By now, everyone who rides a dirt bike knows that there are a lot of differences between the smokers (2-strokes) and the thumpers (4-strokes). Obviously, the thumpers are easier to ride because of smoother power and engine braking. But, there are some other gray area factors and details that go into understanding these two, very different, and yet similar, iron horses.
When Did 4-strokes Make Their Comeback?
Until the mid-60s, 4-strokes were the kings of the track. Then the lightweight 2-strokes began to take over. The 2-strokes could put out as much horsepower with a fraction of the weight. It didn't take long (by the late 60s) before they captured the hearts of all dirt bike riders. Naturally, from that point forward, all the Research and Development went into the new, light weight, aggressive machines. It didn't take long for two major improvements to develop, the reed valve and the power valve.
By the early 70s the reed valve (located between the carburetor and the cylinder) was on all production bikes. The reed valve greatly increased throttle response. Next, sometime in the early 80s, 2-strokes made the second breakthrough invention with the power valve. This power valve is located in the exhaust port. It changes the opening of the exhaust port according to RPMs. This awesome valve has smoothed out the hit of the two stroke's power band a great deal. The valve creates a lot more power in the first half of the power range and still allows the power to keep going way upstairs. This is because the valve stays closed through the first half of the RPM range, and then it opens the exhaust port volume for the second half of the RPM range. For bottom end power the exhaust port needs to be small. For top end power, the exhaust port needs to be large. But in order to get a lot of horsepower out of that 2-stroke engine they still have a certain amount of "hit" to the power band.
The 2-stroke dirt bike
Why, How and When Did 4-strokes Become King of the Mountain Again?
With all this great 2-stroke evolution, why are 4-strokes on top again? The "why" is because, just after the turn of the century, the United States Congress was backing the Environmental Protection Agency (EPA) to crackdown on air pollution. One industry in their sites was 2-stroke smokers. That's when the entire dirt bike industry decided to put their R&D efforts and future plans in 4-strokes. That was the beginning of the end for 2-stroke domination.
The "how" is because the AMA and FIM underestimated the potential of the 4-strokes. They set rules that a 250 4-stroke could race in the 125 class. Another misguided rule against 2-strokes, was 450 4-strokes could race in the 250 class. Once these rules were official the dirt bike manufactures spent thousands to make dyes and castings to create and sell 4-stroke models.
This unfair 4-stroke advantage, coupled with all the R&D shifting from 2-strokes to 4-strokes, and hiring all the best riders to race them, well, I think you get the picture! And to add insult to injury, 2-strokes are not allowed to race in Pro Motocross or Supercross events. Obviously, the EPA crackdown on 2-strokes never materialized! Most of the Motocross and Supercross racing industry would agree that lowering the displacement rules for 4-strokes would create a more even playing field for the 2-strokes. But, it's too late for that, as the re-manufacturing of all the dyes and castings would be way too expensive. Thankfully, 2-strokes are still popular for the kids from 50s to 125s. They are also popular in many off-road classes. Another, hanging on by a thread fact, is that, KTM, Husqvarna and Yamaha are still creating new big bike models.
The Differences Between 4-strokes and 2-strokes Explained
The 4-stroke is smooth, tractor like power, right off idle, to all the way upstairs. This is why the 4-strokes depend less on the clutch and more on throttle control. You don't have to help a 4-stroke into the power, with the clutch, nearly as much as with the 2-stroke. In order to understand and feel this concept, you have to be able to ride at a high enough skill level, so you can run tall gearing. For example, at the transition of a corner, when you first apply the power, you should be in a tall enough gear that the power comes on at the bottom of the power range. This enables the gear to pull smoother and longer before you need to shift. Using the clutch and throttle together, enables you to better control this critical area.
The other difference is with engine braking. When you're going down a straight-away and you shut the throttle off on a 2-stroke the back pressure from the engine will not slow you down much at all, but on the 4-stroke, this engine braking is much stronger. This will make the most important part of a corner easier (the transition, where you go from braking to accelerating). You don't have to be quite as precise with the brakes.
In a negative way, this engine braking effect could affect you on jumps too. If you shut the throttle off on the takeoff part of a jump, it could throw the front-end down much more than on a 2-stroke. This won't be as noticeable if you're in a higher gear. But, if you're in a lower gear and the engine is revving, it's like applying the rear brake on a 4-stroke! Since there is tractor like power, you don't have to be as precise with the clutch and throttle or the brakes on the easier to ride 4-stroke. At this point in time that's the main differences between the 2- and 4-strokers. The actual riding techniques are the same.
The 4-stroke dirt bike
To Sum It Up...
I know I said in the beginning of this riding tip that, the thumpers are easier to ride because of smoother power and engine braking. But, there are some other grey area factors and details that go into understanding these two, very different, and yet similar, iron horses. Okay, that being said, the other factors and details are cost (bikes and parts), and maintenance (frequency, time and difficulty). I know some 4-stroke owners have a dealership do most of their maintenance!
Back in the age of the dinosaurs when I was 15, my dad bought me a new 400 Husqvarna. I rode and raced that bike for the entire spring, summer, fall, winter (hare scrambles) and the next spring. It never had any new parts, except chains, sprockets, air filters, cables, grips, and shock springs, and maybe handlebars, levers, and stuff like that, from crashing. It never broke down, not even once, and I rode and raced that one bike an awful lot!
Then when I was 16, I got sponsored with free bikes and parts from a local CZ dealership. I could split the case on my CZs and rebuild the entire engine, crank, rod, tranny, clutch - everything. Compare those so called, Good Old Days, when Motocross was the fastest growing motorsport in America, with today, and its only common sense that these factors are keeping less and less people from buying, riding and racing dirt bikes.
Then consider fewer places to ride and spending the entire day at a local race to have a 10 minute crowded practice session, and short Motos. Back in the late 60s and early 70s, there were tons of places to ride, local races had two long practice sessions and three 20 minute Motos per class. There were only four or five classes. By the mid-70s, it seemed like everyone had a dirt bike. I'll say it again - Motocross was the fastest growing sport in America!
Today in 2019, I still ride my RM250, but I'm still guilty of riding the 4-stroke wave. I held on to the smokers as long as I could, but in 2006 I got a 250 and a 450 CRF. I still had two RM250s, and would ride all of them pretty equally. By 2010 I had two KX250Fs and the two aggressive screaming RMs sat in the shop.
Here's a video from when I broke the RM back out years of storage. It explains and shows from a rider's standpoint what the riding differences are between these two types of iron horses. I hope they never become extinct! The instructional part is over half way through the video.
Ride hard, ride smart and have fun.
About Gary Semics:
Learn our GSMXS time tested and proven practice and training methods to improve your riding skills and race results. How? Through our hands on Motocross School Group and Private classes, with located in six countries. Through our Techniques and Training DVDs.
