Why do more cars not have "switchable" 4WD?

[white_goodman]

Many 4x4s (and I expect a few still do) used to have a transfer box to switch from 2WD to 4WD (as my 2001 Jeep Grand Cherokee does). However, why is this technology not very common in cars? The PH consensus seems to be that RWD is best but in wet/icy and especially snowy weather, AWD would be preferable. So you either have a RWD car, which is pretty useless in the snow or an AWD car (like a Subaru), which is less efficient most of the time. The new Cadillac ATS (a 3-Series rival not available in the UK) has three electronic settings. Touring: predominately RWD but the front wheels come in to play if the rear wheels slip; Sport: permanently RWD and Snow: 50:50 split (like a Subaru). This would seem to give you the best of both worlds. When you come to a halt, AWD is engaged for maximum traction off the line and the system has a magnetic coupling and is very light and efficient. You can get just a normal RWD ATS but the drop in fuel consumption to AWD is minimal. So why don’t more cars (such as M-powered BMWs and AMG Mercs) use a similar setup and are there any drawbacks to this system other than a slightly higher cost ($3000, about 2000 pounds)?

[Mave]

Well, lots of cars do have "smart" systems which send drive to the "other" wheels when necessary, but are 2wd most of the time, I think Haldex based systems for example. I'm guessing the downsides are additional sensors and actuators to package / go wrong, and from my experience driving an Audi s3 a less linear feel when driving near the grip limit. I've got an Audi with a torsen based 4wd system, and I'm happy to pay for a little bit more fuel for what feels like very consistent handling in the wet, dry, or snow without any levers or buttons to be fiddled with :-)

[GTIR]

Because the vast majority of people, the type who use paragraphs, don't want that feature.

[Crafty_]

Cost, Complexity, Weight.

[jamieduff1981]

A lot more unneccessary weight and a bad lay out are two good reasons to stick with RWD only. To poke driveshafts through the front wheel hubs you need to put the front wheels somewhere a long way behind where you'd want them otherwise - or move the engine forward. Either spoils handling.

There are some cars now with AWD versions of RWD cars for markets which think AWD is a substitute for competence but these are still heavier and add more complexity that modern cars could really do without. It will make them even more expensive to work on for anything other than routine servicing.


I think people need to be careful what they wish for because even what were nice cars are steadily morphing into weird amalgamations of so many conflicting consumer demands that the end results try to please everyone but end up pleasing very few - who then wonder why.

[jamieduff1981]

A lot more unneccessary weight and a bad lay out are two good reasons to stick with RWD only. To poke driveshafts through the front wheel hubs you need to put the front wheels somewhere a long way behind where you'd want them otherwise - or move the engine forward. Either spoils handling.

There are some cars now with AWD versions of RWD cars for markets which think AWD is a substitute for competence but these are still heavier and add more complexity that modern cars could really do without. It will make them even more expensive to work on for anything other than routine servicing.


I think people need to be careful what they wish for because even what were nice cars are steadily morphing into weird amalgamations of so many conflicting consumer demands that the end results try to please everyone but end up pleasing very few - who then wonder why.

[Captain Muppet]

I managed just fine in an Indiana winter (foot of snow every day) in a 210bhp FWD auto, as did everyone else. In the UK I've never been stuck in the snow even in the Elise on summer tyres. I found an MX5 with winter tyres and a welded diff to have enough grip to carry on driving until beyond the limit of ground clearance, at which point body damage was more of an issue than traction.

Those people who need 4WD in a normal car are a minority, and already have loads of cars to choose from. There are limited sales to be won by adding a new selectable 4WD system to cars which don't already have 4WD, and a few sales to be lost by increasing price, CO2 and reducing MPG. Not to mention the cost of increasing OEM fleet average CO2 by lugging around hundreds of thousands of mostly unused diffs and driveshafts.

Even when drive is to just a pair of wheels the unused driveshafts and diff will still be rotating - they aren't turned directly by the transmission, but are turned by the wheels.

[300bhp/ton]

white_goodman said:

Many 4x4s (and I expect a few still do) used to have a transfer box to switch from 2WD to 4WD (as my 2001 Jeep Grand Cherokee does). However, why is this technology not very common in cars? The PH consensus seems to be that RWD is best but in wet/icy and especially snowy weather, AWD would be preferable. So you either have a RWD car, which is pretty useless in the snow or an AWD car (like a Subaru), which is less efficient most of the time. The new Cadillac ATS (a 3-Series rival not available in the UK) has three electronic settings. Touring: predominately RWD but the front wheels come in to play if the rear wheels slip; Sport: permanently RWD and Snow: 50:50 split (like a Subaru). This would seem to give you the best of both worlds. When you come to a halt, AWD is engaged for maximum traction off the line and the system has a magnetic coupling and is very light and efficient. You can get just a normal RWD ATS but the drop in fuel consumption to AWD is minimal. So why don’t more cars (such as M-powered BMWs and AMG Mercs) use a similar setup and are there any drawbacks to this system other than a slightly higher cost ($3000, about 2000 pounds)?

It's worth noting that not all AWD systems are equal and they work in rather different ways and produce rather different results in ability, traction and handling.

[scherzkeks]

Mave said:

Well, lots of cars do have "smart" systems which send drive to the "other" wheels when necessary, but are 2wd most of the time, I think Haldex based systems for example. I'm guessing the downsides are additional sensors and actuators to package / go wrong, and from my experience driving an Audi s3 a less linear feel when driving near the grip limit. I've got an Audi with a torsen based 4wd system, and I'm happy to pay for a little bit more fuel for what feels like very consistent handling in the wet, dry, or snow without any levers or buttons to be fiddled with :-)

Haldex systems are not 2wd most of the time. They are permanent AWD systems that run at about 60/40 most of the time (and in steady state, straight-ine crusing run at about 85/15 for fuel economy). In high-slip situations they can do almost 100 percent to the front or rear. The key is that the torque split isn't static and changes constantly based on many variables.

I've owned both Torsen and Haldex Audis and to me, until the crown gear system and sport diff. came out, Haldex was better. The older Torsen Audis feel agricultural and are too limited in how they shuffle power. Good in inclement weather, but not better than Haldex for sporty driving.

[Krikkit]

scherzkeks said:

Mave said:

Well, lots of cars do have "smart" systems which send drive to the "other" wheels when necessary, but are 2wd most of the time, I think Haldex based systems for example. I'm guessing the downsides are additional sensors and actuators to package / go wrong, and from my experience driving an Audi s3 a less linear feel when driving near the grip limit. I've got an Audi with a torsen based 4wd system, and I'm happy to pay for a little bit more fuel for what feels like very consistent handling in the wet, dry, or snow without any levers or buttons to be fiddled with :-)

Haldex systems are not 2wd most of the time. They are permanent AWD systems that run at about 60/40 most of the time. In high-slip situations they can do almost 100 percent to the front or rear.

I've owned both Torsen and Haldex Audis and to me, until the crown gear system and sport diff. came out, Haldex was better. The older Torsen Audis feel agricultural and are too limited in how they shuffle power. Good in inclement weather, but not better than Haldex for sporty driving.

The whole point in standard Haldex is that it is as close to 2WD as possible to try and push economy up and drivetrain wear down. The split is generally ~95% front bias (depending on generation and individual manufacturer specs, I noted the Golf IV R32 is apparently 80/20), once the front starts to lose traction the system kicks in to move it towards 50/50.

Edited by Krikkit on Wednesday 17th July 09:32

[scherzkeks]

jamieduff1981 said:

A lot more unneccessary weight and a bad lay out are two good reasons to stick with RWD only. To poke driveshafts through the front wheel hubs you need to put the front wheels somewhere a long way behind where you'd want them otherwise - or move the engine forward. Either spoils handling.

There are some cars now with AWD versions of RWD cars for markets which think AWD is a substitute for competence but these are still heavier and add more complexity that modern cars could really do without. It will make them even more expensive to work on for anything other than routine servicing.


I think people need to be careful what they wish for because even what were nice cars are steadily morphing into weird amalgamations of so many conflicting consumer demands that the end results try to please everyone but end up pleasing very few - who then wonder why.

Unless your RWD car is mid-engined, it's engine placement is also compromised. Modern Audis like the S4 have a weight dist. of about 55/45, so not seeing any real difference to the common RWD layouts.

Also, the benefits in traction and the ability to late brake in a corner give AWD vehicles an inherent advantage over RWD on the track.

[Krikkit]

scherzkeks said:

Unless your RWD car is mid-engined, it's engine placement is also compromised. Modern Audis like the S4 have a weight dist. of about 55/45, so not seeing any real difference to the common RWD layouts.

Also, the benefits in traction and the ability to late brake in a corner give AWD vehicles an inherent advantage over RWD on the track.

It's not just about Front/rear distribution, it's about how much weight is between the axles. A big engine slung out beyond the front wheels makes for tricky handling, moving it further back makes it much easier for the dynamics.

[scherzkeks]

Krikkit said:

scherzkeks said:

Mave said:

Well, lots of cars do have "smart" systems which send drive to the "other" wheels when necessary, but are 2wd most of the time, I think Haldex based systems for example. I'm guessing the downsides are additional sensors and actuators to package / go wrong, and from my experience driving an Audi s3 a less linear feel when driving near the grip limit. I've got an Audi with a torsen based 4wd system, and I'm happy to pay for a little bit more fuel for what feels like very consistent handling in the wet, dry, or snow without any levers or buttons to be fiddled with :-)

Haldex systems are not 2wd most of the time. They are permanent AWD systems that run at about 60/40 most of the time. In high-slip situations they can do almost 100 percent to the front or rear.

I've owned both Torsen and Haldex Audis and to me, until the crown gear system and sport diff. came out, Haldex was better. The older Torsen Audis feel agricultural and are too limited in how they shuffle power. Good in inclement weather, but not better than Haldex for sporty driving.

The whole point in standard Haldex is that it is as close to 2WD as possible to try and push economy up and drivetrain wear down. The split is generally ~95% front bias (depending on generation and individual manufacturer specs, I noted the Golf IV R32 is apparently 80/20), once the front starts to lose traction the system kicks in to move it towards 50/50.

Edited by Krikkit on Wednesday 17th July 09:32

Audi uses a standard 85/15 split for its normal cars. S and RS models use different ECU specs that are not published.

The key to Haldex is that anytime the car is doing anything but cruising at a steady state, the torque bias is going to be adjusted to accommodate and it is not limited to 50/50. When cornering, torque shifts rearward, and the car can even be rear biased in such a situation. Another example is during a hard launch. Launch a TTRS hard and use a computer to measure torque at the rear wheels. Tell me what you find out.

Also, when you refer to losing traction, what you really mean is axle slip. The last post in this thread provides a very good and educational run-down on Haldex: http://forums.audiworld.com/archive/index.php/t-27...
Basically mirrors the info. I read in an interview with an engineer from the company a few years back. Lots of misinfo. out there though.

Edited by scherzkeks on Wednesday 17th July 10:22

[300bhp/ton]

scherzkeks said:

The ability to late brake in a corner give AWD vehicles an inherent advantage over RWD on the track.

How does this work?

[Super Slo Mo]

jamieduff1981 said:

A lot more unneccessary weight and a bad lay out are two good reasons to stick with RWD only. To poke driveshafts through the front wheel hubs you need to put the front wheels somewhere a long way behind where you'd want them otherwise - or move the engine forward. Either spoils handling.

Not necessarily, IIRC BMW's answer was to run a driveshaft forwards from the gearbox (presumably from a transfer box mounted on the rear/side of the gearbox) then have a diff beside the sump (it might actually have been in the sump).
Obviously like all 4WD systems, it's a good bit of extra weight, and the transfer box can lead to more driveline shunt than you'd expect as the mileage accumulates. There's not really all that many benefits on the road except in really bad conditions, although that's been done to death on here anyway.

[scherzkeks]

Krikkit said:

scherzkeks said:

Unless your RWD car is mid-engined, it's engine placement is also compromised. Modern Audis like the S4 have a weight dist. of about 55/45, so not seeing any real difference to the common RWD layouts.

Also, the benefits in traction and the ability to late brake in a corner give AWD vehicles an inherent advantage over RWD on the track.

It's not just about Front/rear distribution, it's about how much weight is between the axles. A big engine slung out beyond the front wheels makes for tricky handling, moving it further back makes it much easier for the dynamics.

No disagreement there.