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    Sprint rates for circuit (timeattack)

    I know I'm opening a can of worms and it's all subjective but I'm really trying to figure out what would spring rates would be the best for a track dedicated car.

    The car is Alfa Romeo 156 2.0 which has a thread in racers forum here and it's not the most common car therefore it's hard to find a lot of folks racing them locally in QLD. I mean there are a few folks running V6 versions but none of them did look into spring rates too much.

    Before I go any further I'd say that front motion ratio is ~0.75 so it does differ a fair bit from usual McPherson struts.

    NSW and VIC do have a special series just for these cars and have controlled suspension with 10/7 kg/mm springs which equates roughly to 1.3 Hz and 1.82 Hz front and rear suspension frequencies respectively. But they also have control tire, and from memory that's 205 mm wide 595 RSR or something similar.

    I plan to go for R compound semis next year, likely to be Nankangs AR1.

    The general consensus is that 2 - 2.5 Hz is suitable for a circuit car so 1.3 seems a bit on the low side. I also checked their photos and I believe that they have too much body roll.

    I've checked what some coilover companies offer

    stock 3.5/2.65 kg, 0.77/1.12 Hz
    BC Racing 10/3.5 kgг, 1.3/1.29 Hz
    Yellowspeed Dynamic Pro Sport 12/5 kg, 1.42/1.53 Hz
    Yellowspeed Clubsport/XYZ Racing Tarmac Rally 16/8 kg, 1.64/1.95 Hz
    Yellowspeed Premium Competition/XYZ Circuit Racing 20/14 kg, 1.84/2.57 Hz

    While 20 kg seems high, the motion ratio of 0.75 brings it down to fairly reasonable wheel rate.

    I currently have 11.5/5 kg springs and is somewhat happy. But what if I'm missing something? Going from ST suspension kit with almost stock springs rates to 11.5/5 did improve my lap times a lot.

    What rates seem reasonable for QLD tracks? QR and Lakeside aren't particularly smooth in some parts.

    The car currently has no aero but I plan a splitter and rear spoiler soonish. Apart from that - I have polybushes all around and strut braces.

    I would've tried just changing springs but my coilovers are shot so I need a new set. And a new set can be valved to match spring rate of my choice hence my attempt to get in the ballpark at least.

    Or should I get actual damper/wheel travel figures first?
    Last edited by warsch; 08-11-19, 10:30 AM.

    #2
    Motion ratio is just the start. How did you get the wheel frequencies? You need measurements of the swaybar, sprung and unsprung corner masses at minimum. If you want to listen to some concepts, check out FCM (Fat Cat Motorsports) YT channel. He also goes through to measure on an E46 BMW which is probably very similar and directly applicable to the 156. He firmly believes in "flat ride" which is basically equal or a little lower front wheel rates between front and rear to avoid things like pitching and jacking which make as much different plodding along on the hwy and track performance (apparently), and on a MacPherson strut that usually means introducing a stiffer front roll bar. He also explains valving for a given spring rate is a misnomer. Usually for R-comps/semis he only revalves a specific profile and leaves the rest as is, I always forget between high/low.

    The control series is probably based on a soft corner spring/big bar concept to bring back some roll stiffness, not the best on bumpy tracks. For QLD tracks, I'd be weary of this.

    For functional aero, you want the car to be efficient and that almost always means front splitter as low as possible to the ground. This more often than not messes with the MacPherson strut roll centre. You'd probably need to offset the lower control arm(s) to correct roll centre.
    Last edited by gmx; 09-11-19, 10:31 AM.

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      #3
      I got the frequencies using the formula. I did omit sway bars in my calculation, but got the corner weights off the internet specs, weighted some parts of sprung weights and guesstimated the rest. I've tweaked weights and the frequencies remained within 0.1, so I believe it works for initial ballpark.

      I'll check that YT channel, thanks.

      As far as I recall, control series allows for sway bar change, but there's not much options available off the shelf anyway.

      Great point about aero, I didn't quire realize that. 156 has double A-wishbones in front and multilink at the back, but I still need to measure/calculate roll centers.

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        #4
        If you're serious about measuring up all the ratios, rates, sprung and unsprung weights, the I can send you an excel spreadsheet that I've created that will calculate it all out for you in terms of sprung and unsprung ride rates, and also roll deg per g of cornering plus front/rear weight transfer percentages.

        I created this all based on Miliken's book 'race car vehicle dynamics'

        Will allow you to perform a lot of what if checks on spring and bar rates.

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          #5
          Oh, and your ride rates seem a bit low for examples you've posted. Id say your motion ratio calcs are way off. As you've suggested, Try 2-2.5 hz unsprung ride rate as a start for car with minimal aero and r comps for supersprint/hillclimb/rallysprint/purposes.

          I can say you'll almost certainly not generate any real down force unless you go a deep front air dam and undertray, sideskirts, rear wing to balance it all plus louver vents on bonnet. Otherwise you're just tinkering around the edges and reducing lift a bit.

          You can test car sprung ride rate easy enough if you can disconnect dampers. But not easy on most setups.
          Last edited by 200MPH; 09-11-19, 05:07 PM.

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            #6
            I've build a spreadsheet based off eibachs suspension calculation web page, I'm pretty sure it's the same as Milliken's.

            =187.8 * SQRT (( Spring_Rate (N/mm) * Motion_Ratio^2) / Corner_Sprung_Weight ) / 60

            I haven't calculated the rest yet, but I'm really keen to. Understanding it all is part of the fun for me.

            I'd be happy to get your spreadsheet to check my formulas against.

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              #7
              I might try contact Dave Fillipetto, he ran a diesel version, no aero on R comps back in the production era of the 12hr and he might give you a baseline to work from. You should be changing springs to suit each track or for change of conditions anyway if you're totally serious rather than the 1 size fits all set-ups, that you look like you intend using. Plugging numbers in to a calculator is all very well and good, by the time you've measured CoG, bar rates and sprung and unsprung weights, you might be further ahead by just sucking and seeing with a few hundred bucks worth of springs anyway.

              Comment


                #8
                I am not opposed to buy a few extra springs to find the perfect match, though I might add that measurements and calculations are free.

                Now as for changing springs for each track, it would be really problematic. This year I had 20 track days, so changing springs would not be practical at all.

                However, as I said, I really want at least to get into the ballbark and then go from there.

                Comment


                  #9
                  Originally posted by warsch View Post
                  I've build a spreadsheet based off eibachs suspension calculation web page, I'm pretty sure it's the same as Milliken's.

                  =187.8 * SQRT (( Spring_Rate (N/mm) * Motion_Ratio^2) / Corner_Sprung_Weight ) / 60

                  I haven't calculated the rest yet, but I'm really keen to. Understanding it all is part of the fun for me.

                  I'd be happy to get your spreadsheet to check my formulas against.
                  Pm me your email and I'll flick you some spreadsheets if you like.

                  Comment


                    #10
                    Thanks 200MPH. Your spreadsheet helped me find the error in my calculations due to unit conversions.

                    Now that I have correct frequency figures, it seems like my current rates are about right so I can just go from there.

                    The correct figures are:
                    stock 3.5/2.65 kg, 1.2/1.8 Hz
                    BC Racing 10/3.5 kgг, 2.1/2.1 Hz
                    Yellowspeed Dynamic Pro Sport 12/5 kg, 2.3/2.5 Hz
                    Yellowspeed Clubsport/XYZ Racing Tarmac Rally 16/8 kg, 2.6/3.2 Hz
                    Yellowspeed Premium Competition/XYZ Circuit Racing 20/14 kg, 2.9/4.2 Hz

                    Comment


                      #11
                      I've never bothered with all those textbook equations, but have a couple of rules of thumb that seem to work.

                      Generally with any decent modern car, about two to three times the stock spring rate is as high as you need/want to go for semis and track days (without aero). I've only had one car where I was over three times the stock spring rates. This seems to be regardless of the type of suspension used - ie includes cars with strut, superstrut, and double wishbone front suspensions, and multilink, semi-trailing and strut rear ends.
                      Yours are about 3.3xF and 1.9xR. So your fronts are probably as high as you'd want to go in my experience, and the rears are probably on the soft side, which brings me to my second point...

                      Something near the factory F:R balance is generally about right, also regardless of layout/platform. For example, out of all my cars, FF Corolla AE111, MR SW20 MR2, and FR 180SX, JZA80 Supra, JZZ30 Soarer and VZ Commodore to name a handful, the widest I've gone away from factory spring balance was 2.5xF and 3xR on the FWD AE111. And that was wildly different from anything an off the shelf setup provided, so I wouldn't put too much stock in what the aftermarket seem to recommend. Eg common off the shelf rates for that car were 6/4kg - that's 2.5 and 1.75 times the stock rates. I started with those, then added a big rear swaybar to try and reduce understeer, then went to 6kg rear springs and finally 7kg rear springs.
                      Yours are 3.3xF and 1.9xR, which is a long way from the stock balance. I'd expect that to be very understeery on a FWD car. Normally you need to increase the rear roll stiffness more than the front to dial out the inherent FF understeer.
                      That's my 2c.
                      Last edited by hrd; 18-11-19, 09:10 AM.

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                        #12
                        Thanks hrd. You're spot on that rear springs need to be firmer. While I think that the car's balance is neutral, I may very well be wrong due to the lack of skills.

                        I was able to find out that one of the local Alfa guys used to run 11.5/16 springs and UK guys who build and rent Alfas also run 11.5/15 springs. So that seems to be the consensus.

                        Thus I'm definitely keeping the fronts where they are, not too sure about tripling the rear rate from my current setup, I'm a bit of a chicken here as I'm not sure I can handle oversteer too well. I'll likely follow your advice and go 3x stock rate so 9-10 kg.

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                          #13
                          Great food for thought in this thread. Thanks

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                            #14
                            Agreed. I'd never calculated the wheel rate before but it turns out I'm at about 2.4/2.5Hz on my FD with 17/14kg springs, so in the right zone once I've added some mild aero.
                            LS Powered FD3S RX7

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