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Do you want *some* kinematic toe?

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  • Do you want *some* kinematic toe?

    I recently had an alignment done on my 993. The shop adjusted the rear kinematic toe so it has almost no change in the usable stroke at its current ride height. Since my last time on track in the car (January) I have had this corner balance/ alignment done and added an APR GT250 wing blade. Previously it had a factory style, very low and narrow blade. Think of it as going from an s2000 CR wing to a GT250. Substantial.
    So now the car is very stable but the one thing I would like is the ability to quickly adjust the angle of the car with the throttle. I think it just needs some real front aero but Iím curious if anyone has any thoughts on dynamic toe being good to an extent?

  • #2
    All cars have a bump steer curve in the rear suspension, even live axles. Sometime a shallow "S",l shape, usually a shallow "C" shape. Dictated by suspension pick up points, not something that an alignment tech can modify. Your alignment tech will only change the baseline setting, the curve remains.

    The factory will design the suspension to put the rear suspension in a specific spot in that toe curve at static ride height, acceleration, cornering (bump) and braking (droop). Modern, long travel multi link suspensions do the best job of generating the ideal bump steer curve for all driving conditions.

    Some cars benefit from altering pick up points to change their OEM bump steer curves when lowered. Most AP1 owners have near death experiences they can share to illustrate that idea.

    The 993 rear has a sophisticated multi link design. Unless your tech installed aftermarket parts to alter a few pick up points, you probably just have a bit more toe in than stock.
    WWW.949RACING.COM
    SuperMiata

    Aside from their cost I never understood why people race them.
    But obviously I just dont get it. -fatbillybob

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    • #3
      I posted this despite realizing Iím asking a very general question on something thatís very specific to car, driver preference and overall modifications. I just looked and unfortunately I donít have the before figures or the measured K-toe. Itís now set to 4mm total toe in on the rear.
      Emilio, you said you like a ďcĒ curve on your cars? Meaning it toes out on squat and in on droop? I guess the question Iím asking is do you think the toe (kinematic or static) is a good place to look if you want to make your car more responsive to throttle steering?

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      • #4
        The 993 is a 5 link complicated thing and autometrics makes the cheap thing to hook to the car to make the measure if you DIY. Otherwise you have to go to a shop that not only knows porsches but the 993 specifically. I'm pretty sure that car uses the expensive alignment stuff to set it up and most shops have no idea treating it just as any car. Iirc that KT effects camber and rear caster too. The factory settings were to make it safe and understeer. Racers do something else so you can get that throttle steer and use sphericals to take out the compliance from rubber bushings that are effected by these settings. In other words KT is less of an issue when you run sphericals. There should be plenty of Porsche guys who can help. I'm not a porsche guy just been around too many of them.

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        • #5
          Originally posted by JJ1 View Post
          Emilio, you said you like a ďcĒ curve on your cars? Meaning it toes out on squat and in on droop? I guess the question Iím asking is do you think the toe (kinematic or static) is a good place to look if you want to make your car more responsive to throttle steering?
          I didn't state a preference. What you are fishing for is weight transfer under steady state cornering. Better place to start is spring rate, sway bar rates and rake.

          Note that rake affects RC (Roll Center) to CG (Center of Gravity) relationship. As you lower a car, the RC drops faster than CG. So the leverage the CG has over RC increases as you lower it. This has the effect of lowering roll stiffness. So if you increase rake by lowering front more than rear, you have effectively increased rear roll stiffness relative to front. Roughly the equivalent to reducing front sway diameter more than you reduced rear sway diameter. Rake is a fine tuning adjustment, pretty much done last at the track. I prefer to keep effective rake near OEM so RCA (Roll Center Axis) remains as close to OEM as possible. Notice I wrote effective rake. A strut front will change its RC more than a multi link for the same amount of ride height change.

          A lot to factor in so, I'd suggest not messing with camber curves just yet. You are over thinking it. Get your spring rates dialed in for the RM (Roll Moment) you are generating with your tires and optimized for the track surface you are on. Grippier tires increase RM. Rough tracks require more compliance, softer springs. Then dial in sway bars for transitional response and steady state balance. Fine tune rake for changing track conditions. If all that fails to give you the balance you seek, then start playing with compliance bushings, KT and whatnot.

          The early Miatas have one strategically placed soft compliance bushing in the rear suspension that compresses and creates a wee bit of toe in under high cornering loads. This is not KT, this is compliance steer. One of the first things we do for track prep is to replace this softy with a firm one that matches the durometer of the of the other bushings.


          WWW.949RACING.COM
          SuperMiata

          Aside from their cost I never understood why people race them.
          But obviously I just dont get it. -fatbillybob

          Comment


          • #6
            Originally posted by emilio700 View Post
            The early Miatas have one strategically placed soft compliance bushing in the rear suspension that compresses and creates a wee bit of toe in under high cornering loads. This is not KT, this is compliance steer. One of the first things we do for track prep is to replace this softy with a firm one that matches the durometer of the of the other bushings.
            Can you share which bushing it is?
            www.TrackHQ.com

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            • #7
              Originally posted by ucfbrett View Post

              Can you share which bushing it is?
              Not legal in SM. Rear suspension, front lower, outer. Lower OEM durometer on that one relative to rear lower outer allows a bit of toe in.

              https://supermiata.com/energy-suspen...ngs-miata.aspx
              WWW.949RACING.COM
              SuperMiata

              Aside from their cost I never understood why people race them.
              But obviously I just dont get it. -fatbillybob

              Comment


              • #8
                Originally posted by emilio700 View Post
                I didn't state a preference. What you are fishing for is weight transfer under steady state cornering. Better place to start is spring rate, sway bar rates and rake.

                Note that rake affects RC (Roll Center) to CG (Center of Gravity) relationship. As you lower a car, the RC drops faster than CG. So the leverage the CG has over RC increases as you lower it. This has the effect of lowering roll stiffness. So if you increase rake by lowering front more than rear, you have effectively increased rear roll stiffness relative to front. Roughly the equivalent to reducing front sway diameter more than you reduced rear sway diameter. Rake is a fine tuning adjustment, pretty much done last at the track. I prefer to keep effective rake near OEM so RCA (Roll Center Axis) remains as close to OEM as possible. Notice I wrote effective rake. A strut front will change its RC more than a multi link for the same amount of ride height change.

                A lot to factor in so, I'd suggest not messing with camber curves just yet. You are over thinking it. Get your spring rates dialed in for the RM (Roll Moment) you are generating with your tires and optimized for the track surface you are on. Grippier tires increase RM. Rough tracks require more compliance, softer springs. Then dial in sway bars for transitional response and steady state balance. Fine tune rake for changing track conditions. If all that fails to give you the balance you seek, then start playing with compliance bushings, KT and whatnot.

                The early Miatas have one strategically placed soft compliance bushing in the rear suspension that compresses and creates a wee bit of toe in under high cornering loads. This is not KT, this is compliance steer. One of the first things we do for track prep is to replace this softy with a firm one that matches the durometer of the of the other bushings.

                I assumed the toe curve was not what I should look at to gain better throttle steering but it crossed my mind and using the toe curve as a tuning adjustment is something Iíve never heard discussed before (except trying to reduce it as much as possible). Next time I get the car out I will be sure to play with the sway bars. Thank you for putting so much into that post. Iíve never considered relative front/ rear RC changing differently based on suspension type.

                Comment


                • #9
                  Originally posted by emilio700 View Post
                  A lot to factor in so, I'd suggest not messing with camber curves just yet. You are over thinking it. Get your spring rates dialed in for the RM (Roll Moment) you are generating with your tires and optimized for the track surface you are on. Grippier tires increase RM. Rough tracks require more compliance, softer springs. Then dial in sway bars for transitional response and steady state balance. Fine tune rake for changing track conditions. If all that fails to give you the balance you seek, then start playing with compliance bushings, KT and whatnot.
                  can you elaborate more on this? how would you calculate this matched spring rate?
                  what suspension frequency you would suggest to get in the ball park for tires like A7 or A052?

                  and for tuning rc height by adjusting ride height, is there a practical way to do this at the track? since changing ride height would change the alignment as well.

                  Comment


                  • #10
                    Originally posted by AlpineFD View Post

                    can you elaborate more on this? how would you calculate this matched spring rate?
                    what suspension frequency you would suggest to get in the ball park for tires like A7 or A052?

                    and for tuning rc height by adjusting ride height, is there a practical way to do this at the track? since changing ride height would change the alignment as well.
                    You you are overthinking it. I don't perform any math when adjusting rake. Universal rule that everything else being equal, increasing rake will free the car up. Decreasing rake will tighten it.

                    I also don't do any math to calculate bounce frequencies. I tend to focus more on the simple ratio of front to rear spring rate. This ratio determines the basic steady-state balance. Then using whatever experience I have with that platform and estimating roll moment, we raise or lower all of the spring rates.

                    Weekend warriors that are diving deep into bounce frequencies thinking it will solve a problem, usually are not factoring in the amount of flex from chassis, control arms, bushings, subframes, wheels. These are all undamped. They also do not know the absolute spring rate of the tire they are using. These variables are all accounted for when a pro team puts a car on a 7 post shaker rig. Precise engineering data from the tire manufacturer is factored into the simulation models.

                    The weekend warrior would be better served learning how to simply feel what the car is doing, understanding basic vehicle dynamics and learning how to tune at the track.

                    https://trackhq.com/forum/the-equipm...ng-terminology



                    WWW.949RACING.COM
                    SuperMiata

                    Aside from their cost I never understood why people race them.
                    But obviously I just dont get it. -fatbillybob

                    Comment

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