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Ducati
Desmosedici RR - analysis Introduction. Ducati presented a direct descendant of its motoGP bike, the Desmosedici RR. This commercial version looks very similar with the race bike and it shares the same technology and technical background. Although the engine, the frame, the suspension and other major components appear similar between the two bikes there are some key differences.   This article is based in data and images provided by Ducati. Photos are from the official Ducati press kit and various press releases and the schemes are from robotpig.net. Engine. The engine is based on the race bike's. It is a 4 cylinder in V layout with a 90 degree spacing between the two pair of cylinders. It uses as most, if not all, ducatis a desmodromic valvetrain and uneven ignition spacing between the four cylinders. In the 2003 race bikes the V4 engine operation simulated a large two cylinder. Ignition took place simultaneously at each bank of two cylinders. The purpose of this design was the maximisation of the rear tyre's traction. It appears that racing tyres (and possibly road tyres) can operate with better results if loaded with pulses of thrust rather than a continuous stream. The "twin pulse" as called by ducati marketeers groups the thrust of two cylinders together in contrast to a conventional in line 4 cylinder where the pulse from the ignition of each cylinder is evenly spaced in time to produce a stream of frequent small pulses to the rear tyre. In the RR bike and also in the later race bikes an intermediate layout is used. It has also larger intervals between thrust pulses than an inline 4 cylinder but the pulses come more frequent (and are smaller) than the 'twin pulse' or a similar twin cylinder engine. This is achieved by spacing the ignition timing in pairs for every two cylinders but the ignition between the cylinders of every pair doesn't take place at the same time, there is a small gap (following by a much larger time gap when the next pair of cylinders is to ignite).  The commercial bike's engine is possibly wider because it should last much longer than the almost expendable race engine. The camshafts use a much calmer profile in order to reduce the loads of the valves although the maximum lift is similar. The race engine's camshafts reaches very fast the maximum valve opening but it also submits the vavetrainto great loads because of the accelerations it has to endure. The desmodromic valvetrain also requires frequent readjustment (with catastrophic consequences if neglected) so the calmer operation of the RR's engine apart from the longevity of the components allows a less demanding maintenance. RR's engine produces more than 200ps at 13.000 rpm and is one of the most powerful (if not the most powerful) engines in production. It is however lacking 50-60 ps in comparison with the original motoGP bike. Electronic launch control won't be aveliable at least for this year because it is one of the most sensitive and closely guarded technologies of the race version. Frame. According to Ducati, RR's frame shares the same basic structure and geometry with the 2006 racing bike. However there are same major differences between the two. Of course they both use the famous Ducati trellis frame from steel tubes. The most important difference is at the brackets where the engine is suspended from the frame. The engine is a stressed member of the chassis and is also used to support the rear suspension. It also is used for the complete triangulation of the engine-chassis structure. In the following image you can see on the left the RR and on the right the frame of the 2006 race bike.  The blue arrows point at the first difference between the two bikes. In the race bike's frame the tubes end in a cylindrical element though at the RR's frame there is a cast part with strengthening ribs. Apart from supporting the RR's heavier rear subframe this part is amplifies the local rigity. With the green arrows another possible alteration is shown although because this remark is based on the photos provided is in effect speculation. In the racing frame there is a -slightly bent- tube that connects the front engine mount to the rear. In the RR this element seems to be absent or it is placed upwards, possibly connecting the rear engine mount with the node above the front engine mount, where other tubes connect also. The result either way (total absent of the tube or higher mounting) is a notably flexier local mounting point. There are possibly two reasons for these two differences between the race bike and the commercial bike. The engine is mounted directly into the frame and it also lacks any balancer shafts. This alteration on the frame stiffness possibly reduces or dampens the engine vibration in order to provide a smoother ride. The road bike must also be controllable with road tyres and most importantly with riders who lack the motoGP level of experience. The alteration in frame stiffness looks similar to the design path that most Japanese sport bike manufacturers follow. The rear part is made as rigid as possible and certain flexibility (at specific planes) is introduced to the front part of the chassis. The flexibility in the transverse bikewise axis allows the wheels to follow road disturbances (rare if at all existent at the track). The flexibility in the lengthwise axis assists the operation of the front suspension. Even though the chassis flexibility is minimal in absolute terms these specific flexibility paths allows the bike to be more friendly and controllable in road conditions and under the use of inexperienced (in comparison with factory drivers) users. It should also be noted that the resistant to twist lengthwise is very big thus providing the bike with great stability under sudden lean angle changes or under acceleration or braking.  Another minor change is the possibly wider frame at the front engine mounting bracket of the RR in order to accommodate the wider engine. In the photo above the blue arrow shows the two brackets. Aerodynamics. Motorcycles are inherently problematic in their aerodynamic behaviour. They don't have a streamlined body (rider included), they have a small wheelbase and exposed moving parts (the wheels, especially the front when steering, the rider, etc)  Apart from aerodynamic drag equally, if more so, important is the aerodynamic balance and stability. The Desmosedici race bikes use to have very high top speed (they achieve the highest top speed in many races) and this is largely because of the very low drag of the bike. They also have a lot of destabilisation problems and the Ducati riders have some trouble in this area, especially in fast circuits. Every team makes a compromise between low drag and good stability (or even low lift). For example the Honda RC211V, the most successful motoGP bike has a quite un-streamlined fairing, the rider is largely exposed to the air stream and the surfaces are more crude and angular in comparison with the desmosedici. The RC211V design produces more drag but it disrupts the flow from forming a quasi-airfoil profile thus reducing lift or sudden boundary layer separation. The RC211V has lower aerodynamic lift (and thus better traction) and better aerodynamic stability in exchange of a lower top speed (concerning the drag factor of this matter). The commercial RR version isn't going to endure motoGP race conditions, however its shape is almost identical with the race bike with one major difference, the position and direction of the exhaust. Exhaust. The exhaust position is important for packaging and aerodynamic reasons. There are mainly two options for positioning the exhaust, the one is lateraly, beside the engine end just after the rider and the other is at the back of the bike, in the tail. Of course there are numerous variations and also combinations. In the first version of the desmosedici race bike (2003-2004) the exhaust was in the tail of the bike, as shown in the image below:  In the following 2005-6 version and under the influence of the new, relaxed in matters of noise, regulations the exhaust pipes were much simpler and the gas momentum was greater. The exhaust exits are now four, two in the back (in the same place with the previous version's exhaust) and two laterally beside the engine.  The exhaust gases momentum from the two exits in the back (about half of the total flow) should be approximately the same as the one in the single exhaust of the previous version which although it received the total gas flow it has to reduce its momentum for noise regulations. The result is that the two versions should have similar aerodynamic behaviour considering the influence of the exhaust gases. The exhaust in the RR is placed at a very unusual position. Although the exhaust in the rear on the bike is now almost mainstream, here we have the exhaust facing upwards. It should be noted that even the track only exhaust of the RR is subjected to noise regulations much more strict than the race bike's.  The exhaust is a substantial source of momentum for the airflow. A 1000cc engine rotating at 16.000 rpm produces approximately 8.000 litres of gas every minute. In the final section of this article we will analyse qualitatively the aerodynamic influence of the exhaust placement. It is important here to notice that the aerodynamic of a body is defined, apart from its geometry, from sources of fluid mass/momentum and attached vortices (not examined in this article). In the image below you can see the motorcycle/rider and also the equivalent aerodynamic body (shown with a lighter shade and indicated by the pink arrow marked 'EAB'). This represents the complete effect that the total body of the motorcycle/rider and the source of mass/momentum (the exhaust) have in the airflow. The centre of gravity (circle with black and white), the centre of aerodynamic pressure (white circle) and their respective distance (green line) is also pictured. The red arrow shows the direction of the gases exiting the exhaust. The following images are for examining the subject qualitatively, they are not based in actual experiments or simulations and the shapes and sizes appearing on them are not necessarily real. A very important factor for the stability of the motorcycle (or vehicle in general) is the distance between the centre of gravity and the centre of pressure. The more the centre of pressure is aft of the centre of gravity, the greater the recovering torque that restabilises the bike under any perturbation. In this first image the exhaust is positioned laterally, as in many commercial superbikes, especially in the past.  The flow in this case is not affected from the exhaust because the gas momentum is released outside the wake. In the rear of the bike there is a lot of turbulence because the geometry is very steep and the flow cannot follow it. In the following image the exhaust is positioned at the back of the bike facing backwards, as used by many sport bikes. Here the momentum from the exhaust gases is released at the area where there was a separation problem. Thus the flow is not disturbed and the separation occurs at a greater distance aft of the bike. The equivalent aerodynamic body now is more streamlined, producing smaller amounts of drag and also the centre of pressure moves backwards increasing the stability of the bike. The effect of this exhaust position simulates and behaves similar to a longer and more streamlined body.  The Desmosedici RR uses a variation of the case described. It also places the exhaust at the back of the bike but the exit is facing also upwards thus changing the profile of the equivalent aerodynamic body.  In this case we have a profile that largely simulates that of an inverted airfoil and it should improve stability and traction by decreasing the aerodynamic lift. It is obvious in the last two cases that the aerodynamic behaviour is closely related to the operation of the engine. With full throttle and at high rpm the stability of the motorcycle is augmented because of the large momentum produced by the exhaust gases. Even though this at first seems that the behaviour of the bike could be unpredicted in reality the throttle/rpm interaction shouldn't be a concerning factor. In high speeds where the stability is most wanted the engine is operating at high revs and with full throttle in order to overcome the drag. Also in sudden acceleration the aerodynamic stability enhancement from the exhaust is produced accordingly and on the contrary, when braking modern sportsbikes use almost exclusively the front tyre so any aerodynamic assist wouldn't be really important. In conclusion we can say that even though dependant from the engine operation, exhaust gases could provide a useful assist over the aerodynamic stability of the bike.  Epilogue. The Desmosedici RR even though it isn't a ground breaking motorbike, it consist the showcase of Ducati technology level and race expertise. Many of its features will filter down the rest of Ducati range and possibly other motoGP teams will produce race replicas in the future. comment this article register at robotpig.net/forum . © robotpig.net - All rights reserved. No part of this service may be reproduced, reprinted or republished without written permission |
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I.K.
Erripis 271006 |
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