Performancebikes.co.uk | OCTObeR 2014

sPeciaL / APRILIA RS485 YPVS

44

OCTObeR 2014 | Performancebikes.co.uk

With a 485cc stroker motor packed into the chassis of an RS250, this home brewed GP replica has a feisty surprise hidden under its fairing

Words Gary Inman / Photography: Paul bryant

 ‘I WANTED TO give the bike an engine it deserved,’ explains Dave Bimson of his 1996 Aprilia RS250 Mk1. ‘I’ve owned the bike nine years and love the handling, it worked impeccably, but lacked a bit of power.’ Dave, who has owned GSX-R1000s, VFR400s, Aprilia

SXV550s, Futuras and Milles, continues, ‘being a road rider, not a fan of riding on track, it meant staying with mates on Fireblades and GSX-Rs was not an easy task. The 250 had the edge on entry, mid-corner and really tight stuff, but it was always a struggle to keep up on the exits. So, four years ago, my mate Steve came up with the idea of fi tting a bigger engine.’

Four years… That’s how long it took to fi nish, on and off, stop-start. ‘I ended up going around and around and around, because I’d fi t one thing and it would make the part next to it look scruffy, so I had to replace so many different parts.’

Putting big engines in small two-stroke

chassis is nothing new, the British are masters at it, but this concoction – of big-bore RD350LC in an RS250 is much rarer. Apparently there are another two or three running in the UK, with another two in development hell. There are only a handful elsewhere in the world.

Aprilia RS250s are such a rare sight now, I haven’t seen on one the road or at a bike meeting for years, that it’s tricky for me to spot the changes forced on the bike by the new engine. The bottom of the fairing is cut and widened to fi t in the huge Tyga expansion chambers while nearly everything else looks close to factory.

Sat in isolation with no other bikes around it, the RS250 (Dave’s ‘kiddie dream bike’ that he bought years after his adolescence, when he could afford such toys) looks big. Large capacity sportbikes have shrunk so much, and so incrementally, that this pre-millennial tiddler has as much presence as an RSV4. The long seat dates the design, but still fl oats

THE LITTLE

LARGE SHOWA N D

the boats of men of a certain age. A gentle swing on the kickstart changes

everything though. There’s no other word to describe the noise than crackle. Maybe add and adjective - evil crackle. It’s an exhaust note full of portent. It makes my nerves jangle like wind chimes in a tornado. It’s impossible not to blip the Domino twin-cable throttle, imagining you are a nonchalant, Italian mechanic in crowded pit-lane. Swarthy and you know it.

Even though I’m the correct age, I’m not one of those people who has a special place in their heart for two-strokes. I spent too long pushing and fi xing the lousy ones I bought for that. I bought a CBR, then a GSX-R and didn’t look back. But that doesn’t mean blipping this reimagined Aprilia’s twistgrip isn’t doing something to me.

I pull on my helmet and prepare to roll out of the sleepy West Country village, that Dave’s bodyshop and paintwork business is based in. ‘It’s feisty,’ warns Dave...

45

what’s an Rsw500?

Dave took the paintjob from the Aprilia RSW500, Aprilia’s frst attempt at premier class competition. After winning numerous 125 and 250 world titles, Aprilia employed lateral thinking to chase glory in the 500cc class. They were up against, among others, Rossi and his NSR500. Yes, quite.

The project was headed by Jan Witteveen, later joined by Gigi Dall’Igna (now top man at Ducati’s MotoGP effort). The thinking was, a nimble V-twin, with a lower weight limit, could outfox the brutish, heavier 500. In theory, and on a perfect lap, it could. McWilliams put the twin on pole in 2000 at Phillip Island and scored two

thirds in the same season, at Mugello and Donington (where he was just 1s off the winning race time). The Ulsterman also crashed 14 times, an illustration of just how on the edge he was riding. Ouch.

The main problem, it seemed, was the Aprilia could put in blistering laps,

but if a V4 was parked mid-corner, the Aprilia’s lost its mid-corner speed and it would be blitzed on the exit and down the straights.

With the dawn of the diesels, Aprilia shelved the project to concentrate on the MotoGP RS3 Cube. It was far less successful than their RSW500.

PR

eS

S A

SS

OC

IAT

ION

McWilliams leading eventual winner Rossi and kenny Roberts Jr at Donington 2000

tyga cans emit an evil crackle - and lots of smoke the fairing was widened to fit the huge spannies Maxton modified Fireblade forks slot straight in

Performancebikes.co.uk | OCTObeR 2014

The bike is seriously cramped compared to the four-strokes I’ve been riding recently. It’s made for teens with more-meat-on-a-butcher’s-shoe physiques. I suck in my proto-gut, pull out of the yard and realise the side- to-side steering is tighter than my leathers.

The engine feels unsophisticated. That, for many, is the attraction. It’s as simple as a ditch pump. Minimal number of moving parts for maximum impact. The Hinson clutch is the only heavy thing on the bike. After a while the temperature gauge is showing 55 ,̊ the self-imposed limit Dave has set his engine to run at. I turn onto a straight lined with trees that are so green they seem fluorescent. It’s a perfect English summer’s day, but no longer peaceful. Tugging on the throttle the engine wakes up with all the grace and manners of Godzilla dealing with a TNT enema. I forgot my earplugs and I’m

regretting it. This thing is scaring the life out of me, but I know I would be going at least as fast on an S1000RR. This isn’t a racetrack, it’s not my bike, no one has a stopwatch on me, no one’s even looking, I’m just trying to get a taste of ‘what this thing will do.’ And I’ve been given the answer, loud and clear.

The trees to my left are a TT-onboard blur and I’m in a bubble of noise. I know if I sat further back and tugged the bars it’d lift in third. I’m finding it quite easy to reign my urges in. I admit it, tuned two-strokes are out of my comfort zone, but this ride is pressing some buttons I didn’t know were still wired up. The powerband rips. It would take a long time to get bored of this bike.

‘I like the prettiness of it,’ says Dave, rather unexpectedly, when I return it in one piece. He’s kept everything that was good about the original and improved the rest. ‘I added parts

that I liked from other bikes. I really liked the front end feel when I rode a 2006 Fireblade, so I fitted them.’

Makes sense, I suppose. Handily, the forks slotted straight into the stock yokes. The original forks, on this year of RS250, had minimal adjustment. Dave sent the Honda Showa forks to Maxton for them to be modified to suit the weight of bike and rider.

The same decision process led to Dave mating the Fireblade Tokico front brake calipers with a master cylinder from a Ducati 999R. The wavy discs are 320mm diameter.

After doing some online research, Dave let his plans be known on a forum and was contacted by John Corrin of VSK Corrinaldi, who said, ‘I’ve done conversions like this, let me know if you want any tips or advice.’

Dave took John up on the offer and used his services to rebuild the early YPVS motor. ‘Johnny’s been brilliant,’ says Dave.

The barrels are CPI – Calvin Pollet Industries, original destined for a Banshee quad. ‘There’s a massive market for drag and dune quads in the States, and Calvin Pollet deserves a lot of credit, but sadly, he passed away recently,’ says John, before adding, ‘but it’s not as easy as bolting them on as the ports need altering to suit a motorbike.’

The long-stroke crank is another US-made item, designed for quads. ‘There’s a shopping list of parts you can order from the States, then it’s just a case of bolting it all together properly. That’s what I do,’ says John Corrin. With bigger bores and longer stroke, the capacity is up to 485cc. The bores are fed through V Force 3 reedvalves by brand new 38mm Mikuni carbs with Ramair filters.

‘I haven’t had it on a dyno,’ admits Dave, ‘but I’m expecting 90 horsepower.’ John concurs, saying 80-90 rear wheel horsepower, based on what his modified TZR535, featured in PB, makes. ‘But horsepower figures are bullshit,’ reckons

46

OCTObeR 2014 | Performancebikes.co.uk

BSt carbon wheels are half the weight of aprilia’s the devil is in the detail and Dave’s a perfectionist Close eyes, blip, imagine you are a GP mechanic...

‘The engine wakes up with all the grace and manners of Godzilla dealing with a TNT enema’

47

sPECIFICatIOn DAVE BIMSON’S APRILIA RS485

EnGInE ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||

485cc Yamaha RD350YPVS; CPI barrels tuned by John Corrin; CPI pistons; long-stroke crank; Mikuni 38mm carbs; PMA radiator; Yamaha R6 fan; Zeeltronic programmable ignition; Johnny Corrin Powervalves; V Force 3 reed-valve petals; Ramair f lters; Suzuki RGV250 thermostat housing; uprated mechanical water pump; engine built by John Corrin of vkscorrinaldi.co.uk; engineering by John Finch, Dunkeswell (01404 892958)

ChassIs ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||

2006 Fireblade forks re-valved by Maxton; Aprilia RS250 yokes; Ducati 999R master cylinder; Hagon rear shock; bST carbon wheels; Talon sprockets; Michelin 2CT Power One tyres – medium rear, soft front; brackets by Rob Marsh and owner; bodywork modif ed by owner; TRW rearsets; Nuvo screen; Domino throttle; paint by Db body’s (01823 681499)

Front end

Maxton sticker almost guarantees

a planted front end. 2006 Blade

forks slid straight into 1996 Aprilia

yokes. Easy

hagon shock is 15mm longer to avoid scrapes Mikuni 38mm carbs feed v Force 3 reed valves the slight scuff is due to an initial lack of ride height

one of the coolest frame plates in the bike world

John, ‘It’s more important if it’s rideable and this bike is.’

Corrin made the bespoke powervalves and the Tyga pipes, ordered from the Far East, and a Hinson clutch fi nish the package.

Fitting the engine into the frame required new engine hanger brackets. ‘They’re more L-shaped than boomerang-shaped like the originals,’ says Dave. A kit is available, but, he says, it’s not complete. John gave Dave the dimensions to make the missing parts.

Dave is a fundamentalist when it comes to cooling his bike. It’s swallowed a lot of time and money, and the owner wants to look after his labour of love. ‘Keeping it cool

is key,’ Dave tells me. He had a new, larger and more effi cient radiator made by PMA in Leicester. They made it to suit the gap between the modifi ed fairing and it fi ts like a glove. Dave then searched for a slimline fan, plumping for one from an R6. ‘It fi ts with 5mm to spare all round,’ he states, proudly. It’s linked to a thermostat marketed to kit car builders and wired into a modifi ed loom.

‘The wheels were the last thing I changed and they should have been the fi rst,’ says Dave of the BST carbon wheels. ‘They’re very light, not far off half the weight of the originals. When you’re fl ip-fl opping though tight bends it’s like someone’s helping you.’

Being a painter for a living, that was a job Dave could easily handle, he just had to make the time. The factory replica paint dates from 2000. The bands of colour are all paint, the logos are stickered on, but Dave gave eight coats of lacquer, each carefully fl attened off, so the surface is like glass. There is no ridge where the sticker starts, like most race replicas. Dave also purposely inserted a couple of mistakes into the paint scheme that only one person has ever noticed without prompting. Write in if you spot them.

The fi rst meeting we had planned was postponed. Dave was cagey about the reason, but when we do meet he admits it’s because on the fi rst quick run through his favourite set of bends he heard a ‘chink!’ as the expansion chamber touched down. As he now realises, leaning a bike over at standstill to check ground clearance doesn’t replicate a bike at speed, under compression. The Aprilia was still running the stock rear shock. It was replaced with a 15mm longer unit, with an additional 20mm of adjustment should he need it, from Hagon.

With that fl y fi shed out of the ointment Dave only has one thing on his mind, ‘I spent four years building it, now I’m going to spend the next four riding it.’

Performancebikes.co.uk | OCTObeR 201448

radiator

The PMA radiator is a work of

art. VKS Corrinaldi made the

neat V-shaped panel to fit the

wider gap between the bottom

fairing panels

motor madness

The RS250 has given way to a

RD350 YPVS engine with CPI

barrels, CPI pistons and a long

stroke crank taking it up to

485cc and around 90bhp

paint work

A Windermere of two-pack

lacquer was laid on and rubbed

back to guarantee a perfectly

smooth finish

Zeeltronic programmable ignition cleans things up the seat was re-trimmed by Custom Covers Uk Dave kept the rim widths the same as the originals

Dave looks on at the result of a youth’s

dream built with an adult’s wisdom

OCTObeR 2014 | Performancebikes.co.uk 49