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    1. #1
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      Total Lateral Load Transfer Distribution

      I've been working on setting up my Camaro again after a front roll center height change. The car pushes quite badly now and does not roll much in the front. As a result the outer edge of the front tires is much hotter than the rest of the tire. So it appears that my roll rate split (front/rear) is too high. This brought up a thought about the relation of roll rate front to rear.

      Some call this the magic number, some call it the balance, Milliken calls it "Total Lateral Load Transfer Distribution". The roll rate split is expressed as the percentage of the roll gradient taken up by the front suspension of the car.

      What kind of numbers are y'all running? Do you set up your car for mid corner understeer, corner entry oversteer, etc?

      Electrical/Mechanical Engineer
      1968 Camaro RS - Flat Black


    2. #2
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      Pushes where? Entry or exit or middle. Also, is this autox or trackday or both.

      What were your roll rates before and after the roll center change?
      Donny

      Support your local hot rod shop!

    3. #3
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      Quote Originally Posted by dontlifttoshift View Post
      Pushes where? Entry or exit or middle. Also, is this autox or trackday or both.

      What were your roll rates before and after the roll center change?
      The car pushes everywhere: entry, mid corner, exit. It is worst on entry and mid corner. This mainly pertains to autocross.

      Previous front roll rate was about 3°/g. Not sure what it is now, I will have to calculate it. Probably somewhere near 2.75°/g now.

      Front roll center was moved up about 1-1/8". This was mainly a consequence of improving the camber gain curve up front and was not the main objective, but it comes with the territory on a factory subframe car.
      Electrical/Mechanical Engineer
      1968 Camaro RS - Flat Black

    4. #4
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      ^ That is what the tuner in me would think and try to react to at the track.

      Regarding actual TLLD numbers you are looking for when setting up at the shop, mine have changed so much since the last time it was put into simulation software I don't have an actual number any longer. I just know we work really hard at getting the front geometry perfect for the best contact patch at turn in and also getting the inside rear to release at just the right time and for the correct amount of time. These front heavy big HP cars we are messing with take a bit of a different approach that the small light 50/50 cars that are meant to do this kind of thing (and suspension gurus mainly talk about adjusting).
      Lance
      1985 Monte Carlo SS Street Car

    5. #5
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      Two things stand out to me by what you's said so far.

      First, the outer edges of the front tires hot is a clue you don't have enough caster in the car. When you turn the steering, the KPI overrides the caster and the outside front goes positive camber. Second, by raising the front roll center you've changed the diagonal roll and therefore the inside rear tire may not be disengaging enough and is still pushing hard enough to override the outside front tire.

      With out good pictures of the car on entry and mid turn, it is really hard to tell...but the two above would surely make it push like a dump truck if extreme enough.
      Lance
      1985 Monte Carlo SS Street Car

    6. #6
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      I'm not an engineer, I think it's important to note that. I am a firm believer that most push on entry is driver induced and have beat that drum for years here.........but you say it's tight everywhere and worse since you made the change in roll center.

      That's not a huge change in roll center and I would have thought it would have been offset by better camber gain. Conventional theory would be with the higher roll center the car would want less spring or bar or both to get the roll rate back in line with the weight transfer at that end. This is math I have never done but comparing lateral weight transfer on the front before and after the RC change should tell you what you want your wheel rate to be in roll and then you can make an appropriate change.

      Understeer through the middle is the worst......can't mash the gas if you are waiting for the front end to point the right direction. In general, loose is fast, you just need fast hands to keep up with it. I don't have those so neutral to loose on throttle is easiest for me to drive.
      Donny

      Support your local hot rod shop!

    7. #7
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      Quote Originally Posted by SSLance View Post
      Two things stand out to me by what you's said so far.

      First, the outer edges of the front tires hot is a clue you don't have enough caster in the car. When you turn the steering, the KPI overrides the caster and the outside front goes positive camber. Second, by raising the front roll center you've changed the diagonal roll and therefore the inside rear tire may not be disengaging enough and is still pushing hard enough to override the outside front tire.

      With out good pictures of the car on entry and mid turn, it is really hard to tell...but the two above would surely make it push like a dump truck if extreme enough.
      I don't think the issue is castor related since I have not changed that setting in years really. It is very likely the issue is related to the parallelism (or lack thereof) between the mass centroid axis and the roll center axis. Even so, I think the main issue is related to how the front and rear carry the lateral load.

      I have attached pictures. The two of the car turning right are from the most recent event. You can see camber on the outside front appears to be positive. The front is rolling, but the front suspension is not really compressing. The picture of the car turning left is from March before the front RC change.

      Static Alignment for all pictures:
      -1.5° ±0.1° Camber
      +6.1° ±0.1° Castor

      0.000" ±0.015" Toe

      Quote Originally Posted by SSLance View Post
      ^ That is what the tuner in me would think and try to react to at the track.

      Regarding actual TLLD numbers you are looking for when setting up at the shop, mine have changed so much since the last time it was put into simulation software I don't have an actual number any longer. I just know we work really hard at getting the front geometry perfect for the best contact patch at turn in and also getting the inside rear to release at just the right time and for the correct amount of time. These front heavy big HP cars we are messing with take a bit of a different approach that the small light 50/50 cars that are meant to do this kind of thing (and suspension gurus mainly talk about adjusting).
      Most suggest a TLLD related to the static front/rear weight distribution, and a roll axis parallel to the mass centroid axis. This tends to keep the car pretty well balanced regardless of static weight distribution front to rear. Only minor tweaks from there.

      My fix for this issue is to raise the rear RC in proportion to the front RC movement to get the roll axis more in parallel with the mass centroid axis as it was before, and to add about -1/2° of camber. That should stiffen the rear up and hopefully migrate the balance away from understeer.

      There is an autox next weekend where I'll test out the change fully. I am doing some street testing this week, but you can only go so far with that.

      I appreciate the suggestions. Keep em coming. Very interested to see how other set up their rides to match their driving style. Learning as much as I can is always the goal.
      Attached Images Attached Images      
      Electrical/Mechanical Engineer
      1968 Camaro RS - Flat Black

    8. #8
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      Quote Originally Posted by Sleeper68 View Post
      My fix for this issue is to raise the rear RC in proportion to the front RC movement to get the roll axis more in parallel with the mass centroid axis as it was before, and to add about -1/2° of camber. That should stiffen the rear up and hopefully migrate the balance away from understeer.

      There is an autox next weekend where I'll test out the change fully. I am doing some street testing this week, but you can only go so far with that.
      So what happened?
      Donny

      Support your local hot rod shop!

    9. #9
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      Quote Originally Posted by dontlifttoshift View Post
      So what happened?
      The rear RC was raised 1" from 12.5" above ground to 13.5" above ground. Kept the rear ARB full stiff. The front got some more camber and I changed the toe.
      Alignment for the event:

      Camber: -2°
      Caster: +6.1°
      Toe: 1/8" out , measured at the rim edges, approx 0.4°

      The car did really well overall. There were only two entries for CAMT so the event guys put me in CAMC. Ended up 3rd, about 0.45s off of 1st place. I have attached the results for CAMC.

      The car would still push on some corner entries but I think it was driver induced to some degree. The car seems very well balanced now. I could get some more rotation when letting off neutral throttle slightly in some corners. Probably not loose enough for autocross, but I think it would do fantastic on a road course. This course favored right hand turns heavily but the car still worked the right hand side tires to a good degree based on tire temps. Unfortunately I was unable to get an infrared temp gun in time for the event. I have one on the way and will have it for the next event to log tire temps.

      I will try to attach a link to a video of my last run (fastest) in the next post. I drove alot better overall too I think this time. Much smoother than last time and tried to shorten distance wherever possible. There is still alot of dead time in there where I need to be accelerating (either positive or negative) usually on corner entry. Definitely could have won the class if I used that dead time better. There is still a bunch of time out there.

      Thanks to all for the suggestions. Certainly open to driving critiques.
      Attached Images Attached Images  
      Electrical/Mechanical Engineer
      1968 Camaro RS - Flat Black

    10. #10
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      Quote Originally Posted by dontlifttoshift View Post
      I'm not an engineer, I think it's important to note that. I am a firm believer that most push on entry is driver induced and have beat that drum for years here.........but you say it's tight everywhere and worse since you made the change in roll center.

      That's not a huge change in roll center and I would have thought it would have been offset by better camber gain. Conventional theory would be with the higher roll center the car would want less spring or bar or both to get the roll rate back in line with the weight transfer at that end. This is math I have never done but comparing lateral weight transfer on the front before and after the RC change should tell you what you want your wheel rate to be in roll and then you can make an appropriate change.

      Understeer through the middle is the worst......can't mash the gas if you are waiting for the front end to point the right direction. In general, loose is fast, you just need fast hands to keep up with it. I don't have those so neutral to loose on throttle is easiest for me to drive.
      I also thought the camber gain improvement (from -0.35°/in to -0.78°/in in bump, right off ride height) would offset the RC change as well. It appears I was wrong. Yes, I could reduce the front spring rate or front ARB rate to soften the front back up. My thought was to raise the rear RC up to match the front to get the balance back. The overall roll gradient would be less but not by much. The better camber gain curve should allow for less static camber with the stiffer total roll rate.

      And yes, it was awful. The entry understeer made slaloms hard. Had to use alot of throttle and brake to get the car to rotate which made me sloppy.

      The push in the pictures above was also annoying for the same reason you said. Made it hard to get in the throttle early. I tried to use throttle steering again which sometimes made me push out more than I wanted.
      Electrical/Mechanical Engineer
      1968 Camaro RS - Flat Black

    11. #11
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      I am also surprised that your camber gain/roll center height change made the car push so much more. What did you do to increase camber gain? Did it have much impact on bump steer? Could your inside front be toe-ing in now under roll, giving you less front end grip than you had before?
      - Ryan

    12. #12
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      Quote Originally Posted by stab6902 View Post
      I am also surprised that your camber gain/roll center height change made the car push so much more. What did you do to increase camber gain? Did it have much impact on bump steer? Could your inside front be toe-ing in now under roll, giving you less front end grip than you had before?
      The UCA pivot was moved down 1/2" in rear - 5/8" in front, a variant of the Guldstrand mod if you will. This increases the angle of the UCA and makes camber gain more aggressive, by doing so the FVSA shortens and the RC moves up. Yes this change did affect the bumpsteer curve. I have adjustable height tie rod ends so I moved the outer tie rod down to a point where bumpsteer was minimized around ride height after moving the RC.

      Camaros typically toe-out in jounce and toe-in in bump when the outer tie rod is not low enough. Conceivably "camaro style" bumpsteer could cause understeer if it was bad enough.
      Electrical/Mechanical Engineer
      1968 Camaro RS - Flat Black

    13. #13
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      Quote Originally Posted by Sleeper68 View Post
      The UCA pivot was moved down 1/2" in rear - 5/8" in front, a variant of the Guldstrand mod if you will. This increases the angle of the UCA and makes camber gain more aggressive, by doing so the FVSA shortens and the RC moves up. Yes this change did affect the bumpsteer curve. I have adjustable height tie rod ends so I moved the outer tie rod down to a point where bumpsteer was minimized around ride height after moving the RC.

      Camaros typically toe-out in jounce and toe-in in bump when the outer tie rod is not low enough. Conceivably "camaro style" bumpsteer could cause understeer if it was bad enough.
      Okay good - sounds like you really thought it through. Some people switch to tall lower ball joints to increase camber gain, which exacerbates the stock bumpsteer issue if you don't make complementary changes to the tie rod ends.
      - Ryan

    14. #14
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      If you don't mind a less educated person joining in...
      With the car pushing at entry it does not surprise me that you can't take advantage of a better camber curve. Once the tire slips you are not really going to transfer much weight to that corner. I like playing with the math and modeling suspension in CAD but I know I can get caught up in the numbers and they don't always transfer completely to the real world. As you already know you changed the balance of the car and I don't think you can really tell what you achieved unless you take some wheel rate away from that end (assuming you at least have a decent roll axis inclination). Like Donny said loose is fast. I have typically ran 1 hour wheel to wheel races and I can't drive a loose car that long at the level needed without messing up but man can I get a fast lap time.

    15. #15
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      Quote Originally Posted by CSG View Post
      As you already know you changed the balance of the car and I don't think you can really tell what you achieved unless you take some wheel rate away from that end
      Or add wheel rate to the rear?

      Quote Originally Posted by CSG View Post
      (assuming you at least have a decent roll axis inclination). Like Donny said loose is fast. I have typically ran 1 hour wheel to wheel races and I can't drive a loose car that long at the level needed without messing up but man can I get a fast lap time.
      I think my roll axis inclination is incorrect now since the front RC moved up and the rear did not move.
      Electrical/Mechanical Engineer
      1968 Camaro RS - Flat Black

    16. #16
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      Quote Originally Posted by Sleeper68 View Post



      I think my roll axis inclination is incorrect now since the front RC moved up and the rear did not move.

      I would agree with that, the pictures really show it.

      See how the inside rear is up and front is planted here?



      And see how the inside rear is planted and the front is rolling more than the rear here?




      This is what my car looks like on corner entry, this is a 1.2G turn at about 50 mph entry.

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      Notice how the outside front tire stays in negative camber even with roll and steering input and how the rear is rolling more than the front releasing the inside rear tire.

      My front tires wear completely even and the car doesn't push anywhere. It's set about about -1.5* static camber and 9.75* positive caster. The front roll center is about ground high and migrates under the inside front tire with roll, the rear roll center is about 18" above the ground.
      Lance
      1985 Monte Carlo SS Street Car

    17. #17
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      My camber gain is more aggressive than yours and I run -2.7* static with about 7* caster. Yokohamas want more camber yet. Pic is on Goodyears and while it looks a tick positive, the tires wore well all weekend.



      I would soften the front before I raised the rear roll center. In general I want to work on the end of the car that is not working......removing grip from one end to make the other feel better is not the way to go fast. If the front sucks, that's what needs fixing, then work on the rear to find more speed. However...

      You already made changes so lets see what happens.
      Donny

      Support your local hot rod shop!

    18. #18
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      Quote Originally Posted by dontlifttoshift View Post
      My camber gain is more aggressive than yours and I run -2.7* static with about 7* caster. Yokohamas want more camber yet. Pic is on Goodyears and while it looks a tick positive, the tires wore well all weekend.
      Car looks good in that corner. Rear is not rolling much more than the front. Outside front seems very happy. Inside front has a lot of camber to it.

      This is close to how my car cornered before the change. Looks like your setup is stiffer than mine. My ride freq's are pretty low compared to alot of others, about 1.5Hz front 1.5Hz rear. This correlates to 600lb/in front springs and 185lb/in rear springs with my corner weights and motion ratios. Wheel rates: ~210lb/in front , ~100lb/in rear.

      Quote Originally Posted by dontlifttoshift View Post
      I would soften the front before I raised the rear roll center. In general I want to work on the end of the car that is not working......removing grip from one end to make the other feel better is not the way to go fast. If the front sucks, that's what needs fixing, then work on the rear to find more speed. However...

      You already made changes so lets see what happens.
      I essentially made the front a little stiffer, in addition to the camber gain improvement the car needed so I could run less static camber. I ran about -3° camber when I was on 245s. I went to less camber with the 275 Rivals to improve braking because the car would lock up the inside front badly before with so much static camber.

      Why do you think the front sucks? I see it as the front and rear are not jiving well together.

      Let's say my roll rate before was 3°/g front 3.5°/g rear and now it is 2.5°/g front 3.5°/g rear. The much stiffer front now could possibly be the cause of the understeer.

      Now let's say your setup is 2°/g front 2.5°/g rear. Your setup is still stiffer than mine, but since the roll rates are more in line with your car's static weight distribution the balance is good.

      So one could argue that by making my rear stiffer, I improve the balance of the car. This doesn't necessarily mean I am removing grip from the rear because my setup is still softer than yours.

      Obviously the numbers may be off but I think this is what is going on. What do you think?
      Electrical/Mechanical Engineer
      1968 Camaro RS - Flat Black

    19. #19
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      This picture shows clearly why I like running a bunch of caster. See the contact patch of the inside front tire. This along with a super stiff front sway bar really hook the front of this car up.

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      It also corrects the contact patch of the outside front without having to run a ton of static camber which hurts braking as well as inside front contact patch.
      Lance
      1985 Monte Carlo SS Street Car

    20. #20
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      Lance, I really don't see how you can make those assumptions (about Sleepers car) without the context of what is happening with the throttle or brake pedals. I'll let you guys talk about RC migration and ride frequencies and how much more the front is rolling than the back or vice versa. I have nothing to offer there.

      Sleeper, you said the front [pushes all the time] sucks, not me. Before you had this issue, if you raised the rear roll center did the car loosen up?.......that wasn't because it had more grip. It may understeer less, but I doubt it will be faster. It's all theory till proven otherwise.
      Donny

      Support your local hot rod shop!

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