Rear diffusers in formula 1 – in controversy

update – Diffuser hearing result is out. My article “Diffusers legal in F1”

So what exactly is the rear diffuser controversy in formula one? I did some reading and understanding about that and would like to present a basic understanding about it.

  • Aerodynamics has two major functions in a F1 car:
  1. increase downforce of the car i.e. push the car down on the road providing better grip and thus handling
  2. reduce the drag on the car and consequently improving top speed of the car.
  • Diffuser is a component involved in improving the aerodynamics of the car.
  • One way of producing the required downforce on the car (other than front and rear wings) is what I can be simply explained as the ‘converse of the airplane effect’ – The air flow over and below the car is adjusted in such a way that the pressure over the car is higher (air flows at lower velocity, in accordance with Bernoulli’s principle) and the pressure below the car is lower (air flows at higher velocity). Thus, with a higher pressure on top, it pushes the car downward generating the downforce. This downforce can account for as much as 25-30% of total downforce produced.
  • So the principle is to accelerate the air flow beneath the car, to make it it reach larger velocity to get a higher pressure difference. This is done by a narrower flow cross section in the undertray of the car.
  • Then the diffuser comes in. The diffuser as the name suggests is basically a passage with increasing cross sectional area. This slows down the air and increases its pressure. Slowing down the air to a similar speed as that coming from the top helps in preventing flow separation from the undertray profile. Flow separation can lead to heavy drag losses so the diffuser has to be precisely designed for this. The increased pressure also fills up the wake region behind the car and prevents drag losses due to difference in nose pressure and low tail pressure caused due to wake formation.
  • Another way of aiding this process is by opening the exhaust of the car into this diffuser which helps in sucking air more quickly from underneath the car.
  • Now the larger the volume of the diffuser – the better its performance both in terms of reducing flow separation and also through design achieving a higher air velocity at the bottom (i.e. a lower pressure). This is exactly what some of the teams (Brawn, Toyota and Williams) seem to have succeeded in doing.

Firstly it is very important to see and understand a rear diffuser as per rules. As you shall clearly see it does not make any use of the rear crash structure for its aerodynamic passages. Take a look McLaren MP4-24 rear diffuser design – FIA report

There are two aspects to this controversy – the external design and the internal design

  • External – the teams have done is that they have utilized a central section tunnel. This is one point of dispute. However, as per rules a 175mm central section is permissible. Another point of dispute is the extension of the central diffusion section to within 150 mm of extra body work.

(Williams have cleverly employed a secondary or  ‘double decker’ diffuser by modifying the rear crash structure which exceeds the limits.) – for the pic – FIA reports

(Toyota have extended their bodywork to exploit the 150mm rule. This provides an extended diffuser section allowing it work more effectively. There is also a very low level splitter for more smoother air-flow) for the pic – FIA reports

  • Internal – The diffuser in itself consists of many sections. What the above teams have done is that for the central section of the diffuser they have used a incoming hole. And interpreted the rule in such a way that only the diffuser volume is only the central section and not all three sections. This I believe has helped them to have a higher flow rate possible as well as lower pressure. Also the hole coming from the side will help prevent flow separation.

(The internal hole in the Williams diffuser design – FIA reports)

  • All other teams have taken the spirit of the rule and interpreted it simply as all sections must have similar height and width with no entry tunnel. – FIA official report
  • My verdict – If the judging is fair, and only technical stuff considered, this should be allowed. Other teams should play catch up and all the big guns should accept that they have been out-smarted.
  • Another important point was made the other day on commentary – Imagine 10 teams with about 500-700 of the most elite designers and top class automotive engineers in each of the teams’ R&D. That makes about 5000-7000 geniuses pitted against how many FIA rule book officials – 15! Definitely people will find loopholes to exploit!

For more information, clarification, pics – check out my references

Wikipedia – Diffuser

Wikipedia – Ground effects

2009 regulation changes for diffuser: FIA

Excellent pic showing Williams’ Diffuser on car.

Good aerodynamics drawings and parts zoom in view

The actual diffuser on the race car with more

Everything u need to know about F-1 cars ( a seven part awesome series, aerodynamics in part 7) – Click on “taking the lid off F-1 on the website



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  3. amusana said

    Hi Vanishd!, Hope this is not yur real name..:-) !

    This is pretty impressive stuff, re, yur explanation on the this Diffuser saga!
    Am an F1 fan, and have followed with keen interest this controversy on the Diffuser..then my mind suddenly began the quest – what is this diffuser itself, what’s the impact, on car performance.e.t.c Being a Computer Engineer, and love Physics while I was at school, I enjoyed every bit of yur explanation, starting with the basics – Bernouli principle, the FIA reports/rules, pics of the rear diffusers of the McLaren – MP4-24, and then pics of the rear diffusers of Williams and Toyota, and how they have interpreted FIA rule….Brilliant stuff, well rounded, couldn’t ask for more! God bless!…
    My own verdict: Wiiliams and Toyota out-smarted the other teams, and is legal until that FIA rule is revised!These teams capitalised on the loop-hole in that FIA rule. Though, I’ll quickly add, their interpretation is not intrinsically binding, but sort-of relatively binding – Because, they truly did exceeded the 175mm allowance, though the rule made room for it! …My own verdict… Akin – Dublin

  4. Avnish said

    thanks Akin! My name is Avnish btw!

    Our verdicts are on similar lines… And I believe that when people talk about ‘spirit of the rule’, it then comes totally under the judges discretion. The race stewards at Aus GP have already given their verdict, haven’t they. The big guns should just put their head down, focus and come up with their own design. BMW recently announced that they are nearing completion with their own diffuser system in case the FIA rules the diffuser legal…

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