How To Convert A Dirt Bike To A Street Legal Supermoto

Motocross Hideout includes anything dirt bike related, even if it is ridden on asphalt. So, just what is a Supermotard? Yep, a dirt bike with sport-bike style wheels and tires for riding on the street. If you’ve ever ridden a dirt bike on the street, you’ll know how exhilarating it can be when you whack the throttle and the front wheel comes off the ground. Motards are the ultimate hooligan bike, and can get you into a lot of trouble if you don’t follow the rules. However, if you want to legally ride on the street or go to track-days on a road course with your dirt bike, here’s some things you will need to replace or install.

Wheels

If you want to convert your dirt bike to a supermoto, you’re going to need ‘Sumo’ wheels and street tires. 17 inch wheels, front and rear, are the most common for supermoto, and are much better for street riding than the stock 21/19 that are stock on most full-size dirt bikes. They lower the bike, they handle better, and they’re wider so you can lay more rubber down. 3.5″ width is typical for front supermoto wheels, and 4-5″ are the most common for the rear, with several in between. You can go wider or narrower, but you may run into tire width issues, as well as the wheel being too wide and rubbing on the chain and/or swing-arm.

Tire size selection is important in order to make your motard ride right. If you have a rear tire that’s too wide, it can actually hinder the performance. For example, if you have a 4.25″ rear wheel with a 160/17″ tire, the wheel will be pinching the bead of the tire because it is wider, thus creating a more extreme profile with a smaller radius, and ultimately less road surface. However, if you put a 150/17″ tire on the same wheel, it will have more contact surface on the road and perform better than the wider tire. This is research that you should do for your specific year and model bike to determine what combination is best.

Supermoto Wheels
Supermoto Wheels

 

There is also the more extreme route for those that have the machining ability and want to possibly save a few bucks. Street bike wheels from certain bikes can be retrofitted to work with your dirt bike. However, there is usually machining involved to make clearance to fit on the swing-arm and/or forks. Bearings are fairly easy to swap out, but you’ll either have to find wheels with the same size bearings as your dirt bike to fit the axle, otherwise you have to buy some bearings with the correct size inside and outside diameters. Another problem that may come up if you choose to do this conversion is mounting the caliper. This depends on what size rotor you’re going to use, or if you want to put the stock caliper on the wheels you’re using. I can’t recommend a certain set-up for everyone, as there are so many different bikes and combination of wheels. Unfortunately, that is where YOU will have to read-up on as well. I don’t recommend going this route unless you are mechanically inclined and have the machining capabilities/resources. Lastly, cast wheels are not as strong as spoked supermoto wheels, and they will not be able to withstand the abuse of jumping and off-road riding; just something to keep in mind as well.

Lights

Lights are required if you’re going to ride on the street. Every state is different, but the usual requirements are a headlight (hi/low beam), tail-light/brake-light, mirror, turn signals (hand signals are OK in some states), and reflectors. Headlights are fairly easy to install if you carefully read and follow the instructions (I know, that can be hard for the guys…). However, if you are using a motocross bike, you will either need an alternate or bigger power supply. An upgraded or rewound stator is probably the easiest way to go because that alone will power the lights. You’ll need to get a regulator to go with it so you don’t burn the lights out.

Lighting Kit
Lighting Kit

Unfortunately there’s not an aftermarket stator for every dirt bike, or at least one that’s reliable. Another easy way to get usable lights for your street legal conversion is with battery power. You can buy an aftermarket lighting system for most bikes, such as the DRC EZ electric wire kit. It is powered by a battery pack of 8 AA batteries that are easy to swap out. They will last several hours, depending on how many lights you have and what kind of wattage they pull.

Another option would be using rechargeable Lipo batteries, such as ones used in RC planes and cars. They should be 12 volt (4 cell) to match the lighting system. This route requires a little more initial cost, but being able to recharge the battery makes it easy, especially since you don’t have to worry about carrying around a bunch of small AA batteries.

Brakes

In order to go faster, you must be able to stop faster. If you’re converting your dirt bike to ride on the street or asphalt, you will want to upgrade the front brake. The rear brake isn’t as important because it’s not doing as much braking, so it’s adequate for most riders unless you’re highly competitive in supermoto racing. The brake-system is fairly simple in regards to modifications. You have the master cylinder, brake line, caliper, and rotor. The quickest way to get more braking power is by replacing the rotor with a bigger one because it produces more leverage. 320mm is a popular rotor size for motards to have, but there’s some things you’ll have to consider.

Most bikes can use the stock caliper with a larger rotor, but the stock location will not work. This is why you must buy a relocation bracket in addition to the rotor. Be sure you buy the correct bracket, though, as the size/brand of rotor and caliper may vary. Calipers with four or six pistons are a common upgrade, as this will also produce more stopping power.

A steel or aluminum braided brake line is a fairly inexpensive accessory to your braking system, and they perform better than the stock rubber hose. They are built to withstand more pressure, and give you a more consistent and less “mushy” feel compared to stock.

Upgrading the master cylinder can get expensive, but if you are racing and want the most power with less effort, a better Master will complete the package. There’s quite a few options out there to choose from, and generally, you get what you pay for. On the flip side, many riders on a budget find an MC from a different dirt bike, street bike, or even a quad that is better than what they already have. Remember to do some research as to what works with your specific bike if you decide to upgrade.

Sumo Front Brakes
Sumo Front Brakes

Accessories

Last, but not least, are the ‘accessories’ that you must add to your motard to make it “street legal” and pass inspection if needed. Every state is a little bit different, so I will just hit the basics on what’s required. Almost, if not every, state will require a working horn. This doesn’t have to be anything fancy. In fact, a lot of riders buy a cheap bike horn from wal-mart. Turn signals may or may not be required. Hand signals are legal in some states.

Most states need a headlight with a hi-low beam headlight, as well as a working tail/brake-light that works with the rear brake. Reflectors are a good idea, whether they’re required or not. It will make you a little easier to see, and if you get pulled over, the deputy may take you more seriously if you add little safety things like this. You will likely need a mirror to be legal, so a simple handlebar mounted or bar-end mirror will work.

A road-legal exhaust is a good idea if your bike is being inspected. Most dirt bikes have “For off-road use only” stamped on the stock exhaust, which isn’t going to fly if a cop scrutinizes your motard. Also, bikes that carry passengers must have an appropriate seat set-up and passenger pegs. I know that supermoto’s are solo-bikes 99.9% of the time, but it’s just something else to keep in mind.

Now it’s time to start building your bike! All of these parts add up quickly, which is why properly set-up supermotards (especially when plated) are so expensive to buy used. You probably won’t get your money back when you go to sell it, but it can be worth the money to know that you built it with all of the parts that you want. If you do some shopping around and have the whole winter to piece your bike together, you can find some good deals on used or even new parts. Besides, if you buy a complete supermoto kit for your dirt bike, you can always go back to the stock set-up if you want to ride in the dirt again.

I know that was a lot to take in, so you should bookmark this page/website and come back to it throughout your build. If you just want to see all the parts involved in making a supermoto, here’s a simple list (how far you want to go down it is your decision):

  • Supermoto wheels and street tires
  • Street gearing/sprockets (depends on what kind of riding you’re doing)
  • Headlight
  • Tail/brake-light
  • Upgraded stator or battery system for lights
  • Larger front brake rotor and relocation bracket
  • Better front brake caliper and master cylinder
  • Turn signals
  • Mirror
  • Horn
  • Hi/low beam headlight switch
  • Reflectors
  • Street legal exhaust
  • Passenger seat/pegs

If you have any questions, either post a comment below or click the Contact tab above and shoot me an email. Have fun building your own Supermotard, and ride safe!

-Tom Stark

My Dirt Bike Won’t Run – Diagnosing A No-Start

Any day your dirt bike won’t start is a bad day. The problem could be as simple as turning on the gas. However, if regular maintenance hasn’t been performed, you could have a much bigger underlying issue. Not changing the oil in 20 some hours can do a good deal of damage to the engine. If you just haven’t ridden the bike for a few months and it’s just been sitting in the garage since you last rode it, you can often find and solve the problem in less than an hour.

In order an engine to run, it needs air, fuel, and spark. If your dirt bike is not getting just one of these, you can kick it over all day long and it won’t start. In order to save some possible time, we’ll take a quick look at each of these areas to see if we can spot something simple. That way we won’t spend an hour trying to fix one thing when the problem could be something completely different.

Oxygen

Air comes first, so pop the side cover and/or seat off and take a look at the air filter. This may be a dumb question, but is there a complete air filter there? It’s easy to stuff a rag in there while working on the bike and forget about it when you go to put everything back together. All I can say is, stranger things have happened, and it’s always safe to check first, especially since it only takes a minute. If the air filter is there, how dirty is it? If it’s caked with sand and mud, that alone could be preventing your no-start problem. Clean the filter and try starting it again.

A less-common, yet possible cause, could be an air-leak in the system. Check the intake boot for cracks, as well as any bolts or gaskets in between the airbox, carburetor, and engine. On a 2-stroke engine, the reed valve has pedals that wear out over time. They can last hundreds of hours if the bike isn’t ridden hard, but if the edges are chipped off, it could let unwanted air through and not allow the bike to start. This takes longer to remove and inspect, but it’s just one of the many things that can cause a no-start dilemma.

This filter looks a little dirty...
This filter looks a little dirty…

Fuel

The engine requires air and fuel in order for combustion to occur, so the next step is make sure the engine is getting gas/premix. First place to check is the gas tank. Is the tank clean with fresh gas? Gas or premix that has sat for a number of weeks will degrade and start to gum up. If the gas smells like old paint, dump it out before doing anything and clean the tank out. Fuel must go through the petcock to get to the carburetor, so if the tank is empty, now is a good time to take a peek at the seal and filter on it. If they are damaged, leak, or clogged, replace with new parts/assembly. If the gas line is cracked or plugged, replace it as well.

Next up is the carburetor, which is the root cause of many ‘no-start’ situations. Especially on smaller dirt bikes with smaller carbs, jets and passages can gum up in a matter of weeks because of old gas sitting in it. Even if you can see through the pilot jet, there may be just enough crud stuck on it to not allow a sufficient amount of gas through to ignite the engine. Carb cleaner and compressed air are your friends if the carburetor isn’t too filthy. You can often times just loosen up the carb clamps and rotate it to spray out the jets while still on the bike. This can be done in a matter of minutes by removing the float bowl on the bottom of the carburetor.

If that doesn’t work, you may need to take the carburetor completely off and dis-assemble it for a more thorough cleaning. There’s a couple more quick ways to confirm that the carb may be dirty. You can pour a little bit gas in the spark plug hole and kick it over. If it starts or fires for a second or two then you know it’s not getting gas from the dirty carb. You can also try and push start your dirt bike. If neither of those work, it may not be a fuel problem after all, but lets move on to one more quick check before taking the time to clean the carburetor again.

Spark

My dirt bike may be getting air and fuel, but if there’s no spark, it won’t even want to start. A quick way to check if it has spark is by pulling the spark plug off, putting the cap back on, and resting it on the engine while slowly kicking the engine over. You may need to turn off the garage lights to see it, as it will be a small, blueish spark of electricity on the end of the spark plug. If you don’t see anything, be prepared to do spend a good amount of time swapping out parts if you don’t want to replace everything in the electrical system.

If you know someone with the same bike, ask if you could temporarily rob some electrical parts off of theirs. It could be as simple as a faulty kill switch, or a bad ground in the system. However, you may have to swap out the CDI box or even the stator to find the root problem. Once you find the part causing the problem, order a new one and give your generous friend back his/her parts, as well as taking them out to lunch if they lent you a hand in your project because they could have saved you a lot of money by not ordering the wrong parts.

If you still can’t figure out why your dirt bike won’t start, there’s a good chance that it needs a rebuild if it has a lot of hours on it. Low compression is common on motocross bikes that have been ridden for years. If you check the engine compression when the engine is new, you will be able to tell when it needs to be rebuilt when the compression goes more than 25% below what it started at.

If you have any questions, post a comment below… Have fun, ride safe, and keep those dirt scooters maintained!

-Tom Stark