Explained: Aero Bike Wheels (Zipp Enve Mavic Fulcrum)

hello everyone and welcome in this video I'm going to talk to you about wheel and the aerodynamics of wheels now next to a rider the second biggest impact on drag on a bike are the wheel and I'll come on to why in a moment and in this video I'll tell you what the difference is between this lock wheel a very shallow wheel a deep section Aero wheel and also some of the aerodynamics terminology and interest is involved in this and I started the the explanation process I will have simplified things so if you are an aerospace engineer out there I apologize in advance for trying to condense an entire topic into a short video it's not exactly easy so a word on terminology because this is quite important this video it is quite technical this is the top of the wheel and tire I'm going to refer to the tone wheel almost in unison but this is the top this is the front so this will be hitting the airburst this is the back of the wheel tire and bottom is business in contact with the ground the other thing that anything mentioned is what I prefer the leading edge and trailing edge leading edge is what hit the air and trailing edge is where the air lead blast so this will be the leading edge on the front of the wheel this will be the leading edge outside the trailing edge on the front of the wheel at the back of the wheel this is the leading edge ie the carbon and the trailing edge is tire now what we're interested in in bicycle wheel aerodynamics is the force giraffe now drive is something that is designed to slow bicycle down drag this function of a number of things the first one is the density of the fluid that is going through so a swimming to an engineer can be air can be water can be some gap like acetone or nitrogen or whatever it's just a fluid the fact that it is a gap is irrespective we had a property the next one is the velocity that the object is traveling at so if you have no velocity cannot have any drag if you are going up surrounding say ten kilometres per hour your experience of drag squad if you go up 20 kilometres per hour and you keep the same object roughly speaking the drag force that you will experience it will be four times more because drag force is proportional to the speed squared the final one and I guess the one that we're really interested in is a combination of the side and objects of the cross sectional surface area of the object and what the objects shape is right and that determines on a bicycle wheel quite to a large extent how much drugs that you will experience now coming back to density density is quite important because it's not very equal across the planet and the reason I say that is in in density term you can have a variation between sort of one point zero five kilograms per cubic metre and possibly even 1.3 kilograms per cubic meters that's the mass of one cubic meter of air ie 123 1.2 kilograms if you were to take a rider in for example Australia who while strumming along at for argument's sake let's say 30 kilometres per hour and pulling out 300 watt and then brought him back to the sunny climes of the UK in winter and asked him to do the same 300 watts he wouldn't be able to do 30 kilometres per hour typically it would need an extra 20% ie another 60 watt to be able to go the same speed so that's how much of an effect density can have at the same time that's a not really a function of temperature so in Australia and oh maybe 30 degrees in the UK two degrees and that differential has the effect of increasing the power required at the same time the humidity of the air that you are going through also has an effect because humidity has an effect on tentative the more humid air is the air is actually less dense that might seem counterintuitive but if you got into that have a look for our guard rows number that will explain what that principle is so I need to introduce the concept of relative speed in here and the difference between speed and velocity may attend to use of two terms interchangeably but there is different speed is among a quantity that only has magnitude so if it is just a number 35 m/s or 35 miles per hour they start telling sprouts that is it it is called a scalar quantity because it just has a microchip velocity is a value that has both magnitude and direction for Aeronautics principles we need to concentrate on philosophy so velocities for this this bicycle wheel can be expressed in a number of ways usually you just give it a direction with a minus or a plus so if I could find going that way a positive then the top of this wheel is going in the correct direction however if I if I define positive as that way this wheel is going the opposing direction is therefore negative the signs are reversed at the bottom of the wheel because the wheel rotation means that at the bottom we are actually traveling backwards were over the top wheel is traveling forward you might think why is this important well if it bites on rollers and we're doing 30 kilometres per hour the whole wheel is spinning at page sprout the top of the wheel is going to educate that way the bottom is going to change that way when you take the rider out taking away from the rollers put them actually out on the road what happens it is another effect happens you've got as now keeping the bike when the air hitting the bike is the air if the bike is stationary and the tire is not moving air is hitting it completely at 30 kilometres per hour however when there is now rotation ie the wheel is rotating the 30 kilometres per hour which is this hand is now pitching at the tire is also rotating at service killing these pro you have a relative velocity there or relative speed of 60 kilometres per hour at the same time at the bottom of the wheel the other relative velocity of zero because the wheel is going that way at 30 kilometres per hour and air is also coming in at 30 kilometres per hour so by definition there is zero velocity and therefore no drag so to summarize that sort of section basically the top of the wheel top half of the wheel is going in the direction is unfavorable from a drag perspective the bottom half of the wheel in going direction that is favorable from a drag perspective because it's in the direction of the winds of hitting the bike now on a bike there are really only two ways to generate drag one is called skin friction drag and the other is called pressure drag skin friction drag is the friction that the air has to overcome to just go around you basically so if I put my hand like that I've got a frictional force friction area that's just caused by the air going over it that he makes my hand hot of air goes over it pressure drag it's a bit more complicated pressure drag is actually caused by differential in pressure and that can be caused by differential in speed so with the sheets of paper we're going to demonstrate what pressure drag actually feels like I'm going to blow over the top of the page you saw the paper rise and the reason why the paper went thought instead of most people probably think it comes down is I'm flowing over the top of the paper by me blowing over the top of paper actually reduces the pressure on the top surface of the paper because there's a drop in pressure relative to the bottom of the paper which is still seeing high pressure it lifted the paper up on a bike same thing happens you've got areas of high pressure and low pressure when the two interact they generate something called vortices now in nature you see this all the time high pressure low pressure actually causes wind if it's in a very localized scenario or ball Tex is a tornado a cyclone or a hurricane they are all forms of pressure differential so in the middle of a hurricane you've actually got no movement of air because the pressure is fixed there's no differential and people often hit you know talk about their eardrums popping that's because as a hurricane on a tornado going over the head the air pressure is changing and it's the same thing on a bike or we're a lot less ferocious velocity we're going to get to the juicy bit which is to explain why this wheel is a zip wheel is more aerodynamic than that Shimano wheel there and the reason is not probably what you would think the first thing to consider is let's consider the path of the air that comes over the center of a wheel so air comes over here first thing here is this little area where the clincher clinches into the rim bead that is going to generate a bit of turbulence and we go across here now zip claimed that this is to reduce drag I can categorically tell you that marketing bollocks and because the flow over here is not going to be in a Reynolds number which is not complicated to wear that would make a difference so I think that's just marketing rubbish so we go over here and - from this place and then carry on going past the hub and then over towards the back of the wheel the actual aerodynamic advantage in a deep section wheel is apt is nothing to do with this sexy toroidal shape that has very very little effect it will have a small amount but not very much a bigger advantage comes from pure geometry and that is because of the what would what is called aerodynamically is the cord linked to the aerofoil so if that is an aerofoil section we would determine that as the chord length so horizontally that chord length to the edge of the tire is about on this wheel 10 centimeters however as we move down the wheel this chord length gets longer so that's 14 centimeters there at the problem is the maximum is 40 centimeters so you've got an area where you get detachment if you minimize that area of detachment which you do by increasing the depth of the wheel you will make the wheel more aerodynamic which is why a disc wheel is fundamentally more aerodynamic than any of this type of wheel so I'll repeat the process on the Shimano wheel now on the Shimano wheel and the flow is actually coming from that direction that way but the cross-sectional area there is around about 6 centimeters so that's my quarter links there I like longest point which is around there it's 30 centimeters so we've instantly lost 10 centimeters but it's 10 centimeters multiplied by the height as well so 6 times 30 is roughly 180 this is 11 times 400 which is 440 and that kind of rough estimation is how you determine the aerodynamics of the wheel that's far more effect than the section this is very very akin to a box section wheel so there's hardly any tapering like that on this wheel one of the subjects that a lot of marketing is based on is have been called your angle your angle is the relative direction with a point is traveling in relation to the direction the wings command so if we were to go square on your angle will be zero if we were to text the bike slightly so that the air is coming the other way and my bike's pointing kind of like that way and what happens is the air starts to detach from the bicycle wheel rim the more your angle you have the more detachment you're likely to get a certain angle of your it is aerodynamically inefficient to have a deep section wheel and that is dependent on the geometry of the profile of the bike wheel at the same time the other experience the other thing that you experience is it is a lot of your angle is the bike will become unstable and effectively crosswind will try and hit the bike over one of the other hot topics that is the flavor of the month at the moment is fitting why tires too wide rimmed now the actual drag is caused by this dimple here and the Bulge in the tire when you get round to the back of the wheel basically the basic guidance is if you're going to fit a wide time make sure your rim is wide don't fit a wide tire to a narrow rim because it'll balloon and the ballooning will cause drag on the far side so you rejoin me in what I can best describe as the engineering Mecca that is my computer route and unfortunately I don't have a wind tunnel so what we're going to do is go through this on SolidWorks with some CFD and that will illustrate some of the flow patterns a bit better so what we have here is a wheel that I'm drawn obviously there's some assumptions in here because there's no hub and there's no spoke I apologize for the fun modes as you can running SolidWorks is not exactly a non processor intensive process so this section is typical of what Shimano section would look like so let me show you taking a section through the wheel so this is a tire this is that typical Forge that you would get if you fitted a tire to a slightly narrow rim this is the brake track this is a tapering section and just a bit of rounding there and just to make it look a bit better that's typical of a Shimano will probably be 50 although this ring depth is actually 80 millimeters turn off once you have that wheel you can on cfd run an analysis where you can put flow through the wheel add some rotation and see what the airflow does and that's what I've done now I have prepared this earlier because it takes an actually age to run it so for a wheel at this complexity it takes about three hours to run if you've got a faster computer it'll probably go a bit faster than that so what we have is the line the lead line probably look like viscously from a weather map book in reality what they are are streamline showing vorticity and vorticity is effectively where the drag comes from now earlier I mentioned the drag comes from pressure drag and skin friction drag the skin friction drag is small in comparison to the pressure drag so a pressure drag caused by a difference in velocity causes these things which are vortices and you can see one more texture another loss external aspect now a useful thing with SolidWorks is and you can actually see where the vortices are at any point in the wheel so if we start the bottom at the bottom the prophecy that I predicted was there will be very little drag and there is actually a small amount drag and that's because the velocity is not quite the same as the in for free stream velocity but you can see the very good distribution of a stream line from the right of the leading edge of the wheel tire combination to the trailing edge of the tire combination and this blue that you can see around it that is whether cut through the wheel and tire it I'm going to move this up and you can see that you know in this area where we have no sort of free space and there is just one continuous section there is very little drag okay as we get towards the cut point and when I say the cut point that is where there's a gap between the leading edge of the sort of the trailing edge and the front of the wheel and the leading edge of the back of the wheel down here there's nothing but as we go up there's a definite point and I'm just screwed out of the difference there so now we have clear air between the leading edge and trailing edge of the further back wheel where we have that almost instantaneously the globe disturbance and this is detachment so when air comes along it comes along keeps going keeps going and then all of a sudden it could catch it when it be catching all this mess happens so there's a vortex there another vortex that and then it hits the leading edge of the back of the wheel we're carrying on going a bit higher the vortex business becomes more apparent even though the vortex there the start of one there another one there two one here and one here that are either side of this area where the tire comes over and then meets at the bead and also we've set up a slight bow wave that we're compressing the air slightly in front of the tire carry I'm going a bit higher I need the middle I'm just slightly above or below that is the centerline of the wheel entire combination and all the new this is a stagnation point it is very very clear to see you know the vortex streamline positions a that that's just a stagnation point we've got more tech shedding here vortex shedding here vortexes just immediately off the trailing edge of the front of the wheel that flow is disturbed it carries on going towards the back of the wheel and then we have more flow disturbing sort of a neat ish shape but it's an inverted aerofoil that they're touching at the back of the wheel so it's not preferential we go higher this vortex is still immediately apparent and keep going keep going keep going so this is what I would probably call a nasty section of air for for one of a better word this this top section there is some point to do with the geometry of the wheel and the fluid conditions that have applied that causes massive vortex not quite sure what it is because I'm really studied it in a lot of a lot of depth but to tell you I'm going higher and higher and higher so we're back now at the point where we have continual boundaries of continuous on the front of the tire to the back there is no breakage in the wheel and the flow is better than where there's a break in the wheel carry on going higher and higher and higher this is nasty vortex here and also flow disturbance off the top of the wheel carry on keep going keep going so just as we skim the top of the tire there's actually a lot of wake vortex shedding there vortex shedding there this is why I have is what's known as a computation don't make so I just trimmed the section in this blue rectangle here I was to make it wider you would see another vortex there but that increases the computational times I haven't done it so family it's working garner is you know the number of things the main thing to garner is on the bottom section of the wheel the sort of the airflow conditions are favorable because the wheel is rotating in the same direction of travel so the vortex shedding a vortex development is nowhere near as bad and so looked if you look here as we come back down the wheel as we get near the bottom it's hardly any policies because the wheel spinning is a little in the direction travel okay when we get about quarter the way up we're starting to get vortex development but in here look at that this is called C's everywhere so that's why the top of the wheel has a lot more drag now one of the things that I have done is I try to make a wheel that has a very very sexy aerodynamic profile and that profile it was known as a knack of profile so that the profiles are bringing this up is this the National Advisory Committee for Aeronautics it's basically the precursor to what is now matter in here the airfoils of well study so you've got lift coefficient of drag coefficient CD a CL all that kind of stuff and if you've got time and you understand that you understand what Reynolds number or what kind of company environment look at it but what I've done is I've taken this airfoil profile and made my wheel let's show you that so this is an accurate wheel that if I take a section through the NACA will you can see that it's a beautiful airfoil profile now commercially look the brand's bicycle frame brand and pail brand look use this profiling some of their bike frame and a blow tube for if you're interested that double low means it's a symmetrical airfoil and a two and four determine how thick it is and the point of which the maximum technically and you look it up on Wikipedia it's very well documented anyway this is a knack of profile it effectively modeling a 45 millimeter deep wheel Mavic tend to use this on their wheel rim they see almost a modeling of a Malick wheel I've done the same thing on this one so let's start at the bottom the flow characteristics on this wheel are a bit different now what happens on the bottom of this wheelie yes we have a small amount of what's known as compression vortex generation I wouldn't really get to press upon that because it's hardly anything and also this weight turbulence behind it again it's very very uniform and probably a limitation of the geometry and the CFD that I've used have you know made a lot of assumption here and to try and give you a model that will work as opposeed and spending a you doing all the simulation of the take a week so you know I'm taking the whole bouncing the spokes out and I haven't got some other little features in there on the same day right then what we can do is we can do the same thing so we can move this book so we're going book and we still got this micro vortex generation here again we haven't dissected the wheel at any point we are still attached we've got all of this happening over here but there's no voltages remember is disturbed and flow but no vortices and that's what we want to earn how we keep on going again still no voltage II while they were on the point of having more disease so we've got flow separation flow separation here and we have got flow separation because there's no material between these two points and we've got some turbulence as being generated in front of the wheel now this wheel doesn't have that sort of pinch point where the tie it meets the rim Carey I'm going now we have got some vortex generations with a Vortech generation here here's here and here the reason we have this vortex generation is because this aerofoil is basically into the wind backwards so as coming from the right hand side we've got some small vortex generation but from the trailing edge of this aerofoil you can see this there's no vortex generation there is closer brush with no vortex generation we carry on going bit higher this vortex here and here become more apparent and that's because again the parable is facing the wrong way now around here we have the start of vortex generation so that is can cause drag but if you look very very long and slender and it's also in the direction the wheel travels in not going out at 90 degrees so that's much more controlled carrying on going higher and higher again now you're looking to see if there's anything worth talking about now okay so now we're approaching center section of the wheel almost banging line we've got a fairly strong vortex there but again you can see it's in line and that's what you want you don't want it to be going up at 90 degrees keep going higher and higher okay now in the unfavorable section of the wheel because the wheel is now rotating in the oncoming direction of the wind and okay we haven't got much more text generation there book over here we're now starting to see vortex generation that off to the side of the wheel there are things can only get worse from here and showing up they do so we've got massive vortex generation here in here and here here everywhere of there is the more turbulent fact that having the front and keep doing the business zoom out we're going higher and higher and higher so in this region the aerofoil profile is very very much longer than it was originally anticipated including or Texas have diminished somewhat obviously of the trailing edge we still have that and we are backing to the region where we have no free space between the front and the back of the wheel fish and one unit keep going we are at this point here at this point here what we have is a skim off the top of the tire I see is massive vortex generation over here so that's where you've a basically bicycle frame will be about here so the flux will be right here and that would be hitting it and there's that almost what you would call a Helmholtz vortex so that concludes this kind of CFD bit of the video so summarize in this video you've learnt quite a lot first thing to consider is from the airlines around tire and wheel combination and that is the top of the tire wheel generate vast majority the drags and certainly the top part of the wheel type generate the vast majority of the total drag the bottom of the tire generates various drugs because the relative velocity is very small the tyre data which is very important and especially this little groove in here you take the groove out there is a measurable advantage now the you see is you found magic from putting strips in there to improve the air flow but if you're just on an evening time trial and you put some silicone or something like that in there just to bridge the gap and I think the marshals will say very much the rim depth the rim depth has far more effects on and any sexy toroidal shape or in a aerodynamic profile if you are going to go for their own arm profile I recommend an ocular profile and NACA profile especially Maddock wheels tend to use that this kind of dimpled surface let's put on their wheels it increases the skin friction coefficient but they designed to reduce the pressure dress do I think it work non chance absolute rubbish and think it marketing crap I wouldn't go with that I really hope you enjoyed this video I'm adding some questions to ask below if you want a question use the comment box below I'll do my best to answer them thank you very much for watching until next time

Channel: Hambini

Views: 131,770

Rating: 4.9108591 out of 5

Keywords: Aero bike wheels, Zipp Wheels, Enve Wheels, Enve, Mavic, GCN Wheels

Id: BUih_emc54M

Length: 31min 38sec (1898 seconds)

Published: Sun Mar 19 2017