Floors and Diffusers – The basics explained

An F1 car is a complex vehicle, a lot of emphasis is placed on the things we can see, the wings and bodywork. Sometimes we can talk about less visible items such as engine, gearboxes, suspension or even electronics. But perhaps the least visible and detailed part of the car is the underbody. The floor and diffuser, that together create nearly half the cars downforce, for almost no drag. Underbody aerodynamics have been the key to F1 car’s ever faster laptimes. All we ever see of the underbody is the exit of the diffuser and sometimes, if seen from a low angle, the step under the cars floor. To aid explanations in my other articles on underbodies, I have summarised and simplified what the underbody consists of.

Reference plane

Reference plane: Red

This is the datum for the cars dimensions and is effectively the lowest part of the cars floor. When the old flat bottom regulations, dating back to the banning of ground effects in 1983 were revised in the wake of Senna’s 1994 crash, the floor has had to have a step along its length. So we see the stepped shape of the car in frontal profile, with the reference plane sitting lowest in the middle of the car. This step cannot be wider than 50cm or narrower than 30cm, the reference plane must by flat and run continuously from behind the front wheels to the rear axle line. The Reference planes leading portion, also forms the splitter, also known as the T-Tray or Bib.

Step plane

Step Plane: Yellow

Above the reference plane is the step plane, this is effectively the underside of the sidepods. This must sit 5cm above the reference plane. Again the surface must be flat and run from the complex regulated bodywork zone around the front of the sidepods to the rear axle line. A large clearance is mandated around the rear wheel to prevent teams sealing off the floor against the rear tyres.

Step or Transition

Step: Orange

In between the reference plane and step plane, is the step itself or transition. Simplistically there must be a vertical surface in between these two planes. Any intersections of these surfaces are allowed to have a simple radius to be applied, with a 2.5cm radius on the step plane and a 5cm radius on the reference plane.


Plank: Brown

Not considered part of the floor for measurement purposes, the plank is a strip of wood placed under the car to enforce a minimum ride height. The FIA technical term for this part is the skid block, although this term is rarely applied. Holes in the plank allow the cars reference plane to sit directly on the FIA scrutineering jig, for legality checks over the course of a GP weekend. Titanium skid blocks are allowed to be fitted in certain places in the plank and their wear is measured to ensure a car is not grounding from excessively low ride heights.
The plank can be made in two parts to make removing the floor easier, bit the front section must be at least 1m long. This must be made of a material with a specific density, to prevent excessivley heavy or hard planks producing a performance benefit. Typically the plank is wood based, eiterh jabroc a laminate of beechwood, although more exotic blends of woods and resins not unlike MDF have been used. The plank is 30cm and 5mm thick, any holes made into it must conform to a FIA template.


Diffuser: Yellow

A purely flat floor would probably produce lift rather downforce, so the rules have allowed a diffuser to be fitted to the rear of the underbody since 1983. Before that date there were no rules demanding floor dimensions and diffusers were the full length ground effect tunnels that typified the wing cars of the late seventies and early eighties.
A diffuser creates downforce by creating a pressure differential, with low pressure beneath and higher pressure above. The larger a diffuser is, the more expansion ratio is has, thus more potential to create downforce. Diffusers were limited to a simple 100cm width, 35cm length and 17.5cm height from 2009. Then for this year the height further reduced to just 12.5cm. This massively reduces the potential of the diffuser to create downforce compared to the previous rules. Diffusers are allowed to have fences, but the fences and the diffuser itself must not form undercuts when viewed from below. Which is why we see the simple vertical fences and jelly mould curvature.

Other rules around floors
Overriding all of the above rules are broader regulations covering holes and flexibility. No unsprung part of the car can be visible from below the floor. Typically this means anything, but the suspension and additionally the wing mirrors. This means that no holes can be made into the floor to let flow in or out. The underbodies surfaces are termed bodywork within the rules, there is no term ‘diffuser’ or ‘wing’ mentioned in the rules. Just as with any bodywork in the rules, these parts are not allowed to move or flex. For the floor in comparison the wings, there are few deflection tests commonly carried out, the main one being the splitter deflection test.


Double Diffuser

Over the past two year these rules have been exploited by teams. Firstly the interpretation of holes in the floor and continuous surfaces. This lead to the openings that allowed double diffuser. Effectively the step formed two separate, but individually continuous surfaces, allowing airflow to pass up above the step plane into the upper deck of the diffuser. This rule has been clarified for this year and a single continuous surface must be formed under the floor.
Additionally the flexibility of the splitter has been brought into question, teams were believed to be flexing the splitter upwards, new more stringent tests were introduced in 2010 to stop this.

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16 thoughts on “Floors and Diffusers – The basics explained

  1. Great article Scarbs. I think 2011 diffusers were reduced from 175mm to 125mm in height and I don’t think the plank is wood anymore so I guess it’s some sort of carbon fibre perhaps.

    One thing that I find hard to spot in pictures is the gurney flaps on the diffuser, and are there any regulations on them?

    • 2011 diffusers are not any different that the original 2009 diffusers (obviously not counting Brawn, Toyota and Williams).

    • Well pointed out on the 2011 height, ove amended that and clarified the plank too.
      Gurneys on the rear edge of the diffuser only need to meet the max bodywork height for their specific span of the floor. They tend not to be very tall, I’ll have to check, but for most of the diffuser its probably limited to 25mm. But taller for the middle 150mm section.

  2. Great article Scarbs! This is the best, most easily understandable explanation of the diffuser that I haver read. It’s the first time that I actually understood double diffusers!

    Could you possibly also explain how the low pressure is created? Cheers,

    • The diffuser “creates” low pressure as it is a larger volume than the space the car. As the fixed amount of air under the car expands to fill the diffuser, the pressure drops to less than normal atmospheric pressure. Air always wants to be at atmospheric pressure, so the diffuser volume tries to (1) pull more air into itself or (2) reduce its volume. As it pulls at the ground to try to reduce the diffuser volume downforce is created (or the car is pulled down to the ground since the ground can’t be pulled up.)

  3. Thanks a bunch for this one Scarbs! I’ve not quite fully understood how a diffuser works or looks for a long time – mainly because you just never see the underside of an F1 car! This has been a really informative post and has finally shed a light on the diffuser. Keep up the good work please!

  4. A million thanks to your ever timely and professional posts!! Really help engineering idiots like me 😉 “add oil” (hong kong way of “keep up the good work”)

  5. Hi Graig,

    Thank you for your site and blog. This is just what I need to satisfy my F1 OCD. 🙂 I do however have one suggestion. Can you prepare a glossary of terms used in F1 and were these terms are found on a F1 car? I have no idea what exactly is meant by a gurney flap for example. I do know that you are busy, if you are able to do that in the future or just incorporate those descriptions in your drawings it will help a lot. 🙂

    I discovered your site in November and I have read everything, including everything on your old website. It was an enlightening journey… Thank you so much for that.

    Incredible work.

    Kindest regards

  6. Great article, written with the appropriate level of technical explanations!

    Regarding the plank, would a team use several planks over the course of a weekend?


  7. Great article, as with others this is the first time I really understood how a diffuser works. You have a real talent for getting to the heart of a question with just a few sentences and a relatively simple drawing. Thanks!

  8. Hi

    I have a question for you Scars 😉 How would you compare the downforce generated by the diffusers pre 2009, in 2010 and currently? I know that teams won’t provide any real numbers, but they often say that they have x% less downforce due to rule changes etc.

    It’s quite interesting how much the extra channels in DDDs were worth, or how much more/less downforce they’ve generated compared to the diffusers in 2008, Or how much more they gain by blowing the diffusers.

  9. Great article, though I have to admit my understanding is slightly different. I was always lead to believe that the floor generates downforce by accelerating the airflow through the restricted space between itself and the road (Bernoulli’s principle). The high speed air would be a lower pressure than the slower air above leading to a downward force. How would a flat floor lead to lift unless the air along the top surfaces of the car was travelling faster?

    Also I thought the main purpose a diffuser was to control how the fast moving air under the car was rapidly decelerated as it expanded at the back of the car? By controlling the flow I thought this helped both reduce drag and ‘drive’ the airflow ahead of it creating more downforce. The longer the diffuser the more scope there being for controlling the flow. Is that right?

    I was always lead to believe that there wasn’t any significant downforce created by the expanding airflow as there was too much scope for the sounding air (at atmospheric pressure) to bleed into the diffuser from the back or underneath the streaks, killing the effect – hence why the ground effect cars were so dependent of skirts.

    (No criticism indented, I’m just genuinely interested!)

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