Why a motorcycle doesn’t fall down?
Motorcycles, like bicycles, are designed with various features that help them maintain stability and prevent them from falling over. These features include:
- Gyroscopic stability: Similar to bicycles, motorcycles also have spinning wheels that create gyroscopic stability, helping to keep the bike upright. The spinning motion of the wheels creates angular momentum that resists changes in direction, helping to stabilize the motorcycle.
- Steering and balance: Motorcycles are steered using handlebars, and the rider’s input through steering helps to control the motorcycle’s balance. By shifting their weight and making adjustments with the handlebars, the rider can help maintain stability and prevent the motorcycle from tipping over.
- Suspension system: Motorcycles have suspension systems that absorb shocks from the road and help maintain stability. The suspension system allows the wheels to move up and down independently, helping the motorcycle to maintain contact with the road and preventing sudden jolts that could cause the bike to fall over.
- Engine placement: The placement of the engine on a motorcycle also contributes to its stability. Typically, motorcycle engines are positioned low and centered, which helps lower the bike’s center of gravity and provides better stability when the motorcycle is in motion.
- Rider input: As with bicycles, the rider’s body position and movements play a crucial role in maintaining motorcycle stability. Skilled riders use their body to shift weight, lean into turns, and make adjustments to help keep the motorcycle balanced and upright.
- Anti-lock braking system (ABS): Many modern motorcycles are equipped with ABS, which helps prevent the wheels from locking up during braking, reducing the risk of skidding and maintaining stability during sudden braking maneuvers.
Overall, the combination of the motorcycle’s design, the rider’s input, and various stability-enhancing features work together to keep a motorcycle upright and prevent it from falling over while in motion.
MOTORCYCLE STABILITY AND CONTROLLABILITY: WHAT THEY DEPEND ON AND HOW TO INFLUENCE THEM
Why some bikes steer well and others not so well? We take apart motorcycle chassis geometry: reach, rack, trail, base (and other unfamiliar words), touching on a little more than the typical internet copypastes, simpler and without formulas.
Why doesn’t the bike go down? “Because it either rides or stands on its footing, and when it rides, its directional stability is maintained by two large gyroscopes (wheels) spinning in the same direction. And the faster it rides, the more accurately it holds its original course and position! Physics – 7th grade! They teach this at school, it’s all clear!” – any knowledgeable motorcyclist will answer and be right… by 12%. Or thereabouts. The gyroscopic effect influences on motorcycle stability from 10 to 12%.
The rest is the merit of correctly selected geometry of chassis parts: frame, rear swingarm, and especially front fork. Their shape, sizes and mutual position determine whether the bike will be obedient and twirling, or as stable as a steam locomotive on rails. Changing just one parameter by 2-3 cm, from two motorcycles of the same model you can get two completely different “characters”. Now, let’s break down how this is done. Without tedious formulas and drawing force vectors. Only the principle – how the key dimensions of motorcycle chassis are interconnected.
Rack, trail, carter, base and center of gravity – let’s repeat what all this means.
It is necessary to explain from simple to complex, so at first I will remind (or explain for those who are not familiar with terminology) what are important parameters of motorcycle “geometry” and what they are called.
Every modern two-wheeler with an engine, regardless of class and cubic capacity, has those very two “gyroscopes”, front (controlling wheel rotation) suspension, rear suspension and the frame “suspended” on them, rigidly linking the front and rear.
Frames can be of any shape and design. They can be made of steel, aluminum, carbon fiber. They can even be devoid of swingarm and rear suspension altogether (dry).
But every frame has a steering column, the seat for the bearings that turn the front fork.
The angle at which the steering column tube is welded to the frame sets the position of the entire front fork: how many degrees it will deviate from the vertical. This is the first of the important parameters: the rack.
Even motorcycles with automotive (cantilever) type of front wheel suspension have their own steering column. It is a “pivot cam” and its axle, also inclined at an angle to the vertical. Smaller than on a motorcycle, but also mandatory.
The English rake angle translates to “lean angle” or “rake.” It’s an old motorcycle and custombuilder term. It is more accurately defined as the angle of the steering wheel pivot (aka “front fork outreach”). It’s easier to operate with the term “rack”, don’t you think? It is also measured simply: one straight line goes vertically through the center of the front wheel axle from top to bottom until it hits the ground (floor, asphalt), another straight line goes through the center of the steering column (where the bearings rotate) from top to bottom until it also hits the surface on which the bike stands. The upper angle between these two lines is called the “rack”, (aka “caster”, which is measured on the car during the camber adjustment).
The vertical line from which the camber is measured does not have to be taken from the wheel axle. It is even possible to “throw” a perpendicular down from the very center of the steering column. But “beating it off” from the axis, we will immediately get a reference point for measuring the other two important parameters: the trail and the base of the motorcycle.
Traverse rack (aka extra rack, aka “overhang” for some authors) is the angle of the fork pens themselves in the traverses, which gives an additional advance of the wheel relative to the steering column or shifting it backwards, closer to it. The more the traverse bar “extends” the wheel, the more it affects the length of the trail. The wrong traverse bar can seriously worsen the steering performance (but more on that later).
Trail is also an English word definition that means “drag trail,” and it is probably the closest deciphering of what a trail “does” and how it “works” in motorcycle geometry. Trail is measured in millimeters or inches from the point where the perpendicular through the front wheel axle “fell” to the point where the axle from the center of the steering column intersects the ground.
The more trail the bike has, the less the front wheel “walks” on bumps in a straight line, the smoother it stays on course, and the more effort you have to make to turn it right-left. The smaller the trail, the sharper the front wheel reacts to the slightest turn of the steering wheel (or a bump in the road!), the less force you need to apply to the steering wheel to turn, and the smaller the turning radius.
The bigger the trail, the more stable the bike, the smaller the trail, the more maneuverable it is. In turn, the length of the trail depends on the rail: more angle of the fork (if it doesn’t change in the transoms) – the trail is potentially longer; the fork stands with almost no tilt – the trail will potentially be very short.
Why “potentially”? You have to look at the design of the front suspension here. Parallelogram, short- and long-lever fork-herder, torsion and the famous BMW “Televers” in the course of work move the wheel not strictly parallel to the axis of its tilt, not so critically affecting the length of the motorcycle base. Their “behavior” includes several other factors that affect handling. A description adapted to each of these types would be cumbersome, so let’s consider the most massive and popular designs now – telescopic forks.
Wheelbase is the distance between the centers of contact of the front and rear wheel with the ground surface. For a stock motorcycle, it is easiest to find out by looking in the manual, and in fact – measure with a tape measure, “throwing” a perpendicular from the center of each wheel axle to the asphalt. The force of gravity acts on the technique strictly vertically, so regardless of the class of motorcycle, its rack or type of suspension, the middle of the spot of contact will be exactly under the axle.
The bigger (longer) the wheelbase of the motorcycle, the more stable it is in a straight line. And the less willing to leave this straight line, when it is necessary to “fall down” into a turn. In proportion to the growth of speed, the faster you go, the harder it is. A long bike, especially with a low center of gravity, is very difficult to fall into a turn. You have to make an effort, shifting your own weight to keep it on the trajectory, which it constantly tends to “straighten”.
If a big trail is added to the long base, the motor simply turns into a “streetcar” that does not want to “leave the track” and turn. Even defeating its resistance, pre-braking almost to zero and “dumping” the bike into the turn, the driver will find that the machine is just not able to pass it on a small radius, turning at a steep angle.
Because of the difference in “behavior” of the front wheel, which, turning, “folds” the bike around the axis of the steering column, and the rear, pushing the rest of the length of the bike in a straight line, in a turn the rear and front wheel always pass on a different trajectory. The smaller the base, the smaller the difference between these two trajectories, the smaller the turning radius. And, with the same center of gravity, the bike with the shorter base will need to tilt at a smaller angle in a turn.
Center of Gravity
The center of gravity (CG) is at the point where you “hang” the bike by which you can distribute the weight of the machine evenly in all directions. It’s like balancing a pencil on your finger so that none of its halves are outweighed – then you will find this conditional “point” in the center, above your finger. The center of gravity is sometimes mistakenly called the center of mass, but these are two different concepts.
In statics, most motorcycles have their center of gravity near the engine and distribute the weight between the wheels about equally. Roughly, since a 50/50 distribution is almost unheard of. On average, the axles are loaded with varying percentages ranging from 40/60 to 60/40 (depending on the class of the motorcycle, the number of seats and the availability of luggage).
The motorcyclist also has his CT, located in the area of the abdomen, at the height of the lumbar (on average). The weight of a motorcyclist can be about one-third of the curb weight of the motorcycle. And in the case of low-cubic capacity – even more than half. Therefore, the rider’s weight will greatly affect the overall distribution of load on the suspension (by axle). On how close will be located CT bike and CT rider, depends on the stability and controllability of the bike on the road. By getting on the saddle, the rider “creates” a common center of gravity with the bike, which can be influenced by leaning over, standing on the footrest and shifting forward and backward on the seat.
Thus, the bike’s center of gravity changes its location during acceleration and braking, unloading and loading the front or rear suspension. When the front fork is loaded, the bike’s base shortens somewhat, and when the rear suspension is loaded and the swingarm is “folded”, it lengthens somewhat.
The more you lay (tilt) the bike, the more speed and on a smaller radius you can pass a turn. But the lower the bike’s center of gravity, the more effort the driver has to make to do it, shifting his CT far beyond the axis of inclination of the bike, up to scraping elbows and knees on the asphalt.
At the exit of a turn, it is not enough to “let go” of a bike that it “levels out by itself”. The same “gyroscope effect” cannot compensate the moment completely, even with intensive addition of gas. Besides, by adding gas in the turn, at extreme inclines, there is a danger of breaking the rear wheel.
Loss of traction of the wheel with the road leads to its “drift” under the influence of centrifugal force outside the turn, and after the displacement of the “contact patch” of rubber with asphalt to the side will go all the rear of the motorcycle. If the front wheel “looked” inside the turn, the roll will worsen, and seconds later the whole crew will slam on the ground with the side in which he turned, catch low-side. If the rear wheel again gets too much traction at the moment of drift, it will be even more funny: the pilot will be highside in all its glory, with flying over the handlebars and landing separately from the bike.
Highside, lowside and some more unpleasant situations, to liquidate of which the courses of counter accident driving are directed, are more likely a topic for a separate article, but further we’ll talk about geometry. So: it is the trail and the base that “help” to “lift” the motorcycle from the turn. The longer they are, the harder it is to beat the resistance of the wheel to the turn, and the more it tends to go straight again, having risen vertically.
How does this work?
Why is the behavior of such a large bike affected by the length of some abstract section that doesn’t even physically contact the asphalt surface? How does the trail “force” the wheel to turn in the opposite direction? Why is a small trail “bad,” and when there isn’t one, even worse? Why is a large base both good and bad at the same time? How does suspension affect cornering, and why will improper tire selection turn the ride into agony?
The “perfect motorcycle” does not exist.
The “perfect suspension” does not exist.
The “perfect rubber” does not exist.
Santa Claus… although there are options here…
For each type of road and riding conditions motorcycle geometry, type and rigidity of suspension, the size of rubber are chosen based on what is more relevant. Stability and maneuverability – two “poles”, balancing between which the engineers get the best handling in a particular category. It is not important for motorcycles for drag racing to “be able” to turn, but it is important to be stable and “load” the front in order not to tip over at the start. That’s why their designers don’t care about trail, the main thing is base and weight distribution. It’s vitally important for trail bikes to be agile, lightweight, able to turn not simply on the “patch”, but on the spot. That’s why they have the smallest trail and the shortest base in comparison with others.
How the trail “works”
To put it simply: the front wheel has a center of contact with the surface. It is under the center of the axle. But the wheel does not turn around the vertical through this center, but tends to turn around the axis of inclination of the steering column – at the point where the projection of the blue line in the figure above “falls” to the ground. When we turn the steering wheel to the left, for example, the contact patch left “behind” this wheel pivot axis extends to the right, a greater or lesser distance, depending on wheel diameter, rubber height and steering intensity. As soon as the contact patch and the whole part of the wheel that remains behind the projection of the steering column axis begins to “pull” to the right – there is an oppositely directed drag force, tending to “return” everything “as it was”. The greater the distance between the center of the contact patch and the projection point of the steering column axis (trail), the greater the “leverage” that this force “uses”, the faster the wheel aligns.
“What are you clamoring for: trail-trail…” – it’s about time you got indignant – “Isn’t there something more important about motorcycle geometry?” There is. The balance between trail length and wheelbase length. It determines the bike’s stability factor. “Trail again?!” Yes, there it is again. It’s the one metric that is interrelated with just about everything in the bike’s running gear. The wheelbase doesn’t change if you install a different profile tire. The reach of the fork feathers won’t change if you just move them further in the transoms, and the angle of the… um… front fork bars won’t change if those feathers are lengthened by even a meter. The trail will definitely react to all these changes. It will get bigger or smaller in relation to the base. Because of this, the stability coefficient will increase or decrease, and the “behavior” of the bike on the road will change proportionally.
Which is more important – the rack or the trail
In 1982, English engineer and motorcycle racer Tony Foale, who devoted more than 50 years of theory and practice of construction of motorcycle chassis, the author of two popular textbooks on construction decided to check experimentally what would happen to controllability, if you “play” with the steering geometry?
“By “sacrificing” his BMW R75, on which he radically changed the rack to zero, and after some time to 15-degree (compared to the stock, 32 degrees), he tested the bike for a year with different trail value.
For the purity of the experiment, Tony involved other riders in the tests. In the end, he found that no matter what the angle of the fork, the bike stayed manageable and predictable as long as it stayed at least 3.5 inches long.
Stability and controllability
Stability is what allows you to steer with one hand on a motorcycle and perform the childish, “Mom, look, I’m going hands-free!” trick on a bike; it’s what helps the bike come out of a turn and come back from an incline to a normal perpendicular road position. A stable bike is balanced and can “water” in a straight line on its own, without any driver intervention.
Stability is when the bike is not “shifting” when braking, does not “wobble” at speeds above cruising, and does not “shimmy” the steering wheel, forcing her to drop. Stability – it’s good, but only to a certain limit, because further it reduces the maneuverability and does not give a quick rearrange or fit into a sharp turn at a decent speed.
Maneuverability is a motorcycle ability to turn sharply “straight away” before an obstacle, turn on a small “square”, “fall” to the maximum possible angle of inclination and get out of it without crashing into low-side, keeping controllability – the most important characteristic, the balance between agility and stability, not giving to pull the second childish trick: “Mom, look, I go without teeth!
What drivability depends on
Manageability is predictability, clear feeling of a rider’s “feedback” and confidence that the motor will obediently go exactly in the way and exactly where it is “ruled”. Maneuverability is “golden mean” between “locomotive” stability and “nervous” maneuverability, when a motorcycle sharply reacts to the slightest push of a rudder, phenomenal turning, but does not “forgive” a slope error of literally one degree. Excessive maneuverability is a big minus at high speeds and on bad road. High maneuverability is shown in practice exactly by bikes with short… “Trails?!”. Them. Everything really “rotates” around it, as well as all motorcycle – around axis of a steering tube. There is only a small range of its value, within which the bike is controllable. Anything less is a “nervous monkey”, anything more is a “streetcar”.
The manageability of a motorcycle, besides the trail-to-base ratio, depends on (curl your fingers up): wheel alignment, center of gravity; stroke length, suspension serviceability (and availability), brake effectiveness, ABS availability; weight and riding position, handlebar width, total load distribution, clutch serviceability, engine and gearbox condition (wear)… everything? No, not all. It also depends on the rigidity and material of the bike frame, the shape of the tread and the quality of grip of the rubber on the road, a little – from the fairness of the bike shell (aerodynamic drag), a little – from the longitudinal or transverse location of the engine … A little bit – longitudinal or transverse position of the engine.
I’ll repeat once again the idea that the ratio of the trail and the bike’s base is interconnected with almost any change in “geometry” of its key points. There is such a racing trick, as trail braking: to make it easier to “put” the motorcycle in a corner, just before entering it to brake intensively, loading and squeezing the front fork. The distance from the steering column to the ground will be reduced, and, accordingly, the length of the trail in relation to the base will be reduced. The force of resistance to turning will become less. But the efficiency of this method depends on the condition of the front suspension: if the fork is “shrunk” – instead of smooth compression it will be “rocked”, and then no steerability is out of question.
A question arises at once: “If we have to reduce this trail, why do not they make it small at once? Because with a small trail, wobbling (aka “shimmy”, aka “skewed” steering) occurs more often. A wheel with a small “leverage” is more difficult to resist small lateral jolts from road bumps and resonance from suspension vibrations.
Is there a conclusion?
“I see, basically, we take a bike with a short base, high center of gravity, and the biggest trailing arm we’ll ‘squeeze’ before every turn, and we get a cool handling motorcycle, right?” No. If it were that easy – the technical departments of motorcycle manufacturers wouldn’t be fighting over every gram millimeter and horsepower. For example, the same “gyro effect” that gives stability in a straight line and “steers” the wheel toward the roll is directly related to the weight of the wheel. The heavier it is, the stronger the stabilizing effect of its rotation. But, at the same time, the more weight and diameter of a wheel, the worse acceleration and braking dynamics, more unsprung masses, more fuel consumption and in general – the load on the engine. Taken together, the disadvantages outweigh the pros.
Using the knowledge of the role of trail in the geometry of the motorcycle, you can calculate for each particular bike its stability coefficient. To do this, you need to measure the total length of the base, together with the trail and the trail separately. Then divide the less by the more and multiply by 100. The resulting number is the percentage. If you are not satisfied with the obtained index, you may correct it within reasonable limits. At modern “classics” and road motorcycles it is from 6,3 to 7 %, at cruisers 7 – 7,4 %, at neutrals and civil versions of sportbikes from 6 to 6,5 %, and everything that is less than 6 % concerns purely sports or “ring” models.
How to change the geometry without interfering with the frame structure
If you change rubber to a high-profile, lengthen or shorten the swingarm, even if you change rear shock preload, reducing its stroke by 1-2 cm, you will change the ratio “base to trail”, making the bike either more stable or more maneuverable.
You can increase the trail by “raising” the rear of the motorcycle at the expense of the suspension stroke. By changing the rubber, or by installing a front wheel with a slightly larger diameter. Decrease – move a couple of centimeters of the feathers in the transoms (up) or by replacing the fork on a “shorter”, lengthening the rear swingarm. The main thing is to do it with a preliminary calculation and the possibility to “win back” changes, if the result proves unsatisfactory.