I calculated to forces applied to a strut towere in a 1G turn
11-19-2003, 01:20 PM
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I calculated to forces applied to a strut towere in a 1G turn
Well, I finally calculated the lateral force on the top of the strut tower during a 1G turn. The following web site gives great info on the suspension calculations and set up a spreadsheet to do the calculations.
I measured the ball joint to the ground (5.2”) and the ball joint to the top of the strut (23.5”) on my wife’s 951 and plugged these and the weight (2899) into the sheet.
In a 1G turn the outside strut tower is subjected to 641.48 pounds of lateral force. Now, we can measure the deflection in the tower by attaching a small bracket to the top of the strut tower and applying 641.48 pounds of force pulling outward and seeing how far the tower flexes. Before we get into the discussion about both towers flexing please read the discussion at this web site.
http://www.e30m3performance.com/myt..._bar_theory.htm
Ken
I measured the ball joint to the ground (5.2”) and the ball joint to the top of the strut (23.5”) on my wife’s 951 and plugged these and the weight (2899) into the sheet.
In a 1G turn the outside strut tower is subjected to 641.48 pounds of lateral force. Now, we can measure the deflection in the tower by attaching a small bracket to the top of the strut tower and applying 641.48 pounds of force pulling outward and seeing how far the tower flexes. Before we get into the discussion about both towers flexing please read the discussion at this web site.
http://www.e30m3performance.com/myt..._bar_theory.htm
Ken
11-19-2003, 01:27 PM
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11-19-2003, 09:00 PM
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Ken,
Ok, ok! you got me! I will buy one of your bars.
Let me see......where did I put Skips number??
(really cool tech stuff though, thanks for posting)
Ok, ok! you got me! I will buy one of your bars.
Let me see......where did I put Skips number??
(really cool tech stuff though, thanks for posting)
11-19-2003, 09:55 PM
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Ken,
Assuming that your wife’s car has 4 wheels, has a 50/50 weight distribution and 80% (guessing here) of the of the weight goes to the outside wheels at a 1G turn, then your number needs to be multiplied 50% x 80% = 0.4 equivalent to an outward force on the tower of 257 pounds.
Assuming that your wife’s car has 4 wheels, has a 50/50 weight distribution and 80% (guessing here) of the of the weight goes to the outside wheels at a 1G turn, then your number needs to be multiplied 50% x 80% = 0.4 equivalent to an outward force on the tower of 257 pounds.
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11-19-2003, 10:19 PM
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Since the discussion is up here there is something I have often thought about when I see the strut tower braces. Where does the actual strut tower flex at? If you could tie the bolts on the strut tower back to the outside fenders with 2 bars would it reduce the flex any? Would it reduce the flex as much as a solid bar from tower to tower?
Just some things to think about.
I love the braces though, more for the look than anything, and I may have to get one myself with the current sale and all.
Just some things to think about.
I love the braces though, more for the look than anything, and I may have to get one myself with the current sale and all.
11-19-2003, 11:02 PM
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One of my professors told us that most of our job as an engineer in the field will be to make the correct assumptions to solve a problem. Looking at the given webpage I think most of the assumptions are wrong.
He correctly assumes that when an object is at rest all of the horizontal forces will cancel each other out and also all of the vertical forces will cancel each other out. Then he states that the sum of torques about a point must also be zero, but he fails to recognize that vertical forces will also cause torques that must be considered.
Neglecting the vertical forces allows him to incorrectly assume the direction of F3. This should be obvious if you think about the way the car leans. How can the car’s body leaning toward the OUTSIDE of the turn produce a force pointing to the INSIDE of the turn?
I also think that his examples of deflection are exaggerated. I would be very interested if you could give test results for deflection of the strut tower for different loads both inward and outward. And any compression of the strut tower would give more negative camber to the outside wheel.
I’ll try to make some diagrams and post a better way to calculate stress on the towers a soon.
By the way, when you used his equations you forgot to divide by 2 for the cars front/rear weight distribution.
He correctly assumes that when an object is at rest all of the horizontal forces will cancel each other out and also all of the vertical forces will cancel each other out. Then he states that the sum of torques about a point must also be zero, but he fails to recognize that vertical forces will also cause torques that must be considered.
Neglecting the vertical forces allows him to incorrectly assume the direction of F3. This should be obvious if you think about the way the car leans. How can the car’s body leaning toward the OUTSIDE of the turn produce a force pointing to the INSIDE of the turn?
I also think that his examples of deflection are exaggerated. I would be very interested if you could give test results for deflection of the strut tower for different loads both inward and outward. And any compression of the strut tower would give more negative camber to the outside wheel.
I’ll try to make some diagrams and post a better way to calculate stress on the towers a soon.
By the way, when you used his equations you forgot to divide by 2 for the cars front/rear weight distribution.
11-20-2003, 01:08 AM
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pedro951,
Good points both with respect to remembering all the assumptions and the missing vertical forces. However the number for the outward force on the tower with the modifications I added in my prior post is close to being true, since the ball joint is close to the centerline of the wheel.
Another minor “modifier” is the angle of the strut which pushes the tower slightly inwards.
It is fair to assume that that auto manufacturers have true and tested computer programs that calculates these kinds of things and more with input menus that ensures no omissions.
I remember that when Nissan and Mitsubishi under much fanfare came out with 4 wheel (active) steering, Porsche claimed that they already had that in their Weissach rear suspension (passive steering) for the 928. For that you would need to take rubber bushing compliance into account and I certainly would hate to attempt hand-calculations on that.
Good points both with respect to remembering all the assumptions and the missing vertical forces. However the number for the outward force on the tower with the modifications I added in my prior post is close to being true, since the ball joint is close to the centerline of the wheel.
Another minor “modifier” is the angle of the strut which pushes the tower slightly inwards.
It is fair to assume that that auto manufacturers have true and tested computer programs that calculates these kinds of things and more with input menus that ensures no omissions.
I remember that when Nissan and Mitsubishi under much fanfare came out with 4 wheel (active) steering, Porsche claimed that they already had that in their Weissach rear suspension (passive steering) for the 928. For that you would need to take rubber bushing compliance into account and I certainly would hate to attempt hand-calculations on that.
11-20-2003, 12:42 PM
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pedro951, you are right that the link is incorrect in a number of things. but first, yes there will be a force in the opposite direction of the turn. this is called a coupled force, and it brings the moment to zero around the hub. and yes, the forces are going to be zero in the x and y directions or else the car would be moving and not be static. BUT, the forces in the y direction acting at the top of the strut tower will also create a moment around the hub (it is because of the angle of the strut and it is not directly above the hub), further reducing the force he claims the strut tower brace counters. also, the deflections he claims are no where near what they really are. i have never measured the deflections, but if it really were a 1/2 inch, then there would be some massive wear on all the suspension parts, and the strut tower would have a huge amount of stress most likely to the point of deformation and cracking in the tower. from the factory, they most certainly design the strut tower to handle large loads in all directions, and therefor the deflection will be very minimal. my guess is that a strut brace helps, but is very minimal.
because i really have never done any calculations on the front suspension i don't know what really goes on as far as stresses and deflections, but i know the link posted is much to simplified. what would sound good to me would be to take a strut tower brace, attach some strain gauges, hook them to a laptop with datalogging, and take the car around the track a few times. this would be the deciding factor, as far as the forces applied to a strut tower brace. also, i'm sure i could make something to measure the deflection of the strut tower without the brace in a hard turn.
hmm... i need a design II project for my last simester of school... any companys want to sponsor a project to measure the forces/deflections in the strut towers?
because i really have never done any calculations on the front suspension i don't know what really goes on as far as stresses and deflections, but i know the link posted is much to simplified. what would sound good to me would be to take a strut tower brace, attach some strain gauges, hook them to a laptop with datalogging, and take the car around the track a few times. this would be the deciding factor, as far as the forces applied to a strut tower brace. also, i'm sure i could make something to measure the deflection of the strut tower without the brace in a hard turn.
hmm... i need a design II project for my last simester of school... any companys want to sponsor a project to measure the forces/deflections in the strut towers?
11-20-2003, 02:59 PM
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Kevin-
I think the experiment you are considering was done by a rennlister a little while back (this summer?). Don't know if I read it here or on the mailing list, but the result was a surprising amount movement at the top of the strut mounts, iirc. Their method wasn't nearly as technical as using strain gauges, but I'm sure you could set up something a little more scientific pretty easily (deflection and force measurements would seem pretty easy to make). The resulting data would be pretty valuable towards clearing up this topic.
I think the experiment you are considering was done by a rennlister a little while back (this summer?). Don't know if I read it here or on the mailing list, but the result was a surprising amount movement at the top of the strut mounts, iirc. Their method wasn't nearly as technical as using strain gauges, but I'm sure you could set up something a little more scientific pretty easily (deflection and force measurements would seem pretty easy to make). The resulting data would be pretty valuable towards clearing up this topic.