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Have you ever been curious how fast a 250cc dirt bike can go? Well after some testing I am ready to answer your question.
While max speed will vary slightly based on the type of engine, a 250cc dirt bike will come in between 70-78 miles per hour. 250cc 2-strokes will be faster than 240cc 4-strokes, but only by 5-10 miles per hour.
There is a lot that goes into max speed, and we are going to dive into all of those reasons as well as ways to increase it.
Dirt bike engines come in two different kinds of strokes: the two-stroke and the four-stroke. Because of the way they’re built and the way they function, each one has different “specialties” that have an impact on acceleration, top speed, and multiple other variables.
Two Stroke Engines
Two-stroke engines are the much less complicated engine of the two options. They are designed to complete their engine cycle in two piston movements, instead of four, like the four-stroke.
The two-stroke engine begins its cycle with a power stroke. The ignited air, fuel, and oil mixture force the piston down until the mixture reaches the exhaust port, an opening on the side of the cylinder. The piston travels downward, and it pressurizes the air, fuel and oil mixture that was previously drawn into an attachment on the side called a “crankcase”. The mixture was pulled into the crankcase during the most recent compression stroke, the one prior to the current power stroke that hasn’t completed yet.
An exposed intake transfer port lets the next air and fuel mixture into the cylinder, right as the crankshaft begins its next rotation, pushing the piston back up. This action blocks off the exhaust and intake ports, enabling the piston to compress the fuel and air mixture. The upward action of the piston pulls in the next fuel and air mixture from the carburetors and keeps it underneath the piston. The currently compressed air and fuel “charge” above the piston is ignited by a spark plug, and the whole thing repeats itself, over and over again.
This operation produces less waste than a four-stroke engine of similar or equal power output. They have a more efficient build, enabling them to be assembled and maintained using thirty to fifty percent fewer moving parts than a four stroke. Two strokes are the easiest of the two types of engines to clean. They do, however, have elevated fuel consumption, and because of the way the stroke of the engine works with the openings in the chamber, it produces more emissions.
Due to the intake and exhaust ports being open at the same time, with each piston rotation a portion of the air and fuel mixture escapes out the exhaust port without being used to power the engine. That being said, direct injection and catalytic converters are the exceptions to those rules and will reduce the number of unburned hydrocarbons in your emissions.
Due to the way they’re assembled, they require fuel and oil mixture. Two-stroke engines generally require more maintenance, but the parts are cheaper. Bikes with two-stroke engines are generally lighter and faster, with more of an initial kick to the “get up and go”. While they require half the strokes to accomplish the same purpose as a four-stroke engine, they do make twice as much noise. Two-stroke engines will give you more torque at a higher RPM.
Four Stroke Engines
A four-stroke engine takes four piston movements to achieve a singular engine cycle. On the first, or “intake” stroke, the piston lowers, pulling in a mixture of air and fuel. Next, it raises for the “compression” stroke, which ignites the air and fuel mixture. This ignition forces the piston down for the “power” stroke, followed immediately by the “exhaust” stroke, pushing the exhaust out of the engine. These cycles repeat over and over again, the whole time the bike is running.
Four stroke engines “fire” every two revolutions of the crankshaft, which delivers a steadier and more easily managed output of power. Typically a better bike for beginners, a four-stroke engine is great for both trail riding and racing. When trail riding on a four-stroke, you’ll rarely ride above second gear, because they can rev higher than a two-stroke can.
This means less work for you and your engine. There are more parts to a four-stroke engine, which enables you to spend less time braking and shifting your bike when you have to slow down.
Maintenance is needed much less frequently since the work the engine is doing has been spread out through more parts. Unfortunately, this also means that when maintenance IS needed, it is generally more expensive, since more parts need to be worked on and replaced. These additional parts also make bikes with four-stroke engines heavier than their two-stroke counterparts.
Four stroke engines are a very clean burning engine from the viewpoint of emissions testing. This is because they have a higher fuel efficiency than the two-stroke engine, and you don’t have to mix oil with your fuel in order to make the engine run.
Four stroke engines are heavier, which results in a heavier bike when similar cubic centimeters are concerned, sometimes weighing 50% more than comparable two-stroke engines.
They are also much harder to clean, due to the higher number of parts they’re built of. Four stroke engines will give you more torque at a lower RPM.
Sometimes, after all, you can do to your bike, your the one that might need an “upgrade”. You may have the best, newest, most expensive model of dirt bike out there, but if you don’t know how to use it, you aren’t going to be able to use it to it’s fullest potential.
As painful as this is to address, your weight is going to have an impact on how fast your bike is able to go. Now really, it generally will only have a derogatory effect on your experience if you’re racing in a straight line. The technique will have much more of an impact on your riding experience than your weight will if you’re racing on a trail or a dirt bike track. That being said, weight does make some difference, no matter how you look at it, even if it isn’t always going to be the deciding factor.
Like I mentioned previously unless there is an incredibly large disparity in weight between you and the other riders (in their favor), skill is going to be the deciding factor, not your unfortunate fast food habits. So make sure you’re getting in your practice! You may be able to ride your dirt bike without falling off, but that doesn’t mean you have any kind of skill outside of basic balance, and knowledge sufficient enough to get your bike’s engine running.
What do you need to do to get better you ask? Well, like any mother who has ever tried to make her child learn the piano will tell you, PRACTICE! The practice may not make you perfect, but it does make progress, and enough of it may even make you proficient. Are you practicing for a race? See if you can practice on the track. Want to be faster on the mountain trails you and your buddies frequent? You may have to make some extra runs.
Once you have things down like shifting, braking, leaning into turns, and knowing what it’s like to really move with your bike, things become more of an instinct and a reaction, rather than an action that requires conscious thought and decision making before you can decide on your next move. Those decisions may only take you split seconds, but often times split seconds are the only differences between first place, second place, third place, and not placing at all.
While improving yourself is a necessary and often overlooked part of improving speed and your overall dirtbike experience, there are also alterations that can be made to your bike as well.
That’s right, we don’t discriminate here. There’s a possibility that you AND your bike may be able to lose a few pounds. Remember, since we’re talking about weight, dirt bikes with two-stroke engines are always going to be lighter than their four-stroke counterparts. So if weight is what you’re wanting to focus on, make sure to keep that in mind.
Now, to be clear, these are not recommendations. I’m going to give you ideas and examples of how to make your bike lighter based on my experience and the experiences of others, but make sure you do your research and are responsible with the alterations you make to your bike. Just because you can do it doesn’t mean that you should. Structural stability should always be maintained.
- Lighter tires – Will this make a HUGE difference when cutting down weight? Probably not. But if you can make lots of little changes, the weight you’re subtracting from your bike will start to add up. In a good way. You can also not add tire mousse or tire slime to your tires. Does this come with some inherent risk? Yes. But so does riding your bike in general.
- A lighter seat, or a lighter seat cover – There’s an above average chance that the seat you’re using is the one your bike came with. There’s nothing wrong with that, but if you’re looking to cut down weight, this is a good area to do it. Not only will it not have an effect on the mechanics of the bike, but you might actually even find something more comfortable than what you’ve been using.
- Your plastics kit – Like your seat, if you’re still using the plastics that came on the body of your bike when you bought it, there’s a good chance there are lighter options out there. In addition to that, not only can you find a lighter option, but there are dirt bike plastics kits that are made with aerodynamics in mind. Not only will you be losing weight off your bike, but you’ll be improving the shape, which will help you in all areas of your riding experience.
- The exhaust – This one will be quite spendy, but it’s one of the most effective ways to take some weight off your dirt bike. A dirt bike exhaust system is typically made out of aluminum or stainless steel, and while they are strong, they are not light. A titanium exhaust system will take quite a bit of weight off your bike, unfortunately, it will also take quite a bit out of your wallet.
Gearing is an alteration you can make to your bike that doesn’t really impact the weight of the bike, but rather the acceleration or top speed of the bike. This is done in part by helping your bike maximize grip by changing tire rotation to achieve the best speed.
As this will make a difference in how your bike shifts, please be careful and do your research, or have someone help you who has previous experience changing out the gears on a dirt bike. You don’t want to do something wrong and burn out your engine, break a chain, or strip one of your parts.
If you’re changing out your gears, you’re going to be altering what’s called your “gearing ratio”. For example, if your rear sprocket has 40 “teeth”, and your front sprocket has 10 teeth, you’re going to have a gearing ratio of four. Four rotations in the front for every singular rotation in the back.
Check out this video for a more visual explanation:
Higher Top Speed Threshold
A higher top speed threshold is exactly what it sounds like. You’ll be able to reach higher top speeds, but it will come at the expense of initial acceleration.
If you’re looking for higher top speed, you’re going to need to change out your current sprocket setup for a larger sprocket in the front or install a smaller sprocket in the back. You’re essentially going to be doing the exact opposite of what you would want to do if you were trying to go for faster acceleration.
What you have now is called a “higher gearing ratio”. You would make this change to your dirt bike if you are planning on riding in the desert, or sandy areas and arenas. If you’re racing in mostly straight lines or doing lots of riding in places where you aren’t making many tight turns, and you’re able to actually open up your bike for some speed, a higher gearing ratio would be a good decision for you to make.
Having a quick “get up and go” can give you the edge in quite a few situations you may find yourself in as the owner of a dirt bike, specifically if you’re really interested enough in speed to start altering your bike for specific sets of circumstances.
If faster acceleration is what you’re shooting for, you’re going to want to replace your front sprocket with something smaller, or your rear sprocket with something larger. You would want to gear your bike this way if you see yourself doing track races where you’re making lots of turns, mountain biking on tighter trails, situations where you’re going to need an edge in short bursts of speed, but not a long, straight shot. This creates a lower “gearing ratio”.