Typical roll and pitch centre heights? 2

[dzine]

I know roll centres have been covered many times before now, but I have a slightly different question.

I hope I am not over-simplifying this too much, but could anyone help me on some typical heights of the instantaneous roll centre of different types of cars? Approximate values here are fine, as I understand they could all differ by a reasonable margin.

Forumla (eg F1, GP2 etc)
Sports (eg Corvette and Aston GT1)
Touring (eg Mazda MX-5)

Any if anybody has information on typical pitch centres also, that would great.

Thanks in advance.

 

[sbozy25]

Check with the OEM

Cabbages, knickers, It hasn't got A BEAK!

 

[BrianPetersen]

Unlikely that this information would be available publicly. Where possible, measure it or deduce it approximately from photographs.

Formula 1 double wishbone suspension. The upper and lower links are near horizontal, implying instant center very far out beyond the opposite side of the car, implying roll center near ground level. I don't know if the wishbones have pivot axes that are inclined in side view but I don't think they are inclined much, if at all, implying a pitch center near infintely behind the car, implying a pitch center near ground level. These cars don't rely much on the geometry to do "anti" anything - they have really stiff spring and antiroll rates.

True sports cars have their suspension secrets covered up. Don't know anything about those. It would be quite unsurprising if they use the same approach as Formula 1 ... no or insignificant geometrical "anti" anything, but high spring and antiroll bar rates.

If by "touring" cars you mean basically street cars then the roll center heights will be all over the map depending on what type of suspension is used.

From what I can tell from the outside looking in, the roll center height isn't a primary design criteria. The suspension is designed based on other considerations (including the forces acting on each link and how they interact with the contact patch when cornering) and the instant center / roll center height "is what it is".

 

[dzine]

Thanks Brian

From what I know, I think you are quite right about F1 not having much "anti" geometry (they don't have any Ackermann either), but rather rely on a very stiff setup (and flat tracks!). So perhaps an assumption of near ground level would be about right for Formula cars.

In looking at some basic images on the net, it seems that the front and rear IRC for sports and tourers is anywhere within approximately the height of the wheels, give or take. Maybe that is just not a primary dsign consideration like you say, but rather there are many other things that affect the balance more than this.

Thanks for the help.

 

[Warpspeed]

Anyone know how to measure the actual location of the mass centroid axis ?

Only way I can see of doing this is to spin balance the whole sprung mass in the vertical axis.

Is there a less dramatic way of doing this.

 

[patprimmer]

I think you can weigh the car at all four wheels with it exactly level,

Then weigh it again at a specified tilt angle and weigh the wheels at that angle. The weight transfer gives the required data. The maths is beyond me though.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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[NormPeterson]

Maybe there is something in here that you can use

http://www.nhtsa.gov/DOT/NHTSA/NRD/Multimedia/PDFs/VRTC/ca/nhtsa_inertia_database_english.pdf

Norm

 

[Warpspeed]

I think you can weigh the car at all four wheels with it exactly level,

Then weigh it again at a specified tilt angle and weigh the wheels at that angle. The weight transfer gives the required data. The maths is beyond me though.

Thanks Pat.
I need to think about this some more, but that should be possible.

I might make up a small rigid model with a known tilted mass centroid, and try measuring some corner weights in various planes, and see if I can get anything that makes sense from it.

 

[dzine]

Norm, that's some good information there. Thanks very much.

Although not covering roll/centres specifically, it's definitely very helpful.

 

[cibachrome]

Roll centers are very much a primary design specification, especially if there are going to be multiple levels of suspension for the vehicle model. The consideration is based on having appropriate front/rear ride rates and ride frequency ratio AND a 'No Rear Antirollbar' suspension package on the base model. Other consideration(s) are based on tire load transfer sensitivity (i.e. "big" tires have lower load transfer) and may require more anti roll moment AND you don't want a sewer pipes front or rear for a bar (mass consideration). Big doesn't necessariliy mean larger size, it's a part of the tire construction's sidewall and section recipe and includes useage pressure. The statistics of thousands of roll center height measurements for many and most all vehicles manufactured and sold (no matter by whom or where designed) is pretty interesting. Claims of this or that are not usually borne out by measurements. That tells us 'insiders' that some folks have no idea what the values are (or should be) but state something just to look educated. Measure 5 identical models and getting 5 different sets of values (and even signs) usually says something about build tolerances and repeatability, too.

To measure the inertia ellipsoid, use a trifilar pendulum and take many readings with the vehicle suspended in quite a few orientations. (Empty the fluids, please). The different periods measured in each orientation correspond to the orientation angles of the axis of the inertia ellipsoid. Try it with a watermellon first. You'll get the idea. This is commonly done for engine/transmission modules so that engine mounts are properly located.

More useful information is the inertia cross products. Get them by recording the vehicle wheel weights vs time as the vehicle sits on a roll and pitch platform. The multiple fm periods are used to calculate the signs and magnitudes of the cross products.

I believe there is an SAE paper published by the Govt and an industry partner who makes an inertia measuring apparatus. They show the Math involved in this proces.

This is the 'Insiders' point of view...

 

[Warpspeed]

Thank you cibachrome.
The trifilar pendulum is the solution to a problem that I have puzzled over for quite some time.