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Design of a mechanical 4-wheel steering system for 4WD RC cars to compensate for understeer under acceleration and oversteer under decelleration

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  1. 1. 4-WHEEL STEERING Top View Profile 1/2M 1/2M Vehicle of mass M
  2. 2. 4-WHEEL STEERING Top View Profile L<<1/2M L>>1/2M Vehicle of mass M ACCELERATION - UNDERSTEER
  3. 3. 4-WHEEL STEERING Profile L<<1/2M L>>1/2M ACCELERATION - UNDERSTEER Tires slip Normal Turn Understeer
  4. 4. 4-WHEEL STEERING Normal Turn Profile L<<1/2M L>>1/2MUndersteer By turning the REAR WHEELS in OPPOSITE DIRECTION by an angle proportional to the ACCELERATION, it becomes possible to correct the UNDERSTEERING. Because of the high load, the rear tires have excellent grip. Only a small angle is necessary to compensate for understeer ACCELERATION - UNDERSTEER Tires slip
  5. 5. 4-WHEEL STEERING Profile L>>1/2M L<<1/2M When the vehicle is under STRONG DECCELERATION while turning, There is not enough load on the REAR axle. The REAR tires slip. The vehicle OVERSTEERS. DECELLERATION - OVERSTEER Tires slip Normal Turn Oversteer
  6. 6. 4-WHEEL STEERING Profile L>>1/2M L<<1/2M DECELLERATION - OVERSTEER Tires slip Normal Turn Oversteer By turning the REAR WHEELS in the SAME DIRECTION by an angle proportional to the DECCELERATION, it becomes possible to correct the OVERSTEERING by moving the vehicle “crabwise”, compensating for the tightening turning radius.
  7. 7. 4-WHEEL STEERING 4-WHEEL STEERING is not NEW. Many modern cars use some form or active or passive 4-wheel steering for a different purpose however. In most applications, the goal is to increase the stability of the vehicle at high speed and increase the steering response at low to very low speed. LOW SPEED HIGH SPEED WHY?
  8. 8. 4-WHEEL STEERING The understeering under acceleration and oversteering under deceleration effect is more noticeable if: 1. The vehicle has a higher center of gravity (i.e. the mass transfer between acceleration and braking is more important) 2. The vehicle has a high Power/Weight ratio In the US, power to weight ratio is actually expressed in weight to power ratio (it makes perfect sense…) or LBS/HP. Some examples: 1. Ferrari F12 Berlineta 730HP/3,362LBS 4.6LBS/HP 2. Chevrolet Corvette ZR1 638HP/3,324LBS 5.2LBS/HP 3. Porsche 911 (997) 340HP/3,300LBS 9.7LBS/HP 4. Dodge Magnum STR8* 425HP/4,379LBS 10.3LBS/HP 5. Ford Focus ST 252HP/3,250LBS 12.9LBS/HP 6. VW Golf GTI 200HP/3,100LBS 15.5LBS/HP 7. Toyota Camry 4 cyl. 178HP/3177LBS 17.8LBS/HP * My car. The perfect compromise: Fast as a Porsche but take a full load of two-by-fours from Home Depot
  9. 9. 4-WHEEL STEERING Only VERY EXPENSIVE EXOTIC VEHICLES have a power to weight ratio exceeding 10LBS/HP but some inexpensive vehicles do…… RC CARS (Remote Controlled Cars), for example: 1. TRAXXAX T-MAX 3.3 3HP/10LBS 3.3LBS/HP! AND…. A HIGH CENTER OF GRAVITY! Shown without bodywork
  10. 10. 4-WHEEL STEERING The T-MAX 3.3 from Traxxas is a 1/8-scale off-road buggy. It is a 4-wheel drive vehicle with front and rear differentials (no central dif.) and long stoke double wishbone suspensions using oil filled shocks and anti-roll bar on the rear axle. It is an off-road vehicle and the soft suspension/high center of gravity amplify the effect of mass transfer under hard acceleration and braking. The knobby tires are designed to provide good grip on the dirt, not on a smooth surface ( concrete or tarmac). On a smooth surface, under maximum acceleration, the vehicle will go straight even with the front wheels fully turned. In addition, it is difficult to control the vehicle under hard braking, often resulting in a full 180° rotation. The high power to weight ratio/soft suspension/high center of gravity results in MASSIVE UNDERSTEERING AND OVERSTEERING.
  11. 11. 4-WHEEL STEERING • The T-MAX 3.3 is powered by a 3.3cm3 2-stroke “nitro” engine delivering up to 3HP at 50,000RPM. • “Nitro” fuel used by model engines is a mix of Methanol, castor (or synthetic) oil (10 to 20%) and Nitromethane (10 to 30%.) • A quick word about the mythical Nitromethane. • Since an engine's cylinder can only contain a limited amount of air on each stroke, 8.7 times more Nitromethane than gasoline can be burned in one stroke. Nitromethane, however, has a lower specific energy: Gasoline provides about 42–44 MJ/kg whereas Nitromethane provides only 11.3 MJ/kg. Therefore, Nitromethane generates about 2.3 times the power of gasoline when combined with a given amount of oxygen. • In addition, Nitromethane has a high combustion velocity making it suitable for high-RPM engines and a high heat of vaporization providing cooling to a high performance engine. • Nitromethane 4CH3NO2 is an oxygen rich fuel. The amount of air required to burn 1 kg of gasoline is 14.7 kg (or 11.4m3!), but only 1.7 kg (or 1.3m3) of air is required for 1 kg of Nitromethane.
  12. 12. 4-WHEEL STEERING There are two strategies to implement a 4-wheel steering system of an RC car. 1. Software/electronics (modern) 2. Mechanical (old school) ORIGINAL SKETCH OF A PURELY MECHANICAL 4-WHEEL STEERING SYSTEM
  13. 13. 4-WHEEL STEERING 1. Software/Electronics Modern high-end radio transmitters are very sophisticated and programmable allowing “servo mixing”. This technology was originally developed for remote controlled helicopter in the form of CCPM (or Collective-Cycling Pitch Mixing.) On a helicopter, the swashplate is controlled by 3 servos. In the past the motion of the swashplate for collective or cycling pitch was controlled by a complex system of mechanical linkage (mechanical mixing). Nowadays, with CCPM, 3-servos directly connected to the swashplate (usually at 120°) control its motion for collective and cycling pitch. The software in the transmitter “mixes” the signal to the servos. For example, for collective pitch, the servos moves together raising or lowering the swashplate. This is electronic mixing or CCPM. All modern high performance RC helicopters use CCPM today. The advantages are obvious. By eliminating the complex mechanical linkage, CCPM saves costs in manufacturing, reduce the mass, increase reliability* and provide better control. SERVO MIXING is the best and easiest solution for implementing a 4-wheels steering system on an RC car. swashplate servos *However, a single servo failure results in complete loss of control
  14. 14. 4-WHEEL STEERING 2. Mechanical This is OLD SCHOOL… But then I was born in 1961, in the past millennium! The picture on the right shows a high-end RC helicopter using mechanical collective/cyclic pitch mixing. It is indeed complex and requires precise machining and assembly of the parts as any slop or play in the various linkages results in poor control of the helicopter. It is easy to understand why electronic CCPM has replaced mechanical mixing as the only advantage of mechanical mixing is that the failure of a single servo does not necessarily result in complete loss of control and destruction of the aircraft (with risk of injuries) as a good pilot may be able to safely land the helicopter. In addition, a mechanical system is not flexible. Any modification or upgrade requires designing and machining new components. An electronic system offer more control and tuning by adjusting parameter in the software. swashplate servos *Several years ago in Houston, a RC helicopter pilot died, his head almost completely severed as another pilot lost control of his aircraft. The large RC helicopters have a rotor diameter of 1.4m. These are dangerous toys, easier to fly today thanks to advanced electronic (3-axis gyro, electronic compass and GPS receiver) but still difficult to master.
  15. 15. 4-WHEEL STEERING Simplified Servo Setup for Front Wheel Steering, Throttle and Brake of an RC car A single servo controls the throttle and brake Steering servo BRAKETHROTTLE
  16. 16. 4-WHEEL STEERING OLD SCHOOL (Mechanical) setup for a 4-wheel steering system. YES! It is not simple! This single servo controls the throttle, brake and the proportion of rear wheel steering (opposite to the front wheels under acceleration and parallel to the front wheels when braking
  17. 17. 4-WHEEL STEERING OLD SCHOOL (Mechanical) setup for a 4-wheel steering system The light blue “banana shaped” part rotates around the darker blue shaft
  18. 18. 4-WHEEL STEERING OLD SCHOOL (Mechanical) setup for a 4-wheel steering system RIGHT STEERING - NEUTRAL (NO THROTHLE OR BRAKE APPLIED) 1. The light blue “banana shaped” part rotates when steering is applied 2. However, this has no effect on the rear steering as the vehicle is coasting (No brake/No throttle
  21. 21. 4-WHEEL STEERING Graphical solution for a different system of 4-wheel steering
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