Non-Zero King-Pin Angle

[DrJB]

Kingpin isn't just for that. It also is responsible for the steering to return to center in cars (if you ever wondered why your steering wheel returns to the center position).

That is true, I only gave one explanation. The tire has a self-aligning moment (SAM), and that, together wit the scrub radius, determines how much overall return-to-center you have. If you really want to add other mechanisms that control the return to center, you must include the tire's inherent PRAT (Plysteer Residual Aligning Torque). This is very important in the US, where most roads are straight and vehicles sometimes tend to drift, because the drivers only hold the steering wheel lightly. In Europe however, because roads are typically very sinuous, the drivers hold on tightly onto the steering wheel, and typically cannot sense the pull/drift caused by PRAT ... Well, two more things: The Crown of the road (how non-flat it is), and the front-end alignment BOTH affect Return-To-Center ...

Isn't that the Caster?

Also it's a bit disturbing that you only use "kingpin" when you really refer to "Kingpin inclination". King pin (according to wikipedia) is something old cars had.

Or am I the only one who didn't know that "kingpin" is the synonym for "kingpin inclination"?

True, Kingpin is for old cars. But the terminology stayed and is still in use today, at least with NA auto-makers. True, very few modern vehicles have an actual kingpin. What is referred to in this case, is the line joining the articulation with the upper and lower control arms. Very often also, such kingpin 'axis' is not fixed and tends to move, depending on load on axle ... etc, especially in multi-link suspensions.

Edited February 3, 2014 by DrJB

[nicjasno]

The kingpin doesn't move. It's very well defined by the upper and lower pivot point.

The return to center is of course regulated by caster, camber, kingpin and toe. But the kingpin has the biggest effect on this.

Edited February 3, 2014 by nicjasno

[DrJB]

The kingpin doesn't move. It's very well defined by the upper and lower pivot point.

The return to center is of course regulated by caster, camber, kingpin and toe. But the kingpin has the biggest effect on this.

In a conventional suspension, with upper and lower A-arms, the king-pin is fixed. In a multi-link suspension however, the king pin does move when you steer the tires.

[nicjasno]

No, it does not. :)

[DrJB]

well ... I need to pull out my vehicle dynamics books/notes and refresh my memory a bit ... will get back to you. :)

[nicjasno]

While the links move around, the virtual pivot point stays in the same place. So it's the same thing as in a double wishbone.

Audi and many manufacturers use a multilink, to move the pivot to an area where it otherwise could not be, because there is no room for a ball joint there.

[DrJB]

For argument sake, picture this: a very simple suspension with a single A-arm on the top, and two links at the bottom. The top pivot does not change and we can both agree on that. Now if you extend the lower 2 links and find their intersection point, it will define a second point for that virtual king-pin. Now, if you steer the tires totally to the left, then totally to the right, that second point (virtual) will absolutely/positively move. I can sketch it quickly and post it, but it's a rather easy exercise. It is obvious to me that as the tires steers left/right, the virtual point (intersection of the two links) will move in the fore-aft direction.

What are we missing here?

I am fully aware of the many EU manufacturers using such geometry to 'offset' the virtual king-pin. I even wrote a paper published in SAE about this very same topic ...

Edited February 4, 2014 by DrJB

[Lipko]

Nicjasno, what do you mean by kingpin? The inclination, or something else? Or all the geometry parameters? You use "kingpin" as a parameter of the geometry, but it's something (if we use it as a synonym for the pivoting axle) every since automobile has.

And I agree and disagree with DrJB.

As far as I remember from my mechanics courses, the place of a pole point of a multi-link mechanism (the point around which a specific part is turning, for example the wheel hub) is only momentary. Though the pole point may only move a few millimeters. But sometimes the advanced geometry stuff only means a few millimeters.

I disagree, because the pivoting point is definitely not simply the intersection of the links.

EDIT: the pole point is called Instant centre of rotation.

Oh, wait.... DrJB is right. The virtual center is really just the intersection point of the elongated links (of course if we are talking about a trapezoid linkage)

Edited February 4, 2014 by Lipko

[nicjasno]

This calls for a mockup model.

By kingpin i always mean the virtual axis going from the upper to the lower pivot point.

[DrJB]

Oh, wait.... DrJB is right. The virtual center is really just the intersection point of the elongated links (of course if we are talking about a trapezoid linkage)

That's exactly it, and I said that in post #32. The kinematics is really easy here, and the intersection point MOVES when you steer ...

@ nicjasno

You cannot really build this and look at it. The clearances in the small ball joints will make it a bit difficult.

It is much easier to understand it on paper ... If you've taken a kinematics course, it should be very easy.

Edited February 4, 2014 by DrJB

[nicjasno]

I haven't, i'm afraid :) But i can build it with liftarms and proper black/blue pins to eliminate slack. I need to come to the bottom of this, because i see a lot of multilink suspensions in my future. :P

[Thirdwigg]

This topic is pertinent to a project I am currently working on.

This first picture is a setup to try some of the above mentioned steering features. While it is clear TLG's ready made hubs do not allow for these features, a little work an a custom hub can accomplish much.

This setup uses a short/long A-arms setup which allows for decreasing camber as suspension travel increases. There is a slight positive caster angle. Also the kingpin inclination is present so the steering axis is further to the outside of the tire. The scrub radius is still positive with a 44772 wheel, but its better than "stock" TLG options.

Here you can see the system with some slight modifications placed in a chassis. You can see how the shock has been mounted, and how the steering is added. Again, I have decreasing camber as suspension travels, and increasing toe-in as suspension travels. Also I have positive caster. I have removed the angled kingpin which took away the improved scrub radius as seen in the first photo, as I could not get the steering lock I wanted the the top set-up.

I hope this helps the conversation.

[nicjasno]

If you have increasing toe in, you need to fix that. That is not good. The steering link must be ideally as long as the suspension arm that its closest to, to minimise bump steer.

Edited February 4, 2014 by nicjasno

[Thirdwigg]

That's not my design. Bump steer is mitigated as the steering arm is close to the 6.5 A-arm on the bottom.

As the car goes around the corner, the outside will dive down. As this happens, the contact patch needs to remain consistent. This is done by the decreasing camber as rolls moves outside. The toe in is neutral at no depression. The increasing toe-in during depression allows the car to oversteer the outside wheel during body roll.

[nicjasno]

This is not something you'd want. I'd be scared if my car did that in real life.

[DrJB]

This discussion has suddenly become very interesting ... I did spend few years of my life working on exactly that, and this is rather refreshing. Back then, such 'knowledge' was not truly public as wheel/tire settings was kept 'secret' by the various auto-makers. I worked on balancing various tire performances including handling, noise, and irregular wear. While most European vehicles are 'tuned' for handling, many US cars are often tuned for comfort and max wear life of the tires ... Keep the discussion going, very interesting.

Edited February 4, 2014 by DrJB

[nicjasno]

Sure. But basic geometry applies in both setups. The comfort stuff is mainly achieved with different spring and shock rates, not with geometry.

[Thirdwigg]

I only have geometry to work with, not spring rates, other than the two offered by LEGO.

[DrJB]

Sure. But basic geometry applies in both setups. The comfort stuff is mainly achieved with different spring and shock rates, not with geometry.

I beg to differ (one more time). Sometimes the geometry is used to 'alter' the effective stiffness of the suspension. In the past, it used to be that the balance between handling and comfort was achieved only by one factor: The aspect ratio of the tire. Nowadays, and with advances in suspension geometry/components, SUV vehicles use Run-Flat tires with very stiff sidewalls (poor comfort). The comfort is then provided by the suspension. There are so many performance measure/attributes in the tire suspension system that it is often difficult (from my own experience) to say simply 'this affects this performance and not that' ...

Edited February 4, 2014 by DrJB

[nicjasno]

I'd take a look at this page (the suspension walkarounds):

http://www.edmunds.c...ts/sitemap.html

Some highlights:

Audi A7:

http://www.edmunds.com/car-reviews/track-tests/2014-audi-rs-7-suspension-walkaround.html

Corvette stingray:

http://www.edmunds.com/car-reviews/track-tests/2014-chevrolet-corvette-stingray-z51-suspension-walkaround.html

Lexus 350f:

http://www.edmunds.com/car-reviews/track-tests/2013-lexus-gs-350-f-sport-suspension-walkaround.html

Ford focus:

http://www.edmunds.com/car-reviews/track-tests/2013-ford-focus-st-suspension-walkaround.html

Toyobaru:

http://www.edmunds.com/car-reviews/track-tests/2013-scion-fr-s-suspension-walkaround.html

Lamborghini Aventador:

http://www.edmunds.com/car-reviews/track-tests/2012-lamborghini-aventador-suspension-walkaround.html

Mustang Shelby gt 500:

http://www.edmunds.com/car-reviews/track-tests/2011-ford-shelby-gt500-suspension-walkaround.html

Camaro:

http://www.edmunds.com/car-reviews/track-tests/2010-chevrolet-camaro-ss-suspension-walkaround.html

Porsche Panamera:

http://www.edmunds.com/car-reviews/track-tests/2011-porsche-panamera-suspension-walkaround.html

Edited February 4, 2014 by nicjasno

[Thirdwigg]

That's a helpful site. Thanks.

[nicjasno]

That site is pure suspension porn :P

[jesse66058]

The details of that website are truly astonishing!

[Lipko]

Nicjasno:

If that instant centre is moving, it doesn't mean your understanding of the topic is wrong. It's easy to pick a point on the hub that's not moving in the driving direction like crazy, but moves in a more or less straight line that's perpendicular to the vehicle direction (axial to the wheels).

Every suspension geometry tricks have their tradeoffs. I bet there's another whatever angle that compensates for that axial movement trapezoid linkages have. Or maybe multilink suspensions are more complex 3D linkages.

Anyway

Linky

And look for section 4.3.6 HERE

EDIT: what I tried to say: as far as I know, everything in suspension geometry is just approximation and the designer's goal is to find an optimum of all the imperfect solutions for the specific types of vehicles. It's possible that there is a good-approximate center point in trapezoid linkage that's not moving much, even though it's not the centre mechanical-wise.

Edited February 4, 2014 by Lipko

[Thirdwigg]

Nicjasno:

If that instant centre is moving, it doesn't mean your understanding of the topic is wrong. It's easy to pick a point on the hub that's not moving in the driving direction like crazy, but moves in a more or less straight line that's perpendicular to the vehicle direction (axial to the wheels).

Every suspension geometry tricks have their tradeoffs. I bet there's another whatever angle that compensates for that axial movement trapezoid linkages have. Or maybe multilink suspensions are more complex 3D linkages.

Anyway

Linky

Ans look for section 4.3.6 HERE

Correct.

My understanding of complex movement is limited, but based on what I have learned, what I have recreated is just about as good as you can do at this scale.

Regarding toe-in, I am still trying to figure out the movement of a rear five-link set up to compensate oversteer/understeer.