Improvement of dynamic wheel loads and ride quality of heavy agricultural by suspending front axles

G. RILL, D. SALG and E. WILKS, Regensburg, Germany

The development of agricultural tractors clearly shows the tendency towards faster vehicles. Heavy agricultural tractors driving on uneven roads at 40 or 50 km/h do not offer any ride comfort at all. High wheel load variations generate stresses and strains in roads and reduce ride safety to critical values. By means of a newly developed suspended front axle it is possible to considerably improve ride safety and ride comfort. The proven standard design of tractors is maintained. Field tests and computer calculations prove the effectiveness of a suspended front axle.

INTRODUCTION 6. With a suspended axle, as currently in development 1. Today the farm tractors are all working machines by ZF Passau, the ride comfort and ride safety of a mostly destined for agricultural use. Business with or without mounted implements are clearly developments, however, s how an increas ing tendency improved. For a perfect adjustment of the suspension, to multi-purpose vehicles. Besides the usual field tests are made together with computer agricultural service, transport work is also carried out calcul ations. to an increasing extent. This way of using tractors gains in importance. The vehicle speed has an STANDARD TRACTOR increasing influence on the economy of a tractor. Concept 2. Vehicle speeds of 30 km/h are usual to date, but, 1. Today most tractors are made acc. to the so-called vehicle speeds of 40 km/h are prevailing in those modular design. Rear axle, and engine countries where permissible by law. Today some are rigidly connected to each other. For equalizing vehicles are even offered with permissible maximum extreme ground unevenesses the front axle is movable speeds of 50 km/h or 70 km/h resp., Fig. 1. round an axis in longitudinal direction of the vehicle. 3. The permissible maximum speed is first of all 2. The suspension of the tractor and the damping of limited by the ride comfort and the the ride safety. The the vehicle vibrations is exclusively made by large ride comfort is mostly increased by suspended driver volume tyres. Different tyre sizes on front and rear seats and in some cases suspended driver cabs are axles as well as the driver's cab located in the rear used. third of the vehicle are the essential features of these 4. Ride comfort and ride safety can be improved by vehicles, Fig. 2. actively influencing the three-point hydraulics. Of 3. Various mounted implements can be located both course, this is only possible if a heavy-duty mounted in the rear and front area. Hydraulic lifting devices implement, e.g. a plough is attached. ensure an exact positioning of the implements. By 5. Currently the non-suspended design causes high means of P.T.O. shafts driving is also possible on these wheel load variations at the tractors. Due to this there two main mounting areas. In practical service this are high stresses and strains in the road and the ride results in a variety of loading alternatives, Fig. 3. safety is severely affected.

Fig. 1. JCB FASTRAC Frg.2 Standard tractor

116 Heavy vehicles and roads: technology, safety and policy. Thomas Telford, London. 1992. DYNAMIC LOADS

4. With an empty tractor the axle loads are rather evenly distributed on front and rear axle. In the case of heavy mounted front or rear units the . relating axle is loaded up to 80% of the total weight,[1J. The Consequencies of these extremely axle loads are considerably reduced ride safety, a lower ride comfort and high loads on roadways.

Dynamic Wheel Loads and Ride Quality 5. Fig. 4 shows the driving behaviour of a standard tractor on an uneven roadway (country road) at a speed of 50 km/h. The road profile is shown in Fig. 6. The front and rear-mounted implements result in a static load of 47 kN at the front axle and of 44 kN at the rear axle. 6. Considerably reduced ride safety and enormous load on the roadway are the consequencies of these high wheel load variations.Furthermore the high shock load values in the vehicle body are unhealthy to the driver.

Fig.3. Some loading alternatives

60 60 front front 50 50 40 1\ A 40 r1 ~ lA. ~ 30 t\ f\ ) A A A z \ 1\ z 30 2!:, \ \ I \ I "1 2!:, '\J\ 'f\j" h,.f\ 20 20 1\ J\r M If\f\ ~ IM 1/\/\v UJ VI \ UJ IV v \I v ""0 I ""0 10 , \ 10 .Q'" V V ~ I~ V .Q'" 0 0 (jj Q) Q) 60 Q) 60 .!: rear .!: rear ~ 50 ~ 50 u u 'E 40 'E 40 AA i\ ~ c 11 III c Jl '" 30 >.'" 30 € ') I t\ 11 \ { 1'\ n n n /~ AIM f ""0 i\ I \t!I f)A "AA :/\ f\ 1/\/\ (\1"'1 tAtrll .f\ f\ III 1\ 20 20 v V ~ IJ V . I{ \J ~ \ If {'v I { \ V V 10 V \fv V .L Jl 10 I" V Y y 0 0 hub motion (m/s2) hub motion (m/s2) 6 6 4 1"- 4 111 M } J 2 ~ ~ ! " A 1\ 2 [I 0 \ f ~ .~ rlA Af\ 1\0 1'A .~ (\1\ 0 "'If "'~ \ V "1J ~ vv .~ 1M 1 Ih, VU \J J V 11 -2 r c -2 IL , c 0 If ~ v 0 -4 ~ V :;::; :;::; 11 -4 l- I- '"Q) '"Q) -6 (jj -6 (jj pitch motion (degree/s2 ) u u u pitch motion (degree/s2 ) u 150 150 '" A A >'" 100 ""0 € lA /1 1\ 0 100 0 A 1\ .0 .0 A ~ 50 r 50 I '\ t lA t\ I' I'i (\...... h .A .h. M J1 0 0 V"" I) VI IV V 'vi y f f I '\J "V V -50 I -50 V V V V V V IV 'I -100 V V V -100 V -150 -150 o 2 3 4 5 6 7 8 9 10 o 234 5 6 7 8 9 10 time (s) time (s) Fig. 4. Ride characteristics of standard tractor Fig. 5. Ride characteristics of standard tractor at high speed (50 km/h) at low speed (20 km/h)

117 HEAVY VEHICLES AND ROADS

6 E 4 ~ A ~I .. .tu M. .! ~ 2 ,,.. -\.i= V'\ ~\tJ 0 \ .1\. L 0 If' I' \, a. 1'II'If'I ~ -c -2 ~ '"0 L -4 -6 o 14 28 42 56 70 84 98 112 126 140 space coordinate (m) Fig. 6. Road profi le

7. A vehicle speed decrease, e.g. to 20 km/h, obviously reduces the wheel load variations and im­ prove the ride comfort, Fig. 5. 8. Longer transport times due to lower driving speeds, however, will reduce the economy of a tractor. Due to the installation of suspended front axles more ride safety and ride comfort can be obtained. Fig. 8. MB-Trac

TRACTOR WITH SUSPENDED FRONT AXLE 3. Only in 1972 a vehicle with front axle State of the art suspension was produced in the Federal Republic of 1. The characteristics required for a suspension Germany, i.e. the MB-Trac, Fig. 8. Also in this system in the tractor are based on other case a mechanical suspension is obtained by two objectives than for passenger cars or for coil springs. commercial vehicles. The variety of possible 4. A new vehicle in Trac-design is the combinations of mounted implements results in a JCB-FASTRAC, Fig. 1. Depending upon the vers ion large range of static axle loads in service. this vehicle reaches driving speeds up to 80 km/h. Furthermore rigidity as well as no influence at all With a mechanically suspended front axle and a on the control hydraulics of the mounted hydro-pneumatic suspension on the rear axle this implements are the main demands. vehicle reaches a relatively high ride comfort. 2. Due to the modular design there are very Rear-mounted implements directly attached to the few possibilities to suspend the rear axle. The axle will certainly result in too high dynamiC wheel tractor manufacturers have realized in time the loads on this axle. necessity to use at least a suspended front axle. MAN and Fendt e.g. already offered corresponding ZF-Axle Layout vehicles in 1950, Fig. 7. In this case the front axle 5. The modern tractor design includes all-wheel was connected with the body by means of a leaf drive and level control. The newly developed front spring, as done by other manufacturers. However, axle APL -2000 from ZF Passau fully meets these this concept could not penetrate the market, [5J. requirements, Fig. 9. 6. The axle carrier is supported at the body by two hydro-pneumatic spring cylinders. Driving and braking forces are absorbed by the torque tube. The torque tube joins the body on one end and the axle carrier on the other end via a ball joint. The lateral guiding of the axle is made by a .

Fig. 7. MAN tractor with suspended front axle Fig. 9. Torque Tube Axle

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60 I I I HD High pressure storage element (main 50 front sprin~) ~D Low pressure storage element 40 (additional spring for load compensation) DR Pressure regulator 30 - Pressure regulator .,. \10,.,,- IIA du. .11 !JIA, ,.w. !A ,!Ui! ,!A,~oI. !j~, ,.,.. - Limitation of max. and min. prt'ssure z 20 - Damping valves 20 - Shifting spring suspension/oscillation 10 U) :\'R Level regulator ""0 0 FP Switch: Spring suspension/osciDation (\j ..Q 60 I I I P.T Encrg)', supply central hydraulic unit ID 50 rear QJ ..c 3; 40 ~ Ih 30 A ~ Fig. 10. Hydro-pneumatic suspension (\f\ j\ r \/1. ~ .MA ~\ It A1\ !J. A/'\0 I AA I 20 .. j \J ~ ~ V ~ \f ~ WV 7. The axle suspension is makes the spring 10 V engagement and disengagement possible as well as " 0 the free movement round the torque tube axle. hub motion (m/s2) 8. If the panhard rod is not attached to the 6 axle carrier but to the wheel body, the axle 4 carrier moves in lateral direction when . By means of the pivoted torque tube at the body, an 2 additional steering movement of the axle is 0 I!J\ i\, lA tA. ~~ ~~ l'd lA (\ IL, \J.\A ANI A, ~ c V ~ ~ ~ iN \J \ ""\, V ~ 'V produced. Due to the lateral displacement of the 0 'J V V V :;::; -2 V~ axle more clearance between vehicle body and the (\j L inner tyre wall is obtained, such that a larger QJ -4 ID steering angle is possible. As a result the turning u u -6 circle of the vehicle can still be reduced. Steering III pitch motion (degree/s2 ) is alternativly made through double-acting steering ~ 150 0 cylinders or a turning lever steering. ..Cl 100 9. The suspension is hydro-pneumatic, Fig. 10. The spring accumulators are filled with nitrogen. 50 1\ (\ .11 By means of a special regulating valve the 0 lA /1. AA J\ ~fI '\ A~ rif'\ lA t1 .f.. M~ \J v 'I' \11., V \ v y \I V pressure is adjusted to the static axle load. A ~ \ \J ~ -50 \I~ level control valve is compensating the different vehicle loads. The axle movements are absorbed by -100 a throttle valve. This design ensure good vibration -150 damping characteristics even under extreme o 2 3 4 5 6 7 8 9 10 different axle loads, For special work the time (s) suspension can be locked. The axle will then behave like a standard swing axle. Fig. 11. Tractor with suspended front axle at high speed (50 km/h)

Dynamic Wheel Loads and Ride Comfort 10. Fig. 11 shows the driving behaviour of a tractor with suspended front axle on an uneven z 50 roadway at the speed of 50 km/h. Even driving at 20 suspended high speed the wheel load variations at the front axle U) 40 = QJ axle are considerably lower than for the standard -2 I (\j standard tractor at a lower speed. The wheel load variations > :1 30 = axle at the rear axle are also lower in comparison with .:£. I III I §I those non suspended vehicles. QJ ~ 0.. I :il 11. First of all the driver, almost sitting over 20 :i static ""0 :~ -:=i III I-- :s -- the rear ax le in standard tractors, profits from ..Q .= wheel :~ -I the suspended front axle by the reduction of the 10 '§i ~ load ID =i body pitching. Since the rear axle remains non­ QJ =i-5 "1i -5 I ..c ""'!t -= ;1 suspended, the shock loads at the driver's seat 3; 0 .. =i are only insignificantly affected by the suspended 0 10 20 30 40 50 60 front axle. In this case a suspended driver's cab static front axle load (kN) would compliment the effectiveness of a suspended front axle. 12. As the hydropneumatic suspension system Fig. 12. Effect of suspended front axle at automatically adjust to different static axle loads different loading conditions. the advantage of a suspended front axle holds for Driving over an obstacle (25 km/h) any loading condition, Fig. 12.

119 HEAVY VEHICLES AND ROADS

SIMULATION MODEL 5. By means of Jourdain's principle, see [3], Concept the equations of motion are obtained in form of 1. The tractor body, the front axle and the four wheels are considered as rigid bodies, Fig. 13. K(y) *Y = z , 2. The coordinates xR' YR' zR' "'R' I3 R, YR M(y)* z = Q ( y, z, w ) (4) describe the location of a reference frame fixed in w = R ( y, z, w ) . the tractor body with respect to the inertial frame. The movements of the tractor body are The vector of the generalized speeds is given by unconstrained. Rear axle and driver's cab are . T rigidly connected with the body. The angular Z = [vRx,VRy,VRZ' WRx,WRy,WRz' a.A'~A' 81J (5) speeds of the wheels are described by ° 1, ° 2 , 03 The kinematic matrix K(y) directly results from the and 04 . At point G the front axle is pivot generalized speeds chosen. mounted with the body. The remaining possibilities 6. Due to the skilled choice of generalized of axle movement a.A' ~A' YA are limited by a speeds, the mass matrix M(y) only depends very panhard rod. slightly on the position vector y. Hence, M can be 3. If the panhard rod is fixed at the wheel bo­ considered as constant for the simulation. The. dy, a constraint condition for the yawing movement remaining state variables, like the angular wheel of the axle is obtained in form of speeds ° 1, ° 2 , ° 3 , 04 and the lateral tyre YR = YR (a. R ' ~R ' 81 ) . (1) deflections YRE1' YRE2' YRE3' YRE4 are summarized in the auxiliary vector Where 81 describes the rotation of the left front wheel (wheel 1) round the king pin. Steering kine­ w = [°1,°2 ,°3 ,°4 , YRE1'YRE2'YRE3'YRE4JT . (6) matics entail another constraint condition The lateral tyre deflections are making possible a = (2) dynamic description of the lateral forces, see [2]. 4. Thus the tractor model has f=9 degrees of 7. The tractor model is described by a set of freedom, which are summarized in the position 26 non-linear first order differential equations. The vector nonlinearities are resulting from kinematics (stee­ T ring and front axle suspension) and from the for­ y = [ xR'YR,zR' 8 (3) a.R'~R,'YR' a.A'~A' 1 J . ces (tyres and hydro-pneumatic suspension). By means of generalized speeds the equations of 8. During simulation random road unevennesses motion can be simplified, see [2]. In this case it are generated by means of presettable spectral is adequate to apply as generalized speeds the densities and wavinesses via interferences of band components of limited noise processes, see [4]. = 9. Numerical solution is made by means of a and (3) modified semi-implicit Euler-formula, [3]. Due to = this a run-time minimized computer code is making possible the optimization of the axle suspension. The vectors vOR' wOR describe the velocity and the angular velocity of the reference frame with respect to the inertial frame O. The subscript R Model Quality separated by a comma means that the components 10. Fig. 14 and Fig. 15 show a comparison of vOR and wOR are expressed in the reference between calculations and measurements. A tractor frame. with suspended front axle and an axle distribution as indicated above is driving on an uneven roadway with a speed of v = 50 km/h.

3 60 50 measurement 04t 40 J 30 °3 .~, •.Mo, ~JIi .. l~ ~ rfII\J V. z 20 ""'-'" ~ 10 I/) "0 I1l 0 .Q I 60 I I I °2 I YA (ij QJ 50 simulation '¥ J.,. 81 .£: G '{ I ~ 40 ~A 30 lli,~ L~.I AAIA "l1li. ~A k!UJA I,Ud, 11 .. ,I. 20 '" LA ...

10 °1 wheel 2 J 00 wheel 1 2 3 4 5 6 7 8 9 10 time (s) Fig. 13. Tractor model Fig. 14 Front axle wheel loads

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5. Paul, M.; Wilks, E. Driven Front Axles for 6 Agricultural Tractors. ASAE (American Society of 4 Agricultural Engineers) Distinguished Lecture Series, Tractor Design No. 14, 1989. 2 C\J 6. Wimbeck, M. Vergleich zwischen Messung und Vl "'- 0 Fahrsimulationsrechnung am Beispiel eines S -2 vorderachsgefederten Schleppers. Diplomarbeit an c der FH Regensburg, Regensburg 1991. 0 -4 :;:; III L -6 Q) Qj 6 u u simulation III 4 .0 ::J 2 .£ ~.J, 11,.., A, 0 11.."" lA A ~~ ~AI lA f1 i\.-IA lA N\ ~ >. V ~' "VU V\ '''\ V ~yv "U V ItJ V V V V 0 V\ .0 -2 -4 -6 o 234 5678910 time (s) Fig. 15 Body accelerations above front axle

11. The results of simulation are in good conformity with the measurements. Both the wheel load variations as well as the body accelerations are correctly represented. The somewhat higher acceleration values of the measurement are supposed to be due to friction in the axle suspension. 12. With a tractor model verificated at a proto­ type now the front axle suspension can be adapted to each tractor version without any essential work to be done.

CONCLUSION 1. This paper shows that by use of a suspended front axle the ride comfort and the ride safety of heavy agricultural tractors are considerably improved. Even at high driving speeds an improved ride safety and almost the same ride comfort are obtained as with non-suspended tractors at a lower driving speed by means of the perfectly adjusted hydro-pneumatic front axle suspension. An increase of the driving speed , however results in a higher economy, since times to be spent only for transport work are reduced. 2. The front axle developed by ZF Passau offers a nu mber of advantages. By means of the simulation program ATRACTION (Agricultural Tractor Simulation) The front axle is perfectly adaptable to all vehicles.

REFERENCI ES 1. Ehrlinger, F.; Carlson, R. Improved Economy and Mobility in FWD Axles. SAE Technical Paper Series 821064, 1982. 2. RILL G. Steady State Cornering on Uneven Roadways. SAE Technical Paper Series No 860575, 1986. 3. RILL G. Vehicle Dynamics in Real-Time Simulation. In: The Dynamics of Vehicles on Roads and on Tracks. Ed.: Apetaur, M., Lisse: Swets-Zeitlinger 1988. 4. RILL G. Demands on Vehicle Modeling. In: The Dynamics of Vehicles on Roads and on Tracks. Ed.: Anderson, R.J., Lisse: Swets-Zeitlinger 1990.

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