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    1. #1
      Join Date
      Apr 2006
      Location
      Yankton, SD
      Posts
      240
      Country Flag: United States

      3-link anti-squat adjustment, upper vs lower link

      The 3-link I designed for my car has reached the quoting stage for some laser cut parts and I want to double check something before I commit my green. On my design, there are settings in-between but think of the rear suspension having two positions, normal / auto-x (AS 45-55%) and drag racing (AS =>100%)


      Most of the references I have seen regarding adjustable anti-squat on a 3-link involve adjusting the upper center link. I think that making the upper link front attachment adjustable generally compromises the strength of the upper bracket where it connects as there is very little existing structure in most cars to attach to. If my thinking is correct, you want a strong upper link and attachment when drag racing as all the tension load has to go through the one link. To that end, I designed my suspension to have adjustable lower link attachments on the axle end. When I want to drag race, I drop the rear of the lower links from the top hole (link parallel to the ground) to the to the bottom hole and AS goes above 100%.


      If I understand correctly, this will increase rear roll steer, but does that really matter when drag racing? Any thoughts?





    2. #2
      Join Date
      Apr 2006
      Location
      Yankton, SD
      Posts
      240
      Country Flag: United States
      Oh come on, this is the internet, someone MUST have an opinion one way or another.

    3. #3
      Join Date
      Oct 2012
      Posts
      434
      Country Flag: United States
      My opinion is that you should spend more time showing me how to use CAD programs...


      Oh, you meant opinion on your setup? Sorry, I'm of no help there

    4. #4
      Join Date
      Oct 2006
      Location
      Kansas City
      Posts
      286
      There's more to it than just A-S. You'll want to look at your side view instant center location and swing arm length relative to CG and see what it does when adjusting the upper link vs the lowers. In addition, load transfer is greatly affected by your spring rates and your shock settings. Is the car manual or automatic? How much horsepower? Even if you provide all this information, I won't have the answers, but I know those are most of the inputs required for the guys who are experts at hooking small tire radial cars with large power. I've just started messing with this by putting on relocation brackets on my 4 link, but have no road time.

      I think what you'll find is there's no clear cut answer for your combination. A lot of cars like standard 4 link rears in fox bodies or G-bodies have a lot of on-track testing data that allows for people to find optimal combinations. On a p-t type car where handling is a top priority, drag strip settings are going to be compromised.

      In my opinion, which isn't worth much, I'd be willing to sacrifice the roll steer for the A-S gain at the track. Being around 100% should make a big difference. My 4 link A-S dropped to 77% with the car lowered, and it now will lightly spin 17" drag radials leaving at the track with only 300 lb-ft of torque. When the car had stock springs and closer to stock ride height, probably more like 100% A-S, it would 60 foot better with junk BFG Radial T/A tires. My relocation brackets with a 2" drop take the A-S up to like 141%... more extreme than I wanted (wanted 110-120%), but it was the only brackets readily available aftermarket. I won't be able to do any testing until next year. I'm very concerned with rear brake hop with that much A-S and a side view instant center length of 41", so I may have to switch the arm locations back and forth.

      It really ends up being all about how the car hits the rear tires and what it needs to keep them planted, which you probably will only find out via testing or enough analysis to earn you a Ph.D. I think John P and the II Much crew had planned something out where they could switch between a 3 link and 4 link depending on drag strip and handling duties, but I'm not sure any more details or how it worked out for them. It may have been a Morrison chassis... would be interesting to see what numbers they targeted when designing the suspension.
      Luke
      '83 Hurst/Olds
      SC&C Stage 2, Blazer spindles/C6 brakes, Lee box, Eibachs, Currectrac rear arms... Olds 433 coming soon.

    5. #5
      Join Date
      Nov 2012
      Location
      Sacramento, CA
      Posts
      1,918
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      Hi Guys,

      To have this conversation properly
      … we need to first discuss & understand anti-squat, resultant thrust angle, leverage lift point & lift/push torque distribution from the housing.

      Most veterans know you can achieve the same ant-squat percentage yet end up with different swing arm lengths based on where the IC ends up relative to the wheelbase and also different thrust angles. Because these are so key, I learned a long time ago to treat anti-squat percentage as small player in the 12 factors in how a rear suspension works. So a target anti-squat percentage is never my primary goal. That throws most people off, as they have been taught forever to focus on anti-squat exclusively. But “resultant thrust angle” & lift point (like Luke mentioned) are way more important.

      The lift point of the swing arm is critical (position of IC relative to wheelbase & CG) ... because this determines how much of the cars weight is actually available to the lever (swing arm) ... and whether or not it's lifting the rear end of the body/chassis.

      Resultant thrust angle is the direction the rear suspension is pushing the chassis. It is always some percentage up (lift) and some percentage forward. The anti-squat percentage matters … but it’s just a byproduct of getting the desired resultant thrust angle … not the priority.

      All rear end linkage tuning is based around 3 simple strategies:
      1. Where am I lifting
      2. How much housing torque is distributed to lifting the car?
      3. How much housing torque is distributed to pushing the car forward?

      1. Where you’re lifting is normally the IC in most suspensions, except in a torque arm, where it is the attachment point.
      • The shorter you make the swing arm, the quicker & harder it will load the tires, but with less weight & for a shorter period.
      • The longer the swing arm, the slower & softer it will load the tires, but with more weight & for a longer period.

      2 & 3. The location & angle of the links … and location of the pivots … define how much of the housing torque is distributed to lifting the car & how much is distributed to pushing the car forward. You can not change one without affecting the other.
      • More torque distributed to lift … plants the tires quicker & harder … leaving less torque to push the car forward.
      • Less torque distributed to lift … plants the tires slower & softer … leaving more torque to push the car forward.

      These sound similar, but 1 is different from 2 & 3. Where you lift does not necessarily define torque distribution. It does define how much weight of the car can be used to load the tires. This can be a little cloudy, but I think it will be clear & simple when you see how to tune it with a 3 or 4 link. With high powered cars, I usually set the IC under the CG as a starting point … so I have the full weight of the car to load the tires … then I work out the torque optimum distribution from the housing for chassis lift & forward push. Here is how to tune it …

      More lift/less forward push:
      A. Increase distance of top link housing mount pivot above the axle CL
      B. Increase distance of top link housing mount pivot behind the axle housing CL
      C. Decrease distance of lower link housing mount pivot below the axle CL
      D. Increase distance of lower link pivot in front of the axle housing CL
      E. Increase the distance of the lower link chassis mount pivot in front of the axle housing CL compared to top link chassis mount pivot in front of the axle housing CL
      F. Increase downward angle of top links
      G. Increase upward angle of lower links

      A, B, D & E: Can be achieved without affecting the IC arm lift point, if built into the design.
      C: Has a minor shortening effect on the IC arm lift point & raising the IC.
      F: Shortens the IC arm lift point significantly … and has no impact on rear steer.
      G: Shortens the IC arm lift point significantly … BUT adds positive rear steer. Only do this if positive rear steer effect is desired too.

      Less lift/more forward push:
      H. Decrease distance of top link housing mount pivot above the axle CL
      I. Increase distance of top link housing mount pivot in front of the axle housing CL
      J. Increase distance of lower link housing mount pivot below the axle CL
      K. Increase distance of lower links behind the axle housing CL
      L. Decrease the distance of the lower link chassis mount pivot in front of the axle housing CL compared to top link chassis mount pivot in front of the axle housing CL
      M. Decrease downward angle of top links
      N. Increase downward angle of lower links

      H, I, K & L: Can be achieved without affecting the IC arm lift point, if built into the design.
      J: Has a minor lengthening effect on the IC arm lift point & lowering the IC.
      M: Lengthens the IC arm lift point significantly … and has no impact on rear steer.
      N: Lengthens the IC arm lift point significantly … BUT adds positive rear steer. Only do this if positive rear steer effect is desired too.

      I tune on this lift/forward push balance … with the IC arm pick up point starting under the CG.
      • If I run out off lift adjustment range … and still need more lift for this application … that’s telling me I need a shorter IC arm pick up point.
      • If I run out off forward push adjustment range … and still need more forward push for this application … that’s telling me I need a longer IC arm pick up point.
      • Like all tuning, we’re looking for the best compromise for each individual application.

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

      Guidelines for torque distribution:
      • When you have more engine torque to rotate the housing, you need less torque directed to lifting & more going to push/drive the car forward.
      • When you have less engine torque to rotate the housing, you need more of that torque directed to lifting & less pushing the car forward.

      Also:
      • When you have softer sidewall tires … like drag slicks … you need less torque directed to lifting & more going to push/drive the car forward.
      • When you have stiffer sidewall tires, you need more of that torque directed to lifting & less pushing the car forward.

      Engine power output, curve, tire design, track conditions, etc … all play a role in defining the optimum setup any given day at the track.
      • The key is finding the optimum balance for each individual application.

      In high powered Track Cars on road courses, with wide slicks, you need more push & less lift. In AutoX & Track Cars on TW200 street tires, with harder sidewalls, I find we need to plant the tires a little harder.

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

      Let's discuss the effects of roll/rear steer on traction. First the basics for our readers following along:
      • Lower links at an angle running uphill going forward creates a rear steer effect as the body & chassis roll during cornering. Larger uphill link angles produce higher degrees of rear steer.
      • As the car achieves roll angle, the rear steer effect helps the car to turn while cornering, by pushing the outside tire rearward, the inside tire forward and both tires pointing to the outside of the corner. This is rear steer. This also adds a slight loosening effect.
      • Lower links at an angle running downhill going forward creates a counter rear steer effect, by pushing the outside tire forward, the inside tire rearward and both tires pointing to the inside of the corner. This is counter rear steer. Larger downhill angles produce higher degrees of counter rear steer.
      • As the car achieves roll angle, counter rear steer effect reduces the car’s turning ability. This adds a slight tightening effect.

      * This next explanation does not apply to drag cars if they have controlled their body rotation with preload, springs and/or sway bars. This applies to all other cars that run on Road Courses, AutoX. Street, etc … where you are trying to accelerate out of corners under power.

      Even though running the lower links at an uphill angle … applies more force applied to lifting & planting the tires harder … there is also the different action of rear steer happening simultaneously … providing a counter effect. The small gain in traction from increased lifting force (for a shorter duration) is over powered by the larger rear steer effect loosening the car up. The typical result is more grip at throttle pick up, but loss of traction at some point trying to exit the corner under power.

      When the loss of traction occurs … is defined by how much rear steer is in the car and how long the corner exit is (therefore how long cornering grip must be maintained). On tight corners it possible to achieve a setup that provides rear steer & good exit grip. I find small amounts of rear steer on short, small corners, can work good, as long as everything else is optimized.

      We run into problems is when we:
      • Get greedy with the amount of rear steer utilized.
      • Run any rear steer on high speed, long sweeping corners.

      For these reasons, I suggest:
      • Rear steer can be good tuning tool for short tracks with small tight corners.
      • Avoid any rear steer on big tracks or road courses with long, fast sweepers.
      • In fact, counter or negative rear steer can be used effectively on tracks with primarily long, fast sweeping corners.
      • Avoid any rear steer when drag racing too.
      • In fact, counter or negative rear steer ... or one or both sides ... can be used effectively to control torque steer on launches.

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

      Centered 3-Link:

      Assuming the 3-link is designed with the proper range of adjustment holes …. we can make any of the adjustments … A through N … with a 3-link (4-link too) to achieve the optimum pick up point combined with the optimum lift/push torque distribution.
      • We can achieve 50/50 lift/push torque distribution … while maintaining a longer swing arm under the CG.
      • We can achieve greater than 50% lift distribution with a 3 or 4-link for moderate & lower powered cars.

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

      Now, finally … Offset 3-links:

      As we all know, the rear end housing wants to rotate the same direction the driveshaft is turning & applying the engine torque … counter clockwise from the rear view, clockwise from a front view. So with ALMOST all rear suspension designs … 3-link, 4-link, torque arm, etc … as torque is applied … the left rear tire is loaded more & the right rear tire is loaded less.

      This makes the car want to “drive” to the right, a degree, under hard acceleration. As you make left hand turns the car has more “forward bite” during corner exit … than right hand turns, which have less “forward bite” during corner exit. If it isn’t counteracted … the effect amplifies with increased power output.

      When designing a 3-link suspension … the upper link can be offset (still parallel to the chassis centerline) to the passenger side to counteract the torque transmitting through the rear end housing on acceleration. My rule of thumb is 7-12% of track width. I can provide much more detailed information on the what, why & how ... if you'd like.

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

      I covered the pros, cons & tunability of Watt’s links & panhard bars in much greater detail in other posts. But here is the summary …


      With a centered 3-link, 4-link or Torque Arm & a adjustable panhard bar: You can use the panhard bar angle to effectively counteract "torque steer" … to a degree. The keys to doing this are simple:
      • Keep the center height where you want the roll center.
      • Lower the side you want to load the tire more on & raise the other side the same amount.

      This strategy is always a compromise. But the suspension already had a compromise. You’re just shifting where the compromise is. Think of it this way:
      • If you eliminate 100% of the torque steer, you will have 100% of the effects provided by an unbalanced panhard bar set-up.
      • If you balanced panhard bar set-up 100%, you will have 100% of the of the torque steer.
      • Most racers shoot for a 50/50 balance and then tune for track conditions.
      • You can always shift the balance to gain here & give up there.


      With a centered 3-link, 4-link or Torque Arm & a Watt’s link:
      • You will have even loading on entry & middle for both left & right hand corners.
      • You are stuck with torque steer on corner exits.

      With an offset 3-link & a Watt’s link:
      • You have zero torque steer
      • You have even loading throughout both left & right hand corners.

      With an offset 3-link & an adjustable panhard bar:
      • You have zero torque steer
      • You can achieve a “Balanced Panhard Bar” for even loading throughout both left & right hand corners.
      • Plus, you can tune tire load balance with panhard bar split if the situation calls for it.

      All of these are really good rear suspension set-ups for street, road course track days & AutoX.
      • For optimum track performance the offset 3-link is the best, offering the most benefits & least compromises.
      • Adjustable Watt’s links are the best centering device for most.
      • Adjustable panhard bars are only better for extreme tuners.


      Make sense?


    6. #6
      Join Date
      Apr 2006
      Location
      Yankton, SD
      Posts
      240
      Country Flag: United States
      Ron: Thanks for your always complete and way-over-my-head-on-the-first-10 reads response. I'll spend a lot of time digesting this before I can even attempt to respond intelligently. I'm sure I speak for many members here in saying how much I appreciate the knowledge dump you provide.

      83hurstguy: Car isn't done yet so I have to make some estimates regarding weight and CG. Can't seem to make on my mind on the engine so I don't have any idea on the HP either but I doubt very much it will be above 500. I don't think I will mind switching the arms back and forth. as long as I make it easy to do.

      AS is an easy number to understand and it gives inexperienced people like myself something to latch onto as a place to start. I hope to build in enough adjustability but all of this bolts together so I can remake parts if needed. Based on the CG assumptions I've made, this is where I am at as a starting point for the CAD screenshot linked above. I will probably shorten the center upper link a little so I can tie into the structure.




      In a drag only situation (rear of bottom link in bottom hole), it doesn't sound like I'll be giving up much with the increased roll steer so I'l proceed with this. Most of the time the link will be in the top hole.

      mitch_04: Thanks! If I wasn't able to lay this out in a CAD program first, I probably would have given up a long time ago or not gotten myself into this situation in the first place. It is pretty easy to convince oneself you can build something as long as you can model it. As it turns out, not true in many cases. But again, you have to start somewhere.

      Experience: The thing you get right after you needed it.