Adrian Newey’s lateral thinking in 2009 gave rise to the modern iteration of pull rod rear suspension. Although handicapping the double diffuser, the solution remained on the Red Bull cars for 2010. With double diffusers being banned for next year, other teams are looking at the concept. Lotus Technical director Mike Gascoyne has even cited the opportunity to exploit pull rod suspension as a reason for going with Red Bull Technology for the supply of their 2011 gearbox. Pull rod may well be the buzz word at the launch of many of the 2011 F1 cars.
Red Bull have been running a pull rod rear suspension since 2009, while not a new solution, no team had run this set up for many years, as the aerodynamic demands of the rear diffuser drove designers to place the spring and damper hardware up above the gearbox to create space for tunnels beneath the car.
F1 rear suspension.
F1 cars operate substantially similar suspension front and rear, the packaging varies each end but the main components are the same. Double wishbones control the wheels attitude and from the outer end of the wishbone a rod controls a rocker that then activates the various elements that control the suspensions compliance. Firstly the springs are in the form of torsion bars, these are like straightened coil springs and their resistance to twist provides the springing medium to support the cars mass. Then the dampers, one for each wheel, these control the movement of the wheel as it raises and falls (bump and droop). The antiroll bar controls the amount of weight transfer from one side of the car to the other. Lastly the third spring, also known as a heave damper control the pitch movement (both wheel bump or droop simultaneously) This is especially important to prevent the downforce load pressing the car against the track and bottoming the car on the ground at high speed. Teams may also fit an inerter in this position to offset the uncontrolled bounce of the tyres having an effect on the chassis.
When the rods operating the rockers start out low at the outboard end of the wishbone and rise up towards the rocker, this is known as pushrod, as the rod pushes the rocker when the suspension is in bump. Conversely when the rod falls from the upper wishbone to operate a low placed rocker, this is known as Pull rod as the rod pulls the rocker.
Pull rod is nothing new, it first appeared in 1974 when Brabham designer Gordon Murray applied to the design to the front of the BT44. Murray admitted he saw the idea in a Hill climb car and simply applied his version of it to the F1 car. The alternative suspension designs of the time were either an outboard spring\damper, which was un-aerodynamic and restricted damper movement to that of the wheel. Or rocker arm suspension, with required large and heavy upper cantilever arms to operate an inboard spring\damper. This was heavy and only provided a low ratio of wheel to damper movement, but was moderately better aerodynamically. The pull rod employed light wishbones, placed very little structure into the airflow and gave the opportunity to alter the rate and ratio of wheel to damper movement. Murray subsequently turned Pullrod upside down to create the pushrod for the front suspension of the 1983 BT52.
Red Bulls Adoption of Pullrod
When the aero rules changed significantly for 2009, most teams adopted fairly conventional approaches in the chassis design to accommodate the changes. One of the major aero changes was the switch to a much smaller rear placed diffuser, The loss in potential downforce from the smaller diffuser, made the rear wing performance a greater contributor to the cars total downforce.
Newey’s thinking for the RB5 was to create a low-line rear end, by placing the differential unusually low and switching from pushrods to pull rods. With the smaller diffuser runnels and moreover the tunnels starting as far back as the rear axle line, well behind the main body of the gearbox. This gave Newey the space to package the pull rod hardware and not interfere with the diffusers tunnels. As a result the airflow over the top of the gearbox to the rear wing was far less obstructed by the pushrod operated springs and dampers. This solution was clearly valid as the RB5 was the only car with a single deck diffuser to challenge the Brawn cars. However it exactly the reason the Brawn was so fast, that undid Newey’s low-line rear end philosophy. As the Brawn had a Double Deck diffuser (DDD) this solution found a loophole in the rules that created a secondary diffuser tunnel starting much further forwards and rising much higher. Suddenly in the race to also exploit this loophole, Newey found his Pullrod set up was occupying the exact same space that the DDD needed for the upper tunnels. Newey chose not to design a completely new rear end, and compromised the design of his DDD within the constraints of his pull rod suspension.
For 2010 the car was designed with a DDD in mind, Newey was able to repackage the pull rod set up for even larger tunnels. He said that the choice of Pullrod for 2010 was still not the obvious way to go, but the team decided to stick with a proven pull rod rear end, rather than have to design an all new rear end. Other teams also looked at the feasibility of a Pullrod rear end, However no other teams followed this design path, with the exception of the Toro Rosso team who used the RB5 design in 2009 and simply revised it for their 2010 car. For 2011 the DDD is banned, with revised wording in the technical regulations outlawing the openings beneath the car to allow air to flow into the upper diffuser deck. Thus again we will see teams consider the pull rod layout for better airflow to the rear wing.
Which is better – Push or Pull
In terms of their effectiveness as controlling the wheels, both are equal. In terms of effect on aerodynamics each has its merits depending on the prevailing rules and trends. However both have different benefits and demands on the chassis. Pullrod clearly provides a lower CofG, although access can be an issue. In Red Bulls case they place the 3rd spring and inerter horizontally across the front of the gearbox. This means one sits above and the other below the shaft connecting the engine to the clutch. These can only be accessed when the gearbox is removed and are subject to a lot of heat. Although Newey tells me that they do not suffer unduly because of this. One difference is in the load passed through the wishbones.
As per Newton’s third law, the rod has to react to the force of the springs. This passes back from the rocker to the mount on the wishbone. In pushrods case, this reaction force is in the opposite direction to the force fed from the wheel into the chassis, the two offset each other. With Pullrod the force from the rod and the wheel act in the same direction, this doubles the load in the upper wishbone and resultantly in the mounting the gearbox. This can be accounted for design and weight of the final wishbone design. However Pushrod also has its structural problem, the pushrod when the suspension in in bump (wheel rising) the rod is in compression and would tend to bow outwards. The pushrod was the first suspension component to have carbon fibre cladding for reinforcement, again design and weight is needed to offset this load. Suspension experts point out that Pull rod suffers similar compression bending when the suspension is in droop (wheels falling), but droop is considered less critical in wheel control, than bump. There’s no one answer to which is best, you look at your design requirements and pick which solution works, best. Next year the best car is not necessarily going to be the one with Pullrod rear suspension.
Pullrods at the front?
This was a favoured design for many years, even after Murray innovated with the BT52. However designers found they could slim the nose cross section by mounting the spring\dampers above the drivers legs and no longer to each side of his shins. This improved access, even if it did compromise CofG slightly. Then as the raised nose aerodynamic concept took hold, teams found the gains from a high nose, offset the CofG gain of pull rod suspension. Arrows campaigned their A21 in 2000 with pull rod front suspension, and latterly Minardi ran the PS01 with a relatively low nose in 2001. Each team subsequently moved to a fully raised nose pushrod suspended car. Now the front of the chassis is raised too high for a pull rod to work, the angle from the upper wishbone to the chassis is nearly horizontal. This geometry meaning that almost no movement of the pull rod will occur as the suspension moves. Making the set up structurally inefficient.