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TR-6 autocross car turn-in response

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  • TR-6 autocross car turn-in response

    I am seeking information that will improve the turn-in of a modified TR-6 autocross car. The caster at the front end has been increased to 6 degrees by modifying the shape of the upper control arms and by relocating the bolt holes in the frame mounting brackets for the lower A arms. Care was taken to make sure the geometry changes did not alter the relative positions of the rack and pinion relative to the pitnam arms, so in other words when the car is at nominal ride height the tie rods are parallel to the ground. The current front-end alignment is toed out 1/16 of an inch and camber is -1.5 degrees. Front springs are supposedly British Leyland ralley springs with one coil cut off and closed. The front sway bar is a 7/8 inch diameter Addco and the shocks are the adjustable Koni dampers that were manufactured as after market replacements for the originals. All of the preceding changes helped turn-in but I am still looking for more.

    The car is powered by a normally aspirated 3.8 liter Buick V6 with loads of mods, and the trans is a Muncie M21 alloy-case close-ratio unit. The car has a roll cage chassis stiffener, the differential is a Dana-Spicer model 90 from a Corvette, and the rear suspension is custom-designed three-link IRS with custom-made half shafts and Summers Brothers stub axles. Without the driver the wheel scales indicate the weight is distributed 48.5% front 51.5% rear. More details and photographs are available in condensed .pdf format if desired.

    Regards,

    Jim Prettyleaf
    Cupertino, California

  • #2
    I am seeking information that will improve the turn-in of a modified TR-6 autocross car. The caster at the front end has been increased to 6 degrees by modifying the shape of the upper control arms and by relocating the bolt holes in the frame mounting brackets for the lower A arms. Care was taken to make sure the geometry changes did not alter the relative positions of the rack and pinion relative to the pitnam arms, so in other words when the car is at nominal ride height the tie rods are parallel to the ground. The current front-end alignment is toed out 1/16 of an inch and camber is -1.5 degrees. Front springs are supposedly British Leyland ralley springs with one coil cut off and closed. The front sway bar is a 7/8 inch diameter Addco and the shocks are the adjustable Koni dampers that were manufactured as after market replacements for the originals. All of the preceding changes helped turn-in but I am still looking for more.

    Jim,

    Wow that's quite a beast. My father has raced a Jag E-type w/a Chevy SB for the past 35 years (first in SCCA, now vintage) and I have crewed for him during the whole time, so I can sense some of the challenge you are facing. As you probably know, your front settings (castor, camber, toe-in, springs, etc) have a great effect on handling (and turn-in). While it appears that you have been tweaking the possible settings on your front suspension, I sense that you feel that you have optimised that end of the car as much as you feel you can. Perhaps you should now go to the rear end of the car and start making some changers at that end, as rear settings also have effects on the handling and turn-in. Remember to follow good practise and change only one item at a time when testing, so that you can properly evaluate the change. Some racers have found an available computer program, SusProg3D ( http://www.bevenyoung.com.au/suswin.htm ) helpful. The program lets you step the supension through the possible angles and "see" wh

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