Push vs Pull type rear suspensions... which is better?

[Belatti]

A little question on pullrod suspensions:
Why does a pull rod suspension need rods, couldn't they use some kind of strings/wiring (don't know correct term) as the rod is always under tension.

Nope!

Pull also pushes, push also pulls. The difference between both is if the rod is under tension or compresion when the shock is in compresion. Both types experiments positive and negative normal tensions.

[wrcsti]

I think next years cars will most all have pull rod if the DDD loophole isnt fixed as it gives tons more room to make a bigger upper deck.

[ISLAMATRON]

pull rod seems to be the better solution regardless of the legality of the DDD

[Richard]

A little question on pullrod suspensions:
Why does a pull rod suspension need rods, couldn't they use some kind of strings/wiring (don't know correct term) as the rod is always under tension.

That may be work for a static car. however when it starts to go over bumps, then their is rebound and the forces with reverse.

Also, there will be lateral forces fro wind, and knocks from other cars. You want something that is strong enough to resist those other forces. Note that even buildings/bridges that use tension cables always have an element of pretensioning to resist any likely load reversal from dynamic or wind forces.

[Ian P.]

I think next years cars will most all have pull rod if the DDD loophole isnt fixed as it gives tons more room to make a bigger upper deck.

I think it is the other way "round-way".
Push rod systems allow more room for MDD (multy deck diffusers) while the Pull rod systems give the benefit of lower cg placement of the suspension components. This does take up valuable aero space for tunnels and air passages.
Newey took ages to design a system around their Pull-Rod rear suspension and even then he admits it is all about compromise.
Wouldn't surprise me to see Newey go back to a push-rod rear for next season unless he can re-design the gear box and back end to accomodate the air flow tunnels that will be required.

[Jersey Tom]

I wouldn't expect many if any more teams to switch to pullrod next year. It's not like this is new stuff. Multiple manufacturers have used them before.

The reason they use pushrod is because for their chassis, its the best option.

[dp35]

It amazes me that (apparently) no one has been able to take a photo of the red bull's rear suspension. Like others have said, I'm fascinated at how the 3rd damper, etc has been packaged, and would love to see photos and/or diagrams. Anyone?

[DaveW]

Experience would suggest that a pull-rod suspension has a lower installation stiffness than the equivalent push-rod layout (for various reasons). That was certainly true for the JB Arrows front suspension, & is true for the current Dallara IRL front suspension (also pull-rod). Installation compliance reduces the "efficiency" of the suspension when dissipating disturbance energy, & is, therefore, a property to be minimized or, at least, contained.

I confess that I was unimpressed when I heard that the RBR rear suspension was to be pull-rod, & expected that performance would suffer as a result. History shows that I was wrong, & I must apologise humbly to AN for questioning that decision. With the apology out of the way, it is worth speculating what the RBR layout brings to the party.

Simply changing the rear suspension to pull rod is unlikely to achieve much (other than aggravating mechanics), because the revised position of the suspension elements will simply displace other hardware - hence negating (or at least reducing) any c.g. height benefit.

However, suppose it is wished to achieve as forward a centre of gravity as possible. A neat way of achieving that would be to move the rear wheels back (relative to everything else), as pioneered by Ferrari in 2007. A limit to that idea will be imposed by the resulting drive shaft angles, which can be alleviated by moving the bearbox aft. But moving the gearbox aft will negate (in part) the benefit of moving the wheels back. However, moving the gearbox aft will create space between the engine & gearbox which can be used to accomodate the rear suspension (no doubt amongst other things). Placing the suspension low in otherwise unused space between the engine & the gearbox will benefit the longitudinal c.g. position, & will also lower the c.g. height (as has been suggested by others). A further (& perhaps significant) benefit of moving the suspension elements forward is the greater lateral space that can be made available without upsetting aerodynamicists (unless, that is, they have to install a slotted diffuser). I guess mechanics are not paid to have an opinion.

Comments?

p.s. buckled pull-rods look ugly when the wheels are off the ground.

[tok-tokkie]

I seem to remember reading that the pull rod system freed up space under the wing which permitted better aero design in that area. It was that aero advantage that was wanted. (But then the DDD loophole completely negated that - except it seems not to have.)

[xpensive]

I believe the pull-rod suspension became de rigeur in F1 in the 70s and 80s, pioneered by Brabham I seem to remember.
Rods should in engineering principle be pulled and not pushed to avoid buckling, just like coil-springs should be pushed and not pulled.

Until the higher strength of carbon fiber as well as the superior adjustment flexibility of the push-rod arrangement, spring-units on top rather than on bottom, took over.

Then of course there's the element of center of gravity, but with modern light-weight torsion bars, I think this is of minor importance, no?

[timbo]

Until the higher strength of carbon fiber as well as the superior adjustment flexibility of the push-rod arrangement, spring-units on top rather than on bottom, took over.

Today with raised nose I think push-rod is default. However transition to push-rods happened earlier than raised noses became popular. I think this had something to do with the desire to lower the footwell and front monocoque lower as well as adjustment ease.

[xpensive]

I think you are correct timbo, the ease of ridehight and stiffness adjustment
was surely a vital point before the raised nose?

But come to think of it, anyone remember the "top rocker arms" of the late 70s? like the Lotus 79?
Can you imagine the poor stiffness of those. And how to adjust them, christ!

[DaveKillens]

Obviously, the top arm is constructed to take a tremendous load. As well, the pivot location is short of being neat and integrated into the chassis structure. The Lotus 79 set the standard for it's day, but it's obvious that a bit of triangulation would free up that suspension arm from it's requirement to be so strong and heavy. That's where pull and push rods come in. And as xpensive pointed out, working on the suspension would be a rough deal.
And although a Formula One car's only purpose in life is to go fast, building in maintainability and ease of repair is also desirable. If it takes mechanics two hours to change a shock package, that just won't be acceptable on race weekend. The car has to be on the track, not in the garage.

[xpensive]

I can recall stories by Mario Andretti, on the lack of stiffness of the whole shabang.
He tied up a fishing line diagonally from one suspension pick-up point to the other, jacked the one wheel up just to see the line slack. Which it did in a dramatic fashion.

Peter Warr and the engineers simply hated that, but Mario was the only one who could ever do it, Ronnie would probably have no clue as to what he was about doing at all.

[riff_raff]

I believe most F1 chassis use a three spring suspension system that gives a soft rate in roll and a stiffer rate in pitch. This keeps the aero downforce balance front-to-rear stable during pitch changes caused by braking dive or acceleration squat. And it also keeps all of the tires on the track during chassis roll while cornering.

Since a three spring suspension is coupled side-to-side, a pull rod would not work well since it has very limited compressive stiffness.

I may be wrong about current F1 suspensions using a three spring system, but that's what they were using a few years ago. And I don't think the basic rules have changed since.