12 replies to this topic

[Paolo]

I am in the need of knowing approximate CG and roll centers height for various types of cars.

Any model goes , even an unidentified "90's F1 " or " front drive B segment car" would be OK.
No problem if the info is approximate.

What I need to know is, for every model :

CG height from the ground
CG location around longitudinal axis
Front roll center height above ground
Rear roll center height above ground
(the latter two, assuming it fixed, of course)

If possible, also plausible roll stiffness of front and rear suspension would help.

Thanks

[prathi_psg]

Do you want this data for a Formula 1 racer or a passenger car? I guess I have data for Ford Focus. Would be glad to tell you if you need it.

Regards,
Pradheep.

[Paolo]

Any car will do, thanks. Of course I have my own estimates, but need to compare with others.

[Greg Locock]

The appendix to this book http://books.kelkoo....s_16272185.html

includes parameters for many cars

[prathi_psg]

Here you go.... this data is for Ford Focus.

CG Height from ground: 663 mm
CG location from Front axle: 1050 mm
Front Roll Center Height: 95.8 mm
Rear Roll Center Height: 170.9 mm
Roll Stiffness without Sway Bar: Front: 400 Nm/deg
Rear: 403 Nm/deg

Hope this helps.
I looked into the book, referred by Greg and wow, it had all these details. Thanks Greg.

[Paolo]

Thanks Prathi and Greg.
I'll fetch that book in the near future.
About the Focus : I forgot to ask what the wheelbase is ...

[hydra]

261.6cm or 103"

[Greg Locock]

That CG height looks rather high, off the top of my head.

[Powersteer]

How to calculate roll center? how to visualize it?

[Greg Locock]

How to calculate it:

For a double wishbone suspension,in front view, extend the arms until they meet. Draw a line fromt he intersection back to the contact patch. The intersection of that line with the centreline of the vehicle is the geometric roll centre of that suspension, and its height off the ground is the roll centre height.

The higher it is, the more the lateral forces at the CP are reacted by the suspension arms, and less is taken via the spring and antiroll bar and shock.

How to visualise it? Don't know. It is such a borderline useless concept IN PRACTICE that I have never got a good mental picture of it. Rather more useful is the force based roll centre, in which you work out the effective height of the lateral forces in the suspension, ie Mx/Fy. This tells you where the forces are going in reality. FBRC and GRC do not necessarily move the same way during a manouevre, which is why I don't worry overmuch about the GRC

I have written a spreadsheet to work the GRC out for a double wishbone suspension, called SLARCK. Get it here.

http://www.geocities...wnloadpage.html

[Powersteer]

Thanks, really useful.. I was figuring how roll center can play a part in creating a slight toe-in for the rear wheels during a corner making it lean slightly rearward biased. If roll center is calculated then steering arm can be calculated and adjusted. This of course asuming rear has higher(or lowr?) roll center than front which seems to be the common case.

[red300zx99]

Greg,
Could you explain FRC alittle more, and maybe give some insight on it's calculation and how it is used to transmit forces to the springs?

[Greg Locock]

"Could you explain FRC alittle more, and maybe give some insight on it's calculation and how it is used to transmit forces to the springs?"

Can I just point you at a cached discussion of this?


http://66.102.7.104/.....centre"&hl=en

also :

9.4.28 ROLL CENTER : The point in the transverse vertical plane through any pair of wheel centers at which lateral forces may be applied to the sprung mass without producing suspension roll. (See Note 16.)

9.4.29 ROLL AXIS : The line joining the front and rear roll centers.

16. The roll center defined in 9.4.28 constitutes an idealized concept and does not necessarily represent a true instantaneous center of rotation of the sprung mass.

The springs shocks and a/r bar have to provide all the forces that the suspension arms don't, when you draw your freebody diagram