Roll Center design theory

[Salt Racer]

...I'm presently doing 2D sketches in Solidworks to get my design started. My figuring is that eventually I can flesh out the structural design in 3D. Is this a good idea?....

Yes, but think of it as only a part of suspension design. When I design scratch-built suspension, I constantly go back and forth between suspension program and drafting or solid modeling software once basic parameters are set (tire size, rim offset, LCA length, etc). Watch for John Parsons' article in May issue of PHR.

...It shows how little change change to x,y,z numbers results in substantial change of different angles. IE, just an inch of chassis drop can totally change the handling characteristics...

Yup, that's why it's important to evaluate the modification on geometry before you actually make the physical change. It's so easy to overdo something.

...I'm using different figures though since I'm anticipating a higher pin height on the B- body spindle (+1in I think)...

This may or may not be the case. I don't know exactly what kind of spindles Dennis had, but it needed 3.0~3.5" drop to make LCAs level. I doubt you're planning to use 23" tall tires. Better get the parts in your hand before you finalize the design.

...Raising the spindle more and more will shorten it. This would be the general trend for any setup given this case, no?...

Assuming you're talking about raising UBJ, then yes. If you're talking about raising the whole spindle (w/o changing its height) relative to the chassis pickup points, that would be generally yes also.

...In fact, I once saw one of the nationally ranked mod drivers lose his left front tire due to loose lugnuts. He completed the entire heat (10 laps on a 1/2 mile) and kicked everyone's butt doing it. Yeah, he's good. He had some dirt packed in his LCA though :-)...

That is just too cool!! Having good suspension helps, but it all comes down to driver's ability in the end.

...Another important note is that we don't get to 0 bumpsteer, just tolerable limits...

That's what I do too when I'm just modifying what exists.

...When you guys talk about RC migration how much do you personally like to see on a full body car?

No more than a couple of inches. Less than that on paper or computer b/c more than likely the finished product isn't as dimensionally precise as what you intended.

[wendell]

Thanks Saltracer,
Thats what I'm planning to do. Right now I'm crawling around with a tape measure trying to get dimesions and entering them in Solidworks. I'm sure everybody is going to be interested in John's article. Might have to break down and buy the may issue.

[Salt Racer]

...Might have to break down and buy the may issue.

Tell me about it. The price of magazines are outragous these days. There's not enough tech articles to justify subscription, and it's tough for me to spend $6 for an issue of one of those magazines when I can buy an issue of Racecar Engineering for $2 more.

No offense to magazine editors. It's just an opinion coming from a non-typical occasional reader. I don't enjoy reading about pampered hot rods with loud barks and weak bites. I modify my cars to go faster on tracks, not to get attention from people like most "hot rodders" seem to do.

[CoryM]

No more than a couple of inches. Less than that on paper or computer b/c more than likely the finished product isn't as dimensionally precise as what you intended.

Good to hear. The setup I have been poking at is 30mm (1.2inch) of movement. Just designing it for the learning experience and I have had a hard time finding out what you usually expect to see for allowable movement.
Cheers.

[Mean 69]

Good to hear. The setup I have been poking at is 30mm (1.2inch) of movement.

In which direction, and in what circumstances? Better yet, what does it do in 2" bump and 2 degrees roll? Them's good numbers, but it depends upon the input critera.

M

[wally8]

CoryM,

Those are good numbers. Give us some pivot locations if you would.....

796spdbu,

Racing of any sort will put you far ahead of the average guy especially if you read a few books along with it. Worked for me anyway :-)

Whatever you do put something on a some kind of track somewhere. You don't want to be one of those lawn chair racers at the car show......

Wally

[CoryM]

Sorry. I should have mentioned that the theoretical setup is not GM based and is for a (at this point also theoretical) very small car. Also I am pretty green at this stuff so its possible (probable?) that I am missing something important. That was at 3degrees of roll 3" of bump. I will need to buy a real program when I start buying parts for the car and can measure the spindles I want. Hopefully I will know enough by then to figure it out.
Cheers.