Pros and cons of push & pull rod rear suspension

[TEAM78]

as per title, jusf wondering what are the advantages and disadvantages between the two systems for a non aero car, im fully ware that the winged cars are forced by the rear diffuser to have push rod suspension but why should a car without a rear diffusser not have pull rod suspension. I assume that winged cars also have outboard brakes for the same reason as the dampers or else they would be inboard

[Bob Pearson]

I don't think there was ever a single reason for moving away from pull rod suspension, it was a number of small things like the suspension units were always somewhere where they weren't really wanted. In the wind somewhere or in with the driver's feet whereas the push rod units can be put on the g/box or above the drivers feet. The pull rod will need to have a greater strength than a push rod which is acting mainly in compression.
I thought outboard brakes tended to cool more effectively being in the middle of a rotating wheel, and had the advantage of inducing less wear in the drive shafts than inboards which are trying to stop the car via the drive shaft couplings.

[avsfan733]

Its partially a weigth distribution/cg thing. its going to be lower using pullrods. the pullrods theoretically needn't be as strong. the issue isn't the ultimate/yield strength of materials, its a buckling issue with the pushrods

[Bob Pearson]

You're right about the buckling issue with pushrods, not sure I agree that there isn't a tension problem with pullrods, maybe one cancels the other out. What is the normal wall thickness for pullrods, the Tatuus pushrods are 2mm thick, whether it would get away with less I don't know Assuming you're right, pullrods should be more favoured than pushrods, with the weight lower down. Apart from what I said about the units always being where you don't want them. Never had a pullrod car, so I don't have too many definate opinions

[Goran Malmberg]

One issue dont talked so much about is the loading of the A-arm. Its a little of topic maybe, but it is easier to make a "rod" centreline to cross the centre of the spindle ball joint. This takes the bending load out of the A-arm. I think push or pull-rod is more of a total suspension layout situation than it is + or - between the two.
Goran Malmberg

[TEAM78]

Thanks Goran, thats a really relevant issue there, Its always important to consider the suspension as a structure as well as the kinematic issues with suspension design, Id have to agree that the packaging issues do tend to drive the layout more so than which type is best, The lower arm is loaded more so than the upper arm hence making the lower wishbone heavily loaded, also where is it best to place the toe link? say for example the toe link was inline with the upper wishbone is it better to have a pushrod and say to have a pullrod its best to have a lower toelink??.
From a structural point of view I would say this to be correct

[DAE]

Pull rods or pushrods? Its really down to the installed stiffness of the link and control of the damping. Control in rebound is more important to the vehicles performance.

[Goran Malmberg]

Originally Posted by DAE

Pull rods or pushrods? Its really down to the installed stiffness of the link and control of the damping. Control in rebound is more important to the vehicles performance.

Sorry, I am not really sure what you mean. What effect should the rebound dampening have on the push or pullrod?
Goran Malmberg

[TEAM78]

What do you mean DAE? I have never heard that before, not saying its true or anything but Id be grateful for an explanation,
Goran how do you go about a force analysis? Is it best to do it 2D for each view or attempt to do it 3D, Ive built a speadsheet but its failing to work in 3D where all the rods are treated as links, does the push/pull rod contribute in any way to any other loading other than the vertical load?

[avsfan733]

I think hes talking about the loading of the contact patch through the rod from the action of the suspension spring.

[DAE]

To explain in detail:

The link, be it a pushrod or pullrod, which transfers the loads from the contact patch to the spring/damper assembly is not infinitely stiff, it flexes under the applied load.
This link effectively becomes a spring in series with the road spring, the deflection in this link is not damped. The deflection in this link does cause fluctations in the load seen at the contact patch, no amount of adjusting the road spring and damper rates will completely solve this problem.

So now the question is which is more important for the optimum lap times, compression or rebound of the suspension system?
Under compression a pullrod system is stiff, while a pushrod system is more flexible.

And from testing on rigs and on circuit, it has been proven that the control of rebound damping is more critical to performance, so the link must be stiff under rebound conditions.
So a pushrod system is more efficent as it gives a higher stiffness to component weight ratio under rebound conditions, which permit the suspension damper to have more effective control of the the chassis and wheel deflections.

But a well packaged pullrod system produces a vehicle with a lower centre of gravity, thats one of the reasons why this system was fitted to the active control F1 Lotus.

Quite simple really. ;)

[TEAM78]

WOW, just how sensitive does this have to be though for it to be considered on the ametur scene? Im checking on the buckling side of things but im surprised that the deflections in the rod would be affectting the contact patch. how does it adversely affect the contact patch in rebound and why should rebound be so critical, I was always under the impression that the tyre should be returned to the ground asap, how does the pull rod fail to do this?

[Goran Malmberg]

Originally Posted by DAE

Under compression a pullrod system is stiff, while a pushrod system is more flexible.

the link must be stiff under rebound conditions.

So a pushrod system is more efficent as it gives a higher stiffness to component weight ratio under rebound conditions

Just to clearify your statement, you mean that under rebound condition the pushrod is subject to pulling force, and thereby producing less flex.
Goran Malmberg

[DAE]

Goran, that is correct under rebound conditions the pushrod is under tension.

Team 79, yes the tyre should be returned to the ground asap, but in a controlled manner to try to make the changes in the contact patch smooth.
Components should be as stiff as possible, with minimum freeplay, and all movements, including deflections, damped. How many race tracks have really smooth surfaces which would allow for smooth translations between compression and rebound.
The pullrod under compression produces a small flex, which results in a small change of contact patch load.

Normally a good driver would notice a 0.5kg change in each contact patch, a club driver - who can say, some are very good, others......

How relavent is this to a club driver, I don't believe that it is relavent at all.
Just get out there, learn the track by completing a test before a race day. With reliable simple data logging, you will learn far more and improve your laptimes then get on and enjoy your weekends racing. Now the designers, they can worry about these small but important details and use the test rigs, wind tunnels and simulations to find the best compromise in the cars design.

[Goran Malmberg]

Originally Posted by DAE

Goran, that is correct under rebound conditions the pushrod is under tension.

To me this isnt is not the case, the pushrod is almos allways under pushing forces, even during rebound. At rest we got a constant pushing force, during bump an increase and during rebound a reduction in pushing force.
During bump the pushing forces can be tremendous, as it is working against the mass of the car, but to create a pulling force we need the car to get airborne, using the mass of the wheel tying to get back to the ground.
Goran Malmberg

[rcr286]

I take Goran's position on this one. While what DAE says may be valid in extreme conditions, a push rod should almost always be loaded with some compression, while a pull rod should experience both. I should also mention that rods typically work best in tension, as seen by the undampened flex of pushrods, and that the compression loads a pull rod experiences are nowhere near the loads of a push rod. Off the top of my head, the only time I think a tension rod would be loaded enough to create appreciable flex, is when the wheel is off the ground from heavy 'curbage' (here the swaybar, and not the spring should have the greatest influence on pull rod compression) or from flight with MASSIVE rebound rates. In either condition, damping rates are fairly null and void as seen by the tension rod.

My vote is for pull rods whenever they are convenient (typically on a sports racer/prototype). Lower center of gravity, possibility of lower frontal area on sports racers, rods are working in a more effective manner as the rod is in tension while upper control arm, that can be more effectively designed/modified, is already in compression.

-Roger

[TEAM78]

interesting points there, I do agree that pull rods would be the way to go but many race cars still use push rods, I believe this is due to the ability of having a more inclined rod to the vertical, also on the rear suspension it is very difficult to package a ARB and it a,also forces the gearbox internals up, From a force analysis point of view im finding that a push rod is better due to its increased angle relative to the horizontal how ever does suffer as above mention from buckling issues. Weith regards to lower CG that could be recovered by rnning the exhaust system down the side and keeping gearbox interals lower, CG height is damm important though and some times the force disadvantages get pushed to one side for it

[ubrben]

To throw something else out into the mix

In a corner the outside wishbones are heavily loaded in compression. The load from a pushrod will have a component in the opposite direction and will relieve some of the compression load, allowing the wishbone to be lighter.

This is one reason given by Peter Wright in his Ferrari book as to why F1 has gone fully pushrod. The angle advantage is also a big one.

Ben

[Goran Malmberg]

Originally Posted by ubrben

In a corner the outside wishbones are heavily loaded in compression. The load from a pushrod will have a component in the opposite direction and will relieve some of the compression load, allowing the wishbone to be lighter.
Ben

Very right so! The force is times tan-angle of the pushrod and will therfore become quite influencial as F-1 cars have puschrods angle in the neighbourhood of 40 dgr. The A-arms are also very aerodynamically shaped and all reduction in load is wellcome.
But for moost road type, non open wheeler, its not too important.

Goran Malmberg

[TEAM78]

thats a very good point then from the forces point of view, Im hoping to complete some force analysis work this week so i will be looking out for this to prove it to be correct!
Packaging at the moment appears to be the main driver for which option you go for though as really theres advantages and disadvantages between the two which balances things up

[TEAM78]

just out of interest how can a rear suspension run caster on the rear and why would it be done??
Ive also noticed that most teams (formulas such as F1) have the rear suspension front hard points out of plane to the rear hard points (inboard), this then causes roll steer, other than running a smaller gearbox how do the suspension people deal with this problem or do they simply accept it?

[Goran Malmberg]

Originally Posted by TEAM78

just out of interest how can a rear suspension run caster on the rear and why would it be done??
Ive also noticed that most teams (formulas such as F1) have the rear suspension front hard points out of plane to the rear hard points (inboard), this then causes roll steer, other than running a smaller gearbox how do the suspension people deal with this problem or do they simply accept it?

I not understand what you mean, would you mail me a drawing to look at?
hemipanter@hemipanter.se
Goran Malmberg

[RF_Racer]

How times change, Pull Rod design has been used by everyone this season except for Ferarri....Push Rod is no longer an advantage.

[tristancliffe]

Only at the rear, and the benefits are for aerodynamics rather than suspension dynamics. And times will change again, and F1 will revert back to pushrod. And then it'll change it again back to pullrods when somebody remembers them. Pushrod has many advantages over pull rod (which is why most single seaters use pushrods all round, and will continue to do so).

I wonder how long before the fashionable cars (cars that have to sell themselves, and do so by needlessly copying F1 details even though they don't actually do anything positive) like F3 and GP2 adopt it?

[Chipwizard]

Slightly off-topic, but does anyone know what magnitude of force you get typically in a pushrod? The reason I ask is that I'm presently on with equipping my car with a fairly comprehensive data logging system and could do with some rough idea what load the load-cells will have to cope with.

The car uses all the wishbones and uprights from an A1GP/Lola F3000 car but instead of using pushrods or pullrods the dampers are almost vertical and attach to the very outboard end of the bottom wishbones where the pushrods would have originally attached.

The car weighs 680 kg with me in it and has 48.5%F/51.5%R weight distribution and uses 600 lb/in springs on A1GP Ohlins dampers and has a bit of aero, but not a huge amount.

I know how much spring deflection I get but I just have no idea what sort of damper forces are at play in a typical kerb-wolloping situation.

Thanks in advance.