(Editor Note: We have been following along on the build of our friend Bill Holland’s rare TVR restomod as he makes it into an all-around track killer. It’s been awhile waiting on this next installment as the chassis had to be built, so if you need a refresher on the concept of T-Rex, or what a TVR even is, here is a link to the introduction story — it’s gonna be really cool!)
Chapter 2 – The Chassis
The foundation of any high performance vehicle is its chassis. If you were building one of the more popular vehicles that enthusiasts modify, it could be as simple as picking up the phone and ordering a complete roller from a manufacturer. Art Morrison Enterprises, for example, has “bolt-on” chassis for first-generation Corvettes that gives a C1 the handling characteristics of a C6. The AME “GT Sport” line of chassis benefit from computer-generated measurements (Faro arm and 3D scanner) and precision CAD/CAM technology. The body mounting bolts literally drop into place.
Not so easy with our project TVR, which came with a factory-fabricated tube-steel chassis and a fiberglass body that is decidedly asymmetrical. In fact, when they first started mass-producing the V8-powered TVR Griffith in the 1960s, the bodies were bonded to the chassis with fiberglass — no bolts needed.
The Original TVR Design
While the construction methods were a bit on the crude side, the actual design of the TVR chassis was quite elegant. The all-tube ladder-style frame featured a reinforced center “spinal column” that provided the needed rigidity at a minimal weight. Dual wishbones and coilovers were employed fore and aft to provide independent front/rear suspensions. Compared to the AC Cobra, which had 3-inch tubular-steel main rails, the TVR had a decided weight advantage and was far more nimble. The early TVRs had an 86-inch wheelbase, compared to the Cobra’s 90 inches. Later Tuscan, Vixen, and M-series TVRs got bumped to 90 inches as well.
An Improved Version
With the advent of the TVR Vixen (and later Tuscan) series, the chassis was improved, and ultimately, the bodies bolted into place. A TVR engineer named Mike Bigland took the chassis design a step further for the ‘70s M-series by employing 1.5-inch square tube for the main framerails and outrigger braces, with a “backbone” made of 1.5-inch round tubing. That’s what T-Rex came with from the factory.
Of course, in 1974, the TVR 2500M came with a less-than-inspiring 106hp Triumph I-6 engine, which performed reasonably well in the 1,975-pound car. But, with T-Rex destined to get more than 5-times the power, re-using the OEM chassis was not in the cards. Designing something a bit more substantial was in order. Enter, the proverbial “clean sheet of paper.”
Planning The New Chassis
Influencing the design was 13-time SCCA Solo Champion Mary Pozzi and her husband, Dave. The Pozzis successfully campaign a Gen II Camaro equipped with an Art Morrison Enterprises Multi-Link IRS and front suspension. In addition to kicking everyone’s butt at the SCCA run-offs, Mary gushed about how much better the car handled with the multi-link IRS than with its previous 3-link setup.
The Multi-Link IRS design has been around for a while, having first been introduced by Mercedes-Benz and subsequently adopted by other manufacturers of high-end cars, notably BMW, Porsche, and Ferrari. For sure, the multi-link IRS contributes to superior handling — while simultaneously having good ride characteristics. If that’s so, why aren’t more cars equipped with a multi-link IRS? Follow the money. Because of its complexity, a multi-link IRS costs quite a bit to manufacture, but more and more they’re finding their way into the mainstream mix.
Advantages of the Multi-Link IRS
Technically speaking, one prime advantage of the multi-link IRS lies with the decoupled links that allow changes to one parameter without affecting others. For example, the camber curve can be adjusted with little or no effect on the roll center.
Decoupled lower links create a direct-load path that prevents control arm failure. Moreover, most bushings are loaded in their radial direction for maximum stiffness.
Other advantages over a traditional wishbone IRS include superior knuckle support, and the ability to establish functionally-independent camber, caster, and toe curves.
Unsprung weight is also reduced, which requires less spring rate to control wheel movement. Because three sets of bushings are employed, NVH (noise, vibration, harshness) is reduced before entering the driver’s compartment.
The big drawback to a multi-link IRS — cost — has been greatly mitigated by Morrison through employing the center section of a late model Camaro. AME’s chief engineer, Matt Jones, created a unique “cradle” mounting system that lends itself to a wide variety of applications. For really high-horsepower cars, AME uses a Strange Engineering Dana 60-based center section.
The Front Suspension
Up front an AME Sport IFS was employed. This differs from their standard A-arm front suspensions in that the controls arms are 1-inch od (instead of the common 7/8-inch) and designed to accommodate larger front tires. Other suspension tweaks include the anti-dive being set to minimize nose-diving during hard braking, increased caster for improved stability at speed, and camber gain optimized to utilize more of the tire’s contact. AME also uses special tie-rod ends that better match theoretical bump-steer models.
Tying It All Together
It then became incumbent to tie the front and rear clips together, which lead to a chassis design that blends the original TVR design with contemporary technology. It incorporates basic 2 x 4 x .120-inch-wall framerails (same as employed on the clips), 1-5/8 x .120-inch-wall outriggers, a center “spine” that goes from just aft of the front suspension to the rear attachment point of the IRS cradle. A 12-point 1-5/8 x .120-inch-wall rollcage ties into the framerails and outriggers to provide protection, additional reinforcement, and stiffening. The rollcage is designed to be completely removable, and utilizes a network of billet-steel connectors that are welded to the frame. This baby should be rock-solid!
A few other features have been incorporated into the chassis design.
First, there are 4-inch passageways in the rear crossmember (which had to be specially modified to accommodate the front cradle mounts of the IRS) and a 4-inch round tube “half pipe” inserted at the bottom of the main-rails to provide exhaust system clearance. Secondly, from the front suspension forward, the framerails will transition into 2 x 2-inch extension that extends into the nose. A tubular “tree” that fits into the nose has hinges and will allow the one-piece tilt hood to provide necessary splitter clearance when it’s opened. Lastly, a parachute mount and anchor will be added to the back, as SCTA rules require ‘chutes on all cars going 175 mph or more. Wheee!
Looking At The Numbers
There are three other important considerations that should make T-Rex a formidable competitor on the track: low overall weight, outstanding front/rear-weight distribution and a relatively short wheelbase. The TVR is typically described as a “front mid-engine design,” inasmuch as the front of the motor is well aft of the front spindle. The new Mustang GT has a 53/47 weight distribution, the SS Camaro is 52/48 and a C6 Vette is 51/49. With its flush-with-the-firewall engine placement, rear-mounted battery, and relatively lightweight McLeod 5-speed (about 35-pounds less than a Tremec T56), T-Rex’s weight distribution should be 50/50. The wheelbase is 90 inches (same as Shelby’s AC Cobra), but considerably less than the C6’s 105 inches.
By way of comparison, the upcoming 2018 TVR Griffith — also 5.0-liter DOHC Coyote-powered — will tip the scales at a reputed 2,750 pounds, have 50/50 weight distribution and a 102-inch wheelbase. T-Rex will likely weigh in the neighborhood of 2,250 pounds, which would be a distinct performance advantage. Doing a side-by-side evaluation of the 2018 TVR Griffith and the 1974 restomod M-series TVR, will no doubt be enlightening . . . not to mention facing off against Mustangs, Camaros, and Corvettes that weigh a half-ton more.
Adding The Shocks
Shock absorbers are an important element in the configuration of any chassis. While there’s plenty of available data and case histories of various suspension/shock setups on popular production vehicles, when the application is quite unusual — as in the case of T-Rex — you’re (again) starting with the proverbial “clean sheet of paper.”
In the case of a vehicle with coil-over shocks on all four corners, the goal is to select a spring rate that essentially mitigates the forces from the track or road surface and balances the movement. It’s kind of a “Goldilocks” moment where you don’t want the springs too stiff or too soft — but rather “just right.” How does one determine exactly what that is?
Selecting The Spring Rate
The “baseline spring rate” is defined as the pound-per-inch rate at which the spring supports the corner weight of the vehicle with the shock absorber at the correct installed height, without the need to preload the spring. Differences such as how the spring is mounted (installation motion ratio), weight, chassis stiffness, and other factors are weighted to determine the baseline.
With a complete vehicle it’s possible to calculate the baseline spring rate by subtracting the measured loaded spring compressed length from the spring’s initial free length, multiplied by the initial spring rate and divide that by the spring travel needed to achieve the desired ride height.
But at this stage, T-Rex is far from complete. Short of employing a team of PhDs at Cal Tech or MIT to develop a mathematical formula based on T-Rex’s weight, wheelbase, front/rear weight distribution and myriad other factors, the recommended method was to start with a “best guess” baseline and go from there.
Interestingly enough, both Matt Jones, the lead engineer at Art Morrison Enterprises who designed the suspension, and Chris Alston who manufactures VariShocks, both arrived at a 400-pound-per-inch of travel baseline rate as the optimum starting point totally independent of one another. So be it.
While many road racers may not be familiar with the VariShock brand, as most of its “cred” has come out of the Pro Touring market, the company has developed an effective method of building custom shocks for special applications. That’s clearly T-Rex territory.
When designing the chassis, the criteria was to have 6-inch ground clearance from the bottom of the frame rails, with those items hanging below the rails (bellhousing, exhaust, etc.) accounting for another couple of inches. The folks at USCA, who produce the Optima Unlimited Street Car events, recommend a minimum of 3.5-inch clearance to safely make it through the roadworthiness section of their 5-part criteria — being able to navigate over speed bumps is essential.
Determining Installed Height
After doing the math it was determined that the shocks needed a 13-inch installed height in front and 12.5-inch in the back. This brought us to the VariShock Builder’s Guide, which lists the specifications of the seven components employed to configure the optimum shock. This includes the top mount style, hardware, travel length, shock-body style, valving, base style and hardware. Add the aforementioned coil springs to the mix, too.
VariShock catalogs eight different shock lengths, with the shortest having 2.80 inches of travel, an extended length of 11.35 inches, and a compressed length of 8.55 inches. This would provide a ride height from 9.67 inches to 10.23 inches.
The longest shock body available has 9.75 inches of travel, with a ride height ranging from 19.44 to 21.39 inches. That’s off-road territory. For T-Rex, a shock with 4.25 inches of travel and a ride-height range from 11.75 to 12.60 inches, filled the bill — with the desired 12.5 inches falling neatly in the range, and a 1-inch-extended top eye making the range 12.75 to 13.60 inches for the 13-inch ride-height shocks.
Configuring The Shocks
Mounting the shocks in the chassis presents a number of variables, but rather than use rubber or urethane bushings best suited for highway use, spherical bearings were utilized to provide the most responsive “feel.” VariShock uses Com-8 Teflon-lined bearings.
Valving is, of course, of paramount importance. Given the choice of Fixed, Single-Adjustable, and Double-Adjustable bodies the flexibility of the latter rang true. Not only will T-Rex be on a road course where 50/50 bump/rebound is a starting point, but for drag strip-acceleration runs, more extension travel (rebound) helps weight transfer. Also of note is where the car is to be raced. Consider that track surfaces can range from a glass-smooth Formula One venue like COTA in Austin, Texas, to a place like Willow Springs International Raceway near Los Angeles, which was built in 1953, and certainly shows its rough side. The ability to fine-tune a shock’s compression and rebound rate is essential.
Rather than dissect the inner workings of the shock body in terms of fluid dynamics, let’s just say that the VariShock is designed to operate with zero-bleed and is quite precise. Perhaps more important is that each valve has 16 adjustment detents, compared to 24 on some other shocks. The range on both is essentially the same, so what Alston contends is that it’s easier to identify the effects of each increment when you’re dealing with 256 combinations as opposed to 576. An interesting parallel might be drawn in being fitted for prescription eyeglasses, where often it’s very difficult to discern minute A/B lens options and you can end up with compromised performance.
As with any sophisticated suspension, the true measure of its effectiveness will come from tweaking a multitude of things that range from camber to toe-in, bump to rebound, and spring rate to tire pressure (to name but some). However, so far —on paper, at least— T-Rex is looking good!