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role of shocks in heavy on-highway trucks

role of shocks in heavy on-highway trucks

role of shocks in heavy on-highway trucks

there is an interesting disagreement between "mechanics" and a driving team of a major trucking company here in the Western US.
The "mechanics" are claiming that airbag suspension systems don't really need shocks, that the shocks are just there to keep the suspension from over-extending!
The driving team is complaining of ride and handling  problems, and of physical ailments due to the ride problems.
The last set of tire lasted well enough, but were showing odd wear. They looked like they were hopping or something...
 I would think truck suspension would need damping every bit as much as auto suspension- and that air bags would not have the inherent frictional damping that leaf springs do.
feedback or suggestions?

Jay Maechtlen

RE: role of shocks in heavy on-highway trucks

Definitely need dampers.  Air suspension systems can be designed with a certain degree of damping in the system by introducing a fluid and some complex valving, but I am unaware of any systems on HGV's on the road at the moment.  Are you saying that your 'mechanics' are removing shockers from standard road trucks?!?  You have to remember that the airbag is just a spring.  The air inside compresses linearly and returns the same force (basically nws hysterysis etc) so they will 'bounce' like a spring if not properly damped.  As for saying that the dampers are used to limit travel, you will soon destroy a set of shocks with such high inertias from the unsprung weight of a beam axle banging on them, nevermind any possible torque reaction when pulling 50+ tonne!

Not an expert as such, but I have had experience desigining air susps for heavy duty tractors, and they will bounce around like hell with little or no load on and no shockers.

RE: role of shocks in heavy on-highway trucks

yep- the mechanics won't replace the shocks (after about 300,000 miles) claiming that they aren't important, etc. etc...

Jay Maechtlen

RE: role of shocks in heavy on-highway trucks

Shocking! (sorry)  What kind of trucks are these?  What do they transport?  Rigids or semis?  Not that it matters really, but you (they) are asking for serious trouble.  Bad shockers have a major affect on ride, and especially braking.  The axles can vibrate enough under braking to cause the wheels to leave the ground completely (hence the odd wear on the tyres).

I don't know what it's like over there in the US, but here in old blighty the authorities take a dim view on bad maintenance.  Consider the possibility that one of the vehicles is involved in a serious accident, an engineer's report will undoubtably show that your truck is in an un-roadworthy condition.  Best case scenario, you can kiss your insurance cover goodbye, worst case, negligence and even manslaughter.  Regardless of where the blame of the accident lies, you are still in the sh*t.  If one of a fleet of vehicles is found to be faulty, especially if it's deemed to be through lack of maintenance, the whole fleet is checked out as procedure.  

Also, if the drivers claimed for rsi, or other work related injury, you wouldn't have a leg to stand on.

For a set of shocks at 20quid a pop, it really isn't worth the risk, and if that's the attitude of your 'mechanics'...

RE: role of shocks in heavy on-highway trucks

This is a fleet of semis- big Kenworths and the like...

Jay Maechtlen

RE: role of shocks in heavy on-highway trucks

Is the mechanics' point of view supported by shop management?  And what is their basis for opinion as to (the lack of) necessity?  Driving around the yard at a sane speed for conditions isn't going to work the shocks very hard.  Not at all like potholes, patches, railroad tracks, or uneven concrete expansion joints taken at speed.


RE: role of shocks in heavy on-highway trucks

Your mechanics are probably still thinking leaf springs, which have a considerable amount of hysteresis.  It wasn't uncommon to find leaf springs without shocks in some applications.


RE: role of shocks in heavy on-highway trucks

I design suspension systems for coaches and busses (using truck components or similar) and I can tell you (as others have on this thread) that you can not do without shock absorbers, ESPECIALLY on air spring systems.

Shabba doubted that shocks limit travel, well at least in Europe they certainly do. The dampers we use (Koni) even have a rebound system that gives extra resistance in the last 20 mm of outward travel, I expect that the competitors have something like that too.

It could be that american suspension designs have steel cables to catch the axle on outward travel though.

RE: role of shocks in heavy on-highway trucks

No steel cables in sight!
I just had a nice visit with my Sister and her husband- the driving team originally mentioned.
The trucking company is so boneheaded, they conducted the following test:
They test-drove an in-service truck around some course (I think it was on local streets), then removed all shocks from the rear axles, and repeated the same course. They couldn't tell the difference.
Only one minor catch: The truck in test had about 300,000 miles on it. Since the company doesn't ever change shocks, these were the originals...
So, imagine that! They showed that old shocks don't work...
Of course, they didn't have enough sense to install NEW shocks and repeat the test.
The team spent some $500+ of their own money and had new shocks installed on their rig. At least THEY can ride in a reasonable vehicle. Funny- when the shocks were installed, a mechanic took the rig around the yard, and absolutely raved about how great it rode! But, management: I don't think they want to know. Boneheads- it has to reduce life of cab and related equipment, it has to reduce driver safety...


Jay Maechtlen

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