The relative benefits of a pull rod suspension

[Caito]

xpensive I think you're wrong in this one.
Suspension arms do carry vertical load. Let's put it to an extreme condition, suspension vertical, it carries all the vertical load. So in between parallel to the floor, and perpendicular, there must be a transition point.

Despite appearances the pullrod is as effective in moving the rocker for a given wheel travel as a pushrod. The important factor is the angle between the rod and the wishbone is connected to, rather than the rods angle to the chassis.
http://scarbsf1.wordpress.com/2012/02/2 ... ari-f2012/

Also from craig
Intuitively, this looks like the rod would barely move the rocker as the wheel rises. But the pull-rod actually operates by creating a triangle with the wishbone, so in actual fact it's the angle between the wishbone and pull-rod that creates the displacement. When comparing Ferrari's pull-rod geometry to a push-rod, the relative angles are the same and therefore both operate the rocker equally efficiently.

[dren]

...delete

[myurr]

Why was the pushrod invented in the first place? The pull rod came first and was subsequently optimised into the push rod.

You cant think cut and paste. Show me an old school pull rod torsion spring suspension in the REAR of an F1 car.
And then tell me what was optimised from that. If you can do that.
Saying something is much different than seeing something. You cant blanket all pullrod as the same.

I am clearly referring to rear suspension. So we can take it from there.
Also we can look at the motion ratios of each design. Pull rod has better motion ratio in 2 aspects.

Nice cherry pick of a single point there. But anyway, you can't say all push rod solutions are equal either nor that all pull rod designs are better than all push rod designs. What are the clear cut absolutely indisputable benefits of pull rod designs that categorically means anyone choosing a push rod at any point in the future would have to be crazy?

[xpensive]

xpensive I think you're wrong in this one.
Suspension arms do carry vertical load. Let's put it to an extreme condition, suspension vertical, it carries all the vertical load. So in between parallel to the floor, and perpendicular, there must be a transition point.
...

Don't think I am, imagine Ferrari's suspension arms being L-shaped and horizontal from the upright and inwards,
then isolate the forces in vertical direction on the upright, what's left is wheel-load and pull-rod force.

See what I'm saying?

[dren]

The A-arms are seeing mainly horizontal loads. The pullrod sees mainly vertical loads due to the chassis suspension.

The A-arms do not oppose vertical movement within their allowed range (allowed by the pullrod). The pullrod does limit the vertical movement of the A-arms due to geometry. If the pullrod was firmly connected at the chassis, there would be no movement.

So yes, there are some vertical loads seen by the A-arms. The pullrod primarily sees the vertical loading due to chassis suspension though.

[marcush.]

...
Errr no. It's not the angle relative to the nose that is important, it's the angle relative to the suspension arms. When the wheel moves it will be producing the same forces and movement on the pull rod as in a design where the whole suspension was rotated slightly and the pull rod was no longer horizontal. It's visually confusing and I was fooled by it to begin with, but there have been numerous articles and posts by very well informed members that have set the record straight. And now that it's been pointed out to me it's easy to see.

Don't agree, the angle of the suspension arms means nothing, unless you are thinking track-width widening with vertical movement. Other than that, isolate the vertical force on the upright and the reaction force from the shallow angle pull-rod.

Try think this way, what happens if I remove the pull-rod? Car falls down as the suspension arms don't carry load.

Pull-rod force will be vertical load on the upright over sinus horizontal-alfa for the pull-rod, when alfa becomes smaller,
pull-rod force goes xorbitant and stiffness out the window.

my take on it.

[ringo]

Nice cherry pick of a single point there. But anyway, you can't say all push rod solutions are equal either nor that all pull rod designs are better than all push rod designs. What are the clear cut absolutely indisputable benefits of pull rod designs that categorically means anyone choosing a push rod at any point in the future would have to be crazy?

So you can't find a pullrod rear suspension then?

all pushrod have the same operation.
In the context of F1 aeo benefit it was shown the gearbox can be narrower.
there is a cooling benefit due to packaging as well.
COG benefit etc.

Engineers will look at all of this.
If you are choosing a pushrod against all these facts then... crazy is fitting.

Speaking about the future, we will still see the pull rod rear for 2014. The turbos are in the middle of the car. More than likely the room on top the gear box will be the exhaust path.

[xpensive]

....

Don't agree, the angle of the suspension arms means nothing, unless you are thinking track-width widening with vertical movement. Other than that, isolate the vertical force on the upright and the reaction force from the shallow angle pull-rod.

Try think this way, what happens if I remove the pull-rod? Car falls down as the suspension arms don't carry load.

Pull-rod force will be vertical load on the upright over sinus horizontal-alfa for the pull-rod, when alfa becomes smaller,
pull-rod force goes xorbitant and stiffness out the window.

my take on it.

Indeed marcush, I can't see the problem here, rather simple statics isn't it?

If you visualize the suspension arms with an L-shape profile to make them horizontal from the upright and inwards, then isolate the vertical loads on the upright, you end up with the following:

F pullrod = Wheel-load / sinus pullrod angle.

Suspension arm angle has nothing to do with it, right?

[Richard]

If the wishbones are horizontal and the pull/push rod is at alpha to the ground:

Vertical load = V
Rod F= V / sin(alpha)
Wishbone F = V / tan(alpha)

However, the F2012 front suspension is more like an equilateral triangle, the pull rod and upper wishbone are at the same angle above and below the horizontal plane. Lets say the slope is alpha/2, one pointing up, the other down.



So the internal angle created when the meet is alpha, this is what Scarbs is saying.

That means the previously horiz wishbone that carried no vertical component is now has a vertical component. Using x and y as stiffness ratios...

Rod F= xV / sin(alpha/2)
Wishbone F = yV / sin(alpha/2)

Hang on a second, the rod is a single rod attached a spring, while the wishbones are rather bulky and attached to the chassis. So stiffness y is very high and x is very low.

Hang on a second 2.... For equilibrium, the horizontal components of rod and upper wishbone must be equal, so stiffness is irrelevant to force distribution.

Hence F = 0.5V / sin(alpha/2)

Now I recall 35 degrees getting mentioned, lets say alpha = 35

For conventional horiz wishbone and pull rod at 35 deg,
Rod F = -0.57V
Wishbone F = +1.42V

For the F2012 pull rod at 17.5 deg,

Rod F = - 1.66 V
Wishbone F = + 1.66V

The forces are higher in the F2012.

[xpensive]

I stand corrected, the track-width is increasing substantially with suspension movement, I failed to see that.

[marcush.]

I mentioned this already -the suspension has immense scrub in bump travel (increase of track width ) so you are introducing a side force there ,depending on the rotation of the wheel ...a blocked wheel will lead to increasing vertical stiffness...I think this layout is a can of worms tbh..

[PlatinumZealot]

The wheel motion is the same as a push rod suspension. The increase in track width is significant but not large either. The F1 suspension has such small wheel travel.

[xpensive]

I mentioned this already -the suspension has immense scrub in bump travel (increase of track width ) so you are introducing a side force there ,depending on the rotation of the wheel ...a blocked wheel will lead to increasing vertical stiffness...I think this layout is a can of worms tbh..

The other thing is that you now have compression forces in the suspension arms, why you have to consult Euler when designing them. With a pushrod you don't have to bother with that, only the rod itself is in compression.

[marcush.]

The wheel motion is the same as a push rod suspension. The increase in track width is significant but not large either. The F1 suspension has such small wheel travel.

but thats the point -when your installation stiffness is really determining how much control you get with dampers and springs.

this looks really like a system that is dominated by side effects and you have limited influence in terms of sensivity to damper and spring adjustments.(make things stiffer and other components will give..) true you got little wheel vertical travel but you could translate this into a lot of damper travel but that is only workable if you installation does not flex.

[dren]

I mentioned this already -the suspension has immense scrub in bump travel (increase of track width ) so you are introducing a side force there ,depending on the rotation of the wheel ...a blocked wheel will lead to increasing vertical stiffness...I think this layout is a can of worms tbh..

The other thing is that you now have compression forces in the suspension arms, why you have to consult Euler when designing them. With a pushrod you don't have to bother with that, only the rod itself is in compression.

They will see compression forces when cornering in both push and pullrod set-ups.