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.

[yody]

I have a question, in reality when on the track, how much of an actual difference do all the small details make? What would be the difference between a well thought out suspension consisitng of decent wheels and tires, mostly bolt on parts, like tubular control arms, del a lum everything, matching spring rates, matching valved shocks, matching sway bars, decent weight balance, etc.... compared to something where all the calculations have been made, like unsprung weight, scrub radius, backspacing, custom spindles, spline swaybars, everything measured out, and all that other crap. I would assume it also depend on what type of car we are talking as some had better geomety from the factory. I mean If dennis gets $700 custom spindles how much better are they really going to be? Dennis, are you going to take the car to the track first to get a baseline, are you an experienced enough driver to actaully get a consistent baseline? I ask because if you got enough data, you could sell the custom spindles. I wish i understood what you guys are talking about with all this top secret code and such, I just dont really have a mathematical geometry brain. I would like to do the research but I have been so busy trying to finish my car, which was a huge project for me. I stuck with a pretty basic combo which should work pretty good. Everything is proven or is based on a pretty strong foundation like i listed above. I am wondering if i re engineered my entire setup if i would really gain much? Dennis are you teaching classes? I get pretty lost reading the books.....they need a "suspension for dummies"! :rolleyes5

[wally8]

Salt Racer,

Yes, that is by far my biggest problem. Time to order some parts. I'm comfortable enough now to start that process so maybe in another week or so. I think I can end up with something that will work for this application. I'm saving all the really custom stuff for the next car.


Yody,

To answer your question very simply, it's the difference beteen 1st place or some other much less meaningful finishing position, at least in my experience anyway.

Which book are you reading right now? I would recommend one by Don Alexander (ISBN:0760309485) that really starts at the basics and gives you a comprehensive definition of everything and the basics of traction circle theory for the tires. Don is a writer first and a suspension / driving guy second so his stuff is more readable IMO. From there you can pick up any of the other books and get into the math. It's mostly straight forward high school geometry and physics so it's not too bad but then again, I'm an engineer by trade.

Other than that, ask questions to get the answers. Anytime I'm around some fast guy I give them the interview to see what they know that I don't. There's always something to pick up. Some relationship you haven't thought about yet. If there's some abbreviation like FVSA that you don't know what it means, just ask. I've been around this stuff to this level for years and I've found several things that these guys mention that are different from how I refer to them. Different schools as it were.

Obviously there are some knowledgable people here who would be glad to explain things.

Careful though or you'll end up saving for that perfect uber spindle that you have designed to be within 0.0001% perfect :-) There is a reason they compare racing to crack.


Wally

[dennis68]

Yodi, a nicely refurbished suspension (which is what you have) will be a nice car to learn in. "Matched" springs and "matched" sta-bars are questionable. Without hours of track time it is impossible to determine if they are matched. Matched says who, and by what criteria?

If you pay attention to pit strategies this weekend you will see crew chiefs "add a wedge", this a small amount of change, maybe the difference between 1300lbs springs and 1350lbs, it will make a drastic difference in the way the car feels. So, to answer part of your question...Yes, small changes make a big difference.

Setting the chassis up is not a universal setting to begin with, all drivers have different likes/dislikes. Some drivers prefer the chassis be a little loose, some like it a little tight (I'll stop the jokes right here!). So while you may set your car up the way you like, a different driver may not feel comfortable in it.

There are some chassis design criteria that are universal however. Limited scrub radius, negative camber gain, limited RC migration, limited bump steer and a reasonable FVSA are the same regardless of driver or chassis. If any of these criteria are not met you will be stuck with a car that will never handle the way it could or should.

Will the investment I made in having spindles built pay off....I'd be willing to bet with rubber bushings and all OE control arms it would run circles around a "bolt-on" car.

Tires are the deciding factor in lap times, if you can keep the tires in traction with track and have enough HP to push the car around the track you will have the fastest car (actually it will be a truck ) out there.

The trick is to keep the tires perpendicular to the track and the chassis flat, that is where suspension geometry comes in.

Yes, I do not see the new spindles being on the car by April so I will have a good day of baseline runs to compare against.

Do I do classes.....sure, on how to be a dumb ass. I know nothing compared to most of these guys, I just have more time to play at work so my post count is real high

[Salt Racer]

...What would be the difference between a well thought out suspension consisitng of decent wheels and tires, mostly bolt on parts, like tubular control arms, del a lum everything, matching spring rates, matching valved shocks, matching sway bars, decent weight balance, etc.... compared to something where all the calculations have been made, like unsprung weight, scrub radius, backspacing, custom spindles, spline swaybars, everything measured out, and all that other crap...

It depends on many things, like baseline, type of tracks, etc.

It's possible to go fast in a well-sorted bolt-on car with somewhat crappy geometry. Shock tuning, roll rate distribution, and driver ability go a long way. But how long can you keep going? And what would happen if you go over the limit a little while going through a 100+mph sweeper? Would the car remain predictable or go completely haywire? How about transitions?

That's where good geometry makes difference. It makes a car easier to drive fast and consistently, which takes a tremendous amount of load off the driver. This is somewhat irrelevant for casual track users like us, but it makes a big difference in real wheel-to-wheel racing - the difference between 1st place and also-runs as Wally pointed out.

Some of what we talk about, namely scrub radius, side scrub and bumpsteer, also make the car easier to drive even on street. Some even have significant affect on ride quality. Equally-sprung, identical cars would have totally different ride quality depending on geometry.

As for Dennis' spindles, I'd say the return of his investment will be significant, judging from where he started from. In terms of lap time, the new spindles may not pick up 3 seconds, but he will be able to tell the difference (ease of driving) and his lap times will be more consistent. As a secondary effect, this will probably results in slightly better lap time in many cases (more concentration, confidence, maybe faster exit speed, etc). It's guaranteed to make ride quality better. Tire will wear better also.

[wally8]

Denny makes a good point. Putting it on the track is the only way to figure out anything. All of the computational stuff gets you in the ballpark and keeps your waste of time and dollars to a minimum but you can't put a theoretically perfect car on a track and have it be fast out of the gate.

BTW, Wedge is preloading, not an actual change in rate. If I put more wedge in (or bite as we call it) I'm jacking weight into the left rear tire.

And again, not to pick apart what Denny was saying but all of the design really depends on what you're doing. Another circle track example that doesn't apply here is that late models favor a tremendous RCH migration in the rear mainly because they actually desire the jacking component of that action. Denny's example definitely applies to roadracing though.

Collectively this is a fairly high level forum that you should be able to learn anything you want from. There's enough people here to figure it out.


Wally

[yody]

eh good info. BTW i like to think of my suspension as a little more than a "refurbished suspension" alright smart guy? I mean just because you have "race" parts doesn't make a "race car" . It will be interesting at the least to see Denny on the track for the first time. I know my first time was a real eye opening experience! Also riding along with a professional driver is fun too. Those guys really push it. Makes you kinda scared to take your "baby around some of those turns at 80mph(which is a LOT faster than it sounds) especially on a track like infineon. Dennis sounds like your car is going to eventually be pretty radical and handle very well if you don't crash it, are you going to join a racing class(i.e. racing organization, not school)? Wally that is some good info. Once this car is done, I think i will devote some time to studying this stuff, so i can at least follow along without getting lost in these conversations, one of the books i have is the herb adams book. I read most of it, which included skimming a lot, and seemed to get lost pretty easily. I think it would pay off better to hang around soem people that i can talk too better.

[dennis68]

Sorry Yodi but in the opinion of most, bolting on tube arms and Teflon bushings is not doing anything to improve upon the inferior geometry which leads to classifications like "refurbished suspension".

BTW, not my first time on the track....just the 1st time in a decade and the 1st time in this chassis.

Sorry Wally, wrong use of verbage. Meant to say "add a rubber", ot "take a turn".

No competetive racing for me, just for fun. I can't afford to compete.

[yody]

okay, sort of like your refurbished body work? I think you could join NASA, kinda expensive but much cheaper than really trying to compete, with NASA i believe you can just enjoy the car at track days.

[dennis68]

Or, I could just go out without joining any group and run whenever I want anyway.

BTW, the body work is hack, not refurbished. Then again I'm not about bling-bling or how good it looks sitting still.

[wendell]

Looks like this thread is starting to head south.

Katz,
"Racecar Engineering"?
I took it easy on PHR in my post. My first urge was to say that its nother more than a catalog to look at when American Hot Rod isn't on.


It should be noted that the fact that Denis has "custom" spindles in and of it self means nothing. What he has of value is a KPI that will minimize scrub and a height that will get him the FVSA he's after. Thats the important difference between his spindles and a pair of tubular UCAs.

It interesting to hear about the addative effects of the different design parameters and how they will play out on a street and on a track.

Can any one get a "deal" on performance trends? I can get a "deal" on Solidworks.

[dennis68]

I'm up for a deal on Performance Trends. I use the free trials as needed but I'm running out of computers to put in on.

[796spdbu]

Hey guys
This thread is a real eye opener for me,cause when i started my 79 mali project iwas like damn i got real trick stuff here.Now its like oh....it just looks better than stock stampings.I've always like to tryand engineer stuff but i didnt take any college coarses for it,i took business management.Now i'm looking into ITT tech for C.A.D and some kind of mechanical engineering.You guys have any suggestions as to where i can learn more.wheredo i get this computer software at.How about these books how bout BORDERS.I'm thinking about building some kinda of race car/streetcar.You guys have helped me out in getting me motavated.I've built and helped build some street rods and trucks and know I've been doing it on my own for a year,but nothing in the wat of performance handling,no one has come to the shop looking for any thing like that.I do mostly air bag and conventional drops.Know if i get better at suspension design and theory,maybe i can apply it to my shop.I dont think there are an tracks in lower ohio for road caorse racing that i'm awhere of? Mainly drag racing and dirt track.Speaking of dirt track...Do you guys think that if i get into dirt circle track racing it would give me a hands on with suspension tuning,friend has offered to lert me drive his old street stock car.Thanks for any input.

[dennis68]

Check out the racing section of this forum, there is a list of different clubs who put on events all over the place.

As far as I know Performance Trends is only available from them on the web, books are available everywhere.

[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.