Quote:
Originally Posted by Ron Sutton
Some people like to look at the car as a unit. I look at it as two halves. Here are some examples … using a typical 3500# Pro Touring Car with 53% front weight… to provide more clarity:
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Quote:
Originally Posted by Ron Sutton
Tuning with Rear Roll Centers:
This limited tunability of the Watt’s link requires the tuner to rely more on other tuning items such as springs, shocks & sway bars. It reminds me a little of torque arm suspensions. They work well, just not much tuning adjustment. I think they’re the ticket for car guys that want to get a good set-up & just drive it.
As a tuner, I like to have a full tuning “tool box” at my disposal, and a panhard bar (or track bar, depending on where you came from) is more tunable. With the right adjustable mounts, I can move the RC 1/16” if I wish. I can make it super low … super high … or anywhere in between.
Direction … regardless if you’re using a Watt’s link or panhard bar:
a. Raising the RC, places it closer to the CG, reducing the CG leverage, reducing roll angle … and working the rear tires less.
b. Lowering the RC, places it farther from the CG, increasing the CG leverage, increasing roll angle … and working the rear tires more.
c. With low roll suspensions utilizing mean stiff rear springs … or medium springs & significant rear sway bar … for faster corners found at big road courses I’ve found the happy window to be 9” to 12” … and 7” to 10” for tight AutoX events.
d. Softer sprung, higher roll rear suspensions run higher rear RC’s.
*KEY NOTE: For optimum cornering ability, you need to position the rear RC low enough to work the rear tires … allowing just enough rear roll angle to disengage the inside rear tire to a degree … but high enough to prevent the outside rear tire to roll over so much that the car rolls diagonally … and unloads the inside front tire.
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Ron
Starting with the first statement about looking at the car in two halves. Correct me if I am wrong but I'm assuming you are speaking about the need to tune each end of the cars roll resistance based on each ends independent RC and not that the effect at one end is independent of the other end of the car.
A well built car should have a stiff chassis that allows the suspension to do it's work without the added variable of the frame flexing. But that stiff frame translates load from one end to the other.
Looking at you diagram of roll axis it seems obvious that increasing the length of the moment arm at either end would introduce some additional roll at the opposite end as the frame resists twisting.
If that line was a piece of pipe running through the car as I twisted one end clockwise the other end would follow in a clockwise direction.
Is it the front that unloads the inner rear tire on corner entry and loads it on corner exit?
If the balance is correct, would work something like.
The front have enough roll resistance going into the corner to keep the front flat and load both tires.
The rear also have the amount needed to keep it flat as the front first turns in.
The front roll translated through the axis provides enough added force to unload the inner rear tire which helps prevent overloading the outside front tire.
As you unwind the front on exit and that translated roll force diminishes the inner rear tire in again loaded for better corner exit.
In you statement about tuning the rear I think what you are doing is tuning the rear so that is have just enough of a moment arm to be overcome by the force from the front but not so much where it is overloading the front.
I'll wait on the answer to this to ensure I fully understand it before I ask what effect wheelbase would have on this relationship.
Let me go find my hat
I may be wearing it soon.
Brian Hobaugh SCCA National Tour June 2014
First Hemi 'Cuda Convertible Ever Built
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