Trixie Tech: Yamaha TRX850R
Over Racing boss Kensei Sato fitted high compression 12.5:1 (stock: 10.5:1) three-ring pistons cast in Japan, where the titanium con-rods made by a company supplying most of the local F3000 car teams, were also sourced. The cylinder block was bored 1mm over stock to 90.5 x 67.5mm, for a capacity of 868cc, while the stock TRX crank – already lighter and stronger than the TDM'S – was retained.
The generator was removed to reduce rotating inertia which meant we ran a quite hefty sealed lead-acid battery on a total loss basis giving two hours' running time, with the stock Yamaha CDI retarded by three degrees.
The 270-degree crank not only replicated a 90-degree V-twin's lusty traction in parallel-twin form, it also had reduced pumping losses and thus less internal friction compared to a 360-degree motor like thetdm's, and thetrx engine was also lighter, partly because the balance shaft fitted (and retained in the racer) could weigh less. There remained a slight buzz from the race engine at low rpm which wasn't present in the road bike, but this smoothed out at higher revs, andtrixie was completely untiring to ride in this respect. It also had a very individual exhaust note, which race fans told me sounded great from trackside.
As on any performance motor the serious power came from the 10-valve DOHC cylinder head with offset chain cam-drive on the right. The Over race camshafts were transplanted from the OV-15A TDM racer, but with the lobes rotated to suit the 270 motor. The five valves per cylinder (three inlets, two exhausts) were all 2mm oversize compared to stock, and the head had been ported, but not so much, said Sato, because on a five-valve motor there's not a lot of meat.
Stock valve springs were employed, each with a washer to increase tension, and those race pistons had deep valve cutouts. The twin 41mm FCR Keihin flatslides were fitted with accelerator pumps, but perhaps because of the valve timing, the throttle response wasn't as snatchy as on other bikes I'd ridden fitted with these carbs – the fairly high compression ratio helped out of slower turns, but you didn't have to worry about kicking out the back wheel if you gassed it hard leant right over.
Throttle action was great: because of the big cylinders, I'd expected quite stiff return springs, but the Yamaha could almost have been fuel-injected, its throttle was so light and responsive.the clutch operation was an object lesson to a certain Italian manufacturer: you didn't feel you needed to limber up on the hand-grips before squeezing it, and the stock oil-bath plates were man enough to take the nearly 40% increase in power Sato initially extracted from the TRX850 motor (83bhp at 7500rpm in street-legal form on the Over dyno, compared to Trixie's
original 115bhp at 8500 rpm at the gearbox.)
For the 1997 Sound of Thunder World Series, Over improved the engine performance significantly, lifting peak power to 121bhp at 8750rpm. The water-cooled oil radiator was worth two to three bhp, said Sato, while Trixie's large radiator sourced from a ZXR750 Kawasaki race kit was vital, because Over had experienced overheating problems running theirtdm racer in the Suzuka 8-Hours. In cooler European conditions the TRX motor still picked up heat very quickly.
The stock TRX chassis had been heavily braced, with two extra tubes behind the steering head covered by metal sheeting which also added stiffness, another diagonally under the seat, and one half-section pipe reinforcing the cast aluminium swingarm pivot. The TRX850'S tubular steel space frame has been accused of ripping off Ducati technology, but in fact it's an adaptation of a composite Segale-type design, later adopted on his own admission by Massimo Tamburini on the MV Agusta F4, with the upper spaceframe using the engine as a fully-stressed member, with Trixie's braced-up standard extruded-aluminium swingarm pivoting in a pair of alloy plates doubling as rear engine mounts.
The wheelbase was reduced 10mm from stock to 1420mm, and the rear ride height raised 30mm via extensions to the rear link, which steepened the effective head angle for the Over-modified upside-down Showa fork to 23.5 degrees with 94mm of trail (vs stock 25 degrees/99mm), thus increasing the front-end weight bias to 52/48%. The fully-adjustable forks came from Over's XJR1200 Supernaked racer: both forks and the Öhlins rear shock, sourced from an OW-01 Superbike were well sorted.
Brakes were Brembo's best of the time: the floating 320mm twin cast iron discs up front, gripped by four-piston calipers gave magical stopping power and were able to out-stop a much lighter Ducati in the dry. I only used the rear TZ250 disc very occasionally in chicanes. Theyamaha wasn't quite as nimble as the Ducatis, mainly because of the extra weight, so it needed a little muscle to make it change direction fast, but it wasn't a tiring bike to race: the riding position was comfortable, you sat in the bike rather than on it, without too much weight on my arms, and it steered very well – measured rather than quick
The slant-block engine layout actually placed the centre of gravity quite low (that heavy steel tank didn't help), making thetrx pretty stable round long, fast sweepers like at Assen. But throwing money at it would surely have slashed the weight by a good 15kg with carbon bodywork and titanium fasteners, some right-sized forks that didn't have to be dropped so far through the 26mm offset yokes sourced from the Over OV 15-A Eurotwin roadster, and so forth.