McLaren diffuser, 2014

How teams are clawing back rear-end downforce

F1 technology

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While much attention has inevitably been focused on the new hydrid engines F1 introduced this year, other changes in the 2014 rule book aimed to reduce the amount of downforce being generated at the rear of the car.

Perhaps the most significant of these concerned the positioning of the exhaust. In previous seasons teams have exploited exhaust gasses for aerodynamic gain, directing the hot air around the diffuser in such a way to make it perform better.

In 2014 this trick was effectively banned as the positioning of the exhaust is now tightly controlled. The 2014 technical regulations dictate that the exhaust exit but be between 170-185mm behind the rear axle line, 350-500mm above the floor and along the car centreline.

This has led some teams to get creative at the back of the car and exploit other means of generating downforce. The rear of the car is still hugely important aerodynamically and is responsible for about 40% of the downforce generated. The drawing below shows the back of the McLaren MP4-29.

McLaren diffuser, 2014

The exhaust is positioned above the rear crash structure along the car centreline. Although the rules are clearly written to limit the extent to which teams can use their exhausts to generate downforce, what is learned is never forgotten, and designers have deployed aerodynamic structures around the exhaust exit to try to use the gasses for aerodynamic gain.

In the case of the MP4-29, a flap behind the exhaust serves this purpose. The high-speed exhaust gasses create a low pressure zone below the flap, which helps create downforce. In addition the plume is directed upwards where it will interact with the flow structure from the rear wing, again adding to downforce (although this interaction will be small as it occurs some way aft of the rear wing).

Another source of downforce at the back of an F1 car is the brake ducts. Ostensibly for brake cooling, in recent years the inner wheel area has seen a proliferation of carbon fibre in the name of grip.

Kevin Magnussen, McLaren, Monza, 2014The MP4-29’s has five distinct aerodynamic surfaces that are clearly not designed to help cool the brakes. One advantage of the brake ducts is that the downforce is generated on the unsprung part of the car so is transmitted straight to the rubber (as opposed to being transmitted through the suspension arms).

The final part of the downforce equation is the diffuser. The dimensions of the diffuser is tightly regulated but teams do have choices as to the shape. McLaren have opted to have rounded outer edges in an attempt to keep the airflow inside the diffuser attached, which helps with downforce – turbulent air in the diffuser kills downforce.

The MP4-29 also sports ‘fat’ suspension arms. The intent of the fat suspension is to block airflow above the diffuser. This creates a low pressure area above the diffuser thereby reducing the pressure gradient the diffuser has to work against. Air in the diffuser is more likely to stay attached and generate grip.

This comes at the cost of considerable drag from the suspension, and it appears the trade-off may not be worthwhile. No other teams have copied the McLaren design and the performance of the MP4-29 suggests the invention will not be retained for the 2015 campaign.

The last detail to note is the location of the cooling exit. With the advent of the turbocharger the thermal footprint of the 2014 formula has increased. Like many F1 teams McLaren vents hot air out of the rear of the car. This comes at the cost of having slightly expanded bodywork at the rear of the car, although the McLaren engineers have raise the vent to ensure a reasonably deep sidepod undercut.

With all the focus on engines, one of the unspoken successes of the 2014 regulations has been neutralisation of exhaust blown diffusers. Unlike the front wing and nose, which will change for 2015, there are few regulation changes at the rear of the car. That won’t necessarily stop some clever teams doing something inventive in an attempt to get an edge.

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  • 26 comments on “How teams are clawing back rear-end downforce”

    1. Just have a question can you change the end shape of the Diffuser or allow for wholes in the diffuser ? Is the air allowed naturally to flow through the diffuser if it was to be changed ?
      Good to see teams coming up with different ideas though, McLaren with large suspension arms and you have to wonder if its that good why haven’t other teams gone with similar ideas

      Thanks

      1. Holes, other than the starter motor, is prohibited.

    2. I get so frustrated when I see the words ‘attached’ and ‘low pressure zone’ in random context in aero-related articles.

      E. g. ‘The high-speed exhaust gasses create a low pressure zone below the flap’. Yes, per se, but the most important thing here is that it’s still relatively high pressure compared to the zone behind the flap, which is created by the flap itself and helps suck exhaust gasses upwards and towards the low pressure zone behind the main rear wing element (and this is only possible if the air behind the flap has even lower pressure than the gasses themselves).

      And I’m far from being an aero expert…

      On the other hand, this particular article, I think, is well-written otherwise and gives a nice round-up of what’s going on at the rear of the cars.

      1. I beg to differ that the works ‘attached’ and ‘low pressure zone’ were used in *random context*. They were used to characterise what are the consequences of certain design decisions on a F1 car …

      2. Sorry, not just, or not mainly, here, but mostly elsewhere. As I said, I found this article pretty much OK apart from the one example I brought up above.

    3. Thanks for another great article John, I’d been wondering how effective the McLaren’s fat suspension arms were going to be this year but if no-one else is copying them it’s probably a good indication that they’re not that great.

    4. It’s a nice article, but I also feel some things are missing, some detailing.

      Like for instance the strakes on the floor in front of the rear wing. They are actually very important since it partially creates a sealing effect on the diffuser. Also something more could have been told about the lowdown beam wing. And also other solutions could have mentioned, like Mercedes’ unique starting hole solution, Red Bull’s “under the exhaust winglet”, diffuser VG’s, etc.

      1. IMO especially the latter point that other team’s solutions aren’t mentioned, is a bit of numbing the article for me given the title. Cooling solutions vary a lot across teams, diffuser shapes, gurney tabs, etc.

    5. I find it interesting that the fat suspension arms have not been copied and therefore have not been successful, yet Mac has kept them. Just goes to show that they can’t simply remove them, as other aspects of the rear design would then falter. ie. they need a whole redesign in order to get rid of the fat suspension arms. No surprise though, but just interesting, just as teams not being able to drastically change a front wing without that change in airflow over and under the rest of the car drastically changing how it would perform, including at the diffuser and the rear suspension. Teams still pretty much need to nail their package out of the box, or struggle all season to make meaningful gains over the competitions’ gains as development goes along.

    6. what I find interesting is the front end, the wing, a few years ago a ban was placed on all these flaps all over it, and they have reappeared, it looks far too busy,

    7. I think Merc and RBR had a very good rear end downforce and should’ve been the benchmark. But McLaren still good though

    8. I have been surprised to see aero on the brake hubs, I thought aero on the unsprung parts of the car were banded back in the ’60s when hub mounted wings were band and in the ’80s with the banning to the Lotus 88!

      1. I think the rules say something like the primary function must not be for aerodynamic purposes, which gives the teams just enough wriggle room to make parts that have a different primary function but have a secondary use as an aerodynamic device.

        1. This is the bit that get’s around it;

          3.15 Aerodynamic influence :
          With the exception of the driver adjustable bodywork described in Article 3.18 (in addition to minimal parts solely associated with its actuation) and the ducts described in Article 11.4, any specific part of the car influencing its aerodynamic performance :
          a) Must comply with the rules relating to bodywork.
          b) Must be rigidly secured to the entirely sprung part of the car (rigidly secured means not having any degree of freedom).
          c) Must remain immobile in relation to the sprung part of the car

          There is no aero constraint in 11.4 only the size and positioning of the duct.

    9. How does turbulent air in the diffuser cause the loss of downforce ??

    10. John, can you please explain how turbulence in the diffuser kills downforce ???? PLEASE :-)

      1. Sure. The diffuser generates downforce through a process called the Venturi effect. To simplify a little when air flows through a constricted volume the pressure reduces. When it then expands the pressure increases. The pressure between these two regions are related by the Bernoulli equation which relates pressure to fluid speed.

        Anyway, enough on theory. The diffuser is a device to manage the transition from the low pressure area to high pressure. If airflow in the diffuser becomes detached or turbulent this reduces the speed of the airflow and increases the pressure. This has a knock on effect across the floor of the car. Obviously one wants low pressure under the car as this pulls the car to the ground generating grip.

        1. I have read in a whole bunch of diffuser academic papers from the SAE and SouthHampton University that the base pressure (the wake) of the car is essentially constant/fixed with respect
          to the diffuser/underbody (although the Sauber F1 team on youtube told me it does change a little bit with changes to yaw, pitch, ride height, etc.). Obviously separation in the diffuser results in less expansion and less “pumping down” at the diffuser inlet. Specifically what about the turbulence in the diffuser reduces the speed of the airflow ? There is a paper from SouthJHampton where they measured forces and pressures on a front wing assembly downstream of diffuser equipped bluff body. The air measured in between the two in the middle of the wind tunnel indicated a horizontal velocity deficit and increased “upwash” velocity the higher they measured in the vertical y-axis direction. But what is going on ? How does the air’s energy get divided among the two ? Which is worse ? Turbulence from the rear tires or from the air upstream of the underfloor ?

          1. Turbulence doesn’t *always* result in downforce destruction. Look at vortices which are turbulent but can energise the air in a positive way.

            However, the reason why turbulence is often unwanted is that turbulent flow is chaotic so you can’t really predict where the airflow will go, which is obviously not good. And then the kinetic energy (velocity) of the flow is converted in to internal energy – typically linked to the shear stresses of the flow. Again this loss of KE acts to reduce pressure.

            On your last question it is hard to say which is worse … upstream air is critical for the flow regime under the car — a poorly design rear wing will be punishing to downforce. Also if rear tyre turbulence seeps into the diffuser that will also be very undesirable. Which is worse depends on exactly what is happening. However you only can do so much with the rear tyres so more aero time will be invested on the upstream airflow structures …

            1. Thank you so much John !

              I have often read turbulent air has less energy or is less energetic. I will have to read more about it. The thing that confused me was that several papers from Stanford University (California, USA) in the 1960’s suggested turbulence upstream of a two dimensional diffuser on a glass water table increased peak pressure recovery. I have also read in several places that a turbulent boundary layer can help delay or suppress flow separation. So I found that confusing. A much more modern book was by David Japikse called “Diffuser Technology” in which he wrote that generally turbulence hurt the performance/pressure recovery of the diffuser. I am just beginner who reads this kind of stuff as a hobby (for the time being) but the Standford guys induced turbulence upstream of the water table diffuser by putting a rod in the flow stream to shake it up.

            2. A vortex is not in and of itself a turbulent flow structure – probably not the best illustration for an otherwise valid explanation.

    11. You hear teams talk in terms of ‘philosophies’ in car design. They plan the airflow from front to back as a whole which is why you don’t see successful concepts simply copy and pasted between cars, or not so successful concepts dropped immediately.

      McLaren’s fat suspension obviously doesn’t give an advantage over traditional car designs, but it will in itself generate down force which is why McLaren can’t afford to just give it up unless they can figure a way to claw it back elsewhere.

      The other teams haven’t copied it because even though it will give down force, their own simulations have probably shown it would disrupt their existing airflow philosophy too much to warrant, especially as it probably causes drag.

      McLaren though have a habit of ditching concepts and I think the lack of carry over each year is why they are currently chasing their own diffuser.

      2011 they probably had the fastest race car, it just wasn’t great at qualifying. The following year they abandoned the U shaped side pods.

      2012 they had the fastest car, just unreliable. The following year when everyone evolved their designs they scrap it completely dropping the low nose philosophy and going for pull rod suspension.

      And then this year they’ve abandoned pull rod and got this rather unusual rear suspension. I bet none of it carries over to next year.

      They need a design philosophy that they can stick to for several years.

      1. Yes – I agree with this. I also get the sense that McLaren, in a desperate bid to catch Red Bull, and ride the wave of ‘trick innovation’ invested all its time on what the latest wheeze could be – F-duct, U side pods, Octopus exhaust, fat suspension etc … without focusing on core aero performance.

        Red Bull had a given design and iterated over the years and when they came up with a brilliant idea eg, EBD they integrated it into their design philosophy as you say

    12. Why is a low pressure zone above the diffuser good? Doesn’t that reduce the downforce? I would have thought the fat suspension arms force the airflow upwards from the diffuser to gain downforce, like the rear wing?

      1. It is good because the pressure gradient across the diffuser is lower so air will stay attached (in the vernacular) i.e., doesn’t become chaotic and turbulent. The pressure behind the diffuser will still be higher than that under the floor, but higher than what it would have been without the fat suspension arms (in the case we are talking about).

        If you look at a cross section of the Mclaren suspension arms they are mushroom shaped rather than aerofoil shaped so don’t generate downforce. There are quite stringent rules as to the shape of the suspension which prevents teams from re-creating wing aerofoil profiles

        1. I think it’s more helpful to describe the low pressure as being behind the diffuser, not above. Most of the diffuser is ahead of the suspension arms, so on the top surface of the diffuser the pressure is mostly increased, which itself contributes downforce. Low pressure on the top surface in the short space behind the suspension arms does indeed take away downforce, but as John says, the main effect comes from the improved conditions inside the diffuser.

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