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How important is F/R weight ratio?

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  • How important is F/R weight ratio?

    240SX's tend to run around 55% weight on the front. I finally got my total weight down to 2280# within the SM ruleset but I still have the 54% on the front. I am considering some drastic measures to put more weight on the rear. There is nothing easy about moving weight rearward and my ideas are going to draw some controversy in SM. 2280 is my legal minimum weight.

    So, Is it worth the trouble or not? Will the car handle better at a better F/R ratio or can I tune the suspension to work just as well where the weight is now. I believe I can get to 51 or 52% on the front. Is 250# more weight on the front really that big a deal?

  • #2
    not as important as having that weight close to the center. You could have ballast in the trunk to make it any distribution, but that'd make it a more neutral slow-turning car.


    • #3
      Depends on the usage, but in general it's going to make your front tires work harder, and it's going to tend towards understeer all things equal.

      I wouldn't add weight, but what are your ideas for moving it back?

      When I could move weight, I usually choose a more central location rather than sticking it way in the trunk. Less polar moment of inertia, and more importantly it lowers the CG that little bit. Thinking things like battery.
      '18 Chevrolet Volt - Electric fun hatch for DD duty!

      DefSport Koni Sleeve and Spring Perch Buy!!!


      • #4
        But Def aren't all fixed weights considered as one big piece with one CG ?

        Like Def said I wouldnt add weight but I must say having a car 50/50 feels great. Example Driving a Miata or a Civic both cars are around the same weight, but the fact that the CG of the miata is 50/50 F/R makes its much more nimble then my civic. And like Def said, it also influences the tire loads.

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        • #5
          GC, moment of inertia, and weight distribution are all different things.

          A car with a 50/50 weight distro has the center of gravity in the middle of the car. A car with a 60f/40r has the CG closer to the front axle. (read FWD car)

          Moment of inertia can be easier understood this way.... A 15" wheel with a 24" tire and a 18" wheel with a 24" tire both may have the same weight, but the 18" wheel will take more energy to start and stop because the moment of inertia is higher- the mass is farther from the center of gravity (which is the center of the wheel).

          Make sense?
          Originally posted by SoSideways
          I don't care what color they are as long as they are LONG AND HARD.
          '04 G35 Sedan 6MT- The DD
          '96 240SX- The Track Toy


          • #6
            Personally, I wouldn't add weight to get a better weight distribution. Unless you were under the minimum weight limit and needed the weight anyway.

            Is there room to move your engine back any at all?

            Have you already posted somewhere what you have done to get your car down to 2280lbs?


            • #7
              Haven't done anything crazy to get the weight down, just the normal things. I haven't even done anything with the stock headlights yet. I have my back seat in also.....sort of. It looks like a back seat but there is no metal and very little foam left. I love to see the reaction when I go somewhere new to autox and my seat is still there.

              I have set my engine back 1".
              My battery is a Harley 9# on the shelf behind the back seat.

              The easy moves to put weight towards the middle are done.
              I was very surprised to see my weight was 54% front after the 12.5# hood but I put the rear Z32 uprights on at the same time so I lost 10# back there to the 28.5# loss in the front.

              My car does turn pretty well, 1.27 G's at a recent autox but don't really know if that is a lot. A Miata did 1.5 on the same course but I accelerate better and beat him. I can't help but think; What if I could accelerate like my car and turn like the Miata? I wonder how to get that. Thus the above question. The Miata won Nationals in CSP in 08.
              Last edited by logr; 06-04-2010, 08:57 AM.


              • #8
                My club is having an Evolution Driving School this weekend and the guy that won CSP in the Miata in 08 is interested in co driving with me the day after. Maybe I can learn a bit this weekend. He built the Miata himself, Brian Peters.


                • #9
                  What about adding caster to increase the wheelbase a little bit?

                  I know that's what the Mercedes team did on their F1 car couple of races ago and they managed to move the weight around a little bit by doing that.


                  • #10
                    A lot of classes must have a speficied wheelbase. Mine's listed in my logbook. That said, I'm not sure how close it has to be. I'm running more caster than factory, but not a ton. not sure exactly what the wheelbase is on my car opposed to what it's supposed to be.

                    As said above as well... I would rather have less mass unevenly distributed than I would added weight and perfect balance.

                    logr, how did you get the engine moved back? Are you still running the KA? (I forget who has what!) Any clearance issues against the firewall? i.e. how much of a PITA will it be if you have to pull the tranny?
                    Originally posted by SoSideways
                    I don't care what color they are as long as they are LONG AND HARD.
                    '04 G35 Sedan 6MT- The DD
                    '96 240SX- The Track Toy


                    • #11
                      The tranny is kind of a pain either way but I still think I can get it out. It is hard to tell I moved the engine by casual glance. I moved the mount bracket on the subframe to move the engine. I also lowered the mount to make up for the Nismo mount height. I have an SR.

                      I am not even considering adding weight. I will go to great lengths to move things around and put things where they don't belong if it will make a big difference.
                      I still need lighter front brakes.
                      Power steering, drivers and radiators don't necessarily have to be where they start life.

                      The car does NOT understeer. Oversteer/traction is always an issue. I usually only run 10-13 lbs on a T28 with eveything else stock, puter/maf/injectors/etc. It will accelerate evenly with a Z06. We have tried it numerous times.

                      I didn't like much castor. I keep it around 5 now to make the turning more responsive. I also don't seem to need a ton of camber because of the castor and I check it regularily with a pyro. The rears wear faster than the fronts and I have been going down on camber there due to pyro readings and traction.

                      The ABS stopped flatspotting completely so I tend to get 80-90 runs on A6's but the best runs are below the 50 mark. I had on 75 run tires when I did the 1.27 G reading. My Z06 friend had on equivelant tires and did 1.1G's.

                      My fuel tank , seat and wheels are all very light.

                      I run the Kodiak's and 275/35/15's so wheel moment is low and the total weight is under 35#. This also puts my car on the ground without crazy lowering. My fender heights are front 25.3" and rear 24.3" or less.

                      Did I answer all of the questions?


                      • #12
                        For front brakes, you might think about some aluminum hats and 3/8" thick solid/drilled rotors and throw a set of Wilwood calipers over them. you should be able to save 15+lb/corner over stock brakes.

                        something like these..
                        Rotor No: 160-3455, rotor, rotors, carbon ceramic, carbon ceramic rotors, iron rotors, steel rotors, rotor hats, directional vane, staggered, straight vane, srp drilled, disc rotor, drum rotor, internal parking brake, steel rotors, aluminum rotors, bedding, bedding-in, mounting, mount

                        Team them with some tiny Dynalites or Superlites and you've got some pretty light brakes.
                        Part No: 120-6813, calipers, dynapro, dynapro 6, big brake, dynalite, dynalite pro, forged dynalite, d52, superlite, sl6, sl4, tc6r, promatrix, w6ar, rotors
                        Last edited by Matt93SE; 06-04-2010, 02:17 PM.
                        Originally posted by SoSideways
                        I don't care what color they are as long as they are LONG AND HARD.
                        '04 G35 Sedan 6MT- The DD
                        '96 240SX- The Track Toy


                        • #13
                          Is the weight distribution you reported with you in the car?
                          If not, how big a difference will you being in the car make?
                          Wheelman (Ken Wheeler)
                          73 240Z, LT1, 5-Speed
                          92 240SX SE, KA24DE-T


                          • #14
                            With Driver
                            front 1360#
                            rear 1120#

                            Talking with Brian Peters today, he didn't think F/R was all that big a deal. It's more just tuning to make it work.

                            Tom Kotzian (Nats SS Champion for 09) drove it today and absolutely loved the car. He thought it was very neutral handling and that I shouldn't change a thing. He really liked the no lag and plenty of power thing with power steering and ABS on top. He is driving it at autox tomorrow and it may also be driven by Ryan Buetzer(sp) because Tom raved so much that Ryan had to try it and now he wants to drive it tomorrow too. We are possibly having rain so no telling how things will go. It is gratifying to have this caliber of driver think it is pretty well set up. Now I just have to learn how to drive the thing........................... but I still think it can be better. I have more ideas, don't I always.


                            • #15
                              The 4% difference in negligible at best. I wouldn't worry about it. If the car is neutral and fits your driving style, drive it as is. Cross weights make a bigger difference when all is said and done.
                              Tristan Smith
                              T-Rex Racing
                              SCCA ITR 300zx #56
                              Former SCCA ITA 240sx #56
                              IMSA RS 1978 Datsun 200sx #58