Six wheels. Five-and-a-half tonnes. Drum brakes.... and 1200 horsepower!
There's an argument that goes along the lines that the more pure a racing machine is, the less exciting it is to watch. If superb suspension, aerodynamics and tyres give great grip, then cornering lines will be maintained with relative ease and passing manoeuvres will be rare. The corollary of the argument is that a vehicle whose genesis is a metaphorical mile away from a race track should be pretty bloody spectacular to see racing.
And so it proves with truck racing. You just ain't seen nothin' 'til you see these leviathan monsters lurching and powering their way around a track, front tyres smoking on turn-in and rear tyres smoking on exits! And that's not to mention the braking from 160 km/h at the end of the straights......
We went along to Mallala Raceway in South Australia to see just what make these vehicles tick.
Joe Scarcella (pictured) drives the Nigel Valois-engineered Kenworth W924. The other member of the team is Rob Russell. While superficially their Kenworth looks like any other interstate hauler, it's considerably modified.
Using a 14 litre Cummins diesel, the four valves per cylinder engine gets a massive power hike from the use of two turbos. These aren't arranged in parallel or even sequentially - instead one turbo blows through the other in what's called a compound arrangement.
Both the Holset primary turbo and the Garret secondary turbo are huge. The turbos run without wastegates and together produce a boost pressure of no less than 60 psi! To keep intake air temps at a sane level, water/air intercooling is used. The inlet manifold contains a heat exchanger which transfers the intake heat into water. This is circulated through a front-mounted radiator by an electric pump, with intake temps of around 65-70&°;
C maintained by this system.
To cope with the huge boost, the engine runs a lower compression ratio of 12.5:1 (it's a diesel, remember!) with the change in compression achieved by machined pistons. The fuel system is modified to provide a higher fuel pressure and so greater flows. The camshaft uses a lift of around 370-400 thou, while the other engine mods fall into the "not for telling" category. We sure got the impression that the engine was balanced on a knife-edge of destruction, though!
Backing the 1200hp engine is a Allison 5-speed auto transmission. Using a 1:1 top gear and a lock-up torque converter, the trans is near standard. Mods were tried in the past to achieve quicker shifts, but this tended to result in locked rear wheels as the trans changed down. The trucks already look unsteady enough under brakes as it is - so locking the rear wheels on down-changes sure as hell wasn't wanted!
The diff is locked and runs a 3.2 ratio with bigger-than-standard axles. It's suspended on leaf springs which run only two leafs instead of the many normally fitted to the truck.
Koni adjustable dampers are used at both the front and back, and the sway bars at each end are also adjustable.
Continental race tyres (designed for this application) are used at both the front and back.
When you see the punishment that the tyres on these trucks endure you soon realise that anything not purpose-designed would last only a few minutes.
Brakes remain drum but are 9 inches in diameter front and back. This compares with 6 inch and 7 inch drum normally fitted to the front and back, respectively. The brakes use special race linings and are cooled by purpose-designed air scoops and water sprays onto the drums. Also looked after on the cooling front is the intercooler radiator (sprayed with water), and the power steering, tranny and engine (all fitted with oil/air coolers).
Inside the cabin it's all business, with a roll cage, sparse instrumentation and controls for the water sprays, transmission and not much else. The brake pedal is a huge checkerboard aluminium plate to make left-foot braking a bit easier in high G-load corners.
Also out there with the big boys are the Light Trucks - almost universally Isuzu SBR cab-over models. Compared with the Super Trucks that weigh up to 6 tonnes, the smaller vehicles are "just" 3 or 3.4 tonnes.
They are allowed to run rear aerodynamic aids and with a max speed of 160 km/h allowed for all of the racing trucks, the huge wing really works.
To say that they're spectacular is an understatement - belching black diesel smoke and often wreathed in white tyre smoke, the racing trucks are amongst the most spectacular vehicles to be found on a race track!
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