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Kenny Brown AGS 4.5: Have I Been Living Under A rock Or Is This Brand New?

Mad Hatter

Gotta go Faster
Santiago, Chile
Thats it in a nutshell! Especially in the case of beating the next guy off the corner. Pretty much the strongest area for our stick axle mustangs.
Let me say right up front that I am no suspension guru but I have always been taught that three basic things are true:

1. Unless you have aero, try to use as soft a spring rate as possible to maximize mechanical grip, provided body roll is reasonable and suspension geometry is maintained;
2. Increased weight transfer = reduced lateral grip, since the inside tire loses more grip than the outside gains; and
3. Assuming similar tires, the only things affecting total weight transfer (and thus total grip) are weight of the car, width of the car, center of gravity, and speed. Period. Using things like spring rates, rollbar sizing, and adjusting roll centers, you can tune which end of the car is carrying what percentage of the total weight transfer (understeer vs. oversteer), and whether weight transfer manifests as body roll versus jacking forces - but you cannot reduce the total weight transfer.

If these things are true, I am having trouble seeing how dramatically dropping the roll center on one end will dramatically increase total overall grip? I can see how it would reduce the percentage weight transfer on that end, similar to removing a roll bar or softening springs. And I can see adjusting front versus rear roll centers as one technique to achieve front/rear handling balance. But as @Fabman suggests, I am having trouble seeing how a super low roll center combined with super stiff springs to control roll is a net gain, and it seems as if it could be a net loss if #1 is true? (In fact, isn't this one reason we work to raise the roll center in the front with things like extended ball joints, so we don't need to run so much front spring rate?)

I know KB really knows their stuff and it is likely I am missing something (many things?) but for now put me in the Watch and Learn camp I guess.
Exp. Type
Exp. Level
3-5 Years
Nashville, TN
I look at it like;
Let the car rotate around the roll center without reducing load on the inside tire. This is why I don't typically run a rear sway bar. (never say never though)
Its job is to add load to the outside tire by taking it from the inside tire and adding that spring rate in with the total roll couple. Seems counter productive to me.
With a high roll center you share the load more equally but load transfer from body roll is expressed at the contact patch rather than around the roll center.
Too high and the car just skates. Others like big rear springs to keep a balanced chassis. But again, too much and it skates and as a nice bonus you sacrifice forward bite.
I like low roll centers and soft springs. I'll deal with the body roll and understeer as long as I continue to beat the next guy off the corner.
However, I continue to test and try different approaches and combinations in the quest for lower lap times....who knows what I might find.
And that's the fun of it....right?
That's why we like tweaking! Interestingly, one of the first things Kenny Brown always said was "throw away the rear sway bar and lower the rear roll center." One would think it would push like a pig, but if it works, it works.

A higher rear roll center actually loads the tires *less* evenly because of the jacking effect (assuming the CoG is higher than the roll center, that is). A higher rear roll center has the same effect as a stiffer rear sway bar: more load on the outside tire relative to the inside tire.

If you look at the Roush Boss 302R cars from the Grand-Am days, you'll see them move all over the rear axle. Their bodies "sat" a lot lower at the rear than even everyone else's Mustangs...yet they still passed ride height tech (hmmmm...).

Anyway, two takeaways from what I saw of their cars: Billy Johnson drove those cars on the outside rear. The inside front off the ground...a lot. Even on level turns. I suspect they ran the rear roll center much lower, *and* they ran the rear of the car very soft. It moved around a lot like a contemporary NASCAR (which, given their area of chassis engineering expertise, makes sense). The cars were fast, but they also looked like your average Joe would complain about massive push. Billy would dive the car down in the corners, mash the gas, and drive it out on the outside rear tire. That probably minimized the understeer.

(Merely my observations trackside and in photos.)
Agreed on all least in theory. What's weird: accounts of how this setup works on real cars leads me to believe there's more going on than we may realize. I'm growing more convinced that the extremely low roll center benefits a live axle car more than an IRS because live axles seem to be more sensitive to the jacking effect with above-ground roll centers. Maybe the corner exit bite from minimizing jacking the inside rear wheel outweighs the loss forward bite from stiff(er) rear springs. Going back to theory, however, if both rear wheels are more evenly loaded (from the low roll center), then the rear diff can be "looser," which also reduces understeer.

All food for thought!
the beauty of the K-link is not just the vertically adjustable rear roll center, to better tune to the front roll center, but also the fact that because of the linkage design, the chassis is free to roll around the rear roll center without upsetting the weight on the inner and outer rear tires. I can put my race-jack at the A pillar on the jacking rail, jack the left front tire 5" off the ground, and while the left fender may be raised almost 5", the left rear tire is flat on the ground. When running hard, rear tire temps run fairly even across the rear, and at times maybe even slightly higher temps on the inside edge of the rear tires.

So yes, there's more going on than we realize. I went to 650# spring rate in rear when I started running Pirelli slicks and Trofeo's. I ran with 450# in rear when running with R888R Toyos and it worked fine.
Keep an eye out for the BMR A-arms. They should be on your track day inspection list.
Yes, I have been. The car goes up on my lift after each event, and to do a full inspection, including the welds on the lower arms. I may opt to install the BMR upgraded arm, but I'm also considering fabricating full gussets and welding them in place this coming off season. Jury is out right now. Thanks for the heads up!


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