Bir yarış otomobilinde hidrolik sönümleyici karakteristiklerinin çekiş performansına etkileri
The effects of hydraulic shock absorber characteristics on tractive performance of a race car
Yazar: ÖMÜR CAN KAYAOKAY
Danışman: DOÇ. DR. ÖZGEN AKALIN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2014
Konu: Makine Mühendisliği = Mechanical Engineering ; Mühendislik Bilimleri = Engineering Sciences ; Spor = Sports
Dizin: Taşıt süspansiyonu = Vehicle suspension

Özet:
Bu çalışma kapsamında otomotiv sektöründe kullanılan süspansiyonların özellikle sönüm özellikleri ve hidrolik sönümleyicilerin karakteristikleri üstünde durulmuş, temsili bir yarış otomobilinin bu hidrolik sönümleyicilerle beraber çekiş performansı ele alınmıştır. 
Bu çalışmada, Türkiye'de üretilmekte olup dünyanın çeşitli ülkelerine satılmakta olan bir yarış otomobilinin, hidrolik sönümleyici karakteristiklerinin değişmesi sonucu öngörülen yol ve hız şartlarında elde edebileceği çekiş performansını inceleyebilmek için bilgisayar ortamında modelleme yapılmıştır. Örnek alınan yarış otomobili Avitas Motorsport'un ürettiği CONTROL 4 modelidir. Bu yarış otomobili, 1980'lerde rallide kullanılan otomobillerde de benzerleri görüldüğü gibi, ortadan motorlu ve dört tekerleğinden tahrik edilen bir otomobildir. Bu altyapıya sahip otomobiller günlük şartlarda kullanılan önden motorlu önden çekişli otomobillere göre farklı dinamik özellikler göstermektedir ve bu farklılık iyi anlaşılmalıdır. Referans modelin temel tasarım parametreleri kullanılarak, ilgili testlerin gerçekleştirilebilmesi amacıyla, MSC ADAMS/Car 2012 yazılımında otomobilin barındırdığı her bir altsistem oluşturulmuş, sonrasında da birlikte uyum içerisinde çalıştıkları Çoklu Cisimler Dinamiği yapısı oluşturulmuştur. Tezin içeriğinde otomobilin tasarımında göz önünde bulundurulan temel özellikler ve elde edilen çıktılar aktarılmıştıtr. Otomobilin sahip olduğu hidrolik sönümleyici karakteristikleri literatürde yapılmış çalışmalar fikirsel örnek alınarak bir deneme kümesine bağlı kalınarak değiştirilmiş ve öngörülen testlerde çekiş performansı karşılaştırılmıştır.
Günümüzde çok çeşitli yeterlilik beklentileriyle tasarlanmakta olan hidrolik sönümleyici sistemleri tanıtılmış ve otomobil sporlarında kullanılan tiplerde görülen özellikler belirtilmiştir. Yarış otomobili süspansiyonlarından beklenenler, stabilite ve lastik-yol temasının her şartta mümkün olan en kuvvetli düzeyde olmasıdır. Testlerin ilerleyişinde sırasıyla, sönüm oranı, kapanma-açılma asimetri oranları, doğrusal, azalan üstel ve artan üstel sönüm eğrileri süspansiyon yay katsayıları değiştirilmeden kullanılmıştıtr. Testlerden elde edilen veriler çekiş performansına etkileri irdelenerek kıyaslanmıştır. 
Denemelerde iki çeşit yol tipi kullanılmıştır. İlki otomobilin iz açıklığında dalgaboyuna sahip engebeli bir yoldur. Bu özellikte bir yol, otomobilin hareketinin düşey yönde olmasını sağlamaktadır. İkincisi otomobilin yer ile temasının kesilmesine sebep olan tepe benzeri bir yoldur. Birinci yolda, otomobil düşük bir hızdan aks rezonans frekanslarına denk gelen hızların üstüne çıkacak şekilde hızlandırılmıştır ve hidrolik sönümleyicilerin engebelere ne kadar hızlı tepki verebildiği incelenmiştir. İkinci yolda ise, otomobil yer ile temasını tekrar sağlaması durumunda şasi düşey ivmelerinin hidrolik sönümleyiciler tarafından sönümlenme süresine bakılmıştır. Alınan sonuçlar oluşturulmuş hidrolik sönümleyici karakteristiklerine göre yorumlanmıştır.
Denemelerden, düşük sönüm oranı, düşük açılma katılığı ve düşük hızlı sönüm hareketlerinde düşük katılığın hidrolik sönümleyicinin hızlı tepki vermesini sağladığı görülmüştür. Otomobilin yoldaki bozukluklara hızlı tepki verebilmesi, motor gücünün yola daha iyi aktarılabilmesini sağlamıştır. Ancak tepeden zıplama ve yer ile temasın tekrar oluşması durumunda ortaya çıkan düşey hareketin kısa sürede sönümlenmesi ve çekiş gücündeki dalgalanmanın durdurulması için yüksek sönüm oranı, yüksek açılma katılığı ve düşük hızlı sönüm hareketlerinde yüksek katılığın gerektiği görülmüştür. Otomobilin meydana gelen düşey hareketi kısa sürede sönümlemesi çekiş performasının değişkenliğini daha hızlı engellemiştir.
Yarış otomobili olarak ele alınan örnek bir ralli otomobilidir. Rallide sık karşılaşılan durumlardan ikisi, bir tepenin üstünden geçmek suretiyle yer ile olan temasın kesilmesi ve tekrar yol zeminine dönülmesi ile yüksek hızlarda karşılaşılan, yoldaki yüksek düzensizliklerin üstünden geçilmesidir.
Çalışmanın en önemli yanı, otomobil modellemelerinde genellikle yol zemini düzensizliklerinin düşük, hızların can güvenliğini riske atmayacak seyir hızlarında olduğu durumlar öngörülürken, bu çalışmada bir ralli otomobilinin yarışmak durumunda olduğu farklı yol şartlarının ve bu yol şartlarındaki çekiş performansının üstünde hidrolik sönümleyicilerin etkisinin benzetilmesinin amaçlanmasıdır.

Abstract:
In this thesis, especially the damping characteristics of the modern hydraulic shock absorbers and the properties of the suspensions used in automotive industry were emphasised and tractive performance of a race car has been investigated in correlation with these hydraulic shock absorbers. 
In the aspect of this thesis, to be able to understand the effects of hydraulic shock absorbers' characteristics on a tractive performance of a race car which is being manufactured in Turkey and sold to a various countries around the world, a dynamics model has been created in a computer software basis. Referenced car is Turkish company, Avitas Motorport's model, named as CONTROL 4. This race car uses mid-engine and 4-wheel drive layout as seen amongst the cars on the 1980's rally cars. This layout shows different dynamic properties due to rear-shifted weight balance with respect to common front-engine front-wheel-drive automobiles and therefore these differences should be well understood and managed. By using the main design criterias of the reference model such as constructional points, weights and lengths, all the subsystems (namely front suspension, rear suspension, anti-roll bars, powertrain and chassis) are defined and created in a computer software and assembled in communication with each other as a Multibody Dynamics system to make the investigate the car's reaction to changing shock absorber characteristics. Important properties consired in high level race car design and calculated outcomes are given in the content of thesis. 
Hydraulic shock absorber properties are differentiated within a trial matrix, while this trial matrix is being created by the help of earlier studies found in literature. Used concepts in real-life and literature are accepted as examples for possible different designs. Taken values are in a range from commercial cars to race cars to investigate every combination. By doing the pre-determined tests with these combinations, tractive performance of a race car is compared with respect to changing hydraulic shock absorber parameters.
The thesis consists of five main topics. In detail, topics are the importance and motivation of the subject, literature review, general information about the hydraulic shock absorbers and suspensions in use, theory and definition of the problem, creation of the model, creation of the cases and analysis, results and discussion of the results. 
Hydraulic shock absorbers that are being designed with the various standards and expectations for modern automotive industry are explained and types used in motorports are pointed out. Expectations from race car hydraulic shock absorbers are vehicle stability, controlability and maintainig high level of tire-road interaction. Achieving these results require different types of damping characteristics on different excitations. On the tests in this thesis, linear, progressive and degressive curves are used for shock absorber characteristics along with different rebound/bump stiffness combinations. Results are compared and discussed by their effect on race car's tractive performance.
Reference model used in the thesis is an example of a rally car. It has lightweight tubular chassis frame, composite materials for outer and inner shells, all time front-rear locked four wheel drive system with no central differential, sequential gearbox and wide wheel travel limits. By the influence of 1980's tubular chassis rally cars and their success, reference model is also based on a mid-engine layout.
In the beginning of a rally motorsport journey, roads were already bumpy and mostly gravel. Rally is a single-run track race typically which is being held on the country routes instead of highways. For the purpose of creating a challenge for both driver and the car, generally bumpy and curvy roads are being used. When these roads meet the intention of shortest-time-fastest-race, loss of tire-ground connection and bump impacts often occur. These situations are extreme with respect to a common state of regular drivers. In this thesis, especially the damping characteristics of the hydraulic shock absorbers and the properties of the suspensions of automotive industry were emphasised and tractive performance of a race car has been investigated in correlation with these hydraulic shock absorbers. 
Modern automotive industry creates its shock absorber standards for optimised comfort, ride quality, stability, traction and fuel consumption. Optimization means all of the aspects of the system is being balanced in favour of the customer. As soon as we step into the world of motorsports and exclude comfort from the design criterias and emphasise the importance of traction, modern-day commercial car standards shortcomes.
Generally rebound stiffness of the shock absorber is greater than the compression stiffness. They are designed this way because springs tend to release their stored energy and push the wheels into the potholes found in the country roads. Driver wants to drive over a pothole as he does over a speed bump. If the bump and rebound stiffness were equal, he would still manage to go over a bump but may get stuck in the holes in the road as they tend to stop the vehicle. Same situation still applies for the rally cars but with a different approach. If the rebound stiffnesses are significantly higher than the bump stiffnesses, constantly bumpy roads cause the springs to compress but do not give them enough time to decompress as another bump had already hit the wheels. As we can see, off-road suspension systems need to obtain large wheel travels and somewhat closer bump and rebound stiffnesses. In this thesis, different rebound-compression ratios and different characteristic curves are tested over a take-off situations and harsh bumps.
Four-posters were introduced first in Formula 1. At first they were being used for shallow but high frequency inputs. Up to now, by the help of powerful designs, they found use in modifying suspensions, improving chassis properties, determining life of critical components on asphalt motorsports applications, commercial automobiles and off-road trucks. While they are capable of simulating large range of road data input through axle/tire vibrations and accerelation/inertia effects through chassis exicitations, rally sport is still out of limit with its extraordinary vertical movements and accerelations.
At first, basics of suspension design parameters are studied. Quarter-car-model and 2 degree-of-freedom pitch-bounce model were used. Front and rear axle ride frequencies are calculated with predetermined parameters to see if designed car is in correlation with the actual studies and cars built. After that, damping coefficients for the hydraulic shock absorbers are selected to create the first linear damping characteristic. By using linear damping characteristic values, different rebound and compression ratios, degressive and progressive curves are created. While creating such characteristics the examples found in literature are considered and common span of values were used. After that, for the testing purposes, Multibody Dynamics simulation program, MSC ADAMS/Car 2012 software is used. In simulation aspect, first step was building typical road types in rallies. In order to reflect realistic road profiles into the simulation, real life rally roads and rally pilots' experiences are considered. Road surface properties are not used in simulations because nearly in all cases higher tractive force results in greater performance. Evaluation of tractive performance is thought to be based on smooth and high normal load on tires. Normal loads' fluctuations are undesirable because they are the results of unbalanced and unsettled car dynamics. Results are compared with their fluctuations and convergence times. Fluctuations basically mean that, forces that are being carried to the chassis over the suspension mounts are changing their directions. They may cause unpredictable results for the driver. Also, settling times indicate that when the rally car will be available to turn-in to another corner with control or face another bump properly.
Two types of roads are used in the simulations. First road has the perturbation wavelength equal to the automobile's track length thus creating mainly vertical motions. Second road is designed like a hill to cause the automobile to jump. On the first road, automobile is accelerated from the low speed to the speeds exceeding its axle resonant points and hydraulic dampers' agility to follow the irregularities is examined. On the second road, chassis vertical accelerations and damping times are followed while the automobile is settling again on the road. Collected data are then compared with regards to hydraulic damping characteristics.
With the analysis results, it has been seen that low damping ratio, low rebound damping and lower damping forces on lower damper speeds manages to keep up with the irregularities found on the road surface. This ensures that the engine power is being applied to the road which means higher tractive force. Although lower damping forces result in higher tractive force in road irregularities, hill jump state requires higher damping ratio, higher rebound damping and higher damping forces on lower damper speeds. Analyses revealed that if the damping time of the chassis vertical motion is shorter, force being directed to the ground is normalized quicker thus regaining tractive force.