Determining roll stiffness distribution

[BillyShope]

This is about the only forum (other than eng-tips) where I could post something like this and it will be appreciated. And, you might have a good laugh at my expense.



Now that I'm retired, I get a kick out of helping those interested in suspension design, but who do not have access to all of the fancy fixtures and software available in the industry. So, I considered it another accomplishment when I added Page 28 to my site. With only a floor jack, a set of wheel scales, and a tape measure, I provided a means to determine the roll stiffness distribution. I had it all figured out as to how I would compensate for different fore and aft locations of the jack. The software determined the proper jack placement. So, the user had to input the wheelbase and then locate the jack at the proper distance back from the front wheels.

Then, it hit me. It wouldn't matter if the user was somehow able to place the jack between the front wheels, so that the total weight on the rear wheels was unchanged. The rear roll stiffness would still have the same effect.

So, all of those fancy compensatory equations simplified down to one simple equation and the user is free to put the jack anywhere he likes (fore and aft, anyway).

If any of you guys ever looked at that page and wondered why I asked for the wheelbase and gave you that screwy jack location information, now you know.

Almost forgot: The answers haven't changed. I just went around the barn a couple more times to get there.
http://www.racetec.cc/shope

[David Pozzi]

Billy,
Thanks so much for this one!
I'll check it out. I've been thinking of doing something like this, but you saved me the trouble.

Am I correct in assuming this gives an overall stiffness distribution which includes roll center position contribution to roll stiffness?
David

[BillyShope]

Billy,

Am I correct in assuming this gives an overall stiffness distribution which includes roll center position contribution to roll stiffness?
David

When considering only vertical forces (as in this case) or the fore and aft distribution of the driveshaft torque reaction, roll center height does not enter into the picture.
http://www.racetec.cc/shope

[elacruze]

I have very limited bandwidth here so I haven't been able to access the referent material. Why do I need to know my roll stiffness distribution?

[BillyShope]

Why do I need to know my roll stiffness distribution?

The average racer doesn't, but, if you're really "into it" and want to use, for instance, the equations for wheel loads resulting from a lateral acceleration in the Millikens' "Race Car Vehicle Dynamics", you'll need the roll stiffness distribution as a start.
http://www.racetec.cc/shope

[elacruze]

The average racer doesn't, but, if you're really "into it" and want to use, for instance, the equations for wheel loads resulting from a lateral acceleration in the Millikens' "Race Car Vehicle Dynamics", you'll need the roll stiffness distribution as a start.
http://www.racetec.cc/shope

...So wheel loading during cornering is affected by roll stiffness distribution as a function separate from weight distribution, right?
I could conceivably pass more of my front-end weight bias to the rear tires by increasing torsional rigidity?

I'll read your entire website when I get the opportunity. Good stuff, One always better understands their status quo when studying what lies beyond.

[BillyShope]

...So wheel loading during cornering is affected by roll stiffness distribution as a function separate from weight distribution, right?

I suppose you can consider them separately, but they work together, along with front and rear roll center heights.

I could conceivably pass more of my front-end weight bias to the rear tires by increasing torsional rigidity?

No. In fact, you could DECREASE the torsional rigidity and achieve your goal if you INCREASE the ratio of rear roll stiffness to front roll stiffness.
http://www.racetec.cc/shope