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MX EDUCATION: WHY BIKES TURN, WHY THEY DON’T & WHAT YOU CAN DO ABOUT IT

April 23, 2012
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Gottlieb Daimler may have invented the motorcycle in 1885, but when it came to handling, his wooden buck frame and feet-dragging riding style just didn’t cut it. From his founding-father perch 126 years ago, Gottlieb had no way of knowing what AMA professional motocross racers would do with his rudimentary design. When Daimler invented the first motorcycle, his benzene-fueled wooden kludge needed training wheels to help him negotiate the gentle bends of his villages’ cobblestone streets.  

The dynamic interplay between a motorcycle’s steering geometry, the gyroscopic effect of its spinning wheels and its center of gravity was not known at the turn of the 19th century. Old man Daimler’s invention, with its vertical fork and tiller handlebar, steered more like a boat than a bike. Today, its direct descendants can negotiate a crowded pit lane in first gear faster than anything in Gottlieb’s world could go in a straight line.

THE WIZARDING WORLD OF FRAME GEOMETRY

Frame geometry is the magic that makes it possible to roost corners, slam berms and survive the whoops. Forget engine technology and set aside suspension, because it wouldn’t be possible to get past second gear without the self-stabilizing forces that are built into a modern motocross chassis. Without the right blend of head angle, trail, fork offset and weight distribution, your motocross machine would be hard pressed to out handle a Walmart shopping cart. In fact, it would need a pair of Gottlieb’s training wheels to get out of its own way.

To understand what motocross bikes are all about, you need a rudimentary understanding of the ins-and-outs of frame geometry. Read on and discover what handling is all about.

WHAT IS CASTER AND WHY IS IT SO IMPORTANT?

“Caster” is the effect that causes the swiveling wheels of a shopping cart to steer in the direction you deflect the cart. Motocross bikes use the same physics of caster as shopping carts.

WHAT IS TRAIL AND WHY IS EVERYONE TALKING ABOUT IT?

“Trail” is what causes the front wheel of a motorcycle (and shopping cart) to align itself with the direction that the vehicle is traveling. Trail exists when the contact patch of the tire is well behind the steering axis.

In the case of a shopping cart, the steering axis is vertical and the axle is offset about an inch behind the axis. When the cart is pushed, the wheels instantly swing into alignment. The same holds true for a motocross bike. Take your hands off the bars, and your motorcycle’s front end will snap into a straight line for the same reason. “Caster” is the effect. “Trail” is what makes it happen.

WHY IS IT CALLED TRAIL?

As will become apparent, trail is called trail because it is derived from an equation that measures how far behind the head angle the tire’s contact patch is. In simple terms?how far the contact patch “trails” the head angle.

Unlike a shopping cart, the steering axis of a motocross chassis is angled, but the fact that its front axle and the tire’s contact point lie well behind the centerline of the fork creates the same trail equation.

WHAT IS HEAD ANGLE?

“Head angle,” “caster angle” and “rake angle” are all the same measurement. Head angle is the difference between the front side of the steering axis and a theoretical line drawn perpendicular to level ground in degrees.

Motorcycles need to be stable at high speed, so it is necessary to slow the steering way down by raking the head angle forward. The degree that the steering axis is raked forward is called the “head angle.” A chassis with a slacker head angle steers less when you turn the handlebar and wants to remain in a straight line (think chopper). A steeper head angle turns quicker, feels lighter at the handlebar and is less stable at speed (think trials bike).

Head angle works in conjunction with “fork offset” to cause the front wheel to steer into a turn when it is leaned and straighten out as you exit.    

HOW DO YOU MEASURE HEAD ANGLE?

You measure the head angle from an imaginary line perpendicular to the ground to where it intersects the angle of the steering axis. The numbers are based on a 90-degree right angle. There is some confusion among designers as to which side of the fork the head angle should be measured from. For example, a CRF450’s head angle is listed as 26.52 degrees, because Honda measures it from the front of the fork. But, in the bicycle world, this would be called a 63.48-degree head angle (as measured from the back side of the fork).

The confusion comes when people don’t understand that the head angle is derived off of a 90-degree base number. A 26.52-degree head angle is the same as a 63.48-degree head angle (90 ? 26.52 = 63.48). It is simply two ways of describing the same angle.

WHAT ARE TYPICAL HEAD ANGLES?

Motocross bikes have head angles in the range of 27.5 to 26.0 (62.5- to 64-degrees). A 27.5 head angle is not as steep as a 26-degree head angle. Steeper head angles turn more instantly and in a smaller arc. The slacker the head angle, the more the front wheel will resist turning, the larger the turning arc will be, and the easier it will be to hold a straight line.

Motocross bikes of the ’70s typically had 30-degree (70-degree) head angles. They were called “slack” head angles. Slack is the converse of steep.

HOW IS TRAIL MEASURED IN MOTOCROSS?

It’s a simple measurement: Extend an imaginary line down the center of the steering axis to the ground. Then, drop a perpendicular line through the center of the front axle to the ground. The distance between where the two lines intersect the ground is the trail measurement. As a rule of thumb, it should be between four and five inches.

WHAT EFFECT DOES TRAIL HAVE ON A BIKE?

A longer trail measurement means greater caster effect, heavier and slower steering, and a more stable feel in a straight line. Reducing trail causes the bike to steer lighter and quicker and the front end to feel more nervous in a straight line.

All geometry measurements are taken from a bike that is standing on level ground. At the factory, trail is calculated on an engineering program, taking into consideration what the engineers feel will be the correct static sag setting. Some chassis engineers also refer to trail as “wheel caster.”

WHAT IS FORK OFFSET AND WHY DO PEOPLE CHANGE IT?

Fork offset is the measurement of how far the fork leg sits ahead of the steering axis. To obtain the correct number, measure the distance from the center of the fork leg rearward to the center of the steering stem.

Offset is used primarily to determine the correct amount of trail for a given head angle. Motocross front ends typically use between 18mm and 25mm of trail.

More fork offset reduces trail for a quicker steering feel and less stability. Trail is increased with less fork offset. Fork offset is a term that is used more by aftermarket triple clamp makers, because they offer clamps with various offsets that are used to create handling characteristics that are different from stock.

WHAT’S THE HIDDEN SECRET OF FORK OFFSET?

A little considered fact is the role fork offset plays in the steering process apart from establishing trail. The weight of the wheel and most of the fork is offset above the steering axis, while the wheel contacts the ground behind the steering axis. This creates an interesting relationship. As you lean the bike into a turn, the weight of the bike leaning off to the side automatically forces the wheel to turn into the lean. The more the wheel is offset, the more the fork will steer into a turn when the bike is leaned.

IS IT BETTER TO ADJUST OFFSET AT THE FORK OR AT THE AXLE?

Fork offset can be changed at the triple clamp (by moving the fork legs closer or farther away from the steering stem) or at the axle lug (by casting the lugs so that the front axle is a specific distance away from the center of the fork legs). In 2004, Honda wanted to change the offset on their bikes, but rather than change the triple clamps, they had Showa pull the front axle back 2mm to give the CRFs more trail.

IS IT BETTER TO CHANGE OFFSET AT THE CLAMPS OR AT THE AXLE LUG?

As a rule, changing trail at the axle is better than at the clamps. Why? The more offset at the triple clamps, the more the rider will feel the weight of the fork tubes when steering. Although there needs to be some offset at the clamps to give fork clearance for turning radius, the less offset the better to decrease the fork’s moment of inertia. The lightest steering bikes will put as much of the given offset as possible into the axle clamps.

CAN YOU CHANGE TRAIL BY RAKING THE FORKS?

Yes, but it is rare. Some works triple clamps are designed to angle the fork tubes differently from the head angle. Obviously, raking the forks away from the head angle would increase trail and vice versa. But this is not the way most manufacturers adjust for trail or offset. And, it should be noted that changing the angle of the fork in the triple clamps (by raking them), does not change the head angle of the bike in any way.

The biggest effect of raking the forks, apart from a change in trail, wheelbase and weight bias, is the affect on suspension action. Slacker fork tube angles make the fork less responsive to chatter bumps, flat landings and slap-downs?but more yielding to square-edge and large whoop impacts. Steeper fork tube head angles cause the forks to compress more when braking into corners and react better to flat landings.

WHAT FRAME NUMBERS MAKE A BIKE TURN?

The perfect steering geometry is the correct relationship between the bike’s head angle, weight bias, overall wheelbase, front center (distance from the axle to the crank center), fork offset and trail. Thus, changing one number will affect all the others.

It’s no secret that how the bike is set up can alter the steering geometry. Something as simple as more race sag in the rear shock, a different tire profile or even lower tire pressure can be used to fine-tune the way the front end handles. Any adjustment that drops the rear of the bike will slacken the head angle, slow down the steering and increase the trail measurement. Likewise, trail will be reduced and the steering will feel quicker if you run more shock preload or a larger-diameter rear tire.

HOW IMPORTANT IS WHEELBASE?

Wheelbase is the distance between the front and rear axles. A longer wheelbase turns slower, tracks smoother over rough terrain and is less affected by rider position. Shorter wheelbases are more responsive to weight shifts, turn quickly and ride roughly. Motocross wheelbases typically fall into the 58-inch range.

EXACTLY WHERE IS THE CENTER OF GRAVITY?

The center of gravity is, roughly, the point where the combined mass of the rider and motorcycle are balanced in all directions. Saddle height, the position of the engine within the frame, and ground clearance establish the center of gravity (CG) when the rider is on the bike.

Ground clearance: The amount of ground clearance is largely determined by the amount of suspension travel. To prevent your feet from being swept off the footpegs when the suspension bottoms out, ground clearance will always be an inch or two more than the suspension travel.

Seat height: Motorcycle engineers choose a seat height that positions a rider of average height and weight at a balance point near the center of gravity, where small body movements will have a strong affect on braking, steering and acceleration. If the CG is too low, the rider won’t be able to transfer weight properly over the front or rear of the bike. If the center of gravity is high, too much weight will transfer from front to back during accelerating and braking, causing the bike to bounce all over the place through the bumps.

Engine position: Computer programs are now responsible for engine position. They must factor in not only the weight of the engine, but the gyroscopic effects of its internal motion. Once the computer selects the optimum position, factory test riders confirm it during test sessions.

Changing suspension sag, handlebar height, footpeg location and the thickness of the seat are options for a rider looking at raising or lowering a bike’s center of gravity.

WHAT IS WEIGHT BIAS?

“Weight bias” is how the percentage of total vehicle mass is divvied up between the front and rear wheels. It’s kept as close to a 50/50 bias as possible?with and without the rider on board. With shared bias, it takes less input from the rider to make the front stick in corners or the rear tire hook up out of corners. The rider controls the front/rear weight bias by moving forward or back over the seat.

HOW DOES GROUND CLEARANCE RELATE TO THE OTHER NUMBERS?

Motocross geometry must be built around ground clearance. If you have 12 inches of suspension travel, you need at least 13 inches of ground clearance and a bit more peg height. Bikes with less travel and lower ground clearances will have a lower center of gravity. A lower center of gravity shifts less weight from front to rear when accelerating and braking, and because of this can get away with a shorter wheelbase. But, before you run out and lower your seat and slide the rear axle forward to improve cornering, you must factor in the mass of the vehicle and ask, “How will these changes affect the weight bias?” Remember, moving the rear wheel forward in the swingarm moves the engine backwards.

Among the many mass considerations involved are engine weight, engine position, gas tank volume, fuel position, seating position, footpeg position, overall bike weight, wheel weight, handlebar position, rider weight, and the rated horsepower. One change, say moving the rear wheel forward, will beget many other changes: (1) Moving the rear wheel forward will lessen leverage on the shock and make it feel stiffer. (2) Moving the wheel forward will make the wheelbase shorter, but also change the weight bias from 50/50 to something approaching 45/55 (by moving the gas tank, engine and rider closer to the rear wheel and farther from the front wheel). (3) Moving the rear wheel forward will shorten the wheelbase and make the bike easier to fold into tight turns.

CAN THE ENGINE AND WHEELS AFFECT A BIKE’S HANDLING?

Yes. Horsepower and torque influence weight bias and suspension performance. The rotating inertia inside the engine creates a gyroscopic effect that impacts how much the bike resists turning and how it behaves in the air. High-rpm engines slam bumps harder and need to be mellowed out through frame geometry. A designer must also take into consideration the riding styles that will most likely be used on that displacement bike. Smaller displacement bikes are shorter, quicker turning and more agile to match the way they are ridden.

The weight of the tires and tubes affects cornering. Factory riders can feel the difference between heavy and light tubes. Heavier tubes increase the gyro effect. The heavier spinning weight keeps the bike straighter and more resistant to leaning.

HOW CAN THE WIDTH OF HANDLEBARS AFFECT HANDLING?

The arc, that the ends of the bar form when turned side to side, will be smaller with a narrower bar width. Conversely, wider bars turn a larger arc. That means the rider’s hands move a greater distance to turn the wheel the same amount if his bars are wider.

On the track, narrow bars feel quick and nervous. They also transmit more feedback from the track surface to the rider. Conversely, wider bars slow down the steering input (because the hand movement is longer and slower) and mask some of the feedback that comes from the track through the steering geometry.

Handlebars can also be used to tune the center of gravity. Straighter, more upright bars, or forward-set bar clamps, will move the rider forward on the bike and shift weight bias to the front. Depending on the exact suspension setup, this could make it easier to stick the wheel into corners.

HOW DOES ENGINE SIZE AFFECT THE CHOICE OF STEERING GEOMETRY?

Steering geometry is not the first thing on the designer’s mind. Not until the remainder of the bike has taken shape will the engineer finally decide on the exact steering geometry. With the input of test riders, it will be tuned for quicker or slower steering action, or for more or less stability to best match the characteristics of the bike and rider.

A good comparison is the Honda CRF250 and CRF450. The CRF450 is heavier and faster, with more weight bearing down on the steering axis and front tire. It is powerful, heavy and blessed with greater mass in motion. It must be ridden with more subtlety than a smaller or less powerful bike. The lighter CRF250 doesn’t need to be toned down like its big brother. It weighs less, has less power and less mass in motion. It can get thrown around without biting back. Thus, the CRF250 has a steeper head angle, shorter wheelbase and a half-inch more trail. The combination of a steep head angle and more trail balances the quick steering with more self-straightening action.

The geometry equation used on small displacement bikes prefers quicker turning and greater caster effect to keep the lighter bike and rider more stable. Big powerful bikes, by their very nature, have slacker head angles to avoid oversteer.

WHAT ABOUT WORKS BIKES?

A famous factory rider might be required by the rules to use the same frame that you do, but he can adjust his rake by using eccentric steering bearings, his head angle with longer or shorter shocks, and his offset with different triple clamps. Because factory suspension settings are so stiff, factory riders like to run steeper head angles so the front settles more into corners.

WHAT CAN I DO TO ADJUST THE GEOMETRY ON MY BIKE?

Lots of things:

(1) The weight bias and head angle can be influenced through preload adjustment, suspension settings, bar position and by sliding the forks up and down in the triple clamps. If you make the bike taller in the back and set the controls so the rider weights the front, the bike will turn quicker and be less stable. If it’s taller in the front with a more rearward weight bias, the bike will turn slower and be more stable.

(2) Removing or adding a link in the chain makes it possible to lengthen or shorten the wheelbase. Short wheelbases turn sharper, hook up better out of the corners and feel more nervous. Longer wheelbases are more stable.

(3) Aftermarket triple clamps are available in a wide variety of offsets. Depending on the fork offset, aftermarket clamps can speed up or slow down handling. More offset results in quicker steering. Less offset makes the steering heavier but more controlled.

(4) The center of gravity moves every time the rider moves. Rear set footpegs can take the load off the front of a bike, slow the steering input and change the weight bias rearward.

(5) Aftermarket shock linkage can lower the rear of the bike to kick the head angle out. The rider can then raise or lower the forks to choose the head angle that he wants?while also lowering the center of gravity.

SHOULD YOU CHANGE YOUR BIKE’S GEOMETRY?

Yes. MXA test riders do it all the time. It was MXA who pioneered offset changes on late-model KTM and Honda CRF frames (changes that the factory incorporated over time). It was MXA that cut the 1979 Honda CR125 in half to change the head angle so that the oddball 23-inch front wheel could be dumped. It was MXA that railed against rearward bar mounts. You can follow MXA‘s lead with small steps. For example, if you have a problem with head shake, try sliding your fork legs down in the clamps to slacken the head angle. You are only a victim of your bike’s handling characteristics if you don’t understand them.

That said, a lot of testing and engineering went into the frame geometry of the modern motocross bike. Small changes will make a big difference in the way your bike handles. But, if you start messing with every aspect of your frame’s geometry, come race day, you might get lapped by the ghost of Gottlieb Daimler.

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