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The finite dynamics of car tuning for the track...

Discussion in 'Other Racing' started by rob lay, Apr 6, 2004.

  1. rob lay

    rob lay Administrator
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    Dec 1, 2000
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    Rob Lay
    I think others can contribute to this thread more questions to be answered, but I'll start.

    Tire pressures - I always thought you want more pressure in the front tires to dial in more oversteer or less understeer. On my SRX7 this past race weekend I was running 27 back pressure and 29 front pressure cold. Hot pressures were 31 back and 35 front. The car was pushing like you wouldn't believe. Just for an experiment in practice I went opposite by going 28 in back and 27 in front cold. Once the tires warmed up the car handled better than it's ever handled before (I've always started with higher front pressure). The warm pressure would end up pretty balanced at 32 back and 33 front. My lap times showed it to as I took another second of my previous setup's time.

    What the heck?

    Have I been wrong the entire time thinking more pressure in front meant less understeer? Is it really the opposite as it seems my experiment shows that by having less pressure in front I'm actually getting more steering?

    Thanks,
    rob
     
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  3. Texas Forever

    Texas Forever Four Time F1 World Champ
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    Apr 28, 2003
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    Texas!
    Are you sure that you know the front from the rear? I've heard stories of you going backwards. :)
     
  4. 62 250 GTO

    62 250 GTO F1 Veteran

    Jan 9, 2004
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    Neil
    If you want more bite up front, take some air out. If you want to loosen up the back end, put some in.
     
  5. 62 250 GTO

    62 250 GTO F1 Veteran

    Jan 9, 2004
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    I have found that if your car doesn't handle the way you want it to, making small changes one at a time is the best way to sort out your trouble. Also if the car is almost where you want it but not quite, you may have to start over from scratch to find the best setup for your car. ie: You just can't "tweek" a stock car, all the way up to a perfect setting.
     
  6. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,058
    There is a point at which adding more air pressure starts lifting the contact patch and reducing the amount of rubber on the road beyond this point more air pressure only reduces traction.

    The way to tell is to get a pyrometer and use it to measure the temperature profile on the tires. You would be looking for an even spread from the outside to the inside of 15 dF to 20 dF where the center is the average of the outside and inside readings. That is the correct tire pressure for that alignment. Use a probe tiped pyrometer to poke into the tire tread or cords to get the real temperature not an infrared pyrometer. In the best case, enter the pits after a set of hot laps and measure the temps immediately upon stopping in the hot pit area. A cool down lap will make pyrometer readings unuseful.

    Once you find the profile, if the center is not the average of the inside and outside, then add or subtract pressure to bring it into the average. Once the air pressure is correct, the gradient of the profile tells you about the camber and toe. Gradient too small -> more negative camber and vice versa. {{Can't remember how to use pyrometer to set toe}}

    The reason the outside temperature is lower is that it gets more cooling form the air running by the car, while the inside is shielded from the wind. Formula cars may or may not use profiled temperature gradients.
     
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  8. Schatten

    Schatten F1 World Champ
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    Apr 3, 2001
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    You have more racing/track experience than I, but you don't tune the front pressures for oversteer/understeer characteristics. Generally, you tune the front pressures for optimal turn-in/steering characteristics and then adjust the rear for understeer/oversteer characteristics. That's at least what I've always been taught and practiced.
     
  9. Brian C. Stradale

    Brian C. Stradale F1 Rookie
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    Mar 17, 2002
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    There is a point of pressure that maximizes traction... lower or higher results in less traction. Very often with stiff R-compound tires, the point of maximum traction is the lowest pressure such that the tire doesn't start to rollover onto its sidewalls. Hence the common trick of putting shoe polish on your sidewalls and lowering pressure until you start rubbing off the shoe polish. You can do somewhat better using a pyrometer to compare inside, center, and outside temps.

    Once you have all four tires at max traction, then you get a feel for over/under-steer. To fix, you add pressure to the front to correct oversteer and add pressure to the back to correct understeer. Again, you can do somewhat better using a pyrometer to compare temps front to back.
     
  10. writerguy

    writerguy F1 Veteran

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    #8 writerguy, Apr 6, 2004
    Last edited by a moderator: Sep 7, 2017
  11. ctkellett

    ctkellett Karting

    Jan 2, 2004
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    Chris K.
    I ran an SRX7 for the past four years in the MARRS series and I may be able to help you here. Which tire are you using in your class down in Texas? Toyo or Kuhmo? I am going to make a few assumptions here that will more than likely bite me in the but later. I guess that the car has been scaled and alined in the near past and that we are using new, shaved tires. That said- Where are you having the problems, fast corners or slow ones? Are you running a rear bar? Are you sure that the rear is 100% straight and that the 4 rear links are not binding? IF you go too low in the rear they will bind and the car will do some weird things in the corner. The 81-83 rears used a thiner axles than the later ones (84-85) and they do not take a beating well. What Camber, caster and toe settings are you using? As the others have pointed out , tire temps will fill you in on what should be happening tire temp wise, It would be wise to spend a day doing this once the car is sorted. I remember that the Kuhmos were looking for a hot pressure reading of 36psi and the Toyos were a bit lower, 31-33 psi. The Kumhos fell off pretty fast after a few laps of betting them too hard, and the Toyos were a bit more forgiving.
     
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  13. ctkellett

    ctkellett Karting

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    I remember the days well, interesting line thought the corner for a few of them.
     
  14. rob lay

    rob lay Administrator
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    Thanks ctkellett! Good to see another SRX7 racer on here. The #12 in Jim's picture is my car.

    We're running Toyos now. I love them, as you say they can be beat up more. The #12 is a 85 and I do run the rear sway bar. Camber is at our max by rules at 1.5 degrees. Zero toe and I think my caster is 4 degrees.

    The corner I was pushing in was a very slow corner. It only happened last half of the race when the tires were the hottest.

    I'm thinking my fronts were up to 36-37 pounds and back was only 32. Hearing what has been said about Toyos and lower pressures on R tires, I think the combination of having hot pressure closer to 32 pounds and not having such a big increase between back and front is what improved the car so much.

    I've just been wrong about my assumption the past 3 years. You think after 10+ AX's and 50+ club races I would of figured it out by now. I've been the fastest SRX7 the past three race weekends, they better really watch out for me now! :)

    More SRX7 pics (my #6 and old #12 R.I.P.)...
     
  15. ctkellett

    ctkellett Karting

    Jan 2, 2004
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    Chris K.
    Ouch! I have been down that road as well, I feel your pain. Once you get the tire pressures sorted out I would try running without the rear bar. The car likes a bit of slip angle and we have found it much faster thought fast turns (like turn 10 @ Summit Point) which is a better trade off for lower lap times. Most of the 1st gen rx7s that I know (ITA & SRX7) have long since lost the rear bar all together, it is a must lose in the rain. Unlike the Kumhos, the Toyos seem to get faster the more heat cycles you get into them, and we are getting 2-3 times the life out of set.
     

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