Porpoising; what is it and how do you stop it happening?

Discussion in 'Car Information and physics updates' started by Alex Hodgkinson, Dec 18, 2020.

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  1. Alex Hodgkinson

    Alex Hodgkinson Sector3 Developer

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    Porpoising
    Verb
    To move through the water like a porpoise, alternately rising above it and submerging.
    "the boat began to porpoise badly"
    or in our case to drive along the road, alternately rising and diving.

    This is a phenomena which is specific to cars which rely heavily on ground-effect to create downforce.
    A ground effect car uses a sculpted lower surface in order to create an area of low pressure beneath it. This low pressure area causes the car to be 'sucked' down towards the ground:

    [​IMG]

    The forces generated by ground effect are immense. At the peak of their development in the early 90s ground effect Group C cars were producing more than double the downforce of anything that's racing today.

    However, ground effect development is far from plain sailing. This style of downforce generation can be hugely sensitive to the pitch, yaw and roll of a car's body. So much so that a Williams F1 engineer once admitted that if the ride height changed as much as a few mm, downforce generation could drop by as much as half. One of the biggest issues that GT, Prototype and Touring cars have with ground effects is when the front of the car gets too close to the ground. Air flow to the underbody is cut off, which quite literally switches off the downforce. This is less of an issue with single seaters as air can flow more freely around the front of the car given that the front wheels are not enclosed.

    This sudden switching off of downforce causes the car to jump upwards as the springs are no longer being compressed by the huge downforce. Once the ride height has risen up and the airflow has re-attached, the downforce builds quickly again, forcing the car back towards the ground and the whole cycle starts over. To an onlooker, the car can be seen to be physically jumping up and down, like a Porpoise swimming in the sea. Pay particular attention to the high speed sections in the video below, especially this section to see what this looks like inside RaceRoom;

     
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  2. Alex Hodgkinson

    Alex Hodgkinson Sector3 Developer

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    Aerodynamic solutions

    So how do you solve it? For the first race teams to experience it, it was a real conundrum. Considering how much downforce the GTP cars of the late 80s and early 90s were making and how stiffly they were sprung to keep them from scraping the ground, the porpoising effect was incredibly violent.

    Once they had worked out that the route cause was cars running too close to the ground, they increased spring rates and damper rebound settings. The stiffer springs would increase the ride height at speed, and the rebound would stop the car from springing up so violently should the underbody stall. That however was a less than ideal solution as it dramatically increased the load and also the load variation of the tyres, meaning mechanical grip reduced and tyre failures became more frequent. Some teams also used very small bump rubber gaps meaning the cars had effectively no suspension at speed.

    Something better was needed.

    One example is the Nissan NPTI team's solution of installing front diffuser vents, shown in the picture below above the car's headlights. Referred to as BASSholes, they syphoned off the airflow from inside the front wheel arches even when the splitter was running extremely close to the ground, thus reducing the possibility of total downforce loss. Louvres placed over front wheel arches are a more common solution which is still seen nowadays and which performs a similar task.

    43904634.XRFHmugP.jpg

    The most common solution applied to most flat-floored race cars since the mid/late 90s is to scallop out the centre section of the splitter, highlighted below in yellow. Air flow in though this scalloped out section will not be blocked off should the rest of the splitter touch the ground, so downforce production can continue.

    r8splitter.png
     
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    Last edited: Dec 18, 2020
  3. Alex Hodgkinson

    Alex Hodgkinson Sector3 Developer

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    Third springs

    614250e4bd0a3a02fe939f4bb31c6da9.jpg

    I'm often asked what the third spring on a race car is for, and this is it's intended purpose; to hold the car at an optimal ride height so that the underbody downforce production can remain optimal and consistent.
    Look at the picture above and you'll see that the third (central) spring is connected to both suspension pushrods as well as both left and right damper/spring assemblies. There's also a pivot on that connection which means the left and the right damper can move independently, but should a force act on both left and right damper the central spring is also compressed. So any force pushing the car down towards the ground equally on both corners of an axle needs to compress left spring, right spring and central spring.
    This now means that there's no need to run exceptionally stiff 'corner' springs as the central spring handles that load. Thus suspension compliance can be increased and mechanical grip will improve.
    Third springs can also be easily tuned to change the pitch of the car as speed changes, allowing drag to be reduced at speed.

    For reasons of cost-control, third springs are outlawed in a lot of race series these days. This has meant that cars have had to resort to bump rubbers to mimic the effect. There's a lot I'd like to say about this, but seeing as most of my knowledge about this has been gleaned from the engineers who design race winning ADAC, DTM and GT3 dampers for KW, I think I'll hold off telling the world their secrets!

    Active suspension

    There are many types of active suspension system, all of which are outlawed in modern race series with no exceptions. One example would be a system which 'learns' a circuit as well as a car's aerodynamic attributes. Springs and dampers are replaced with linear electromagnetic motors which are programmed to hold the car at predefined ride heights in various situations or points on the circuit. Takes a huge amount of number crunching to get right, but the results are total optimisation, or if you're a 1992 Williams FW14B crushing domination.
     
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    Last edited: Dec 18, 2020
  4. Alex Hodgkinson

    Alex Hodgkinson Sector3 Developer

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    What you can do about it

    Let's be clear, a little bit of porposing at high speed in a straight may not have any detrimental effects. It might make your motion rig shake a lot or your buttkickers go crazy, but that is what it's supposed to do.
    It becomes a real issue when it starts to happen in the middle of turns and it can send you straight to the scene of a huge high-speed accident before you even know what's happened. Now hopefully we've spent enough time setting up our cars so that doesn't happen on our default setups, but should you start to tinker it may well become a factor.

    If you find a car is porposing so hard you loose control, here's what you should do:
    • Raise the ride height, particularly at the front
    • Increase spring rates of corner springs and/or third springs if the car has them
    • Increase high-speed rebound damping
    • Porposing is often triggered by hitting a bump at high-speed. See if it's possible to drive a different line which avoids it.
     
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