Saab White paper on FWD vs RWD
December 29, 2012 in Saabology
Saab’s first chief engineer, Gunnar Ljungström, in 1960 authored a white paper covering many aspects of basic chassis design.
Mr Ljungström came from a family of engineers. Both his uncle and father were accomplished engineers who among other things designed turbines, an automatic transmission and apparently a bike with a freewheel hub. It was Mr Ljungström’s team who engineered the first Saabs and they must have done something right…
I accidentally stumbled across Ljungström’s whitepaper last summer. Anders Isaksson, a former Saab employee, kindly made available a copy of the whitepaper on his blog.
Unfortunately the document is written in Swedish. As a Norwegian I can understand most things written in Swedish, but half-way thru I abandoned my translation project due to the complexity involved. (sorry)
But even so, I wanted to draw some attention to this little gem. Who knows, maybe a good English translation exists out there somewhere?
In any case, the document can be summed up in one sentence: “FWD good, RWD bad, mmm’okay?”.
Some of the key elements discussed:
- Stability. A car should have a low center of gravity and a good suspension system. FWD helps by eliminating the long drive shaft.
- Comfortable. The lack of a drive shaft through the middle of the car usually means more room for the passengers.
- Weight distribution. Putting the engine up front means 60% of the weight rests on the front wheels. FWD means the drive wheels have more grip. Useful when pulling the car through snow on a slippery surface.
- Center of gravity. The lack of a long driveshaft makes it easier to put heavy parts of the car closer to the ground, as well as have the driver and passagers seated in a lower position. This helps reduce side movements and the ride feels more comfortable.
- Under-steering is easy to correct. Just let go of the big pedal.
“When loosing grip due to applying too much throttle, or even due to engine braking, a RWD car can easily reach a state where it becomes impossible to correct its course.” (I actually managed this with my 9000 once. My rear fenders were packed full of ice, snow and slush. The braking effect of all the gunk caused my rear wheels to loose traction now and then, and the end result was me pulling a 180 degree turn on the main road. This made me realize the importance of putting the best tyres on the rear wheels, not because I think they’ll magically keep my fenders clear, but because I want to avoid loosing grip!)
Mr Ljungström acknowledges that a RWD car might be able to get up a steep hill covered in snow. As the angle increases, more weight will rest on the rear set of wheels. A driver of a RWD car can ask someone to sit in the back, or put a bag of sand in the trunk, “but where to find a spare passenger late in the evening? It is easier to simply ask your potential passengers to step out in case you encounter a particularly troublesome hill”.
Of course, a lot has happened over the last 50 years, but I find it difficult to not notice the many German cars stuck in the right lane after a little snow has been added to the mix. It isn’t easy to beat fundamental physics.
theSandySaab said on December 29, 2012
Yes a pearl indeed. An interesting read for the (swedish speaking) technically inclined.
)
I remember when I got to borrow my parents FG 1979 900 GLs (dual carburetors, normally aspirated) 5 door to go skiing with friends in central (northern) Sweden and we got stuck with at least another dozen cars, mostly rear wheel driven Volvos, at the bottom of a fairly steep hill. It was getting dark and had snowed and was just below freezing, but the many attempts of climbing the road on the hill had made the snow hard compressed at best, with icy patches. Nobody made it up that road, even with spiked tires. Finally, someone in my group of 4 young guys, came up with the idea to back up the hill. It almost worked, but we finally made it by putting two of my friends on the hood while reversing uphill – we where first on the top! Everyone was looking big eyed at our successful experiment and some tried to imitate the recipe, reversing uphill with one or two persons on the hood.
Funny enough it did not seem to help all the (RWD) Volvos….
theSandySaab said on December 29, 2012
This must have been in 1980, when Volvo still produced the 240 series…
Still can’t believe my parents let me go in their virtually brand new 900…
Hans H said on January 1, 2013
I have done that with a V4.
Angelo V. said on December 29, 2012
Climates without much snow—-RWD has many benefits. Hitorically, RWD has required much less maintenance and has shown greater durability and handling. That’s why fleets like taxi cabs and police cars always wanted RWD cars. Racing cars and sports cars always opted for RWD because of the better handling characteristics too. When it comes to everyday driving—particularly where slippery conditions exist—no comparison. The confidence that FWD instills is so superior, again, no cmoparison. I don’t care about traction control and other “advancements” on a RWD platform. Any FWD car will pull much better in snow because….well because it’s PULLING! My first car was a Chevy Monza hatchback—a comparitively small RWD car with a cast iron V6 engine in it. Very heavy in the front, very light in the back, rear wheel drive. It was a disaster on the snow and as an inexperienced driver, I wrecked it twice in about a month on slippery snow covered roads. Had it fixed and kept it for many years. Loved it so much, a few years ago, I bought another one—-same year, same color, but with a V8! I still have that one—-reserved for driving on nice days. Returns me to my youth.
minienigma said on December 30, 2012
As others have pointed out, there are advantages to RWD. While it is true, the weight of the engine gives better traction to front wheels, but RWD makes it easier to achieve 50-50 front-rear weight distribution. The 50-50 weight distribution allow much better handling characteristics on dry roads. Likewise, with RWD, it is easier to perform a controlled loss of traction to the wheels, which is useful during specific situations for trained drivers.
Personally, I think FWD is safer in general and in most situations, but RWD is probably more fun.
saabonaut said on December 30, 2012
As far as my memory serves me, it was in Top Gear, as they testet the 9-5 I with quite a load of HP, when they quoted a swedish engineer, that Front Wheel Drive with more than 230 HP simply doesn’t make sense because of loosing traction and not bringing it down to the road. The tested AERO had far more HP and they demonstrated how it would pretty heavily understeer in curves. This wasn’t quite the best advertising for us, i guess …
Rune said on December 30, 2012
Top Gear has a tendency to make dubious claims, which is why I have stopped mentioning their ‘test’ of the BMW xdrive system where they got stuck trying to drive up a grassy knoll (while a land rover happily pulled up without a problem). It is a very entertaining (and addictive) TV show that I never miss a single episode of, but I doubt their research always holds up to the light.
Logically speaking, shifting down to third or second, and then push the big pedal all the way down is going to get you into trouble with almost any Saab navigating a roundabout. XWD buys you a few more seconds before everything goes poo-shaped, but even so… I rarely see anyone taking a roundabout at my speed. Except once, and that was Bohlin in his 9-3 as I rarely enter speeds that can get my driver’s license revoked.
On the autobahn, where the turns can be taken at 250 kph, extra horses in a OG9-5 will only add to the fun. There is no limit then as far as I can tell. (MapTun can get your FWD 2.3T OG9-5 to 470 bhp / 580 Nm)
Accelerating from a stand-still, you of course have the problem of weight being pushed to the rear set of wheels. FWD is a handicap in that situation, but… I somehow doubt most of us are that into drag-racing anyway. (compared to all other drivers I see on the road, I get going plenty fast already and don’t really need the extra attention a 2 second 0-100 would earn me)
Either way, Ljungström’s assertion that understeering can be corrected simply by reducing throttle input still holds. In the snow, Saab’s FWD design makes driving as comfortable as if you were driving in the summer. And let us not forget the success Saab enjoyed on rally circuits all around the world in the 50s, 60s and 70s — often competing against cars with bigger engines. But yes, they never stood a chance at drag racing (except perhaps in the snow).
Angelo V. said on December 30, 2012
My ’04 9-5 wagon has 220 HP and for real world driving in the U.S., I’ve never felt the need for more power. My car has very decent interior volume—excellent off the line performance and the power to pass on the highways—-and on trips, can get over 30 MPG. Its handling is the best of any FWD car I’ve ever driven—-feels very light and balanced—-sporty and comfortable. I imagine on the autobahn, some extra horses can come in quite handy—and with added aftermarket performance or Aero instead of ARC, that can be achieved. Really, I’d feel better if the car could take regular gas instead of premium—-but the owners manual REQUIRES premium, it doesn’t merely suggest it. I understand it runs better and offers greater longevity with the premium over the regular. So I stick with premium.
Rune said on December 30, 2012
I’ve only managed to push my NG 9-5 XWD to 260 bhp (Hirsch stage 1), and the extra 40 bhp is not immediately felt afaict. Still, when driving on Norwegian roads, I will take every grain of extra ‘oomph’ I can get. Those roads (curves mixed with lots of hills) rarely presents opportunities for overtaking other traffic, and decent 80-120 kph acceleration is absolutely vital (unless you enjoy being stuck behind a lorry for several miles).
I do share some of theSandySaab’s objections below. Originally I wanted to compare 9-5 FWD and XWD head to head before “pulling the trigger”, but everything was moving slowly back in the summer of 2010 and I already hated the idea of waiting three months to take delivery (how to test properly in the summer anyway?). So I shrugged my shoulders and figured that although FWD is good enough for me, I’d sure as heck would love to try my hand at XWD. So XWD I got. Looking back, I probably would have been better off with FWD (220 bhp FWD probably has the same amount of ‘oomph’ as my 260 bhp XWD), but as a Saabista I am happy that my particular 9-5 has almost all the bells and whistles on offer at the time.
(basically I agree with you, but my playful side would not mind exploring the 300 bhp or 400+ side of things
)
saabonaut said on December 31, 2012
yeah, rune, I get your point and you definitely know what you’re talking about – so, thank you, this makes very much sense.
Just for the sake of completeness: it was TopGear s03e03, and it was a Saab-engineer who told Clarkson about the absolute bhp-limit for a fwd, which would be 220 bhp.
However, you made the right point and I think that Saabs video about how the XWD works pretty explains the advantages about rwd and fwd and how they beautifully come together in the XWD. – for which I really envy you, as we have the regular fwd-160bhp diesel and xwd looked really awesome. Anyway the way the new 9-5 fwd handles and moves through corners and how well it “sticks” on the road is just impressive and we love it.
theSandySaab said on December 30, 2012
Virtually all powerful FWD are restricted in the lower gears in terms of torque. This is where a tune can be a fun experience, which will reduce/remove these restrictions… I’d love to try a MapTuned OG 9-5 with 470/580 performance, even on this FWD vehicle.
In terms of XWD on other surfaces than complete ice, my experience it is really easy to keep a nice 4 wheel drift. The XWD is by far the best 4WD system I have experience in terms of driveability on roads (maybe less so in terrain or grassy knolls), it makes a car (Saab) a point and push vehicle. XWD is a great system, while very advanced and complex, which again with my own experiences (leaking rear diff) makes me wonder about its longevity. It is a vast step from KISS (Keep It Simple Silly) and “Less Is More” that Saab used to be…
On dry surfaces very few FWD cars can compete with RWD when in comes to feel and sensitivity. The best FWD car I have driven that, in my opinion was better than the RWD MB C300 (both 2011 models) was the NG 9-5 (TiD, fwd) – it had a brilliant chassi, very nice to drive with very good feel…
(just my .1€)
Bernard said on December 30, 2012
There are two seemingly contradictory characteristics of FWD cars that are rarely understood.
First, FWD is much safer and more predictable close to the limit. You can hoon it up in a FWD car with tyres smoking and squealing way past the point where a RWD car will have spun-off and self-destructed against a tree.
Second, FWD is actually harder to drive at the limit. You get to a point where rules change: lifting the throttle produces oversteer, so the way through a corner is to accelerate even harder.
I don’t buy the “fact” that RWD will handle better on dry roads, at least when dealing with sub-F1/Nascar horsepower numbers. I remember 10 years ago when FWD and 4WD cars were competing together on tarmac stages in the WRC. The FWD cars humiliated the 4WD cars.
On a Saab-related note, some will remember that the Saab 99 won SCCA Showroom Stock championships against some of the all-time great RWD cars: 240Z, 510, 924, MGB, TR7, Celica, 2002, 320i, etc. If I recall, the 99 had a power deficit as well as an “architecture deficit”, so it must have all been down to driving…
Rune said on December 30, 2012
On a slightly related note, last year I researched RC cars trying to figure out if anyone had made a nice Saab 1:10 scale model.
http://en.wikipedia.org/wiki/Yokomo had this interesting snippet to offer:
“Yokomo are also known for the infamous and very rare YR-F2 chassis line. Despite being a full time front-wheel drive RC, the YR-F2 is banned from certain racetracks as it was too fast for other RC’s.”
So, at least now I have some idea of who should make the chassis for the next RC Saab.
GerritN said on December 31, 2012
Okay, my two cents.
You build a shitty car, i.e. wrong weight distribution etc, and it doesn’t matter whether it’s FWD or RWD. I’m in the lucky situation of driving two great fun cars, a Viggen (with reinforced engine mounts and fatter rear swaybar) and a Porsche Boxster. Both cars are a hoot to drive but in very different ways. The Viggen has to be driven as an WRC car, just ignore that the front wheels like to live their own life under full torque and try to rip the steering wheel from your hands. This can be a huge amount of fun but the understeer you have to deal with does get tiring after a while. The Boxster with its really mid-mounted engine has a completely different character. Where the Viggen is a blunt performer around corners the Boxster is precision tool. On dry tarmac you can basically steer the car by driving against the limit or by slightly drifting while the front wheels give unbelievable feedback that is not compromised by these wheels trying to apply torque to the road.
So, both cars perform very well when driven to the limit on dry tarmac, with the Boxster the clear winner of effortless cornering. However, confidence in the Boxster quickly disappears when the road gets slippery, it is unbelievable easy to loose control of the backend with some really embarassing results. Of course the Viggen with its enormous torque is also a handfull in the snow, but with some careful throttle control it does keep plowing on.
Conclusion, on dry tarmac both cars are very satisfactory to drive but in a very, very different way. On a slippery surface the FWD Viggen instills much more confidence, the Boxster just stays in the garage until better weather comes around.
The only pleasure I had with a AWD was in a small Honda. I borrowed this car from a colleague when I had just moved to the Boston area in January and it was snowing like hell. This was an amazing little car and pretty much unstoppable even when plowing through big snowdrifts. But, keep in mind that AWD adds a lot of weight and friction to a car. Most of us just don’t need that kind of traction for most of the time, so the performance and efficiency penalty is just not worth it.
RS said on January 3, 2013
I’m often doing long distances on icy roads and it got me thinking isn’t a big part of the fun also to be able to retain regular speed in compromised weather without breaking a sweat?
Understeer and good chassis balance provides the luxury of not having to pay much attention to keeping the car on the road.
In my experience many owners of other makes, especially RWD, often think Saabers are maniacs or bullies only because slippery conditions don’t force us to slow down as much and/or change driving style.