Inside JRi’s Radial-Specific Shocks On Our Project BlownZ06

Back in August, we took a deep dive into the science of radial tire racing, analyzing the intersect point between suspension tuning and power management in our piece, What Goes Up: The Black Art Of A Wheelstand-Free Radial Tire Run, to reveal all of the magic involved in producing a safe, successful, and potentially record-breaking run on drag radials. There, with the help of some of the most talented and decorated individuals in the sport, we dissected the fine balancing act between the two elements, illustrating how the suspension setup sets the table for power management.

Unlike Pro Stock and Pro Modified where the finer details of the suspension package are kept under lock and key, radial racing tends to be a little more transparent — perhaps because it’s more a case of how you use what you have, than what you have. Only the most successful of racers have that formula figured out. And so, without anything to hide, we’re going to roll back the curtain on the custom suspension setup that’s been developed for our forthcoming Radial versus The World-style 2017 Z06 Corvette, giving you a detailed look at the shocks and struts that JRi has assembled for us and what factors went into their selection.

We’ll be utilizing both front struts and four-way rear shocks from Charlotte, North Carolina-based JRi Shocks on our Project BlownZ06 drag radial C7 Corvette.

At the heart of a suspension system for a radial tire car is the very principle upon which such cars operate; the opposite of their slick tire counterparts, a radial car separates the rear suspension at launch, thus placing “bite” into the rear tire. Also unlike a bias-ply slick, a radial tire doesn’t cope well with wheel-speed, and so all efforts in the chassis and suspension center around planting the tire and avoiding any slippage. Thus, a radial car is the only form of drag racing machinery that in fact separates the rear shock and maintains a separation at or above ride height for the duration of the run. A radial car also relies on front end suspension separation on marginal race tracks to help transfer weight to the rear of the car — again, to plant the rear tires into the racing surface.

For these reasons, radial tire racers utilize shocks and struts with more total length than a big-tire car. Once you take a peek inside, their level of uniqueness only becomes more obvious.

The Front Struts

In a best-case scenario on the best of racing surfaces, radial tuners attempt to drive the nose of the car into the pavement, in effect creating an ‘attitude” in the chassis akin to a wedge. Because the track is so good, additional weight transfer to plant the tires is unnecessary, and with the nose being driven down, they can pour the power to the car with less risk of a wheelstand. In such situations, more extension (rebound) in the strut can be used to keep the nose down. But this is the real world, and not every track is a drag racer’s Disneyland.

On a sticky track, you want the strut to be a lot tighter, as it will try to hold the front end down more. You want to minimize how much the front end can raise. – Marc Smith, JRi

That’s where the struts become vital.

Specifically for the radial application, JRi produces a 6-inch stroke version of their Ultra strut, which offers 3-inches more extension travel than is typically utilized on a slick car. In doing so, extension can be softened to increase travel, thereby allowing the front end of the car to rise and shift the vehicle’s weight rearward. Of course, doing so on a sticky track while also trying to rotate the earth, power-wise, could end in disaster.

“On a sticky track, you want the strut to be a lot tighter, as it will try to hold the front end down more. You want to minimize how much the front end can raise. But when you get on tracks that have less grip, you want the strut to extend out to help get weight on the rear tires. When the rear shock extends, it essentially shortens the wheelbase, because it raises the rear end up and drives it under the car and pushes the front end down,” explains JRi’s Marc Smith.

 

The 6-inch, double-adjustable, monotube strut we’ll be running offers both high- and low-speed rebound (extension) adjustment, resulting in one of the most highly adjustable drag racing struts on the market. Similar to how the wastegate on a turbocharger works, wherein increased air pressure opens a spring-loaded pressure relief valve, this strut features an adjustable shim stack atop the internal piston that bleeds off oil at greater or lesser rates (depending on adjustment), thereby allowing for adjustment of “high speed” rebound operation. In JRi’s struts, this uniquely-designed, pre-loaded shim stack offers a range of adjustment that’s less than you’ll ever use and more than you’ll ever use, leading the crew at JRi to promote it as offering “three times the adjustment” of any other strut.

This degree of adjustment is important when you take into consideration how violent a radial car can be upon launch, instantly separating the front and rear suspension.

“The extension in the front can keep the front end down. When the tracks lack bite and the car tries to wheelie, you want less extension, because that keeps the front end together,” Smith explains. “On a bad track, the rear tires are spinning more anyway, so you want to get the weight transfer on the rear tires to stop that from happening. You’re not worried as much about the thing flipping over backward because the tires are spinning and you’ve likely pulled out some horsepower.”

Although the low-speed circuit can be set to adjust either compression or rebound, JRi uses the low-speed adjustment for rebound, because very little compression is used on the front of a radial car — this allows the front end to settle going down-track. Plus, running more compression would also require more gas pressure. As Smith explains, “We use both the high- and low-speed adjustments for rebound, because once you get your high-speed set where you want, the low-speed adjustment offers more fine-tuning.”

JRi has utilized a strut housing from Strange Engineering for the build and pairs it with their own strut body, complete with their own shaft, piston, and internals. JRi also welds a bung on the body to place a Schrader valve so the gas pressure can be checked. As part of their design, JRi uses a sealed head bearing — in essence a misalignment bearing — that prevents the strut from binding during its range of compression and extension. This setup takes pressure off of the seal atop the strut body, often a weak point in binding situations. Their low-friction seal design also has less “stick” than other struts on the market.

Rear Shocks: Where the Magic Happens

Like the struts, JRi’s rear shocks for radial cars also have a unique length: 21.5 inches overall, providing maximum separation.

“The big issue with a radial car is on the hit, because if you don’t get enough separation (extension) in the rear shock, the front end is going to come up and you won’t be able to save it if the car bites too much. But on a marginal racetrack, you don’t want it to separate as much, because you want to try getting weight off the front end with the strut extension so the tires will grab,” Smith says.

The high speed cage design on compression and extension controls the high frequency, low amplitude motion at a higher coherency rate which results in quicker reaction and better tire grip. – Marc Smith, JRi

Like the front struts, these shocks are also a four-way design, which means both the high- and low-speed circuits can be adjusted on both compression and rebound. An external canister on each shock serves as the pressurized overflow under compression and is where the low- and high-speed compression adjustments are made, via a needle and jet setup.

In a four-way shock, the shock body features shim stacks located on both sides of the piston. A “cage” sits atop the rebound stack, limiting the flow of oil passing through. This high-speed adjustment is done simply through a pre-loading of the shim stacks. The low-speed rebound is managed through an orifice on the shaft of the piston — an adjustable needle and jet screws into the end of the shaft to accomplish this, in effect creating a bypass around the piston. The low- and high-speed compression utilize the pressurized external canister — an overflow, if you will, that helps make this degree of adjustment possible. The low-speed, as with the rebound, is done through a needle and jet that’s located in the canister, while the high-speed is managed through a similar spring-loaded shim stack and cage design.

Seen at right, the four-way shocks utilize an external, air-pressurized canister to control the low- and high-speed compression adjustments.

“The high-speed cage design on compression and extension controls the high frequency, low amplitude motion at a higher coherency rate which results in quicker reaction and better tire grip,” Smith explains.

JRI uses many interchangeable parts between the rear shocks and the front struts, and those that aren’t interchangeable are of the same design but simply sized to fit the specific body.

Going back to the violence of a radial car, the initial hit, Smith confirms, is a high-speed circuitry operation of the shock as it’s “trying to rip the rear end apart.” But, by tightening up the low-speed adjustment, you can slow that motion down to control how abrupt the shock separates at the launch. By fully opening the low-speed circuit, you could essentially remove all resistance.

Here, you can see a view of the internal piston and the shim stack “cage” design.

Because separation is so important, compression is tightened on a radial shock to help keep it separated going down the racetrack, with only minor compression over undulations in the racing surface (well under an inch of compression in total, as Smith confirms). By and large, compression only occurs upon deceleration in the shutdown area at the close of a run with a radial car.

Essentially, with a big tire you run softer compression and stiffer rebound — radials stiffer compression softer rebound. – Marc Smith, JRi

A Pro Modified car, by comparison, will generally run a 19.5-inch rear shock, with a ride height of around 16.5-inches; Smith says you’ll typically plan for about three inches of travel in either direction. A radial car, however, because it never goes below ride height, only needs 1/2- to one-inch of exposed shaft at ride height. The only time you’d go less than ride height, he says, if you were rolling around in the pits, when you don’t want your shocks to bottom out. “We’ve set up the shocks on the Corvette to have about an inch of shaft showing, so that will give it six inches of extension (only about three to four inches of actual extension occurs, meaning the shock isn’t trying to pull itself apart, in reality). What you don’t want is for them to top out, because then it messes the whole pass up.”

The rear shocks that JRi builds for radial tire cars measure 21.5-inches in total length — two inches longer than a typical Pro Stock or Pro Modified shock.

“The primary difference is that on a slick car, the compression is softer and the rebound is stiffer. And the low-speed rebound jet, you would never open on a high-horsepower Pro Mod — you would close that jet off. But when you switch to a radial car, you would open that rebound up to provide the oil that bypass, and soften the high-speed rebound. You would then stiffen the compression up to hold it up there once it goes up on the shocks. Essentially, with a big tire you run softer compression and stiffer rebound — radials require stiffer compression and softer rebound.”

Armed with the right tools for the job — the best parts and pieces that JRi has to offer for the no-holds-barred radial tire market — there’s no question we’re well-equipped to put our BlownZ06 Corvette down the eighth-mile in its debut later this year. JRi has provided racers with the virtually infinite adjustment that’s necessary to fine-tune these race cars to achieve the proper balance between traction and wheel-speed, and to properly transfer weight to achieve such without pointing to the moon. It’s all about how you use what you have, but a well-engineered piece with a lot of thought behind certainly goes a long way.

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About the author

Andrew Wolf

Andrew has been involved in motorsports from a very young age. Over the years, he has photographed several major auto racing events, sports, news journalism, portraiture, and everything in between. After working with the Power Automedia staff for some time on a freelance basis, Andrew joined the team in 2010.
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