extreme roll center movement

[Murray]

As the roll center moves below the ground plane, it experiences extreme lateral movement. It moves so much with respect to the cg, it seems hard to believe that the moment arm would really increase from a few inches to, say 6 or 7 feet in less than an inch of vertical chassis movement. Is this really what is going on?

[Murray]

As the roll center moves below the ground plane, it experiences extreme lateral movement. It moves so much with respect to the cg, it seems hard to believe that the moment arm would really increase from a few inches to, say 6 or 7 feet in less than an inch of vertical chassis movement. Is this really what is going on?

[Mike Sadie]

what are you modeling the kinematics in? Crossing the ground plane with the roll center introduces certain problems, but this doesn't sound right. Approximately, how long are your upper and lower control arms?

[Murray]

The upper control arm is 10.25 in and the lower control arm is 16 in. I am modeling the kinematics in a SolidWorks sketch. When one instant center drops just below the ground plane, and the other instant center is still just above the ground plane, the lines between the tire contact patches and the instant centers intersect at a large distance from the chassis. Does this really mean that this is where the roll center of the car is?

[ben]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Murray:
Does this really mean that this is where the roll center of the car is? </div></BLOCKQUOTE>

In a word - no.

Ben

[Dave K]

I am only a beginner in chassis design, but wouldn't you want your roll center to be as close to the CG as you can get it?

[flavorPacket]

if your RC is at the CG, then you have no elastic weight transfer. Most people choose not to do that.

[Chris Lane]

You'll find that you have a roll center for both the front and the rear, and these connect to form a roll axis. Likewise, you can have a pitch axis also.

Solidworks isn't really the best solution for kinematic analysis. I was using it extensively until my uni managed to purchase Optimum K.

I now use a combination of both Solidworks and Optimum K, to design my suspension.

I had the same problem as you for a while (wild front roll center movement in the Y plane). I found my wishbones were too similar, with a upper/lower ratio of around 0.9. I increased the lower arm length, and decreased the top arm length a bit to get a ratio of around 0.85, which is working quite well.

Try playing with your wishbone lengths, roll center heights, and FVSAL's.

[J. Vinella]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by ben:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Murray:
Does this really mean that this is where the roll center of the car is? </div></BLOCKQUOTE>

In a word - no.

Ben </div></BLOCKQUOTE>

What Ben might be alluding to is a force based roll center method of defining the roll axis, not using the traditional kinematic roll center. I botched the force based roll center question they were asking at West 07. Ben you are lucky in Moto GP knowing where your roll center is and all that camber thrust! http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

Dave K,

The higher your RC from the ground plane also increases your jacking forces.

[HenningO]

The past two years we've used the FBRC approach when modelleing our suspension. It sure makes the already tricky art of suspension design even trickier! Mainly because your tie rod location affect the roll center.

SAE paper #962536, however, shows that a force based representation is far more accurate. So I hope it's worth the extra struggle.

[Tintin]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Ben you are lucky in Moto GP knowing where your roll center is and all that camber thrust! http://fsae.com/groupee_common/emoticons/icon_biggrin.gif
</div></BLOCKQUOTE>

Hope this is not going to be too OT but if somebody in MotoGP knows where the RC is then the driver is on either installation or outlap and the bike's not sliding at all.... http://fsae.com/groupee_common/emoticons/icon_wink.gif

I had several lengthy discussion on this subject over the last few years, all very amusing but also rather inconclusive.



Tim

[murpia]

4 pages to read here (http://fsae.com/eve/forums/a/tpc/f/125607348/m/47110403711)...

Regards, Ian