Strut Brace test methodology
12-11-2002, 12:20 PM
#46
I still want to see how much rotation there is around the pivot (heim joint) on each mount. I am still thinking there will be rotation around the joints. Obviously this will negate to some degree the effort to stiffen the chassis since you are still allowing motion in the tower tops.
A strut tower brace with heim joints (rod ends) is comparable to how your tie rods connect to your steering knuckle. The entire steering unit (rack and tie rods) never changes length, it only changes in regards to how much is on one side of the car versus the other. If the tie rod ends were not ball joints (in other word they can allow rotation) then you would not be able to steer the car.
Every strut brace I have seen for our cars have some sort of connection that allows rotation (usually heim joint), thus allowing the towers to move. If one tower is forced up the joint will allow movement that will enable the other tower to rotate in and down. Yes the distance between the tower tops is constant, but they become horribly misalligned.
I still feel the best design will be one that is rigidly mounted to the strut tower tops. If you feel like drilling and adding X bracing, have at it. I think the bases on the brace in question in this thread is a good place to start as they are held in place by all four fasteners of each bearing plate. Now change the cross bar so that it is fixed to the base plates. If fit is going to be in question, then add a threaded fastener in the center of the bar to allow adjustment for length
A strut tower brace with heim joints (rod ends) is comparable to how your tie rods connect to your steering knuckle. The entire steering unit (rack and tie rods) never changes length, it only changes in regards to how much is on one side of the car versus the other. If the tie rod ends were not ball joints (in other word they can allow rotation) then you would not be able to steer the car.
Every strut brace I have seen for our cars have some sort of connection that allows rotation (usually heim joint), thus allowing the towers to move. If one tower is forced up the joint will allow movement that will enable the other tower to rotate in and down. Yes the distance between the tower tops is constant, but they become horribly misalligned.
I still feel the best design will be one that is rigidly mounted to the strut tower tops. If you feel like drilling and adding X bracing, have at it. I think the bases on the brace in question in this thread is a good place to start as they are held in place by all four fasteners of each bearing plate. Now change the cross bar so that it is fixed to the base plates. If fit is going to be in question, then add a threaded fastener in the center of the bar to allow adjustment for length
12-11-2002, 12:50 PM
#47
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Napoleon
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I agree with you 100% and I like the Porsche design they used. Simple and effective. Though I would probably have used tubular steel to keep it from deflecting to much, or at least U-channel stuff.
12-11-2002, 01:49 PM
#48
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Mike, Sid.
I am the designer of the KLA bar and the guy that did the sharpie pin test. So please take the results as coming from a potential biased position. I did, however try to be totally fair. PLEAS UNDERSTAND that this test was not close to science. I would love to see the test run with a dial indicator with a tattletale that could be put into a computer. If you can also use a G-meter then we are getting close to science. The sharpie test was simply a way to get an idea of how much one tower will flex more than the other. The strut brace is not designed to eliminate all movement of the front end. It is simply designed to transfer the load from the outside strut tower to the inside tower so both working together will eliminate as much flex as possible. We drove the car on a smooth road with no potholes. We did not drive it over a curb or do anything that would induce flex into the body. We did drive the car to 50 MPH and made hard right and left hand turns to the point where the tires started to loose traction. I wanted to get a ballpark estimate of the movement. I'm the guy that guessed 4-mm to 8-mm after doing some rough calculations looking at the geometry of the front end.
The theory is that in a hard turn you have a bending moment induced into the tower. The outside tower has greater forces on it inducing a greater bending moment (flex) than the inside tower. This results in the camber changing more on the outside tire than on the inside tire. The theory is that the inside tower is not stressed to its point of flex so it has additional (excess) strength that can be used to reduce total camber separation between the front tires. By adding the brace you take stress that the outside tower cannot hold without flexing and transfer it to the inside tower. By doing that you are allowing the inside towers excess strength to reduce that total flex in the outside tower. You will then reduce the camber change in the outside tire and also reduce the camber split between the 2 tires.
Again, not only will you reduce the camber change in the outside tower but even if both towers flex some the camber change split will also be reduced. In other words, both tires camber will change the same amount and you will not experience some of the ill handling effects of an improper camber split.
I agree that the Porsche bar will help eliminate the camber change by being hard mounted but I didn't think that many people driving on the street wanted to drill holes or weld on their car.
I have the capability of re-producing the Cup bar if you want one. I'll offer it for the same $125.00 as the standard bar. I just don't want to take any responsibility if the holes start to crack out after driving it on the street.
Ken
I am the designer of the KLA bar and the guy that did the sharpie pin test. So please take the results as coming from a potential biased position. I did, however try to be totally fair. PLEAS UNDERSTAND that this test was not close to science. I would love to see the test run with a dial indicator with a tattletale that could be put into a computer. If you can also use a G-meter then we are getting close to science. The sharpie test was simply a way to get an idea of how much one tower will flex more than the other. The strut brace is not designed to eliminate all movement of the front end. It is simply designed to transfer the load from the outside strut tower to the inside tower so both working together will eliminate as much flex as possible. We drove the car on a smooth road with no potholes. We did not drive it over a curb or do anything that would induce flex into the body. We did drive the car to 50 MPH and made hard right and left hand turns to the point where the tires started to loose traction. I wanted to get a ballpark estimate of the movement. I'm the guy that guessed 4-mm to 8-mm after doing some rough calculations looking at the geometry of the front end.
The theory is that in a hard turn you have a bending moment induced into the tower. The outside tower has greater forces on it inducing a greater bending moment (flex) than the inside tower. This results in the camber changing more on the outside tire than on the inside tire. The theory is that the inside tower is not stressed to its point of flex so it has additional (excess) strength that can be used to reduce total camber separation between the front tires. By adding the brace you take stress that the outside tower cannot hold without flexing and transfer it to the inside tower. By doing that you are allowing the inside towers excess strength to reduce that total flex in the outside tower. You will then reduce the camber change in the outside tire and also reduce the camber split between the 2 tires.
Again, not only will you reduce the camber change in the outside tower but even if both towers flex some the camber change split will also be reduced. In other words, both tires camber will change the same amount and you will not experience some of the ill handling effects of an improper camber split.
I agree that the Porsche bar will help eliminate the camber change by being hard mounted but I didn't think that many people driving on the street wanted to drill holes or weld on their car.
I have the capability of re-producing the Cup bar if you want one. I'll offer it for the same $125.00 as the standard bar. I just don't want to take any responsibility if the holes start to crack out after driving it on the street.
Ken
12-11-2002, 02:14 PM
#49
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I can also provide a optional end that will still have right and left hand adjustment capability and a solid 2 point mount on the bracket. I estimate that these will cost about $30.00 to build in small numbers. Here is a rough shape.
I'm not sure if we are debating theory or if you want a solution so here is a solution. As you can see I will have a flat plate laser cut and add a piece of high tencel rod stock. Have these welded together and thread one right hand and the other left hand. Have 2 holes laser cut in both the bracket and the end. We will then need to send the ends out for heat treat and plating. and then you can hard mount and end. Again, I don't want to be responsible for cracks in the tower. When you eliminate normal flex from one part you induce it into another. Where we induce it might not be able to handle the strain.
Ken
I'm not sure if we are debating theory or if you want a solution so here is a solution. As you can see I will have a flat plate laser cut and add a piece of high tencel rod stock. Have these welded together and thread one right hand and the other left hand. Have 2 holes laser cut in both the bracket and the end. We will then need to send the ends out for heat treat and plating. and then you can hard mount and end. Again, I don't want to be responsible for cracks in the tower. When you eliminate normal flex from one part you induce it into another. Where we induce it might not be able to handle the strain.
Ken
12-11-2002, 02:15 PM
#50
Big thirst, Sore Thumbs
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No reaon you can't combine the two. Use your current brackets, combined with a rigid bar. I think it would work much better.
As for the test I can get an accelerometer easy like, with hand held readout. Combining the two would be a little more difficult.
However if we do the test local to me I can get a portable computer (Read "not a laptop") With data acquisition hardware capable of reading not only the potentiometer but the accelerometer. And just about any other gauge you can think of or get your hands on.
And if you want to get fancy. I could probably swing a $30k high speed color video camera, that would unfortunately have to mounted on the inside of the car. But we could at least get video out the windshield to what the road was doing and have data points tied in per frame.
Could also use the video camera to examine the hood to fender gap and measure how it closes/opens in high speed turns, once again tied into accelerometer data. (Two cameras at same time not a problem)
Keeping in mind a test with that kind of equipment would have to be in a closed track. No way these guys are going to risk $50 k of hardware on a truck driver pulling out. But they have already promised that I can use the equipment, and they would personally love to have the video.
Funny I can get the cameras easier than I can get the strain gauges.
As for the test I can get an accelerometer easy like, with hand held readout. Combining the two would be a little more difficult.
However if we do the test local to me I can get a portable computer (Read "not a laptop") With data acquisition hardware capable of reading not only the potentiometer but the accelerometer. And just about any other gauge you can think of or get your hands on.
And if you want to get fancy. I could probably swing a $30k high speed color video camera, that would unfortunately have to mounted on the inside of the car. But we could at least get video out the windshield to what the road was doing and have data points tied in per frame.
Could also use the video camera to examine the hood to fender gap and measure how it closes/opens in high speed turns, once again tied into accelerometer data. (Two cameras at same time not a problem)
Keeping in mind a test with that kind of equipment would have to be in a closed track. No way these guys are going to risk $50 k of hardware on a truck driver pulling out. But they have already promised that I can use the equipment, and they would personally love to have the video.
Funny I can get the cameras easier than I can get the strain gauges.
12-11-2002, 02:27 PM
#52
I occured to me that someone is actually offering a bar as I described. BUT, I think the base plate you all have designed is WAY more substantial and a much better design. Take a look at the <a href="http://www.stable-energies.com/stable/camber.html" target="_blank">Camber Truss</a> and tell me what you think about probucing a bar like the one they show for the 944 except using your base plate design. I cannot imagine your base plate shearing, especially since it will be fastened at 4 points versus the 2 points on the Camber Truss. And if fit (tolerance) from car to car poses a problem, then install a center adjuster as I described (here is one for a VW, it looks <a href="http://www.autotech.com/stresbar.htm" target="_blank">Like this</a> on their bar). Obviously that would wind up being the weak link. I really doubt fit should be a problem though (unless the car has been wrecked) and the adjuster can most likely be left out.
Edit I want to make sure I was clear that I am talking about using your existing base plate design with a fixed bar instead of one that uses heim joints. This way there would not be any need for drilling into the cars sheet metal.
Edit I want to make sure I was clear that I am talking about using your existing base plate design with a fixed bar instead of one that uses heim joints. This way there would not be any need for drilling into the cars sheet metal.
12-11-2002, 04:19 PM
#53
I love Rennlist...
Does anyone know if the Cup cars had reinforced shock towers? I rather doubt it, so I would think that the solid mount bars would be just fine, and quite frankly, I think Mannings idea just about fits the bill. Ken, if you are interested in making up a solid mount bar, let me know as I would want one so that I could compare the two. My honest guess is that there would be little difference between the heim bar and the solid one, but I am a little too involved in this debate to back out now.
Regards, ...Scott
Regards, ...Scott
Does anyone know if the Cup cars had reinforced shock towers? I rather doubt it, so I would think that the solid mount bars would be just fine, and quite frankly, I think Mannings idea just about fits the bill. Ken, if you are interested in making up a solid mount bar, let me know as I would want one so that I could compare the two. My honest guess is that there would be little difference between the heim bar and the solid one, but I am a little too involved in this debate to back out now.
Regards, ...Scott
Regards, ...Scott
12-11-2002, 04:21 PM
#54
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I'm going to jump in and show off my ignorance...
Scott, I have the strut bar on my car as well-I too have noticed a more "solid" and "controlled" feeling with the bar on. I also admit I just think it looks cool
I guess I understand what the STB does mechanically, but I always wondered if the whole setup (looking at the front of the car) moves like a paralellogram under load. In other words, the bar keeps the towers an equal distance apart, but can both towers be moving the same direction under load, and what is lost by that?
Greg Schlemmer's brown 930 had a STB on the front, that also had a connection to the suspension mounting point in the tub. I assume (dangerous) that his bar not only kept the towers equaldistant from each other, but also from a fixed point that would be the neutral center of the car.
I wonder what the difference would be if the STB not only went tower to tower, but also triangulated back to the cowl. I think the purpose is to keep the front of the tub from flexing under load, and I think that is what you want to measure. BUT, I feel that if the tub can flex significantly tower to tower-wouldn't it also be flexing at other places? What about the towers movement side to side, front to back, up down as you are looking at the front of the car?
Scott, I have the strut bar on my car as well-I too have noticed a more "solid" and "controlled" feeling with the bar on. I also admit I just think it looks cool
I guess I understand what the STB does mechanically, but I always wondered if the whole setup (looking at the front of the car) moves like a paralellogram under load. In other words, the bar keeps the towers an equal distance apart, but can both towers be moving the same direction under load, and what is lost by that?
Greg Schlemmer's brown 930 had a STB on the front, that also had a connection to the suspension mounting point in the tub. I assume (dangerous) that his bar not only kept the towers equaldistant from each other, but also from a fixed point that would be the neutral center of the car.
I wonder what the difference would be if the STB not only went tower to tower, but also triangulated back to the cowl. I think the purpose is to keep the front of the tub from flexing under load, and I think that is what you want to measure. BUT, I feel that if the tub can flex significantly tower to tower-wouldn't it also be flexing at other places? What about the towers movement side to side, front to back, up down as you are looking at the front of the car?
12-11-2002, 04:37 PM
#55
Race Director
The 944 Turbo S does have chassis stiffeners. These are small 1" by 5" triangle bits of sheet metal that extend from the shock tower fwd at the frame rail. They are not much but there are there.
Intersting point about STBs
I installed the cambermeister on my race car. When I removed the center bar to get my blown engine from the the car I found STB bar had alot of pre-load in it. What I mean is that the bolts were hard to remove. When I got one out I saw that the bar was about 1/4 too long such that the holes did not align. Now this brace has threaded rod ends so I could match the proper length exactly during installation for no preload in the system which is what I did. (at least I think I did)
So why was there bar longer than the distance betweent the towers?? Not sure?
Possible reasons
1) I removed the bar with the car on is wheels, but installed with it on jack stands and towers were spread appart on first install
2) Temperature. The thermal growth of the the car frame and bar are different and the temperature was much warmer when I installed it.
3) Chassis flexed during racing and deformed some
4) I am an Idiot and did not install the bar right the first time.
Honestly not really sure which one it is!
Intersting point about STBs
I installed the cambermeister on my race car. When I removed the center bar to get my blown engine from the the car I found STB bar had alot of pre-load in it. What I mean is that the bolts were hard to remove. When I got one out I saw that the bar was about 1/4 too long such that the holes did not align. Now this brace has threaded rod ends so I could match the proper length exactly during installation for no preload in the system which is what I did. (at least I think I did)
So why was there bar longer than the distance betweent the towers?? Not sure?
Possible reasons
1) I removed the bar with the car on is wheels, but installed with it on jack stands and towers were spread appart on first install
2) Temperature. The thermal growth of the the car frame and bar are different and the temperature was much warmer when I installed it.
3) Chassis flexed during racing and deformed some
4) I am an Idiot and did not install the bar right the first time.
Honestly not really sure which one it is!
12-11-2002, 04:42 PM
#56
Race Director
[quote]Originally posted by hplj123:
<strong>Well I have completed my test on a 1986 951. I used a length of plastic pipe and a sharpie pin. I bolted the pipe to a bracket and slit the bar into the plastic pipe to make a slip joint. I wraped the bar with bubble wrap to keep it centered. I then drilled a hole in the plastic pipe the size of a sharpie pin. I used duct tape to hold the sharpie against the bar. Ken</strong><hr></blockquote>
Ken nice attempt at a test. Only problem is that I think most of the movement you saw was from vibration or movement of the Pen. The securing of it was not really that good.
Honesltly I think we are trying to measure something very small and it is going to be hard to factor out many of the other factors that could cause movement in your measurements. The only way around it is to use a very scientific approach with strain gages and such, but this is really to expensive to do so we are kind-of stuck.
<strong>Well I have completed my test on a 1986 951. I used a length of plastic pipe and a sharpie pin. I bolted the pipe to a bracket and slit the bar into the plastic pipe to make a slip joint. I wraped the bar with bubble wrap to keep it centered. I then drilled a hole in the plastic pipe the size of a sharpie pin. I used duct tape to hold the sharpie against the bar. Ken</strong><hr></blockquote>
Ken nice attempt at a test. Only problem is that I think most of the movement you saw was from vibration or movement of the Pen. The securing of it was not really that good.
Honesltly I think we are trying to measure something very small and it is going to be hard to factor out many of the other factors that could cause movement in your measurements. The only way around it is to use a very scientific approach with strain gages and such, but this is really to expensive to do so we are kind-of stuck.
12-11-2002, 04:52 PM
#57
Big thirst, Sore Thumbs
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A potentiometer with each end anchored on something similar to the sharpie test should work okay. With the advantage of being able to ignore the noise until the corner.
I'd rather go the strain gauge route (Have to be fiber under those enviormental conditions) but as you say the cost is prohibitive. I can reduce that by half though if people are interested. And the video stuff integrated with data I can do for free and get a couple of high powered PHD types to help as well.
BTW the test should be done on a hard corner with the clutch depressed and the engine idling on good motor mounts.
I'd rather go the strain gauge route (Have to be fiber under those enviormental conditions) but as you say the cost is prohibitive. I can reduce that by half though if people are interested. And the video stuff integrated with data I can do for free and get a couple of high powered PHD types to help as well.
BTW the test should be done on a hard corner with the clutch depressed and the engine idling on good motor mounts.
12-11-2002, 05:02 PM
#58
Ben,
My thinking is that if you allow the deflection that heim joints will introduce then you are allowing the camber angles on both side to change under load, one to positive and one to negative. If you eliminate the heim joints then you create a more rigid structure. Regarding additonal bracing I think folks, including myself, will be resistant to having to hack up the front bulkhead (firewall) to install a cross brace. The brace I am guessing dude with the 930 is using is the Camber Truss for a 911 which doesn't actually bolt into the bulkhead, but rather uses compression to keep it triangulated to the front trunk floor.
Scott,
I don't know for sure that you will notice a difference between the two. I do feel that from an engineering standpoint the fixed cross brace is a better, more solid, design.
My thinking is that if you allow the deflection that heim joints will introduce then you are allowing the camber angles on both side to change under load, one to positive and one to negative. If you eliminate the heim joints then you create a more rigid structure. Regarding additonal bracing I think folks, including myself, will be resistant to having to hack up the front bulkhead (firewall) to install a cross brace. The brace I am guessing dude with the 930 is using is the Camber Truss for a 911 which doesn't actually bolt into the bulkhead, but rather uses compression to keep it triangulated to the front trunk floor.
Scott,
I don't know for sure that you will notice a difference between the two. I do feel that from an engineering standpoint the fixed cross brace is a better, more solid, design.
12-11-2002, 05:14 PM
#59
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I see what you are saying now Michael-
By having a solid bar with no joints, it would be "impossible" for the towers to move together, because it would be like a solid bar welded in...
good point. The only way would be if a bolt popped.
I mention the bracing to the firewall only because I have seen it on a lot of race cars. In truth, the more solid we make the suspension, the harsher the street ride. I just wonder if there is a way we can make it better. I can't imagine the cars flexing that much, but If they do-I feel that the towers would also move front to back, etc. not just closer/farther apart.
By having a solid bar with no joints, it would be "impossible" for the towers to move together, because it would be like a solid bar welded in...
good point. The only way would be if a bolt popped.
I mention the bracing to the firewall only because I have seen it on a lot of race cars. In truth, the more solid we make the suspension, the harsher the street ride. I just wonder if there is a way we can make it better. I can't imagine the cars flexing that much, but If they do-I feel that the towers would also move front to back, etc. not just closer/farther apart.
12-11-2002, 10:13 PM
#60
Race Director
[quote]Originally posted by HY M8NC:
<strong>In truth, the more solid we make the suspension, the harsher the street ride.</strong><hr></blockquote>
But in this case, you are doing nothing to the suspension. You are stiffening the chassis the suspension attaches to, and that's a good thing. It makes the suspension actually do it's job rather than have chassis flex do part of it.
<strong>In truth, the more solid we make the suspension, the harsher the street ride.</strong><hr></blockquote>
But in this case, you are doing nothing to the suspension. You are stiffening the chassis the suspension attaches to, and that's a good thing. It makes the suspension actually do it's job rather than have chassis flex do part of it.