Internship reportDesigning a mannequin for aerodynamic bike testing

H.J.M. BaanUniversity of Twente

Enschede, The Netherlands

May 10, 2013

AbstractTo determine the aerodynamic drag of a cyclist tests can be carried out on the renewed bike rig in theYacht Resarch Unit in Auckland. With these tests the difference in drag for several cyclist positionscan be determined and the position of the cyclist can be optimized. Since these tests take a lot of timewhich is of course inconvenient for the cyclist, especially since he or she has to sit still in the sameposition during each run. To solve this problem a mannequin is manufactured that can make the samemovements as a real person. Furthermore are can the joints of the mannequin be locked individually, soit can be setup in a certain fixed position. The joints are made using a 3D printer, which is availableat the Yacht Research Unit. Hereby broken joints can easily be replaced. The mannequin is made of adisplay mannequin, but also an investigation is carried out concerning the making of a mannequin usingCNC technics, to be able to create a custom mannequin, eventually after a specific person.Tests show that the mannequin is usable for dynamic tests, however, to reduce weight and for bettershape properties a CNC manufactured mannequin is more desirable.Besides designing and manufacturing the mannequin also some changes are made to the bike rig con-cerning the stability and versatility.

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SamenvattingOp de vernieuwde fiets test opstelling in de Yacht Research Unit in Auckland kunnen testen wordenuitgevoerd om de luchtweerstand van een fietser te bepalen. Hiermee kan het verschil tussen verschillenderij posities worden onderzocht en aan de hand van de metingen verbeteringen worden ingevoerd. Omdatdeze testen veel tijd in beslag nemen kan dit erg vermoeiend zijn voor de fietser, vooral omdat hij ofzij gedurende een testrun in de zelfde houding moet blijven zitten. Als oplossing is er een mannequingefabriceerd die de zelfde bewegingen kan maken als een echt persoon. Daarnaast zijn de afzonderlijkegewrichten van de mannequin vast te zetten, zodat deze in een vaste positie opgesteld kan worden.De gewrichten zijn geproduceerd met de een 3D printer die aanwezig is op de Yacht Research Unit.Hierdoor kunnen defecte schanieren gemakkelijk vervangen worden. De mannequin is vervaardigt uiteen etalagepop, maar er is ook gekeken naar de mogelijkheden om een mannequin te produceren doormiddel van CNC technieken. In de toekomst zou er een een mannequin op maat gemaakt kunnen worden,eventueel van een specifiek persoon.Uit testen blijkt dat het concept van een bewegende mannequin voor het uitvoeren van dynamischetesten goed haalbaar is. Voor gewichtreductie en betere vormeigenschappen is een CNC gefabriceerdeechter mannequin wenselijk.Naast het ontwerpen en fabriceren van de mannequin zijn ook enkele aanpassingen aan de fiets testopstelling gemaakt met betrekking tot de stabiliteit en veelzijdigheid.

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Contents

1 Assignment 11.1 Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Literature research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Bike rig modifications 22.1 Leveling the base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.2 Adapt for heavy load testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3 Mannequin design 43.1 Body . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3.1.1 Casting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43.1.2 CNC manufacturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43.1.3 Display mannequin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3.2 Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53.2.1 Fixed and free rotating mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53.2.2 Strength analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53.2.3 Different joint locations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.3 Skeleton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.4 Skin and finish . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.5 Attaching the mannequin to the bike . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4 Testing 114.1 Mechanical testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.2 Veracity of the mannequin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.3 Aerodynamic testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

5 Conclusion 125.1 Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

6 Recommendation 13

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Chapter 1

Assignment

1.1 DescriptionThe Yacht Research Unit in Auckland is well known for their yacht testing capabilities, but road biketests are performed over here as well. Since reducing drag is an important topic for cyclists, a lot oftesting is done to reduce the loses caused by drag. The new bike testing rig is in operation since early2013. However, all cyclist tests have to be performed using a real test person to carry out a test. Theproblem is that the person has to sit tight during the whole test, since a small change in position willsignificantly influence the results. Another issue is the repeatability of the tests, since it is very hard tostay in exact the same position during several tests.The solution to these problems can be found in designing a mannequin which can be set up in a specificposition. One should be able to set the mannequin in different fixated cycling positions, but it shouldalso have the option to unlock the joints for peddling.The main use for the mannequin will be cycling, however it may be used for other sports later on.Therefore ideally the mannequin has to be able to imitate any human pose. Since the mannequin willonly be used for aerodynamic purposes the weight should be as low as possible to assure easy handling.

In the ideal situation a complete set of mannequins will be made later on to fit each member of theNew Zealand biking team, and carry out personal tests for each member. Therefor a quick and easy wayof manufacturing is preferred.

1.2 Literature researchOf course a lot of mannequins - both full body and specific body parts - are used for aerodynamic testing,so for bike testing. However most tests are done using a mannequin in a static biking position, withoutthe ability to peddle during the test. Garcia Lopez et al [1] pointed out that there is a reasonabledifference between a static test and a dynamic test in which the cyclist is peddling. Except for the studyof Martin et al [2] al tests are done in a static way.These tests show a significant difference in drag results for the static and dynamic test method, thereformost the test carried out at the Yacht Research Unit will be done in a dynamic way.

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Chapter 2

Bike rig modificationsThe bike rig which is used at the Yacht Research Unit is a quite new one since it has been completelyredesigned last year to serve for bike testing. However, there are some issues which need to be solvedbefore serious testing could be performed. One of the problems is that the floor on which the turntableis resting is not completely level and horizontal. Since the turntable rests on four air pads a not levelbase results in a wobbly turntable. The turntable should be able to yaw to different angles, therefor thefloor underneath the air pads needs to be completely horizontal to prevent the turntable to wobble atany angle.The drag force of the cyclist and bike is measured using in inverse pendulum system with four arms withlength l and a hinge with specific rotational stiffness (k). The change in angle by changing wind speedsor riding positions can be converted into a drag force, see figure 2.1 for an overview.

Figure 2.1: A schematic overview from the working of the bike rig

At set up the angle is measured which is the result of the weight of the bike and the cyclist (m). The dragcaused by the wind adds a drag force Fd. We can express the angle of the pendulum by the followingfunction:

Fml cos θ + Fdl sin θ = 4kθ (2.1)

Fm cos θ + Fd sin θ = 4klθ (2.2)

For small movements θ ≈ π/2 we can try to linearize the equation and take sin θ ≈ 1 and cos θ = θ−π/2as can be derived by a Taylor series expansion around θ = π/2. This results in:

Fm(θ − π/2) + Fd ≈ k

lθ (2.3)

However we still have a function θ which has a term Fmθ. Therefore we do not have a linear relationbetween drag force and angle. Ideally we want a setup which is independent of Fm, which is not thecase.Furthermore, with the bike rig setup does Fm change with changing center of gravity, which happenswhen a cyclist changes position, which is inevitable during cycling. Therefore we want a driver with afixed know weight so there is no need to calculate Fm again each time. Also a as light as possible driveris an advantage, since when his position changes the center of gravity of the bike plus driver changesrelatively less then with a heavy driver.

2.1 Leveling the baseThe previous section amplified the importance of a level floor to avoid a shift in θ. The floor of the bikerig in the original situation consisted out of just concrete slabs, with some cracks in between. To create

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Internship report 2. Bike rig modifications

a horizontal and level surface a thin layer of self leveling compound was applied. Since the result wasnot satisfying a second layer was applied which was also sanded. The final result is a level surface, butthere is still about 2mm difference between the highest and lowest area’s and therefore not completelylevel.

2.2 Adapt for heavy load testingThe turntable of the bike rig is designed to be used for bike testing, however it is also desirable that itcould be used for testing other setups, for example a skier or a motorbike. The turn table is death flat,only the wheels of the bike rest on the force measurement system. An adaption is made to the setupwhich makes it possible to connect the turntable to the force measurement system, so that it is possibleto test different (heavy) setups and situations by inserting two connection beams. Changing from biketesting to heavy load testing can now be done in just a few minutes.

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Chapter 3

Mannequin design

3.1 BodyOf course one of the most important design decisions is the material the body is made of. Since themannequin is only used for wind tunnel tests a strong but light design is preferred. A mannequin madeof a high density foam would be the ideal candidate, but first a way of manufacturing such a mannequinneeds to be found.

3.1.1 Casting

Figure 3.1: Bodycast test of the leftforearm

A common used way to make mannequins is body casting.This is however a time consuming process in which a lot ofmaterial is used. A cast of the left forearm was performedusing plaster of paris to check the feasibility of a full bodycast, as can be seen in figure 3.1. It turned out that a lot ofmaterial was needed to make a good mould. Also staying inthe same position to let the mould dry for over 20 minutes istoo long. Especially for casting the torso or other large partsit is hard to stay in the same position for that timespan. Thesecond problem is getting the mould off without damaging itor the person, which is almost impossible. Better results maybe obtained by professional body casters, but this can be veryexpensive. Furthermore, since a mould can normally only beused once the risk of having to make a new mould due to someproblems during the casting is very high.

3.1.2 CNC manufacturingAn easier and better repeatable way is to create a 3D model ofa person is to scan the person - in biking position if possible -and use a CNC machine to cut a copy out of high-density foam.In this way one can create the optimal shaped symmetric man-nequin of which all model data is know as well which also givesthe option for a CFD comparison with the computer model.A 3D model was created using a cheap Microsoft Kinect Xbox3D sensor and the software ReconstructMe [3]. These give a rough model which can be modified lateron using dedicated software. But since the requested quote from several companies exceeded the budgetfor making a mannequin, and so another option had to be found.

3.1.3 Display mannequinSince the previous two options where complicated or too expensive an other option needs to be found.After some investigation the decision is made to buy a cheap display mannequin and chop that intoseveral parts. The best of course would be a mannequin in biking position, but these are very rare andvery expensive. Therefore a standing mannequin is picked which was quite symmetric. The mannequinwas cut in several parts, these parts where attached together using a special designed skeleton and jointsas can be read in the next sections.

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Internship report 3. Mannequin design

3.2 Joints

Figure 3.2: The mount used for Go-Pro cameras.

An important part of the mannequin are the joints, since they givethe mannequin the ability to move but do also have the optionto lock it in a certain position. At first a study is done on whatjoints are available already to obtain ideas. Since there is a 3Dprinter available the joints can printed easy and cheap.

As starting point the GoPro mount as shown in Figure 3.2is taken, this mount is based on a ball joint which is clampedbetween two shells. However this joint is designed to do threedegrees of freedom, where most joints in the human body do onlyone or two.

Another requirement was that the joints needed to be uniform,so that every joint of the mannequin is almost the same so theycan easily be replaced in case they are broken or a small modifi-cation is made.In Figure 3.3 are the different versions of the designed joint dis-played. In Table 3.2 a short description of the modifications inthat version is given.

In general there is one uniform joint with some small modi-fications to fit every situation. A computer render is shown inFigure 3.5. This joint is used for every joint of the mannequin. In Figure 3.4 a simple overview of themannequin is given with the positions and orientation of the joints.

(a) Joint version 1 (b) Joint version 2 (c) Joint version 3

Figure 3.3: Different versions of the joint during the design process

3.2.1 Fixed and free rotating modeThe ball of the joint is designed with little ribs which only cover a part of the surface. An overview ofthis is given in Figure 3.6. If the ball is rotated 90◦, the ribs on the ball are either locked in the ribs ofthe shell or on the free surface of the shell and free to move. In this way each specific joint could easilybe changed from a fixed in a free rotating joint by loosening the screw and rotating the ball around itsaxis.

3.2.2 Strength analysisStrength tests point out that the joint is able to withstand a torque of 7 Nm, which is enough for thejoints. At higher loads the profile on the ball will just be destroyed and make the joint unusable. Forplaces where a high load is applied like in the back a new stronger version of the joint is designed with alarger profile area on the ball and shell. A comparison between the normal ball and stiff ball is shown inthe bottom of Table 3.2. The stiff joint is no longer able to rotate freely, but this is not required for thespine. The bold which holds the joint in place does not have be be tightened very much, since the profileholds the ball in place. To much tightening could break the shell, but when the tightening is done byhand without the use power tools breaking the joint is not likely according to the Ansys analyses. Anoverview of the expected deformation is given in Figure 3.7.

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Internship report 3. Mannequin design

Version no. Changes made Detail

1 The asymmetric slot at the top caused the shells tobend not equal at both sides, resulting in bad grip.Making both sides of the shell identical and symmet-ric solved the problem.

1 The shell was too thin at the cutout for the bottompart which caused the shell to crack at high loads,which was also confirmed by an analysis using Ansys.

1 The size of the hole for the threaded rod was in-creased to 12 mm.

2 The joint dimensions where scaled down by a factortwo to reduce the amount of material needed.

2 A rib was added on the top of the shell for extrastiffness and exchanged the hexagon shaped nut holeto a normal hole with the nut on top, since the nuthole suffered from wear.

3 Removed material in the center of the shell to reducethe required amount of material.

3 Added profile to the ball and to the shells to improvethe grip.

3.1 (stiff) Increased the profile area on the ball and shells toimprove the grip even more.

Table 3.1: The most important changes made to the joint during the design process

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Internship report 3. Mannequin design

Figure 3.4: An overview of the joints in the mannequin. The dashed joints are stronger joints with norotating mode.

Not only a strength analysis has been carried out, also a small wear test have been performed to seeif the joints can operate in free rotation mode for a decent amount of time. To do this the ball head(without profile) has been attached to a drill and rotated at 600 rpm for two minutes within the shellenclosure, the same has been done for the bottom part.The surface looked still fine after the test and did not suffer any damage. Therefore it is expected thatno problems will occur during intensive usage of the joints in free rotating mode.

3.2.3 Different joint locationsMost body joints need only one degree of freedom, but the shoulders and hip joints have to be moreadvanced. A schematic overview is given in Figure 3.8. In this figure one can also see what part of thebody material around the joint should be removed to make the desired angle of movement.

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Internship report 3. Mannequin design

(a) Final joint assembled

(b) Final joint without top shell

Figure 3.5: The final joint

Figure 3.6: Schematic overview of the balljoint in its shell. Red means fixed and green is free to move.

Figure 3.7: An Ansys analysis for the deformation of the shell when tightening the bold.

3.3 SkeletonThe skeleton consists out of 16mm thick aluminum tubes, which are connected by the joints. Both sidesof the aluminum tubes are provided with internal thread to connect to the M12 threaded rod which isattached to the joints.Pieces of MDF are glued into the ends of the chopped up mannequin parts, and equipped with centerholes, through which the skeleton is inserted as shown in figure 3.9. One can see that the skeleton is fixedby just tightening the screw so the MDF is clamped between the tube and the nut. The connectionsbetween the spine and the legs or arms is done using a piece of a steel bar which connects the tubes as

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Internship report 3. Mannequin design

α

α

α

(a) Joint for both the hips

α

(b) Joint for the ankles

r

(c) Joint for the shoulders

(d) Joint for the wrists

α/2

(e) Normal joint for elbows and knees

Figure 3.8: The different joint locations for the mannequin.

shown in figure 3.10. The whole skeleton is easy to disassemble and to modify by undoing the bolds orscrews. This makes it easy to replace or exchange (broken) parts or make modifications to a certain partof the mannequin.

Figure 3.9: Joint attached to the MDF with aluminum tube and nut.

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Internship report 3. Mannequin design

Figure 3.10: Connecting element for the skeleton.

3.4 Skin and finish

Figure 3.11: Black morphsuit

Since the mannequin is used for wind tunnel testing the skin needsto be smooth and resemble a real person. Therefore a morphsuit isused: a tight suit (see figure 3.11) that covers the whole mannequinand gives it a smooth finish. At the joints there are gaps, resultingin pits at the surface. To keep the original shape the gaps are stuffedwith foam rubber, which gives the joints the possibility to rotate andkeeps the shape of the person.

3.5 Attaching the mannequin to the bikeTo attach the mannequin to the bike a special seat post is designedwhich is attached to the mannequin. In this way the mannequincan easily be mounted in a fixed position. The fingers are made ofsteel wire (figure 3.12) which can be bended around the handlebarsto secure the upper body. The feet are put into biking shoes and canbe attached in the common way.

Figure 3.12: Hand with steel wire fingers.

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Chapter 4

Testing

4.1 Mechanical testingThe mannequin is mainly designed for bike testing. Therefore it should have the ability to sit in bikingposition and pedal as well. The common biking positions are tested and checked if pedaling is possible inthis position. The cuts for the hips are adjusted to make it also possible to pedal in time trial position.One of the difficulties was keeping the feet horizontal while pedaling. Normally we use our muscles to doso, but if the pedals are moving the feet around they tend to flip over. To keep them straight a rubberband is attached to the back of the feet and the bar of the bike behind it, and from the front of the feetto the front bars of the bike. In this way the feet stay horizontal. In figure 4.1 one can see the mannequinset up on a mountain bike for testing.

4.2 Veracity of the mannequinSince the mannequin is made of a tall display mannequin (almost two meter) it is taller than an averagecyclist, this is something to keep in mind. Secondly the display mannequin is one in standing position.Therefore the back is of a man in standing position, instead of driving position. Also the shoulders arestanding more outward than one usually has when riding a bike.

4.3 Aerodynamic testingAerodynamic testing has not been done yet, so a comparison between a human and the mannequin isnot made yet.

Figure 4.1: Mannequin set up on a mountain bike without the morphsuit.

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Chapter 5

Conclusion

5.1 SpecificationsThe mannequin meets the specifications that where required. The overall weight is around 10kg, whichmakes the mannequin easy to handle. The joints work as expected and can hold the mannequin in thedesired position.

With this project it is shown that it is possible to create a mannequin for wind tunnel testing justout of plain materials. Wind tunnel test have to point out how valid the results will be compared to areal person. Due to the low required manufacturing cost the mannequin is a bit amateurish looking andthe shape or build of the mannequin is determined by the display mannequin.

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Chapter 6

RecommendationAs already mentioned in the design process: the best way to create a completely customizable mannequinis to create a foam one, although this can be expensive. If this is not an option the existing mannequinneeds some fine tuning, especially the skin part to make this as smooth as possible. For strength themannequin may be filled with a rigid foam to make it more rigid. For now the skeleton is attached to theouter shape by just some pieces of glued MDF, which is of course a bit fragile. Furthermore the screwsin the joints could be replaced by hex-headed screws for easy adjustment using an Allen key.Of course the mannequin should be equipped with real cyclist clothing, helmet and shoes when performingtests.

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Bibliography[1 ] Garcia-Lopez, J. A. Rodriguez-Marroyo, C.-E. Juneau, J. Peleteiro, A. C. Martinez, and J. G. Villa.

Reference values and improvement of aerodynamic drag in professional cyclists. Journal of SportsSciences, 26(3):277-286, 2008.

[2 ] Martin, J. C., Milliken, D. L., Cobb, J. E., McFadden, K. L., & Coggan, A. R. Validation of amathematical model for road cycling power. Journal of Applied Biomechanics, 14, 245-259, 1998.

Websites

[3 ] http://www.reconstructme.net (01-02-2013)

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