thehelix112

Hey, The better half got me a stencilling kit for my Birthday over a year ago. This is the first thing I've done. Excuse the mobile-phone-quality pic. Dave

Logan, That is not at all how I understand the function of vortex generators. A vortex generator does not create turbulent flow (when I say `turbulent flow' I am basically referring to sporatic reversion) it creates flow in the shape of a vortex/helix. Essentially the same shape as the groove in a drill bit. This simply adds energy to the flow stream by accelerating it. Ie, instead of taking the shortest path between two points, the flow takes a diagonal line, but it must cover the same longitudinal distance in the same time, hence it accelerates. This increased inertia can then be used to flow up and unfavourable pressure gradient, such as is found on the rear windscreen. We can discuss how the wing would compare with a vortex generator: Wing will induce much more drag/lift on its surface. Vortex generator does not induce any lift. Wing is much more likely to have a significant influence on separation point down the rear window. What I am also interested in is how the wing compares to not having the wing at all. Since its on a competition car (WRC) one would assume this is favourable, but lets have a ponder on it shall we. I don't believe you ever get anything for nothing, and I think that an important reason that wing is there is that its moment about CoG is much smaller than the main wing's. Thus although it is inducing lift in order that the rear wing can induce more negative lift, I believe that the rear wing's moment about CoG is larger, and hence it has a positive overall effect on downforce. As for drag, I am not familiar enough with CFD equations to be able to guestimate whether the larger drag pressure over the small wing area would be larger than the reduced drag pressure over the larger rear windscreen. Can anybody help on that one? Dave

Cary, The topic does clarify what I am interested in, and yes, I am sure it will have an effect on suspension, but thats not what I am primarily concerned with. I am not talking about compliance. I am talking about how (note: not how much, I know it will be insignificant) the forces are applied from the tyre to the chassis, and how they change with caliper mounting position. Does that help? Dave

Cary, What I am talking about is how the force applied by the tyre affects the strut assembly, and then distinctly, how this affected strut assembly affects the chassis. Dave

cygnusx, Acknowledged, but is that the way the forces would be applied? Dave

I am glad we all agree that the position of the caliper is irrelevant to its longitudinal effect on suspension. Now back to my original post on the lateral effect; a caliper that has been clocked lower will lower the centre of mass of the strut assembly. I believe this will affect how the lateral force from the contact patch gets translated to a moment on the strut assembly. Ie, if the centre of mass is exactly at the stub axle (spindle) level then no moment will occur and the force will be transferred directly to the ball-joint/LCA and strut tower. If the centre of mass is above the spindle, there will be a moment attempting to rotate the strut top in the opposite direction of the turn. For example, the car is turning left, therefore the contact patch/wheel/spindle are exerting a force left. This force is below the spindle, so the strut will see a moment that is attempting to move the strut top right, and the ball-joint left. Once again I fear I need to either re-write that, or draw a diagram, neither of which I have time for. I hope you get the drift of what I am asking (for clarification on). Dave PS. I realise there are many more significant things to consider when tuning a car, and that considering this is perhaps less than supremely practical. I still want to understand whats going on more fully.

Fly Wheel Horse Power zprace. Dave

Not quite. I am thinking the strut is angled forwards, thus any force perpendicular to the strut axis will have a vertical component. Unless the car is going to move, this vertical component will be reacted by the strut tower and the suspension will compress. No? Dave PS. Thanks for bearing with me.

Ok I am still obviously not completely understanding this. In that case that the strut has positive castor, won't the twisting moment cause an effective change in length due to the vertical component of the end of the struts arc of movement? Damn I need a diagram. If that makes no sense I'll draw one. Dave

Yay. Looks like we're all on the same page, I may have started reading from the back of the book (and hence have no idea whats going), but atleast we're there. The next question cygnusx, is will this twisting moment cause ANY suspension movement (however slight it may be). Dave

Jon, I understand what you are saying about a twisting moment, but I disagree that it won't compress the strut. As I said: . Ie, when the strut is `twisted', the end of the strut ends up being lifted (by 1-sin(90 deg - castor) I believe). I stand to be convinced, but I believe that in order for the strut top to stop the strut from twisting under this moment, the reaction force vector (applied as a moment about the spindle) will have a downwards component. Dave

That is exactly how I understand it. Dave

I would just like to point out that I agree that it makes no difference where the caliper is mounted, which is not how I read cygnus's first post: . Dave

Cygnus, I apologise if you thought I was inferring you were being a dick, I greatly appreciate you taking the time to explain something to me. Honestly. I also think your explanations are in-depth and interesting. I do see what you are saying, and I do understand that the spindle (as in stub axle?), strut housing and caliper are all the one body. But the disc/wheel is not part of the same body, and this is where the force is coming from. If you had a bare strut in free space and you were rolling it along. Say someone put the brake on, are you telling me that the strut assembly will NOT try to flop forwards about the spindle? It seems logical to me that it would. Ok another question/thought, you say that the wheel will try to pivot around the brake pads. But as the wheel is attached to the road, what is to stop the brake pads (and caliper and strut) from pivoting around the wheel (spindle)? Dave

cygnus, Great analogy. Thinking about it like that almost makes sense, but I have a counter-analogy ( ), I have a bottle of milk in one hand and I am swinging my arm around in a vertical circle (up behind me and down in front). If I move my arm in front of my legs as it comes down and smack myself in the knees, would I not be knocked onto my face? I appreciate that you are a mecheng, thats great. Personally I believethe true test of knowledge on a subject is being able to explain it to someone less knowledgeable. And how long you've been doing anything I find irrelevant to the discussion, if someone has a valid point, they have a valid point, end of story. Thats the beauty of the internet, it lets me (the PhD CompScI fool that I am) annoy educated engineers like yourself into explaining things I don't understand. Just so we're clear, I am unlikely to ever get offended by a post on the Internet, and I do not intend to offend anyone else. Dave

It seems to me that you will NEVER have just a force unless it is being applied at exactly the centre of mass. If you apply a force anywhere else, you generate a moment about the object's centre of mass. Simple analogy: Get a pen on your desk right now and place it vertically away from you, flick it from the side at approximately the middle. The whole pen moves. Now flick it at one end. The whole pen spins about its centre of mass. This is exactly what is happening with our struts. For the `flick' given to the strut (about its centre of mass) not to rotate the strut, it must be reacted by something else capable of exerting a moment on the strut (strut tower, ball joint/LCA, castor rod, steering arm). Now the interesting part is that as far as these `something elses' are concerned, they are exerting only a force, which then becomes a moment using radius as the distance from application to the strut's centre of mass. Now here is where I get fuzzy, and is why I started this topic, this means that as far as the strut tower is concerned it is only exerting a force vectored in the opposite direction to that applied by the caliper. If the car is rolling forwards then the moment applied by the brakes to the strut is going to be anti-clockwise (when viewed from the drivers side), this means that the reactive force will be clockwise, which means for example, the strut tower will be pushing backwards/down? Bah, I am always open to correction, but I need more convincing that the brakes are no generating a moment on the strut about its centre of mass, which seems to be the main point cygnus and jt1 are making. Guys? Dave

Primadonna, You make some good points. I have some questions though, is the black on the sides behind the front wheel and infront of the rear wheel functional? And more importantly is the one behind the front wheel a vent? I can't tell from the pics. It would seem pointless to add the small wheel well vent at the top when the place strip is exactly where it should be. There are always a number of improvements to any design that can be made, and the primadonna Z is no exception. Not bad for 30 years ago though? Dave

Very interesting pics there Sven. Dave

Correct me if I am wrong (obviously I don't think I am), but vortex generators are used to delay flow separation, just as that flap will. True, but that would seem to defeat the purpose of putting it on there. My main question to this is, wanna test my race car in your wind tunnel after I've built it? Dave

Diffusers built into the wing? Where? I see lots of vertical strakes which appear to be a lot more focussed towards the spoiler surface than the wing. Dave

Jon, I've read the thread (and also contributed). As I said, this is not a serious concern for me, and definitely not worth my time (or anybody elses most likely) to real-world test. What I asked, I did so from a theoretical design perspective. I agree it most likely serves the same purpose as the vortex generators, but achieves the desired effect in a very different way. I can't help but feel that unless you spent a long time tuning such a flap and a very short time tuning the vortex generators, that the VGs provide a neater/cleaner/more efficient solution. Dave

I was walking down Lincoln Blvd in Santa Monica to get myself a coffee when I noticed this WRX. The small flap is designed, I believe, to redirect air down the rear windscreen, doing two things. 1, reducing the drag caused by the rear windscreen (at the expensive of drag caused by the flap itself), and 2, to increase the amount of air that flows over the rear wing. I'd been mulling over this idea on and off, but in my case, there is no reason why I wouldn't just move the wing up into a clearer stream. My questions on the flap are as follows: 1. Is it likely(possible?) that the additional downforce created by having more flow to the rear wing would negate the additional lift caused by the flap? Is this likely because the wing is further back and will hence have a larger moment for the same pressure than the flap? 2. Is it likely(possible?) that the additional drag caused by the flap would be less than the drag reduction caused by seperation of flow from the rear windscreen? Thanks, Dave

jt1, That was a typo, I fixed it to say `castor'. Can you explain why you think the caliper will exert a force rather than a moment? Are you thinking along the lines of centripetal forces, as thats what it sounds like, but I don't see how it applies. Dave

jt1, As zguy has said, the caliper only exerts a moment on the strut assembly. What could be happening however, is that due to the castor of the strut tube, the strut tower/body is required to exert a force that is not perfectly horizontal to cancel this moment. If you have positive castor, then the moment on the strut will be attempting to lift the end of the strut, fairly clearly this will result in compression of the suspension. Can anyone see a flaw in my thought process? That the moment on the strut caused by the caliper is likely to result in suspension compression seems (to me, at the moment) undeniable. What IS questionable however, is whether the location of the caliper on the disc changes this suspension compression. I can't think of a reason why it would on strut suspension. Dave

Application of braking force: not too sure what you mean there, will have to wait for the article unless you can remember and be bothered expanding? Inertia: Depends what sort you are talking about. It will definitely affect the moment of inertia to turn the strut(wheels). This will be at a maximum at 3/9 o'clock + degrees castor, and at a minimum at 12/6 + degrees castor. I can also imagine it affecting the inertia of suspension travel as it affects the centre of gravity/mass of what has to move, but my brain is a little fuzzy this morning. I look forward to you finding said article, and on smarter brains than mine chiming in. JohnC? Dave