Designing for G Loads in Passenger Vehicles 1

[Schmity]

Been searching and lurking and have yet to find a credible answer to my question.. which is:

Is there a standard (design/safety factor) that automotive manufacturers use for calculating the necessary load bearing capability for critical mounting points (i.e. control arm ends, engine mounts, etc.)?

I understand that these will forces will be effected by the design's geometry and load paths, loading conditions, etc. I have also heard that baja cars employ a 50G standard, but this seems unreasonably high for a vehicle that is not intended to become airborne.

Could someone point me in the right direction?

 

[GregLocock]

Current values are proprietary. For circuit cars many cars were designed for 3g vertical, 2g longitudinal 1 g lateral at the contact patch.

You then need to cascade those loads through the suspension. Note that the worst case load on a given arm may be any combination of those three cases, or their negatives, or zero.

A suspension designed by these rules may fail if you kerb it and 1g lateral looks a bit light on for modern tires. FWIW I've run vehicles designed to 3-2-1 rules for thousands of miles of public roads without a failure in the metal parts that were calculated.

A similar process is used by some car manufacturers, using loads calculated statistically from previous similar cars, and real data. It's a pretty broad brush, in fact we'd only use them now for prototypes. Once you have a prototype we can then measure the loads and design the production car using the measured loads.

I don't know how engine mounts are designed, but I suspect that the maximum driveline torque *2 would be a great starting point. Again measuring the loads is easy once you have a proto.

It goes without saying that some parts are fatigue limited, others are strength limited, many are stiffness limited, and some have a maximum strength requirement (ie they are mechanical fuses).

Cheers

Greg Locock


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[Schmity]

Thanks for your response, Greg.

The source of my concern is that an under-built design will cause irreparable damage my prototype. I am currently developing a method for integrating a trailing arm suspension to a composite monocoque chassis. If this vehicle were to undergo a 'worse case scenario' (say a pothole while decelerating in a hard corner) the forces would cripple the chassis... unless intended failure points were designed into the suspension members.. but that is further than I would like to go on a prototype.

Your response was very helpful, and I greatly appreciate it. But I also need a cite-able source to justify my design. I have Milliken's Chassis Design on the way, but it may take awhile for me to dig through it.

 

[GregLocock]

I don't think Milliken covers abuse loads.

I have measured data for square edge pot hole (50 kph, wheels locked, 100-150 mm deep) for several cars, and have spent weeks attempting to simulate it with partial success.

http://forums.autosport.com/lofiversion/index.php/t121354.html

Note the importance of suspension tune in my results - I can stop the wheel falling into the pothole (lots of low speed rebound damping) it'll see a lot less load when it hits the upcoming kerb.



Cheers

Greg Locock


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[GregLocock]

http://www.eng-tips.com/viewthread.cfm?qid=149427&page=16

Cheers

Greg Locock


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[Schmity]

Great thread!

But it still leaves me wondering. As of right now I plan to strategically over-design the critical interface points to avoid damage to the chassis (8G's in each direction). The vehicle is intended to have a kneeling pneumatic suspension with 8+ inches of travel, so super low rebound rates may be out of the question. Also, what if the road irregularity is a curb rather than a pot hole? Wouldn't low rebound rates then exacerbate the loading by essentially not damping out the force??

 

[GregLocock]

8g sounds very sensible to me, and is not overkill.

Yes Curb is a worse case than square edge pothole.



Cheers

Greg Locock


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[Schmity]

Greg, I truly appreciate your insights. Would you be comfortable providing me with some credentials so that I may cite our correspondence?

 

[Schmity]

In case anyone is interested here is a test specimen, with attached interface device, fixed to an MTS. The specimen is a simulated frame rail segment made of polypropylene/e-glass composite, the interface device was developed to test a method for affixing a suspension mount to a composite chassis.

 

[Schmity]

 

[GregLocock]

BA(Cantab)

Cheers

Greg Locock


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[sbozy25]

Schmity - FYI, you will find nothing concerning appropriate g-loading of hydro-pneumatic suspensions in any reference guide of book. We have been looking for good sources for years at my company and thus far have had to work from tribal knowledge.

From the hydraulic experts and PE's we have hired and had evaluate our systems, as well as various sets of vehicle mounted strain gauges and string pots on vehicles at proving grounds we have a set of criteria in our facility:

Maximum GVW
On Road:
3.7g V
2g Lat + 1 G Long
2g Lat + 2 G brake on 20% grade

Off Road:
5g V
2g Lat + 1 G Long
2g Lat + 2 G brake on 20% grade

FOS no less than 3:1

This is all at the tyre patch and must be resolved to the components in question. If you contact any of the hydraulic suspension component manufacturers or SAE experts, they will probably tell you something similar to this.

Cabbages, knickers, It hasn't got A BEAK!

 

[GregLocock]

See also SAE 2007-01-1647, for a modelling approach although they don't give specific numbers.

Cheers

Greg Locock


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