Push-rod OR Pull-rod suspension

[DB1]

Currently working on a project involving designing and optimising the suspension system for a IMechE FS car. I've come up with a few concepts with regards to pull-rod and push-rod systems. My intuition is telling me that the standard push-rod config increases a racing car's CoG and the pull-rod config lowers its CoG.

[CBeck113]

Give your intuition a high-five from me. But a suspension has a lot more to do, so the CoG should not be the reason for choosin gthe design. As McLaren have experience this year, and Ferrari last year, pull rods can be difficult to set up, and a good car has to have an easily adjusted and very flexible (no pun intended) suspension in order to be fast on all tracks.

[krisfx]

I can't see you'd make massive gains by choosing one over the other at the "circuits" FSAE actually runs on, you should design for the easiest to set up. You can have the lowest CofG but if you just end up with a bucket load of understeer, you've not really gained anything?

[DB1]

It makes sense to go for the more easily-accessible push-rod config for the formula student race car. The project is to test for vehicle dynamics, so looking more in to how the chassis behaves under acceleration and braking forces with different suspension set-ups. I'll play around with different designs/configs and see what I get. cheers guys

[RideRate]

My argument is always that both can be designed and setup to give you the exact same system level behaviors. IE the net spring and damping rates at the wheel. So in reference to vehicle dynamics it's not relevant.

Therefore I argue that pullrods have two small but worthwhile gains over pushrods. They are a reduction of total weight and better location of weight used.

Pushrod's positive argument always include words like "ease of" and "simple". So if that's your desired qualities...

For my taste I like the system choice that give all the small performance gains with clever packaging. My car had 4 pullrods.

[Jersey Tom]

My argument is always that both can be designed and setup to give you the exact same system level behaviors.

Can be in theory? Sure. Typical in practice? Not so much, probably particularly so with young/inexperienced design engineers - even if it's done up "optimally" in the CAD or computer world.

There's no free lunch, or every "rod" car would use a pull-rod because it would be a no-brainer - but it isn't, so they don't. Always compromises. Maybe with the rockers mounted low you now have to invert your T-style FARB system and that puts mass higher in the car. There are other packaging compromises. There will certainly be an easy of service compromise.

It makes sense to go for the more easily-accessible push-rod config for the formula student race car.

So far this discussion has been hand waving either way. You could do it a bit more objectively with say a decision matrix. Rank some criteria, and rate how different options (push-rod, pull-rod, etc) meet those criteria. See which scores higher. Could even put some slightly out of the box alternatives in there, like having the coilover almost like a "strut" and getting rid of the "extra" rod and rockers all together...

[RideRate]

Can be in theory? Sure. Typical in practice? Not so much, probably particularly so with young/inexperienced design engineers - even if it's done up "optimally" in the CAD or computer world.

I agree, but our big weight plus showed up in the resultant chassis design, not in the suspension parts themselves.

When we get into road cars I'd argue the push/pull rod dilemma not even be considered. There is plenty structure now to go direct actuated and eliminate links, rockers, hardware, etc. But when we're stuck to inboard mounting without aero penalties for either solution it becomes mostly a trade-off of small performance versus the ease of design/implementation and service.

I still like to see pull rod designs. To me it shows more of an effort in pulling out all the stops, which I personally like. The negative with my design was the packaging because we weren't working in "free space". But that's one more reason why I liked it. The serviceability wasn't compromised at all.

[xpensive]

How many threads like this one do we have had at this fine forum now richard?

[DaveW]

There's no free lunch, or every "rod" car would use a pull-rod because it would be a no-brainer - but it isn't, so they don't. Always compromises...

Totally agree.

One challenge encountered by the designers of the current crop of F1 vehicles is the geometry of the rear suspension. The pull rod is angled to pick up the forward location of the rocker. This might, in part, explain the poor installation stiffness (which, in one case, was estimated to be a third of the equivalent GP2 figure).

The other challenge is access. At a typical rig test of a GP2 car it would be normal to assess around 50 suspension set-ups during the course of a day. That was reduced to 11 for an equivalent F1 rig test.

... And I didn't even mention words like "ease of" and "simple"....

[Tim.Wright]

One thing I've noticed recently (which is actually quite obivous now I think about it) while playing about with these geometries in my own designs is that with roll centres where they typically are (near the ground), a push rod normally almost always give a higher gain of p/rod movement per wheel movement compared to a pull rod. This means the pull rod is almost always subjected to higher forces and as a result you need heavier parts or you lose out in installation stiffness.

Tim

[mep]

How many threads like this one do we have had at this fine forum now richard?

Many and still people hardly talk about aerodynamic reasons to chose one over the other.
Currently, I would say pullrod has aerodynamically a clear advantage. Whereas weight might be higher and stiffness lower than with previous installations. However advantages in aero win about all these disadvantages.

[DaveW]

In my view, installation stiffness apart, a pushrod layout does little to help mechanical performance. This would be for several (mostly regulatory) reasons. On the other hand, I suppose that the alternative pullrod suspension is more likely to keep debris (e.g springs) contained with the current layout.

Currently, I would say pullrod has aerodynamically a clear advantage.

Certainly that is the perception. Unsurprisingly, statistics don't really help:

Ferrari points (percent of total)

• 2011, pushrod, 19.54
  2012, pullrod, 19.80
  2013, pullrod, 18.45

I wish someone would explain the "clear advantage" to me in simple terms.

[DaveW]

An interesting couple of paragraphs extracted from http://mercamgf1-fans.com:

GEARBOX WITH SECOND CARBON SKIN
Engineers Brackley have adopted this approach for having the opportunity to change the angles of the rear suspension without having to redo the merger of the the transmission case, but the main benefit that it provides is that it makes it possible to work on the rear of the car without having to replace the gearbox that needs to remain the same for five GP, otherwise you pay a penalty of five places on the starting grid. In fact it has emerged that this second structure could have decreases significantly the degradation of the tires.

MODIFIED REAR SUSPENSION
As mentioned previously for the Monaco Grand Prix Mercedes has brought a completely revised solution of the composite structure that should stiffen the rear, without affecting the gearbox. The leadership of the German House, in fact, have lobbied on the team because the excellent performance test, also consistent with identical returns in the race. Bob Bell has asked his men a big commitment that did not concern only the gearbox, but also the brake ducts that they must “draw” air in an area that is affected by the presence of hot gas exhaust (Coanda).

Tyre degradation improved by increasing installation stiffness, apparently...... Perhaps mechanical set-up does have a part to play in F1, after all.

[mep]

I wish someone would explain the "clear advantage" to me in simple terms.

Sorry, I was talking about rear suspension only.
Aerodynamically its better to have pullrod on the rear. Whereas on the front pushrod might have a certain advantage.
That is all driven by way how the floor generates its downforce.