Push rod on upright suspension - an examination

Here are our CFD links and discussions about aerodynamics, suspension, driver safety and tyres. Please stick to F1 on this forum.
PlatinumZealot
458
Joined: Thu Jun 12, 2008 2:45 am

Push rod on upright suspension - an examination

Of late I have been seeing people hailing this "push rod on upright" (POU) suspension arrangement as made popular by Mercedes. Fine, no problem with the hype. However I am seeing a lot journos boldly stating that it is for steering and jacking up/or down the car. Right or wrong, F1 suspension is a complex thing, so journos should not be so careless making these claims without asking a suitably qualified individual to examine the kinematics first.

For a system like this, we have to go into vector geometry. Where the position of the push rod mount is relative to the king pin axis (or pivot axis of the upright) will determine whether the wheel will tend to steer left or right in bump (compression of spring).. and it will ONLY steer in that one direction in bump because of geometry. Imagine the push-rod resisting the translation of the wheel as the wheels comes closer to it. You wont see any jittery double-jointed snaps from left to right obviously. Steering in one direction alone (choose one) if the wheel is in bump obviously cannot be good. Why would you want the car to steer right if your suspension compresses in a left turn? Crazy. Agreed?

This is not the same as capitalizing on bump steer with the track rods on the rear wheel. And is not the same as capitalizing on steering arms lowering raising the car in corner (because this can be the effect you want in either turn direction left or right, and you can actually design this in - toe in or toe out, whatever). Also, in Formula one, 99% of the time you will notice that the steering arms are mounted in nearly the same plane as one of the control arms.This eliminates bump steer, and actually if the wheel is steered by outside forces, whether drag or this "POU," all that steering force would have to be resisted by the driver and whatever the other wheel is doing.

For example..if the left wheels is under compression, trying to turn right.. the right wheel will be pulled right by the tie rod. The right wheel will also by lifted by the anti-roll bars. Now. what are the external effects on the right wheel though? is it under compression too with the left wheel? (dive) or is it under extension (roll) opposite to the left wheel, or in between? It will try to push its tie rod( or the steering rack) to turn left if it is under compression. It will try to pull the steering rack right if it under extension. The forces of the tie rod, driver's hands, and anti-roll will have to be resolved to see what ultimately happens here. And remember depending on which side of the king pin axis the push rod mount is this effect will always act one way with bump. (there is a little way around this I will explain later)

Common sense would tell you to position the POU, so that its line of action coincides with the lower ball joint of the upright or anywhere along the king pin axis. It avoids all this steer, since the forces will be taken by the control arm ball joints. Mercedes has a curious shape for it POU mount. I have been looking at it, but I have not found any photos of the bare metal upright to see if this little mounting point is connected to the upright itself or the lower control arm. With the length of that little mounting arm it would be tricky to keep it aimed at one axis (if we wanted it to). And remember my second paragraph, if it is not in the axis, it will always bump steer in one direction ONLY. There is way to make it switch sides of the axis desirably though (I know a way, anyone else figured it out?)..but then again do we want this? You tell me.

I also encourage you reader to think of what other reasons they would position the push rod like this. It could be a very good one.

Manor Push rod on upright aligned to the king pin axis it seems:

A Sauber push rod on upright

Then we have the highly evolved Mercedes upright. A naked one is highly sought:

Tim.Wright
464
Joined: Fri Feb 13, 2009 5:29 am

Re: Push rod on upright suspension - an examination

A couple of tricks you've missed.
1. The pushrod has zero kinematic effect on bumpsteer.

2. Setting the toe link in the same plane as the control arms doesn't eliminate bumpsteer.

The trick of positioning of the pushrod off-axis from the steering axis to control ride height is called steer-ride and has been in F1 for at least 10 years from what I've heard.

The idea is to try and kinematically actuate the spring in the rebound direction with steer. This will cause the wheel loads to drop with steering input and the car will lower itself to regain weight force equilibrium.

There's a good interview with James Allison who explained the concept and why it's hard to drive in a straight line. I'll post it if I can find it.
Not the engineer at Force India

Tim.Wright
464
Joined: Fri Feb 13, 2009 5:29 am

Re: Push rod on upright suspension - an examination

This should be the interview with James Allison where he explains the concept. I'm geoblocked so I can't verify.
https://www.skysports.com/watch/video/s ... ial-part-2
Not the engineer at Force India

zibby43
zibby43
420
Joined: Sat Mar 04, 2017 11:16 am

Re: Push rod on upright suspension - an examination

Tim.Wright wrote:
Sat Feb 15, 2020 6:40 am
A couple of tricks you've missed.
1. The pushrod has zero kinematic effect on bumpsteer.

2. Setting the toe link in the same plane as the control arms doesn't eliminate bumpsteer.

The trick of positioning of the pushrod off-axis from the steering axis to control ride height is called steer-ride and has been in F1 for at least 10 years from what I've heard.

The idea is to try and kinematically actuate the spring in the rebound direction with steer. This will cause the wheel loads to drop with steering input and the car will lower itself to regain weight force equilibrium.

There's a good interview with James Allison who explained the concept and why it's hard to drive in a straight line. I'll post it if I can find it.
Is that why the Mercedes looked so darty and nervous on the straights in Melbourne last year?

Tim.Wright
464
Joined: Fri Feb 13, 2009 5:29 am

Re: Push rod on upright suspension - an examination

Yup I'd say so. The system creates a "negative" steering torque which effectively means the driver doesn't feel a centering force at the wheel like normal cars but an anti-centering force which tries to increase the steering angle by itself. The driver has to fight against this to keep the car going straight.
Not the engineer at Force India

zibby43
zibby43
420
Joined: Sat Mar 04, 2017 11:16 am

Re: Push rod on upright suspension - an examination

Tim.Wright wrote:
Sat Feb 15, 2020 8:37 am
Yup I'd say so. The system creates a "negative" steering torque which effectively means the driver doesn't feel a centering force at the wheel like normal cars but an anti-centering force which tries to increase the steering angle by itself. The driver has to fight against this to keep the car going straight.
Fascinating stuff. Thank you for the explanation.

MtthsMlw
846
Joined: Wed Jul 12, 2017 5:38 pm
Location: Germany

Re: Push rod on upright suspension - an examination

Pick one

PlatinumZealot
458
Joined: Thu Jun 12, 2008 2:45 am

Re: Push rod on upright suspension - an examination

Tim.Wright wrote:
Sat Feb 15, 2020 6:40 am

2. Setting the toe link in the same plane as the control arms doesn't eliminate bumpsteer.
It can be done among other ways. Its just geometry. I did not mention the pivot points and length of the rod in relation to the arms. But you can just calculate for this. But it is easiest when the tie rod is close to the same plane because one variable (rotatation ofr the upright is elimianted)

Minimizing bump steer
http://www.longacreracing.com/technical ... ?item=8162

PlatinumZealot
458
Joined: Thu Jun 12, 2008 2:45 am

Re: Push rod on upright suspension - an examination

Cant see anything here.
Show us some without the coverings.

269
Joined: Sat Jan 02, 2010 12:57 am

Re: Push rod on upright suspension - an examination

I don't quite understand. It sounds like you're saying they build in bump-steer. I have always been of the opinion that they do everything they can to eliminate bump-steer. Why would they want the car to steer itself?
To achieve anything, you must be prepared to dabble on the boundary of disaster.”
Sir Stirling Moss

Zynerji
Zynerji
80
Joined: Wed Jan 27, 2016 3:14 pm

Re: Push rod on upright suspension - an examination

Sat Feb 15, 2020 7:38 pm
I don't quite understand. It sounds like you're saying they build in bump-steer. I have always been of the opinion that they do everything they can to eliminate bump-steer. Why would they want the car to steer itself?
Hasn't Merc rear-steer been hailed as a huge thing? Self-steer seems like it would be very helpful if it could be exploited.

Pyrone89
3
Joined: Fri Jul 05, 2019 8:44 pm

Re: Push rod on upright suspension - an examination

Which teams have this on their 2020 car? Does the RB16 and SF1000 have it?
Best WDC-drivers in F1 history:
Schumacher, Senna, Fangio

Driving a dominant car in the most dominant team ever, helped by favorable rule changes, against subtop teammates does not make you the GOAT (but still superb). It just helps you inflate/skew your stats.

PlatinumZealot
458
Joined: Thu Jun 12, 2008 2:45 am

Re: Push rod on upright suspension - an examination

Tim.Wright wrote:
Sat Feb 15, 2020 8:37 am
Yup I'd say so. The system creates a "negative" steering torque which effectively means the driver doesn't feel a centering force at the wheel like normal cars but an anti-centering force which tries to increase the steering angle by itself. The driver has to fight against this to keep the car going straight.
So how does the setup know whether it's turning left or right?

Sort of a trick question...

Greg Locock
Greg Locock
188
Joined: Fri Jun 29, 2012 11:48 pm

Re: Push rod on upright suspension - an examination

What's the advantage of rolling the car when turning? Why is this better than CGZ/RCH/sta bars?

Well, further thought says that this will be speed dependent, SWA and hence wheel steer is greater at low speeds. But I'd have thought rolling the car at low speeds is not especially beneficial.

zibby43
zibby43
420
Joined: Sat Mar 04, 2017 11:16 am

Re: Push rod on upright suspension - an examination

Pyrone89 wrote:
Sat Feb 15, 2020 8:35 pm
Which teams have this on their 2020 car? Does the RB16 and SF1000 have it?
The SF1000 has it. Ferrari had it last year, too.