Pushrod - pullrod
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| Pullrod suspension |
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| Pushrod suspension |
Pushrod or pullrod, the difference as the name suggests is the whether the rod push up to the rocker or pull down to the rocker.
Pull rods were first brought to Formula 1 by Gordon Murray with Brabham in the 70s but now all formula one teams make use of the push rods, as pull rods are quite hard to implement in a high nosed car. The advantages of a pull rod lie in the possibility to make the nose lower, assemble most suspension parts lower to the ground and thus lowering the height of the center of gravity.
Pull rod set up has a strut from the outer end of the upper wishbones that runs diagonally to the lower edge of the chassis and "pulls" a rocker to operate the spring\damper.
A pushrod is the opposite; the strut runs from the lower wish bone to the upper edge of the chassis.
Choice between the two is geometry and CoG (Centre of gravity). Also a pull rod will flex in droop (wheel going down) and push rod will flex with the wheel in bump (wheel going up) hence F1 push or pull rods are large carbon molding to withstand the flexing from the high wheel loads.
Most F1 cars now use push rods. The high nose makes it impossible to locate a rocker low enough in the chassis and still have the right geometry. Minardi and Arrows used pull rods with low noses to lower the centre of gravity. These two teams are a last two to use this concept.
Pullrod suspension has been ignored because of the installation problems with high nose cars. The critical geometry in spring\damper layouts is the installation angle of the push\pullrod. In the picture of Minardi PS1 below, the pullrod has an angle of around 35-degrees which is fairly steep. Notice how high on chassis is the the point where it meets the upper wishbones, on some cars these are now almost as low as the floor of the monocoque. Then the keel below the square section of the monocoque reaches down to pick up the lower wishbone, the space created below the nose is what allows the front wing to be so efficient as there is little to obstruct the wake forming. The keel is made to be as slender as possible to improve flow under the nose . Should the nose be made lower the aerodynamics would be crippled.
When pullrod suspension were adopted with a good installation angle, the rocker would need to be located near the keel (upsetting the aerodynamics) or appear pick up point of the pullrod would need to be located somewhere above the tire, clearly a upper wishbone mounted pullrod with a rocker inside the monocoque would never be able to work efficiently due to the poor installation angle.
Access problems used to be a common problem with pullrod suspensions: The removal of the dampers and rockers being hindered by their low location inside the monocoque.
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Jos Verstappen in Arrows20, Year is 2000 and designer of this car is aerodynamicist Eghbal Hamidi |
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European Minardi PS01 Designed by Gustav Brunner. Drivers: Fernando Alonso and Taso Marques |
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In the typical torsion bar pushrod set up described below the torsion bars pass through the centre of the rockers and fix to the front of the chassis. The Rocker pivots on the torsion bar. The push rod pushed the rocker and twists the torsion bar to provide the spring in the suspension, the rocker then compresses the damper and operates the antiroll bar if the car is in roll.
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Black - Pushrod Yellow - Rocker Dark Yellow - Rocker splinted to Torsion bar Light grey - torsion bar Red - Damper Blue - antiroll bar linkages |
Now is preseason testing for year 2009 and RedBull Racing with Adrian Newey like technical director and chief designer, brings pull-rod rear suspension back to Formula One for the first time after 10 years. The last pull rod dates from 2000 with the low nosed Arrows A20 which featured it.
With the diffuser moving rearwards under the revised 2009 regulations, the pullrod layout allows for cleaner aero packaging for rear suspension area. See circle for more common pushrod set-up witch normally occupy more space under the engine cover, preventing designers to make clean airflow toward rear wing because suspension rockers and dampers are positioned on the top of the gearbox.
Because of Red Bull's choice to create very low sidepods at the back end, a pushrod did not make much sense as the suspension components would prevent a clean design. The pullrod version has allowed the designers to put more components close to the car's floor, lowering its centre of gravity. The pullrod suspension is lighter and, along with a gearbox that is now 15cm lower, helps to improve handling. The top wishbone is a single piece and is attached very high at the rear of the car (see upper arrow), actually they connect above the car's bodywork, in the most ideal position, forming a wing-like section that works together with the lower section of the rear wing. With some imagination, if you would turn the car upside down, the rear suspension is effectively connected at inverted single keel. The keel is here supporting both upper wishbones.
This solution was clearly valid as the RB5 was the only car with a single deck diffuser to challenge the Brawn cars. However it was exactly the reason why 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 out that 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.
Also note the very low and rearward positioning of the exhausts (lover picture, right arrow), which exit just under the front arm of the rear wishbone, and the lack of a central pillar to the rear wing, which is instead mounted via the wing's two large endplates, which attach directly to the diffuser. This concept will evolve next year (2010) to very successful blown diffuser, critical part in driver and constructor championship win by Red bull Racing.
For 2011 the DDD is banned, with revised 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. During pre season 2011 presentations of the new cars, all but Ferrari and Sauber are going with pull rod.
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. Sometime flor have to be removed. 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.
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.
