VEHICLE DESIGN AND SKID RESISTANCE
F. Gauss, TU Hannover
On slippery roads the limits of manoeuve nering forces and locked braking for rability are reached much earlier, lower ces are existing for tires of different cornering speeds result, stopping distan design but for the same purpose of use. ces become longer and dangerous motions This becomes obvious at the limiting of vehicles occur more frequently. Some conditions of cornering and at emer experimental and theoretical research gency braking. To reduce as far as work published by various authors in the possible these differences only caused last years has contributed for better by tires and wet roads would present a understanding of the interaction be very efficient contribution to improve tween tire, vehicle and road. traffic safety.
The vehicles show a typical behaviour when reaching the limits of breakaway and beyond it. When driving on a cir Skidding and long stopping distances of cular path the steady state driving motorvehicles depend to a great extent on steering angles of understeering cars in the tire-road friction which can differ and crease progressively with increasing change in a wide range according to the speed whereas the steering angles of over type of road surfaces and to weather condi steering cars decrease rapidly after tions. Besides at given friction conditions passing a maximum value. For both stee the possible manoeuvers of vehicles depend ring characteristics the torque which is on the design of the vehicle. On the basis felt at the steering wheel is always of several papers published recently a re passing a maximum followed by the loss port shall be given on the origin and de of feel for steering response. The be velopment of dangerous driving conditions haviour of the car when the limiting and above all on the wide ranges and vague condition is approached is determined by nesses which make prediction difficult. the location of the center of gravity, the suspension design with its kinematics, springs and shock absorbers and the tire Critical Driving Li mits characteristics. In order to force the breakaway intensified conditions of On principle there are two distinguish driving were applied, individually or able limits for the feasibility to drive combined, such as starting from a circu on roads: the limit of the tire-road fric lar path steadily growing steering angle, tion and the limit of directional stability. sudden lack of skid resistance, emergen The directional stability of the ve cy braking. Under these conditions, the hicle motion, as given by the fact that by vehicle deviates from its intended cour short time interfering forces without stee se, the understeering car continues on ring correction no increasing breakaway will a wider circle whereas the oversteering be introduced, is determined by the tire car turns around its vertical axis. When cornering force characteristics and by the braking with locked wheels the center of vehicle design which could be different in gravity of the car moves tangentially mass distribution, wheel kinematics and to the bend. The forces transferred from suspension systems, and in front or rear tire to road and their dependences are drive etc. We can proceed from the fact of decisive importance. The available that all modern passenger cars show direc max. values of the traction coefficients tional stability under normal operating range between 0.1 on ice up to 1.0 on conditions. However approaching the cri dry surfaces. They depend differently on tical driving limits with too high speeds the speed, on the cornering forces and and at reduced tire-road friction coeffi the braking or traction forces. On wet cients - that means any time when due to an roads important differences in cor- 19 20 interfering force the slip angles of the sistant one, the conditions for stability tires at the rear axle increase more than do not change either. In general a reduc at the front axle - the limit of stability tion of friction will reduce the critical is exceeded at a critical speed individual speed of stability. If the gradients of the for the vehicle. Especially tail heavy rear cornering forces change their sign as they driven vehicles and those suspensions, wheel usually do for high slip angles, the motion alignments and tire characteristics which of the vehicle always will get instable as have an unfavourable effect tend in that indicated by these conditions. When approa direction. The course of the vehicle after ching the critical limits of friction the having exceeded the limit of stability can limit of stability can be exceeded in some be described approximately by a sum of ex cases and both conditions occur at the same ponential functions in the shape of time. Without separating them, these effects will be treated in the following by considering particular cases of vehicle mo tions. A vehicle can break away or skid if the steering wheel is turned too much or the brakes have to be activated too strong or if both actions occur at the same time The coefficients Ai determine the extent of particularly if the available tire-road instability. Provided this is low enough friction is to low. the vehicle motion can be kept under con trol more or less by fast steering reac tions. Vehicle motions when cornering Simple criterias for the directional stability of real vehicles do not exist be On a circular path the required stee cause of the great number of design in ring angle is a criterion. It can be co fluences which work partly in the same di ordinated to the centrifugal force as rection partly in the opposite direction. shown in Figure 1 for three different ve A paper (2) recently published shows hicles (!). that simplified mathematical substitute models describe the tendencies mainly cor rect. For a single track substitute model Vehicle Drive load on front axel % with nonlinear tire characteristics easily ready for permissible surveyable conditions are given in (l) service gross weight
A front 62 53 B rear 53 48.5 C rear 39 45
There are two cases clearly distinguish- able. In the first case, at the critical range the required steering angle decreases steeply after a maximum value. This is ty pical for vehicles and driving conditions with the denotations: M vehicle mass, for which a transition from a stable to an k radius of inertia, 1 wheel base, a and instable motion occurs. In the second case b distances of the center of gravity from the required steering angle increases the front and rear axle, v vehicle speed. steeply. Obviously the steering torque The formulae which is felt at the steering wheel, re duced by the steering gear ratio, decreases always rapidly in the critical range, Fi gure 2. The maximum available value of cen tripetal acceleration never can exceed the dF, peak value of the friction coefficient, =--, Figure 3. dol1 For these calculations (2,4) it has been presupposed that tire characteristics chan ge in relation to the maximum friction co efficient which certainly is not complete ly exact, but nevertheless it is showing the tendencies. Some measurements indicated denote the gradients of the cornering for a good correspondence (4). ces F 1 (front) and Fz (rear) in relation to A typical behaviour-of the vehicle can the correspondi ng slip angles oG.1 and oc'..2: be ovserved when the circular path (2) shall The exponential coefficients .\..i also depend be narrowed by a steadily increasing-stee on the values ¢ 1 and ¢ 2 . That means that for ring angle, Figure 4, and when for the pur the stability of the state o f motion a t pose of aggravating of the conditions addi which static balance of forces at slip angles tionally the skid resistance falls down oe1 and ol 2 exists, not the figures of the suddenly to a low value after the beginning side force coefficients are decisive but the of this steering manoeuver. The more the gradients at both axles. If at small slip skid resistance decreases the less the ve angles for moderate centrifugal forces these hicle follows the steering control. In gradients do not change when proceeding from addition, greater or smaller turnings of a skid resistant pavement to a less skid re- the longitudinal axis of the vehicle in re- 21
Figure 1. Calculated steering angle at Figure 2. Calculated steering angle and the front wheels at steady state circular steering torque at the fron·t wheels for drive versus lateral acceleration for va steady state circular drive (~). rious vehicles with the loads Fr (ready for service) and Fp (permissible gross weight), ( (j_) vehi cle foo d t --A Fp ~ 10° - - -- C Fp +---+-- -i-+-t--- -t------i vehicle load ~ ---A Fp - ·-·- C I Pr thin lines : other spring rates 13 ---A 8° D lifting of the bend inside -+---#----t-1-- ...... +-- - --t ----8 front wheel -·-·-C 12° R0 =65m - ,- ,-c 50 µo = 0.8 __,_ __ _,_ __..,... _ ._.,....__.-+---+----<
o calculated limit of traction t 11 ----.,-t---t----1 c lifting of the bend inside ... front wheel ~ 4o l-----l--::::::;...-'1"'=--- -1--....,.,.:;....:...~ .::..,i,,f-- 'f-l-ll------l 0i I . 10 R0 = 30 m C I I o ~-~~- I "ii"' µ 0 = 1.0 CII ~ 2o f--- - +----t-----t----t---+-t--+-t------1 C ~ 9of-----+---+---+----1-_,,.--11-J1- 1-1-..,_---1-__-1 ·.:: ~ c0 ~ N·m 1-----1---+---l-""7~""1-- --'rlir----.l------1 Figure 3. Calculated steering angle at the Figure 4. Calculated course of the front wheels at steady state circular drive vehicle and its position starting from a for various friction coefficients µ 0 versus circular path with friction coefficient lateral acceleration, (il. µ 0 and a steadily increasing steering 1'0.-----,--"""T""---r---r------r------angle. After one second the friction co vehicle A other spring rates efficient jumping (from its initial value (see Fig. ) µ 0 ) toµ. load 13 Fp ---- Fr 120 •0.8 •1,0 t ,201----+----+-µ.0 •0.5 m t ~ 11°1----I---+-- ),: t•5s Cl) :;or,,action .!!! u Cl) 100 C: --t---1'-f----,~1----+-----.-F--- =constant Cl) 0 -c: 1rf' .... µ!-1 ~ -!!! .,.()P. I ... '"C) C: en ine brakin 0 90 .::: 90 Cl) ...C: 0 80 .!! 90 0, vehicle grad C: 0 70 ::_-:-.: A /3,r1,8d!!£9, f 0, 70 ·;::C: Cl) - ·- ·- C /J;Fl02"3~·t Cl) 60 U) - R = 260m 0 V = 100 km/h 50 0.3 0.4 0.5 0.6 0.7 g lateral acceleration aq - The location of the center of gravity has an important influence leading to nose heaviness or tail heaviness with their known tendencies of relatively reducing the available corne ring forces. When cornering tire loads on the outer wheels become higher than those on 0 10 20 30 40 50 m the inner wheels. The distribution of the distancey- torque due to the centrifugal force to front and rear axles depends on the wheel suspen sion spring rates and on the torsion bar experiments. A high numerical accuracy can stabilizer as anti-roll spring for this pur not be expected because in view of the sen pose. According to the wheel suspension kine sibility of the motion processes at the matics the vehicle roll can be connected critical limits of driving small deviations with effects comparable to an additional of the test conditions when proceeding from steering effect. Near the critical driving one to another measurement can cause a limits the tire load relations can be chan distinct effect. ged by the progression of the springs rates and by spring stroke limitations thus A test report (6) on observations with changing also the vehicle behaviour, Fi- 4 vehicles of different design features gure 2. Undoubtedly tire cornering forces nearly corresponding to the preceding exam differ at different tire inflation pres ples has been published which with its de sures, and at the free rolling wheel and at fined extreme driving conditions is com the wheel transfering circumferential forces. parable to a certain extent with the afore The tire-road friction causes the greatest mentioned calculations. The following types range of differences. Each of these in of vehicles were tested: F (front drive, fluences can be madeeffective in the same di nose heavy), S (standard rear drive, nose rection as another or in the opposite di heavy) R (rear drive, tail heavy), M (rear rection. drive, central engine, slightly tail Provided that axle loads differ only in heavy), see Table 1. an usual rate not exceeding 0.5 ± 0.10 a On a steady state circular drive with moderate tail heavy vehicle may show a be a radius of 32.5 m on a wet asphalt pave haviour comparable with that of a nose ment the largely nose heavy vehicle F heavy one, as it is the case with the tail reaches the highest speed and centripetal heavy vehicles A and C in Figure 1. acceleration, the slightly tail heavy ve The results of the calculations men hicle M reached a little lower though tioned up to now are verified at least in higher values than the nose heavier vehicle their tendeJX¥bY observations and special S. The largely tail heavy vehicle R reached the lowest speed. On a skid pad with an 23 Table 1. Driving limits of similar vehicles with different design features, (£) front drive stan:iard rear drive rear drive rear drive, mid.engine nose heavy (1') nose heavy (S) tail heavy (R) slightly tail heavy(M) Audi 100 BMW 1802 vw 411 vw- Porsche ( load on axle % 60 40 54,5 45,5 42,5 57, 5 47,5 52,5 ~ ae< &"' ~ ~ mass kg 1099 1020 1115 985 circular course, wet asphalt, 65 m ¢ : speed km/h 52. 1 51 .o 50.0 51. 3 2 G. ~10 6. 18 5.95 6.3 lat.ace. m/s basal, 75 m ¢ : speed km/h 51. 2 41. 2 49.2 50.0 2 lat.ace.mis 5.45 5.00 5.00 5. 15 _aguaolanin9: on circular course 190 m ¢, deviated of the 5 m lane at a speed of: 30 km/h after a distance of m 8.25 9.5 8. 7 10. 6 90 km/h after a distance of m 5.0 3.0 6.0 6.6 a9 uao l a n i n9 on straiJht course anJ a.mer turn of the loncrltudinal axis a few deg swerving 540° 9oo0 up to 'J,')o ( slalom course distance: 18 m, speed lc...'11/'.1 55. 1 54.0 53.0 56.2 distance: 36 m, speed km/h 105.4 107.5 101 .o 108.8 hard eacked snow, avera9e a3cent 7.4% anJ benus { average speed ]c.1/i1 I 32.2 28.4 32.4 30.9 Figure 5. Calculated course of the vehicle Figure 6. Calculated lane change motions and its position starting from a circular on a dry road (a) and on a wet one (b) at path R with locked steering angle BR and emergency braking with and without an anti the fr~ktion coefficient jumping from its lock device (BV). Standard vehicle (rear initial value µ 0 to µ drive,front engine) with equal axle loads, (~) 120 m without SV ~ ,/ t m f•8S / 100 "., / u / C: an9f• at the / ~ // [_,rr Ill -..... with BV --+--- ~ ~·~ 1.,.,_;'---- - 80 ~~ ~ .. I wttnout brakinT braking initlot•d -,- ai/ \bl ,, ~ - ,- I/ with BV r w\v I. "I:. I 2 1__,.,.- L--- =--:Gi~ul BV 0 10 20 30 ,o sa I/ti' 70 ilO !J{) ,00 m R0 =65m initial circular vehicle A distance - nose heavy 40 1-----«'>-#---+--I-course tail heavy µ 0 = 0.8 v0 = 72 km/h aquaplanig section representing a sudden f3R= const. complete loss of friction the distance was 20 .----,~+------4---- - +------1------1------measured up to the point when the braking away vehicle reached the periphery of the aquaplaning section. At a speed of 80 km/h vehicle M showed the slowest lateral off 0 .____.__ ___..J..._ __ _ .i.__ __..__ ___ J._ __ _.. set, vehicle F the fastest one. At a speed 0 20 40 50 80 m increased only slightly to 90 km/h the distance y- measured distances shortened but above all 24 vehicle S did not follow the expected scheme Figure 7. Braking forces coefficients re for which no reasons can be given without lated to wheel load versus slip on various the knowledge of all particular details. wet roads. At emergency braking when driving 1,0 -----~------straight on an aquaplaning test course ve hicle F turned around its vertical axis on ,,, .•..._ .. ly a few degrees. Vehicle S turned up to 90 ( degrees swerving once but mainly moved ·--·--- straight on. Vehicle R turned one and a half times, vehicle M two and a half times .... On a road covered with hard packed snow c:: with bends and 7 % ascent together with CII descending sections the vehicles F and R ·u showed the best performance. Vehicle M was less fast and vehicle S was the slowest one. :E ··- .. CII The tests showed also remarkable differen C) -··- ces in the requirements for skill in stee ~ tu---+----1------'F:....,,.,,_....--+----~ ring the vehicles. The vehicles Sand R cu needed the highest requirements in this u ~ respect. ~ In general these vehicles demonstrated f a behaviour as expected. However it is re gi0,2 I markable that the numerical values in the ~~-~~ ~-~ ~~~~=r ~,,~~ ~n~ ~~~~ :i;: I .....--~~~--1 ~.._ ...... ':J._ ~--- ··------the effects caused by the differences in the location of the center of gravity can ! of~---~--~---~--~--~ be balanced well by other design features. 0 0.2 o., 0.6 0.8 slip s Tire and Road Surface Note: English measurements, (l). - asphalt a a 48 km/h a 2 16 km/h 1 b 16 km/h Vehicle motions are decisivly determined as~halt b b 1 48 km/h 2 by the characteristics of the forces between Swiss measurements, (2_) • tire and road surface which differ on more asphalt c or less skid resistant surfaces. Of course concreted well patterned tires should be used, and snow e therefore only such tires will be considered ice f in the following. The transferable traction and braking forces increase with growing slip as shown in Figure 7 for different road sur Figure 8. Max. friction coefficient faces. After reaching a maximum value these µma available by an anti-lock device and forces decrease on wet surfaces more or less loc~ed wheel sliding coefficient µG versus to the value of locked wheel braking. The speed on dry and wet asphalt (water film respective maximum values and the dependence 1 mm) and concrete pavement, (~). on slip are determined by the condition of the road surface and by the tire. The fric tion coefficients range from 1.0 on well sur ) ,.o I faced dry roads to less than 0.1 on ice and ;f 0.8 1--+---+--l----+-- -+------1--~ J--1---l 0.2 on snow. The maximum value is a very un stable point in the force development thus ~ ;-~,.._....::_d,..-_-+---+- -t_-_,_..._ :::,;:j,__'.:.. -:'.-~.:::~.f--== -~.-~:-,= drj being difficult to measure exactly. This Q6 1---.:::::::.--1-·--=-.=1._~_ _::: __ ~...i,~~~~;-_~-+=7_=~~=+~--- -_-_-_-1--_) ---1.1,__. point can be apporached better by using an I ~_~-=.. Q - -- r----_ -- ..... ___ anti skid device and thereby the differences ..., 0. .4 1-....::::=t--~-1--="'4~-l--+----1---4....:..:::::0j..... wet _ between the practically attainable maximum c · ·- ·- .,... :: :::::1::::::::,.. ..,_ (wal•rfilm: value and the locked wheel value had been .!,! '""'f,-.--1---LF=:,:-·· 1mm) found. Both values decrease with increasing :l1 02 I speed as shown in Figure 8 (8) for cross- ~ . asphalt _!L -~~ I concrete-·- ·- - .. - R,_ ply tires. - 0 '----'--....L.-...... L--L---'----'--...... I.--L....-..L.....-...J The considerable differences of road pro 20 30 40 50 60 70 80 90 100 km/h perties can be derived from the sliding co speed v -- efficients which were measured in dependence The great differences of the values and of on speed using a standard tire in crossply the dependence on speed is significant for construction on wet roads wetted by con various types of radial tires (.lQ), Figure trolled and defined watering (9), Figure 9. 10. It ist unknown in detail which influen The wide range between 5 % and-95 % of the ces of construction, pattern, and rubber statistical collective of the tested roads compound cause these variations. with values from 0.2 to 0.6 at 60 km/h for example is remarkable. This statistical graph The cornering forces of the tire in does not show the differences in the decrease crease strongly in the usually used range of the sliding coefficients with increasing of small slip angles, Figure 11. They re speed on certain roads. These values are not main approximately constant or decrease at absolutely valid any more for modern tires, greater slip angles until the area of la because at speeds from 60 km/h to 80 km/h teral sliding. On wet roads the cornering modern radial tires reach about 25 to 30 % forces are smaller, and on icy surfaces on higher friction values than crossply tires ly a fraction of the normal forces is trans which is to consider looking at Figure 9. ferable. On dry roads the speed is of minor 25 Figure 9. Sliding coefficients of a sta Figure 11 . Max. cornering force coeffi tistical collective of wet road surfaces. cients at a speed of 60 km/h. Swiss mea Standard test with locked wheel trailer surements on roads, (_~) . and crossply standard tire , (1). c· t 1.0 V) ....::t. C: 0.8 .! u 0.8 ~ % ~ 0.6 ( u .. ______10 I ·· --·- CII 20 range of variety IJ t 30 ... (!) ,o for wet surface ~ ::t. 0.4 60 0) ... 0.6 70 C: .s 80 ... .! QI ice IJ 90 ...C: ------' ;;:: 0 -- ... IJ ~- CII 0 t \ IJ 0) 0.1. C: 00 20° ~ slip angle ex - V) Figure 12. Maximum values of cornering ( 0.2 force coefficients versus speed, measure ments on internal track drum, (ll). )( ~ 1,0 V, ::t dry 'i-,..... 20 ,o 60 km/h 100 C o.e "~:.: speedv- .! ~""- I.) ...... -:..: , :::: r-,, ...... ·-. !'-...-..... water film : ~ 0.6 Figure 10. Locked wheel sliding coeffi IJ ------. -·Q2mrr: cients for various steel and textile belted Qi ', ", IJ ' tires. Measurements on internal track drum ... \ ', machine, water film 1 mm, (1.Q). ~ 0.4 --tomm gi \ ·.:: 1.0 CII ' l: 0.2 " ' ...... , 3 -2.omm )( t 0.8 t, (.'.) E 0 ::t 0 20 40 60 80 100 km/h ..... speed v- C: 0.6 CII The development of the cornering force ·-u depends to a high extent on the tire con ·-.... struction and may differ considerably for CII 0 0.4 dif ferent types of radial tires designed u for the same purpose of service, as shown 0, in Figure 13 (10) for the modern tires of C: Figure 10. ~ ·-"t, 0.2 The resulting forces at the coinci ·- dence of cornering forces and forces in -1/) the direction of the motion, Figure 14, apparently are determined by the vectorial 60 80 100 km/h strain characteristics of deformations in 40 the tire contact area and can be described speed v --- coherently by calcula tions based on mea surements (2). The results are elliptic influence. On wet roads and on surfaces with curves which show minor decreases for the low skid resistance the cornering forces de cornering force s when small longitudinal crease with increasing speed, (ll), Figure for ces coincide and sudden decreases to 12. gether with large longitudinal forces. The amount of these decrea ses is smaller on wet roads than on dry roads. As demonstrated for example by the side forces at slip 26 Figure 13. Cornering force of various wheel. However especially on slippery roads steel and textile bleted tires at a the torque does not correspond to the for speed of 80 km/h, measurements on inter ces transferable by the tires, Figure 15, nal track drum, water film 1 mm,(.l.Q). and the steering feel can be lost comple tely. -- Figure 15. Tire self aligning torque versus slip angle. a - on dry and wet road, tire size t N 155 SR 15 ( 11) b - on ice, temperature -5 deg centi ~ grade, tire size 155 SR M+S (~). C1J 20001---- -l-----.lle---- ~.-=.~-=..:..="1---I I.).... b) ~ -~.... ClJ C:.... 8 10001----1cu---i---+---+---+---1 5/ipangl•cr- ~ ,. 6• a• 10• slip angl• er ___. 20 40 50 eo 100 This survey shows that the forces slip angle a - transferable by the tire can vary in a very wide range, and therefore the tire-road Figure 14. Calculated cornering force co- friction has the strongest effect on the efficients µs versus braking force co controllability of the vehicle at the cri efficients µ for various friction co tical driving limits and represents the 8 , main source of insecurity . efficients µ 0 speeds and slip angles; calculations supported by measurements. The whole range is given by the various tire constructions, by dry and wet roads with their different skid resistance va t 0,6 lues, by the wintry conditions with ice and snow, and by the special case of aquapla : O,S l'""'=~cc....c=t===--t--.:=t---+------11-----t------l---1 ning of the tires at thicker waterfilrns. It is not possible to establish a sim ple evaluation for the tire-road friction which could be exactly characterizing for all critical conditions because of the different dependencies from cornering for ces and longitudinal forces on dry and wet roads, furthermore because of the depen dencies on the speed and the tires. There fore the sliding coefficient at locked wheel braking which can be measured suits best as an approximate figure for the skid resistance. 0.1 (l6 0.7 0.9 braking force caefficient µ8 - Conclusions What can be done to prevent the break away of the vehicle fran the controlled course angles of 2 and 6 degrees the changes do and to ensure short stopping distances? not develop proportionally. A further addi First of all at all critical driving tional reduction of the side forces occurs limits the breakaway motions should not with increasing speed. occur suddenly, and should develop in a The reduction of friction due to vi goodnatured manner announcing its impen brations combined with wheel load fluc dence. Control by countersteering should be tuations only should be mentioned, as this possible in due time in order to diminish effect is of minor importance on even roads. the effect of the breakaway motion. This is It would be favourable for the controlla effective only if the available tire-road bility of the vehicle on surfaces with low friction is sufficient. Above all the v e skid resistance if the driver could rea hicle should not turn around its vertical lize the loss of friction for instance by axis when the friction decreases. As exam the change in the torque at the steering ples show that can be achieved to a high r 27 extent by design features such as the loca 11. W. Gengenbach. Das Verhalten von Kraft tion of the center of gravity wheel suspen fahrzeugreifen auf trockener und ins sion kinematics, spring rate adaption and besondere nasser Fahrbahn. Dissertation brake layout. TU Karlsruhe, 1967 The motions of the vehicles are deter 12. R. Weber. Der KraftschluB von Fahrzeug (' mined decisively by the characteristics of reifen und Gummiproben auf vereister the transfer of the forces between tire and Oberflache. Dissertation TU Karlsruhe, road, therefore the improvements by radial 1970 tires are remarkable. It would be useful for the optimizing of the whole system ve hicle-tire-road to reduce the considerable variety in the cornering force characte ristics of the various tires for the same purpose of service. As the road surface is of decisive in fluence it is important to provide an al ways as much as possible constantly high skid resistance. This uniformity to which the driver can adjust himself seems to be more important than individual road sec tions with very high skid resistance followed by others with lower skid resistance which occurs not seldom in wet road conditions. That demands a road surface with adequate roughness which is advantageous at thin wa ter films and moreover after their sudden freezing. The road surfaces should be as much as possible even but drain of water should not be prevented. Especially the de velopment of puddles is very dangerous be cause of the risk of aquaplaning. Provided these requirements are ful filled as far as possible the inevitable dangers of the weather conditions remain. They can be met only by the drivers attention and skill. ( References 1 . P. Lugner. Zur Stabilitat der Kurven fahrt. Vehicle System Dynamics, Vol. 1, 1972, Nr. 1, 17 pp 2. K. Rompe. Das Schleudern der Kraftfahr zeuge bei zu schneller Kurvenfahrt. Habilitationsschrift TU Hannover, 1975 3. H.B. Pacejka. Simplizied Analysis of Steady-State Turning Behaviour of Motor Vehicle. Vehicle System Dynamics Vol. 2, 1 9 7 3 , Nr. 4 , 1 7 3 pp. 4 . K. Rompe. Verhalten von Kraftfahrzeugen bei Kreisfahrt im Grenzbereich. Deutsche Kraftfahrtforschung und StraBenverkehrs l technik, 1972, Vol. 227. 5 . P. Wiegner. Ober den EinfluB von Blockier verhinderern auf das Fahrverhalten von Personenkraftwagen bei Panikbremsungen. Dissertation TU Braunschweig, 1974 6. H. Eicker. Auto, Motor und Sport, Vol. 13, 1972. 7. K.E. Holmes. Braking Force/Braking slip. Road Research Laboratory Crowthorne (England) Report LR 292, 1970. 8. J. Grandel and E. Horz. Gleitbeiwert, maximaler KraftschluBbeiwert und Brems schlupf. Deutsche Kraftfahrtforschung und StraBenverkehrstechnik, 1974, Vol. 242 9 . K.-H. Schulze.Griffigkeits- und Rauheits messungen ... StraBe und Autobahn 1/1973, 31 pp 10 . H.J. Kaeppeler. Mot-auto-journal, Vol.11, 1974 l.