standard stresses on suspension parts? 2

[stanglou]

I have designed a suspension in catia which I would like to stress test. As I am not an automotive engineer, is there some standard stress that you put on certain places of the suspension in order to determine that it is capable. If so, where would I find these numbers?

 

[MacGyverS2000]

I've been dreaming of replacing certain cast suspension components with slightly beefier (but lighter) parts milled from billet aluminum. Interested in the answer to this one...


Dan
Owner

http://www.Hi-TecDesigns.com

 

[BobM3]

Look for SAE papers on "road simulators", "durability testing", "multi-axial fatigue", "component testing".

 

[Warpspeed]

I am not sure there is a complete answer to this.

The highest stresses will be shock loads from when you hit something solid, but what is reasonable ?

A race car may never be expected to hit a curb or gutter, and if they do, it is usually at frightening speed.

Would it be reasonable to expect a vehicle to get completely airborne and then land heavily on one wheel?

Normal fatigue limits are one thing, but one time destructive overloads are something rather different.

 

[MarkTEng]

Typically you start with a 10 to 100 occurence loading of 3g vertical and 2g Fore/Aft through the center of the wheel to simulate a pothole event. Also working with 1g lateral loads for cornering and max braking loads in forward and reverse driving directions, which can be upwards of 10,000 occurences for these events. You may even want to do an analysis for curb push offs and dry scrub turns. These events will give a pretty good estimate of the durability of your parts.

 

[MacGyverS2000]

Sidestepping the original question for a moment... does it seem reasonable to run a stress simulation on the original component and material, then use those values to create a new component from a different material? Seems logical to me, but I would like verification...


Dan
Owner

http://www.Hi-TecDesigns.com

 

[patprimmer]

Seems very logical to me to obtain a design that is known not to fail. It will not however reduce over design or excess weight at certain points from being carried through.

Of course a bit of excess weight might not be of concern.

Regards

eng-tips, by professional engineers for professional engineers
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[GregLocock]

turczinator - those values are fine for a circuit car (although 1 g lateral may need to be adjusted), they are a bit light for a road car, where I'd be inclined to use 4-5g vertically, 3-4g fore-aft, and 2g laterally. The range covers low profile tires with stiff suspensions (the high end) through to more sensible designs.

Dan - that is a reasonable approach, but it is twice as much modelling work. 3-2-1 , or as I have suggested, 5-4-2, is a pretty good approach, designs based on that rarely need significant re-designs. Make sure your billet aluminium has a guaranteed elongation at failure of at least 7% right through the billet.




Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[NickE]

WOW! -- Greg thanks. Great info.

 

[MacGyverS2000]

Greg,

Just wondering how difficult it would be for me to re-engineer some components on my car. I have a decent mechanical background (degrees are in EE, not ME), but very little when it comes to computer simulation of part stresses, etc. My idea would be scan the part in (either by multiple caliper measurements, cheap access to a laser scanner, etc.), enter appropriate data (such as materials, load points, etc.), and get typical/max stress values. Once I have the worst-case for that material, begin redesigning a piece out of aluminum that meets (or exceeds) those max specs, but is obviously lighter.

This isn't a money making idea, simply something I would do in my spare time to play with my car (though selling a few extra pieces to pay for mill time wouldn't be a bad idea). As always, I just love learning new stuff.

Dan
Owner

http://www.Hi-TecDesigns.com

 

[GregLocock]

It is certainly possible - many people, including me, do it.

However, FEA has many pitfalls, and for the interested amateur, even if you avoid the modelling problems, the biggest difficulty is getting hold of good correlation data.

As an example, two of us have just run a correlation exercise on a ladder frame chassis. Building the FEA model probably took 20 hours (off the top of my head). Building the test rig and getting the results sorted out for the correlation took two or three times as long. I'm very glad we did it though, my FEA was nothing like as accurate as I'd hoped it would be. Once I knew where the problems were it was very easy to improve the accuracy. Now it is a reliable basis for further modifications.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[NickE]

"I'm very glad we did it though, my FEA was nothing like as accurate as I'd hoped it would be. Once I knew where the problems were it was very easy to improve the accuracy. Now it is a reliable basis for further modifications."

Amen Brother

 

[BobM3]

Pothole events that cause jounce bumber inpacts can impart over 30 g's vertical acceleration (measured on a control arm near the ball joint). Those same events will produce over 3000 lbs longitudinally (at the tire patch).

Check out MTS Corporation's "SWIFT" Wheel Force Transducer. Their brochure lists the load capacities for vertical, lateral and longitudinal forces.

 

[MacGyverS2000]

I'll start a new thread centering on my questions when I get home... I have more info to share and more questions to ask.


Dan
Owner

http://www.Hi-TecDesigns.com

 

[stanglou]

so, that all sounds good. Using solid works, and the cosmos stress testing package, I am going to put the suggested number of G's on the parts, and see how it goes. On second thought though, how do i convert g's into pounds(so that it is useable on cosmos)I am no engineer after all, this is new to me.

 

[BobM3]

It's not easy. For vertical loads you would have to know the "load impedance" of the unsprung mass in the vertical direction. If there was no damping or spring loads then you could take the g's times the unsprung mass (reflected to the tire centerline) to get the force. So, for 100 lbs of unsprung mass and 30 g's, the vertical load would be 3000 lbs. The impedance is nonlinear. Below the sprung mass natural frequency (usually about 1.5 hz) the body moves with the spindle so there is no spring deflection or shock damping and vertical loads are developed primarily from acceleration (but now you must add sprung and unsprung mass). At higher frequencies the body doesn't move so you'll have some spring and damping loads (and maybe jounce and rebound bumber loads) in addition to the acceleration loads. When I was in the simulator design business we figured 5000 lbs of vertical load could handle the severe events.
For lateral and longitudinal loads most data is taken with strain gaged components and not with accelerations. The load paths for these directions are stiffer than the vertical load path. Again, pot holes and curb strikes cause the largest loads. A big part of the lateral load is from reacting the moment created by the accelerating vertical mass striking the jounce bumper. We used about 4000 lbs for lateral inputs and 6000 lbs for longitudinal inputs.
You would not want to design your parts for millions of pot hole strikes. Maybe 100's.

 

[GregLocock]

Everything Bob says is true... but it is too damn hard in practice. 3-2-1 is a static analysis, traditionally.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[MarkTEng]

My 3g vertical and 2g fore/aft that I referred to was not soley based upon the unsprung mass, but the corner weight of the vehicle. Though I do agree 4/5g vert and 2/3g fore/aft would be more reasonable for pothole events, but I must disagree that the lateral load is do to the reation of the moment created by the accelerating vertical mass striking the jounce bumper. It is due to the bump steer event as the tire impacts and exits the pothole and the moment at the tire patch about Mx.
I use a simulator that has 18Kip vertical (after the 2.5:1 bellcrank), 11Kip Fore/Aft, and 7.8Kip Lateral capacity and wish it had more capacity, 5 Kip would barely tickle a suspension vertically and still get the required velocities.