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    1. #1
      Join Date
      Jan 2005
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      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


    2. #2
      Join Date
      Aug 2004
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      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.

    3. #3
      Join Date
      Jan 2005
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      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

    4. #4
      dennis68 Guest
      Typical??? 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.

    5. #5
      Join Date
      Jan 2005
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      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

    6. #6
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      Jan 2005
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      Boise, Idaho
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      Not 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
      -Nate
      71 Firebird.....in progress

    7. #7
      Join Date
      Aug 2004
      Location
      Boston MA
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      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

    8. #8
      Join Date
      Aug 2004
      Location
      CA
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      452
      Dennis,
      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.

    9. #9
      dennis68 Guest
      I 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.

    10. #10
      Join Date
      Aug 2004
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      957
      Total height is 10.25". KPI is 13*ish.
      With ball joints? With a few degrees static camber too, or is this the static design? I have to say, I STILL have not studied the stuff you sent me, though the numbers you show look darned good.

      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

    11. #11
      Join Date
      Aug 2004
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      CA
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      Quote Originally Posted by dennis68
      ...it's been awhile. Looks like I'll have my spindles this time next month
      Yeah, it's been a while plus I've been so freakin busy. I forgot exactly what we did. It'd be cool if you show up at Buttonwillow with new spindles on!

      Quote Originally Posted by Mean 69
      ...My personal feeling is that the roll center height, scrub, and migration characteristics far outweigh the need to have a cool "camber gain" figure...
      Couldn't agree more. I actually use long FVSA and short SVSA when I design suspension from scratch for a dual-purpose car.

      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.

    12. #12
      Join Date
      Aug 2004
      Posts
      957
      All this tech talk is really making me want to fix the front end of my car...
      Me too. But at this point, I'd really just like brakes, and for my new headers to fit. And to beat the poop out of the car on the track. Aye-aye-aye.

      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

    13. #13
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      I can't wait to see what Den and Katz came up with as a spindle. Im also looking into the SED units too.
      Den how much is a pair going to run you. I was pondering several ideas over the years and how improvments can one make to pivot points on a stock frame to improve geometry but using a tall GM F, G body,or any other production spindle? I guess what im trying to ask is, Is a new spindle design really necassary?
      Mark, I have seen the circle track spindles and they look basic and do improve geometry but are they designed to sit with in a certain guide line as per rules and regulations of the race class? Congrats on your new shop, Im going through the same thing now, hey if your rack hunting, go with a drive on type with center point lifts,with all of the suspension work you have infront of you. Trust me you will not regret it. Your brakes still giving you problems? I had the same thing happen to me, turned out to be a bad master cylinder...I guess new don't mean sheet!
      Thanks for the info guys
      MrQuick ΜΟΛ'ΩΝ ΛΑΒ'Ε

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    14. #14
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      Jan 2005
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      Okay. Thanks for all the input so far.

      I'm down with the low roll center and I'll start shooting for 2".

      I agree with the order of priority for the design principles as well. We just adjust static camber with tire temps and work on it until they come out even. If everything else is close then it's pretty easy.

      Now for some questions:

      1. Maybe I'm missing something but how do you raise spindle height, maintain stock UCA pivot point and come up with an FVSA of 110"? I must be doing something wrong here. I'm using A-body numbers as well and they are very similar to Denny's (mine's a 65 for reference) I assume that you're using stock length LCA's as well? (15.5in) This would seem to shorten FVSA substantially.

      2. I'm trying to get a handle on actual spindle measurements and I don't have any sitting in from of me yet so it's been a little tough. Are the heights you're talking about from UBJ pivot point to LBJ pivot point? I see that's the case with Salt Racer's numbers. From my research that would be pretty close to a B-body with tall UBJ's and LBJ's. Of course, I could be way off since I don't have one physically present.

      3. If so, Denny, do you have numbers for a B-body spindle with stock BJ's? I was under the impression that B-body spindles were about an 1.5in taller than my stock spindle which would make it about 8.75 in. tall (no BJ's, just the spindle measurement)

      4. I was actually getting some decent numbers with my stock spindles and tall UBJ's and LBJ's. Comments?

      5. I thought an FVSA of 77 was pretty good given the recommendation of 2 to 3 times track (1/2 track actually which is 29 in this case). 77 is about 2.5 times. This is a Van Valkenburgh reference IIRC (<--- thanks Denny, I got to use that reference <---)


      Admittedly I'm handicapping myself by sticking with stock spindles but I haven't found a big enough reason to pursue something different yet. This won't be an all out track car so there are some compromises (weight being the biggest one).

      Good discussion so far.



      Wally

    15. #15
      dennis68 Guest
      OK, here is the scoop. As many know I am on....limited funds (limited by my kids and wife, my W2 shows I should be a damn millionaire-where the hell is all the money)? I thought about this a long time and evaluated all the different possibilities. Buying a pair of 1 off custom spindles built exactly to my spec for under 500 bucks is a way better use of funds than trying to cobble together something that may or may not work.

      The only problem is that the steering arm and ball joint tapers are not negotiable, they have a jig and it cuts the taper and you have to find ball joints to fit; small trade off in my opinion.

      After enough hours in front of this damn LCD to make my eyes jump out of my head I came to a few conclusions;
      1) No matter what you do to the stock spindle it will never be ideal
      2) No matter what you do an F or B spindle it is even worse than stock
      3) Relocating control arms is feasible but spindles require much less work and with relocating all of the pivot points that leaves a great deal of room for error.
      4) Katz is a great guy and I'm grateful for his advice and time over the last month.
      5) My elco is gonna whoop ass at the track……let the smack talkin’ commence!!!

      Wally, I'm pretty sure the B spindles are only an inch taller, maybe 1.25". Mine are 10.50, that’s about 3" taller than stock.
      110", I wish. My FVSA is under the magic 70" mark but scrub is under 2" and bump steer is almost 0 (.026" at 2.00" of dive) so I'm not terribly worried about the violent change, again a trade off for something else. I suppose if I were going to do a lot of high speed runs (ala Nevada style) I would be more concerned running that short of an FVSA but for street driving I don't mind the stiffness and most of tracks i will visit are nicely paved.

      If I had to do this all over again I might have just gone with tall ball joints upper and lower, no on second thought I wouldn’t only because I like to be different. Using the stock spindles with tall ball joints is a huge improvement over the typical “tall” spindle conversion.

      Good luck, I’ll post pics of my spindles when they show up.
      Last edited by dennis68; 02-16-2005 at 06:44 PM. Reason: Wally's post

    16. #16
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      spindley

      ok but why? We had so much better handeling (as it seemed till now) with a GW upper and a tall spindle. There has to be something other than having a spindle made. How about other production ones? C4, C5, Mustang, 4th gen Camaro...I know strut style but hey I though i'd throw it out there.
      I hope your Elco kicks something,it better! The only thing I usually wanna kick (down) is my previous lap time. Remember the only thing tha hauls more trash (talk) would be a garbage truck, and they would benifit from a lower roll center too.
      Wally sorry for high jacking your thread.
      MrQuick ΜΟΛ'ΩΝ ΛΑΒ'Ε

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    17. #17
      dennis68 Guest
      Quote Originally Posted by MrQuick
      ok but why? We had so much better handeling (as it seemed till now) with a GW upper and a tall spindle. There has to be something other than having a spindle made. How about other production ones? C4, C5, Mustang, 4th gen Camaro...I know strut style but hey I though i'd throw it out there.
      I hope your Elco kicks something,it better! The only thing I usually wanna kick (down) is my previous lap time. Remember the only thing tha hauls more trash (talk) would be a garbage truck, and they would benifit from a lower roll center too.
      Wally sorry for high jacking your thread.
      Vince, you find a spindle with all the correct number and I’ll try it out. I honestly don't think one exists.

      I'm gonna save my trash talk for the GG show.

    18. #18
      Join Date
      Jan 2005
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      160
      Hey Denny,

      Man! Ain't that the truth!

      Would you care to expand on a couple of your points?

      Specifically

      1. I'm not looking for ideal, not that it's not a good idea, obviously. This is one of my compromises. What area do you think the stock spindle falls shortest?

      2. This is clearly no where near the case in the dirt world. B-spindle geometry is considered far superior to any other stock spindle (stock spindle classes being the assumption here) What makes it worse than stock for a road racing app, in your opinion?

      3. This may raise the skill level but I see it as something that has to be done. What sort of compromises have you found by leaving it stock?

      4. No explanation needed :-)

      5. I wish I was in a position to get mine on a track soon but it'll be awhile. I'm sure it will be faster than yours though ;-)

      I know you're pretty passionate about your setup so don't think that I'm hacking on you. I'm just trying to figure some stuff out given different compromises. You've picked a different set of parameters than I have so I'm genuinely interested.


      Wally

    19. #19
      dennis68 Guest
      Quote Originally Posted by wally8
      Hey Denny,

      Man! Ain't that the truth!

      Would you care to expand on a couple of your points?

      Specifically

      1. I'm not looking for ideal, not that it's not a good idea, obviously. This is one of my compromises. What area do you think the stock spindle falls shortest?
      BIggest gripe is spindle pin height. By the time you drop the spring height down to a reasonable level the whole front geomtery looks like hell. Mine had a FRCH of like -12.00 or something with stock spindles and a pretty big spring drop

      Quote Originally Posted by wally8
      2. This is clearly no where near the case in the dirt world. B-spindle geometry is considered far superior to any other stock spindle (stock spindle classes being the assumption here) What makes it worse than stock for a road racing app, in your opinion?
      They aren't even great for dirt track, you guys just don't notice it in that controlled 4 wheel slide you call racing (looks like drifting on dirt, albeit fun as hell looking). Bumpsteer is friken crazy once you add some spring drop, FRCH is still down in hell somewhere, steering ratio drops to like 25:1 as well as losing 5-8 feet from your turning circle (try steering out of oversteer conditions with a real crappy steering ratio) and they are way heavy.
      Quote Originally Posted by wally8
      3. This may raise the skill level but I see it as something that has to be done. What sort of compromises have you found by leaving it stock?
      with or without the tall ball joints? With tall ball joints they work pretty well. Without tall ball joints, see #1

      Quote Originally Posted by wally8
      4. No explanation needed :-)
      we agree
      Quote Originally Posted by wally8
      5. I wish I was in a position to get mine on a track soon but it'll be awhile. I'm sure it will be faster than yours though ;-)
      dream on dirt boy.

      Quote Originally Posted by wally8
      I know you're pretty passionate about your setup so don't think that I'm hacking on you. I'm just trying to figure some stuff out given different compromises. You've picked a different set of parameters than I have so I'm genuinely interested.


      Wally
      Can't hack on me....I'm Superman.

    20. #20
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      Someone needed to rate this thread, so I did.

      And Dennis, you're not superman. You're Underdog.
      Allen Ortega
      Meanstreets Performance Fabrication

      ---------------------------------------

      Vegetarians are the reason for global warming

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