Is parking a loaded truck overnight bad for the suspension?

[digger242j]

This is a topic that's been debated among some people I know, and having found this site it occured to me that there's no better group of people to ask than automotive engineers.

Is it bad for the suspension, to load a truck and then park it for any length of time? This is just a general question. Apply it to a dumptruck loaded with dirt, or a pickup truck loaded with lumber. Assume we're not talking about exceeding the truck's GVW, at least not by much.

My thinking has always been that being parked would be less stressful than actually driving--while driving there are all sorts of dynamic factors at work that would be much more abusive. Once it's loaded, sitting for five minutes or five days wouldn't make any difference. The opposing viewpoint is that, by being left loaded overnight, the springs will take a "set", much like when you bend a paperclip. I just can't see it.

Thanks in advance for your professional opinions.

 

[patprimmer]

The steel springs do gradually sag under load, so leaving exposed to the load for an unnecessary amount of time, reduces the potential life.

Air suspension is another story altogether

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
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[mloew]

The mechanism you are interested in learning about is referred to as Stress Relaxation (also called load loss). This relaxation for springs (other than air-springs) should be available from the spring manufacturer or material supplier and will likely take the following form:
stress relaxation vs. temperature for a constant period of time and constant stress
stress relaxation vs. stress for a constant period of time and temperature

There is a standard test published by ASTM: E328-02 Standard Test Methods for Stress Relaxation Tests for Materials and Structures that may be helpful. Try also searching on Google with the keywords: stress, relaxation springs (ignore the second hit ;-)).

Members TVP or CoryPad may be able to provide more information.



Best regards,

Matthew Ian Loew
"Luck is the residue of design."
Branch Rickey

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[digger242j]

Thanks for the replies.

I did the Google search and got a tension headache from weeding through all the results that wanted me to go to some hot springs someplace and relax, but I did get a handle on the concept of "spring relaxation" as it applies to this question.

My original question was pretty general--not specific to any supplier or manufacturer. The replies didn't really provide the general kind of answer I was looking for. I'll try to re-state the question in a more succinct fashion.

I would have thought that truck suspension systems would be robust enough in their engineering--component life of several hundred thousand miles while being subjected to all the rigors that such service imposes--that the portion of spring relaxation, or load loss resulting from being parked, stationary, with a load, would be negligible.

I understand that leaving the system exposed to any load for an unnecessary amount of time *will* lead to decreased life, but unless they are sitting unused on a wharehouse shelf, suspension components have a finite lifespan anyway.

Have I badly underestimated the adverse effects attributable to spring relaxation, and if so, am I to understand that as an operational practice, parking a truck with a load on for any appreciable amount of time must be avoided?

(An answer of "Yup" or "Nah", will be understood, but if anyone cares to elaborate, it would be appreciated. Like, "How long is too long?" or "How much of a difference is there between the adverse effects at 50% of designed GVW as opposed to 120% of designed GVW?", etc...)

 

[NormPeterson]

. . . Assume we're not talking about exceeding the truck's GVW, at least not by much. . . .

Perhaps this needs to be more specifically addressed. As in it not being sufficient to merely be approximately at (or at least not grossly exceeding) the vehicle's GVW - you also should be considering the distribution of that load and not be severely overloading one axle while leaving the other(s) lightly loaded. Obviously, this goes to the "stress relaxation vs. stress" side of things.

Norm

 

[JayMaechtlen]

sag=stress relaxation= outer "fiber" of the spring undergoing low temperature creep.
I don't know if creep is linear as function of time*stress, or more like time*(stress^n), where n>1
Anyway, the more it is loaded, and the longer it is loaded, the more the springs will sag. There may also be some effect on bearings, bushings and so forth.
(I wonder if small defects will tend to turn into cracks more quickly, as function of (time*stress^n)?)

More importantly: how long does the company keep the vehicle, how valuable is the convenience of keeping the vehicle loaded for longer periods of time, and is there any reduction in resale value of the vehicle?

For reasonable periods of ownership, maintenance/cost of ownership probably won't be affected by the practice of leaving the vehicle loaded overnight.
For long-term ownership, load-carrying rubber bushings will probably fail more quickly, and springs will sag and/or fail more quickly.

Jay Maechtlen

 

[GregLocock]

I am baffled here. Typically metal accumulates damage as (number of cycles)*(stress range)^6, and even an overloaded truck is unlikely to be over (say) twice its design load. Yet a bump can easily exert at least 3, and sometimes 5,g

So the pro-saggers here are suggesting that one cycle of 2g is somehow more important than the greater number of bigger bumps seen in normal use.

To change the length of coil springs we have to squish them up to coilbound, even then some don't take a set.






Cheers

Greg Locock

 

[NormPeterson]

I'd guess that creep or stress relaxation is a different mechanism than cyclic fatigue since it's a longish-term effect that occurs under static load. And quite separate from the fatigue usage that you could estimate from taking the same load and let the truck go bouncing down the road with.

As perhaps an extreme illustration, creep is a one-time event that's involved with determining permanent 'sag' in the fuel pool girder of a Mark II BWR power plant (which is rather heavily-reinforced concrete IIRC) over a 40-year design life. Half a cycle of alternating stress intensity?

Cyclic fatigue may well be the more important design consideration for mechanical bits, since that can result in catastrophic failure as opposed to simply causing excessive deformation.

I wonder if it's important to know whether the truck(s) in question have leaf springs . . .

Norm

 

[NormPeterson]

Edit to the above - make that 1/4 of a cycle . . .

Norm

 

[harrisj]

I'm sure that like most mechanical phenomena, the springs will do exactly what the theory states. They will yield according to the load, temperature and duration, but as Greg points out, the stresses should be nowhere near the elastic limit and the effect will be small to infinitesimal. On the railways, loads are left in wagons for months or years without any appreciable effect on suspension sagging - at least the wagons remain serviceable with adequate suspension travel.

Tyres are a different matter and I would be more concerned about the effect of leaving tyres in hot climates with heavy loads, especially with sunlight, ozone etc. A truck tyre left in the heat for a month or two becomes noticeably out of round - when the vehicle drives off, the vibration can be felt for some hours until the road has battered it back to the original shape.

ps This probably applies to tires also.