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Need some help determining strut tower load

  1. Feb 25, 2013 #1
    Ok, I have a question pertaining to the load a Macpherson strut tower sees VS. the load a particular double wishbone car sees. Basically since the macpherson strut is required to hold the weight of the car and control the cornering forces imparted through the tire/ upright, I was under the impression the strut tower is seeing a greater load force than the wishbone car.

    Here are pics of both systems in question

    1. Macpherson strut from nissan 240sx

    S14_frontsuspension_zpsc01bd228.jpg

    2. particular double wishbone from the fd3s rx7.

    fd3sfrontsuspension_zps6c4b933d.jpg

    Im assuming that both cars weight the same, are capable of the same g-force limit, have the same wheel/ tire package, and are the same weight distribution on the chassis. Basically I want to look at this from the stand point that the front suspension is the only variable in the equation.

    I was wondering if the top of the shock tower would see the same exact load on both suspension systems. I want to think that the macpherson strut will see a higher load acting on the strut tower since the cornering force, and vehicle weight is being imparted through the same load member (the strut/ spring assembly). My natural instinct is to think since the double wishbone car (fd3s rx7) only uses the shock/ spring to control the vehicle weight, and the top wishbone is controlling the lateral force through the big frame rail, the rx7 shock tower would see less load than the 240sx tower?

    I know that the the force going into the chassis through the tire is identical (under my listed conditions), but I would think since the load is being dispersed over multiple points in the rx7 vs. the 240, the strut/ shock tower is being loaded less. I guess Im basically wondering if I were able to quantify the force acting on the top of the shock tower, would both shock towers have the same number even though the macpherson is a direct connection, and the rx7 is dispersing the force in multiple points?

    Thanks for the help. I can provide more pictures if needed.
     
    Last edited: Feb 25, 2013
  2. jcsd
  3. Feb 25, 2013 #2
    I figured I would post a picture of both car's from the engine side. These are not my car's just a reference from the net. I notice most macpherson strut cars have steel in front of the strut tower, and many double wishbone cars do not i.e 350z along with rx7. Even the older macpherson rx7's have this steel. I have always heard that macpherson cars have a greater load being placed on the strut tower due to the more direct/ localized force path.

    s14/ 240sx macpherson strut car

    s14enginebay2_zps87b4a187.jpg

    fd3s double wishbone car

    fd3senginebay2_zpsc81a77b5.jpg
     
  4. Feb 26, 2013 #3

    Ranger Mike

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    The simple answer is mc strut design has higher load due to number of contact points vs double wish bone design...and...you have differeance in motion rates between the two.
    what are you shooting for?
    why the question?
     
  5. Feb 26, 2013 #4
    Basically two different ideas of what needs done to recreate a shock tower on a tube frame car. I basically just stated that a macpherson car is not used widely in tube frame applications for a large variety of reasons, one being all of the extra weight up high due to needing a much stronger tower. My friend building the car is fully convinced that the force on both towers are numerically the same in the same spot. I realize both cars are seeing the same force entering the chassis (according to my above scenario), I just stated that the wisbone more widely disperses the same load (meaning less on the tower).

    He is trying to say since the frame rail is connected to the tower the load is equal on both, but there has to be something accounting for less square inches being affected in the macpherson car.
     
  6. Feb 26, 2013 #5

    Ranger Mike

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    ifin you are building a tube chassis car..why would you want the mcstrut??
    no way to have best handling combo.....only reason auto manufacturers went to is was because it is cheap, small foot print in cramped engine room , short assembly and alignment time, and overall...it is cheap...no way is it for competition...go with dbl wishbones
     
  7. Feb 26, 2013 #6
    Im fully with you on that. I basically built my car for something to do. The s14 chassis is a piece of paper when stock, and I needed more room for the engine. My car ended up meeting my goals. The car turned like a jet, and had the most smooth and responsive steering I have felt with it ever.

    Here is a picture of the car last year. I realize there are most likely many things on my car that is way less than ideal. I really did not want to do a double a-arm conversion due to my limited understanding of suspension. I basically kept factory geometry and lightened/ stiffened the car. I have had this car for some time so I was able to drive the car before and after the semi-tube frame. I can say I definitely met my goals with the car, I feels amazing compared to before.

    I was just trying to get some insight on shock tower loads. I purposely wanted to try and over shoot the durability of the shock tower, due to me thinking a mac-strut car sees a greater load. My friend basically said it does not, it sees the same load. This was my disagreement with him. If I had an rx7 I would have gone a bit lighter on the tower than what I did on my s14. I was able to drop the weight to 2400lbs wet fully loaded of my car. We also jigged the chassis and fastened it down before any cuts were made, pretty important also.

    DSC03044.jpg

    Like I said, Im not trying to justify my car (I know mac-strut is less than ideal), it's what I had, and I got bored. I just wanted to know if my line of thought on the strut tower loads was in the right direction. I just could not see a wishbone car putting the same stress into the tower.

    If it's no problem for you could you give me an explanation on the math used that would give me the load number on both towers?

    Thanks for the help.
     
  8. Feb 28, 2013 #7
    The direct acting McPherson strut has a lower vertical load and a higher lateral load than the wishbone strut (essentially a coilover, not really a McPherson strut).

    When changing from direct acting to wishbone, the vertical force increase on the top of the strut tower is the inverse of the change in motion ratio, i.e. a motion ratio of 0.7 means there will be 43% more force acting vertically on the tower to hold up the car with the wishbone suspension. Keep in mind that the required spring rate goes up by the square of the motion ratio change.

    The lateral loading on the strut can be calculated by measuring the distance from the spindle to the centre of the lower ball joint and comparing that to the distance from the spindle to the top of the strut. Let's say the distance to the lower ball joint is 1 unit and the distance to the top of the strut is 7 units and the cornering load is 8 units; the top of the strut would see 1 unit of lateral loading and the lower ball joint/lower control arm would see 7 units.
     
  9. Mar 1, 2013 #8

    Ranger Mike

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    Mender..i want to understand and not torque you off so please bear with me.
    With a double wishbone suspension , the top wish bone is simply guiding the top of the spindle thru proper suspension movement and does not have a lot of heavy duty lifting. We crashed at the race track one time and the top ball joint was torn out of the upper A-Arm ( wishbone) and had no replacement upper ball joint, We were amazed to see the thing held together.

    The mcstru has 3 points of contact with a whole bunch of force acting on the top anchor point of the coil over ( tower). The double wishbone has 8 points ( two coil over spring mount, 4 wishbone mounts, two ball joints) of contact and the top of the coil over to frame mount point takes the most vertical force. And I agree you will have more vertical load at the coil over points than the mcstrut but not at the top bj or top wishbone mount points.
    Lateral force is spread out over the 6 mount points and provides more control of suspension movement.

    You can adjust caster and camber as well as anti squat a whole lot easier with this arrangement.
    Mcstrut is almost impossible to use when you go to wide tires ..huge steering offset if you have to go to wide wheels. and camber control is good but not as good as the dbl wishbone arrangement.
    I do not have the math, nor time to do the calculations but I know that the double wish bone was been refined over 90 years of racing and when you can spread out the same vertical and lateral load over 8 points vs. 3 points you have better control and less stress on the components...my opinion.
     
    Last edited: Mar 1, 2013
  10. Mar 1, 2013 #9
    First of all, Mike, as usual you're spot on with the load being distributed at multiple points and a wishbone suspension being much better for racing, etc. but I thought the OP was asking specifically about strut towers with the intent of making sure his were strong enough for the loads incurred.

    And of course the title of the thread. The math is pretty straight forward, all we're dealing with are Class Three levers. :)

    ETA: I just spotted what may have caused the confusion; I should have said "The direct acting McPherson strut has a lower vertical load and a higher lateral load on the strut tower than the wishbone strut (essentially a coilover, not really a McPherson strut)." My apologies. :)
     
    Last edited: Mar 1, 2013
  11. Mar 1, 2013 #10

    Ranger Mike

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    excellent :approve:
     
  12. Mar 4, 2013 #11
    Thanks for all of the info guys.
     
  13. Apr 4, 2013 #12
    what is the affect of varrying load on the steering of mc phersion strut...plz reply..!
     
  14. Apr 5, 2013 #13

    Ranger Mike

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    same results as on all steering...why not look at race car suspension class on automotive forum?
     
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