Maintaining Vehicle Stability PDF

Hcnv does a skid liappen? A skid develops when one lyre or more loses normal grip on the road, causing an involuntary inovemenl of the vt Chapter 7...

Hcnv does a skid liappen? A skid develops when one lyre or more loses normal grip on the road, causing an involuntary inovemenl of the vt Chapter 7 'lliis happens when the grip of tyres on the road hccoiiies less than the force or forces acting on the vehicle, Maintaining vehicle stability 'these forces act on your vehicle whenever you operate the controls - the brake, llie accelerator, the clutch or the s wheel. If you brake or accelerate while steering round a bend or corner, two forces are combined. There isoniy lin tyre grip available so if these forces become too powerful tliey break the grip of the ly on the road. Remember' res grijs trade-olf. The diauraiti belo'v sho'vs how each ol tliese forces alfcct.s the vehicles stability anti reduces lyre gri \ Learning outcomes The learning in this chapter, along with driver training, should enable you to: explain why active safety features can interfere with driver behaviour Braking... explain how to avoid the actions that reduce a vehicle's stabilit)' and tyre grip identify the causes of skidding and how u> minimise the irsk I dc.scribe the principles of anti-lock braking systems, traction control systems and electronic stability programmi acccl6r3ting... (Amlrollingyour vehicle’s stability A vehicle’s stability is reduced when you brake, accelerate or steer because these actions produce forces that alter l vehicle’s weight distribution and balance, and reduce tyre grip. A vehicle may skid when one or more of the tyi e.s I normal grip on the road, i.'t si'. Vic lyfcgiip New vehicles are lilted with a growing range of active safety features to increase vehicle stability. The second part i ch.aptcr hrielly explains the principles of anti-lock braking systems (.AB.S), traction control systems (TCIS) and eles and cornering are the forco.s that stability programmes (E-SP). The principle.s of each type of sy.stem are similar but there are significant ditlerences 1 cause a vehicle lo lose ivre grig. maniifaclurcrs in how their particular.system is activated and how it behaves. Willi r.ipul cluuiges in technology, it i.s viial dial you reler to llic inaimlaclnrei s h.iiulbook to know which salely 1- are liiled to your vehicle and what dUal Ihcv will have on I he vehicle. He aw.ire that many police and other eiiierg 'Cl', ice hides may have a dilfcrenl specilicjlion from standard models..Auiiiuics lo vehicle safety technology Rcsearcli has shown that safety systems can give some drivers a false sense of.security, causing them to become ov dcpeiideni on these features and lake more risks than they would in a vehicle sviihoui them. Safet)’ features cannot change llio latvs of physics - they don’t make a vehicle perform better or increase a drivers you’re on the point of losing control of the vehicle, you h.vvc misjudged the situation. Safety devices can help you I Gnp available Grip used lor braking or regain control, hut only ifyou iinderslaiid the specific fe.Hiire and know how to use it correctly. ; for steering accQlerating AvoidingO skiddingO Never drive to the limits of the lyre grip available -.ihvays leave a safely margin lo allow for the unforeseen. Avoiding.1 skid by driving.safely U far better than having to correct one. Know your own and your veliicle’s limits It takes much less force to break tiie grip of the lyres on a slippery road surface. road, traffic and weather conditions. Often it isn’t poor road or weather conditions that cause skids but the driver’; rcsjKm.sc to (hem. Skidding is caused by excessive speed, coarse steering, harsh acceleration or excessive or suddei - brakmg. or s gorntrinatumofthaw.-'i MOW U) mliuinisv die risk ui skUldiiig Aim to control your vehicle so that it does not skid. This becomes more difficult when road or weather conditions detevioiale. Hut you can minimise llie risk by driving more slowly and using your.skills of observation, aiiticipatif wlannim. Check vour vehicle  check t>TC tre.ids ami tyre pressure regularly Each skid is unique and every vehicle respond.s differently. Hmv you apply the principles and techniques outlinei clieck the vehicles brakes before you drive - defcciive brakes are especially dangerous on slippery surfaces. this chapter will depend entirely on the circumstances and on the vehicle you are driving. Avoid skidding in the ftr.st place - use ob.servaiion, anticipation and planning to adjust your driving when the roa surface inay be slippery. Cause: driving too fast for the circumstances li K At higher speeds you need more tyre grip to corner or stop. When surface grip is low, altering speed or direction c Observe - weather and road conditions lo watch for exceed the available grip, causing a.skid. Tlic faster you go the more likely this becomes, 'flic vehicle’s own weight change in road surface can reduce the grip oflhe front or back lyres. Weight in the boot will alter the vehicle’s nor Skidding IS more likely in bat! weather conditions and on slippery road surface.s. \\'alch out for: balance and lyre grip. snow, ice, frost, heavy rain wet mud, dami> leaves or oil, which can create.sudden slippery patches on the road surface cold spots in shaded areas, under trees, on slopes or hills - watch how other vehicles behave in icy weather Cause: harsh acceleration dry loose dust oi' gravel Harsh acceleration can cause the wheels to spin, even at low speeds. a shower or rain after a long dry spell - accumulated rubber dust and oil mi.xed with water can create a very slip' surlace tvorn road.surfaces that have become polisheii smooth concrete - may hold surface water and become slippery, especially in freezing conditions cobbled roads - these become very slipper)’ when tvel Cause; exceSvSive or sudden braking changes in the road surface (e.g. on bridges) and how this alTecLs lyre grip. in older cars without ABS, excc.ssive braking for the road conditions may cau.se skidding because the tyres lose thi You are at greater risk from these hazards at corners and junctions because you are more likely lo combine brakin as'cclcrating and steering in lhe.se situations. Anticipate ami plan - adjust your driving to the road condition.s Use \*our nb.scrvation.skills - vratch out for aiui Cause: coarse steering.issess poor weather and road conditions accur-slely ancl adjust vov accordingly: A moving vehicle uses least tyre grip when travelling in a straight line. As soon as you.start to corner you place ex demands on the lyre grip. If you steer too sharply for the speed you will cause the vehicle lo understeer or overste leave plenty of room for manoeuvre, reduce your speed and increase the distance you allow for stopping to male may break the lyre grip and the vehicle will then go into a skid- Aim to make your steering as smooth as possible, road conditions - on a slippery surface a vehicle can lake many limes the normal distance to stop in dilfereiil vehicles, you may need lo negotiate the same corner at different speeds. use lower revs in slippery conditions to a\ oid wheel spin, especially when moving off. Use a higlier gear when ir at low S[>eed.s on a slippery.surface aim lo brake, steer and change gear as smoothly as possible, so that you don’t break the lyn use the principles of cornering (see Chapter 10) lo negotiate corners carefully in slippery conditions. Undcrsleer and overstcer Recognising the cause of a sld(d Understeer is the tendency of a vehicle lo turn less, and oversiecr is the tendency of a vehicle to turn more in re.sp a given turn of the steering wheel. Tliis can happen even at low speeds. If your veliide loses stability and a skid begins to develop, you need to recognise the cause of the skid. Ihe coinnu causes of skidtling iuc: Hie tendency lo understeer or oversiecr is a characteristic of the vehicle itself coupled with the driver using exces; for the circiim.stances. Most front wheel drive vehicles undersleerand some re-ar wheel drive vehicles oversteer. H driving too fast for the circumstances some modern vehicles are designed lo compensate for these tendencies. Make sure you know the different steerin characteristic.s of each vehicle you drive and adapt your driving on aimers and bends. harsh acceleration In a front wheel drive car, you will increase understeer if you; excc.ssive or sudden braking enter the bend too fast coarse steering. mneirp" the bend A skid can be caased by these actions individually or in combination. steer loo sharply. You can reduce this understeer by reducing power and/or steering. But if you reduce power loo much and loo sue Anti-lock braking systems you may convert the understeer to oversteer (‘lift-off ovcrstcer’). This is because weigh! transfer from the rear to tl the front wheels and reduces grip at the rear wheels, allowing the rear to lose traction and slide. Almost all modern vehicles are filled with ABS. Tliis is an electronic safety device that adds to the conventional h) increases grip at iiraking system by giving you.some ability to steer during harsh or emergency braking. A rear wheel drive car initially behaves in the same way. but if you apply loo much power on a slippery surface an understeer may convert quite suddenly to oversteer. Ihe foot brake applies the brakes to all four svheels at once, but ABS controls the braking applied to individual wh works by sensing when a wheel is slowing down and about to lock up Wlien this happens, ABS releases the brake Four wheel drive cars provide better traction all round for acceleration and cornering, but not for braking. When wheel before It locks up fully. It re-applies the brake once the wheel starts to rotate again, to extremes they behave in a similar way to the front or rear wheel drive model from which they are derived. The advantage of ABS is tliat it allows you to steer the vehicle under full braking power, because it prevents the wl locking up. Once you activate ABS, you must maintain maximum pressure on the brake pedal throughout. ABS may reduce c different road surfaces but it lengthen the stopping distance of the vehicle compared with conventional brakes on the driver to retain some steering control. ABS cannot increase the grip of the tyres on the road, nor can it fully prevent the possibility of the vehicle skidd) When ABS is activated, you will see a warning ligln on the da.sliboard and will feel the brake pedal vibrate or judd momentarily as the system modulates the brake line pressures. If you become aware that the ABS is cutting in, yoi should learn from this and reduce your speed for the rest of the journey. Emergency brake assist New vehicles now have emergency brake assist (BA or liBA) as well as ABS. EBA increases braking pressure in an Understeer - the vehicle runs Oversteer - the rear of the vehicle emergency situation. If you apply the brake tvilh speed and force. EBA will cut in and fully apply the brakes until i takes over to prevent the wheels locking up. wide of its intended course steps out Traction control systems How active safety systems work When you accelerate, it is possible for the power transmitted to the driving wheels to exceed the amount of as-aUal grip. This is more likely when moving off on icy or slippery roads, on a steep hill or accelerating out of a corner. T cause the dris’cn wheels to spin. Wheel spin reduces both the vehicle’s ability to accoleiaie and its siabilit)-. Ifyou activate a s.ifety device, you arc losing control of the v chicle. Safety devices are not a replacement for Rodd competences. A traction control system works by controlling excesswheel spin on individual tvhecis. It appUes independent bra the spinning wheel. Some systems may also limit the wheel turning power of the engine to increase lyre grip. Manufacturers arc constantly seeking to improve vehicle stability with active sajery features that can help safety ar TCS allows you to make maximum use of tyre grip, especially on slippery surfaces or where the friction of the roa stability during braking, acceleration or steering. The active safety systems explained below are: surface is uneven. An example U where one wheel can grip the normal surface but the other slips on icc or snow. the dashboard. If traction control cuts in when you pull aw «anii-lock braking systems If you activate the TCS. you’ll see a warning tight on a standstill, reduce pressure on the accelerator to regain control of the steering. Note that if you switch the sysi traction control systems the warning light will remain constant to advise you that you no longer have its assistance. electronic stability programmes. If an active safety system is fitted, you tvill see an icon light up on the dashboard when you turn on the ignition or Electronic stability programmes the engine. If more than one device is fitted, they may be di.splayed separately or combined in a singje warning lig An ESP is an active safety system which incorporates conventional anti-lock braking ami traction control systems designed to help vehicle stability by delecting when the vehicle is driven to the limit of its physical capabilities. Vehicles fitted witii active safety devices behave differently from vehicles without There is more variation between types of ESP than between t>-pes of ABS or TCS. Your vehicle’s handbook may ex what the manutacturer’s sy.stcm does. Skid control in a vehicle fitted with one or more active safety systems will depend on tlie exact features fitted. Diff systems intervene at different points and some models have a deliberately delayed point of intervention. 11 you dri Sensors.it each wheel work in combination with a sensor that monitors the rotation and pitch of the vehicle - call dilierent vehicles, you must take note of the oranufaciiirer’s advice, and guidance in the driver’s vehicle handbook i.»w caoi veliKlr iv likely to IhIi ive extreme cii lust.'nces yaw sensor. Another sensor on the steering assembly detects the driver’s intended path. If these^seiisors detecUha correct umiersteer or oversteer and realign the vehicle. It U therefore crucial Uial you steer in the direction you go. Ifyou activate any of these active safety sy’stems, you may not be driving within the safe limits of the vehicle or tl road conditions. Most sv.slems interact with the enaine manaaemeiU.svstem. reducing or increasina the eneine power to the driver  ESP detects your intended path so it is crucial that you steer in the direction you want to go. If you drive a vehicle beyond its physical capabilities. ESP does not guarantee that the vehicle will remain stable i under control It cannot defy the laws of physics. I Key points «If you brake too hard, ABS prevents tlie wlieel from locking up. I if you accelerate too harshly, traction control prevents the wheel from excessive spinning. If you steer too sharply, ESP can help prevent the resulting oversteer or understeer from developing into a skid. Never rely on active safety devices. Always drive in such a way that they are not necessary- Build-Up of standing water on the road surface causes aquaplaning. If you activate a safety system, workout why you misjudged the situation. Honestly assess your driving behavioc ask yourself what human factors may have led to the activation. Hotv could you avoid repeating this mistake in future? Check your understanding You should now be able to apply learning from this ciiapter in your driver training so that you can: explain why active safely features can interfere with driver behaviour Correcting a skid in a vehicle without active safety featur explain how to avoid the actions that reduce a vehicle's stability and lyre grip If 5’ou drive an older vehicle that’s not fitted with any of tlie active safety features discussed in this chapter, the acti identify the causes of skidding and how to minimise the risk take to correct a skid will depend on whether the skid is a rear wheel, front wheel or four svheel skid. You need to describe the principles of anti-lock braking systems, traction control systems and electronic stability progrr to recognise different l>'pes of skid in the early stages so that )’ou can respond appropriately and regain t>Tc grip a: as possible, Tlie safest way to gain experience is ihrougli skid (raining from a qualified instructor. Correcting a ski should never be practised on a public road. Each skid is unique and every vehicle responds differently so the action you take will depend on the circumstan< on the vehicle you are driving. Aquaplaning Aquaplaning occurs where a wedge of water builds up between the front tyres and the road surface, often because or worn tyre tread. Wider tyres are more likely to aquaplane, and it can happen with brand new lyres, if the lyre li grooves are not able to disperse water quickly enough. Whether you brake or steer, the vehicle will not respond. The safest solution is to remove pressure from the accelerator, allowing the vehicle to lose speed and the tyres to n their grip. Don’t turn the steering ivheel while aquaplaning because the veliicle will lurch whichever way the whee pointing when the tyres regain grip.  1 turn into a left-hand junction? Chapter 8 ♦ carry straight on, having forgotten to cancel the last signal? D river*s signals I ite- } 'KS? i The mdicator i signal on the red car is ambiguous. ' Learning outcomes Use the position i and speed of the The learning in this chapter, along with driver training, should enable you to: vehicle to help ; you inteiprel what.demonstrate appropriate use of the full range of signals available to you in different situations the driver intends.show appropriate responses to and caution in interpreting signals given by olher.s to do.. show that )ou make active use of courtesy signals. Developing your competence at using signals Using signals may seem to be a basic competence, but many drivers don’t use the ftili range of as-ailabie signals consistently or to best effect. This chapi 'vill help 5'ou improve your competence at using signals. cr Giving information to other road users Is a key part of information proce.ssing in the system of car conlroi. S(t Chu^i/rZ 77i«w./n of car romml. pagi; 27. 7/if oj information. W.rii„ (J,a/KCT If. EnKfumy mivmr.Jarfurlhcc about Ihc ngtuHi araitoHc lo cmergca.y Kiponu utuationt. ^rhe range of signals The purpose of signals The signals available to you are; Signals inform other road users of your pre.sencc or intentions. Think before you signal Indiscriminate signalling indicators helpful to anyone. hazard warning lights Give a signal whenever it could benefit other road users. brake lights headlighl.s If you decide a signal is necessary, signal clearly and in good time. Alwa)’s make sure the meaning of your signal i; Sometimes a signal is not in itself enough to make your intentions dear and other road users may use your positic position of your vehicle speed to interpret what your signals mean. When negotiating a roundabout, for example, )”our signals maybe horn signals misinterpreted if you haven’t taken up llie correct position for your intended exit. arm signals Key points courtesy signals (for example, raising a hand to thank another driver). Select the most effective signal for the job. You must give your signal in plenty of time if it is to benefit other road Consider the need to give a signal on (he approach to every hazard, and before you change direction or speed. Be aware that when you change the.speed or position of your vehicle you are also giving information to other roa< Give a signal whenever it could benefit other road users. Note tliat heavy goods vehicles (HGVs) initially move to the right in order to turn left. Remember that signalling doe.s not give you any.special right to carry out the actions you indicate. FoDow the Highway Code ~ check your mirrors before you signal or manoeuvre. Using the indicators The Highway Code advises you to give a signal when another road user could benefit. Use observation to anticipat a may be needed. This encor:,,,.-. v,u. i., k- -.w,.,.. a , also reduco.s uiinetes.sar) hand movemeiit.s and signal duller. You also need to be cautious about how you interpret the signals of other road users. For cx.imple. does a vehicle 1 If in doubt, it’s better to sign.nl than not to signal. the left-hand indicator mean that llie driver intends to: Thp nnrnt>«p of