JMortensen Posted December 7, 2006 Share Posted December 7, 2006 Maybe they just don't know the answer... (that ought to flush them out!!!) Quote Link to comment Share on other sites More sharing options...
cygnusx1 Posted December 7, 2006 Share Posted December 7, 2006 My post has already explained what REALLY happens. I can even give you diagrams and actual force numbers but I don't have the time right now to put it together. I am a trained Mechanical Engineer and have worked on cars and designed automation machinery for over 12 (edit: 15..i am getting old) years. The caliper is not pushing up down or sideways on any strut or suspension member. It can't, the caliper is bolted to the strut. Here is an excersise that can help you understand: Hold one arm straight out in front of you. Take your other arm and push down on the arm that is straight out. Is it making you fall forwards? Answer: No. Why not? Quote Link to comment Share on other sites More sharing options...
JMortensen Posted December 7, 2006 Share Posted December 7, 2006 I think I just figured out how to prove Cygnus right. Pull the ebrake and step on the gas and load the suspension. I remember my auto shop teacher did this in high school with a 280ZX. Semi-trailing arm suspension IS affected by braking, that was his point. He kept putting it in drive, then reverse, and the back end of the car was lifting up and setting down. It was pretty funny actually. I don't believe that our strut suspension is going to move at all, other than bushing flexing and that sort of thing. Quote Link to comment Share on other sites More sharing options...
cygnusx1 Posted December 7, 2006 Share Posted December 7, 2006 That movement in the suspension you saw JM, was force from the axles. They are an "outside" force and have nothing to do with the brake. It would do the exact same thing no matter where the caliper was mounted on the clock, or if you wedge a stick in the spokes of the wheel against the strut tube. I think people have a generaly difficult time mentally separating forces that are coming from the contact patch with the internal forces of the brakes. Yes, when you brake, the nose of the car dives, the TC rod gets compressed, a trailing arm would want to lift up, but NONE of these forces are coming from the caliper. They are from the contact patch loading. Quote Link to comment Share on other sites More sharing options...
jt1 Posted December 7, 2006 Share Posted December 7, 2006 posted by cygnusx1 I am a trained Mechanical Engineer and The caliper is not pushing up down or sideways on any strut or suspension member. It can't, the caliper is bolted to the strut. Huh? If the caliper doesn't apply any force to the strut, why bolt it on to start with? What resists the force of the pads friction with the rotor? John Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted December 7, 2006 Author Share Posted December 7, 2006 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 Quote Link to comment Share on other sites More sharing options...
JMortensen Posted December 7, 2006 Share Posted December 7, 2006 This is, I'm sure, what my vague recollection at the start of this thread was about. Brake jacking in single pivot mountain bikes explained (if you care): http://forums.farkin.net/showthread.php?t=35572 I've read about the floating brake solution and I still don't understand how or why it works... (?) I don't think this has anything to do with caliper placement at all, and I think Cygnus is right that the ZX would do the same thing regardless of where the caliper was placed. I don't know why Nino with the Ducati was convinced to relocate his calipers. Still haven't decided 100% that he was full of it, but I'm getting there. I'm thinking that this is not a factor in our Z suspensions because the control arm mounts are perpendicular to the braking force. Quote Link to comment Share on other sites More sharing options...
cygnusx1 Posted December 7, 2006 Share Posted December 7, 2006 I am only trying to explain. It is tough to explain obviously but when I know something, I wan't to pass on my knowledge. When I don't know something I listen. This time I am trying to make people understand. The main thing to understanding this is that the caliper AND the spindle are both the same body. The caliper is bolted to the strut housing and the spindle is welded to the strut housing. All of the forces applied to the brake pads are transferred to the strut housing through the bolts. The wheel wants to pivot around the brake pads when squeezed, but it can't because the spindle holds it in place. The force of the wheel against the spindle exactly counteracts the force of the caliper. This cancellation happens all inside the wheel-struthousing-caliper relationship. The strut itself (the piston) sees none of what is going on. I am not trying to be a d*ck. I just want to make it believable and understandable. Quote Link to comment Share on other sites More sharing options...
tube80z Posted December 7, 2006 Share Posted December 7, 2006 Cygnusx1 I think you did a good job describing what happens. Ortiz has an article about this I'll try and find. The reader had a similar question and he determined it didn't matter where the caliper was mounted. Cary Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted December 7, 2006 Author Share Posted December 7, 2006 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 Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted December 7, 2006 Author Share Posted December 7, 2006 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: The "direction" of the jolt will be determined by the position of the caliper on the clock.. Dave Quote Link to comment Share on other sites More sharing options...
zguy36 Posted December 7, 2006 Share Posted December 7, 2006 As I said before, caliper placement can have no effect on the compression or extension of suspension. The first thing they teach engineers is to draw a free body diagram, label all of your parts and all of your vector forces. If you do this, then you will see that all forces are counteracted inside of the braking system (caliper, hub, knuckle) As for caliper placement on the rear swingarm of a motorcycle, this is a different story. There will be an effect of placement, but in a much simpler manner. The caliper has weight, so the closer you put this weight to the pivot of the swingarm, the less equivalent unsprung weight you have. Quote Link to comment Share on other sites More sharing options...
zguy36 Posted December 7, 2006 Share Posted December 7, 2006 I just had a thought, and maybe this is where some confusion in coming into play. There are forces exerted on the strut assembly from the caliper, but these forces are a moment. This moment however, acts on the centerline of the spindle. It is the caliper mounting brackets and the knuckle that bring this moment to the centerline. So the resultant force on the strut is trying to twist the whole assembly in the same direction that the wheel is spinning. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted December 7, 2006 Share Posted December 7, 2006 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)? The control arm outer pivots and the strut top are what prevent the suspension pivoting. But they're trying to stop a rotating force that comes from the tires hitting the pavement. If that force didn't exist, the brakes wouldn't be doing any work at all, right? If the suspension pivots are inline like on the bicycle or a motorcycle, then the rotating force coming from the tire can affect the suspension. If the mounts are perpendicular, then the force coming from braking will stress the mounting points in a twisting manner, but it won't compress the strut. Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted December 7, 2006 Author Share Posted December 7, 2006 I just had a thought, and maybe this is where some confusion in coming into play. There are forces exerted on the strut assembly from the caliper, but these forces are a moment. This moment however, acts on the centerline of the spindle. It is the caliper mounting brackets and the knuckle that bring this moment to the centerline. So the resultant force on the strut is trying to twist the whole assembly in the same direction that the wheel is spinning. That is exactly how I understand it. Dave Quote Link to comment Share on other sites More sharing options...
rudypoochris Posted December 7, 2006 Share Posted December 7, 2006 This might be a dumb question... but how is one going to bleed it? Detach the caliper and then retach??? Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted December 7, 2006 Author Share Posted December 7, 2006 Jon, I understand what you are saying about a twisting moment, but I disagree that it won't compress the strut. As I said: 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.. 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 Quote Link to comment Share on other sites More sharing options...
cygnusx1 Posted December 7, 2006 Share Posted December 7, 2006 Originally Posted by zguy36 I just had a thought, and maybe this is where some confusion in coming into play. There are forces exerted on the strut assembly from the caliper, but these forces are a moment. This moment however, acts on the centerline of the spindle. It is the caliper mounting brackets and the knuckle that bring this moment to the centerline. So the resultant force on the strut is trying to twist the whole assembly in the same direction that the wheel is spinning. New Quote: That is exactly how I understand it. Dave And that "twisting moment" will be there, cancelled by the same exact vector, no matter where the caliper is mounted on the clock. Quote Link to comment Share on other sites More sharing options...
olie05 Posted December 7, 2006 Share Posted December 7, 2006 All the forces that the suspension sees and that are large enough to effect handling are from the tire contact patch and the cars center of gravity acting on the suspension, NOT the caliper. This is the only bit of information you need to understand the whole situation here. The only way that the caliper could directly jack the car up or pull it down is by mounting the caliper to the sprung assembly, and still have it act on a rotor attached to the unsprung hub. This turns the caliper into an external force, as seen in the 280zx example Jon mentioned. Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted December 7, 2006 Author Share Posted December 7, 2006 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 Quote Link to comment Share on other sites More sharing options...
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