My last post was about the surrounding airflow and aerodynamics of an open exposed wheel mounted on a 4 wheel vehicle, its effects and how it interacted with the vehicle. Due to the size and shape of a motorcycle and the size and shape of its tire and rim, its wheels will act much differently and the surrounding airflow will be influenced in different ways than a 4 wheel vehicle. It would help to review my earlier post on exposed vehicle wheels before reading this post.
This post will pertain to a motorcycle in straight running with the following conditions, a front wheel with spokes, no front fender, telescopic front fork and a rear suspension. We will look at the way aerodynamic forces interact with the front wheel, how it affects braking and the stability of the vehicle.
The world’s fastest single person, human powered, unpaced vehicle record belongs to a Canadian cyclist, designer and builder, Sam Whittingham. Sam is the holder of several world records, on September-18-2009 he pedaled a streamliner recumbent bicycle named Varna Tempest, 82.8mph for 660ft with a flying start. Sam also holds the 1 mile flying start record set on June-10-2001 at 78.6mph. Thus far, Sam is the first and only person to break 1/10 the speed of sound, human powered, when he pedaled to 82.3 mph on September-18-2008.
On a motorcycle the main contributors to overall drag are the rider, the cooling radiator, the fairing and the wheels.
Aerodynamics are more important at high speeds than power, because the power required increases with the cube of the speed and increases in speed can be more easily attained by improving aerodynamics rather than adding horsepower. At high speeds the aerodynamic forces on the motorcycle have the largest influence on its top speed and acceleration. These forces are the pitching moment (nose-up and nose-down rotation of the bike), yawing moment (rotation to the left or right about a central vertical axis) and rolling moment (roll to the left or right in a crosswind) as well as the side force, lift and drag are the combined forces that determine the aerodynamic loads on the motorcycle and will influence top speed, acceleration, crosswind handling and stability. Normally, a motorcycle with less rolling and yawing will move around less in crosswinds and if it has less front end lift will normally respond better to steering movement at high speed and be more stable. A motorcycle that is taller will normally have more front end lift and an increased rolling moment.
The motorcycle will have 3 to 6 times the power to weight ratio as most vehicles. The tire and wheel is the most important part of the entire vehicle, everything else is built around them. Most of the braking force is thru the front tire and the acceleration forces are thru the rear tire. If a tube is installed inside the wheel tire combination it will cause an increase in the rolling resistance and build up more heat inside the tire, if possible a tubeless tire would be much better. A radial ply tire will generate less heat and normally be lighter weight than a comparable diagonal cross ply tire. Under acceleration power is applied thru a forward direction in the tire and applied in a rearward direction under braking conditions. Due to sidewall flexibility the tire can make up 8% to 10% of the absorber and suspension system and this can be tuned with air pressure to increase or decrease the flexibility. A new tire will have more rolling resistance than a used tire, if the tire is worn 50% to 60%, it will have about 20% less rolling resistance than when new. Tire cord stresses account for about 30% of the rolling resistance and rubber compound elasticity accounts for about 10% and tread compression and flexing make up about 40% of the resistance. Rolling resistance of a cross ply tire will be greater than that of a radial tire. Lighter wheels and tires will improve steering and suspension response because they are part of the unsprung weight. There will be a self-aligning force generated at the center of the contact patch due to the force trailing behind the pivot point. Depending on speed and lift forces generated on the front tire the amount of trail will change and this will influence the characteristics of the steering. Under normal operating conditions the tire will bulge at the side wall at the ground contact patch, due to this, the side wall will approach the ground at an angle and this will cause the greatest force to be applied at the edge of the tire and the leading edge of the contact patch while the forces will be less at the center of the contact patch. To help them bank around curves, the tires have a rounded cross section. Compared to the average car, a motorcycle will apply double the power to the ground for the same size contact patch with much less weight to generate traction, but the motorcycle has a small fraction of the frontal area as a car and thus generates much less aerodynamic drag for the tire contact patch to have to overcome at high speeds. Due to the motorcycle tire being smaller and very rounded, as compared to a car tire, it is much more aerodynamic than a car tire, it will generate much less lift from the base of the leading edge, at the contact patch, because there is less surface area for the pressure to act against and generate the upward lift, but mainly due to the tire being convex and very rounded, it will bleed the high pressure air off to the outside edges of the tire before it could build up very high and generate very much lift. Because the tire is convex and very rounded, the high pressure stagnation point at the front leading edge of the tire will be much smaller than normal and not generate as much drag, due to the high pressure air bleeding off around the front and over the sides of the tire.
I do not know if rules would permit it but if motorcycle tires could be fitted to the front of an open wheel car it would lower the amount of lift and lower the drag forces also. If motorcycle tires were placed on the front of a vehicle they would make it more susceptible to side winds and lower the braking force of the tires due to having a much smaller contact patch and the very rounded contour.
To determine how the air flow around a front wheel of a motorcycle can affect performance, the whole vehicle package should be looked at. The front wheel is the part of the vehicle that the oncoming air sees first. The aerodynamic performance of components behind the front wheel will be affected by events occurring upstream at the wheel. The drag associated with the rider, assuming that the riding position has been optimized, is dictated by the fixed size and shape of the rider. The design of the front wheel can be optimized to improve overall aerodynamic performance and handling characteristics, the optimum wheel rim depth and cross sectional profile, spoke count, size and shape, the location and shape of the caliper and the shape and size of the brake rotor, tire air pressure, wheel diameter and tire size will all effect airflow around the wheel. The optimization of the front fork, frame and brake caliper for the individual wheel used on the vehicle would lead to performance improvements also. The rear wheel is designed to withstand a combination of loads and a driving torque that is needed to transfer the force from the hub to the rim of the wheel which propels the vehicle forward, as well as the rim of the wheel also supporting the tire and the weight of the vehicle. During hard acceleration, for maximum tire grip, the rear wheel will have to spin a little faster than the actual road speed; the tire slip will be about 10% to 15%. The spokes of the front wheel, while very small in area, do account for drag forces which are comparable to those of the wheel hub, though the spokes have no impact on vortex shedding from the wheel. The air flow around the center wheel hub will be greatly impacted by the vortices from the upstream leading edge of the wheel. The highest drag occurs on the leading edge of the wheel. At the top of the wheel the drag forces will be close to zero and will actually create lift in this area due to early airflow separation, because of the forward tire rotation, causing a low pressure area. Variations in the track surface will lead to vertical movement of the wheel, thus varying the tire contact patch area and affecting its aerodynamics due to changing the shape of the contact patch.
Tire air pressure can have a big effect on how the vehicle reacts to lateral side winds and gusts, if air pressure is to low it can allow the vehicle to move about quite a bit in side winds. As the air pressure is raised in the tires it will increase the lateral stiffness and help to prevent this unwanted movement. The tire rolling resistance will increase by 1% to 3% for a 1 psi reduction in tire pressure from the recommended standard pressure settings. Rolling resistance increases with tire width and decreases with wheel radius, nearly 50% of the tires rolling resistance come from stresses in the tire belts or cords and the elasticity of the rubber compound. The aerodynamic drag contribution from the wheels will be 15% to 25% of the total drag of the vehicle.
A small rider will have an advantage over a larger rider; if you're big it will be harder to tuck in, causing extra drag, the smaller rider will have an advantage. Wearing tight clothing will help to reduce the "balloon effect," and can provide a reduction in drag. The rider refining and perfecting their riding position will have a big impact on aerodynamic forces of the vehicle, decreasing drag and increasing stability. It is important to work on riding position if you want to go fast, neatly tucking in behind the wind screen or arching the back of your body up in the air a few extra inches to improve airflow and keep it attached to the back of the vehicle, reducing the low-pressure areas behind the fairing will get that last little bit of speed from the chosen combination of parts. Adjustments to the drivers’ position, even minor ones, can result in a decrease of the aerodynamic drag, which can lead to what can be considered as big performance gains once you are approaching 200mph.
Below 60mph to 70mph the tires rolling resistance and friction from the drivetrain are the primary force your vehicle will have to overcome to maintain speed. Over about 70 mph the increasing aerodynamic drag is the primary factor in the top speed and acceleration of your vehicle. To go faster you can add more power, which is not very effective in small amounts at high speeds, reduce the frontal area or reduce its drag co-efficient. If you wanted to double your speed, you need to increase the power of your engine eight times. It is much more effective to try and reduce drag than to add horsepower, especially at very high speeds where added power in small amounts will not be very effective but small changes as far as drag is concerned will see much greater returns on performance.
Side winds will produce a torque that can make the vehicle fall over away from the wind, to maintain balance it will have to be leaned into the wind. The amount of lean will depend on the location of the center of pressure and the center of gravity. The center of pressure is the point through which the wind forces on the side area of the vehicle act. Centre of pressure should be behind the center of gravity, on a motorcycle you will be very limited but the greater the distance between the center of pressure and the center of gravity the more stable it will be in side winds.
The side force contributions by the wheel components will have the hub contributing the least to overall side forces. The aerodynamic side forces of the wheel will usually increase in a near linear fashion with increasing yaw of the wheel. Aerodynamic forces changing with the yaw angle of the wheel and creating more lift will only act on the wheel and do not generate lift on the hub or spokes. The aerodynamic forces will always act downward on the hub at most yaw angles. The aerodynamic side forces are the greatest on the trailing edge of the wheel for all yaw angles and will increase with the speed of the vehicle. These increases in aerodynamic side forces on the trailing, inside part of the wheel will have an effect on the handling and maneuverability of the vehicle at high speeds. There is an area of transition where the aerodynamic forces increase unwanted lift at low yaw angles, leading to problems with control and maneuverability. There will be an area of high pressure generated in the leading edge of the wheel at the ground contact patch that will create lift and due to airflow separation creating a low pressure area at the top of the wheel generating lift also and a low pressure area at the rear trailing edge of the tire generating drag also.
By increasing the yaw angle of the front wheel will have a strong influence with the amount of flow separation occurring on the trailing edge, side of the wheel. The air flow can be attached to the wheel and start to become detached as the yaw angle increases. The drag will increase gradually with increasing yaw angle. There will not be much difference on the drag of the wheel whether the fork is present or not. The passage of the wheel spokes will have a strong effect on the fork drag. The rim will interfere with the drag on the fork but the fork has little effect on the drag of the rim.
as the yaw angle increases, a clockwise (as viewed from behind the wheel) vortex coming off of the outer edge of the wheel, and a counterclockwise vortex coming off of the inner edge of the wheel rim on the suction side. Both vortices, once separated will move in a downstream direction, being carried along by the surrounding flow. These vortices will be carried along with the forward rotation of the wheel. For most yaw angles there will be a vortice generated at the top of the wheel from the impact of the air being dragged forward by the outer edge of the rotating wheel into the oncoming free stream flow. In the upper part of the wheel, airflow passing the outer edge of the wheel will either join the vortices already there, or be dragged along downwards, following the rotation of the wheel. As the yaw angle increases the vortex will roll down along the front of the wheel, extending from the upper area of the wheel down to the ground.
Due to the front tire rotation towards the ground, the differences in the air speed at the leading edge of the wheel will be at its highest at the top of the wheel, and decreases to a minimum at the ground contact patch. At the center leading edge of the wheel the air flow speed will be zero causing a pressure build up and a slight lift generated due to causing a rearward rotation of the motorcycle at high speeds. At the ground contact patch the air flow speed is zero and the pressure will be high generating a lift there. In the upper area of the wheel, airflow shedding from the inner rim occurs sooner due to the greater difference between the wheel speed and the oncoming air. As the difference in speed decreases, the shedding of the vortices from the inner rim is delayed. The timing between the shedding of the structures will be the lowest at low yaw angles and increase shedding as the yaw angle increases.
Along the trailing rear area of the wheel will be another pair of counter-rotating vortices with one being generated by interaction of the flow being pulled along the inner edge of the wheel and the oncoming air, and the other resulting from the flow on the outer edge of the wheel meeting the oncoming air. Flow in this area of the wheel will be highly disrupted, due mainly to the vortices from the upstream areas of the wheel.
The amount of tire wear, tire construction and tire pressure will affect the handling and stability of the vehicle. With the addition of a streamlined fairing it can lead to a high speed instability on what was a stable vehicle. Severally worn tires can cause a handling problem at higher speeds due to the non-uniformity of the tire. As the tire width increases there will only be small changes to the camber stiffness.
Many stability problems of the vehicle will be due to gyroscopic force and how it relates with the steering. Wobble is the rotation of the front steering turning relative to the rear frame, wobble is a steering oscillation and weave is oscillations of the vehicle that involves steering, roll and yaw and is a fishtailing-type motion that is normally a high speed problem. The flexibility of the frame and forks will have an influence on the speed that the wobble will occur at; normally the more flexible the forks and frame are, the lower the speed wobble will occur at. Weave will becomes more likely when the rear wheel loading increases, or with a higher aerodynamic center of pressure, and wobble will be more likely when front wheel loading increases. Under acceleration at high speeds, the vehicle is much less likely to have front end wobble but more likely to weave due to the aerodynamic loading of the rear wheel. Under hard deceleration the chance of wobble increases and weave will normally not be affected by the deceleration. Wobble becomes more likely under braking and the effects become exaggerated as the deceleration rate increases, the driver tensing his arms and upper body will have little effect on controlling the wobble once it starts. Vehicle weave can be influenced by vehicle weight, wheel base, driver posture and weight. The driver shifting his upper body weight forward can reduce weave. A lighter driver will have more instability problems than a heavier driver. A light driver will have more problems with weave caused by the track surface than a heavy driver. Weave will be relatively unaffected by braking and is affected most at very low speed. Wobble will be caused by the design of the front and weave will normally be caused by both wheels. Tuning the front suspension dampers will not have much effect on wobble but a front steering damper will greatly decrease wobble. The stiffer the vehicle frame is, the more prone it will be to high speed wobble. Weave can be decreased by increasing the damping on the front shocks of the vehicle. By increasing the rear spring rate, adding a steering damper or increasing the force of the steering damper will increase the chance of weave occurring.
Not that we need to worry about it, but under high speed cornering conditions both weave and wobble will be more likely to occur due to the increased tire loading.
Under acceleration the top of the drive chain will be in tension and under engine braking deceleration the bottom of the chain will be in tension. The aerodynamic drag will tend to load the rear wheel, while lightening the load on the front tire. The forces in deceleration are equivalent to the gravitational forces of going downhill and will react the same way.
Aerodynamic drag will do most of the high speed braking, but this task is taken over by the brakes as the aerodynamic drag decreases. Aerodynamic drag will tend to lighten the front wheel and load the rear wheel. As the speed of the vehicle decreases the brakes will take over and do the majority of the stopping. As the aerodynamic braking force decreases the load on the front tire will increase and braking force will have to be increased to have the same level of deceleration. Under deceleration, as the aerodynamic drag decreases, the weight on the rear tire decreases so the front will have to do the majority of the braking. With most of the vehicle weight being on the front wheel most of the braking will be accomplished by the front wheel. Braking with the front wheel at low speeds will have a greater chance at causing wobble so rear wheel braking should be used at very low speeds.