Results 1 to 20 of 84
Thread: Roll Center design theory
-
02-15-2005 #1Starting The Transformation
- Join Date
- Jan 2005
- Posts
- 160
Roll Center design theory
Hey Guys,
What roll center height should I be shooting for here. Again I'm drawing from my dirt oval background here.
We normally shoot for 3.5 to 4 inches static from a geometry standpoint. This is for an A body or G body front stub on dirt. Typically if you were going to put the same car on asphalt it's recommended you go lower, like 2 inches but I've never actually put one on asphalt so I wouldn't know the validity of that number.
How about road racing? What is typical for say an American Iron car?
The numbers I'm working through look best with an Impala spindle (8.75in.), stock BJ's, a level LCA and relocating the UCA inner pivot up about 3/4 inch or so. This moves my IC out to about 2.5 times the track (77in.) and gives me the lowest RC migration, from 3 static to about 1.4 with roll. Not sure where it's going side to side. This seems to give me a minimal jacking effect as well. This is for an A body BTW.
I haven't checked what this will do to my camber gain, however, but I'm assuming nothing too bad. Not too worried about bumpsteer at this point either. I'll do whatever I need to do to shim it out. Having the IC out there like that should make that part fairly easy.
Am I working in the right ballpark on this? Am I way off the mark? I've run too many numbers right now and I'm losing track. Any opinions are appreciated.
Wally
-
02-16-2005 #2Full Blown G-Machiner- Join Date
- Aug 2004
- Location
- Boston MA
- Posts
- 686
WOW, can't believe that no one is touching this. Possibly the most substative post in months. Guess everybody is busy discusing how to tuck 24inch spinners under their first gen.(I'M KIDDING!)
All kidding asside, quite a few people hang out here that can answer this question in spades.
With out any paved or dirt experience and after studying for the last 6 months, this week I started to design a front end with a static front RC @+2inches. I too am looking at maximizing the length of the ICs and minimizing RC migration then clean up bump steer. This is for my first gen Fbody.
The book "Paved Track Stock Car Technology" has been a great resource for me and would be a very quick read for someone with dirt track experience. Also, try to get as much info as possible from the Bush North guys. Heavy, fairly narrow track, high CG... I'm hoping to get some face time with a American Irion racer in the next few weeks. I'll pass along anything I can extract.
02-16-2005 #3Starting The Transformation- Join Date
- Jan 2005
- Posts
- 160
Thanks for the input Wendell.
As I expected my dirt experience is only good for the fundamentals.
I thought about 2" as a baseline also since that's the recommendation for an asphalt modified but I was worried that it might venture below ground dynamically. I don't think that sort of migration would be any good (+ to -).
My thoughts on AI are that it's a fairly similar app to what I'm shooting for although my wheelbase is probably longer.
I might try tuning it down to 2.5" and see what I get from there. My CG should be a little lower than stock since the car will be lowered but I'll have a big ol' steel headed big block in there so my F/R balance is not going to be good.
Sounds like we're designing along similar lines though.
Let me know what you find out, if anything.
Wally
02-16-2005 #4dennis68 GuestTypical??? nothing is typical on asphalt. it depends on who you talk to. There are a bunch of guys out there running whatever advertising has determined to be the latest and greatest. You would be surprised how few folks are actually out there have a basic understanding of suspension geometry....evn fewer understand what works, what doesn't and why.
here is a link to the spindles I designed for my A body, it's the last image of text. link
FRCH came in at 3" with RC migration at less than 4" throughout range of travel. I also got bump to near 0.
You seem to have a good grasp on dirt geometry...not much different. Minimize the difference between RCH and CoG without inducing any unpleasant jacking effects. On the A body I have found it is pretty difficult to put RCH and FVSA where you want it and limit RC migration without lengthening the upper and lower arms.
Also watch scrub radius, it has a little more importance on asphalt.
02-16-2005 #5Starting The Transformation- Join Date
- Jan 2005
- Posts
- 160
Hey Denny,
I've raced for a lot of years now so I'm not surprised at all by how many people have no clue what they're doing with their cars :-)
I don't want to be derogatory or offend anyone but road racing I'm sure has a higher average education level than dirt track racing by far......
Most people are into the "trick of the week" rather than solid theory. I guess that's why we have won more than they have :-)
I was curious to ask you some more questions about your spindle since it seems you're going the opposite of what Wendell and I are thinking about.
Seems you'll have a very close IC (short FVSA) with the spindle you're contemplating thus the reason for your high rate of RCH migration. Are you comfortable with that or is it more a compromise you're making due to not relocating the UCA pivot point? It's pretty common to move that point up from stock on all the race cars I've seen.
I don't care much about correcting the bumpsteer with the spindle directly. I can shim it out easy enough other ways.
How much taller is your spindle than an Impala (8.75in.)? Did you figure in using tall BJ's too? Also, you wouldn't happen to have the KPI spec for an Impala spindle would you?
I'm interested in your thought process for the design, particularly since you've had the Impala spindle on yours (still do?)
Wally
02-16-2005 #6Not trying to go too far off subject here, but other than "Paved Track Stock Car Technology", what other books do you guys recommend reading to learn about this? I am very interested in learning suspension design and theory, but don't have the time to go to school for it. I would rather spend time reading what the knowledged on the board suggest rather than as Wally stated "the trick of the week". Thanks
-Nate71 Firebird.....in progress
02-16-2005 #7Full Blown G-Machiner- Join Date
- Aug 2004
- Location
- Boston MA
- Posts
- 686
Wally,
I agree with looking at the AI cars. The Bush North cars are also a good fit. I'm guessimating my CG @ my cam height.
dennis,
I've been reading your posts for a while. If I could do it all over again I'd be a mechanical engineer. I think this stuff is as fun as it gets!
Not to hijack this post but what is the most universal format to share suspension info? When everything is fleshed out I'd like to solicit oppinions. So much fun!
J
02-16-2005 #8Dennis,
Did you change something from what we did? I thought FRCH was 1.78".
Wally,
I helped Dennis design his spindles based on his measurements. It's 2.5" taller (UBJ center to LBJ center) than what he currently has (B-body spindles?). FVSA came out to be about 110".
Anyway...What numbers to shoot for really depends on what your main objective is. What works good on slow, twisty road courses may not be the best for high-speed open road racing, for example.
77" FVSA may be bit too short for OE type short knuckles, as short knuckles tend to increase the rate of FVSA length change. Check the FVSA length and IC height at 3" bump. I'd say instanteneous camber gain is -0.8* or so with 77" FVSA. If you plan to do more than one type of racing, I'd make the FVSA adjustable.
I normally shoot for RC height anywhere from 1.5" to 3" depending on application. I wouldn't go much higher than 2" with short FVSA.
Vertical RC migration isn't necessarily a bad thing. The goal is to maintain the roll moment as close to constant as possible. So RC should drop 1" down from static when suspension is at 1" bump position. This usually happens on SLA suspension w/o trying. You do want to minimize lateral migration under roll, however.
Both side scrub and scrub radius play a big role in stability on pavement, especially if you intend to run wide rims. Keep LCAs close to level. Getting small scrub radius may be tough with OE GM style knuckles, since most of them wouldn't even fit inside 18" rims. Either steering arm or UBJ will hit the rim.The first step of becoming a better driver is to attend a track event, time yourself, and realize the fact you really suck.
Signed,
A driver who laps Big Willow at 1:42.6 in a 134hp BMW - and I am still considered mediocre.
02-16-2005 #9dennis68 GuestI figured the master would check in soon.
Yeah Katz, I don't remember why now but I did make a small change that bumped FRCH to 3". It was a worthwhile compromise, maybe to further reduce RC migration, it's been awhile. Looks like I'll have my spindles this time next month .
Wendell, I've been playing with Performance Trends, as long as you understand basic design theroy and know the x,y,z measurements it's a pretty good peice of software.
Wally, I did not figure in tall ball joints, I left them out so I can later add them if nesessary. The Impala spindles are the same height as the "b" spindle so mine would be a couple inches taller. Total height is 10.50". KPI is 13*ish. I made quite a few compromises playing with getting scrub as low as possible without lengthening the arms. You are correct in that I did not want to relocate the arm mounts.
02-16-2005 #10Registered User- Join Date
- Aug 2004
- Posts
- 957
Total height is 10.25". KPI is 13*ish.
I'd shoot for as low a roll center as possible, but, as Den and Katz very most likely did, let it float a bit to minimize migration. If it moves all over the place, well, things won't be "linear" in roll, etc. As you probably know, from the dirt track deal (which we are very eager to learn from by the way!!!!), the longer the control arms, the slower things like scrub, IC, etc move around in bump and roll. My personal feeling is that the roll center height, scrub, and migration characteristics far outweigh the need to have a cool "camber gain" figure. Everyone is different though, we all have our preferences.
I'll advertise what I think is an excellent book on suspension, once again, it is called Race Car Suspension Design, or something really close to that, from Allan Staniforth. It is a terrific read, and has an absolutley wonderful section on weight transfer. A great third book to Herb's (first), and a number of the others (Carroll Smith, Van Valkenberg, etc).
Great stuff, keep it coming.
Mark
02-16-2005 #11Originally Posted by dennis68
Originally Posted by Mean 69
As for Dennis' numbers, I belive 10.25" from UBJ center to LBJ center. The vertical span between UCA and LCA inner pivots was like 9.625" average. My computer shows 11.9* KPI with -1.5* camber, so he must have increased it for reduction in scrub radius.
All this tech talk is really making me want to fix the front end of my car...The first step of becoming a better driver is to attend a track event, time yourself, and realize the fact you really suck.
Signed,
A driver who laps Big Willow at 1:42.6 in a 134hp BMW - and I am still considered mediocre.
02-16-2005 #12Registered User- Join Date
- Aug 2004
- Posts
- 957
All this tech talk is really making me want to fix the front end of my car...
So the spindle you guys came up with is a one-off? I know that HRP, Coleman, SCP, etc offer a 9" unit, with a ~10 degree KPI, and a ~3" or so pin height. Hoerr has used this on their T/A cars in the past, though they wouldn't give me the exact, or even close goodies, other than to say that scrub is minimized even with a really deep set wheel (or is that especially because of a deep set wheel?).
I could be in my new shop as early as next week, and I'll tell you, one of the first darned things I am buying is a car lift. I am getting too old to crawl under the car trying to fit big tube headers, bleed brakes, bla bla.
M
02-16-2005 #13