Ifs 101

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    Ifs 101

    Hello everyone. I feel like I got lost in a time travel vortex and when I came back to Pirate I was all alone. It's good to know that the tech spirit lives on here and I look forward to my membership.

    I posted this over there and wasn't getting any activity. I hope it gets some traction here.

    I'm interested in learning more about IFS design theory for a few reasons.

    A, I'm bored not being able to build anything for years and hoping to get back to a point where I can sometime in the next year or two. I can use this down-time to learn.
    B, I kinda wanna build one.
    C, it's probably more applicable to the style of wheeling I have in my future.
    D, I've always been curious about it.

    I've been doing prelim research using several sources. Of course, Herb Adams' Chassis Engineering book, and some great YT videos that I made a list out of here:

    https://www.youtube.com/playlist?lis...iBb_WxXdUF4raB

    There's also several threads here about it, but sadly not nearly as in-depth as I'd wish. I think a lot of people keep their secrets for racing now days. Nevertheless, I've developed an outline of topics and questions that I have. I'd appreciate any light anyone is willing to shed on them. My goals are not racing, just general off-roading. If I ever built anything I would want it to be basically as close to a Polaris as I could get while still using an automotive drive train. Feel free to share thoughts about any venue though.
    1. Camber
      1. Def: Vertical angle of tire when viewed from front.
      2. How important is this within reason for general off-roading? I feel like camber is going to be all over the place in uneven terrain. Beyond the suspension's movement, the terrain itself would cause camber changes even if the suspension stayed stationary. I get that a small amount of negative static camber is generally good for road racing when some camber gain is built into the suspension for turns. But isn't that all kind of out the window when going over whoops, rocks, berms, hills, ditches, ruts, etc.? Not saying that this should be ignored, but maybe just not as obsessed over as it might be on a road-car?
    2. Toe
      1. Def: Angle of front of tires when viewed from above / below.
      2. Again, my hunch is that, so long as toe is relatively neutral, for general off-roading this isn't going to be hugely impactful to stress over given the lack of traction and reduced need for tracking perfectly straight?
    3. Scrub Radius
      1. Def: The distance from where the king pin inclination axis touches the ground to the center of the tire.
      2. I suspect that this is a big concern given tire sizes and the effects that excessive scrub radius can have on handling.
      3. Assume this needs to be minimized if not negative?
    4. King Pin Inclination
      1. Def: Angle of steering axis when viewed from front.
      2. Herb Adams recommends 7-9* but this is for race cars.
      3. Not sure if off-road implies any special considerations here?
    5. Caster
      1. Def: Angle of steering axis when viewed from side.
      2. How much positive caster?
      3. Herb Adams recommends 10* as a rule of thumb.
    6. Roll Center
      1. Def: The intersection of both a-arms, when viewed from front, with imaginary line drawn from that point to the center of the opposite tire patch.
      2. This is what I imagine to be one of the most important design focuses of an IFS for any car, but especially offroad vehicles that have higher chances of rolling.
      3. COG acts like a weight on the end of a pole with roll center being the pivot point on the opposite end of the pole.
      4. Theoretically one would like their COG to be as low as possible with RC to be equal to COG. This would induce the least amount of roll angle in turns and weight shifts. It would also induce a large amount of jacking, which means the COG is moving all around in a semi-unpredictable fashion.
      5. So my understanding is that we want RC to be slightly above the road surface and to design CoG as low as possible to reduce the lever arm. Of course, everything is a compromise.
      6. RC will move around throughout the movement of the suspension. RC change should be minimized for a predictable suspension.
    7. Pitch Center
      1. Def: Same as roll center only viewed from side.
      2. A shorter PC will allow the CoG to have more leverage, and therefore, for fore / aft movement under both braking and acceleration, correct?
      3. I would think that a longer PC with minimal change would be a desirable goal? Herb Adams calls pitch center "side view swing arm." Swing arm either directly correlates to, or actually is ...
    8. Anti-Dive
      1. Roughly equivalent to anti-squat in a rear suspension.
      2. It's been a long time since I've done suspension research. The last time I did it was the general consensus that anti-squat should be relatively neutral and the shocks / springs should be tuned to handle most of the suspensions resistance to movement.
      3. I assume the same applies to the front / anti-dive? This is Herb Adam's general recommendation for race cars.
    9. Ackerman should abide by all the general rules as any other front suspension as far as I know. Thus, I won't go into great detail here as there are threads about that already. Tell me if I'm missing something.
    10. Of course, bump-steer should be minimized.
      1. When viewed from front, if your steering arms intersect with a theoretical point where your upper and lower a-arms intersect, IE all three at one point, theoretically you would have zero bump-steer.
      2. How realistic is this in an off-road rig?
      3. I suspect that using a steering rack helps with this by allowing longer steering rods than would otherwise be possible with just a normal ram?
    11. CV plunge
      1. Should be minimized to avoid heat / wear / tear.
      2. In order to minimize plunge the center point of the CV joints will need to be close to the suspension pivots?
    12. Calculators:
      1. Haven't come across many. I'd be willing to pay for one if someone can recommend it.
      2. Suspension Geometry Calculator

    I'm sure I'm missing a ton, please enlighten me.
    Last edited by patooyee; 3 weeks ago.

    #2
    I'm DuckDuckGo-ing for suspension calculators right now. I've not used any of these yet, just linking them for future reference. I'll update it as I find others:


    Free:

    https://www.racingaspirations.com/ap...ry-calculator/
    http://vsusp.com/

    Paid:
    http://susprog.com/
    https://optimumg.com/product/optimumkinematics/
    http://www.auto-ware.com/software/asgp/asgp.htm
    http://performancetrends.com/
    Last edited by patooyee; 3 weeks ago.

    Comment


      #3
      Good to see you and I have nothing to offer but interested in it.
      You can pull the pin, its not until the spoon flies off things change.........

      Comment


        #4
        Is Isdtbower here? This kind of stuff is right up his alley.

        Comment


          #5
          Originally posted by rockmup View Post
          Good to see you and I have nothing to offer but interested in it.
          Basically this

          I was trying to play with ifs a little on a suzuki sidekick, but it was a McPherson strut, so basically garbage.

          I also have a mild 3rd gen 4runner I'd like to mess with a little, but need to maintain drive ability. It's dual a arm with coil overs for those who aren't familiar.

          I've also thought about scratch built full independent rig. Ie: sxs killer/wannabe. Seems like flat, equal a arms is kinda the go to. I'd be curious about laying the a arms back like an rc stadium truck, basically so the tire moves back under compression.

          Is there a reason why making 8 identical a arms, then building a little caster and camber into the chassis mounts wouldn't work pretty damn decent?

          Comment


            #6
            Finally, the fun begins.
            1. It can't be too bad to have or else there wouldn't be solid axles building it in. Looking at desert suspensions they seem to do what they can to keep it constant.
            2. Above not front. Same as camber, it doesn't hurt to run a small amount
            3. Depends. Desert guys seem to like to run as little as possible. Crawlers seem to like it. Allows you to try to grab traction by turning. Depends on use. Higher means more force on the steering. Negative should probably be avoided.
            4. Higher means more return to center at low speed. Depends a bit on design, but Race-dezert says 10*-15* is the normal range. Clearance and scrub radius guide it.
            5. Geometry governing the straightening of the wheels at speed. I've heard 12* from a solid axle racer on PBB.
            6. Its the intersection of the line from the center of the tire patch to the front view IC of the suspension locating that wheel with the same line on the other side.
            7. Yes. But its higher/lower not longer/shorter. Lower it is , the more pitch the vehicle will experience during acceleration. Its more of a point of interest rather than a design point for most people. The current 4-link calculators don't determine it.
            8. Pretty much. The lower the value the more it will dive when braking. But it will also be softer in rough stuff when braking. With outboard brakes, the line for it goes from the center of the tire contact patch to the side view IC.
            9. Correct
            10. In combination with the outers.
            11. Nothing missing here
            12. From a technical standpoint, its only at that moment in travel. At any other point it will not be pointing at the IC. Road cars can handle this because they have such little travel. From a pure design standpoint, you can create an axis in 3D space along which there will be no bump steer at full droop, ride, and full bump. A steering rack makes it easier to get the rod end on that line.
            13. Pretty much
            14. That one is okay, but doesn't account for caster change. Same with vsusp. CAD that can handle 3d parametric design is by far the best option, just requires some hand (excel) math for stuff like finding shock install ratios and stuff.
            Found this playlist a while back, been meaning to watch it: https://www.youtube.com/playlist?lis...r0uReYyZATRACB

            Comment


              #7
              Originally posted by YotaAtieToo View Post

              Basically this

              I was trying to play with ifs a little on a suzuki sidekick, but it was a McPherson strut, so basically garbage.

              I also have a mild 3rd gen 4runner I'd like to mess with a little, but need to maintain drive ability. It's dual a arm with coil overs for those who aren't familiar.

              I've also thought about scratch built full independent rig. Ie: sxs killer/wannabe. Seems like flat, equal a arms is kinda the go to. I'd be curious about laying the a arms back like an rc stadium truck, basically so the tire moves back under compression.

              Is there a reason why making 8 identical a arms, then building a little caster and camber into the chassis mounts wouldn't work pretty damn decent?
              Usually, the upper arm is a bit shorter than the lower.

              The big limitation of 8 identical arms is the lack of tunability of the values as the suspension moves.

              Comment


                #8
                im glad you got my message

                Comment


                  #9
                  Good to see you here

                  Comment


                    #10
                    Originally posted by rockyota83 View Post
                    im glad you got my message
                    I beat you too it yesterday.
                    88 Ford F-turd50 IDI Crawler

                    Comment


                      #11
                      Hell yeah pat! Nice first thread. This will get good imo.

                      I know you want shorter upper control arms than lowers so your camber is less effected by suspension travel.
                      Another problem is that you'll never be able to delete toe, and will be less under compression, and higher under droop.

                      Comment


                        #12
                        While not 4wd, here is a decent picture of a go fast front suspension on a pre runner i saw a couple weeks ago at a local fab shop.

                        Click image for larger version
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                        Comment


                          #13
                          I have the Performance Trends suspension program. It's an impressive tool. I think I bought it in 08-09, and it was around $500. You can't compare it to the 4-link calculator everyone on here is familiar with. They are worlds apart, even though I'd guess PT is Excel based too.


                          If you don't have it already, you need the Milliken brothers book Race Car Vehicle Dynamics. It's an SAE college level book that's basically God's word on all things vehicle dynamics. It's thick and it ain't cheap.

                          The most important thing to always remember is that everything affects everything else. Getting tunnel vision on something like keeping the RC movement as low as possible, will always bite you in the ass somewhere else.
                          Kevin

                          Comment


                            #14
                            Originally posted by SLOWPOKE693 View Post
                            While not 4wd, here is a decent picture of a go fast front suspension on a pre runner i saw a couple weeks ago at a local fab shop.

                            Click image for larger version
Name:	20201215_125704.jpg
Views:	598
Size:	444.2 KB
ID:	244417
                            I wonder why they chose to package the upper control arm that way. Seems like you could get same-ish strength to weight with the mounts on either side of the shock with possibly more steering angle. My best guess is they wanted to leave room in case they wanted to angle a shock to the rear.

                            Comment


                              #15
                              Originally posted by arse_sidewards View Post

                              I wonder why they chose to package the upper control arm that way. Seems like you could get same-ish strength to weight with the mounts on either side of the shock with possibly more steering angle. My best guess is they wanted to leave room in case they wanted to angle a shock to the rear.
                              I think alot of it has to do with steering. Im thinking once the rack and tie rods are mounted and turned full lock left, there wouldn't be much room to push the coil over forward from its current position before there would be interference with the spring and tie rod. I could be wrong though. This is all just a guess.....
                              Last edited by SLOWPOKE693; 3 weeks ago.

                              Comment

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