Roll Center up-down movement with Pitch Change

[Christopher Crowe]

In some designs, the car's roll center seems to move up and down with the up/down motion of the chassis. That is, as pitch-changes occur, the R/C (and the CG with it) tends to follow, as one.

Other designs don't seem to do this. On these, the rc seems to stay at much the same height, even as the car's pitch changes. As the chassis (with its cg) moves up and down, the roll center height stays relatively the same -- which would cause the moment acting on the springs (as the car goes into roll) to be a constantly changing thing as the car's pitch attitude changes.

These seem to be two very different, almost opposite approaches. On the surface, it would seem that trying to keep the r/c and c/g distance from one another a constant thing (regardless of pitch) would be a good thing... But then I wonder why everybody doesn't do it -- which they don't seem to at all!

Can anyone shed any light on this area of mystery for me?

[Marty Nygard]

As Colin Chapman once said, "any suspension will work if you don't let it". What is your suspension design goal? What magnitude of suspension movement are you using? Are you using force based or geometric analysis?
I say, use the longest control arms you can fit, design for minimal camber change within a limited chassis roll and design a bit of adjustment capability into the chassis.
Of course, I've only design two race car suspensions and neither has won the runoffs.
M

[S Lathrop]

Look at the lateral movement as well as updown. You need to think in 2 dimensions. You will find that the cars that have the RC moving with ride height may have less lateral movement. All this stuff is compromises. Think about what you want your car to do at various positions around a corner and corners at various speeds. There is no right or wrong in this.

[Christopher Crowe]

-- and yeah, I HAVE noticed that a roll center that moves with car pitch DOES seem to have lots of lateral movement instead. And I know this changes the moment working on those springs laterally. Ah, man -- this is one arcane art.

I guess I'm finally at the point where I can't learn a whole more before attaching pickup points and running the car and looking at data.

It's been a long, long haul to get here... and I thank so many people on Apex who have so generously given me help by sharing so many of their hard-earned lessons.

Cheers, gang -- time for me to find out how this mouse-trap of mine works!

[ASRF1000]

Best of Luck Chris!

[S Lathrop]

I forgot to say that the lateral movement is in roll. In short you need to balance the lateral movement against the vertical movement.

Build it and drive it.

[Christopher Crowe]

Is the idea to keep the RC within a pretty small circle in ALL planes? That is, compromise things in a 360 degree view -- so that the roll center moves in as small a three dimensional space as possible... A tight little sphere.

My assumption is this would lessen any big dynamic changes in the moments acting on the springs through all regimes... and make for a smooth, progressive-feeling race car as the various directional loads are being fed in via the four tire contact patches.

Yeah, it IS time to glue on the pick up points and try it -- while being damned observant of what's been proven to work. Still scary...

Thanks for all of your help.

[S Lathrop]

Minimizing movement is what I try to do. If I have to choose, I like to minimize the lateral movement. I pay attention to the vertical movement of the front relative to the rear.

[Reddog]

Can anybody suggest a good software program to obtain for suspension design ... the last time I raced [Trois-Rivieres 1974] Bill Gates was broke ... so I'm clueless

[stephen wilson]

I was going to ask the same question. Is there a better moderate cost solution to Mitchell?

[Christopher Crowe]

I'm not a big fan of this program relative to Bill Mitchell's, but many of the FSAE kids use it -- "Optimum G".

http://www.optimumg.com/OptimumGWebS...tOptimumK.html

One thing that's really terrific about the Mitchell software is that Bill will get on the phone and advise you -- and promptly, almost day or night. He even was goot enough to put some corrections into my data when I got lost concerning not distorting parts that I wanted "locked" (my VD uprights).

Chris

[nulrich]

The program is called OptimumK (company is OptimumG). In my opinion, it's the best available solution under $10k. It costs about $1700 if you're not a student (my understanding is there is a huge discount for FSAE teams). I haven't ever used their tech support, so I can't comment on that aspect.

Nathan

[Reddog]

$1,900 bucks for software seems cheap compared to CM tubing screw-ups ....Thanks

[rmstringham]

Having used both software, WinGeo and OptimumK, I can hopefully shed some light...

Since WinGeo has been around longer, Mitchell has had more time to add/modify features. So WinGeo has more options, including the ability to "design" within the software.

OptimumK has a much more user friendly interface, with less cryptic nomenclature. Also the graphical interface seems to allow for better visualization. For those of you that don't know, many of OptimumG's programmers are college interns and might not be as savvy as Mitchell (and might explain customer support woes). However, I believe their vehicle dynamics program will be the hot ticket once released (for those of you with tire data and aero maps of course).

I preferred the OptimumK interface, but the capabilities are skewed towards WinGeo.

Hope that helps...

[rmstringham]

$1,900 bucks for software seems cheap compared to CM tubing screw-ups ....Thanks

There are plenty of FSAE teams in California and Michigan. Consider contacting them to help with the design/kinematics. Most students will jump at the opportunity to be able to put "real racecar" experience on their resumes.

[Reddog]

Thanks, I have a welding and a machine shop too ... and I am not far from the University of Michigan, great idea, but I hope they invite to a "Toga Party" lol

[rmstringham]

Thanks, I have a welding and a machine shop too ... and I am not far from the University of Michigan, great idea but I hope they invite to a "Toga Party" lol

Great, Ann Arbor is currently ranked number 1 in the world for FSAE. So they are definitely knowledgeable. Just don't hold your breath, in case they have a "too big for our britches, our poo doesn't stink" attitude.

And of course, there is a lot to gain from asking your friends here at Apexspeed!

[Reddog]

So, I'll buy my own software, become a student there, and kick their ass
[maybe] lol

[mousecatcher]

I like the build from parts option with wingeo.