TimZ Posted February 10, 2007 Share Posted February 10, 2007 Torsion bars, anti roll bars are long bars/tubes. Torsion bars and anti roll bars are being used as springs, not structural members. This is not germane to the discussion. Quote Link to comment Share on other sites More sharing options...
Guest Mike Posted February 10, 2007 Share Posted February 10, 2007 Yes... Those are designed to twist and bend to give a little bit. You wouldn't want these solidly locked... at least not on a street-driven car. That would hurt a little:mrgreen: Torsion bars and anti roll bars are being used as springs, not structural members. This is not germane to the discussion. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted February 11, 2007 Share Posted February 11, 2007 I gotta disagree Tim. Cages twist, and the round tubes' ability to attach at any angle and twist without deforming is what makes it particularly well suited to roll cages. This is kind of tangential to the discussion, but I think you're starting to edge nearer to a round tube vs square tube debate. My understanding of tubing and it's strengths and weaknesses is this (please correct me if I'm wrong): A single bar like a strut tower bar isn't (or shouldn't be) used to take a twisting load. It's going to take a pretty simple tension/compression load. It would be strongest if welded, but that makes it tough to do valve adjustments and pull heads or engines, so it's usually necessary to bolt it in. A roll cage structure is different. The structure of a cage can and does twist. The whole idea is that you want to tie the cage in enough different planes to enough structural members so that the torsion spring effect of the cage will minimize the twisting, and you want the tubes to resist deforming in the event of an impact. You might not think of it as a bunch of torsion springs all tied together, but that's really what it is. For that purpose, round is superior to square, because square tube has less yield strength to twisting and permanently deforms from twisting or from impacts at odd angles much more easily. On a single bar that needs to resist bending at set regular 90 degree angles, square tube is better than round. The square or rectangular tubes we see in a ladder frame which attach at regular angles to each other are stiffer in bending than a similar round ladder frame would be. Round tubing handles compression better than square. Round tubing also bends MUCH better than square unless you've got some really expensive bending equipment. http://coloradok5.com/forums/showthread.php?t=87627 Quote Link to comment Share on other sites More sharing options...
TimZ Posted February 11, 2007 Share Posted February 11, 2007 I gotta disagree Tim. Cages twist, and the round tubes' ability to attach at any angle and twist without deforming is what makes it particularly well suited to roll cages. This is kind of tangential to the discussion, but I think you're starting to edge nearer to a round tube vs square tube debate. My understanding of tubing and it's strengths and weaknesses is this (please correct me if I'm wrong): I guess I'm not sure exactly what you were disagreeing with. I thought I was talking about appropriateness of different mounting techniques for a strut bar. Perhaps I wasn't clear enough - sorry. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted February 11, 2007 Share Posted February 11, 2007 Just to put a finer point on this - you don't want to put anything but tension/compression loads on the strut bar (or any long tube for that matter). Trying to control other motions will never be effective, and will eventually fatigue the mounting points. I should have quoted you I guess. I was disagreeing with this statement. A cage will have some fairly long spans and works by attempting to prevent twisting through the structure. Quote Link to comment Share on other sites More sharing options...
driftz240 Posted February 12, 2007 Share Posted February 12, 2007 Holy God 260det Is That A Fj20det? Quote Link to comment Share on other sites More sharing options...
lbhsbZ Posted February 12, 2007 Share Posted February 12, 2007 I'm just unclear as to why anyone would use heim joints to mount anything that isn't supposed to move. Heim joints are low speed bearings for stuff that moves. If a bar is mounted with heim joints on each end, I think it would make the bar less resistant to bowing under load than if a non-pivoting type mount were employed. Quote Link to comment Share on other sites More sharing options...
johnc Posted February 12, 2007 Share Posted February 12, 2007 It's not completely unheard of for someone to differ with John, even if he is right 99.9999999999999999% of the time. If that was my percentage, I would be running a car at Le Mans instead of an autocross at Cal Speedway. Now mine weren't centered the way John's and BJ's were and didn't straddle the 3 piece of the cowl's triangular structure the same way , I think the difference might be in what was used to reinforce/span the cowl box on my old car. It was a .095" thick 4130 plate with four .080" 4130 vertical ribs. That structure itself won't flex under the loads from the strut towers. The load from that structure was spread out over a square foot and tied into the cowl box. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted February 12, 2007 Share Posted February 12, 2007 I'm just unclear as to why anyone would use heim joints to mount anything that isn't supposed to move. Heim joints are low speed bearings for stuff that moves. If a bar is mounted with heim joints on each end, I think it would make the bar less resistant to bowing under load than if a non-pivoting type mount were employed. This doesn't make sense to me. Why would a bearing make at the end make the strut tower bar more prone to bending under loads? Do you think that a suspension link that has heims joints is similarly prone to bowing under a load? How about a tie rod? Let's take it to the extreme. Do you think that the suspension link on a Baja 1000 truck is more likely to bend in the middle because it has heims joints at the ends??? Quote Link to comment Share on other sites More sharing options...
katman Posted February 12, 2007 Share Posted February 12, 2007 A straight bar with heim joints at both ends would be LESS prone to bending than one welded to the towers, because by definition there are no induced moments. It would be what we call a "two force member". Jon is right, for a straight bar. Quote Link to comment Share on other sites More sharing options...
lbhsbZ Posted February 12, 2007 Share Posted February 12, 2007 If the suspension components move, then heim joints are required. Look at it this way. If you have your strut bar mounted with hiems, grab the middle of it and bow the bar. When you do this, you can watch the heim joints move. The angle between the bar and the strut tower will change when the bar is bowed, and the heim joints don't resist the angle change. I think that with a solid mounting plate welded to the bar and welded bolted to the strut tower, it would resist the angle change more than a heim joint, and the therefor resist flexing or bowing the bar. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted February 12, 2007 Share Posted February 12, 2007 That's the whole point. The strut tower shouldn't be taking loads in the middle of the bar. It should be taking tension and compression loads, and the heims joints don't allow any movement in that direction. If the bar is taking other loads then what that means is that the chassis needs further reinforcement in OTHER areas. Quote Link to comment Share on other sites More sharing options...
TimZ Posted February 12, 2007 Share Posted February 12, 2007 That's the whole point. The strut tower shouldn't be taking loads in the middle of the bar. It should be taking tension and compression loads, and the heims joints don't allow any movement in that direction. If the bar is taking other loads then what that means is that the chassis needs further reinforcement in OTHER areas. I think that was all I was trying to say in the first place. Quote Link to comment Share on other sites More sharing options...
Guest Mike Posted February 13, 2007 Share Posted February 13, 2007 A straight bar with heim joints at both ends would be LESS prone to bending than one welded to the towers, because by definition there are no induced moments. It would be what we call a "two force member". Jon is right, for a straight bar. :mrgreen: :mrgreen: CORRECT!!!!! :mrgreen: :mrgreen: !!!!!!YOU GET A CEEE-GAR!!!!!! Quote Link to comment Share on other sites More sharing options...
bjhines Posted February 13, 2007 Share Posted February 13, 2007 You can't make analogies that don't represent the real conditions... Your chassis does not "grab the middle of the bar and bow it"... In fact... by looking at this in that way... You are comming up with exactly the opposite of reality for a straight tube... I see your thoughts on this.. I understand that you figure a rigid structure is always stronger than a hinged structure... But that is simply not the case... Look at large stadiums... The buttresses under the stands are commonly mounted with a large hinged joint... to aviod any tendancy to bend the supports.. which could cause them to collapse... If you must make a bend in the bar/tube... then you are getting into an entirely different situation... If the tube is bent.. then it will gain some strength from a welded/solid mounting... Quote Link to comment Share on other sites More sharing options...
lbhsbZ Posted February 13, 2007 Share Posted February 13, 2007 OK, Now that I think about it a little harder, that makes sense. A straight bar (with no bends) would not, under normal conditions, be loaded in such a way that it would bow in the middle. A bar WITH bends in it would. I have a question that maybe one of you can answer. I'll try to be as clear as I can but I'm not sure of the proper terms for this. A curved bar would be loaded so that it would bow, but a straight bar would be loaded so that it would crush should those loads become great enough (which they won't in a strut bar application). Is there a middle ground between crushing and remaining straight....kind of like the plastic state that aluminum goes into right before it melts? Or will the bar simply remain perfectly straight until it reaches its yield strength and then collapse? How would one calculate the yield strength of bars loaded in this manner? Since it wouldn't have the tendency to bow, then does that enable us to use extremely thin wall (light weight) tubing for the strut bar and other non-sanctioned chassis stiffening members loaded in this same fashion? Quote Link to comment Share on other sites More sharing options...
bjhines Posted February 13, 2007 Share Posted February 13, 2007 Read the rules for the class you plan to enter... Generally speaking... anything NOT attached to the cage is open game... Quote Link to comment Share on other sites More sharing options...
lbhsbZ Posted February 13, 2007 Share Posted February 13, 2007 Sorry BJ, I should have been more clear. I wasn't asking if we'd be able to do it to fit into the rules, I was asking if it would work and be halfway functional. I understand the characteristics of a straight piece of tubing with respect to compression and tension loads, but where is the limit? I'd still like to know how to calculate the yield load for a given tube under compression. Hell, If I can get away with using 1/16 wall 1 1/4 inch aluminum tubing for all my bolt in stiffening members that don't require bends, that seems like the way to go to save weight. Quote Link to comment Share on other sites More sharing options...
bjhines Posted February 13, 2007 Share Posted February 13, 2007 The best bet is to look at what is being used for similar weight and power cars... 1" OD seems pretty common... That is what I used... Swedged aluminum tie-rod tubes with lightweight aluminum rod ends would be great... but they will wear out(threads in tubes gall, aluminum rod ends may pound out in sizes less than 1/2")... sourcing aluminum jam nuts and gun-drilled, ball-milled, titanium attachment bolts would be ideal... Quote Link to comment Share on other sites More sharing options...
dimsum Posted February 13, 2007 Share Posted February 13, 2007 I have a question that maybe one of you can answer. I'll try to be as clear as I can but I'm not sure of the proper terms for this. A curved bar would be loaded so that it would bow, but a straight bar would be loaded so that it would crush should those loads become great enough (which they won't in a strut bar application). Is there a middle ground between crushing and remaining straight....kind of like the plastic state that aluminum goes into right before it melts? Or will the bar simply remain perfectly straight until it reaches its yield strength and then collapse? Wow...you made me dig up my old lab manual. Couldn't find my class notes, though, so I can't give you as much explanation as you might want. Even a straight bar will bend under enough load. It will not crush, like an aluminum can, even if the loads are perfectly aligned along the bar's axis. I cannot remember why this occurs, but it was demonstrated to me in my materials lab last semester. We loaded aluminum and steel bars 5/8" in diameter, from 6" to 21" in length, and they all bent. None of them "crushed." Once the bars reach their yield strength and begins to bow (or deform), though, the amount of load they support either remains constant or even decreases a little bit. When the bar starts to bend, it becomes a spring, and not a rigid structure any more. So, if your bar requires 2,000 lbs before it will bend, once it reaches that point, it will not support any more than 2,000 lbs (and most likely a bit less than 2,000 lbs). How would one calculate the yield strength of bars loaded in this manner? Since it wouldn't have the tendency to bow, then does that enable us to use extremely thin wall (light weight) tubing for the strut bar and other non-sanctioned chassis stiffening members loaded in this same fashion? You can use the following equation to calculate the yield strength (critical load) and predict how much force will cause the bar to bend: critical load (yield load) = EI(pi^2) L^2 where: E = modulus of elasticity, or Young's modulus (specific for each type of material, http://en.wikipedia.org/wiki/Young%27s_modulus) I = moment of inertia (specific for bar shape, http://en.wikipedia.org/wiki/List_of_area_moments_of_inertia) pi = 3.14159 L = length of the bar According to the web site, for a round, cylindrical bar, I = 0.25 x pi x [(outer radius)^4 - (inner radius)^4] Thanks for jogging my memory. Quote Link to comment Share on other sites More sharing options...
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