johnc Posted October 31, 2011 Share Posted October 31, 2011 I don't understand how braking force would not be transmitted mostly through the TC rod. I'm not talking torque here but the weight of the vehicle. Braking force is only torque. Just like acceleration loads are only torque. Those loads are transferred to or from the chassis via the suspension links. And ALL of the weight of the sprung mass of the vehicle is supported by the springs. How could it be otherwise? Quote Link to comment Share on other sites More sharing options...
johnc Posted October 31, 2011 Share Posted October 31, 2011 What would you say the force is on that rod under hard braking? Say the strut sees 600lbs of compression, what would you estimate the TC rod sees at the same instant? 300lbs? Not that much and there's no relationship between the two numbers. The TC rod is a suspension link and only sees loads from locating the spindle. If the TC rod is broken the LCA can still be reasonably located with the anti-roll bar end links or even poly LCA bushings. That shows that the fore/aft loads are fairly low. There are no vertical loads on the TC rod. Quote Link to comment Share on other sites More sharing options...
mark Posted October 31, 2011 Share Posted October 31, 2011 (edited) I recently went off at VIR. The car flew at least 30 feet. Landed nose first. I had Arizona Z car tubular control arms (old style) on the front of the car. The control arms broke The tie rods bent One billet aluminum steering knuckle bent The stock cross member cracked The TC rods held in place by one bolt were fine. Not even bent. I was fine also. Edited October 31, 2011 by mark Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 31, 2011 Share Posted October 31, 2011 Correct. And that does show that the TC rod primarily performs a locating function and doesn't support a large braking load. If an anti-roll bar end link with a class 8.8 10mm bolt can keep the LCA located or poly LCA bushings can keep the LCA relatively in place under braking then then fore/aft loads must be low. Depends on how hard you press on the brakes. Most people, once they realize that there is a problem and especially if they have an idea of what is wrong, do not jump on the brake pedal. I think you're seriously underestimating the forces at work here. Braking produces g forces through the tires. So if you slow a 2500 lb car at 1 g, there is 2500 lbs of forward force that is being resisted at those tires. How is the g force getting to the tires? Through the suspension links. What part of that force goes into the TC rod on a Z? I don't know, but it does seem best positioned to resist the force of the chassis trying to push the wheel fore or aft. If we assume that 80% of the braking power comes from the front wheels, that means that 1000 lbs of force is going to each front tire. My SWAG: 850 lbs? 900 lbs? Once you realize that there is a problem and stop at .2 g, you're probably putting a couple hundred lbs of force on there max, and if you've been outside the car and seen it, the front tire does move all over the place even at slow speeds with no TC rod to hold it in place. Can a poly end link hold 900 lbs of force? It better. I had calculated my sway bar rate at upwards of 350 in/lbs. Quote Link to comment Share on other sites More sharing options...
zero Posted October 31, 2011 Share Posted October 31, 2011 Braking force is only torque. Just like acceleration loads are only torque. Those loads are transferred to or from the chassis via the suspension links. And ALL of the weight of the sprung mass of the vehicle is supported by the springs. How could it be otherwise? Correct me if I'm thinking about this wrong but here's my take on what 30 ounce means. There certainly is quite a bit of torque applied to the suspension when you brake, but that isn't all of the force being applied to the suspension. I think of it as a pulley, you have two torques being applied, at the tire and at the rotor/suspension, but in order for those torques to be applied, the wheel has to remain in the same place relative to the forces. What 30oz is thinking, I believe, is that the locating force isn't insignificant. Sure, the ARB and your tie rods might keep you on the track, but there are forces other than the torque to consider. Consider the case of an IRS with inboard brakes. In this case the torques are not being taken through the suspension, but I think we could all agree that you'd still have lateral force in the suspension under braking, and you certainly would want a semi longitudinal member to take that load. Quote Link to comment Share on other sites More sharing options...
Leon Posted October 31, 2011 Share Posted October 31, 2011 Braking force is only torque. Just like acceleration loads are only torque. Those loads are transferred to or from the chassis via the suspension links. And ALL of the weight of the sprung mass of the vehicle is supported by the springs. How could it be otherwise? Wha? Draw a free-body diagram of a car under braking and there will be more than a torque on the wheel. Brake pads generate a torque on the rotor which is directly connected to the tire. This torque produces a force proportional to wheel radius (lever arm). You must look at the car as a whole. The strut picks up vertical loads, meaning the springs compress because of load transfer caused by brake forces at the tires having leverage on the vehicle's CG. The T/C rod picks up longitudinal loads during braking. Moments produced from the braking forces are reacted by the TC rod, control arm and strut top. Quote Link to comment Share on other sites More sharing options...
johnc Posted October 31, 2011 Share Posted October 31, 2011 I think we are all saying the same thing in different ways. The way I look at it is: the brake caliper is trying to rotate forward on the spindle as a torque load when the brakes are applied. This torque is resisted by the strut (shock, strut tube, top mounting), LCA, and TC rod. EDIT: Are some people thinking that braking loads are the same as if the front tire hits a curb while not under braking? Quote Link to comment Share on other sites More sharing options...
zero Posted October 31, 2011 Share Posted October 31, 2011 Well, the way I'm thinking of it is that the caliper is trying to rotate forward and is resisted in the way you describe. But I think the wheel is also trying to move rearward. I'm not thinking it's the same as hitting a curb, but there is a component of force that is similar to a tire hitting a curb Quote Link to comment Share on other sites More sharing options...
EvilC Posted October 31, 2011 Share Posted October 31, 2011 (edited) Haha, I have not heard that term since school - free body diagram. Check my logic here, I also think some people are saying the same thing in different ways and others are thinking on another page: Take the 2500lb Z car.......going 70mph.....slam on the brakes, at that moment let us all try to agree on what is taking place. The nose of the car takes a dive. What are the G forces there? I think we can agree that all of the natural forces due + braking is not in a perfectly vertical line. Forget the calipers for a second, would it be correct to say that the top load lets say the strut mounts is trying to rotate anit clockwise from the contact patch of tire on the ground? Some of the cars forces (80%)? will be absorbed by the spring/strut but where does the rest of those forces go. Some I believe has to be translated through the T/C rod. In my minds eye, the T/C rod is compressed and twisted during hard braking. After a few cycles of that well we see what happens. I am not sure if we can put #s on the loads but design here has a major part to do with the failure. I have not been around Z cars for yrs and yrs and yrs but do any of you older guys have pics of failed NISSAN STOCK t/c rods or front control arms? Kinda interested to know what they ran back in the day. Edited October 31, 2011 by EvilC Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 31, 2011 Share Posted October 31, 2011 Take the 2500lb Z car.......going 70mph.....slam on the brakes, at that moment let us all try to agree on what is taking place. The nose of the car takes a dive. What are the G forces there? I think we can agree that all of the natural forces due + braking is not in a perfectly vertical line. Forget the calipers for a second, would it be correct to say that the top load lets say the strut mounts is trying to rotate anit clockwise from the contact patch of tire on the ground? Some of the cars forces (80%)? will be absorbed by the spring/strut but where does the rest of those forces go. Some I believe has to be translated through the T/C rod. In my minds eye, the T/C rod is compressed and twisted during hard braking. After a few cycles of that well we see what happens. I am not sure if we can put #s on the loads but design here has a major part to do with the failure. I have not been around Z cars for yrs and yrs and yrs but do any of you older guys have pics of failed NISSAN STOCK t/c rods or front control arms? Kinda interested to know what they ran back in the day. Weight transfer is, as I said before, a side effect of g forces and CG height. If you could design a car that ran on a track with a big ditch underneath the body of the car, you could suspend a weight at or below the hub centerline and you could get 0 or even rear weight transfer under braking. The g forces cause the weight transfer, the weight transfer (and so the spring and strut) doesn't absorb the g forces. Failed stock TC rods. I have yet to see one that isn't bent. Seriously. Every single one I've ever looked at has been at least slightly bent. As far as failures go, both of the ones that I've seen fail and all of the ones on that scary tc failure thread were stockers that were run with poly bushings. Quote Link to comment Share on other sites More sharing options...
johnc Posted October 31, 2011 Share Posted October 31, 2011 All of the TC rod failure that I have seen or heard about were the result of: 1) Bending loads on the arms caused by lack of articulation in the chassis mount. 2) Impact loads from a wheel hitting a curb or a big pothole. In scenario #2 above I agree 100% that the TC rod takes the vast majority of the load. But that is not the same as loads induced under braking. Quote Link to comment Share on other sites More sharing options...
zero Posted October 31, 2011 Share Posted October 31, 2011 I'm pretty sure in most of the TC rod failures I have heard of, the driver experienced a STRONG geometry change under braking. All of the accounts indicated the car pulled HARD in the direction of the failure, implying the wheel was moving rearward in the car. This seems to mean there must be some significant rearward force on the suspension when braking. Frankly, assuming the strut doesn't break, there is really only so much geometry change that can happen from a torque effect. Also, we know that TC rods don't like bending loads and really aren't the right shape to resist them so it can't be contributing much to the torque resistance anyway. Yet, there is still the braking force/geometry change correlation. The TC rod/bucket/frame rail absolutely feel a significant compressive force under braking. I haven't experience a rod failure myself, but I have experienced a TC bucket failure, and the car pulled in a similar manner. Quote Link to comment Share on other sites More sharing options...
Leon Posted October 31, 2011 Share Posted October 31, 2011 (edited) I think we are all saying the same thing in different ways. The way I look at it is: the brake caliper is trying to rotate forward on the spindle as a torque load when the brakes are applied. This torque is resisted by the strut (shock, strut tube, top mounting), LCA, and TC rod. EDIT: Are some people thinking that braking loads are the same as if the front tire hits a curb while not under braking? This is why you have to look at the entire car, not just a section. Your example works for stopping a levatating spinning wheel. This brake torque is also sent through the tire and to the road, and this must be taken into consideration. The torque caused by the brake rotor generates a force at the contact patch which is opposite to the direction of travel. This braking force is what slows the car down, and it is what must be reacted by suspension links (EDIT: along with the other torques and moments). Weight transfer is, as I said before, a side effect of g forces and CG height. If you could design a car that ran on a track with a big ditch underneath the body of the car, you could suspend a weight at or below the hub centerline and you could get 0 or even rear weight transfer under braking. The g forces cause the weight transfer, the weight transfer (and so the spring and strut) doesn't absorb the g forces. Jon brings me to my other point. Weight transfer happens because of a braking force at the tire contact patch. If all you had were torques, then there would be no weight transfer! Brake forces at the wheels have leverage on the car, proportional to CG height. The taller the car's CG, the more leverage the brake forces have on the CG, the more weight transfer you have. I can draw and scan in an FBD when I get home today, if it will make this easier to understand. Edited October 31, 2011 by Leon Quote Link to comment Share on other sites More sharing options...
EvilC Posted October 31, 2011 Share Posted October 31, 2011 John I agree with scenario #2, the T/C rod takes the load there and fails. Seen that plenty of times (thank you nyc potholes). Jon M - thanks I have to go and look for that thread. The weight transfer may be a side effect of the braking g force but are you saying it plays little to no stress on the mounting point of the t/c rod? If the design was better, maybe we would see a bent t/c rod here. The mounting place on the bracket was the weak point. As far as the bending of the r/c rod lets play out a few scenario, would you see a bent t/c rod during: 1. A car standing still? (static) 2. A car accelerating? 3. A car decelerating? 4. A car doing figure 8s at a constant speed? I am trying to start to pin point this convo to when/where/why do we see t/c rods start to bend and possibly fail (stock). From there we can look/discuss the design and why it might have failed. Because I am thinking of the "track" use lots of these cars see and none are used in just a straight line. Quote Link to comment Share on other sites More sharing options...
johnc Posted October 31, 2011 Share Posted October 31, 2011 This is why you have to look at the entire car, not just a section. Your example works for stopping a levatating spinning wheel. This brake torque is also sent through the tire and to the road, and this must be taken into consideration. The torque caused by the brake rotor generates a force at the contact patch which is opposite to the direction of travel. This braking force is what slows the car down, and it is what must be reacted by suspension links (EDIT: along with the other torques and moments). I agree, the equal and opposite reaction thing. I'm not arguing otherwise. I agree. I just look at it the way I describe for suspension tuning reasons. You may not agree with how I look at it, so be it. Look at it your own way. Draw all the little pictures you need. I don't need them. And, back to the original point, a single bolted clevis is just fine for a TC rod attachment to the front LCA. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 31, 2011 Share Posted October 31, 2011 Jon M - thanks I have to go and look for that thread. The weight transfer may be a side effect of the braking g force but are you saying it plays little to no stress on the mounting point of the t/c rod? If the design was better, maybe we would see a bent t/c rod here. The mounting place on the bracket was the weak point. Weight transfer puts no stress on the TC rod, just like taking 100 lbs and laying it on top of the front crossmember puts no stress on the TC rod. The g forces created by braking do and the g forces create the weight transfer, but the weight transfer itself is unrelated to TC failure. Quote Link to comment Share on other sites More sharing options...
Leon Posted October 31, 2011 Share Posted October 31, 2011 I agree, the equal and opposite reaction thing. I'm not arguing otherwise. I agree. I just look at it the way I describe for suspension tuning reasons. You may not agree with how I look at it, so be it. Look at it your own way. Draw all the little pictures you need. I don't need them. And, back to the original point, a single bolted clevis is just fine for a TC rod attachment to the front LCA. I don't need to "draw all the little pictures" personally, I'm just trying to help others understand what's going on here. I agree with the way you look at it, but only for the purposes of analyzing a floating wheel. Sure, it may be good for something like upright design, but what else? Analyzing the suspension when you hit the brakes with the wheels off the ground? Fact of the matter is, the TC rod picks up a majority of the braking forces. There is no way around it. In order to properly design or "tune" a suspension, you must take into consideration the forces that exist at the tire contact patch. Without that, what's the point? 1 Quote Link to comment Share on other sites More sharing options...
RebekahsZ Posted October 31, 2011 Share Posted October 31, 2011 My head hurts. Lots of good bench racing going on, but what are we to conclude? That the brand of LCA that was bent is crap? If any of us are running that brand, should we replace them before we DNF in the middle of rush hour traffic? I've got TechnoToys LCAs based on JohnC's recommendation and they look like they could survive a nuclear holocost. I've got zccjdm.com TC rods on his recommendation, and on closer inspection, the clevis bolt is bigger than I thought. I'm gonna stick with what I've got. I've been 100% very happy with everything that JohnC has recommended for my car, and he ain't selfish: he often recommends products that he doesn't supply himself. Glad I'm in this club. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 31, 2011 Share Posted October 31, 2011 My head hurts. Lots of good bench racing going on, but what are we to conclude? That the brand of LCA that was bent is crap? If any of us are running that brand, should we replace them before we DNF in the middle of rush hour traffic? I've got TechnoToys LCAs based on JohnC's recommendation and they look like they could survive a nuclear holocost. I've got zccjdm.com TC rods on his recommendation, and on closer inspection, the clevis bolt is bigger than I thought. I'm gonna stick with what I've got. I've been 100% very happy with everything that JohnC has recommended for my car, and he ain't selfish: he often recommends products that he doesn't supply himself. Glad I'm in this club. The TC rod was incidental in this particular failure. This failure was caused by the lateral loading of the control arm, probably with some bending introduced by the sway bar at the same time. I would conclude via the long history of clevises being used on Z car TC rods and other race car suspension that the clevis is not a problem in and of itself. Quote Link to comment Share on other sites More sharing options...
jt1 Posted November 1, 2011 Share Posted November 1, 2011 In my opinion.... The control arm failed in bending due to the vertical force applied to the LCA by the sway bar. The plate was notched for the tube to be inserted and welded, and right at the end of the notch the section goes from very large to very small, near the middle of the arm, with a force applied near the minimal section. Bad design. If the TC rod doesn't transmit most of the braking force to the chassis, I don't know what else possibly could. The strut resists the caliper torque, but this force is applied to the chassis in a forward direction. The method of attaching the TC rod to the LCA is fine, and didn't contribute to the failure. jt Quote Link to comment Share on other sites More sharing options...
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