Cal-tracs or Slide-a-links: Tuning for open track use?

[CDMBill]

As a leaf spring rear suspension car with do it all pretensions, I have a *******ized rear suspension with Calvert Split mono-leaf springs, Afco double adjustable shocks and Cal-trac traction bars having removed the Slide-a-links as they negatively affected 60 ft. times at the drag strip. At last years RTTC I ran a 2.08 best road course time which included a spin in turn 1, Lap 2, (Pink Hat for me) which was upper mid-pack.

Does anyone have advice on settings for this type of rear suspension? I set the shocks at 60% firm on compression and 40% firm on rebound for last years' event which was my first open track event with the car. I set the bars with a 1/4 turn of preload and did not experience any wheel hop or brake jacking that I could feel.

Front suspension is relocated upper a-arms, Total control lower control arms and strut rods, 750 in"/lb springs D/A shocks set at 50/50. Baer 6S brakes all around.

My avatar picture is from car Craft's coverage of the event. Its a very heavy (4250#) '71 Mustang convertible with a big motor. (10.0's on motor 1/4 mile uncorrected).

I'm currently running Nitto NT-01 275/40/18 on the front and Nitto NT 555R 305/45/18 Drag Radials on the back. New tires are in order for RTTC 2011 and the T56 will be back in it instead of the AOD with 4500 stall converter as it was last year if it matters. Rear end is a 9" with 4.11 Tru-trac differential for auto-x and road courses.

Thanks in advance.

[79T/Aman]

Spin out?, well it's not hard to spin out but it also may be an indication of oversteer caused by the rear suspension being too tight, the Cal-tracs use a solid bushing in the front leaf spring eye causing bind and oversteer, preloading the Cal-tracs will make this even worse.

[firebob]

Wouldn't a parhard bar create the same condition as a solid spring eye bushing only more so as far as limiting the lateral motion of the rear end? I wouldn't think that would be necessarily a bad thing.

[79T/Aman]

no, the solid front eye bushings cause bind by not allowing the leaf springs to articulate, a lateral locating device only acts as a counter to a applied force

[firebob]

I have the SALs. It's true that there isn't alot of wiggle room in the front spring bushing setup but the spring will pivot around the bolt that goes through. You don't have to wrench down on the nut so hard as to cause a bind of the bushings.

Robert

[CDMBill]

I am not sure I understand the idea of articulation in reference to the leaf spring itself. They do have solid aluminum front 'bushing' which means the leaves act as the lateral location for the rear end, fairly effectively too, as the tires do not rub during high lateral loads despite having less than .5" clearance on either side. A far as front to rear roll stiffness, it hard for me to judge given this was the first time I had driven this car on an autocross or road course. It does have a 1.25" front bar. That 'feeling' of rear roll stiffness is why I haven't added a rear anti-roll bar.

I didn't go with zero preload mostly to avoid harder to predict or feel changes in load while cornering as I assumed, maybe mistakenly, that when an air gap to the bar was used up the effective rate would change suddenly and dramatically. I'm pretty sure I over cooked the first turn of my second and best lap because I had caught the car that was sent out ahead of me on the back stretch of the first hot lap and i was a bit frustrated thinking we only had three hot laps. With the power the car has it is easy to get the back end loose with throttle and I think I a bit of the red mist.

This time out I'll try zero preload and see if that helps overall balance. I'm still trying to make a pig dance and despite the Skip Barber courses and previous auto-x experience with a 911 this is a completely different beast and I have limited track time.

I see a lot of pro-touring cars that still have leaf springs and this kind of traction bar so I'm hoping we can all learn form those with more experience.

[Flash68]

I see a lot of pro-touring cars that still have leaf springs and this kind of traction bar so I'm hoping we can all learn form those with more experience.

I see many PT cars with leaf springs. but ZERO with traction bars... I removed mine the first thing after I acquired my car.

Regardless of that -- I am stoked to see your car at RTTC 2!

[funcars]

There are Mustangs that have used overrider or underrider traction bars on road courses. Some people now run the spring link tractions bars (AFCO type) to help avoid binding with the competing ICs. Look up Cobra Automotive. The old time Trans am guys usually ran panhard rods or watts links to help separate the lateral location and spring/control arm functions. That way the front of the leaf spring could use a soft bushing or spherical so the leafs could work more effectively as springs. Some of the old Steve Smith books have a lot of info about optimizing leaf spring chassis cars.

There is some more guidance for "American Sedans" in my old thrashed copy of Tune to win by Carrol Smith. He says to always use either a panhard or a watts link for lateral control and has some detail guidance for tuning the front end camber curves, how to help tame the badness from the large percentage of front end weight, etc. Worth getting/borrowing a copy. Good luck

[BMR Tech]

I would like to point out an alternative that has worked well for a few of our customers. We make a complete torque arm suspension for the first and second gen cars to eliminate the leaf springs. When developing this kit, the car was first tested with only the torque arm installed for use as a traction device while leaving the leaf springs to control lateral rear end location and spring the car. This setup works very well and doesn't have the dramatic "hit" of a traditional traction bar, making it perfect for a double-duty touring car. The front Delrin-bushed slider allows bind-free articulation and it telescopes to prevent conflicting suspension arc bind. Under normal driving conditions, you can't even tell it's there yet at the drag strip 60 ft/times were improved significantly and corner acceleration produces much less wheelspin. It's not an economical solution by any means but is a great alternative to drag strip-inspired traction bars and it can be upgraded to our full torque arm suspension at any time if you choose to go this route later.

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[CDMBill]

[QUOTE=BMR Tech;756018]I would like to point out an alternative that has worked well for a few of our customers. We make a complete torque arm suspension for the first and second gen cars to eliminate the leaf springs. When developing this kit, the car was first tested with only the torque arm installed for use as a traction device while leaving the leaf springs to control lateral rear end location and spring the car. route later.




I have heard great things about torque arm suspension. I have a 71 Mustang convertible which is usually not a car that most manufacturers have fitments for, but your set-up looks great.

[Rod]

I would like to point out an alternative that has worked well for a few of our customers. We make a complete torque arm suspension for the first and second gen cars to eliminate the leaf springs. When developing this kit, the car was first tested with only the torque arm installed for use as a traction device while leaving the leaf springs to control lateral rear end location and spring the car. This setup works very well and doesn't have the dramatic "hit" of a traditional traction bar, making it perfect for a double-duty touring car. The front Delrin-bushed slider allows bind-free articulation and it telescopes to prevent conflicting suspension arc bind. Under normal driving conditions, you can't even tell it's there yet at the drag strip 60 ft/times were improved significantly and corner acceleration produces much less wheelspin. It's not an economical solution by any means but is a great alternative to drag strip-inspired traction bars and it can be upgraded to our full torque arm suspension at any time if you choose to go this route later.

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??? whoaaa??? now I'm interested, what would that torque arm set run? I have be running a prototype watts link for about 4 months working on lateral locations and angles under daily use and race situations to find a even all a round setup, this would be fun to try!

[BMR Tech]

Glad to hear the word is spreading!
Rod, we have the Torque Arm (http://www.bmrsuspension.com/index.cfm?page=products&vehicleid=3&maincatid=25&catid=77&productid=233) available for 10 and 12 bolt applications for $1k. This works with your existing leaf springs and can be upgraded to the full Torque Arm Suspension (http://www.bmrsuspension.com/index.cfm?page=products&vehicleid=3&maincatid=25&catid=77&productid=231) any time you choose. If you purchase the whole Torque Arm Suspension it runs $2k plus coil overs. This includes the Torque Arm with crossmember, adjustable control arms, adjustable Watts Link, and shock crossmember as shown below.

https://static1.pt-content.com/images/pt/2010/12/TAS001_large-1.jpg