Champions of Tomorrow: Properly Maintaining a Mini Bike

August 8, 2017 / by Alex Witham

Proper bike maintenance is a critical component in racing. Check out these mini bike maintenance tips to keep your young racer running at the races.

Almost every young racer participating in the amateur motocross nationals has the ultimate dream of becoming the next champion. For most, the dream starts when we see our heroes grace the track, flying through the air, taking the checkered flag and raising the number one plate high in the air. It’s no secret that to be the next Ryan Dungey, Ken Roczen or Eli Tomac, you have to devote yourself to a lifetime of training. The dedication and perseverance it takes to achieve this goal is that of an Olympian. However, beyond the physicality of the individual is the equally important health and performance of the motorcycle itself.

Loretta Lynn’s amateur national is arguably the most sought-after championship title in amateur racing, especially in the 85cc and Supermini classes. Young and hopeful racers go through endless practice sessions, consistent training, and many qualifying race rounds just to get the opportunity to compete at Loretta’s.

Many young racers' goal is to make it to the starting line at Loretta Lynn's.
Photo: Ken Hill

Every parent attending the big amateur nationals is aware of the cost involved. Entry fees, fuel both for transport and racing, camping fees, and the bike modifications to be competitive all stack up very quickly. However, these investments mean nothing if the bike doesn’t stay together.

Gone are the days when amateur riders were signed to multi-year professional contracts at the end of their amateur career. It has become more common for the factory efforts to sign a young phenom while they’re still early in the minibike classes with the plan of grooming them into winning a Supercross or Motocross title. This is why racers must consistently make sure their bike is properly maintained, so performance and mechanical issues don’t get in the way. Let’s go over some recommendations for essential maintenance practices for the mini bikes that are being put to the test by young racers.

Engine Maintenance: Crankshaft Rebuild

First and foremost is the engine maintenance. Like most two stroke motocross bikes, mini two stroke race bikes make the majority of their horsepower at high RPM. For minibikes such as 85cc and Superminis, it is not uncommon for them to spend the majority of the race singing at an incredible 12,000 rpm. The result of this is extreme heat and wear on the connecting rod, crank pin, and bearing. Because of this, it is extremely important to rebuild the crank shaft assembly frequently.

The parts commonly inspected and replaced when rebuilding a crankshaft: connecting rod, big end pin, big end bearing, and thrust washers.

Rebuilding the crankshaft involves removing the engine from the bike, removing the top end, and splitting the cases. Be sure not to confuse rebuilding the crankshaft with replacing the crankshaft. Crankshaft assemblies do not need to be replaced every time. If you inspect the crank web of your existing crankshaft and make sure you don’t see any signs of cracking, breakage, or abnormal wear, you can rebuild the assembly with a new crank pin, big end bearing, and connecting rod. These are the parts that experience the most stress and wear during engine operation, and not replacing these after they are worn could result in engine failure.

ProX Racing Parts offers the parts needed for a crankshaft rebuild. ProX connecting rods are made in Japan with Japanese steel, exactly as the OEM rods are. The rods are double forged and have an aligned grain flow, allowing for better tensile strength, meaning lower chances of breaking with bending. ProX crank pins and bearings are also made to OEM specifications, so the rotation of the crankshaft will be smooth and reliable. ProX also has the gasket kits you’ll need when reassembling the engine, including the top and bottom end.

Find out more about ProX connecting rods here.

ProX connecting rods are double forged and come with a big end pin, bearing, and thrust washers.

Although there’s always some possibility of engine failure, it is recommended as a rule of thumb that the crankshaft assembly on mini race bikes be rebuilt between fifteen and twenty hours, but of course always double check the recommendation from your bike manufacturer.

Engine Maintenance: Top End Rebuild

This brings us to pistons and top end parts. Because the 85cc and 100cc bikes have such a small-bore cylinder, they experience heat at a different rate than bigger bikes. A smaller piston means less surface area and less heat dispersion. As a result, heat is more centralized on the piston, rings, wristpin and small end bearing, which increases the risk of failure.

Many ProX 2-stroke pistons come with a low friction skirt coating.

ProX offers complete pistons kits for all of the popular mini bikes, as well as many other big bikes and unpopular models. ProX two stroke pistons are cast aluminum, which allows them to have a higher silicon content. The advantage this provides in two stroke applications is that thermal expansion is decreased, which is necessary for these high heat, high revving two stroke engines. Many ProX piston models also come with a skirt coating, which allows for less friction during engine operation. ProX piston kits come complete with the piston, wrist pin, circlips, and rings.

To optimize horsepower and engine life, the top end should be replaced roughly every ten hours, or two top ends per every crankshaft rebuild. Again, this is just a recommendation. Every engine could vary, so it’s always good to double check the manufacturer recommendation.

Get more info on ProX piston kits here.

Tech Tip: Quick Connecting Rod Inspection

It is important to note, if you’re just replacing the top end and not splitting the cases, you should still always inspect your connecting rod. While the piston is removed, simply grab hold of the rod and try to move it up and down. Don’t try to rotate it, simply try to move it up and down from top dead center. If there is any up and down play, inspect and rebuild the crankshaft assembly.

There will always be some side to side play at the pivot point. However, too much can result in engine failure. To check this, use feeler gauges and make sure the gap between the big end of the rod and the crank lobe is still within the manufactures recommended tolerances.

When rebuilding the top end, be sure to inspect your connecting rod while the cylinder is off. Check the play in the rod as well as the big end clearance.

Another point of inspection is going to be the small end bearing slot. Using a flashlight, inspect the slot to ensure there is no scoring or gouges in the metal. Also, make sure to inspect the sides of the connecting rod for cracks.

Simple Stuff

Air filters are one of the most important and easiest parts of bike maintenance, but they are often neglected. Cleaning or changing air filters is of equal importance to keeping the engine fresh. As discussed earlier, these small-bore engines run at high RPM and have very little lubrication, so keeping dirt and debris out of the engine is a must. High performance engines need clean air to go along with clean fuel. If this requirement is not met, the engine will not perform well and could experience damage.

 A common mistake is to let track conditions dictate the frequency at which the air filter is changed. Don’t be fooled; even a nice tacky track such as Loretta Lynn’s Ranch will kick chunks of dirt into the air box. With the level these mini racers are at, changing the air filter after each moto will help ensure your bike makes it to the checkered flag.

ProX air filters feature the same dual-stage performance as popular aftermarket brands.

Furthermore, air filter choice is important. ProX Racing Parts offers high flow air filters utilizing dual stage bonded foam coupled with a thick, flat foam sealing ring for a tight seal against the air box. ProX air filters provide the high performance of common aftermarket companies without spending much more, if anything more, than an OEM filter.

Check out ProX air filters and other accessories here.

 Engine Maintenance: Clutch

The clutch assembly is yet another key part of the engine that must be maintained. Because these small bore two stroke minibikes produce less torque than their four stroke counterparts, the clutch is very frequently used. This subjects the clutch fiber and steel plates to immense heat. Heat is not friendly to a clutch. Once the clutch plates have been heated to the point of burning, the clutch will start to slip, causing the bike to lose power and even more torque.

Inspection of the clutch begins with removing the outer most cover on the right side of the engine. Once removed, simply remove the bolts holding on the pressure plate and pull the plates from the basket. It is important to inspect every steel and fiber plate for any discoloration or cracks. After plate inspection, inspect the clutch basket for grooving, as well as all other clutch components for notching or cracks. When replacing the clutch plates, it is always best to replace both the steel plates as well as fibers. Always refer to your manufactures service manual when torqueing any bolts as under or over torqueing can cause engine failure.

Here's an example of a burnt clutch. Notice the dark discoloration on both the friction and steel plates.

ProX offers OEM quality replacement clutch components that are made by the leading OE manufactures. You can purchase clutch kits ranging from just plates, all the way up to complete clutch assemblies, including: plates, springs, basket, inner hub, and pressure plate. ProX clutch baskets, inner hubs, and pressure plates surpass OEM quality because they are forged aluminum instead of cast, providing a better grain structure in the alloy and creating much better wear resistance and strength. In addition, ProX clutch springs are 10% stiffer than OEM springs, which gives the rider a more positive and precise feel of clutch engagement.

Full details on ProX clutch components can be seen here.

(Left) ProX clutch baskets are forged and teflon hard coat anodized for smooth strength. (Middle) OEM quality steel and friction plates with 10% stiffer springs. (Right) ProX inner hubs are forged for added strength above OEM.

Drivetrain Maintenance: Chain and Sprockets

Bike maintenance doesn’t stop at the engine. The chain and sprockets experience consistent wear during operation. Because all the torque and power produced by the engine is transferred to the chain and sprockets, it is extremely important to replace them when they show signs of fatigue. Worn sprockets or a stretched chain can become derailed during a race and cause significant damage to the chassis, engine, or even the rider. Be sure to inspect all aspects of the drivetrain frequently to be sure there isn’t any abnormal wear or breakage. It's a good idea to replace the chain and both sprockets together so that you can be sure everything is wearing evenly. However, in some cases with aluminum rear sprockets, wear could be accelerated and require two rear sprocket replacements for every one front sprocket replacement.

One sign of a chain in need of replacement is worn down link pins. For the sprocket, replace it once the valleys between the teeth are visibly asymmetrical.

More on ProX drivetrain components here.

ProX Racing Parts also offers sprockets and chains for many applications. The front sprockets are CNC machined from low-corrosion SCM435 Ni-Chrome Molybdenum steel, resulting in both light weight and strength. The rear alloy sprockets are machined from high grade 7075-T6 aluminum, making them lightweight and durable. ProX also offers steel rear sprockets, stretching out the lifespan of the sprocket. ProX replacement drive chains are designed to fit and work perfectly with their front and rear sprockets, and are manufactured by the leading OE manufacturers in Japan.

Brake Maintenance

All of the previously mentioned maintenance topics play an important role in getting strong and reliable power to the ground. That being said, the final topic of discussion involves stopping power. Often forgotten are the brakes on the motorcycle. Unfortunately, there is no standard time frame when it comes to replacing brake pads. Because of this, it is all that much more important to inspect the pads to ensure that they are not cracked, glazed over, or lacking material. As with most forms of racing, braking is more often than not where time is made up on the track. This is especially true of motocross racing. Most young racers can hold the throttle wide open when permitted, but getting the bike slowed down and set up for a corner is a different story.

Brake pads with a ground down and glazed surface should be replaced.

When inspecting your brakes, you’ll have to remove your front and rear wheels and remove your brake pads from your caliper. Take a flash light and shine it across the pad at an angle in order to see separation in the pad. You’ll also want to make sure you don’t have any glossy or glazed spots on the pad surface. If any of these are present, it is recommended that you replace the brake pads.

ProX Racing Parts offers replacement sintered brake pads made from a combination of metallic materials. Sintering the material fuses metallic particles with a specific combination of heat and pressure. This fusion allows the material to have much more resistance to the heavy heat that brake parts encounter. ProX front pads are designed to be slightly softer, while the rear pads are slightly harder to give the rider strong and controllable braking power.

Check out more on ProX braking components and find them for your bike here.

ProX offers both replacement pads and rotors that are much more affordable replacements than OEM.

By using this as a guide, you can ultimately increase the longevity of your machine and decrease the amount of money spent on repairs. Give yourself every opportunity to succeed. After all, all the effort spent getting to the race doesn’t mean anything if poor bike maintenance ends up hindering your results in the end.

Topics: featured, Powersports, Tech

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Written by Alex Witham