Pullrod/pushrod suspension springs horizontally near floor?

[g-force_addict]

How much F1 springs and shock absorbers weight?

Couldn't the center of gravity be lowered by a pullrod or pushrod suspension with springs and shock absorbers placed (almost) horizontally near the car floor.
Likely spring length might require them to be placed staggered, one in front of the other.

Has it been done before?

[g-force_addict]

Pretty much something like this but turned upside down


Maybe springs can also be placed longitudinally as long as they are mounted low

[Tommy Cookers]

Citroen 2cv & 3cv ?
the disposition of the spring forces wrt the 'cg' would have effects eg on relativity of pitch and bounce periods etc etc

[Tim.Wright]

Citroen 2cv & 3cv ?
the disposition of the spring forces wrt the 'cg' would have effects eg on relativity of pitch and bounce periods etc etc

I doubt that's correct. While I haven't proven this to myself methametically, my feeling is that the orientation of the springs would have a negligable first order effect on the chassis response. Second order effects might crop up such as friction increasing with certain (bad) orientations of the push rod/rocker/damper.

Consider this: Placing the spring horizontally on the floor, you will produce a lateral force applied to the chassis where the spring attaches. The reason that this is of no consequence is that at the rocker pivot, you have an almost exact opposite force. So they largely cancel out.

Would be an interesting proof to look at though...

[marcush.]

I´d think Gustav brunners legacy cars -Ferrari F187/188 Rial Arc01and Zakspeed have a front suspension along your proposal...

[DaveW]

The last(?) generation of Dallara's Indy car had a pull rod front suspension with coil over-dampers arranged horizontally front/rear, I think. Couldn't find a cut-away, I'm afraid.

Access good (from the outside of the tub), installation stiffness not so good (compared with the previous push rod layout).

[Tim.Wright]

Dave, in your experience do you typically see a worse installation stiffness in pull rods compared to push rods? A theory popped into my head the other day which suggested that could be the case.

[marcush.]

I think this does show it quite nicely.I don´t see the problem why it could not be just as stiff as a pullrod in installation stiffness .i think most designers make the mistake to take too much advantage of the system being mainly under tension forgetting about the vibrations and orders ineviatbly the thin rod will dance around and give funny input and may be lured into thinking you could make do with flimsy rockers and rocker bearings.
http://smashingev.wordpress.com/racing/package/

[Tim.Wright]

This is my reasoning as to why I think pull rods often have a lower installation stiffness. In the image below i have a typical double wishbone setup, albeit not really representative of a contemparary F1 design. I have a pushrod in red and a pull rod in orange. Both can be assumed to be connected to the upright for this explanation. The wishbones define an instant centre about which the upright rotates.

Below that I have isolated the rods and drawn in n-t lines to show normal and tangent directions of the rod outboard points. I consider an instantaneous change in the P/Rod length, which will actually translate into a deflection of the spring because the P/Rod is fixed. However the change in P/Rod length is easier to visualise.

The instantaneous change in length of the pushrod w.r.t. the wheel vertical movement is going to be proportional to the cosine of the angle between the 't' direction and the rod itself. I.e. the more co linear the rod is to its current trajectory the more its length will change as the wheel moves vertically. Conversely when the rod and the 't' direction are at 90deg, the rod won't change length at all.

We can see that in this arrangement, the push rod has an angle of about 45deg and the pull rod has an angle of about 110deg. This means that the push rod changes its length a lot more than the pull rod for a given vertical travel at the wheel because its trajectory is more collinear to the push rod compared to the pull rod.



This neccessarily means a higher change in force is required in the pull rod because in accordance with the conservation of energy:
Force@wheel x Disp@wheel = Force@P/Rod x Disp@P/Rod
For a given wheel displacement and force, a lower displacement of the pull rod will require a higher force to maintain the wheel rate.

So here the pull rod has higher forces in it as compared to a push rod of the same overall wheel rate. This is the first step towards a lower installation stiffness. The next step is, as Marcush said, people tend to design the pull rods thinner and lighter since they only need to take a tensile force. All of this adds up to increase compliance in the pull rod and rocker assembly.

[Lycoming]

So then, let's say my suspension system is as you've illustrated, but let's say my chassis is such that the top surface of the chassis is practically the same height as the top inboard wishbone mount. That would mean that the height of the pushrod's inboard mounting point would now be about the same as the top inboard wishbone mount. In that situation, won't the installed stiffness be comparable or slightly worse for the pushrod? What I mean to say is that while your line of reasoning holds for most common configurations, it is not true in general.

[Jersey Tom]

I'm onboard with at least most of Tim's line of reasoning. Though how I'd describe it... you have three orthogonal force at the ground - vertical, lateral, and longitudinal. To resolve those forces efficiently into the chassis, you want to have elements either in those directions or at least with some "orthagonality" to each other. Otherwise, to resolve say a vertical load in a nearly horizontal element the load will indeed be quite large.

Really the question is.. does it make much difference in a typical car topology.



Say you were to replace the UCA and LCA in either case there with a single swing arm. Would either the pull- or push-rod have a significantly larger angle relative to the swing arm?

Eyeballing it here, I'd think they'd be somewhat close.

In any event, getting back to the original question.. sure, you could potentially lower the total car CG marginally with a topology like that. But nothing is in isolation. Those small gains can be dangerous if they're at the expense of something larger...

[DaveW]

Dave, in your experience do you typically see a worse installation stiffness in pull rods compared to push rods?

I had my say here ... Your theory is interesting, and marcush comments are certainly valid. The devil, as always, is in the details.....

[marcush.]

the very best car of recent years ignores what JT says completely so he got a point here.But on the other side tyou see them using MASSIVE rocker installations with a very wide base to counter the potential installation stiffness issuesat the back of the car.

Really the task of the designer is to take into account the awkward angles in his design and i think that´s what RBR is doing .They have a very precise and sturdy rocker assembly and live with a pull rod at an awkward angle .I wonder why nobody yet has replaced the rocker with a flexure -the potential installationstiffness,stick slip and friction characteristics would all help here and you could reduce part count(=weight=reliability)drastically.Now you tell me the movement is too much for a solid joint ? RedBulls expertise in designing flexy parts should allow them to work on this ...

[Tim.Wright]

So then, let's say my suspension system is as you've illustrated, but let's say my chassis is such that the top surface of the chassis is practically the same height as the top inboard wishbone mount. That would mean that the height of the pushrod's inboard mounting point would now be about the same as the top inboard wishbone mount. In that situation, won't the installed stiffness be comparable or slightly worse for the pushrod? What I mean to say is that while your line of reasoning holds for most common configurations, it is not true in general.

Yes of course its not always valid, I was just demonstrating a theory. Though I'd still argue that with the push rod repositioned like you describe, its still seeing a better motion ratio than the pull rod. The reason being that in this design, which was typical maybe until about 10 years ago, nearly always has the upper arm inclined more than the lower in order to set the RC at the desired height. Also in these designs you also nearly always had space to put the push rod above the upper wishbone whereas there has never really been space to put it under the lower wishbone.

These days with both the upper and lower control arms sloped upwards to the centre of the car, I think its almost 50/50 as to which solution will produce the highest forces for a given wheel rate.