UP IN SMOKE
If you’re a two-stroke nut then chances are you’ve spent the last decade or so crying in the corner of a dark room, but there’s a glimmer of hope that you may not need to cry much longer...
It’s been a long time since any major manufacturers cranked up the production lines and wheeled off a stroker, with Cagiva’s SP 525 hooligan learner bike being one of the last examples to make it into the showrooms. Yamaha were still pumping out their headbanging TZ250 racer until quite recently, but unless you wanted to share a prison cell with a hairy misogamist named ‘Bob’, the race focused weapon was little to no use for Her Majesty’s Highways.
If you’re over the age of 30, chances are you grew up riding a two-stroke of one capacity or another, and it’s likely to have left an indelible mark on your biking persona. Be it for their stonking performance or sheer inbility to go for a whole week without breaking down, twostrokes offered an experience of surprise that kept most of us on our toes and filled our daily boredom with memories of both the good and bad variety. If you’re like me, as in slightly fat and balding, then you’re probably old enough to feel a certain nostalgia to Suzuki’s RG500, which I class as the last mainstream superbike to have made it into dealerships. The 250cc two-strokes enjoyed a longer stay of execution but the Japanese big four had abandoned ’strokers and their fans by 1998. Luckily, Suzuki sold the design rights to their RGV250 engine to Aprilia, who continued to use a modified version of the Suzuki engine in their RS250 right up until 2004.
Many aficionados cite the little Aprilia as the purest two-stroke road bike ever made and 13 years on, the RS250 still looks and handles like a race bike that’s escaped from the paddock. Which it is.
Return of a full-fat 500
Aprilia were about to encounter some heavyduty competition to their pre-mix powered monopoly. Bimota, purveyors of exquisite if quirky Italian exotica, took the risky step of deciding their first ever in-house designed engine should be a two-stroke. And not a tiddly, little 250 like everybody had been pushing for the last decade, oh no, they decided to go for a full-fat 500. In 1997, 10 years after the Japanese had turned their backs on big ’stroker lovers, Bimota unveiled the V-Due.
The V-Due’s sales brochure read like a pure race bike: The bike’s 500cc, 900 V-twin engine was to be held in a twin-spar aluminium frame, suspended with fully adjustable 46mm Paioli forks and Öhlins rear shock all wrapped up with mouth-watering Italian styling. Bimota promised that the V-Due would deliver a useful 110bhp punch with a 145kg wet mass. To put that into perspective, it was going to weigh the same as the Aprilia RS250, but with twice the power (but at nearly three times the price). It seemed as though the proposed production run of 500 bikes would be snapped up by (wealthy) two-stroke junkies as soon as they hit the showroom. How could Bimota go wrong with a bike that was going to pack decent power in a top drawer chassis that weighed the same as a 250?
To answer that question, I’m afraid we need to understand a little bit about how two-stroke engines work and particularly why they’ve always had a rep for being a bit environmentally unfriendly. Unlike a four-stroke, a two-stroke engine doesn’t use valves to control its air/fuel flow. Instead, ports (holes) are cut into the engine’s cylinder walls and the piston acts like a valve as it slides past these ports. To be able to accomplish a gas exchange (out with burnt exhaust gas, in with a fresh charge) in just two strokes, the ports that allow fresh charge in (transfer ports) need to be open at the same time as the exhaust port. So, you may be asking yourself, what stops the fresh charge from just heading straight out of the exhaust port then?
Well, nothing as it happens. And here lies the first problem of managing polluting two-stroke emissions. Not only does some unburnt charge escape out the exhaust (causing hydrocarbon pollution) under certain conditions, a significant volume of exhaust gas stays in the cylinder and ‘dilutes’ the next cycle of incoming charge. This is also bad, because it causes the engine to
misfire until the engine has cycled through enough times to result in a charge that can
be lit by the spark plug. This phenomenon known as ‘four-stroking’ also leads to unacceptable levels of hydrocarbon pollution. Now consider that traditional two-strokes burn their lubricating engine oil along with their fuel and you can see why tree huggers hate them so much.
Lethal injection
Got that? Now, back to the V-Due. Bimota took the incredibly brave and noble step to take on the task of cleaning up the twostroke and re-establishing it as a viable bike engine for the future. They did this by equipping the V-Due with electronic fuel injection, a first.
The V-Due’s injection system was doubly impressive because Bimota took the unprecedented step of designing the injectors to spray fuel directly into the cylinder transfer ports rather than the engine’s intake like a ‘normal’ injection system. Not only were Bimota brave and noble, they were sadly naïve and overconfident to believe they could master a problem that had vexed the likes of Honda and Yamaha for over 30 years.
Inevitably, Bimota’s direct injection system didn’t work. Sure, the V-Due passed American EPA emissions tests, but the bike was unrideable. The fuelling was so bad customers universally complained, added to other quality issues, Bimota had to concede that the bike wasn’t fit for purpose and offered everyone who bought a V-Due their money back. By 1999, the V-Due disaster finished Bimota off. Only 340 examples were ever made, 21 of which weren’t even fuel injected, they actually had carbs fitted instead. The clean two-stroke dream entered the millennium still-born. The lesson all major bike makers learnt from Bimota’s great folly was to steer well clear of two-strokes.
Stroker of genius
With the sole exception of Aprilia’s RS250, production sports two-strokes were dead and buried by the new Millennium. Honda even killed off their rare and exceptional CRM250AR enduro bike in 1999. The technology contained within this little off-road single was possibly a twostroke’s best chance to clean up its act. The CRM250AR incorporated technology from Honda’s class-winning experimental 1995 Paris-Dakar EXP-2 two-stroke racer. CRM’s ‘AR’ stood for Active Radical, referring to ‘free radicals’ that are present in still hot recently combusted exhaust gas. The CRM250AR utilised this still ‘active’ exhaust gas to spontaneously ignite the fresh incoming charge under certain loads and engine speeds. In actual fact, the CRM operated
much like a two-stroke diesel from around 10% to 60% throttle! The upshot of this unusual combustion strategy was that the CRM250AR was more fuel efficient and tractable than an equivalent four-stroke, had cleaner exhaust emissions and it weighed less. If Honda were prepared to turn their backs on two-strokes after investing so much time and money in developing their ground breaking ‘Active Radical’ system, you can see why other bike makers concluded that two-stokes had no future. But not so fast, tree huggers; luckily for us two-stroke fans are a hardy bunch and thanks to a few fanatics, the engine that can boast double the specific power output of a four-stroke, didn’t disappear completely. Although, not exactly ‘run of the mill’, a handful of bespoke European engineering firms decided to keep the oily flag flying just to remind us all how weak four-bangers are.
The Swiss Big Cheese
Few may remember Swiss GP rider Eskil Suter for his racing career. But I doubt that bothers him much, because Eskil’s got other skills too. He is the brains behind Suter Racing Technology, the company responsible for designing the Petronas FP1 and the Moto2 chassis that Marc Márquez rode to victory in the 2012 600cc championship.
SRT also helped Kawasaki develop their ZX-RR MotoGP bike and they assisted Formula 1 specialists Ilmor Engineering with the chassis design of their 800cc Ilmor X3 MotoGP bike. STR realised that their expertise shouldn’t be confined to a handful of racers. As a way to showcase their engineering expertise and the potential of two-strokes, STR are crafting a limited run of (just 99) specials called the MMX500. With credentials like the ones above, any road or track special SRT produce should be good, right? They say the Devil is in the detail, which we’ll get to, but just look at the MMX500’s basic stats – 195bhp and 127kg! Even with a 70kg rider, the MMX500 still matches the power to weight ratio of a modern MotoGP bike.
To achieve this incredible feat, STR cradle their own hand-built V4 two-stroke in a fully adjustable CNC ally frame and swingarm. Suspension is top-drawer Öhlins, wheels are forged magnesium from OZ, braking is by Brembo, the custom exhaust expansion chambers are titanium with carbon fibre Akrapovic silencers. The spec’ sheet is endless. If you peek behind the carbon-fibre bodywork and under the carbon-fibre tank, you’ll find a bespoke fuel-injection system with three injectors per cylinder, such is the deluge of petrol required to fuel the MMX500’s mighty 576cc two-stroke engine. But to be honest, if you can afford the £94,000 purchase price, ‘The Beast’s’ thirst for pre-mix isn’t going to worry you.
Legally fast
What if track days aren’t your thing, instead you want a viscously fast, light modern ’stroker to pop to Tescos on or shave seconds off your commute? Well, fear not, the Germans have got you covered (albeit only a very well-heeled 46 of you).
If you’ve followed any of the World GP Legends races you will have heard of Ronax. Their 500cc V4 two-stroke race bike furnished Jeremy McWilliams with a win
at the 2016 Sachsenring Classic races last year. In Jeremy’s hands, the bike was more than a match for the likes of Freddie Spencer and Garry McCoy on their ageing factory 500 GP machines. The significant thing about Ronax isn’t the fact that a small engineering firm can field such an impressive bike in the World GP Legends series, no, incredibly, Ronax’s 500 is available as a road bike, complete with lights, number plates and mirrors. Presumably to watch everything else on the road disappear in. For a trifling £84,000 (plus tax), you can enjoy a full on, 160bhp, 147kg, 500cc GP bike on the road. Wouldn’t it be great if a major bike manufacturer would take the lead and try bringing the two-stroke up to date and to the masses in the way these specialists companies have for the super-loaded?
The good news is that a couple of the big boys never completely ditched the twostroke. Fair enough, Yamaha and KTM only make single-cylinder off-road ’strokers at the moment. But it proves that as far as MX and enduro goes, the market for two-stroke thrills never completely died. Moreover, the evidence suggests a strong resurgence in demand for ‘stink-wheels’ with enduro riders. Here’s the reason: Providing power at every stroke of the piston gives the two-stroke engine access to huge potential – not just searing top-end power, but if tuned correctly, also with monster torque, right off the bottom. It’s not the top-end power that diehards love about the two-stroke enduro bikes still available from the likes of KTM, Husqvarna, GasGas and Beta, but the monster torque available at really low rpm.
Even when bogged right down to tickover speeds, these bikes respond with front wheel lifting torque. Coupled with lower weight, easier starting and simpler maintenance, grunty, powerful two-strokes are just too good for dedicated enduro bike makers to ignore. So good in fact, that rather than take the easy option and abandon two-strokes in the face of strict Euro 4 emissions regulation, KTM are equipping their 2018 250 and 300 EXC models with fuel-injection this year.
Secret stroker
By the time you read this KTM will have released these new models, and all the secrets that come with them, but it seems the KTM’s set up has similarities to the Bimota V-Due’s transfer port injection system. This might seem risky, but transfer port injection is necessary to meet exhaust emissions targets. However, repeating the mistakes made by the Italian firm would be ruinous for KTM.
So it’s no surprise to learn that test bikes from the Austrian maker have been spied competing in enduro events all over the world for several years now. The chance of it running like a bag of spanners like the Bimota did are minimal, but even if this new breed of FI two-stroke off-roader cracks the dirt bike market, what are the chances of it leading to new, road going two-stroke bikes? Let’s not get too giddy about the thought of Suzuki unveiling an all-new RG500 any time soon. First of all, emissions regulations are just getting stricter and stricter. For a high performance road going two-stroke to meet Euro 4, it would need to incorporate complex direct fuel-injection, a direct lubrication system (rather than premix) and catalytic converters in its expansion chamber exhausts. If any two-stroke is still able to outgun a four-stroke after that, the issues with piston, ring and bore wear also need to be addressed. Otherwise major services to a ’stroker are going to cost three times those needed for a comparable four-stroke. And with money being as tight as it is the world over, there’s little chance of any manufacturer heading out down that costly road.
So, get buying those lottery tickets because if you’re desperate to be reacquainted with some GP inspired two-stoke exuberance it’s the likes of Suter and Ronax who you’re going to have to turn to. And they’re not cheap. For the rest of us, we’ll just have to hope that some hardcore stoker fans find their way into the motorcycle industry and start forcing a hand to see this new and cleaner tech being utilised on production road bikes. Until then…