The Ins and Outs of Converting a Motocross Bike to a Flat Track Singles Race Machine

February 15, 2019 / by Chris Cooksey

Flat track racing is one of America's original extreme sports and is quickly growing again. We talked to a flat-tracking father and son duo about what goes into converting a motocross bike to a competitive single-cylinder flat-tracker.

For years, V-Twin machines have maintained a stranglehold on the American Flat track racing Series.  As a life-long motorcycle enthusiast, when you think of flat track racing, you might envision V-Twin motorcycles competing on a mile-long horse racing track. Now, with new competition from manufacturers like Ducati, Indian, and more entering the scene, the popularity of flat track racing is on the rise. One new racing class, the singles class, has caught the interest of many, and peaked curiosity on the process involved to convert a traditional motocross bike into a flat track racing machine.

Singles flat track racing can be found in everything from local-level events to the professional Singles class in American Flat Track.

Lifelong flat track racer, Thomas McGrane, has converted a 2005 Honda CRF250R motocross bike into a flat track racing machine for his son Logan.  Logan is on his way to becoming a top amateur standout, and having a father with endless knowledge of the sport is proving to be an advantage. We leaned on Thomas for information about converting a bike from full motocross trim into a bike that can successfully race the American Flat track series singles class.

Logan McGrane is quickly working his way up the flat track singles ranks.

Wheels

Much like all other racing, wheel size matters.  Specifically, a set of 19” rims for both the front and the rear are required.  But, motocross bikes already come with a 19” rear wheel, right? While the stock wheel is legal per the official American Flat Track rulebook, Thomas quickly explained the advantages of different rim widths.  In flat track racing, the tire size is 130/80-19 on both the front and rear.  Other forms of racing usually allow small variations in tire size to suit conditions and power output. However, changing the rim size has a dramatic impact on the tire curvature. 450 motocross bikes come stock equipped with a 2.15 x 19-inch rear rim, with the 2.15 being the width of the rim in inches.  Flat track rules allow up to 3.5 inches of width. So why would you want to use different rim widths and how does this affect your handling and traction?

For flat track purposes, the motocross bike receives a 19" rim for both the front and rear wheels. The rims are also wider than motocross rims for handling purposes. Read on for details.

Using a wider rim gives the tire less curve and a flatter surface, which allows for a larger contact area and results in better grip.  On longer tracks, it is ideal to have a larger contact area for additional traction at higher speeds.  The downside of having less curve in the tire is the handling, as the bike becomes less maneuverable.  When using a narrower rim, the tire has more curvature and the contact area is smaller, meaning there is less traction.  Common sense may be to always want more traction, but Thomas assured us that on short tracks, having an easily maneuverable bike is more important than pure traction.  Flat track racers typically have a few different rim sizes and use these subtleties to fine tune their motorcycles.  Flat track rims normally function with stock motocross hubs, so if you want to save a few dollars, you can lace up different rims on your stock hubs instead of purchasing two complete wheel sets.

A wider rim gives flat track racers more tire-to-track contact area, resulting in increased traction. However, moving to a narrower rim and giving up some traction for maneuverability is beneficial on some short tracks.

Suspension

Flat track racing requires modifying stock motocross suspension to sit much lower and ride stiffer.

A stock motocross bike’s suspension is too tall and soft for the demands of flat track racing.  Flat track suspension is far stiffer and slower than motocross suspension.  Thomas insisted that flat track bikes need to be lowered.  Motocross suspension is extremely tall due to the rigors of large obstacles. Flat track doesn’t have obstacles remotely similar to motocross, so weight distribution is more important than suspension travel, making the additional travel of motocross suspension unnecessary. Lower suspension provides a lower center of gravity, which is important to the bike’s ability to handle while laying it down low in the corners.

Braking

The next item to analyze is brakes, or lack of brakes.  Our first question to Thomas was why take off the front brake? The reason seems to be more of a flat track tradition than specific performance advantage. Flat track racers have always removed the front brake, therefore, they continue to do so.

This question has come up many times before, including in an interview with American Flat track CEO Michael Lock conducted by Tom Gushue: “I think the sporting argument would be this: This is dirt track racing, i.e. you are on a semi-loose surface. Brakes, which are designed to scrub speed also require decent traction control. Dirt will not provide you that. I think that if anything, the use of brakes on these original dirt ovals would have been counterproductive and would have caused quite a considerable risk in terms of losing control of the machine.” 

According to Thomas and AFT, running no front brake on flat track machines has become somewhat of a tradition stemmed from early flat track racing safety concerns.

By only utilizing a rear brake, flat track racing remained core to its creators while avoiding a potential catastrophic accident. Flat track racers take great pride in their ability to scrub off speed without using the brakes. First time racers who are using a bike for both motocross and flat track can simply remove their lever.  If the bike is being converted into a flat track racing machine, it’s as simple as removing the front brake system. 

Additional changes to the controls include swapping out the handlebars.  According to Thomas, wider handlebars offer additional control. Having control of the bike while backing it in sideways at high speeds is critical, and the wide bars allow for ease of control and reduce chances of over or under-correcting.

In addition to removing the front brake system, stock size motocross bars get swapped out for wider bars, helping to control the motorcycle while sliding sideways around corners.

Engine

Next, we wanted to dive into the engine modifications required to be competitive in the singles class. A slipper clutch was the first mod that came up. Slipper clutches are designed to prevent rear-wheel chatter and skidding during engine breaking, especially after aggressive downshifting. The inner hubs and pressure plates of these clutches are designed with ramps that push the inner hub and pressure plate apart during engine breaking, allowing the clutch plates to slip past each other. Amateur flat track racing does not require a slipper clutch, but professional racing mandates the use of a slipper clutch for safety reasons. Keeping the rear wheel under control at high speeds is critical to avoid potentially catastrophic crashes.

While Wiseco does not make slipper clutch-style inner hubs and pressure plates, Tom chose Wiseco clutch components for his 2007 CRF250R, including a forged basket and a plate, fiber, and spring kit. Tom commented, “Last year, I put a complete Wiseco clutch (in the ’07 CRF250R), with the basket, plates, and everything, and it still looks beautiful right now.”

Tom decided to go with a Wiseco forged clutch basket and pack kit in their 250F to improve reliability and performance. Read more about Wiseco's clutch basket with a lifetime warranty HERE.

Flat track racing can put a lot of stress and accelerated wear on the rotating assembly and valvetrain of motocross bikes. Many riders, including Thomas’ son Logan McGrane, run the bike close to or on the rev limiter frequently throughout each race. Coming out of turns two and four, entering the back and front straights, the bike gets revved up to its limit and held there until it’s time to back it in to the next corner.  

As a result, Thomas changes the oil very frequently, re-rings his Wiseco pistons in his motocross-converted bikes every 6-8 hours, and throws in a new Wiseco forged piston around every 12 hours. As for the crankshaft, Tom has found that a replacement time of around 20 hours is a safe bet for their applications. They do this not only to maintain optimum performance, but also as a safety precaution. Engine failure from worn out parts while on the track at high speeds could result in serious injury.

To help in the performance department and combat fatigue from spending so much time on the rev limiter, Tom runs Wiseco forged pistons and re-rings them every 6-8 hours as a precautionary measure.

When we asked Thomas about the engine in his ’07, he mentioned “Last year I installed everything Wiseco (engine components wise) in our ’07 250, and then freshened up the piston this year after about 14 hours to go to Daytona. It’s got a lot of racing on it, and Logan does rev the crap out of it. The next time it comes apart, I will do a (Wiseco) crank again.”

Find Wiseco components for your engine here.

Tom recently tried Wiseco bottom end components in another one of their 250Fs and was impressed with how they held up to flat track racing. The next rebuild will see another Wiseco crank.

Check out the excessive R&D and QC processes Wiseco cranks go through.

Valves are another major concern when running 4-strokes motocross bikes at constant, high RPMs. The excessive opening and closing of the valves, along with additional heat, puts a lot of stress on valves. Worn out valves can break, causing an engine failure.  Thomas chooses to use steel valves from Wiseco because they are an affordable option over OEM, and the steel offers longer life than their Titanium counterparts.  Titanium valves are also a choice, and some racers may choose them for their lightness and sealing potential, but the material will wear out faster and need to be replaced more frequently to avoid valve failure.

Converting to steel valves is a popular choice to get more life out of your valves and save money at the same time. Check out Wiseco's Garage Buddy Steel Valve Kits here.

After talking to Thomas about motocross-to-flat track conversion, it is no wonder flat track racing is growing in popularity.  How many other elite forms of racing can you purchase a showroom machine, and with a few modifications, race at the highest level?  Good luck converting the family wagon into a competitive NASCAR race car.

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Topics: featured, Powersports, BIKE FEATURES

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Written by Chris Cooksey

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