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

3. 4-WHEEL STEERING
Top View
Profile
1/2M 1/2M
Vehicle of
mass M

4. 4-WHEEL STEERING
Top View
Profile
L<<1/2M L>>1/2M
Vehicle of
mass M
ACCELERATION - UNDERSTEER

5. 4-WHEEL STEERING
Profile
L<<1/2M L>>1/2M
ACCELERATION - UNDERSTEER
Tires slip
Normal Turn
Understeer

6. 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

7. 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

8. 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.

9. 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?

10. 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

11. 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

12. 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.

13. 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.

14. 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

15. 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

16. 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.

17. 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

18. 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

19. 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

20. 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. 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

22. 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

23. 4-WHEEL STEERING
Graphical solution for a different
system of 4-wheel steering

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