Shock absorbers

[SteveW77]

I collected my Swift 530se from the storage site today,a journey of some 5 miles. On entering the van I found that the storage cabinet under the cooker had opened and the content were strewn around the floor, also the glass top on the cooker had opened and the metal rack that the saucepans stand on had come out of its fitting. This has never happened before, but the only thing I did different was to blow the tyres up before I came away from the storage site. The tyre pressure is listed as 60lb psi. Would shock absorbers solve this problem or should I just lower the tyre pressures.

Steve W

 

[lunar2000]

Steve W, What a coincidence. I picked my Lunar up from service and traveled about 20 miles. When I opened the van I found the draw over the fire on the floor with contents strewn about.

I'm not sure if this answer will confuse you even further but, when I got my van the tyre pressures were 60psi so I checked with the dealer and they said 54psi was correct pressure for my van.

I required a new spare tyre and the garage blew the tyre up to 45psi, when I qeeried this I was told that 45psi was correct, so I let my tyres down to 45psi ( draw still ended up on the floor). I have since changed the catch on the draw for a magnetic type and it hold fine. Touch wood!

It would appear that the draw was jumping over the lip of the old type catch possibly when I hit a bump in the road.

Suggest you check your catches for up and down movement, or even sideways play?

Hope you solve it,

 

[Lutz]

Reducing the tyre pressures below the recommended value could upset the stability of your outfit. Whether shock absorbers would solve your particular problem is uncertain but they could and they certainly have other advantages, too.

 

[mcghee]

Shock absorbers are not the answer since they don't actually absorb shock at all. What they do is prevent the suspension from continuing to bounce after the initial jolt. Don't under any circumstances reduce your tyre pressures to less than stated by the caravan manufacturer, excessive compression of the tyre sidewall could cause it heat up to the point of failure.

 

[Lutz]

It may be the bounce that's causing the locker catches to fail under repeated banging by the contents inside. For this reason, it is conceivable that shock absorbers might help.

 

[Allyson]

How do you find out what the correct tyre pressure should be? We have at present an Elddis Typhoon GTI 1992. If anyone has one please can you tell us the correct pressure?

 

[Lutz]

According to the data I have, the max. weight of a '92 Elddis Typhoon GTX is 1232kg but without knowing what tyres it has fitted I can't say what the tyre pressure should be. Perhaps someone has an owner's handbook with the information.

 

[Surfer]

Either way I recommend fitting shock absorbers anyway as it improves stability anyway. I have them fitted to my Lunar and it has made a tremendous difference when towing.

 

[Rich]

QUOTE..

"Shock absorbers are not the answer since they don't actually absorb shock at all. What they do is prevent the suspension from continuing to bounce after the initial jolt."

Shock absorbers or dampers do control wheel travel.Dampers use both compression and rebound damping so if you hit a bump or a hard edged pot hole the wheel is controled through its travel both when compressed and returning to it's neutral position. If you hit a bump hard enough the wheel will move fast enough to lock out on the bump stop, but with a correctly valved damper it will not move fast enough to get that far and will possibly smooth out the ride in the van. It could stop things from being bounced out or opened. Lowering pressure in the tyres may reduce wheel travel so it will be less likely lock out against the bump stops but a tyre has no comrpession or rebound damping and the van will keep bouncing.

 

[RogerL]

Craig's comment that "shock absorbers don't absorb shock" is absolutely correct.

"shock absorber" is an incorrect, but universally used, name for dampers which damp down the oscillations caused by springs.

The world's motor trade call them shock absorbers or "shocks" but they should be known as "dampers".

 

[Rich]

Here is a quote from Showa "Shocks use hydraulic friction to damp out or resist suspension motion. Springs are "load-sensitive" devices while shock absorbers react to suspension velocities. Shocks remove kinetic (or motion energy,) from the suspension. Shock absorbers work in two directions and are critical as working companions to the springs.

Compression (or the closing direction) works with the spring to reduce bottoming and help keep the tire in good contact with road surface

The extension or opening phase is supposed to control the release of spring energy so that the vehicle chassis does not bounce, float or wallow around."

Dampers are speed sensitive (not travelling speed! But the speed the shaft is moved through the actual damper). So you can compress the damper through its travel slowly but if you try and move the damper quickly it will absorb energy and control the travel with the oil, gas and valving which is part of a shock absorbers design by becoming firmer and resisting movement. This action gets stronger the faster the damper is moved. This is why some shock absorbers or dampers have high and low speed compression- rebound damping. So they do absorb shock by slowing the motion of the axle of the van and absorbing the kinetic energy put into the wheel and axle by hitting the bump and the energy put into the spring by the movement of the van. The spring is used to carry the weight of the van and allow the wheel to travel when forces try to make the van bounce, if it had no spring the wheels would bounce off the road and have problems following all of the undulations and bumps. But using a spring only is not perfect as the energy put into the spring by both the weight and then kinetic energy put into the wheel and axel by the bumps in the road will go un controlled until the energy is spent. The dampers need to match the spring rates for the best control but please learn about suspension before posting opinions as they can be taken as accurate and mislead people into thinking they are 100% accurate.

I hope I am right as I am quite new at all this suspension lark. I have only been doing this with some of the teams at world championship level...If this attached image opens the front suspension is valued at

 

[Rich]

Sorry try this URL to see the image I tried to attach....

http://img345.imageshack.us/img345/3158/dsc30424an.jpg

 

[Rich]

Sorry for the long explanation... for those not too technical the spring is "load sensitive" the damper is "Speed sensitive" That is not travelling speed but the speed the axle moves when it hits a bump or is forced to move up and down. The slower it moves the less the damper does and the faster it moves the harder it works. Modern dampers are very highly developed. They can be adjusted for both types of movement high speed moves like bumps and lower speed ones like rises and dips where the suspension extends and compresses over soft undulations or during cornering. Might sound too techical for a van but all vehicles suffer from bumps and can benefit from dampers.

Sorry to sound like a know all but I do not like in-accurate information and during 20 years in motorsport I have learned somethings about vehicle dynamics which I use to improve my own transport to make it better as all production vehicles are a compromise and can be made to work better.

 

[Lutz]

I can't fault Rich's long and detailed explanation other than he fails to point out that he is describing the workings of viscous dampers only, not frictional ones. Whereas frictional dampers on car suspensions went out in the late 30's they are still the norm for use as stabilisers on caravans (like the AlKo or Winterhoff or most blade stabilisers).

In the end it all boils down to the fact that shock absorbers (or dampers), regardless of whether they are frictional or viscous, are designed to 'kill' as quickly as possible unwanted swinging movements that an undamped spring will cause.

 

[Rich]

I have only answered to the post as here it is talking about shock absorbers for the axle, not for use as a stabiliser. Plus the initial poster Steve W was thining about lowering tyre pressures... That would make the problem worse as it would have to cope with the undamped weight of the whole caravan as well.

RogerL and Craig were of the opinion that the damper or shock absorber do not actually absorb shock. They are wrong. As you point out the damper "kills" unwanted movements from an undamped spring....It does go a lot further than that in reality. The real diffrence between the older type of friction damper is that it is not speed sensitive and the damping forces actually decrease the higher the inertia of the moving object and this cannot be compensated for, Plus the friction damper has a problem with what is called "stiction" this is a term from the industy for static friction of the unit when it makes it's first initial movement.

I hope that clears that up......

 

[Rod_5041370]

What vehicles are you guys towing with?

If they have rear leaf springs, this could be the answer.

 

[RogerL]

Rich - the more effective a shock absorber is, the more shock it TRANSMITS, not absorbs.

It's the name that's wrong, not the function or explanation.

 

[mcghee]

The purpose of the shock absorber is not to absorb shock as I've already said. Or at least it does not absorb shock introduced into the vehicle by road undulation. It does absorb shock in as much as it soaks up the enery stored in the spring when it is compressed, thus preventing the spring from extending past the rest position due to the release of stored energy. What they do is prevent the vehicle from continuing to bounce after the initial shock. It is for this reason I say shock absorbers will not help in this situation since the shock introduced by any pothole, etc will still be felt the same, although the vehicle will settle quicker afterwards.

 

[Lutz]

The very fact that the pendular movement is reduced quicker with shock absorbers could reduce the danger of the locker doors coming adrift because the contents will settle down quicker, too, and not keep banging away at the doors from inside.

 

[Rich]

Sorry guys........ You are both wrong in so many ways!!!

RogerL "The more effective a shock absorber is" you are confusing effective with stiffer damping. The more effective a damper is the more control through the emulsion tube and shim stack. The damper has a damping curve which will allow slow axle movements freely but as the axle acclerates the damping will increase and as the axle decelerates and the axle reaches maximum travel the damping will soften. It is about controlling the speed at which the axle can travel but not stopping full movement. A damper at it's most effective is as soft as possible but controls suspension travel to the extent that road shock will not upset the vehicle and deflect its given path, it also allows the tyres to follow the road surface without being kicked into the air off the road when it hits a hard bump. if damping is too stiff the whole wheel will bounce off the road in the same way a trailer with no suspension will bounce down the road. If a tyre is allowed to bounce it will increase tyre wear and cause drive train damage over time.

Craig you are talking like a damper only works in extention and controls an axle returning from it's compressed state after hitting a bump. Dampers work both in compression and extention.

"It does absorb shock in as much as it soaks up the energy stored in the spring when it is compressed, thus preventing the spring from extending past the rest position due to the release of stored energy."

Sorry I do not understand this comment...... How does it do that? it can only do this if there is no compression damping and allows the spring to compress uncontrolled and then when the spring extends the damper works to stop the spring from releasing its full energy... In real life this would lead to "PACKING DOWN" This is where rebound damping is stiffer than compression. The axle will hit a bump and compress the spring but before the axle extends fully another bump will compress the axle again and after a few bumps the axle will be fully compressed and the vehicle will now behave as if it has no suspension at all !! Because it has bottomed out !!!

If you want a full real life lesson in suspension let me know and I will take you to a race meeting or test session where you can see suspension in action with datalogging showing wheel travel and damper speed. You might even get to see a damper stripped to it's core components and told exactly what emulsion tubes, shim stacks, flow trough needle valves, stop washers and pistons actually do. As I have said I have been doing this at the highest level for a number of years and even raced at an international standard and against a 5 time world champion! Sorry guys do not tell me what I do and do not know.......... I work with suspension and suspension technicians. I even supply shock absorbers which cost

 

[Rich]

LUTZ....."The very fact that the pendular movement is reduced quicker with shock absorbers could reduce the danger of the locker doors coming adrift because the contents will settle down quicker, too, and not keep banging away at the doors from inside."

This is correct slowing the movement in a controlled manner will reduce these problems. It is all about control of the suspension travel both through compression and extention.To do this the damper has to absorb shock and slow suspension travel to a point where it is not going to cause bounce or handling problems but will allow the suspension to settle to it's neutral position quick enough not to cause "PACKING DOWN" or the oposite problem "TOPPING OUT".......

 

[Surfer]

Despite all the blah, blah about the technicalities the biggest boon is when travelling on a motorway there is no sway when overtaking trucks or when being over taken. Secondly when towing there is not so much movement. They should be compulsory on all British vans!

 

[Lutz]

Quite right, Ian. There's no denying Rich knows what he is talking about but his explanations are rather longwinded and he does get a bit schoolmasterly in the process.

Shock absorbers (or dampers or whatever you want to call them) are actually one of the legal requirements in Germany to be able to tow at 100kph. Without them fitted, one is limited to 80kph.

 

[SteveW77]

Blimey, I did not think that I would cause so many differing opinions. It's all very interesting, but I am still not entirely sure if the answer to my question is yes or no. No doubt fitting shock absorbers could only improve the overall handling but what about my flying saucepans. I take the point about not lowering the tyre pressures.

Steve W

 

[joeby]

Thanks guy's i was thinking about fitting shocks after reading the last P C mag ,i enjoyed the piece from Rich ,but the last comments from Ian & Lutz have definately made my mind up to go ahead and fit them