Do spoilers on the rear of front wheel drive cars give any improvement to handeling?
Well, they can, but at normal road speeds and normal spoiler sizes and configuration, the effect is minimal. For looks only.
At higher speeds, yes they can act to push the front/rear of the car down onto the road, redirect air from underneath the car (front spoiler), and help the car stay in a straight line.
So the answer is, it just depends on several factors.
I seem to recall a comment of a sports car maker (porsche) that the self employing rear spoiler of a 911 model would add approx. 80 kg (170lb) of down pressure at 200km/h
point being - they work if they are purposeful designed and streamlined in a tunnel - and not designed for looks
cheers
alfred
Sorry, I meant to also more specifically address your question as to front wheel drive cars, as well…at high enough speeds, they can push the rear of that car down, helping the tires grip the road better. This is especially important in high-speed curves.
I have no doubt that there are a number of front wheel drive cars out there that can go fast enough for this to make a difference. I can’t think of any front wheel drive race cars off hand, but there are some hot street numbers flying around.
One other thing I should add is you have to be careful with this. I read some road tests years ago where a car with rear spoiler only had the rear end being pushed down, yes, but enough to cause the front of the car to pivot upwards, causing more drag and getting the front end “looser” thus negating the added stick of the erar spoiler. When they added the front spoiler, the nose came back down and all was well with the world.
That is why I also mentioned configuration of the spoilers in the previous post I made.
In some cases, the spoilers are for improved gas milage. You don’t want a vacuum created behind a car that will cause poorer aerodynamics, the spoiler “spoils” the air to create some turbulance.
But I think most are there for looks.
Sorry I realise now I wasn’t precise enough in the OP.
I meant do rear spoilers have any positive effect on front wheel drive cars?
The fact that the rear tires are pushed into the ground by the spoiler will help marginally during breaking from high speed (breaking is always on all 4 wheels).
Does the spoiler on the back increase or decrease the traction of the front wheels? I guess this will depend on whether the force acting upon the spoiler will tend to push the whole car down, or whether it would tend to pivot the car arround the back axel lifting the front wheels.
Is there any advantage in hadelling for a front wheel drive car to have the rear wheels firmly pushed against the road? I’m guessing the rear tyres would be less likely to slip, but is that a problem with a high performance front wheel drive car?
Yes, there IS an advantage. In a curve, forget about straight-ahead traction from the driving wheels. Think big-ass drag slicks or Indy tires tot ake care of that, OK? But we’re not talking about this. IN A CURVE, the more downforce on EITHER set of wheels is good. It helps traction,period.
Has nothing at all to do with which wheelsets are driven.
The downside is that there is a limit at which the downforce versus drag will slow you down ON THE STRAIGHTS. You have to balance drag and handling.
That’s the short story. Again, there are many, many, many variables to consider here.
But one thing you do not need to consider is which wheels are driving the car in you scenario. Honest, the spoiler(s) don’t know, and don’t care whether the vehicle is front wheel drive, rear wheel drive, or AWD.
The air don’t don’t care.
Hope this helps, just trying to categorize the multiple issues. 
But does any street legal car ever go fast enough for it to make a difference?
Just had to do this, sorry
According to what I’ve heard, almost never.
The rule of thumb I was given (by a guy involved in auto racing who seemed to know the subject) is that the spoilers fitted to street cars do very close to nothing at all below 90mph, and not much until you’re over 100.
Even then, you need to look to marques like Porsche to find spoilers that are properly designed. 98% of spoilers are mere window dressing - the “tailfins of the nineties”.
as long as I can do this one
Car
The theory (understand that I’m not an aeronautical engineer) goes like this. The flow of air over the rear deck of most cars, whether boxy or fastback, is at close to atmospheric pressure (roughly 15 pounds per sq. inch.) If you set up a vertical wall at the trailing edge of the rear deck, the flow of air gets “backed up” ahead of the wall. So the pressure rises to, let’s say, 15 1/2 psi ahead of the spoiler. You multiply that half pound by the number of square inches on the rear deck, and you get a usable amount of downforce.
The front airdam works on a different theory. The frontal area of a car, when you’re figuring drag coeffecients, includes the area from the front bumper to the ground. Air gets crammed into that undercar area, and there’s so much turbulence and pressure down there that it counts as solid bodywork. Now, if you extend the bodywork farther down, and scoop that air to the sides, it subtracts from that positive pressure. If you can cut the undercar pressure down to 14 1/2 psi, then two things happen. First, the flow through the radiator and front brakes is assisted, and second, you have a 1/2 psi downforce on most of the hood (bonnet in the UK.) The front airdam actually gives a bit of downforce without adding to drag.
The faster you go, the more downforce you get, but you do have some at real-life highway speeds. The front airdam gives an improvement in fuel mileage. On my old Mercury Capri, it gave me about 2 mpg at highway speeds (maybe enough to pay for the airdam.)
So …
front airdam good, rear spoiler no damn good
I knew you were going to do that
The OP was asking if a rear spoiler helped a front-wheel drive car. I race cars, both front- and rear-wheel drive. Whether in turns or in straights, the fastest car is the one that is smoothest around the course. In FWD, it’s all about the front wheels. Keep pressure on the fronts, keep them in front, and keep them from smoking. It is common on a FWD car to have 60-70% of the weight up front, due to the engine. The rear tires are mostly coming for the ride, as long as the tire setup isn’t altered.
A rear spoiler on a FWD car is a decoration at best. At worst, it actually pushes the rear down. Putting weight in the back takes it off the front, and that’s bad. It will unsettle the front, making it harder to go, stop and turn.
A rear spoiler on a RWD car, however, helps by putting more weight on the tires that do the work.
Now, the exceptions. Aftermarket spoilers tend to have more rake angle than stock units. Makes it look cool, bitchin, gnarly, awesome or whatever, but generally exaggerates the stock conditions. Without modifications to the other end (whether FWD or RWD) it actually makes things worse.
Stock high-performance cars like Porsche, Ferrari, etc, have little rake in the spoilers because cars today don’t need much. Modern aerodynamics make downforce less necessary than the 70s and 80s. Instead of needing to generate 1 lbf/mph, it is now down to 0.1-0.3 lbf/mph.
My BMW M3 can easily go 150 mph, but it has a “lip” spoiler, barely noticable. It’s because the car’s aerodynamics are so good. My RX-8 can go 160 mph (modified) but I retain the stock spoiler because it’s good enough. My daily driver Neon has a dang surfboard for a spoiler, but it’s a stupid decoration. In fact, the Neon would probably be faster simply for the loss of the spoiler’s weight than for the downforce (if any) that it creates.
Lastly: In the late 70s Lotus or Renault tried a FWD GT racer. It was reasonably successful, but ultimately RWD was a more reliable platform for speed. The FWD GT had spoilers front and rear, and lots of time in a aerodynamic tunnel.