4 ws

4-WHEEL STEERING

Top View

Profile

1/2M 1/2M

Vehicle of mass M

4-WHEEL STEERING

Top View

Profile

L<<1/2M L>>1/2M

Vehicle of mass M

ACCELERATION - UNDERSTEER

4-WHEEL STEERING

Profile

L<<1/2M L>>1/2M

ACCELERATION - UNDERSTEER

Tires slip

Normal Turn

Understeer

4-WHEEL STEERING

Normal Turn

Profile

L<<1/2M L>>1/2MUndersteer

By turning the REAR WHEELS in OPPOSITEDIRECTION by an angle proportional to theACCELERATION, it becomes possible to correct theUNDERSTEERING. Because of the high load, the reartires have excellent grip. Only a small angle isnecessary to compensate for understeer

ACCELERATION - UNDERSTEER

Tires slip

4-WHEEL STEERING

Profile

L>>1/2M L<<1/2M

When the vehicle is under STRONGDECCELERATION while turning, There is notenough load on the REAR axle. The REARtires slip. The vehicle OVERSTEERS.

DECELLERATION - OVERSTEER

Tires slip

Normal Turn

Oversteer

4-WHEEL STEERING

Profile

L>>1/2M L<<1/2M

DECELLERATION - OVERSTEER

Tires slip

Normal Turn

Oversteer

By turning the REAR WHEELS in the SAMEDIRECTION by an angle proportional to theDECCELERATION, it becomes possible to correct theOVERSTEERING by moving the vehicle “crabwise”,compensating for the tightening turning radius.

4-WHEEL STEERING4-WHEEL STEERING is not NEW. Many moderncars use some form or active or passive 4-wheelsteering for a different purpose however.

In most applications, the goal is to increase thestability of the vehicle at high speed and increasethe steering response at low to very low speed.

LOW SPEED HIGH SPEED WHY?

4-WHEEL STEERING

The understeering under acceleration and oversteeringunder deceleration effect is more noticeable if:1. The vehicle has a higher center of gravity (i.e. the

mass transfer between acceleration and braking ismore 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/HP2. Chevrolet Corvette ZR1 638HP/3,324LBS 5.2LBS/HP3. Porsche 911 (997) 340HP/3,300LBS 9.7LBS/HP4. Dodge Magnum STR8* 425HP/4,379LBS 10.3LBS/HP5. Ford Focus ST 252HP/3,250LBS 12.9LBS/HP6. VW Golf GTI 200HP/3,100LBS 15.5LBS/HP7. 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

4-WHEEL STEERING

Only VERY EXPENSIVE EXOTIC VEHICLES have a power toweight ratio exceeding 10LBS/HP but some inexpensivevehicles 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

4-WHEEL STEERING

The T-MAX 3.3 from Traxxas is a 1/8-scale off-road buggy. Itis a 4-wheel drive vehicle with front and rear differentials(no central dif.) and long stoke double wishbonesuspensions using oil filled shocks and anti-roll bar on therear axle. It is an off-road vehicle and the softsuspension/high center of gravity amplify the effect ofmass transfer under hard acceleration and braking.

The knobby tires are designed to provide good grip onthe dirt, not on a smooth surface ( concrete ortarmac). On a smooth surface, under maximumacceleration, the vehicle will go straight even withthe front wheels fully turned. In addition, it is difficultto control the vehicle under hard braking, oftenresulting in a full 180° rotation.

The high power to weight ratio/softsuspension/high center of gravity results inMASSIVE UNDERSTEERING AND OVERSTEERING.

4-WHEEL STEERING

• The T-MAX 3.3 is powered by a 3.3cm3 2-stroke “nitro” enginedelivering 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 Nitromethanethan gasoline can be burned in one stroke. Nitromethane, however, has a lower specific energy: Gasolineprovides 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 highheat of vaporization providing cooling to a high performance engine.

• Nitromethane 4CH3NO2 is an oxygen rich fuel. The amount of air required to burn1 kg of gasoline is 14.7 kg (or 11.4m3!), but only 1.7 kg (or 1.3m3) of air is requiredfor 1 kg of Nitromethane.

4-WHEEL STEERING

There are two strategies toimplement a 4-wheel steeringsystem of an RC car.

1. Software/electronics (modern)2. Mechanical (old school)

ORIGINAL SKETCH OF A PURELYMECHANICAL 4-WHEEL STEERINGSYSTEM

4-WHEEL STEERING1. Software/ElectronicsModern high-end radio transmitters are very sophisticated andprogrammable allowing “servo mixing”. This technology wasoriginally developed for remote controlled helicopter in the form ofCCPM (or Collective-Cycling Pitch Mixing.) On a helicopter, theswashplate is controlled by 3 servos. In the past the motion of theswashplate for collective or cycling pitch was controlled by a complex

system of mechanical linkage (mechanical mixing). Nowadays, with CCPM, 3-servos directly connected tothe swashplate (usually at 120°) control its motion for collective and cycling pitch. The software in thetransmitter “mixes” the signal to the servos. For example, for collective pitch, the servos moves togetherraising or lowering the swashplate. This is electronic mixing or CCPM. All modern high performance RChelicopters 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 steeringsystem on an RC car.

swashplate

servos

*However, a single servo failure results in complete loss of control

4-WHEEL STEERING

2. MechanicalThis is OLD SCHOOL… But then I was born in 1961, in the pastmillennium! The picture on the right shows a high-end RC helicopterusing mechanical collective/cyclic pitch mixing. It is indeed complexand requires precise machining and assembly of the parts as any slopor 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 ofmechanical mixing is that the failure of a single servo does not necessarily result in complete loss of controland 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 upgraderequires designing and machining new components. An electronic system offer morecontrol 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.

4-WHEEL STEERING

Simplified Servo Setup for Front Wheel Steering, Throttle and Brake of an RC car

A single servo controlsthe throttle and brake

Steering servo

BRAKETHROTTLE

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 accelerationand parallel to the front wheels when braking

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

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

4-WHEEL STEERING

OLD SCHOOL (Mechanical) setup for a 4-wheel steering system

RIGHT STEERING – UNDER ACCELERATION

THE PROPORTION OF OPPOSITE REAR STEERING IS FUNCTION OF THE THROTTLE (THE MORE THROTTLE, THE MORE OPPOSITE STEERING

4-WHEEL STEERING

OLD SCHOOL (Mechanical) setup for a 4-wheel steering system

RIGHT STEERING – BRAKING

THE PROPORTION OF PARALLEL REAR STEERING IS FUNCTION OF THE THROTTLE (THE MORE THROTTLE, THE MORE PARALLEL STEERING

4-WHEEL STEERING

Graphical solution for a different system of 4-wheel steering

RETURN

TO

MENU?