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

[xpensive]

If I understand you right Dave, we are talking vertical wheel-load versus flexibility at spring pick-up point?

But still, 2000 N/mm, it's not xactly mindboggling is it?

[DaveW]

xpensive, The idea is that it represents wheel deflection that is not controlled by a damper, since it is, effectively, a "spring" in series with the damper. Your average aerodynamicist might say (has said) that a reduction in installation stiffness can be compensated for by an increase in spring stiffness. Dynamically, however, contact patch load control (hence "grip") can only be maintained by reducing spring stiffness (keeping the ratio constant). The aerodynamicist's logic probably explains where some F1 teams are (or have been - F1 designs are generally better now than they were a few years ago). All of which which explains, hopefully, why it is an important parameter to know.

Actually 2 KN/mm is fine for a normal open-wheeler. Less than 1 KN/mm is certainly not. To be fair, it is a challenge to design a vehicle weighing not much more than 400 kg (dry & unballasted) without throwing away stiffness.

[xpensive]

Pardon me Dave, it's just the neanderthalic engineer in my spine thinking that Aluminium gives me an E-module of some 70 GPa. Which translates to a 0.14 mm elongation of a 500 mm long and 100 mm^2 cross-section rod when subjected to a 2000 N load?

[DaveW]

xpensive: I haven't checked, but I'm sure your calcs are correct. However, for once, I beg to differ. Did you consider motion ratio? The following link should bring up one of Tony Matthews' beautiful sketches that might be instructive (the damper motion ratio will probably be >= 1).

http://forums.autosport.com/index.php?s ... &p=3724945

The rocker ratio appears to be around 3? - which would divide your stiffness estimate by around 3^2 = 9 (ish).

[xpensive]

Very much a static man myself Dave, but I was thinking a bit. Imagine 0.14 mm elongation is a ball-park number for an 11 mm dia and 500 mm pull-rod, but at a 45 degree angle that quickly becomes 0.2 mm of wheel-travel, then adding the compression load of the A-arms as a reaction force from the pull-rod?

Stiffness and clearance from different ball-joints involved and we might be homing in on your numbers after all?

Gotta think about this for a while longer, thanks for the support.

[xpensive]

Dave,
No matter how I twist and turn this, a 500 mm long and 11 mm dia aluminium pull- or push-rod will not elongate/comress more than 0.14 mm at 2 kN, if angled at 45 degrees, 0.20 mm.

A carbon-fibre or steel rod obviously even less and modern-type flextures has eliminated clearance and flexibility in those areas, why I'm not even getting close to your value of 2 kN/mm flexibility from the wheel to the spring pick-up point?

I must be missing out on something here.

[DaveW]

I'm sure I will not answer your question in a way that will satisfy you, xpensive. However, the fact that the rod is angled will increase the load & hence the rod deflection as you suggest, but will also increase the wheel deflection caused by a unit rod deflection - which is why motion ratio is a useful (& lazy) concept. Geometric complexities can be short-circuited by looking at the moment arms of the rod & the damper acting at the rocker, which is why I suggested you look at Tony Matthews' artwork. Referring to that again, & looking at the two moment arms (damper & push-rod), I estimated the ratio to be around 3. It is reasonably safe to assume that the damper motion will be (roughly) 1:1 with the wheel (although that particular layout had a "rising rate" characteristic). Hence your calculated rod stiffness must be divided by motion ratio squared (once for load & once for displacement) in order to convert it to stiffness at the damper (& hence wheel). Which is why I suggested that your estimate should be divided by 9 (I would be prepared to negotiate around the value, but not the principle). BTW I haven't seen flexures used on push-rods, & can think of very good reasons why they wouldn't be (the effect of bending deflection on buckling load).

[xpensive]

Thanks for you patience Dave,
That's really the thing, I don't doubt you numbers for a second, I'm just the kint of enginner who nneds to get his head around such things.

Regarding the flextures, I was thinking the reaction force in the suspension arms, push-rod compression meaning A-arms stretch, right?

A most interesting correspondence however, thanks.

[Greg Sowell]

So what is the difference between the two suspension types. Why has Newey stuck to pull rod for the RBR cars?

[Jersey Tom]

Advantages and disadvantages to both.

RBR went with pull rods as they think they work best with the entire vehicle as the system. Ferrari don't as they believe pushrods are overall the better option.

Why do some people prefer chocolate ice cream to vanilla?

[speedsense]

There’s been a lot of technical discussions on the benefits of push versus pull type suspensions. How the rod is loaded in bump and rebound as well as the loading seen by the outboard end of the wishbone. While the installation stiffness versus weight might vary between the options the main factor is packaging. F1 cars have had front push rod suspensions since the advent of high noses (the Minardi PS01 and Arrows A22 both had a low nose), simply because there was nowhere to package the linkages in a high nose without a poor angle to the pull rod. Pushrods place the parts in a better location for access albeit at a price for Vertical CofG. At the rear the issue has been both a suitable location for the linkages\torsion bars and the third damper (and latterly an inerter), thus atop the gearbox is the ideal solution for a narrow rear end.
This year the need for narrowness is partly negated by the new rules pushing the diffuser 33cm back, leaving space low down to package the linkages etc. However I don’t know how Newey has interconnected the linkages to a third damper and inerter, I guess he has lengthened the gearbox to create space between the engine, clutch and gearbox and passed the two 3rd elements above and below the counter shaft. Access to this lot will be a nightmare, but a sign of true Newey car. Changing torsion bars and dampers will be a floor and gearbox off job.

One issue you missed... how big are the hands of the mechanic that adjusts the suspension and whether or not he cusses a lot or not... Cars that have pushrods, have happy mechanics. Cars that have pull-rod mechanics need a team mental doctor to keep "em happy and from throwing tools, while raiding the first aid kit...

Ever notice that Newey's mechanics always have small hands?

[xpensive]

I think access is the ultimate reason for going the pushrod way, the reason for RBR's somewhat mysterious to me,
when it wouldn't seem to be preferable for the diffuser?

I remember the very first suspensions of this kind, the Brabham BT44 for example, all had pull-rod fronts? But I think that could hark back to the old saying, "springs should be pushed, rods should be pulled".

[autogyro]

I think Neweys rear pull suspension is a big leap forward in suspension design.
It is a lot more than just packaging.
I believe the geometry works much better in pitch than conventional push rod systems, resulting in a much smoother and more progresive application of drive torque, giving higher traction and natural traction control.
Under braking it allows a smooth relative increase in rear ride height, increasing DF from the resulting floor angle into corners.
In a way the car rolls smoothly around the front axle (within a fine mm margin of course). This has allowed the aerodynamics to be better designed around this more controlled mechanical base.
In roll the rear transfer of load on the drive wheels is far more progresive than a hard push rod system, which not only continues to assist traction and traction control but also maintains a better feel of the car through all load changes.
Of course, there is the addition of ride height control, of which the least said the better perhaps.

[riff_raff]

Here's something to consider with regards to pull vs push rods on a modern F1 suspension. A few years back F1 cars used spherical joints instead of flexures for their a-arm attachments. Pushrods were preferred since they could accommodate higher rebound compression forces than slender pullrods. With the flexure joints now in use, the flexure is in effect an additional spring (like the tire sidewall) whose forces are not transferred through the push/pull rod. Thus the suspension spring itself is required to produce less of the total suspension spring force. So a pull rod might be suitable with these lower loads.

[xpensive]

Here's something to consider with regards to pull vs push rods on a modern F1 suspension. A few years back F1 cars used spherical joints instead of flexures for their a-arm attachments. Pushrods were preferred since they could accommodate higher rebound compression forces than slender pullrods. With the flexure joints now in use, the flexure is in effect an additional spring (like the tire sidewall) whose forces are not transferred through the push/pull rod. Thus the suspension spring itself is required to produce less of the total suspension spring force. So a pull rod might be suitable with these lower loads.

Allow me to be slightly doubtful here Terry, is there any way you could quantify this load-bearing capacity of the flextures? I seem to remember that when John Barnard first introduced such on the Ferrari in the 90s, there were arguments of them being illegal due to just that capacity?