[dustom_99]

My bad,i thought conti came in diesel. Rear wheel dirve does make a diff, but just a little bit.

[Sudhindra]

If you were refering about diesel conty, you are absolutely right. Its very sluggish and there is no way it could catch up with other cars..

[GTO]

Here's a witty, brilliantly-made video:

http://www.youtube.com/watch?v=sY2NBNpO2Jk

[wilful]

I was checking out the thread http://www.team-bhp.com/forum/suvs-m...2-lac-inr.html
and was curious to know the difference between 4WD and AWD.

In another thread 4WD V/s 2WD, veyron1 gave a brief explanation which threw some light. Wanting to get some more information, I came across this link which I felt gives a more detailed explanation to a layman like me.
http://www.digitaltrends.com/cars/wh...l-wheel-drive/

Apart from the obvious difference that in an AWD the power to the drive system of the 4 wheels is on all the time, while in a 4WD it can be switched on and off, there are other important differences which is explained in relatively simple terms with the advantages and disadvantages of both.

[shashanka]

Dear members,
This subject has been buzzing around in my head for a while but for one reason or another I have postponed it till now. I am taking the plunge here in the hope that I can put down what I have gathered in informal talks with other buffs who hold definite views on the subject, but have not really given much thought to it!

Speaking for myself, I will confess that I am firmly in the RWD corner. And the reason is simple - I believe that from an engineer's point of view, a FWD arrangement is a bean-counter's solution and not the engineer's. A FWD set-up is easy to manufacture. The engine, clutch, G/B and undercarriage sub-assembly can be made very compact and the set-up lends itself perfectly for volume production! But…..it is not an engineer's solution!. Good engineering practice warrants that the stresses and strains in a mechanical system - a car in this instance - should not be concentrated in one region, but distributed as much as possible in order to keep wear & tear low & prolong the life & durability of the system.

FWD lay-out

In a FWD set-up, the forces – (steering generated & engine torque generated) - are concentrated on the front wheels and its attached suspension, steering and drive-line components. And it doesn’t require too much imagination to visualise the cornering stresses built-up when the car is being driven at full steering lock and at max allowable torque (= throttle), a situation common enough for me when climbing the steep hairpins of Uttaranchal.
While it is accepted that for good wheel articulation (in all three axes), universal couplings/CV couplings at both ends of each half-shaft to the front wheels are needed, the stresses (specially during adverse driving situations as mentioned above) due to steering forces & engine torque remains high in FWD vehicles. And the situation worsens as the wheel base keeps increasing and the vehicle wt approaches max. allowable GVW, the front wheels having to lug all the weight behind the front axle centerline.
The peak (of this engineering folly?) was reached by the Americans in the 70's when they decided to follow the European lead in FWD application. GM installed FWD on their full sized goliaths - the Cadillac Eldorado & the Oldsmobile Tornado - both of which were chassis-on-frame vehicles, were over 18 feet long, had the usual lazy V8s of 7 L displacement & had kerb wts in excess of 2 tons! At the time they were the largest displacement engines in FWD cars, and despite their cumbersome handling & litany of niggles, they had long production runs.
But the FWD trend continues globally today - Skoda Superb, Nissan Teana, Toyota Camry, Hyundai Sonata, Honda Accord, some Audi & Volvo sedans being large WB FWD examples sold in our market. The reason, as mentioned earlier, is the reduced manufacturing costs & compactness of the package. But one must concede the reason for this design's commercial success - it is widely acknowledged that (for a WB < ~2.4 m & overall length < ~3.8 m, in other words, the small hatch-back!), the FWD layout, coupled with a monocoque structure, has distinct advantages in handling & agility.

RWD lay-out

On the other hand, in a RWD set-up, the same situation (stresses due to steering forces & engine torque) is more rationally resolved. The steering induced stresses are all focused on the front wheels and its accompanying suspension & steering components. The engine torque induced stresses are focused on the rear wheels and its associated sub-assemblies - the locating arms/linkages, the suspension components, & in case of IRS, the half-shafts & their associated bits & pieces. The transmission & drive-line (prop shaft) losses are usually ignored as they are minor in comparison. As can be seen, the situation in a RWD set-up is much more in line (in my view!) with rational, practical engineering.

AWD/4WD lay-out

And finally, the AWD/4WD set-up, for tackling the difficulties faced in driving over bad terrain - mud, slush, ice & snow and, of course, off-road "kaccha" surfaces. We have all seen AWD rally cars in WRC events with their stupendous ability to tackle dangerous surfaces. We also know that most of them are equipped with add-ons - limited slip / torque sensing differentials of one kind or another, ESP and other similar software, etc. What some of us may not know is that full-time AWD (the Audi Quattro models, eg) requires a central/3rd differential or a viscous coupling (like the AWD version of our home-grown XUV500) to prevent torque wind-up or "chirping" on smooth tarmac with AWD engaged, while giving full steering lock. Thus it is clear that AWD is rather more complex & complicated (=higher maintenance & niggles issues!) than one would like in one's daily commuter. For the dedicated tourer - who also indulges in the occasional & necessary bits of off-roading in his travels - the investment in AWD may be worth the added expense and maintenance.
I understand that the above expresses my views and therefore it would be worthwhile if members gave their valuable inputs/views/opinions to bring out any/many of the hitherto untouched aspects of this subject!

Regards,
Shashanka

[thatfastdude19]

Nice post.

Well to be honest, FWD is really never the choice when you have performance in mind. It is of course done just for the sake of its simplicity and lower costs compared to RWD/AWD. You can see the Top Gear Review of the GT86 where Clarkson actually explained this.

The most common and logical choice is RWD. But to be honest, nowadays the AWD systems are soo well developed that they are becoming more and more popular in performance cars as they not only allow you to have fun like in a RWD car (Oversteer by simply making the system more rear biased) but also provide amazing amount of grip.

Specially the new Haldex systems used in many cars are quite good. So to be honest I am sitting in the AWD corner here.

[SmartCat]

Under certain conditions like slippery/slushy road and steep gradient, a FWD car is likely to perform better than RWD car because -

1) Engine weight is on the front wheels and that results in better traction
2) You can wiggle out of sticky situations with some accelerator and steering inputs in a FWD car.

Then there is the safety issue - FWD car will understeer when pushed aggressively into a corner, while a RWD car will oversteer. Understeer is much easier to control for aam aadmi behind the steering wheel. He will instinctively take his foot off the accelerator pedal if the car is tracking straight instead of turning. RWD is fine in premium cars because they are equipped with electronic aids that automatically cuts power to engine or brakes individual wheels when it senses 'danger'.

So one cannot say that RWD is better from engineering point of view.

The perfect engineering solution would be - front wheels powered by internal combustion engine and rear wheels powered by lithium ion batteries.

[shashanka]

Originally Posted by thatfastdude19

Nice post.
Well to be honest, FWD is really never the choice when you have performance in mind. It is of course done just for the sake of its simplicity and lower costs compared to RWD/AWD. You can see the Top Gear Review of the GT86 where Clarkson actually explained this.
The most common and logical choice is RWD. But to be honest, nowadays the AWD systems are soo well developed that they are becoming more and more popular in performance cars as they not only allow you to have fun like in a RWD car (Oversteer by simply making the system more rear biased) but also provide amazing amount of grip.
Specially the new Haldex systems used in many cars are quite good. So to be honest I am sitting in the AWD corner here.

Hello fastdude,
Thank you for your input & insight! I make it a point to to remember that FWD's current popularity is a relatively recent development (after Alec Issigonis designed the Mini in the late 50's) and that prior to that, RWDer's were the norm, with the odd FWDer being the exception! I somehow feel that the explosion of FWD popularity is more linked to the evolution of chassis design - once the monocoque fabrication technique caught on, it did not take long for the bean-counters to figure out that even more profits could be made if FWD were integrated with the monocoque structure.

Undoubtedly the Haldex clutch is a clever device, but again, it depends on external inputs - electro-hydraulics - for its functioning & therefore subject to niggles & gremlins. The one device which works on real-time actuation & is completely self-contained, needing no external input whatsoever is the the Torsen diff & its various derivatives. The trade-off is that it absorbs more power than other systems & needs precise manufacturing tolerances - in other words, is more expensive!

[shashanka]

Originally Posted by smartcat

Under certain conditions like slippery/slushy road and steep gradient, a FWD car is likely to perform better than RWD car because -
1) Engine weight is on the front wheels and that results in better traction
2) You can wiggle out of sticky situations with some accelerator and steering inputs in a FWD car.
Then there is the safety issue - FWD car will understeer when pushed aggressively into a corner, while a RWD car will oversteer. Understeer is much easier to control for aam aadmi behind the steering wheel. He will instinctively take his foot off the accelerator pedal if the car is tracking straight instead of turning. RWD is fine in premium cars because they are equipped with electronic aids that automatically cuts power to engine or brakes individual wheels when it senses 'danger'.
So one cannot say that RWD is better from engineering point of view.
The perfect engineering solution would be - front wheels powered by internal combustion engine and rear wheels powered by lithium ion batteries.

Hello smartcat,
You have highlighted two interesting & pertinent points. Engine weight over the front wheels definitely aids traction - but only for smaller WB cars. With larger wheelbases & particularly on inclines with full loads, I've found FWD cars (a Honda City & a Ford Ikon come to mind) in difficulties on hill roads with unfinished gravel surfaces. The smaller FWD hatchbacks on the other hand are very competent & can draw circles around RWD heavy weights (like my Scorpio!) under similar circumstances.

For me, good engineering (as distinct from good safety practice) means durability, reliability & niggle-free existence!. The perfect engineering solution - as you envisaged - would indeed be perfect! Perfect fail-safe characteristics - if the ICE fails, the batteries are there in limp-home mode, and if the batteries fail, the ICE is always there to take on adversity! Ah, if only we lived in a perfect world.

[shashanka]

Originally Posted by veyron1

.......basically, this simple statement should sum everything up: do you, or anybody else, know of any 550 BHP+, 330 kmph+ car that is front wheel drive??? !.....

Hello Veyron1,

It is interesting that you should mention the above! There have been some high-powered FWDers around, made by - who else? - the Yanks! - "....In 1970 the Eldorado introduced the new 500 cu in (8.2 L) V8 engine, the largest-ever production V8, rated SAE gross 400 hp (298 kW) and 550 lb·ft (746 N·m)...." Well not quite 550 bhp agreed, plus of course, it had all the handling characteristics & torsional rigidity "of an over-ripe banana" as George Bishop was quoted as having said of some pre-war convertibles!

[PreethamB]

Originally Posted by veyron1

basically, this simple statement should sum everything up: do you, or anybody else, know of any 550 BHP+, 330 kmph+ car that is front wheel drive??? also, if you look at the titles of the "world's fastest cars", you shall find that none of them are front wheel drive....

Would Nissan's 1250bhp Le Mans racer qualify?

Read more here.

[shashanka]

Originally Posted by vamsi.kona

I have a very basic question. Can any one elucidate the difference between AWD and 4WD, or are they same. TQ

Hello vamsi.kona,
Quite a while has passed since you asked this question and I have touched on this in my recent post (#260) on this thread.

In a 4wd vehicle, the output prop. shafts from the GB/transfer case, going to the front & rear axles are normally revolving at the same speed, which is fine when the vehicle is running in a straight line on smooth tarmac. But when the car is running lock-to-lock steering on smooth tarmac (as when negotiating hair-pins), the front wheels carve out a larger diameter circle than the rear wheels. This means that the front wheels are running at a higher speed than the rear wheels. How is this difference in speeds between front & rear wheels to be allowed for?

In the average bread & butter kind of 4WD SUV (the Scorpio, Bolero, Safari, Gypsy etc), the 4WD function is engaged only part-time (manually via the transfer case) when there is actual off-roading requirement. If the 4WD function in such a car is engaged permanently on smooth tarmac, something called torque wind-up or “chirping” occurs between front & rear wheels due to the difference in their speeds, when making full lock U-turns, resulting in wheel scrub.

In order to overcome this “chirping”, full-time AWD cars & SUVs have a central differential or a viscous coupling (as in our XUV500 AWD model) to allow for this difference in wheel speeds & these cars can be driven with permanently engaged 4WD without any ill effects. But as with everything else in the real world, one can go for all kinds of add-on frills and gizmos with lovely acronyms!

[shashanka]

Originally Posted by PreethamB

Would Nissan's 1250bhp Le Mans racer qualify?

Read more here.

Shock & awe, Preetham, shock & awe! Trust Nissan to come up with something like this - after turning dragon-slayer at Nurburgring by dethroning Porsche with their Skyline GT-R, it's not surprising that they should again cock a snook at the establishment!

[veyron1]

Pretty nice and old thread being resuscitated...

Originally Posted by PreethamB

Would Nissan's 1250bhp Le Mans racer qualify?

Read more here.

Interesting example. The NISMO LMP1 was a hybrid innovation to showcase to the engineering world, and stands as an example of perhaps what SHOULDN'T be done.

The VRX30 is driving the front at a rated 500 bhp with the rear drive electrical unit at additional 750 horses from it. Technically it still uses rear wheel "drive", although it is front wheel "driven".

The age old adage stands true here - why re-invent the wheel? Just make the wheel FASTER...

Originally Posted by shashanka

Hello Veyron1,

It is interesting that you should mention the above! There have been some high-powered FWDers around, made by - who else? - the Yanks! - "....In 1970 the Eldorado introduced the new 500 cu in (8.2 L) V8 engine, the largest-ever production V8, rated SAE gross 400 hp (298 kW) and 550 lb·ft (746 N·m)...." Well not quite 550 bhp agreed, plus of course, it had all the handling characteristics & torsional rigidity "of an over-ripe banana" as George Bishop was quoted as having said of some pre-war convertibles!

There are awesome FWDs, like the DC2, DC5, Eclipse, Tiburon GT, VRS, etc. I am a VRS fan too. But ultimately, a RWD is ahead by design, that's all. I would pick a 500 bhp E46 over a 500 bhp 1U2 for a lifer. But when it comes to VFM & practicality, it's FWD all the way...

[veyron1]

Originally Posted by smartcat

Under certain conditions like slippery/slushy road and steep gradient, a FWD car is likely to perform better than RWD car because -

1) Engine weight is on the front wheels and that results in better traction
2) You can wiggle out of sticky situations with some accelerator and steering inputs in a FWD car.

Then there is the safety issue - FWD car will understeer when pushed aggressively into a corner, while a RWD car will oversteer. Understeer is much easier to control for aam aadmi behind the steering wheel. He will instinctively take his foot off the accelerator pedal if the car is tracking straight instead of turning. RWD is fine in premium cars because they are equipped with electronic aids that automatically cuts power to engine or brakes individual wheels when it senses 'danger'.

So one cannot say that RWD is better from engineering point of view.

The perfect engineering solution would be - front wheels powered by internal combustion engine and rear wheels powered by lithium ion batteries.

From strictly an engineering point of view,

a) the driven wheels whould have the maximum payload over it
b) the steered wheels should not be driven
c) the weight distribution should be 50:50

From that point of view, a mid/rear engined RWD or AWD will be the best cake for all. Money permitting, one 911 GT2 for me please...

It's completely requirement driven & situational, so FWD/RWD/AWD will each be good at its own game.

But strictly performance wise, RWD takes the cake, followed by AWD and FWD. There are exceptions to beat every rule on earth, but they will remain exceptions and may not become accessible to all or the norm...