This post will pertain to a full body vehicle with a flat floor underbody, rear diffuser, 3 inches of ground clearance and 4 wheels enclosed in wheel wells. It will cover the airflow through the rims and around the tires and wheel wells and show some of the dynamics of the tires and how they affect other parts of the vehicle, as well as a glimpse into what is occurring at the tire ground contact patch when running on a deformable track surface.
The fastest speed ever attained by a manned rocket-powered aircraft was by the North American X-15, it was part of the X-series of experimental aircraft and eight different X-15 pilots met the USAF spaceflight criteria and became qualified for the status of astronaut and gained either USAF astronaut wings or NASA astronaut wings in doing so. The X-15 was air dropped by a B-52 and could burn 15,000 pounds of fuel in 80 seconds. On October 3, 1967, William J. "Pete" Knight, piloting the X-15A-2, set a world speed record for an aircraft, 4,520 mph. He earned his astronaut wings by flying an airplane in space, reaching an altitude of 280,500 feet. He was inducted into the Aerospace Walk of Honor for his bravery and accomplishments.
The effect humidity has on atmospheric density is very small, the average amount of water vapor in the atmosphere by weight is less than 1%. At sea level the air density would change less than 1% even with a 100% humidity reading. Moisture content in the track surface, on the other hand, can have a big influence on vehicle performance.
Before reading this post it would be helpful to review my earlier post, on air flow around the wheels of an open wheel vehicle. In my earlier post on airflow around open exposed car wheels, there were 3 sets of counter rotating vortices generated around the tire, a pair from the top of the tire; a second set from the center of the wheel axis and the final set are the high pressure jetting vortices coming from the ground contact patch and following along each side of the tire. When the wheel is enclosed in a wheel well, three out of the six vortices disappear: only the pair of jetting vortices at ground level and the external vortex shedding from the outer wheel axis remains. The vortices from the top of the tire will disappear because the wheel well will interfere with the external air flows ability to attach its self to the upper portion of the wheel. There will be other vortices generated from inside the wheel wells that will travel downstream, but they are not attached to the wheel, only influenced by the wheel.
Applying the engines power to the track surface takes a system of components all working together, the engine, transmission, driveshafts, differentials, springs, absorbers and tires, but in the end it all comes down to those 4 small rubber contact patches and how they react with the track surface beneath them.
Due to the interactions between the tire being deformable and the track surface being deformable, it will cause many problems for the performance of the vehicle. Rolling resistance of the tire will produce heat from the mechanical energy passing through it to the track surface, consuming part of the engines power in the process. All major forces and moments affecting the vehicle, other than aerodynamic and gravitational forces, are applied through the tire ground contact patch.
Hysteresis in the tire is caused by the deflection of the tire sidewall and tread. While rolling, a tire with low hysteresis will have reduced internal heat generation and a reduced rolling resistance. A tire with a shorter sidewall will deform less, therefor generating less heat in the tire and less rolling resistance. A tire with high hysteresis will require more energy to initially deform the tire during loading than the unloading; it can be thought of as deformation and recovery occurring at high speeds, that extra energy will be dissipated or "lost" as heat into the tire, and will be more pronounced under high speeds. Hysteresis and tire slip will be what generates most of the heat buildup in the drive tires. The viscoelasticity of the tire rubber causes the hysteresis and it is the main cause of the tires rolling resistance. During the tires construction, the rolling resistance can be lowered by substituting certain compounds into the tire, such as silica can be substituted for some of the carbon black in the tread compound or nano-clay can be added to the mixture during the tires construction to reduce rolling resistance also. Increasing the air pressure in the tire will decrease the flex of the sidewall and the contact patch and lower the rolling resistance. Severally over inflating the tires may not always reduce rolling resistance because the tire may have more of a tendency to hop and skip over the track surface and the wheel slippage and heat buildup may increase due to the rotational speed changes in the tire and lead to premature tire failure and blistering. Rolling resistance will decrease in the tire as the tire temperature increases, within reason. Hysteresis is the stretching and movement of the belts and plies inside the rubber tire that causes the subsequent heat buildup. At speeds of 80mph to 95mph rolling resistance from the tire can be broken down as 90% to 95% due to hysteresis in the tire sidewall and tread and 5% to 10% due to friction in the contact patch. The rolling resistance from hysteresis in the tire structure can be further broken down into a 70% to 75% contribution from the tire contact patch area, 10% to 15% from the sidewall, 10% to 15% from the tire shoulder and 2% to 4% from the beads. A thicker tread and sidewalls will normally increase the rolling resistance due to an increase in hysteresis losses. Generally a tire made of natural rubber will have a lower rolling resistance than one made of synthetic rubber. A rough track surface will have a higher rolling resistance than a smoother track and a moist track surface will have a higher rolling resistance than a dry surface. Radial-ply tires will have less rolling resistance and heat generation as compared to a bias-ply tire. On a radial tire, as the temperature of the shoulder increases, rolling resistance decreases, up to a temperature of between 160F and 170F before it levels off, the increased tire temperature alters the stiffness and hysteresis of the tire rubber. A tire with a shorter sidewall will deform less, generating less hysteresis, therefor generating less heat in the tire and a lower rolling resistance due to the decreased hysteresis.
The cords in adjacent plies, in a bias-ply tire, run in opposite directions and the diagonal plies flex and rub on each other, this is the main cause of the high rolling resistance in a bias-ply tire, whereas flexing of the tire involves very little movement and rubbing of the belts in a radial-ply tire. On hard surfaces the proper inflation pressure of a bias-ply tire is more important than on a radial-ply tire, for decreasing rolling resistance. The rolling resistance of a radial-ply tire will be 40% to 60% less than that of a bias-ply tire under similar conditions, and the tire life can be double that of an equivalent bias-ply tire. A radial-ply tire will have a more uniform pressure over the contact patch area, whereas the contact patch pressure for a bias-ply tire will vary greatly as the contact patch area undergoes a wiping motion from the cross plies flexing and rubbing over each other in the bulging side wall and contact patch area.
The forces transferred to the vehicle thru its tires and wheels, from the track surface, are the main influence on the vehicles motion, influencing its stability and maneuverability. Vehicle tire sinkage into the track surface and the distribution of pressure in the contact patch is a function of tire slip and the amount of downforce applied to the tire, rolling resistance will increase with sinkage. Because the tire is deformable, when ran on a soft track surface, it will tear up particles from the track surface and displace the material above the original track surface in the wake of the tire. Under acceleration the tire will remove track material from the contact patch and it will be transferred to the trailing edge of the tire. Moisture content in the track surface above 15% to 20% will cause the tire slip to become less dependent on tire sinkage. On harder surfaces there will be more of the tire bulging 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, but on softer surfaces the pressure at the edges of the contact patch will be about 50% of what it is at the center of the contact patch. The compaction of the surface will involve the particles sliding over each other and as the loading and pressure increases it will cause fragmentation of the particles. There will be erosion of the particles due to their low ductility and the shear response will be based on its yield-strength vs. pressure. As the particles fragment they still have the ability to support pressure but its capability to withstand tensile and shear stresses will decrease.
The effect of the rotating wheels, inside a wheel well, will have on the flow past the vehicle will be many and far reaching. The rotating wheels will influence the cooling of the brakes, underbody air flow, track surface deposition on the vehicle, the lift forces and drag acting on the vehicle body.
Tires on open wheel vehicles will generate large amounts of lift and drag, whereas enclosed wheels inside wheel wells will generate an overall increase in downforce, as well as a reduction in drag, and if the wheels are properly optimized for their respective vehicle, even higher downforce and lower drag will occur. The total aerodynamic drag from the wheels is about 25% to 30% for an ordinary passenger car and the rear wheels will contribute about 60% to 65% of that total 30% drag. The airflow from the front wheel can affect the airflow at the rear wheel but the rear does not affect the front. Most of the drag is due to the crossflow through the rims and by optimizing the rim design and size of the wheel wells, will decrease the drag of the vehicle. Wheel covers placed on the outside of the wheel rim only, for all 4 wheels, will decrease wheel drag by 20% to 35% and increase overall vehicle downforce. With the addition of ducts installed inside the wheel wells, to vent the rear wheel well pressure off into the wake behind the vehicle, will lower the rear wheel drag an additional 2% to 3%. Using outer wheel covers in combination with internal wheel well ducting, to vent the high pressure off, will result in a further 2% to 3% decrease in the front wheel drag also. Using moveable underbody panels, attached to the spindle and lower A-arm will help to seal the wheel well from the underbody airflow and reduce the front wheel drag by an additional 1% to 3%. Adding wheel covers to the back side of the rims will decrease the drag and lift even more, especially on the front wheels. A substantial decrease in vehicle drag can be achieved with the proper rim design. The wheels will have a large influence on the overall aerodynamic properties of the vehicle. In the vicinity close to the wheels and wheel housings the airflow is highly dependent of the rim design. When the rims have small spokes and large open areas a significant airflow through the rims will occur, creating large areas of low pressure around the wheels where energy will be lost. By covering the entire rim outer surface with a moon disc or some other aerodynamic cover and blocking the crossflow through the rim, will have positive effects, mainly reducing the drag, but it will have negative effects also. By covering the rims, it will result in smoother airflow along the sides of the vehicle, as well as prevent the airflow underneath the vehicle from being pulled out and spilling towards the sides of the vehicle and therefor maintain a higher flow velocity underneath the vehicle. By blocking the crossflow through the rims, it will result in increased static pressure in the front wheel well, increasing the lift, whereas blocking the crossflow in the rear rim will result in increased downforce due to an increased flow rate through the diffuser, the rear wheel when not running an outer cover will pull air from under the vehicle and away from the diffuser. Blocking the air crossflow through the rims can destabilize the vehicle stability by increasing lift on the front wheels and increasing downforce on the rear wheels, so additional care should be taken to add some downforce on the front wheels when wheel covers are added, such as extra ballast in the front, adding a splitter, inner wheel covers or a front diffuser or underbody vortex generators added to the front underbody. The crossflow through the front rim openings will create large wake structures that will travel downstream, creating turbulence and drag. When the front wheel is covered the wake will be narrower. Installing an aerodynamic cover over the front rim will help to reduce the strength of the jetting vortices coming from the contact patch, thereby reducing lift from that area and the airflow passing the rim on the outside will be less disturbed and be pulled in closer to the side of the vehicle, lowering the drag from the side airflow. Covering the front wheels will usually result in a decrease in drag but at a penalty of an increase in lift on the front axle, covering the rear wheels will reduce the drag even more than only covering the front. The static pressure inside the front wheel well will be higher when the rim is covered and the high pressure will move in front of the wheel close to the contact patch and it will influence the flow around a majority of the side of the vehicle, causing the low pressure that exists there to be partially eliminated. If only the outside of the front wheel is covered, higher pressure inside the back of the rim will occur because the crossflow through the rim that was occuring is stopped, creating higher static pressure and lift, causing a flow reversal, and causing the wake on the inside of the wheel to be slightly wider. By stopping the crossflow through the wheel the flow from inside of the wheel house needs to pass between the wheel and the wheel well. The higher pressure found inside the back of the rim, due to the crossflow being stopped, will increase the lift of the front axle. The higher pressure generated inside the wheel well, will cause more of the air that would normally enter the wheel house, to travel along the outside of the wheel. The drag of the vehicle will increase as the volume of the wheel well increases. The radius of the wheel well will affect drag more than the width of the wheel well. Tire wake thickness and width will increase as the radius of the wheel well increases. Drag will also increase as the wheel well is widened but it has less effect on the drag than the wheel well radius. The air flow and wake from the front wheel will have a big influence on the flow around the rear wheel. By covering the front wheel, it will cause a reduction in drag at the rear wheel of 7% to 21% due to a decrease in the strength of the high pressure region around the contact patch of the rear wheel and a decreased rear wheel wake. If the front wheel is not covered the wake from the front wheel will be larger and will extend all the way back to the rear wheel, this affects the stagnation pressure for the rear wheel. The low pressure area around the rear wheel will become larger when the front wheel is not covered. The difference in pressure at the leading edge and trailing edge of the rear wheel becomes greater, increasing drag and creating a larger wake behind and to the outside of the rear wheel.
Covering the rear wheel will cause the pressure close to the ground, starting from the contact patch, to be slightly lowered. The downforce from the diffuser can be increased by 4% to 10% when the rear wheels are covered because the air underneath the vehicle is more enclosed by the closed rims and the flow through the diffuser is increased, and not being pumped out of the vehicle underbody by the rims crossflow pumping effect. Covering the outer part of the rim will cause the largest reduction in drag. The wake area at the lower part of the wheel will be significantly smaller for this rim due to blockage of crossflow through the rim. In general the drag will decrease with increasing covering area whereas the lift will increase with increasing covering area. Changing the shape or spoke design in small amounts will have little influence on the results. If both front and rear wheels are covered they will have the lowest drag but will be influenced the most by yaw angle, increasing the drag more than if they were not covered. By covering the rear wheels only the vehicle will be the most sensitive to yaw, in terms of lift.
A vehicle with decent aerodynamics will benefit more from the wheel covers, but if the vehicle has very bad aerodynamics and a large base drag, low pressure area behind the vehicle, fabricating and installing a centrifugal fan on the rims to pull air flow through the rear rims, from the outside to the inside of the wheel well, the wheels can be used to pump air into the wheel wells and channel it through ducts, from the wheel well, to exit out the rear of the vehicle, helping to fill the wake and lowering the pressure drag on the rear of the vehicle. This technique can reduce the overall vehicle drag by 15% to 20%. The greater the pressure drag behind the vehicle the better this will work. The installation of the fans on the rear rims will reduce the strength of the jetting vortices exiting the contact patch at ground level also. 60% to 70% of the airflow pulled through the rim by the fan, will attach its self to the tires tread surface, forming a vortice, and be carried forward into the contact patch area where it will split into 2 vortices, reducing the strength of the 2 jetting vortices from the contact patch area, thereby reducing the wake behind the wheel and reducing lift that is generated by the jetting vortices. About 30% to 40% of the air being pulled through the rim by the fan will exit through the duct and out the back of the vehicle into the low pressure wake area. Fans can be installed on the front rims also, but there effect will not be as dramatic as the rear fans, due to not being able to contribute to the reduction of the rear pressure drag, but they can still reduce the jetting, lift and the tire wake size of the front tires.
By virtue of their close proximity to each other, changes in one part of the vehicle will have an effect on other parts. By changing the location or angle of a rear wing or spoiler or the shape of the fender behind the rear wheel will cause significant changes in the drag of the rear wheel. There is an important interaction between the wake of the rear wheel and the wake of the rear body.
A separation of the rubber sandwiched between the belts of the tire is a common failure of a tire at high speeds and will be due to a fatigue crack in the rubber between the belts. The rubber that bonds the belts to the tire plies and the tread surface can separate peel due to the centrifugal force at high speeds, causing the outer belt and tire tread to separate from the body of the tire and this can become more likely with increased tire age and depending on the use and storage of the tire during its life. As the tire ages it will lose elasticity and cause the growth rate of cracks to increase and start to form at the belts, resulting in tread separation. As the tire ages the strength of the rubber between the belts will decrease and the peel strength decreases accordingly. The aging or Oxidation of the internal rubber is very dependent on the temperature that the tire has been maintained at during its life. The tire rubber oxidation will occur from the outside surface in and from the inside pressure surface out, but the oxidation rate from the inside of the tire to the outside will occur much faster due to the high pressure air inside the tire. The use of nitrogen for tire inflation will reduce the oxidation of the rubber between the belts, due to the tire being permeable to the air and the escaping pressurized air accelerating internal tire oxidation. The nitrogen molecules are much larger than the air molecules and will not permeate through the tire as easily and if it does, it will not cause rubber oxidation due to it being an inert gas. Natural rubber is more permeable to air while synthetic rubber has a much lower permeability rate. The innerliner built into the tire is critical in reducing oxidation of the tire rubber; it is made from a special blend of synthetic rubber to reduce the permeability and oxidation from the high pressure air inside the tire. The tire innerliner does not stop the permeation it only slows it down. Time and temperature are the most important factors to internal tire rubber oxidation. Land speed racing would be more susceptible to tire oxidation and failure than any other form of racing, due to the fact our tires can last much longer, in some cases many years, and are normally purchased when a vehicle build is commenced that could take years to complete and when we do race it is normally in climates with extreme temperatures and faster speeds than any other form of racing, putting extreme pressures on our tires. At times there have been shortages of certain tires we require and older tires or any that were available were used. Sometimes a racer may make 2 events in a year, park it for 1 or 2 years dust it off and make another event. The faster the vehicle and the more pressures and loads the tires are subjected to, will make the effects of oxidation more dangerous. Purchasing a tire with a higher speed rating than is required will allow a margin of safety for some oxidation of the rubber. A tire with a taller side wall will have a faster rubber oxidation rate than a tire with a shorter side wall. The oxidation of the tire rubber will increase as the tire inflation pressure is increased and the oxidation will increase with temperature up to about 160F and then start leveling off. Inflating the tire with nitrogen in place of air will reduce tire rubber oxidation 60% to 75%
If the tires have tread grooves and are running on a hard surface there will be deformation of the tread grooves as they enter and exit the contact patch. Due to the tread deformation as it enters the contact patch the air captured in the grooves will be compressed and then expelled back out of the leading edge of the tire contact patch. At the trailing edge there will be air displacement as well, due to the tread expansion, which will generate an inflow effect. The air squeezing out of the compressed grooves and rushing into the expanded grooves will cause air fluctuations. The air pumping effect can occur due to the cavities in the tire tread, as well as being due to pockets in the track surface that the tire rolls over. As the surface of the tire moves toward the ground contact patch at the leading edge, it will transport energy towards the stagnation point increasing the total pressure. The pressure difference between the high pressure area in front of the ground contact patch, and the sides of the wheel forces the flow laterally and causes it to accelerate rapidly, this is called jetting and will cause vortices to form at the ground level, coming out of the front and trailing down the sides of the tire contact patch area. The vortice on the outside of the wheel will be larger than the vortice on the inside of the wheel due to the outward deflection of the approaching air flow down the side of the vehicle.
The friction, which provides the traction, between the tire and ground surface is complex and will depend on many factors, such as the elastic properties of the rubber, camber angle, load pressing on the tire, tire type, tread design, ground surface texture, temperature, tire slip ratio, vehicle speed and tire inflation pressure. The friction between the tire and the track surface will be dependent on the vehicle speed and the slip ratio. On a soft track surface, both the tire and the track surface will be deformable. As the wheel sinks into the track surface under acceleration, it will encounter resistance by the track surface, because it will have to compact the surface as it rolls forward, thereby requiring more energy to overcome. Deformation of a soft track surface, in front of and beneath the tire, as well as lateral movement of the track surface, increases the tire rolling resistance. On very hard surfaces, the effect of the lateral movement of the track does not occur and is of no concern, but on soft surfaces, lateral movement of the track surface will increase the tire rolling resistance. The vehicle tires and track surface having a resistance to deformation will have a resistance to penetration by the tires and therefor generate lift and drag, this effect will increase with vehicle velocity, as the track surface will have less time to react to the movement and at some point the tires of the vehicle can actually be raised up above the surface of the track and start to plane on the surface, much like a boat raising out of the water and planing on the surface of the water at high speeds. The amount of lift and drag will be determined by the amount of deflection from the track surface and the tire load deflection. When the vehicle tires start planing above the track surface, it will lower the drag and smooth out the ride. There will be a fine line between the amount of tire drag and the down force required, whether from ballast or aerodynamic down force, required to keep enough traction for forward momentum. If too much downward pressure is created it will push the tire into the surface too much and create more drag than required, slowing the acceleration rate and top speed of the vehicle. If there is not enough down force, either ballast or aerodynamic down force, the vehicle will start to plane to early and too much and become very loose, spin the tires too much, cause vehicle spin out or even cause the vehicle to become uncontrollable and possibly become airborne. Sinkage into the track surface will be due to the compressibility and shear strength of the track surface in response to the vehicle load. A combination of the track surface, tires and vehicle velocity can combine to form a type of bow wave of particles from the track surface that will be moved along in front of the rolling tires at higher vehicle velocities, this is called bulldozing. Because the force of a moving wheel is acting on the tire at a more inclined angle, a rolling wheel on a deformable surface will sink more than a non-moving wheel, with the same vertical force applied to it. Wheel sinkage will decrease as the wheel diameter increases. The softer the track surface, the less internal tire rolling resistance, hysteresis, there will be due to less rolling resistance. Rolling resistance of the tires on a semi soft surface will be mainly due to a compaction force, displacement effect of soft track surface particles and side wall friction. The moisture content of the track surface will play a large part with tire rolling resistance and the tire behavior in these situations.
Jim Halls Chaparral-2E, had a driver adjustable rear wing that could be adjusted while the vehicle was moving on the track. The driver would use it to generate maximum down force for cornering and then reduce its angle entering the straights, for maximum top speed and less drag. By controlling the amount of airflow into the underbody, with an adjustable rear diffuser angle or adjustable height side skirts, an adjustable wing or a type of moveable canards on the side of a vehicle that the driver can adjust on the move, down force can be adjusted to suit changing track conditions. If a person wants to use ballast for all of the down force, the moveable aerodynamic downforce can be used for a sort of fine tuning device, which the driver can adjust on the move. If he does not have enough downforce and is spinning the tires, there are only so many runs that can be made, instead of going back to the trailer to add ballast make an adjustment while moving on the track, it could make the difference between an aborted run or a record setting run. 600 pounds of downforce could be dialed in by the driver with the push of a button, the pull of a lever or the turn of a knob. According to Wikipedia, the bodywork of the Target 550 streamliner of Marlo Treit and Les Davenport will provide 1000 lbs. of downforce to help prevent the car from becoming airborne. It seems as though a lot of the newer streamliners are building in aerodynamic downforce, just to name a few Target 550, Speed Demon and Mormon missile either with shaped underbody floors or upper body work. The DOWNFORCE (wmts) from the underbody is virtually drag free though.
I think this post should transition nicely into my next post.