After the heavy accusation by Ciro I took some time to do some calculations to create my own primary information. It should back up my theory and show the root of the problem which is in my opinion: The lack of slipstream effectWhy do we need slipstream for overtaking?
Slipstream is the only thing that gives just the following car an advantage and therefore the possibility to overtake. As I mentioned before all other things just create a parallel shift of performance and therefore don’t create an advantage for the following car. By this I mean thinks like for example worse brakes that increase braking distance. Some argue that overtaking happen on brakes and therefore braking distance should be increased. I say the distance will be increased for both cars so there is no physical reason why one car can catch up by this.
JohnsonsEvilTwin mentioned tire grip, which admitted has some effect on overtaking. I will talk about this in the end of the post. However it is not the effect that the leading driver will make more mistakes. Even when this might be true it still does not guarantee that the follower can take advantage out of it.What is the problem with slipstream?
I basically watch two race series which are Formula 1 and DTM. Those cars are totally different but feature the same problem: Overtaking is very difficult. Something else is noticeable. F1 cars usually follow each other with a time difference of 1 second which is dependent on the speed a couple hundred meters. DTM cars can follow each other with a distance of couple cm in corners and a car distance on straights but still can’t overtake. Close following alone is not the magic bullet as some hoped for. The most alarming race was Hungary 2010 well the whole 2010 season was alarming in many ways. At least I was pissed and bored the whole season. In Hungary Vettel was 1 second faster than Alonso but could not pass him even when he was able to stay close in the last corner Alonso could keep the distance on the following straight. A slipstream effect was not notable at all.
To investigate the problem I made a excel program that should simulate overtaking due to reduced drag. As basic car I took the Mp4/13 from 1998. This car had 780PS (573kW) and set a top speed at Hockenheim (old track) of 353km/h. Based on the rules they had that time I calculated the frontal area to 1,25m².
Formel I Aerodynamik Simon McBeathhttp://en.wikipedia.org/wiki/McLaren_MP4/13
With those data above you can calculate the drag coefficient to reach this top speed. In Simon McBeaths book I found a calculation for the lift coefficient of such a car. According to McBeath the lift coefficient is 2,32.
The table will sum up the input data. Sure some figures you need to guess little bit but you have indicators like reasonable acceleration times from 0-100 and 0-200. I won’t go to much into detail of them now but when you have questions to them just ask. Also when you have any sources for data give them.
Car 2 got exactly the same figures except that I reduced the drag and lift coefficient by 20% due to the slipstream effect.
Drag coefficient: 0,592
Lift coefficient: 1,856
Ok, lets have a look on the charts generated by this. First one shows the distance travelled by both cars. Car 2 starts 50m behind car 1. After some couple hundred meters it was able to catch up just because of the slipstream. My basic intention was to show the distance needed for this process but I found some more interesting stuff by doing this. Ah yea some might argue now that it has nothing to do with reality when both cars start with speed zero. I can propose you to imagine any speed you like and read out the time from the speed chart below. Then you can set a starting point at this time and reduce the total distance by the start distance.
On the speed chart you see the velocity over the time. Remember Car 1 is based on the data of the Mp4/13 doing 353km/h at Hockenheim. Speed of Car 2 seems to be little bit to high but that’s in the slipstream of Car 1. However the absolute numbers are not that interesting. I can turn the drag coefficient up to reach any lower top speed values from any other circuit. So if you want to see other results and have the top speed values you can give me those.
Now let’s focus on the interesting stuff of all this. What we notice in the chart is that the blue car has a higher speed at around 200km/h. This is surprising because we expected the red one to be faster which in fact catches up at around 250km/h to go for the lead. So the slipstream effect sets in very late and it’s no miracle we never notice it while watching two cars following each other.
On the acceleration chart this becomes more obvious. We see that the leading blue car in fact can accelerate faster up to a specific speed. The red car is handicapped here because it is in the wake of the leading blue one which does not only reduce drag but also downforce. However things turn at around 200km/h.
Let’s come down to the root of the problem. The following chart shows the forces involved in the process. It will explain us why things are like they are.
Blue shows the force the engine can maximal deliver.
Light blue shows the traction of the car. It includes the mass of the car, the generated downforce and the friction coefficient. Also it includes the weight balance (CoG position) front to rear and the downforce balance (CoP position). It is the max. force the tires can transmit.
Red is the air resistance.
Green is the actual force left for acceleration. It includes the force delivered by the engine, the force that can be transmitted through the tires minus the air resistance. Yes it reminds us of the acceleration seen above.
From the force graph we can see that a F1 car is traction limited up to a speed of around 180km/h when the two blue lines cross each other.
Below that point the high power of the engine is not needed because we don’t get the power on the ground. The traction has a basic component due to the car weight and rises with increasing downforce. Up to the crossing point all the downforce we can get is helpful to make us quicker. This explains why the red car was suffering in the slipstream. The reduction of downforce reduces the ability to bring the force on the road. The engine has sufficient power available so lower drag is not needed yet. We notice this in the acceleration chart when the blue car was able to accelerate faster than the red one.
After the crossing point we are limited by engine power. The reduction of downforce doesn’t matter anymore because we don’t have any power left. The force the engine can deliver falls even more when the speed is rising up to a point where we cross the air resistance curve. That’s our maximum top speed limited by engine power and drag. A reduction of drag by slipstream helps us now to have more force left to accelerate the car. So after the point at around 200Km/h slipstream starts to bring an advantage in acceleration and at around 250km/h we reached the speed of the leading car. Quite late in my opinion when we consider that on a normal track the top speed is limited to around 300-320km/h. We have just this small window between 250km/h -320km/h to catch up on track.
Btw.: The crossing point of traction force and engine power should also be the moment when you push your KERS and lower your rear wing. Let’s see if this turns out to be true. What kinds of solutions are possible?
Difficult to say, many effects are included into this.
1. Somehow we must widen that window where slipstream is positive.
2. We can make the effect of slipstream more efficient.
I reduced both drag and downforce by 20%. The downforce value was much higher (2,32) so there was a higher loss in downforce than in drag (0,74) which caused a loss in acceleration performance. So if downforce and drag values are closer together the negative effect would get reduced. That was possible true for early 1980 cars. I also can’t imagine a common airplane has such a big difference between drag and lift. The reason has to lie in the ground effect which makes the car more efficient. On the other hand it’s possible that the floor of the car is not so sensitive to the wake of a car but that’s speculation.
So how about tires with less grip?
They will lower the traction curve so the car will be traction limited even more and therefore making the window of positive slipstream effect smaller. So I don’t think that’s the way to go.
Increase of car weight.
It raises the traction curve but not the acceleration of the car. I found out that it has very little effect on acceleration but I must admit a drop of friction coefficient is not taken into account yet. When the traction curve in higher our window should get increased and we should be less dependent on downforce so maybe it can help us little bit.
Increase of drag
I think that’s something that should help. Like already said drag and downforce values should lie closer together. In general higher drag means higher reduction cause slipstream. It could be achieved by wider cars. The cars before 1998 had 2m. It makes the tires more exposed what cause the surface and drag coefficient to rise. Also rear wing should be made wider. Wider cars with wide rear wings also makes the cars look much more attractive.
Reduction of engine power.
Yea I think that’s something possible. It will lower the max engine force curve and make the car more power limited. It will also lower the max top speed. Possible that the engineers want to counter that by reduced drag which they achieve with less wings making the cars even more power limited on lower speeds. In the end this could increase the area where the car can profit from slipstream.
I wonder what kind of reactions I get now.
However this can be continued. Maybe someone can give me some data of a 1980 car. We should have a look at them to see why overtaking (or racing) was better during that time. Also I might take more things into account to bring the model closer to reality. Up to now I just felt that I should show you something. It’s not that I say anything without thinking about it before.