Lotus Renault GP: Fluid Inerter

Lotus Renault GP (LRGP) have been on one of the teams most innovative with their suspension over the past decade. As RenaultF1 They introduced the Tuned Mass Damper (TMD) back in 2006 and have since raced conventional Inerters. Inerters are a special component in the suspension to counteract spring effect of the Tyres\Suspension, using a spinning mass on a threaded rod to control these loads.

LRGP have also been one of the teams racing hydraulically interlinked suspension and have looked at other ways to legally alter the suspensions performance. It seems this work has lead to the discovery of a new form of Inerter, primarily using fluid for the Inerter effect. This new development has been termed by the team a “Fluid Inerter”.

A cross section of the patented Fluid Inerter

The intellectual property rights to this development have been safeguarded by Patent, allowing the details to be freely accessible in the pubic domain (the source for the picture at the top of this article). My attention was drawn to this development by Italian Mechanical Engineer Rodolfo De Vita, a specialist in Torsional Dampers and Dual Mass Flywheels (DMF).

Becoming ever more complex the suspension in an F1 car has a number of devices to counter loads fed into the chassis, in order to maintain the ideal conditions at the tyre contact patch. We understand the role of springs and dampers, but there remain other spring effects within the suspension system, not least from the high profile tyres. Their spring effect goes undamped and hence is largely out of the control of the teams in setting up the car. Being able to counteract these uncontrolled forces in a suspension will allow the tyre to main better contact with the ground for more consistent grip. In 2003 Cambridge Universities, Dr Malcolm Smith proposed a mechanical method of controlling these loads via the Inerter. McLaren took up this idea and tested the idea in 2004 then went on to race an Inerter in 2005.
In size and construction the Inerter looks like any other damper. Being placed in the same position as a Heave Damper it was well hidden and unknown to most people. Until the “Spygate” saga in 2007, when the design was referenced as both the “J-Damper” and “a Damper with a Spinning Mass”. It wasn’t until May 2008 that I was able to understand and expose the details of the Inerter concept, publishing its details in Autosport.com (subscribers only http://www.autosport.com/journal/article.php/id/1554 ). Co-incidentally this article is cited in the patent documentation!

A Mechanical Interter

An Inerter can be configured in two ways: a linear and a rotary format. In both guises the device uses a toothed drive to spin a mass. Likened to the same effect as a bicycle bell, the load fed into the bells lever is dissipated by the spinning element. In F1 teams use a cylindrical mass screwed onto a threaded rod inside a damper body. One end of the rod is affixed to one side of the suspension and the damper body to the other side of the suspension. Reacting to the acceleration of the suspension, the Inerter absorbs the loads that would otherwise not be controlled by the velocity sensitive conventional dampers.

The Renault 2006 Front Tuned Mass Damper (TMD) - Copyight: Craig Scarborough

The Inerter predates Renaults TMD, which aimed to achieve the same effect. With the TMD a weight is suspended on spring to offset the same forces being fed into the chassis as the Inerter. Renault first raced the TMD in 2005, its discovery by Giorgio Piola around Monaco of that year; both forced a development race and an enquiry by the FIA. It was subsequently banned on what proved to be false grounds. The FIA citing a movable aerodynamic effect as the reason for its ban.
Unaware of its effect on the contact patch, I initially saw the device as a means to prevent the front wing pitching downwards when braking. The inertia of the suspended mass keeping the nose from pitching downwards during the initial braking phase. This I thought would prevent the car from being pitch sensitive. Despite a lengthy court room case, this “aero” function was upheld as the reason for the ban of the device.
Ironically the McLaren was using the Inerter at the time, and despite it being used for the same function was not banned and remains legal and in universal use to this day.

LRGP’s Fluid Inerter Concept
Reading the detail of the LRGP patent, it’s clear this was at least partly a surprise discovery. The Patent states the discovery was “based on lab testing of another hydraulic suspension device”, when it was found that the effect of the fluid within the system “has a very significant inertia effect”.
I would suspect this discovery was made during the development of the linked suspension system. Where fluid lines are used to link the suspension in a similar manner to the Mercedes system I detailed earlier this year (https://scarbsf1.wordpress.com/2011/10/17/mercedes-innovative-linked-rear-suspension/). Perhaps the longer fluid lines to link front and rear suspension provided the discovery, rather than the very short left to right linking pipework. I understand Renault have had hydraulically linked suspension on the car since at least 2009.
With this insight LRGP have proposed a Fluid Inerter using both the inertia of the fluid and a spinning mass. By making the Inerter device more like a damper, where by a damper rod displaces fluid; this fluid is then piped into a circuit to spin the mass. There by both effects can be created. It is the inertance of the fluid that differentiates the LRGP patent to the conventional Inerter proposed by Dr Malcolm Smith.
Inertance is a new term and I’ll quote the patent for LRGP’s explanation of the effect.
“Hydraulic fluid inertance means” concerns an arrangement in which the presence of a hydraulic fluid provides an inertance, where inertance is a measure of the fluid pressure which is required to bring about a change in fluid flow rate in a system. Between the terminals this translates to an inertial force which resists acceleration.
LRGP found that the fluid used was critical to the efficiency of the design. In particular to make it effective for the lightweight and small packaging volume required to make the device work within the tight confines of an F1 footwell or gearbox. Needing to be incompressible and low viscosity, they have proposed several fluids, such as water and oil, but the preference appears to be for Mercury. Although a metal, it’s liquid at ambient temperatures and very dense. This means smaller fluid lines filled with mercury will provide the necessary inertance, compared to larger amounts of less dense fluids. Passing from one chamber in the damper body via the fluid line to the other chamber, the detail design of the length and diameter of the fluid lines are key in creating the correct tuned inertance effect. Just “1 to 50g” of fluid is required to get the desired effect. The range of inertial reaction is quoted as “10 to 500kg, which is a typical range required in Formula One racing cars”.
As a side note McLaren decouple this inertial reaction force into different measured units. Rather than Kg of Inertial force, McLaren use the term “Zog”, this allows them to hide the actual units set up on their Inerter.
Renault suggests winding the lines around the damper body as one solution for the packaging of the fluid circuit. Additionally a valve or shim stack in the damper rod would also alter the amount of fluid displaced, to further tailor the Inerters effect.
With Mercury having a high coefficient of thermal expansion, the patent suggests using a relief valve emptying into another chamber is used to ensure the system has a constant volume of fluid.
Clearly the emphasis is on the fluid to provide the inertance effect, the patent citing a minimum 50%, up to as much as 90% of the inertance coming from the fluid.

In Detail

The device uses the left hand casing as the fluid cylinder & the right hand casing for bump rubbers

Not only does the Patent contain the conceptual information on the Inerter, but also detailed cross sections. I have simplified these to explain the Inerters construction. LRGP have been able to condense the entire solution into a single self contained component, which fits into the same volume as the conventional Inerter.
The device is made up of a main body and a damper rod. The main body split into left and right sections bolted together. The left hand casing forms the cylinder, not only contains the fluid, but also channels machined in the outer casing form the fluid lines. Such that no external pipework is required. The right hand casing allows the damper rod to pass through and also houses bump stops to prevent the device bottoming at the end of its 16mm of bump travel or 23mm of droop travel. In total the device is just 220mm long (eye to eye).
In cross section we can see the casing is a complex machined part. With the right hand chamber formed with bushes, seals and endplates to create the cylinder for the damper rods to pass through. The damper rod along with its shim stack valve pass through the cylinder like piston as the suspension compresses and rebounds. The mercury within is displaced and passes through channels into the channels machined into the wall of the body.


The Inerter (yellow) is mounted between the rockers

LRGP provide a diagram for the Inerters mounting. This being a typical position between the pushrod rockers. No doubt a similar mounting is found between the rear pull rod rockers. Externally it would be hard distinguish the Fluid Inerter from a Mechanical version. Albeit the Renault front bulkhead design shows almost nothing of the Inerter inside the footwell. The steering rack and anti roll bar getting in the way of the small aperture inside the front of the monocoque. Thus we cannot be clear if the device has raced.

One benefit is the technology is proprietary to LRGP and not used under license via Penske or Dr Malcolm Smith. Thus LRGP are free to use and develop this technology freely.
I couldn’t state whether the Fluid Inerter has any compliance benefits over a mechanical one. Perhaps it’s easier to tune via the shim stack in the damper rod, rather than the fixed specification of the mechanical Inerter. Equally it may be easier to maintain, teams needing to strip clean and re-grease their Mechanical Inerters frequently to maintain their smooth operation.
It seems one advantage to this device might be lightweight. The tiny amount of fluid required would be lighter than an equivalent spinning mass. As Inerters tend to be mounted relatively high a weight saving will aid CofG height, as well as ballast placement.
One negative issue is that Mercury is a hazardous material. Considering the unit is positioned ahead of the driver’s legs, any mercury leakage as a result of a major accident will only complicate the health issues for the Driver and Marshalls. I am not aware of Mercury being specifically restricted by the FIA approved material list. Although with just a few CC’s of the liquid contained within the cylinder, this might not be regarded as an issue by the FIA.

However the team came across this solution, it is a new direction for Inerter development. The solution is totally legal, as set by the precedent of the mechanical Inerter being allowed to race, even when the TMD wasn’t
It will be interesting other teams come forward with new Inerter or linked suspension solutions. The only problem is few teams patent their design to allow us such insight to their design.

A view of the outer casing

A cross section of the Inerter

An exploded view of the parts

More references on Inerters



23 thoughts on “Lotus Renault GP: Fluid Inerter

  1. The technical brilliance of using a cross-linked suspension system as an inerter aside, I’m now slightly concerned that so many Renaults exploded this year, if they have mercury lines running all over the tub..

    But yeah, fluid-driven spinning mass inerters filled with mercury and doubling as active suspension… Hat’s off

  2. If I’m not mistaken, F1 teams that patent innovations – such as this – have to allow other F1 teams a free licence to use the patents in their F1 designs and cars. If this is the case, then LRGP cannot stop other teams from adopting this for themselves. So perhaps the main reason for patenting it is to be able to licence it out to other car manufacturers to use in their commercial products to bring in outside revenues?

    • Do you know where this rule comes from Andy? I can’t find anything relevant in the sporting regulations or technical regulations.

      If this is the case, I’d hazard a guess that the benefits of this device are not in performance, because if it performed better than current inerters then Renault would want to keep it out of the hands of the competition. I guess the reliability/maintainability advantages that Scarbs described are probably the main ones, making it more suitable for licensing outside F1.

      • It makes sense that any patented device is freely available – consider the alternative, teams of patent lawyers running up and down the paddock trying to get an eyeful, court cases, more shenanigans etc etc

  3. Pingback: Lotus Renault GP: Fluid Inerter | Scarbsf1′s Blog « F1Enigma's Insider Notebook

  4. Interesting article Scarbs.

    Can you elaborate on the ‘shim stack’ – is this introducing a tuning effect, like the valves in a shock absorber, or by changing the volume of the liquid? Your drawing also suggests to me that the disc between the bump rubbers could have a damping effect.

    Although mercury may be the most effective liquid, I can’t see its use being tolerated in the increasingly ‘green’ F1 world – at least when such use becomes public knowledge. For that matter, I doubt it would be accepted in road cars either. So either another substance will be an acceptable substitute, or the device is intended for other uses.

    • You’re first assumption was right, the shim stack is for alterign the flow through the device, as with a conventional damper. The disc between the bump rubbers sits is free air, so I doubt it has a damping effect.

      I agree Mercury porbably isnt allowed in F1.

      • Its entirely possible that another fluid has been found to be effective enough, or perhaps more effective than mercury. (I am not certain about the fluid mechanics of mercury, i.e. newtonian behavior and viscosity.) The CTE of mercury could be an issue as addressed in this article as well. I would guess that they list mercury in the patent because it is a good example but a poor practical choice. It is entirely possible that the proprietary information which could give LRGP a boost is this liquid itself which is not disclosed. This could explain why it is patented and potentially still lead the commercial licensing of such technology without any concerns for mercury being used in a production vehicle. All of this, of course, is pure speculation though so take it with a grain of salt.

    • In isolation the CofG benefit will be quite small, however teams look for these small gains and cumulatively they find measurable gain. CofG isnt a massive contributor the laptime, being ~0.012 seconds per mm of lowered CofG height.

    • I’d say yes.
      That’s how it works in the mountain bike world at least – you go over the entire bike with a fine tooth comb and even if you only save 3-5 grams on a 140 gram part, if you do it to enough parts, you’ve shaved half a pound to a pound by the time you’re done.
      It’s obsessive compulsive, but it’s also what it takes when races are being won by a tenth of a second.

  5. Do you know if any F1 team has experimented with non-Newtonian fluids (think cornstarch and water) in their dampers/inerters? One would think that the variable viscosity of these fluids could be taken advantage of. For example, slower corners would yield a lower viscosity allowing more body roll. Higher-speed corners and sudden braking would yield a higher viscosity, effectively stiffening the suspension, reducing body roll and aiding the underbody’s aero efficiency.


  6. Hi Scarbs. Great site and awesome work, as always. I am a Mech. Engr. if you remember from F1-technical. Our Oregon State University team won Formula SAE 2009 & 2010 and Formula Austria events for which I worked on suspension design (interesting pull-rod front). I am very proud, if you cannot tell. 🙂

    I am quite familiar with Inerters and have recently read “Performance Benefits in Passive Vehicle Suspensions Employing Inerters by Malcolm C. Smith and Fu-Cheng Wang” The authors clearly demonstrate that Inerters should be used in SERIES ARRANGEMENT, NOT PARALLEL with dampers for maximum effect. The LRGP susp. diagram looks like it is parallel to me. Is this a real design? Or is there something I am missing?

  7. The fluid in magnetorheological dampers has a high density compared to standard damper oil, perhaps mimicing mercury in a non-toxic, less-corrosive format.

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