MARMARA ÜNİVERSİTESİ TEKNOLOJİ FAKÜLTESİ

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1 MARMARA ÜNİVERSİTESİ TEKNOLOJİ FAKÜLTESİ TAŞIT TEKNOLOJİSİ ŞASİ, ÇERÇEVE ve GÖVDE - KAROSER/İ Şasinin Çalışma Koşulları ve Traversler Yrd. Doç. Dr. Abdullah DEMİR

2 ÖZET... ÖZET... ÖZET... Araçta şasi ve karoserinin başlıca görevi: Yay, amortisör ve askı sitemi bağlantı noktaları gibi veya basit olarak aksın üzerindeki destek noktaları üzerinde özgül ve faydalı yükün taşınması ve dağılımının sağlanması; Frenleme ve tahrik esnasında oluşan boyuna; yön verme ve virajda ortaya çıkan yanal kuvvetlerin karşılanması ve iletilmesidir. Bu görevleri: Tek başına üstlenen üst yapılara taşıyıcı şasi denir. Bir kısmı karoser tarafından alınıyorsa, ortak taşıyıcılı şasili karoser, Tamamen karoseri tarafından taşınması durumunda kendinden taşıyıcılı karoser ismi verilir. Karoserin Görevi: Amaca uygun olarak taşınacak yükün ( veya kişi ağırlıklarının) alınması, Bunların dış etkilerden korunması karoserinin görevidir. Hava şartlarından, gürültüden, kazadan, titreşim zorlanmasından korunmanın karoseri tasarımında etkisi vardır. Prof. Dr. N. Sefa KURALAY, Sürüş Sistemleri Sunumu

3 ÖZET... ÖZET... ÖZET... Şasi, Yapı Elemanları, Malzemesi: Taşıyıcı şasiler ağırlıklı olarak yük taşıyan araçlarda ve römorklarda, seyrek olarak otobüslerde ve yolcu taşıma araçlarında kullanılır. Şasiler: Farklı karoserlerin kullanılması, Karoserinin daha az zorlanması, Mukavemet açısından bakıldığında geniş bir serbestlik, Yani aracın mevcut şasi ile küçük imalat sayılarında üretilmesi gibi avantajları sağlar. Şasiler, çoğu kez ıslah çeliklerinden yapılır. Kamyonlarda bugün hala merdiven formundaki şasi kullanılmasına karşın, Otomobillerde ve otobüslerde kendinden taşıyıcılı karoseri tercih edilmektedir. Prof. Dr. N. Sefa KURALAY, Sürüş Sistemleri Sunumu

4 ÖZET... ÖZET... ÖZET... Şasi burulmaya mukavim veya esnek olabilir. Bunun tercihi, aracı kullanma ve gövde özelliklerine yönlendirir. Burulmaya esnek konstrüksiyonlar daha kolay ve ucuz olmaları nedeniyle tercih edilirler. Açık kasalı kamyon ve römorklarda şasinin burulmasının yük ve karoseri için olumsuzluk teşkil etmediği durumlarda tercih edilir. Buna karşın, kapalı tip karoserlerde daima burulmaya mukavim şasiler kullanılmalıdır. Şasinin burulma katılığı (direngenliği), 1. Boyuna taşıyıcı kesitine, 2. Enine katılığına bağlıdır. Açık enine kesitler aynı miktar malzeme ihtiyacına karşın burulmaya dirençsiz, Kapalı kesitler burulmaya dirençlidir. Prof. Dr. N. Sefa KURALAY, Sürüş Sistemleri Sunumu

5 Şasi Formları ÖZET... ÖZET... ÖZET... Uygulama şekli, araç tipi ve kullanım amacına göre belirli şasi formları kullanılır. Kamyon sektöründe merdiven veya kutu şeklindeki şasiler tercih edilir. St 42 malzemesinden soğuk preslenmiş veya St 50 malzemesinden sıcak preslenmiş taşıyıcılardan ibarettir ve burulmaya esnektir. Ekseriyetle iki boyuna taşıyıcı pek çok enine taşıyıcı ile bağlanarak burulma katılığı, eğilme mukavemeti ve taşıma kapasitesi yüksek, genelde kamyon şasilerinde kullanım alanı bulan bir merdiven formundaki şasi elde edilir. Prof. Dr. N. Sefa KURALAY, Sürüş Sistemleri Sunumu

6 Dr. PaulJ. Aisopoulos, Chassis Design, Powering the Future With Zero Emission and Human Powered Vehicles Terrassa2011

7 Dr. PaulJ. Aisopoulos, Chassis Design, Powering the Future With Zero Emission and Human Powered Vehicles Terrassa2011

8 The characteristics of body connection with the chassis is highly relevant: structural function, insulation and isolation, safety and, partially, aerodynamics (due to floor contribution). The remaining functions are not directly affected. Lorenzo Morello, Lorenzo Rosti Rossini, Giuseppe Pia, Andrea Tonoli, The Automotive Body, Volume I: Components Design, e-isbn

9 Şasinin Çalışma Koşulları Chassis Operating Conditions The design of an automobile chassis requires prior understanding of the kind of conditions the chassis is likely to face on the road. The chassis generally experiences four major loading situations, that include, vertical bending, longitudinal torsion, lateral bending, and horizontal lozenging. Five basic load cases: Bending case Torsion case Combined bending and torsion Lateral loading Fore and aft loading Automotive design - Chassis* design, web.iitd.ac.in/.../3-automotive_chassis-design-...

10 Bir taşıta etki eden kuvvetler, taşıt cinsine göre değişiklik arz etmekle birlikte temelde iki ana gruba ayrılırlar. Bunlar, Statik ve Tekrarlı Dinamik kuvvetlerdir. Taşıtın maruz kaldığı kuvvetlerin büyüklüğü ne kadar önemliyse kuvvetlerin tekrarı da en az o kadar önemlidir. Zira, kuvvetleri periyodik olarak değiştirmek ve değişim sayısını yeter derecede arttırmakla bir malzemeyi statik sınırların çok altında da harap etmek mümkündür. Statik kuvvetler değişken olmayan kuvvetler ile aracın ömrü boyunca en fazla 5x10 3 defa tekrarlanan kuvvetlerdir. Statik kuvvetler, taşıtın kendi öz ağırlığı ve yükü, fren ve kalkış kuvvetleri, viraj kuvvetleri, burulma kuvvetleri, münferit darbe kuvvetleri, çekici ile römork arası bindirme kuvvetleri olarak sayılabilir. Tekrarlı dinamik kuvvetler ise 2 5x10 6 tekrar sayısından başlayan, yol pürüzlülüğü, lastik çevresinin düzgünsüzlüğü gibi sebeplerden dolayı ortaya çıkar. Murat Ereke, Kubilay Yay, «Çiftkatlı Otobüs Gövdesinin Bilgisayar Destekli Gerilme Analizi»

11 Taşıt gövdesi yapısal analizinde, işletme şartlarından doğan kuvvetlere karşı dayanıklılık temel hedeflerden biridir. Hedef, ağırlık açısından uygun, yeteri kadar mukavim bir yapı (hafif yapı) elde etmek, malzeme ve enerji tasarrufu sağlamaktır. Karoserinin karmaşık yapısı gereği, işletme şartlarından doğan zorlanma sonucu oluşacak gerilmelerin hangi yoğunlukta ve hangi şiddette olacağının kestirilmesi büyük zorluk arz eder. Gerilme yığılmalarının olduğu bölgeler kritik bölgelerdir. Bu bölgelerdeki kesitlerin doğru tasarımı için gerilmelerin şiddetleri bilinmek zorundadır. Deformasyonların ve gerilme yığılmalarının tespitinde gövdenin sonlu eleman yöntemiyle modellenerek bilgisayar ortamında analizi modern tasarım tekniklerinin başında gelmektedir. Murat Ereke, Kubilay Yay, «Çiftkatlı Otobüs Gövdesinin Bilgisayar Destekli Gerilme Analizi»

12 Taşıtın Kendi Öz Ağırlığı ve Yükü: Taşıtın düz ve yatay bir zeminde durduğu farz edilirse, şekildeki kuvvetlerin etkisi altında kalacaktır. G ağırlığı, taşıtın öz ağırlığını ve yükü birlikte ifade etmektedir. Ön ve arka aksları birer lastikli olan taşıtlar binek otomobilleri, hafif kamyonlar ve hafif römorklardır. Ön ve arka aks yükleri arasında fazla fark oluşmaz, yani hemen hemen birbirine eşit kabul edilebilir. Murat Ereke, Kubilay Yay, «Çiftkatlı Otobüs Gövdesinin Bilgisayar Destekli Gerilme Analizi»

13 Vertical Bending: Considering a chassis frame is supported at its ends by the wheel axles and a weight equivalent to the vehicle s equipment, passengers and luggage is concentrated around the middle of its wheelbase, then the side-members are subjected to vertical bending causing them to sag in the central region. Sag: Bükülmek, bel vermek.

14 Automotive design - Chassis* design, web.iitd.ac.in/.../3-automotive_chassis-design-...

15 Bending Dynamic loading: Inertia of the structure contributes in total loading Always higher than static loading Road vehicles: 2.5 to 3 times static loads Off road vehicles: 4 times static loads Example: Static loads Vehicle at rest. Moving at a constant velocity on a even road. Can be solved using static equilibrium balance. Results in set of algebraic equations. Dynamic loads Vehicle moving on a bumpy road even at constant velocity. Can be solved using dynamic equilibrium balance. Generally results in differential equations. Automotive design - Chassis* design, web.iitd.ac.in/.../3-automotive_chassis-design-...

16 Longitudinal Torsion: When diagonally opposite front and rear road-wheels roll over bumps simultaneously, the two ends of the chassis are twisted in opposite directions so that both the side and the cross-members are subjected to longitudinal torsion (Fig. 1), which distorts the chassis. Fig. 1: Longitudinal torsion.

17 Burulma Kuvveti Bir taşıtın ön tekerleklerinden biri bir tümsek veya bir engebenin üzerine çıkarsa o tekerleğin dinamik tekerlek yükü artmaktadır. Sağ ve sol tekerleklerin dinamik tekerlek yüklerinin farkı taşıtı uzunlamasına eksen boyunca burulmaya zorlamaktadır. Tekerlek yüklerinin birbirlerine göre farkı ve dolayısı ile burulma momentinin büyüklüğü engebenin yüksekliği, tekerlek iz genişliğinin büyüklüğü, lastiklerin ve yayların katılığı ve taşıt gövdesinin katılığına bağlıdır. Taşıtların burulma momenti engebe yüksekliğine bağlı olarak artmakta olup, her taşıt için aşılabilecek bir engebe yüksekliği sınırı vardır. Tekerlek iz genişliği ne kadar büyük olursa, belli bir engebe yüksekliği için meydana gelen burulma momenti o kadar küçük olur. Lastik ve yayların yumuşaklığı ise, belli bir engel yüksekliğinde gövdenin daha az dönüp dönmemesinde rol oynamaktadır. Süspansiyon sistemi ne kadar yumuşak olursa, burulma o kadar az olur. Taşıt gövdesinin katılığı veya elastikliği de burulma momenti ile ilgili olup gövde elastik olduğu ölçüde moment düşmektedir. Murat Ereke, Kubilay Yay, «Çiftkatlı Otobüs Gövdesinin Bilgisayar Destekli Gerilme Analizi»

18 Burulma Kuvveti (dvm.) Bir taşıt gövdesinin boyutlandırılmasında değişik zorlayıcı kuvvetleri dikkate almak gerekir. Her şeyden önce statik kuvvetler altında gövdede veya şaside kalıcı deformasyonlardan kaçınmak şarttır. Bir otobüs gövdesi için ilk yaklaşım hesabında taşıtın öz ağırlığı ve yükü + %30 münferit darbe kuvveti + %50 burulma kuvveti kullanılabilir Murat Ereke, Kubilay Yay, «Çiftkatlı Otobüs Gövdesinin Bilgisayar Destekli Gerilme Analizi» Otobüs gövdelerinde kutu profilden oluşan kirişler kullanılır. Malzemesi ise St 37 dir. Literatüre göre [1,2], otobüs gövdesi imalatında kullanılan bu malzeme için ortalama gerilme sınırı (ön gerilme) σm= 9 kg/mm 2 ve genlik gerilmesi sınırı σg= 8 kg/mm 2 değerleri alınmalıdır. Akma gerilmesi de σf için de 24 kg/mm 2 değerinde alınabilir.

19 Torsion When vehicle traverse on an uneven road. Front and rear axles experiences a moment. Pure simple torsion: Torque is applied to one axle and reacted by other axle. Front axle: anti clockwise torque (front view) Rear axle: balances with clockwise torque Results in a torsion moment about x axis. In reality torsion is always accompanied by bending due to gravity. Automotive design - Chassis* design, web.iitd.ac.in/.../3-automotive_chassis-design-...

22 Combined bending and torsion Bending and torsional loads are super imposed. Loadings are assumed to be linear One wheel of the lightly loaded axle is raised on a bump result in the other wheel go off ground. All loads of lighter axle is applied to one wheel. Due to nature of resulting loads, loading symmetry wrt x z plane is lost. R R can be determined from moment balance. R R stabilizes the structure by increasing the reaction force on the side where the wheel is off ground. The marked Side is off ground Side takes all load of front axle Side s reaction force increases Side s reaction force decreases to balance the moment. super imposed= birleştirilmiş Automotive design - Chassis* design, web.iitd.ac.in/.../3-automotive_chassis-design-...

23 Combined bending and torsion Automotive design - Chassis* design, web.iitd.ac.in/.../3-automotive_chassis-design-...

24 Lateral Bending: The chassis is exposed to lateral (side) force that may be due to the camber of the road, side wind, centrifugal force while turning a corner, or collision with some object. The adhesion reaction of the road-wheel tyres opposes these lateral forces. As a net result a bending moment (Fig. 2) acts on the chassis side members so that the chassis frame tends to bow in the direction of the force. Fig. 2. Lateral bending Note: In modern production cars a transversal acceleration of 1 g is reached while turning at limit conditions, while about 0.5 is reached in longitudinal accelerations. G. Genta and L. Morello, The Automotive Chassis, Volume 1: Components Design, 351 Mechanical Engineering Series,

25 Viraj kuvveti Taşıt viraja girdiği zaman, merkezkaç kuvvetin etkisi altındadır. Kuvvetin yönü dışarı doğru olduğu için şasi dış putreline gelen yük artmaktadır. Viraj kuvveti hesaplanırken şekildeki gibi ağırlık merkezinin yerden yüksekliğinin (h), taşıt ağırlığının (G), viraj ivmesinin (a) ve iki tekerlek arasındaki mesafenin bilinmesi yeterlidir. / Ref: Murat Ereke, Kubilay Yay, «Çiftkatlı Otobüs Gövdesinin Bilgisayar Destekli Gerilme Analizi»

26 The Automotive Body Volume II: System Design; 2011 incipient: Başlangıç Capsizing: Devrilme

27 For a vehicle travelling on a road with known grip characteristics, the maximum lateral force is given by the lowest of the limit loads given by the Eq. μy max = maksimum yanal adezyon katsayısı Maximum lateral force due to the incipient capsizing In the case of a road with good grip conditions μy max , depending on the ratio 2h/t between the height h of the center of gravity and the semitrack (t/2), the limit of the lateral force is given by the tires or by the incipient capsizing. Considering a vehicle with track t = 1.2 m and a relatively low center of gravity: h = 0.5 m, so the ratio t/(2h) = 1.2 > μy max. The maximum lateral force is then imposed by μy max. For a small commercial vehicle with the same track and a higher center of gravity h = 0.8 m t/2h = 0.75, the maximum lateral force is imposed by the capsizing condition. The Automotive Body Volume II: System Design; 2011

28 Lateral loading Automotive design - Chassis* design, web.iitd.ac.in/.../3-automotive_chassis-design-...

29 Lateral loading For a modern car t = 1.45 m and h = 0.51 m. Critical lateral acceleration = 1.42 g In reality side forces limit lateral acceleration is limited within 0.75 g. Kerb bumping causes high loads and results in rollover. Width of car and reinforcements provides stiffness forces. sufficient to withstand bending lateral Lateral shock loads assumed to be twice the static vertical loads on wheels. Automotive design - Chassis* design, web.iitd.ac.in/.../3-automotive_chassis-design-...

30 Longitudinal loading Limiting tractive and braking forces are decided by coefficient of friction b/w tires and road surfaces (note: b/w = between / with) Tractive and braking forces adds bending through suspension. Inertia forces adds additional bending. Automotive design - Chassis* design, web.iitd.ac.in/.../3-automotive_chassis-design-...

31 Longitudinal loading When vehicle accelerates and decelerates inertia forces were generated. Acceleration Weight transferred from front to back. Reaction force on front wheel is given by (taking moment abt RR) Deceleration Weight transferred from back to front. Reaction force on front wheel is given by Automotive design - Chassis* design, web.iitd.ac.in/.../3-automotive_chassis-design-...

32 Fren Kuvveti Fren kuvvetleri taşıt fren yaptığı zaman ortaya çıkarlar. Düz yolda ideal fren kuvveti dağılımı ile elde edilecek fren ivmesi ortalama 4,5 m/s 2 dir. Başlangıçta bir anlık maksimum değere ulaşan fren ivmesi düz ve kuru bir asfalt yolda 8 m/s 2 ye kadar çıkar. Fren hesaplarında fren yolu boyunca muteber olan ortalama ivmedir. / Ref: Murat Ereke, Kubilay Yay, «Çiftkatlı Otobüs Gövdesinin Bilgisayar Destekli Gerilme Analizi»

33 ÖRNEK BREMACH T-Rex Tanıtım Broşürü

34 Horizontal Lozenging: A chassis frame if driven forward or backwards is continuously subjected to wheel impact with road obstacles such as potholes, road joints, surface humps, and curbs while other wheels produce the propelling thrust. These conditions cause the rectangular chassis frame to distort to a parallelogram shape, known as lozenging (Fig. 3). Fig. 3: Lozenging

35 Asymmetric loading Results when one wheel strikes a raised objects or drops into a pit. Resolved as vertical and horizontal loads. Magnitude of force depends on Speed of vehicle Suspension stiffness Wheel mass Body mass Applied load is a shock wave Which has very less time duration Hence there is no change in vehicle speed Acts through the center of the wheel. Automotive design - Chassis* design, web.iitd.ac.in/.../3-automotive_chassis loads. Resolved vertical force causes: Additional axle load Vertical inertia load through CG Torsion moment to maintain dynamic equilibrium. Resolved horizontal force causes: Bending in x z plane Horizontal inertia load through CG Moment about z axis to maintain dynamic equilibrium. Total loading is the superposition of all four

37 Allowable stress Vehicle structure is not fully rigid Internal resistance or stress is induced to balance external forces Stress should be kept to acceptable limits Stress due to static load x dynamic factor yield stress Should not exceed 67% of yield stress. Safety factor against yield is 1.5 Fatigue analysis is needed At places of stress concentration Eg. Suspension mounting points, seat mounting points. Bending stiffness Important in structural stiffness Sometimes stiffness is more important than strength Determined by acceptable limits of deflection of the side frame door mechanisms. Excessive deflection will not shut door properly Local stiffness of floor is important Stiffened by swages pressed into panels (swage: baskı kalıbı) Second moment of area should be increased Automotive design - Chassis* design, web.iitd.ac.in/.../3-automotive_chassis-design-...

38 Bending stiffness Thin panels separated by honeycomb structure reduced vibration Local stiffness has to be increased at: Door Bonnet Suspension attach points Seating mounting points Achieved by reinforcement plates and brackets. Torsional stiffness Allowable torsion for a medium sized car: 8000 to N m/deg Measured over the wheel base When torsion stiffness is low: Structure move up and down and/or whip When parked on uneven ground doors fail to close Doors fail to close while jacking if jack points are at a corner Torsion stiffness is influenced by windscreens TS reduces by 40% when windscreens removed Open top cars have poor torsional stiffness Handling becomes very difficult when torsional stiffness is low. Automotive design - Chassis* design, web.iitd.ac.in/.../3-automotive_chassis-design-...

39 Chassis types Ladder frames Used by early motor cars Early car s body frame did not contribute much for vehicle structure Mostly made of wood which has low stiffness Carried all load (bending and torsion) Advantages: Can accommodate large variety of body shapes and types Used in flat platforms, box vans, tankers and detachable containers Still used in light commercial vehicles like pick up Side rails frequently have open channel section Open or closed section cross beams Good bending strength and stiffness Flanges contribute large area moment of inertia. Flanges carry high stress levels Open section: easy access for fixing brackets and components Shear center is offset from the web Local twisting of side frame is avoided Load from vehicle is applied on web Avoids holes in highly stresses flanges Very low torsional stiffness. Automotive design - Chassis* design, web.iitd.ac.in/.../3-automotive_chassis-design-...

43 Torsion in cross member is reacted by bending of side frames Bending in cross frames are reacted by torsion of side frames All members are loaded in torsion Open sections are replaced by closed sections to improve torsional stiffness Strength of joints becomes critical Max bending occurs at joints Attachment of brackets becomes more complex Automotive design - Chassis* design, web.iitd.ac.in/.../3-autom

44 Chassis types cruciform frames (cruciform: krusiform, çapraz şekilli) Can carry torsional loads, no elements of the frame is subjected to torsional moment. Made of two straight beams Have only bending loads Has good torsional stiffness when joint in center is satisfactorily designed Max bending moment occurs in joint. Automotive design - Chassis* design, web.iitd.ac.in/.../3-automotive_chassis-design-... Hogging: belerme, bombe

45 Combining ladder and cruciform frame Combining ladder and cruciform frame provides good bending and good torsional stiffness Cross beams at front and back at suspension points are used to carry lateral loads Automotive design - Chassis* design, web.iitd.ac.in/.../3-automotive_chassis-design-...

46 Chassis types Torque tube back bone frame Main back bone is a closed box section Splayed beams at front and rear extent to suspension mounting points Transverse beams resist lateral loads Transverse Back bone frame: bending and torsion beam Splayed beams: bending Transverse beams: tension or compression Automotive design - Chassis* design, web.iitd.ac.in/.../3-automotive_chassis-design-...

47 Chassis types Space frames In all frames till now length in one dimension is very less compared to the other two dimensions Increasing depth increases bending strength Used in race cars All planes are fully triangulated Beam elements carry either tension or compressive loads. Ring frames depends on bending of elements Windscreen, back light Engine compartment, doors Lower shear stiffness In diagonal braced frame s stiffness provided by diagonal element Automotive design - Chassis* design, web.iitd.ac.in/.../3-automotive_chassis-design-...

48 Chassis types Integral structures Modern cars are mass produced Sheet steel pressings and spot welds used to form an integral structure Components have structural and other functions Side frames + depth + roof gives good bending and torsional stiffness Geometrically very complicated Stress distribution by FEM only Stress distribution is function of applied loads and relative stiffness between components Advantages: Stiffer in bending and torsion Lower weight Less cost Quiet operation Automotive design - Chassis* design, web.iitd.ac.in/.../3-automotive_chassis-design-...

49 Structural analysis by Simple Structural Surfaces (SSS) method Many methods to determine loads and stresses Elementary method is beam method, FEM is advanced method and SSS is intermediate Developed by Pawlowski in 1964 Determines loads in main structural elements Elements are assumed to be rigid in its plane Can carry loads in its plane Tension, compression, shear and bending Loads normal to plane and bending out of plane is invalid and not allowed Automotive design - Chassis* design, web.iitd.ac.in/.../3-automotive_chassis-design-...

50 Passenger car More complex than box type van Detailed model vary according to mechanical components Front suspensions loads applied to front wing as for strut suspension Rear suspension (trailing arm or twist beam) loads to inner longitudinal member under the boot floor SSSs varies with body types Vehicle structures represented by SSS Bus or box type vehicle Van Passenger car Automotive design - Chassis* design, web.iitd.ac.in/.../3-automotive_chassis-design-... SSS and Not SSS

51 Reading Text: Flexural stiffness Kf is defined as the ratio between a load applied to the middle of the wheelbase and the deflection of the same point; reaching acceptable values is generally not difficult, if other structural requirements are satisfied, except in the case of very long vehicles. Torsional stiffness Kt is instead the ratio between a roll torque applied to the wheel hubs of the front axle and the consequent rotation, when the rear axle hubs are fixed to the reference system. In this ideal scenario, elastic primary and secondary elements of each suspension are replaced by rigid elements of equal geometry. Elementary scheme for modelling the torsional deformation of a vehicle: kta and ktp represent the torsional stiffness of the two axle suspensions, while Ktc and Ktt represent torsional stiffness of body and chassis frame in a hypothetical vehicle with separated frame. G. Genta and L. Morello, The Automotive Chassis, Volume 1: Components Design, 351 Mechanical Engineering Series, 2009

52 Chassis-frame Design Side-member and Cross-member

53 Chassis frames The chassis frame is the commercial vehicle's actual load-bearing element. It is designed as a ladder-type frame, consisting of side and cross members. The choice of profiles decides the level of torsional stiffness. Torsionally flexible frames are preferred in medium- and heavy-duty trucks because they enable the suspension to comply better with uneven terrain. Torsionally stiff frames are more suitable for smaller delivery vehicles and vans. Apart from the force introduction points, critical points in the chassis-frame design are the side-member and cross-member junctions. Special gusset plates or pressed cross-member sections form a broad connection basis. The junctions are riveted, bolted and welded. U- or L-shaped side-member inserts provide increased framework flexural strength and reinforcement at specific points. Side member: Şasi boyuna taşıyıcı, şasi (yan) kolu, Cross member: Ara taşıyıcı, kuşak, travers Bosch Automotive Handbook

54 Chassis Frame Sections During movement of a vehicle over normal road surfaces, the chassis frame, is subjected to both bending and torsional distortion as discussed in the previous section. Under such running conditions, the various chassis-member cross-section shapes, which find application, include. Solid round or rectangular cross-sections, Enclosed thin-wall hollow round or rectangular box-sections, Open thin-wall rectangular channelling such as C, T, or tophat sections. Side-member Bending Resistance: The chassis side-members, which span the wheelbase between the front and rear axles must be able to take the maximum of the sprung weight. The sprung weight is the weight of the part of the vehicle supported by the suspension system. The binding stiffness of these members must resist their natural tendency to sag. The use of either pressed-out open-channel sections or enclosed thin-wall hollow round or rectangular box-sections can provide the maximum possible bending stiffness of chassis members relative to their weight.

55 Chassis Frame Sections A comparison of the bending stiffnesses of different cross-sections having the same crosssectional area and wall thickness is presented in Fig. A to F. Considering a stiffness of 1 for the solid square section, the relative bending stiffnesses for other sections are, Practically, a 4 mm thick C-section channel having a ratio of channel web depth to flange width of about 3:1 are used as chassis sidemembers. This provides a bending resistance of 15 times greater than that for a solid square section with the same cross sectional area. For heavy-duty applications, two C-section channels may be placed back to back to form a rigid load-supporting member of I-section (Fig. H). To provide additional strength and support for an existing chassis over a highly loaded region (for example, part of the side-member spanning a rear tandem-axle suspension), the sidemembers may have a double-section channel. This second skin is known as a flitch frame or plate. the relative bending stiffnesses Square bar 1.0 Round bar 0.95 Round hollow tube 4.3 Rectangular C-channel 6.5 Square hollow section 7.2

56 Side-and Cross-member Torsional Resistance The open-channel sections exhibit excellent resistance to bending, but have very little resistance to twist. Therefore, members designed both side and crossof the chassis must be to resist torsional distortion along their length. Figure C to F illustrates the relative torsional stiffness between openchannel sections and closed thinwall box-sections. Comparisons firstly between the open and closed circular sections and secondly between the rectangular sections are made, considering the open section has a resistance of 1 in each case. the relative torsional stiffness Longitudinal split tube = 1.0 Enclosed hollow tube = 62.0 Open rectangular C-channel = 1.0 Closed rectangular box-section =

57 Fig.: Chassis-member sections. A. Square solid bar B. Round solid bar C. Circular tube with longitudinal slit D. Circular closed tube E. C-section F. Rectangular box section G. Top-hat-section H. I-section I. Channel flitch plate This clearly explains the advantages of using channel sections over the hollow tube due to high torsional stiffness. The chassis frame, however, is not designed for complete rigidity, but for the combination of both strength and flexibility to some degree.