line
The slowest of all due to the highest losses in the drivetrain. Maybe the fastest off the line because there's zero wheelspin, but this also infers that the engine tends to bog down. Good drivers can slip the clutch just the right amount to keep the engine spinning. (not good for the clutch though!)


Top speed
For obvious reasons, the slowest.


Braking
Roughly in between the FWD and RWD, but as it's darn near impossible to have all four wheels lock up if the clutch is let out at the wrong time it makes them quite safe and stable.


Stability
Again, when corning at the limit of the tyres if you add power the traction limit will be exceeded. In the 4WD's case though, they all tend to start to let go at the same time and so the car will tend to four wheel drift a little. However, it does depend on the torque split front/rear, but in most of the lower powered 4WD's they usually have a 50/50 split. For reasons that I'll explain later though, that tends to make the car understeer more ...


Ease of set-up
Probably tending to be more like the FWD in terms of difficulty, but with more like RWD rates, etc.


Effect of total weight
Quite insensitive. Lighter is always better.


Cost
Due to having components of FWD and RWD, they naturally tend to cost the most to make.





Summary - low power

Depending a lot on what you want the car for.

Public
FWD wins every time.


Club work
Whatever you prefer. I learned to drive in a RWD, and I find it very difficult to drive FWD's quickly. They can be driven very quickly indeed, but it does seem to require a touch more skill to do so. Left foot braking to stabilise the car and control the car is often needed. 4WD's tend to behave more like FWD's, and so you've come off a FWD then you'll be better off than a RWD driver. In motorkhana's, FWD's are very hard to beat indeed.


Racing
FWD or RWD, again take your pick as you have to trade off between power at the wheels and cornering power and braking ability. Since low power cars tend to do better with even a slight amount of extra power, FWD would seem to be best here.





Medium power (150hp to 250hp)

FWD


Straight line
Initially the slowest of the lot due to wheelspin (and that is subject to driver ability!) but once the car is up into 2nd gear there again is that slight advantage of power at the wheels over the other two types. But by that time, usually, the other two have pulled out enough distance so that over the 400metres they can't be caught. Also, if the FWD is traction limited when hitting 2nd gear that will also badly affect the acceleration. Good LSD's again help a lot here.


Top speed
As mentioned above.


Braking
As mentioned above.


Stability
Increasingly worse with more power, as the car is getting to the point where it's able to spin both fronts in corners. This is where the clever LSD's really start to be needed to get the most out of the car. Turboed engines even more so, as the torque rises rather more quickly with engine revs than a naturally aspirated engine does.


Ease of set-up
This is where the FWD's start to become a problem, as you start to have to really compromise things to keep the front end stuck to the ground. Softer springs in the front, stiffer in the rear, etc.


Effect of total weight
Again this is where the FWD's have to start compromising between adding weight to the front to make it stick or having too much weight in the front that makes them understeer more. More weight in the back helps the balance, but reduces traction. Lighter is always better.


Cost
As mentioned above. CV joints that're strong enough start to become a problem.





RWD


Straight line
Getting to the point where traction can start to be a problem, depending on the rear suspension and driver ability.


Top speed
As mentioned above.


Braking
As mentioned above.


Stability
As a general rule, the more power a RWD has the more rearward the weight distribution should be. This is to keep the weight on the rears when cornering hard, to help them stick. This naturally tends to make the car suffer from the pendulum effect even more, so once again some sort of compromise must be made. This is usually made by the driver working the throttle and steering to control the oversteer. LSD's help, though no special ones other than a plain old Salisbury (friction plate) type is often needed.


Ease of set-up
Not all that hard, and again the wheel rates are usually much the same front to rear. Playing with wheel & tyre widths on the rear, front offset, etc, is all a factor in fine tuning. (These same thing apply to FWD's as well)


Effect of total weight
Starting to be a factor. The heavier the car the more the tyres get worked, so as usual the lighter the better.


Cost
Starting to swing towards RWD, as there are not all that many FWD type gearboxes that can handle up around 250hp+. RWD gearboxes are still pretty easy to get that can handle that power. They do, however, weigh a fair bit.





4WD


Straight line
Starting to get hard to beat, as they will start to have enough power to keep the engine spinning along with the wheels until it gets moving. Not always the case though. Otherwise as mentioned above.


Top speed
As mentioned above.


Braking
As mentioned above.


Stability
Ever increasing reliance on the torque split, but generally pretty good. 4WD cars with this much power tend to have more rear bias on the torque, and so also tend to be more stable when working the throttle in corners when on the limit. It also means that you can start to drive the car on the throttle to control the attitude.


Ease of set-up
Pretty much the same as mentioned before, though more attention to LSD ratios, etc, to control on-the-limit under/oversteer.


Cost
Still the most expensive.





Summary - medium power


Public
Take your pick really.


Club work
Again personal preferance, but it depends on what the car is to be used for. Sometime FWD is best, sometimes RWD, sometimes 4WD.


Racing
RWD or maybe 4WD wins every time. The ability to really drive the car through the corners is where RWD is best, but the sheer traction of 4WD can also be a good thing.





High power (300hp+)

FWD


Straight line
HP for HP, the FWD will be the slowest here as they just can't put the power to the ground. (Unless they are specifically set-up for dragging in a straight line, but they'll still be spinning wheels when the other two types are pulling away with all the traction they can handle)


Top speed
As mentioned above, though the really high power cars all tend to be either RWD or 4WD.


Braking
As mentioned above.


Stability
The worst of the lot, as high power cars also tend to have lots of torque, and so keeping the fronts under control tends to dominate the entire chassis set-up. This tends to compromise the car into being a 'one type' car, ie, it's set-up for rally, road, or race, but is only good at the one thing at a time. That's true to a certain extent with all the cars of different types, but even more so with FWD's.


Ease of set-up
Becoming very difficult, as mentioned just above because the chassis has to be set-up around keeping the front tyres on the road, at the expense of a lot of other things such as ride height, ride stiffness, etc. They also tend to wear down front tyres at a fearsome rate compared to the rears. They also typically require much softer rear tyre compounds, to try to get them up to temperature, but again this is a compromise as you can't have the rear sticking too much as it increases the understeer ... as I said, it's hard work!


Cost
Lots - Starting to need rather strong gearboxes here, big soft front tyres, much work on suspension, etc.





RWD


Straight line
Traction starting to become a bit of a problem, but wider tyres & driver control help a lot.


Top speed
As mentioned above.


Braking
As mentioned above.


Stability
Very dependant on suspension, etc, and so varies from rather twitchy to quite good. Lots of power allows the driver to control the attitude of the car quite well though, and so with good throttle control corners of any speed can be taken pretty much as desired.


Ease of set-up
Tending to favour work done on the rear, to keep the tyres stuck to the ground. Rear tyres tend to wear a lot faster than fronts.


Cost
Also getting high, as big gearboxes, diffs, axles, etc are needed.





4WD


Straight line
Impossible to beat, with their great traction off the line giving them too much of a jump over the other two types.


Top speed
As mentioned above.


Braking
As mentioned above.


Stability
Still pretty good, though there is starting to be enough power to spin all the wheels, hence they're often set-up to have more rear bias with the centre diff torque split, so as to give a bit of warning that the whole thing is going to start moving sideways.


Ease of set-up
Usually just fine tuning from lower power set-ups. A lot of the high power 4WD's such as the WRX's, Skylines, etc, have fairly stock geometry and only really fiddle with ride heights, spring rates, dampers, etc.


Cost
Ouch! The great traction they have really kills the gearboxes, so very strong ones are needed, as are the half-shafts, etc. The cars also tend to be relatively heavy, so big brakes are often fitted.





Summary - high power


Public
High power cars and the general public probably should not be allowed, but unfortunately it does from time to time. Just forget big HP FWD's with the public, as they would be wrapping themselves around telephone poles faster than they already do now. Wet roads would see mass slaughter! RWD is liveable, but again it requires a bit of talent to keep under control. Wet roads are again a big problem though. 4WD has to be the winner here, as it allows the average bloke/blokette to drive around in relative safety.


Club work
There's not a lot of applications for high power FWD's here, as for example in a motorkhana it would be rather difficult to control the wheelspin & related understeer. Most club stuff involves accelerating from a fairly slow speed, so wheelspin is always going to be a killer. RWD can be quite good, but again it requires a good driver to get the most out of it. 4WD is fairly idiot-proof, but it's still no substitute for a good driver. It can also get you into trouble rather quickly, but you're more likely to be able to get out of it in such cases.


Racing
RWD or 4WD works very well, depending on the rules and how much power you have.





Specifics


Motorkhana's
FWD's win every time, simple as that! Because it's pretty much low speed stuff, they can point the driving tyres in the direction they need to go, and power off in that direction with the tail end just along for the ride.
RWD's can also do okay, but to rotate the car around a point they tend to waste time thowing the rear around with the wheels spinning.
4WD's are normally pretty crap, as they have a lot of trouble getting a slide happening.
For example all the cars that win the championships are specially made FWD's with very light space-frame chassis.


Drag racing
As mentioned in the above paragraphs, but it's hard to beat a RWD for cost and performance.
I don't know of the rules for the various classes in drag racing, but if there was one where you could pick between all three types you'd go for either 4WD or RWD, depending on how much power & tyre size you were allowed.


Circuit racing
RWD is very hard to beat, though if you can get a 4WD car down to about the same weight they can often be a little quicker. 4WD is banned in quite a few classes though. FWD has a slight power advantage in lower powered car, but as soon as there's any reasonable power the RWD's work out better. For example, in any class where the car is purpose built (and you can make it FWD or RWD) no-one makes a FWD chassis.
An example being my Sports 1300 class and the Aussie Formula Two's; The rules allow for either FWD or RWD (not 4WD though) but no-one has built a FWD because it would just not be competitive.


Rallying
All things being equal (weight, suspension, etc) 4WD rules here - The traction and stability is by far the best out of the three types. With FWD & RWD it's a little harder to pick, as the days of RWD rally cars are long over thanks to the FIA and CAMS. There's also not very many modern RWD cars around to compete against the FWD's, and so there's not been a lot of developement with them. But I think that again with all things being equal, the RWD's should be better.
There was an example given in another post about Ed Ordinski's 4WD Mitsubishi when it ran around in RWD and then FWD when the centre diff was playing up, and how Ed said it was much nicer to drive in FWD configuration. That's an example of it not being all things equal, as the Mitsu's are basically a converted FWD car and so have a relatively heavy front end. A RWD car will have different suspension geometry and weight distribution, and so handle quite a bit better.
At the end of the day though, with any reasonable amount of power in a well set-up car, it's up to the way that the driver likes the car, either FWD or RWD. As I've said, I can't get my head around FWD so they're quite un-natural to me.


Rain
Rain is great because it brings the maximum corning power of the car right down, and so lets you see the handling characteristics of your car at a much lower speed. This is where you can see FWD's understeer off into the gutters, RWD's spin off into the gutters backwards, and 4WD's generally ignore the wet altogether.
Most of the gutter hitting from the FWD & RWD isn't due to bad handling, but more so because of the driven wheels spinning from excess power, thus making them lose grip. The way to check is to find something like a flat roundabout (with no traffic) and gradually speed up around it until you can feel the car start to do something.
A better example is where my club ran an event at the Driver Training Centre at Norwell, just south of Brisbane. They have a nice, big skidpan there with a good sprinkler system. In my AE-86, as I accelerated around it, the car understeered, but as the speed picked up it started to become more neutral, then finally several laps of full-lock were done. (yes, I enjoyed it!) There was a few 4WD WRX's there, and they were all understeering badly around the skidpan. That could have been fixed by more rear torque bias.


Wheelbase
Or to more more correct wheelbase to track ratio. The general trend of cars that have a short wheelbase in comparison to their track is to be fast in a straight line (less weight & aerodynamic drag) and twitchy in the corners. This is one reason why it's difficult to compare modern FWD & RWD cars as they often have quite different ratios, with the FWD's usually being shorter. FWIW, the apparently optimum ratio for a RWD is about 1.7:1, which as an amazing co-incidence (not!) is very close to the AE-86's ratio.





Other examples


Toyota MR2
These are RWD, but of course also have the engine in the rear as well and so use a FWD type gearbox. So, that means that for the same horsepower as something like an AE-86, they'll be putting more power to the ground. However, Toyota decided to make the MR2's a bit porky and so they're not as fast as they should be.


Subaru Impreza WRX
4WD as we all know, but with a 50/50 torque split. They also have quite similar suspension geometry front and rear, and this is why they understeer when pushed. It's tough to try to explain without resorting to pictures, but bascially when the car is cornering and the tyres are running near their limit if you extend a line from the centre of each wheel to the centre of the circle that the car is making , then the point at which the two (okay, four) lines join is going to be in 'front' of the central point of the circle, hence the car is understeering. To make the car neutral and so bring the two points back to the same place, the front roll centre needs to come down a bit and so make the front stick better - or - more torque to the rear to make that end lose traction first - or - more weight to the rear of the car, etc.
Anyway, that's why they understeer when pushed, and that's also why the STi's have much more power going to the rear end than the plain vanilla WRX's.


Falcon Ute's
Saw this one in a previous post, and so I thought I'd have a go at explaining why they handle like a sloppy bucket of crap when pushed. The front suspension is actually pretty good, but the rear is pretty bad! The problem they have, like many other live axle cars including the AE-86, is that the rear axle isn't allowed to move freely in roll. It's because of the need to have the upper trailing arms shorter than the lower ones to fit them under the floor. In the case of the leaf-sprung ute's, the springs are rather stiff in both rate and and because they have multiple leaf's they tend to stick a little to each other before moving in bump/rebound. (That's why there was a lot of effort back years ago to try to get single leaf springs to work, as it eliminated the slight binding effect)
They're also quite stiff to be able to carry the 1,000kgs or whatever it is that they're rated to stick in the tray and not bottom out too much.
Anyway, the end result is that they're too stiff in roll and this tends to make them lift up the inside wheel when cornering, thus making it spin much earlier than what it should. The cure for that is to 'three and a half link' the rear end, which in the case of an AE-86 means putting hard poly bushes in the rear trailing arms, but in the upper left-hand one you leave the factory rubber bushes in and drill them full of holes to let that arm move around a lot easier. This lets all the arms, because they all follow different arcs in roll, move around without binding. You put the poly bushes in the upper RH arm because it tends to hold the diff housing flat when under power.


Honda Integra Type R
They are a bloody quick little car, and it's largely thanks to the very clever LSD they have in the front. It's a very clever adaption of the old tank system of getting power from the engine to both tracks in varying amounts, to let them turn. In the Honda it's bascially three small diffs in one package, and the middle diff is turned by a small electric motor that make the torque go unevenly to one side of the car. By using sensors from the steering wheel, etc, the computer works out which wheel is loaded up more and so diverts more torque that way. Apparently it works really well, as the car suffers from very little understeer. It's understandable as with the system working flat out there's no reason why you couldn't have a good 75% of the torque going to the outside wheel, which of course tends to 'pull' the car into being more neutral.


Brabham BT24
At least I think it's the BT24 ... Anyway, it's the 1984 turbo Formula One car as designed by Gordon Murray. It was the last year of the unrestricted F1 turbo engines, and shortly after the season started the BMW engine that they were using was tested on a dyno at about 1470hp. I say 'about', as the dyno 'only' went to 1450hp! Anyway, Murray knew that this was coming as so he designed the car with just one feature in mind - He knew that the races would be won on the straights, and so he made the car with as much weight as possible on the rear; about 62% in fact! By doing that they could get good traction as 4WD is not permitted in F1. 4WD was banned in the mid 70's I think, with the last effort by Lotus getting erratic results. It was also indirectly banned by the banning of the Tyrell P34 six-wheel car with the change in rules.


Stupidly powerful drag cars
The 6,000hp odd methanol burning rails that do sub five second runs are all RWD, but they woudln't really benefit from being 4WD because the front wheels are hardly on the ground very much throughout the run. Again because of the great power they have they need a very large rear weight bias, and so the fronts are very lightly loaded but a long way out in front to give some control.

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PostPosted: Fri 12 Aug 2005, 7:49 pm 
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What a fabulous guide.

Saves Instantly.....Very good find.

Lots of info, im loving it


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PostPosted: Fri 12 Aug 2005, 8:27 pm 
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Very good read Rob.

Cordias are awsome cars for top speed being light fwd's.I had my share of wrx's and skylines getting freaked out when cordia pulls, rolling :) Most of them never stood a chance, off the line it's a totally different story.


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PostPosted: Fri 12 Aug 2005, 8:30 pm 
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very interesting read, although i think i have read that somewhere else (and no, not the gsr/evo forums :P)

will make a good FAQ :)


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PostPosted: Sat 13 Aug 2005, 1:21 pm 
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Nice find.

Moved to FAQ's.

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PostPosted: Sun 17 May 2009, 7:52 pm 
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Just forget big HP FWD's with the public, as they would be wrapping themselves around telephone poles faster than they already do now. Wet roads would see mass slaughter!


I actually disagree here!. in the hands of the public I think fwd cars are safer than rear wheel drive cars as the rwd's tend to spin the back out easily around corners under power leading to all sorts of trouble.
On a wet day i'd let the missus drive the cordia over the falcon anyday givin it had no traction control


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