dirt bike – AdventureBikeTroop

When buying your first dirt bike, an important question you need to ask yourself is whether a 2-stroke or a 4-stroke dirt bike will be better. There are some major differences between the two that may influence your decision. 4-stroke dirt bikes deliver smooth and consistent torque throughout the rpm range compared to the steep, narrow power band on 2-strokes. 2-stroke dirt bikes have less moving parts in the engine resulting in less maintenance and a lower weight than 4-strokes. Most modern 4-stroke dirt bikes are fuel-injected whereas 2-strokes still have carburetors. While those are the major differences there are more to consider, but first: Let's have a look at what 2-stroke and 4-stroke mean. What Does 2-Stroke and 4-Stroke Mean? 2-Stroke and 4-Stroke refers to the internal design of a dirt bike engine. The piston in a 4-Stroke engine moves up or down 4 times to complete each combustion cycle: Intake, Compression, Combustion, Exhaust. 4-Stroke engines have valves that open and close depending on the position of the piston. The movement of these valves have to be timed perfectly and therefore requires a lot of intricate moving parts in the top-end of the engine. 2-stroke engines don't have moving valves, but rather make use of ports in the cylinder walls. These openings are opened and closed by the piston moving past it during its up and down travel. As a result, a 2-stroke engine has far less moving parts and can move through all 4 stages of the combustion cycle with one up and one down movement. 10 Differences Between 2-Stroke and 4-Stroke Dirt Bikes Before we get too technical, let's have a look at 10 real differences between 2-stroke and 4-stroke dirt bikes that may assist you in choosing the right type of dirt bike for you. #1: Power Compared to Engine Size 2-Stroke dirt bikes generally generate more power than 4-strokes for a given engine size. This is as a result of less moving parts in the engine and therefore less internal frictional losses. Another reason is that a 2-stroke engine crankshaft only rotates once for each power stroke. On a 4-stroke the crank has to rotate twice during each combustion cycle. This is why you often see 125 cc 2-stroke dirt bikes compete directly with 250 cc 4-strokes, and 250 cc 2-stroke dirt bikes compete with 450 cc 4-strokes. While the power output is not identical, it is close. The 2021 Yamaha YZ 250 (2-stroke), for example, develops 41.5 Nm of torque at 7 500 rpm while the 2021 Yamaha YZ 450 F (4-stroke) develops 44.3 Nm at 7 400 rpm (only 6.7% more). The 450 cc 4-stroke delivers 53.1 hp at 9 700 rpm compared to the 48.8 hp (at 8 800 rpm) of the 250 cc 2-stroke. The Yamaha YZ 250 2-stroke and YZ 250 F 4-stroke look almost identical (Source: Yamaha USA) The torque curve on a 4-stroke dirt bike is much smoother and more linear than that of a 2-stroke. Twist the throttle and there's immediate power. On a 2-stroke dirt bike, the torque curve is much steeper and available in a narrower rpm range, referred to as the power band. A 2-stroke requires more rider input with the clutch and throttle to keep the engine revving in the sweet spot. This will heavily influence your riding style. If you intend on during hard enduro and technical riding where you need a sudden burst of power, a 2-stroke may be best. If you plan on long distance full speed rides, a 4-stroke is the way to go. #2: 2-Stroke vs 4-Stroke Weight As a result of less moving parts in the engine, 2-stroke dirt bikes weigh less than their 4-stroke counterparts. On modern dirt bikes the weight difference between 2-strokes and 4-strokes are not as pronounced as you might think. The 2021 Yamaha YZ 250 2-stroke motocross bike weighs only 7 lbs (3.2 kg) less than the YZ 250 F 4-stroke. Most racing dirt bikes that are available with 2-stroke and 4-stroke engines share most of the other components and are therefore nearly identical. The only difference is the engine and therefore the weight difference is as a result of the additional valve train on the 4-stroke. A modern 4-stroke is rarely more than 5 to 10 lbs heavier than a 2-stroke. This may sound insignificant, but if you consider that the average competition dirt bike barely weighs more than 200 lbs then the weigh difference can be noticeable. A 2-stroke dirt bike requires less physical assertion from the rider to maneuver around at low speeds while a heaver 4-stroke will feel more planted racing across the desert at wide open throttle. #3: 2-Stroke vs 4-Stroke Traction The linear power delivery and extra weigh of a 4-stroke dirt bike often results in better traction throughout the rpm range. A two-stroke dirt bike's narrow power band results in a sudden surge of power which, combined with the lighter weigh, requires better clutch and throttle control from the rider to maintain good traction. On a 4-stroke dirt bike, you can twist the throttle at nearly any engine speed and there will be smooth power. Yes, there is a spike at higher rpms, but it is more predictable and easier to get used to than the power band on a 2-stroke dirt bike. Most 2-stroke dirt bikes feel sluggish right off idle and requires more slipping of the clutch to get moving. This may also be necessary during slow corners where the engine speed drops significantly. The maximum torque is delivered in a narrower rpm range and therefore requires more frequent gear changes and clutch slipping to keep momentum. Maintaining traction on a 2-stroke race bike requires better throttle and clutch control (Source: Yamaha USA) 4-stroke dirt bikes produce engine braking due to friction of all the internal moving parts. This can assist the rider in slowing the bike down or controlling to motorcycle down steep slopes. 2-stroke dirt bikes don't generate much (if any) engine braking which can take some getting used to if you are used to it (like when driving a car). #4: Fuel delivery and Emissions Almost all modern 4-stroke dirt bikes have electronic fuel injection, while many 2-strokes still use carburetors to deliver fuel to the engine. There are, however, some modern fuel injected 2-stroke machines available these days, for example the KTM 300 XC-W TPI. Some modern 2-stroke dirt bikes, like the KTM 300 XC-W TPI, have electronic fuel injection instead of a carburetor (Source: KTM USA) Fuel injection adjusts the air-fuel mixture ratio electronically via a computer which means the bike will start easier and always run optimally, regardless of the elevation. On a carburetor, you have to tune the air-fuel mixture by opening up the carb and replacing the different jets. This doesn't mean fuel injection is always better. Yes, fuel injection requires less maintenance and emits less carbon dioxide, but if something does go wrong (like drowning your bike for instance), a carb will be easier (and possibly cheaper) to get going again. It is also easier to tune a carburetor yourself, but then again, it might also be necessary more often. #5: 2-Stroke Dirt Bikes are Easier to Start 2-stroke dirt bikes are generally easier to start thanks to less internal moving parts and only two strokes for each combustion cycle. This is only true if the carburetor is tuned properly and not flooded due to forgetting to close the fuel tank petcock after the last ride. As me how I know! A 4-stroke with fuel-injection will start easier than a carb'd 2-stroke every time, despite more moving parts and double the amount of strokes per cycle. Most modern 4-stroke dirt bikes have an electric starter button while many 2-strokes still only have kick-starters. To kick-start a properly tuned 2-stroke is no big deal, but depending on the terrain you ride in and how often you stop the engine, it can be a pain. If you stall a dirt bike halfway up a slope, an electric start is a big advantage. On a bike with only a kick-starter, you may have to push back down the hill to start the engine. I prefer a bike with both electric and a kick-starter as a back-up. Some modern 4-strokes don't have a back-up kick-starter anymore, which means a dead battery will leave you stranded. #6: Ease of Maintenance and Cost of Ownership Since 2-stroke and 4-stroke dirt bikes share all the same components, apart form the engine, regular maintenance won't differ amongst them. You still have to wash the bike, clean the air filter, lube and adjust the chain, clean and adjust the brake and clutch cables, and service the wheel bearings. The biggest difference in maintenance between 2-stroke and 4-stroke dirt bikes is an engine rebuild. Dirt bikes, especially powerful race bikes, require a top-end rebuild every 60-120 hours of hard riding. A top-end rebuild on a 2-stroke will be significantly less costly than a 4-stroke due to far fewer internal parts. Both 2-stroke and 4-stroke dirt bikes will require a piston and ring set, gaskets and bearings. 4-strokes, however, may require valves, valve guides, and work to the camshaft and rockers. Additionally, 4-stroke dirt bikes require regular valve clearance adjustments. As the cam lobes wear, the valve clearance will reduce until there is no clearance left. At this point, the valves won't fully close and could damage the rest of the components. Frequent valve clearance adjustment can be a pain and if you buy a second-hand 4-stroke, you'll always wonder whether it was done regularly by the previous owner. 2-stroke machines without an automatic 2-stroke oil pump and a separate oil tank will require you to premix the fuel with 2-stroke oil to ensure lubrication and cooling. This is something that is not necessary on a 4-stroke dirt bike. #7: 2-Stroke vs 4-Stroke Cooling Most competition or racing dirt bikes have liquid-cooled engines. This means the engine is cooled by liquid (water and coolant mix) that circulates through water jackets in the engine side walls and through an external radiator with a fan. Liquid-cooling is very effective, but does require more maintenance and are more vulnerable during a crash than air-cooled engines. Most modern dirt bikes, like this Kawasaki KX 250 4-stroke race bike, have liquid-cooling (Source: Kawasaki USA) 2-stroke dirt bikes will run cooler at low rpms due to lower idling speeds and half the amount of explosions per cycle. At faster speeds, there is not much difference between 2-strokes and 4-strokes. Additionally, 2-stroke engines require 2-stroke oil to be mixed with the fuel in order to lubricate the cylinder walls to keep cooler. Carbureted dirt bikes will run hot if it is jetted too lean, so it is important to tune the carb properly when moving to a lower altitude. Incidentally, if you add too much 2-stroke oil to the fuel, the bike will also run lean and possibly get hot. #8: 2-Stroke vs 4-Stroke Sound If you've ever been around dirt bikes, you'll know that there are two very different sounding bikes. 2-stroke dirt bikes have a distinctive hin-hin-hin-hin-sound, that is usually accompanied by some smoke. 2-stroke engines have a higher-pitched sound and cannot be heard as far away as a 4-stroke. For more detail on why the 2-stroke makes that unique sound, check out this post I wrote. A 4-stroke dirt-bike has a deeper (lower frequency) sound that travels further and is often louder. Every afternoon I can hear someone tearing up the track close to our house and it is only a 4-stroke that can be heard over that distance. Obviously the exhaust pipe, and whether the muffler was removed, makes a big difference on both types of bikes. A more free-flowing exhaust will make a louder sound than a stock, restricted exhaust pipe. #9: Current Development Dirt bike manufacturers are spending more time and money on the development of 4-stroke engines due to the ever-increasing regulations on carbon emissions. As a result, some manufacturers are not selling new 2-stroke machines anymore, like Honda and Kawasaki. The European bike manufacturers are still developing their 2-stroke dirt bike technology. KTM for instance, recently starting producing fuel-injected 2-stroke dirt bikes like the KTM 300 XC-W TPI. Husqvarna also developed fuel-injection for 2-strokes like the Husqvarna TE300i 2-stroke dirt bike. Gas Gas and Beta are two other European brands that still produced modern 2-stroke race bikes. Most of the European manufacturers still develop their 2-stroke race bikes, like this Husqvarna TE300i with TPI (Source: Husqvarna) Of the four Japanese manufacturers, it is only really Yamaha that still sells competition 2-strokes like the YZ 125 and YZ 250. Honda, Kawasaki and Suzuki switched over to 4-stroke race machines while still manufacturing 2-stroke kids bikes like the Kawasaki KX 65 or KX 85. #10: Type of Riding Choosing between a 2-stroke and 4-stroke dirt bike will come down to the type of riding that you will do. If you are into hard enduro, where you need a light bike with a massive burst of torque to double-blip over obstacles, a 2-stroke might be best. If you intend on racing flat-out in a desert race for long stretches, a 4-stroke will be more stable and last longer at constant high rpms. The type or riding that you do will influence your decision between a 2-stroke and a 4-stoke (Source: Husqvarna) On a motocross track or forest single track it will come down to personal preference and your own riding style. Personally, I love the kick-in-the-pants of a 2-stroke and the fact that you need to learn better clutch and throttle control to get the best out of the bike. For exploring the woods I prefer the smooth power delivery and reliable starting that comes with fuel injection. Conclusion: Which is Best? It is impossible to pick a clear winner and say which is best between a 2-stroke and a 4-stroke dirt bike. After taking the above 10 differences into account, the best you can do is to take both types of bikes for a test ride and get a feel for the them. I am still undecided and I think the only solution is to get one of each. Happy riding!