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  • square set-up or not?

    Since I started in OTA I have run a square tire set-up on my Mustang (275/35/18).
    I like it on the car. Very little if any understeer and just a little, but controllable (most of the time) oversteer.
    I have seen Mustangs at other events with bigger tires on the back. And I have room. Just wondering if going to 285's on the rear might be perfect?
    So I'm in the research stage. I'm sure others have done this. Was it a good move or should I be content with the set-up I have since it works pretty well?
    Kelly B. GT Class Competitor 2011 - 2014
    OTA Director 2015
    ASN/FIA Committee 2015

  • #2
    Re: square set-up or not?

    I always prefer square to allow front to rear rotation to maximise tire life.I corded all 4 hoosiers equally at the last event of the yr,hate to just throw 2 away.

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    • #3
      Re: square set-up or not?

      Originally posted by OTA Director 2015-2016 View Post
      Since I started in OTA I have run a square tire set-up on my Mustang (275/35/18).
      I like it on the car. Very little if any understeer and just a little, but controllable (most of the time) oversteer.
      I have seen Mustangs at other events with bigger tires on the back. And I have room. Just wondering if going to 285's on the rear might be perfect?
      So I'm in the research stage. I'm sure others have done this. Was it a good move or should I be content with the set-up I have since it works pretty well?
      I would think the bit of oversteer you're getting is more driving style and that going to a 285 rear will probably slow the car down in most tight corners
      sigpic

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      • #4
        Re: square set-up or not?

        Subarus are picky about same diameter tire on every corner, so I always run the same size all around. However, I'll add spacers to the front or rear where I need more grip. (run the narrower track at the end I need less grip)
        sigpic

        Stephen, SPDA VP, OTA Director, CCC Member
        OTA: SGT1 ! -=- CSCS: SSA #842

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        • #5
          Re: square set-up or not?

          Tire set ups are like brake pads: hundreds of options and no 'right' answer (only "what's right for you"). Obvious considerations are cost, extended tire life with fully square set ups, speedo/traction control error, or the need to only bring one spare.

          Two factors that jump to mind based on your question:

          1) a side effect of increasing the front contact patch width is the (possible) negative impact on steering feel. The wider you go, the wider the effective front track width but that also impact the scrub radius of your tires. Scrub radius is the distance between the steering axis inclination and the center point of the contact patch. A greater scrub radius will act as a longer lever for changes in wheel direction. This is more of an issue for front wheel drive cars where change to the scrub radius will exacerbate torque steer, but depending on front suspension geometry it could worsen steering feel (which impacts confidence; and with 300+ hp at MIR, its all about confidence).

          2) wider tires need to be properly sized to the rim width. You mention going up to 285 at the rear, is that on the same sized rim? If so, the marginal increase in width will almost certainly be negated by the less than ideal rim width to contact patch.

          I debated going with a square set up, but ultimately I chose to simply up-size the OEM staggered and raked set up (245/18 285/19 to 265/18 305/19) despite it leading to extremely limited tire choices because I just happen to like the look of 'rake' on the car. For me, it all came down to vanity.
          Greg.

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          • #6
            Re: square set-up or not?

            Front-rear tire size stagger is very dependent on the vehicle - its drivetrain and its weight distribution especially.

            In pure cornering, to create the same slip angles/tire loadings at each end of the chassis, the tire sizes should be biased approximately the same as the weight distribution of the car, which for Kelly's Mustang is maybe 5-8% front-heavy or so. So in this case, the 'square' setup is actually a bit rear-biased to begin with, and should contribute a little bit to understeer.

            Under power though, there is not very much rear grip margin to allow early power application on corner exit, so wider rear tires might improve acceleration out of corners, at the expense of a little extra drag and a bit more understeer. This would be a problem in autocross, but not on a road course. Since the 'Stang is not exceptionally powerful, the square setup is probably pretty good overall. With wider rear tires, you might gain a little lap time, and the handling would feel more stable and a little less crisp.
            Another quality post from Black Dwarf Racing

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            • #7
              Re: square set-up or not?

              ^^ I would add spring rate bias (based on wheel rates, not measured spring rate) to that as well.

              IMO, unless you're willing to play with spring rates to fine tune the balance (or are roasting rear tires all the time) I wouldn't stray from square sizes.

              Some cars are limited to how much tire they can get up front, but struggle with putting power down, but I don't think you have that problem?

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              • #8
                Re: square set-up or not?

                Originally posted by wparsons View Post
                ^^ I would add spring rate bias (based on wheel rates, not measured spring rate) to that as well.

                IMO, unless you're willing to play with spring rates to fine tune the balance (or are roasting rear tires all the time) I wouldn't stray from square sizes.

                Some cars are limited to how much tire they can get up front, but struggle with putting power down, but I don't think you have that problem?
                Yep, just trying to "fine tune" a bit. Always looking for that extra bit of performance. I do have suspension and Scott M. looks after the set-up/corner balance. And I do actually have to be careful on corner exit when applying the power so I don't lose grip - which is why I though a little more rear tire tread might make some difference.
                I spoke to a guy at another track day event who had a Boss 302 with 295's up front and 305's out back. Granted his car was pretty serious stuff, but same principle I think.
                Kelly B. GT Class Competitor 2011 - 2014
                OTA Director 2015
                ASN/FIA Committee 2015

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                • #9
                  Re: square set-up or not?

                  Corner exit could also benefit from softer rear spring rate or even better, maybe a smaller rear bar. I put down power way better in my E30 with the rear bar disconnected. You may want to play with that if you like experiments.
                  _____
                  BMW E30 (#84) | Honda Civic (Chumpcar - Scrap Denial #19)

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                  • #10
                    Re: square set-up or not?

                    Kelly, I don't know that a wider rear tire will help put down power unless the you can lower the rear pressures a lot. The contact patch total area is determined by air pressure and side wall stiffness. The shape of the patch is more determined by tire width.

                    Ever notice how skinny top fueler's tires get when they spin them up? They are low pressure and the contact patch is not overly wide but it is very long which seems to help with longitudinal traction and they are dealing with whole orders of magnitude more power than we are.

                    Personally I would stick with a square set up for your car
                    Mobil 1 Time-Attack # 4, CCC Member

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                    • #11
                      Re: square set-up or not?

                      My 2 cents from running various Mustangs on track over the years ... unless you have a huge tire budget, you're better off with a square setup. They will always eat the outer shoulder of the fronts, even with lots of camber, so the ability to rotate front to rear is important to get maximum life. A 10 mm increase in section width isn't going to accomplish enough to make a difference. Play with pressures, stab. bars, springs & shocks, etc. before worrying about trying a slightly different rear tire size.


                      cheers
                      Ed
                      84 Mustang SVO
                      95 Jaguar XJR

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                      • #12
                        Re: square set-up or not?

                        Originally posted by fast Ed View Post
                        My 2 cents from running various Mustangs on track over the years ... unless you have a huge tire budget, you're better off with a square setup. They will always eat the outer shoulder of the fronts, even with lots of camber, so the ability to rotate front to rear is important to get maximum life. A 10 mm increase in section width isn't going to accomplish enough to make a difference. Play with pressures, stab. bars, springs & shocks, etc. before worrying about trying a slightly different rear tire size.


                        cheers
                        Ed
                        Agree with Ed, and we both came from the same school of racing, Old School.

                        JohnP

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                        • #13
                          Re: square set-up or not?

                          Originally posted by John P View Post
                          Agree with Ed, and we both came from the same school of racing, Old School.

                          JohnP
                          I like Old School
                          Thanks for the advice all.
                          I'll stick with square set-up.
                          Just as well - have some life left in my current ZII's.
                          Kelly B. GT Class Competitor 2011 - 2014
                          OTA Director 2015
                          ASN/FIA Committee 2015

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                          • #14
                            Re: square set-up or not?

                            What if you went to 305's on the rear!
                            sigpicFrank - OTA #259 retired. I'm in BC now!

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                            • #15
                              Re: square set-up or not?

                              Originally posted by nissannx View Post
                              What if you went to 305's on the rear!
                              Believe or not - they would fit! (just) - but then there's new rims etc....
                              Maybe if I ever revive my drag racing days of yore.
                              Think I'll stick to the 275's.
                              Kelly B. GT Class Competitor 2011 - 2014
                              OTA Director 2015
                              ASN/FIA Committee 2015

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