Question

A car weighing 13 kN has an engine with performance characteristics as shown in Figure 1 below. The car equipped with a 5-speed manual transmission system with 90% efficiency, final drive gear ratio (final drive) is 3.0. The equivalent weight of the rotating parts of driveline at 3500 rpm is 300 N with a radius of gyration of 10.1 cm. Each tyres is 66 cm in diameter with a radius of gyration of 25.2 cm and weighs 220.4 N. Each front tyre has a cornering stiffness of 40.3 kN/rad whilst the rear tyre has a cornering stiffness of 30.2 kN/rad. The car has a wheelbase of 175 cm, the centre of gravity is 60.3 cm behind the front axle and 35.4 cm above the ground level. Frontal area of the car is 2.1 m2 and aerodynamic drag coefficient is 0.25. Rolling resistance coefficient is 0.015 and the air density is 1.225 kg/m3.

20 30 40 50 60 70 80 90 Power (kW) 250 982 bsfc, g/kWh (efficiency,%) 785 200 WOT 250 (343) 260 (33.31 270. (31.8) 280 (30.6). 589 150 310.27.2 Brake Torque (Nm) BMEP (kPa) 393 100 350 (245) -400 (1.4) 196 50 .500 (171) 600 (143) 700 (12:41 800-410.7) 1000 (8.37) 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 Engine speed (RPM) Figure 1

Explain the meaning of neutral steer, understeer and oversteer. What should be done when there is understeer problems?                                                                                                (8 marks)

Answer 1

1) Neutral steer definition

The characteristic of a vehicle's slip angles where both front and rear are the same. In slippery conditions, both the front and rear tires will break loose at the same time so that the vehicle slides sideways rather than the rear end swinging around (oversteer ) or the front end wanting to move to the rear (understeer ). A cornering condition in which the front and rear slip angles are roughly the same. Although seemingly an ideal state of balance, perfect neutral steer is not as stable as slight understeer.

2)Understeer – what is it?

This is when the car’s front wheels lose grip through a corner due to excessive speed. This causes the front end to push towards the outside of the corner and for the steering to become useless.

Understeer – what causes it?

Understeer occurs when the front tyres start slipping. This happens when the front tyres are asked to turn while also managing a lot of braking or speed. If you’re going rather fast or braking very hard and trying to turn the wheel, the extra momentum may cause the front tyres to slip in the direction you’re travelling so, instead of turning, the car ploughs straight on.

Understeer – how to stop it?

Slow down! Understeer almost always occurs when the driver is going too fast for the road conditions – but this can be as innocuous as doing 15mph around a damp mini-roundabout. Taking things a little slower will avoid most instances of understeer.

If you start understeering, you should start to reduce the amount you’re pressing the accelerator or brakes – don’t jump off them completely – and reduce the amount of steering angle you’re applying. Of course you might run out of road still, but this will increase your chances of getting control of the car back before that happens.

3)Oversteer – what is it?

Oversteer is an extremely similar set of circumstances to understeer, but affecting the back of the car. It’s the tendency in powerful rear-wheel driven cars for the back end to want to overtake the front during cornering. If you’ve ever seen an episode of Top Gear, the presenters rarely test a fast car without performing some sort of oversteer. Professional racing drivers are trained to hold the car in this state in the sport of drifting.

Oversteer – what causes it?

Oversteer normally occurs on cars that drive the rear wheels and happens when the car is turning and the driver applies more power than the tyres can deal with. This makes the tyres slip and try to push in the opposite direction to the turn, kicking the back end of the car out. The same effect can happen by braking too hard while turning or by suddenly removing your foot from the throttle.

Oversteer looks great on a racetrack or at a drift competition, as the rear of the car slithers about and billows tyre smoke into the stands, but it’s not as much fun on a public road as it usually foreshadows a very big accident.

Oversteer – how to stop it?

Much the same advice applies as for understeer – slow down! Power oversteer is often caused by the driver accelerating too soon in a corner so, if you’re at this stage, you need to reduce your speed. If you’re unlucky enough to get into an oversteer situation try to remember some basic tips – keep looking and steering in the direction you want to go and don’t suddenly jump off the accelerator or slam on the brakes. If you can, try to modulate the throttle to gradually bring yourself to a halt.

While all road cars are prone to understeer, in rear-wheel driven ones you will need to watch for oversteer too, so take particular care if you’re an inexperienced or not especially confident driver if you buy a car that drives the rear wheels. It should go without saying, don’t deactivate your traction or stability control unless you’re at a racetrack.