[Ron Sutton]

Tuning with Rear Roll Centers:

If you have a double A-arm independent rear suspension … then it is exactly the same as the front. Change the term from “spindle” to “upright” and party on. Everything else is the same.

For all other common rear suspension types, here are the quick methods to tune.

Panhard Bar/Track Bar: Each end has a rod end as a pivot point. In race cars, we use a variety of different styles of mounts on the chassis & housing that allow us to relocate these pivot points up or down on both sides. I see a lot of aftermarket suspension kits & frame clips that have no adjustability.

My first thought is “what the … @$&% !”

Then I realized they’re doing this so tuning rookies don’t get themselves in trouble. Frankly, I can not fathom not having the rear roll center adjustable. It is one of the most predictable tuning tools & with the right mounts, simple, quick & easy. It is so easy, that in racing circles, if a racer is “lazy” that’s the tuning tool they use first & most, which isn’t correct, just reality of human nature.

Watt’s Link: Very simple to move the bell crank pivot up or down, as long as there are holes available.

Leaf Springs: Do not have an adjustable RC unless you change the mounting points of the springs or use lowering blocks.

Triangulated 4-links: Do not have an adjustable RC unless you change the mounting points of the links. Great for street performance cars with owners not desiring to tune.

Diagonal Links: Are for drag racing … as they are not independently adjustable for RC in the typical mounting. They mount to the ends of the rod end bolts, so change only when the lower bars are being moved to different holes.

The two best methods of centering the rear end … and have tunability … are the Watt’s link & panhard bar. Many people favor a Watt’s link because it keeps the rear end perfectly centered during travel & roll. I like them, but they’re not my preferred method, because the method of mounting leads to limited adjustability … sometimes only a few holes … farther apart than I like to make changes … and often mounted with the RC too high for modern low roll suspensions.

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.

If … for any variety of reasons … the car is working the rear tires different in RH corners versus LH corners … I can put a little “tilt” in the panhard bar … while keeping the same RC height … and even out how the car works the rear tires in RH & LH corners, making a more balanced, faster track car. Just lower the bar down on the side you want to work the tire more … and raise the bar up on the opposite side the same amount. If I’m at a road course where I need a “lil sumthin” extra in one corner, I can achieve that with a little panhard bar tilt.

As far as keeping the rear end centered … with the newer technology low roll suspensions … if the outside rear tire is compressing much more than an inch during cornering … I’ve got bigger problems than the rear end shifting a few thousandths off center.

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.

Old school, conventional, soft sprung rear suspensions achieve this with higher rear RC’s. Modern low roll suspensions achieve this with stiff rear springs (or or medium springs & significant rear sway bar) to control the roll angle, while utilizing a lower RC to work the outside tire more. In other words, modern suspensions don’t use the rear RC as the primary tool to control the car’s roll angle.

For the hot rod I’m designing & building for myself, I have two rear baseline RC locations. For AutoX it is 8” & for road racing & high speed events it is 11”. Because both sides adjust easy, it is about a 2 minute change.

Please don’t use these numbers for your car. Suspension set-up is all about the total package and not piece meal a set-up together.

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Explaining what roll centers are, how to calculate them & how to tune with them ... front & rear ... was a little complex but very important.

This post got into the detail of roll centers (RC) ... but I think it's helpful for car guys & tuners to take several steps back & look at the big picture of handling ... to better understand the role of the RC with the rest of the car.

Total weight ... weight distribution front to rear ... and height of this weight (CG) act like a lever over the roll center. As discussed earlier, lowering the CG shortens that lever, as does raising the RC ... but works the tires less. Raising the CG lengthens that lever, as does lowering the RC ... and works the tires more.

Your goal is to move them both ... to the degree possible ... where you find the optimum balance of working the tires & roll angle. BUT ... and this is KEY ... modern day tuners do not use the RC height as the primary means of controlling roll angle. They use the suspension tuning items as their primary tools & the RC height as a secondary tool.

So to achieve the optimum balance of roll angle & working the 4 tires optimally ... this all has to work with your suspension ... springs, anti-roll bars & shocks ... and track width ... to end up at the optimum roll angle for your car & track application.

Hopefully, this brief overview, helps clarify roll centers place in the tuning picture.


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