Aprilia RS500

Short Shifts
APRILIA RS500		Words & Pictures: Jim Yeardly
Italy's Aprilia RS 250. Openly raved about as one of the best race rep's available on the shop floor. If you want to embarrass your mates on the twisty bits, and as long as the straights aren't too long don't look any further. The usual quotes apply "rock solid chassis" "stops like you just dropped an anchor". Great little whizzer. Pity it only gets it's power pulses from a stock Suzuki RGV 250 lump. Not that there's anything wrong with the big S's little old V-Twin. Ultra-reliable. Kicks out enough beans to scare most people. Around 55 beans give or take the temperature in the dyno room. See that's the problem. Aprilia have given us a superb rolling chassis, but stuck a 250 mill in the slot. Boy, am I glad that it's not only me who thinks like that. Enter Dave Hosker, Carpet machine fitter extraordinare. By the time Dave's Aprilia was run in he was itching for more. How about a nice little 100% power increase, just to liven things up a bit. Dangerous Dave knew the power plant he wanted, Suzuki's "square four" RG 500 lump.

A few phone calls and Dave had his new mill. Don't think that stock was going to be enough for our Dave, so off to two stroke tuning specialist "Stan Stephens" (01474-854331) went the RG 500. Stan pulled out the stops, stage three tune, close ratio box, high compression pistons, new balanced cranks, new barrels. One trick motor, one big till receipt. As Elvis would say, "Thank you very much"
Midnight oil time. The biggest problem with the "donor" lump was to get it high enough and further back and at the same time keeping the drive line from the gear box to the rear wheel. Dave's answer, chop out the rear frame cross beam/shock mount. This along with a modified clutch cover allowed Dave to slot the RG lump in place. Dave then machined a new billet cross member complete with new shock mount and welded it in place. The new mount looks like it was welded in on the production line. The only major frame mod was a new engine sub frame.

Dave's next head ache was about to rear it's ugly head. Exhaust pipes, Dave knew what he wanted, the new pipes had to look stock. Rolling chassis with new hybrid motor was delivered to exhaust wizard Pete Gibson (01708-372122). True to his talent Gibson produced a set of four curved and hand welded expansion pipes, two exiting Siamese style and two up under the seat. An extra set of stock end cans were shortened by 50mm to exit just clear of the seat unit. The only other real problem with the frame had been dismissed by simply jacking out the rear sub-frame by 70mm to allow for the expansion chambers.
Little Things. No room for the oil tank , ditch it. The RG runs Pre Mix at 40:1. The stock Aprilia curved radiator had to be heated and straightened, the RG's four carbs, they fitted on the intakes but the frame had to be drilled to allow the cables to pass through. The stock air RG 500 air filters, no way.
Dave as usual came up with his own novel air filters. He cut perspex sheet to mimic the shape of the stock oval filters and then mounted them as back plates to the intake mouths. A trip round the auto store got Dave his four filters, from a family saloon car, perfect fit. Only the Dyno would tell if they passed enough air.

Power valve actuator had to be moved from the rear operating cylinder to the front just to let the fuel tank sit back down. Standard RG 500 ignition box mated to Aprilia wiring and switch gear supplies the sparks and lights the candles on Dave's rocket.
Under the fuel tank? don't ask . In Dave's own word's "like a snakes wedding". Remounted ignition coils, cables, cooling system pipes, wiring, radiator filler cap. We've all got skeletons in our cupboards, this is Hosker's.
Remember that stage three motor and high compression slugs? Hosker paid the ultimate price for hiking up the compression. He has to bump start the hybrid Aprilia, why? it keeps shearing of kick start shafts.
Hosker ran the bike for a few hundred miles then strapped it to the dyno for some truth telling sessions. Straight away it was jetting time Hosker settled eventually on 190 mains. The stock 500 runs 120s, idle, needle and slide are all stock. The dyno recorded 90bhp at 11,000rpm which pleased Hosker, he hadn't taken the hybrid past 11,000 on the road and reckoned that was pretty good. He was in for a shock.
The throttle was nailed on the dyno and as the rev counter charged towards the blood line the scream from the four expansion pipes changed into a sharp shrill, as the motor reached it's peak power the graph spat out its trace. 12,000 rpm, 113bhp. In the space of 1,000 rpm the motor had kicked out an extra 23 bhp. Impressive!
Hosker was over the moon. He hadn't even taken the Aprilia to 12 grand. When you think that the RG/RS hybrid weighs in at around 150kgs were talking serious speed from a 500, close to 160mph.
Since then Hosker has made some changes in the power department. He's had the exhaust pipes shortened by 10 mm to give him more mid range, mind you he only had to sacrifice 10 bhp to make his missile as tractable as the stock RS 250. Fancy Hoskers rocket ship? It's up for sale and Hosker is moving to Georgia. So if you see a blue haze being pursued by some EPA officials in a rented sedan, guess who's out cleaning his plugs. How much for the bike Aprilia should have built? 6,500 in old English (11,000 $ US). And remember it's the only one.
For details of Aprilias standard range checkout their excellent web site at www.aprilia.com

A few phone calls and Dave had his new mill. Don't think that stock was going to be enough for our Dave, so off to two stroke tuning specialist "Stan Stephens" (01474-854331) went the RG 500. Stan pulled out the stops, stage three tune, close ratio box, high compression pistons, new balanced cranks, new barrels. One trick motor, one big till receipt. As Elvis would say, "Thank you very much" Midnight oil time. The biggest problem with the "donor" lump was to get it high enough and further back and at the same time keeping the drive line from the gear box to the rear wheel. Dave's answer, chop out the rear frame cross beam/shock mount. This along with a modified clutch cover allowed Dave to slot the RG lump in place. Dave then machined a new billet cross member complete with new shock mount and welded it in place. The new mount looks like it was welded in on the production line. The only major frame mod was a new engine sub frame.
	Dave's next head ache was about to rear it's ugly head. Exhaust pipes, Dave knew what he wanted, the new pipes had to look stock. Rolling chassis with new hybrid motor was delivered to exhaust wizard Pete Gibson (01708-372122). True to his talent Gibson produced a set of four curved and hand welded expansion pipes, two exiting Siamese style and two up under the seat. An extra set of stock end cans were shortened by 50mm to exit just clear of the seat unit. The only other real problem with the frame had been dismissed by simply jacking out the rear sub-frame by 70mm to allow for the expansion chambers. Little Things. No room for the oil tank , ditch it. The RG runs Pre Mix at 40:1. The stock Aprilia curved radiator had to be heated and straightened, the RG's four carbs, they fitted on the intakes but the frame had to be drilled to allow the cables to pass through. The stock air RG 500 air filters, no way. Dave as usual came up with his own novel air filters. He cut perspex sheet to mimic the shape of the stock oval filters and then mounted them as back plates to the intake mouths. A trip round the auto store got Dave his four filters, from a family saloon car, perfect fit. Only the Dyno would tell if they passed enough air.
Power valve actuator had to be moved from the rear operating cylinder to the front just to let the fuel tank sit back down. Standard RG 500 ignition box mated to Aprilia wiring and switch gear supplies the sparks and lights the candles on Dave's rocket. Under the fuel tank? don't ask . In Dave's own word's "like a snakes wedding". Remounted ignition coils, cables, cooling system pipes, wiring, radiator filler cap. We've all got skeletons in our cupboards, this is Hosker's. Remember that stage three motor and high compression slugs? Hosker paid the ultimate price for hiking up the compression. He has to bump start the hybrid Aprilia, why? it keeps shearing of kick start shafts. Hosker ran the bike for a few hundred miles then strapped it to the dyno for some truth telling sessions. Straight away it was jetting time Hosker settled eventually on 190 mains. The stock 500 runs 120s, idle, needle and slide are all stock. The dyno recorded 90bhp at 11,000rpm which pleased Hosker, he hadn't taken the hybrid past 11,000 on the road and reckoned that was pretty good. He was in for a shock. The throttle was nailed on the dyno and as the rev counter charged towards the blood line the scream from the four expansion pipes changed into a sharp shrill, as the motor reached it's peak power the graph spat out its trace. 12,000 rpm, 113bhp. In the space of 1,000 rpm the motor had kicked out an extra 23 bhp. Impressive! Hosker was over the moon. He hadn't even taken the Aprilia to 12 grand. When you think that the RG/RS hybrid weighs in at around 150kgs were talking serious speed from a 500, close to 160mph. Since then Hosker has made some changes in the power department. He's had the exhaust pipes shortened by 10 mm to give him more mid range, mind you he only had to sacrifice 10 bhp to make his missile as tractable as the stock RS 250. Fancy Hoskers rocket ship? It's up for sale and Hosker is moving to Georgia. So if you see a blue haze being pursued by some EPA officials in a rented sedan, guess who's out cleaning his plugs. How much for the bike Aprilia should have built? 6,500 in old English (11,000 $ US). And remember it's the only one. For details of Aprilias standard range checkout their excellent web site at www.aprilia.com