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Two Stroke or Four?

by Paul Robey on 07/10/2009


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Some time around 1996 one of Yamaha Motor Corps’ top engineers walked into an office full of executives to tell them about a new idea he had to revolutionize the world of motocross. He wasn’t holding the plans for a newport-timing, fueling or intelligent ignition system. Instead he would ask them to pitch the future reputation of the company on a new four-stroke engine.

In hindsight that’s like asking Ronald McDonald to replace the Big Mac with a plain salad. Nonetheless, far from slowly inching their fingers towards the security buzzer, the executives were obviously impressed with his plans for the prototype YZ400. I detest the use of cliches, but the rest [as they say] is history.

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Since then the four-stroke has become king, yet at the same time it’s victory has never been quite complete. In one form or another the two-stroke rebellion still lingers on, and it’s for this reason that the manufacturers find themselves in the strange position of shoveling clods of dirt upon the two-stroke grave with one hand, and with the other holding the coffin ajar.

I wanted to take a deeper look at some of the reasons why the four-stroke has not completely captured the hearts and the minds of the motocross public and ask – in the battle between two-stroke and four – which one is really best?

They say that history repeats itself; the funny thing is that it happened in reverse in the 1960s. Back then, the four-stroke traditionalists, with their puddin’ basins and leather britches, railed against the new breed of lightweight stinkwheels that suddenly began to enter the market and thereby threaten the status quo.

Today the two-strokers might be the traditionalists, but they believe their grievance is not so much a reaction to change, but one grounded in arguments as much of the head as of the heart.

The birth of the four-stroke we know today is in part a reaction to the environmental issues pushed to the fore within the political mainstream across the globe. In countries like America, where the manufacturers have to protect a huge and profitable market, it’s not hard to understand why this would be such a pressing issue; particularly in hotspots like California, where sales are high and emissions laws very strict.

Unfortunately, whilst the four-stroke is undoubtedly more eco-friendly than a two-stroke [at least in emissions terms], the truth is it’s not going to save the world. If you weighed up the other modes of transport your average motocross rider used in a year, like the car, the bus, or [particularly] the jumbo jet, then the relative impact of his or her two-stroke emissions would be negligible to say the least.

Of course, the irony is that the four-stroke has brought with it a different kind of pollution – noise. And who would really be brave enough to argue that the detrimental effects to the sport brought on by a symphony of ear-splitting four-strokes has been outweighed by the benefits of pretending what an eco-friendly bunch of people we are with our cleaner four-stroke power-plants?

When you think about it, our sense of the relative merits of the two and four-stroke has been cleverly skew-whiffed by the current race format that suggests a 450 four is in some way comparable to a 250 two-stroke. In truth the MX1 and MX2 classifications clearly favour the four-strokes; it isn’t a fair fight at all.

There are many people that believe the recent introduction of the four-stroke was actually a regressive move by the manufacturers. They argue that the two-stroke is far better suited to the demands of off-road and has a far greater potential for development – even allowing for the emissions factor.

It’s a little known fact that there are already companies out there that have developed new technologies that radically improve the fueling and emissions output of the two-stroke engine, to a level not that far below a four-stroke. These innovations have already found there way into the jet-ski and watercraft industry, which relies heavily on the two-stroke, and which has also been hit by the green lobby.

It’s true that whilst the power valves are a little more fancy and the carburetors have flat instead of round slides, the two stroke motocross engine has remained largely undeveloped for over a decade. In comparison with the changes seen on production road bikes the two-stroke motocrosser is about as cutting edge as Fred Flintstone’s buggy.

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Christian Burnham remembers a nice story of what happens when you invest some time into making a two-stroke work: ‘the Sarholz Honda CR500 that I rode in the 90s was such a good bike. Rather than trying to get more power out of the engine we chose to make it smoother. We had a new cylinder liner made with smaller pistons and increased the stroke to compensate. This bike was awesome. I remember lining up at the first GP of the year in France [1999] between Peter Johanssen’s factory KTM four-stroke and Yves Demaria’s factory Husqvarna and taking a perfect holeshot!’

Sadly, I suspect that one of the reasons why the manufacturers of dirt bikes have not developed the two-stroke is that it means creating separate [and no doubt costly] development departments within their fundamentally road bike [and thereby four-stroke] operations as a whole.

For the man on the street, there are a number of simple considerations when choosing between a two and four-stroke.

One of the first is likely to be cost. Buying a new four-stroke will see you somewhere in the region of £1000 lighter in the pocket, depending on which model you choose. Generally, the relative depreciation between two or four-strokes is not dissimilar, although the exception is the 250 four, whose second-hand prices do seem to nosedive furthest.

Buying second-hand can be a worry no matter what bike you’re after, but there’s no doubt that buying a second-hand four-stroke carries far more risk. Those that have been well maintained can be great mounts; unfortunately, those that haven’t boast the mechanical integrity of a hand-grenade with a loose pin. In fact, there are plenty of dealers out there that are reluctant to take older four-strokes in part-exchange for this very reason.

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Whilst two-strokes can, and do, let go, a four-stroke explosion will have the victim digging far deeper into their pockets for the repair bill. In many cases, older fours that incur serious failure are simply not cost-effective to repair. The strange thing is, both the prices and demand for older four-strokes don’t seem to have suffered too much as a result;  whether this will be an indefinite trend remains to be seen.

If things are already swinging favorably towards the two-stroke it’s time to swing them a little more – let’s talk about maintenance.

In a nutshell, four-strokes are far more complex than a two-stroke and have a far greater number of moving parts. It follows that they will take more time and skill to keep them up to speed.

Top end maintenance is a case in point. For the two-stroke owner, changing a piston is a relatively simple proposition – drain the water, take the exhaust, carb and plug cap off, disconnect the power valve mechanism, unbolt the head, remove and replace the piston and then reverse the process.

For many four-stroke pilots, looking into the top end is like looking at flat-pack furniture instructions, in Swedish. Even adjusting the valve clearance, which should be a routine task [particularly on the 250s which rev to the moon] – and which doesn’t actually involve removing the head – can be a daunting proposition.

On some models you have the luxury of adjusting the clearance via a locknut and rocker arm. However, on most modern fours, like the CRF and RMZ, you have a bucket and shim configuration that means removing the camshafts if adjustment is necessary. You then have to attempt the formula for calculating the replacement shim sizes before ordering them from your local dealer.

In fairness, four-stroke maintenance is a lot worse in the thinking that it is in the doing. However in circumstances where it carries a certain ‘fear factor’, or where riders are pressed for time [I won't say lazy!], the worry is that some might regard the more demanding tasks as those best left to the bike’s next owner.

I wouldn’t dream of speculating on the relative numbers of two and four-stroke bikes that incur catastrophic failure, but I have a hunch, and it probably has far more to do with the actions the owner than any inherent weakness in the machines themselves. I think it’s fair to say that because of their inherent simplicity, two-strokes suffer fools more gladly than fours.

On paper, one of the biggest differences between a two and four-stroke is in the weight department. The 450 four-strokes are generally quoted between 100-102kgs, whereas the 250 two-strokes are around the 96kg mark. The relative difference between the 250 four and the 125 two-stroke is not dissimilar, with the 250F coming in at 93-94kg, whilst the 125 comes in around 86-88kg.

Unfortunately the figures don’t tell the whole story. The problem with the four-stroke engine is not just that it’s heavier than a two-stroke, but that its particular design means the additional weight is carried higher in the chassis – the valve-train is a perfect example. This creates a pendulum effect that hampers the bike’s turning ability – imagine a slalom skier trying to turn whilst carrying a dumbbell.

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The increased number of moving parts, combined with their particular location within a four-stroke engine, also increases its relative gyroscopic mass. This is the kind of force felt when you spin a wheel on its spindle and then rotate the spindle off centre. Again, this increased effect within the four-stroke affects its turning ability both on the ground and in the air.

Two-strokes produce far more power than four-strokes, but in motocross, peak horsepower is a secondary consideration – it’s not so much about how much power gets to the ground, but in the way in which it gets there. A two-stroke’s power explodes over a short period, whereas the four’s is more linear and results in a far wider power-band. Torque is everything, and four-strokes have it in abundance.

The benefits of having usable power across a wider spectrum are many. For starters, on a four-stroke you don’t have to spend as much time changing gears, and therefore you spend less time with the throttle partially disengaged. The linear power also encourages the rear tyre to grip instead of spin, and its predictability makes it an ideal choice for the lesser-skilled rider.

One of the fundamental differences between the two and four is in the level of front end grip when approaching and riding through the apex of a turn. On a four-stroke, shutting the throttle into a turn is like dragging the rear brake. This has the dual benefit of not only slowing you down without the front forks diving [which thereby upsets the handling], but the engine braking effectively ‘holds’ the front wheel into the ground right through the corner.

If you shut the throttle abruptly on a two-stroke you will simply freewheel, so you can’t brake anywhere near as deep into corners without the risk of losing the front. Instead, you have to brake earlier and get on the power earlier so that the front wheel is effectively pushed into the ground by the forward motion of the rear.

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The two-stroke is a bike that rewards a skillful rider, but is much less forgiving for those with poor technique. Anyone that has a road bike background [and is therefore used to riding a bike that has front end grip all the time] and wants to try off-road would probably gel far more with the four-stroke than the two.

Four-strokes are generally far better suited to the demands of riding on hardpack. The softer power delivery encourages the rear tyre to grip and the bike has a far more planted feeling because the engine maintains constant tension on the chain.

When ridden in sand [proper sand!], the extra weight of the four-stroke can be very demanding on the body. In addition, whilst the goal is to get the front wheel ‘ floating’ over the very loose stuff in the corners, the front end bias of the machine can make the front feel like it wants to tuck under. In contrast, the rear end bias of the two-stroke [not to mention its lighter weight] can make riding in these conditions much less of a chore.

As far as riding in mud goes, any bike will feel heavy when it begins to stick, but there’s no doubt a 450 four-stroke will test your fitness to the limit in the worst kind of conditions, particularly if you have to pick the thing up off the floor. The sheer power of a 450 can also cause the rear tyre to spin through mud whereas a 250 two-stroke will often bog down slightly under acceleration, which can then let the rear end dig in and bite.

Ironically, the roles can be reversed for the smaller bikes. The 125 can be a peaky motor to ride when it’s very muddy and its all or nothing power can make you feel like your whipping a tired donkey. The 250 four-stroke is torquier off the bottom than a 125, and as well as being lighter and not so brutally powerful as the 450, can be a great bike in these kind of awkward conditions.

To conclude, even if you look at things in an ideal world, there is really no simple answer to the question of wether a two or four-stroke is best. However, it would certainly be interesting to see what kind of two-stroke a company could come up with if its motocross department were given the same free hand as it had for the CRF450. Then we might have a shoot-out on our hands.

As things stand, for me, making the choice can be as much about emotion as pure logic. My head knows that I’m faster on a four-stroke, but once you’ve reached that point as a rider when you finally accept you’re never going to be world champion, being faster isn’t always the be all and end all. My heart is with the many riders who race a two-stroke precisely because they are considered a spent force.

When you see yourself as the underdog then victory is all the more sweeter. The best thing is that if you can win on a two-stroke you’re a hero; if you lose you can just blame the bike!

In the final reckoning, perhaps we should simply be thankful for the luxury of choice – one day we might not be so lucky.


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