Third springs - what they are and how to use them

Discussion in 'Setup Information (Under Construction)' started by Alex Hodgkinson, Jul 10, 2021.

  1. Alex Hodgkinson

    Alex Hodgkinson KW Studios Developer

    Jun 5, 2017
    +1,949 / 0 / -0
    I'm being asked more frequently of late about the third spring - what's it for, how to use it and so on. So it must be about time to get stuck in and explain.

    The explanation

    Let's begin by picturing one end of the car. You have the two wheels connected by suspension arms to the chassis which is being held up from the ground by one spring on each corner. OK, some cars may have monoshock suspension where both wheels are taken care of by a single spring/damper unit, but that was rarity even at it's peak 10-15 years ago. Example here. Our regular pair of corner spring/damper units soak up the road bumps with their high speed damping, and weight transfer with their low speed damping. How much the suspension compresses is directly related to how stiff the springs are, while the damping rates decide how quickly the suspension is allowed to move.

    That is all well and good until you factor in the huge downforce levels some cars produce. Spring and damper rates which may be perfect at lower speeds are vastly insufficient once the speed increases and the entire car is squashed to the ground. Once underbody aerodynamics are allowed to get too close to the road surface they risk stalling which is as dramatic as switching off downforce. See porpoising for more on that topic. Early solutions to this problem were to run extremely stiff springs which would hold the car up at high speed, but compromise mechanical grip at lower speeds. Then some clever person invented the third element:


    The third spring/damper (I'll call it element from now on) connects both sides of the suspension together and is activated only when forces act to compress the suspension on both sides of the car. So, when downforce is pushing the car to the ground the third element pushes back. When braking and the nose of the car is trying to dive, the third element resists it. Meanwhile each corner spring is free to soak up bumps and deal with body roll without activating the third element. Thus the corner springs can be softer as they don't need to deal with downforce and mechanical grip improves as a result.

    To get the total vertical stiffness of the car's suspension we add the three elements together;

    Stiffness = SpringLeft + SpringRight + 3rdSpring

    How to use it

    At a later date I'll go in to more detail regarding how to tune the third element and it's damper. The damper tuning takes a bit of explaining and I strongly advise you master this method before even attempting to fiddle with the third element damper. Although a useful rule of thumb is that if you increase third spring stiffness you should also increase both damping and rebound (low speed only should suffice). The opposite is true if you decrease third spring stiffness.

    That leaves the springs and plenty of options for tuning. Put broadly, you use the third spring to control how the car's ride height changes at speed. It's best to use a third party data logging app like Second Monitor or RST so you can monitor ride heights throughout the lap. A stiffer third element will result in less ride height loss as speed increases, whereas a softer third element will result in a car which runs closer to the ground at speed.

    This gives us some interesting options/routes to follow seeing as the following is true:

    - We will increase downforce if the rake increases (up to a point)
    - Increased rake will also increase drag

    One option is therefore to run a high rake/high downforce setup at low speed with third elements which allow the rear to squat down a speed thus decreasing drag.

    Another option might be to run a third element setup which gives uniform rake at all speeds at a circuit which is comprised of medium/fast turns with short straights. That way downforce is prioritised as straight line speed isn't a major factor.

    Alternatively you might have a car which had inherent aerodynamic understeer. It may just happen to improve that situation if you allow the front to run closer to the ground at speed.

    Anyway, food for thought. Hope this will be useful for you.

    More soon!
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  2. Zziggy

    Zziggy Well-Known Member

    Apr 27, 2019
    +73 / 0 / -0
    Which cars in RaceRoom do have third springs? Only single seaters?
  3. Alex Hodgkinson

    Alex Hodgkinson KW Studios Developer

    Jun 5, 2017
    +1,949 / 0 / -0
    DTM 2013-2016
    P2 (except Daytona Prototype)
    VW ID.R

    Although some may not be adjustable currently
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