Saab White paper on FWD vs RWD


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
Member
3 years 8 months ago
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… Read more »
theSandySaab
Member
3 years 8 months ago

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
Member
3 years 8 months ago

I have done that with a V4. 🙂

Angelo V.
Member
3 years 8 months ago
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… Read more »
minienigma
Member
3 years 8 months ago

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
Member
3 years 8 months ago

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
Member
3 years 8 months ago
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… Read more »
Angelo V.
Member
3 years 8 months ago
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… Read more »
rune
Member
3 years 8 months ago
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”,… Read more »
saabonaut
Member
3 years 8 months ago
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… Read more »
theSandySaab
Member
3 years 8 months ago
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… Read more »
Bernard
Member
3 years 8 months ago
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… Read more »
rune
Member
3 years 8 months ago

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
Member
3 years 8 months ago
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… Read more »
RS
Member
3 years 8 months ago

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.

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