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Suspension: Pushing and Pulling

By Mav | 13 Feb 2012 | 4 Comments | 19,916 views

Even before the Ferrari F2012 was launched, the team was apologising for its looks. However, it’s not the nose itself that is the most remarkable feature: Rather, it’s the pull rod suspension mounted to it. It’s a case of history repeating itself for Fernando Alonso as the last front pull rod suspension was seen it was in 2001 on his Minardi PS01. Why the change, though? And will it prompt a new fashion?

MP4-27 vs F2012
Click image for enhanced view

Above are this year’s McLaren MP4-27 and the Ferrari F2012 side-by-side for comparison, an enhanced version of the image is available by clicking on the image in order to better make out the suspension differences.

Together, the two historic rivals represent the two extremes of front-end design in 2012. The former baulks the trend for high noses, avoiding the need for a step-change in height to accommodate new rules in the process. The latter ignores thinking of the last decade and reintroduces a front pull rod set-up. It’s a curious decision as the key reason for the push rod’s rise to power came on the back of higher noses – basically, it’s hard to point a pull-rod upwards at a reasonable angle from a nose that is itself high. That means that the MP4-27 is actually better suited to the use of pull rods and yet retains push. However, in McLaren’s case that happens to look at the problem in the wrong order as it seems that the low-nose has been retained in order to keep what they believe is an optimised suspension set-up.

At the start of 1983, the McLaren MP4/2 and the Brabham BT52 reintroduced push rod suspension. By 18 months later, every Grand Prix car would have a system along the lines of McLaren’s.

Historically, the pull rod configuration is nothing new and Auto Union used a version on their pre-war Grand Prix cars. There were occasional examples throughout the early Formula One era too but they really came to attention with the Lotus 72 which carried a system recognisable as pull rod both front and rear, even if it wasn’t termed as such. Then in 1972 two cars came out within weeks of each other. Firstly there was Brabham BT44 with a front pull rod layout (termed tie-rod at the time) while hot on its heels was the Tyrrell 007 with a push rod set-up. The subtle difference appears to have been largely overlooked at the time.

Formula One continued to boast a wide variety of configurations throughout the 1970’s and into the 80’s but 1983’s McLaren MP4/2 use of push rod finally drove opinion in one direction. How much of that was down to genuine insight into the push rod’s benefits and how much was down to McLaren’s 12 wins from 16 in 1984 remains open to debate. However, push rod would dominate both front and rear for the next quarter of a century until Red Bull’s use of pull rods at the rear if the RB5. But why use a pull- or push rod in the first place?

The two layouts for the the MP4-27 and the F2012 are illustrated below. In essence, the pull rod layout is simply the upside-down equivalent of its push rod twin, however, that leads to some notable differences in the way they operate. In the case of the push rod, the suspension member is in compression in order to maintain contact with the ground, hence “push”-rod. The reverse is true with the pull rod which is in tension – hence the early use of the term “tie-rod”.

MP4-27 versus F2012

In turn that has an effect on the wishbones. In the case of the push rod the load is reacted as a tension in the lower suspension arms (Newton’s third law: To every action there is always an equal and opposite reaction.) The pull rod, on the other-hand, reacts as a compression in the upper control arms and steering arm. Materially, that means thicker, stronger upper arms with the pull rod, whereas the push rod puts focus on the lower arms.

However, that’s when the car is travelling in a straight-line and only having deal with bumps. The situation becomes slightly different under cornering where the outer wheel is heavily loaded and applying a side-ways force on the suspension. In the case of the push rod configuration, that relieves the force in the lower wish-bone, reducing strength considerations. The opposite is true for pull rods and their upper-arms.

Early comments on the F2012’s front suspension layout focussed on the low angle to the horizontal of the pull rod. The loading in the pull/push rod is a function of this angle as a vertical push rod would work in the same direction as the wheel’s travel when negotiating bumps. In the case of the McLaren, their push rod is at about 30° meaning that the force in it will be twice the wheel load (sin30°=0.5). The Ferrari, on the other hand, has a pull rod of 8° meaning the force in it would be over seven times that in the wheel load, with a corresponding higher load in the upper suspension arms as the force is reacted.

However, it’s not that simple although it would be true of Alonso’s Minardi PS01 as the wishbones were more or less horizontal. Instead, the upper- and lower-suspension arms have been gradually angled upwards from the wheels as zero-keels became favoured and higher noses desired. This is illustrated by the approximate measurements for Ferrari’s front suspension since 2009:

  Angle of pull/
push rod
Angle of upper
suspension arm
Angle of lower
suspension arm
F60 35°
F10 36°
F150° Italia 38° 15° 12°
F2012 -8° 17° 13°
MP4-27 32°

Angling the wishbones optimises the aerodynamics but at a cost to the the efficiency of the suspension system. This because the wishbones take some of the vertical wheel loading. That’s good in terms of dealing with the loads but it impacts on the car’s effectiveness in dealing with bumps around the circuit. The result is that the F2012’s roughly 8° pull rods may not be as bad force-wise as is initially thought. Indeed, it seems that the wishbones carry significantly more of a bump’s force than the pull rod. How the system responds to those bumps is the question though.

Then there is still the issue of deflection. The pull/push rods role is to transfer the movement of the wheel to the damper and springs and the calculation of this is analogous to the force estimates. For the McLaren, the deflection at the damper would be be half of the wheel’s deflection (sin30°=0.5), for the Ferrari F2012, it’ll be less than a seventh. That motion is amplified by the use of rockers – a lever that translates the push/pull rod’s motion into the rotary force on the torsion springs and the motion of the damper. In order to recover that lost deflection, Ferrari would need to use larger rockers, with an associated weight penalty. Never-the-less, suspension efficiency appears to have been compromised and the question is whether that may surface either as poor handling characteristics or more subtly in terms of front tyre degradation. However, again, the angling of the wishbones is likely to help compensate.

However, let’s say it works – afterall, as the last man to race with front pull rod suspension, Fernando Alonso is in a position to comment on the choice – will the idea spread? Ferrari claim they will gain aerodynamic benefits as well as being able to exploit a lower centre of gravity by having the dampers, rockers and springs located lower. Inevitably, if Ferrari did win the Championship, there would be copy-cats out there – even if rival designers don’t like an idea, team principals have a habit of nudging them in a seemingly successful direction. However, with high noses de rigueur despite the FIA’s attempts to lower the tips, I can’t see that happening.

Then again, I said that when the possibility of Ferrari using pull rods first surfaced and yet here we are…

EDIT: Having had a chance to analyse the geometry in more detail, Ferrari should have few problems: link

Credit: Vodafone McLaren Marlboro/Scuderia Ferrari/Viva F1

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