MacPherson Strut Spring Installation 3

[nenesteamer]

MacPherson Strut front suspension was common on a lot of cars in the 1950s onwards. It is well known that the spring has to be compressed prior to, and during, assembly into the damper/spring unit and held in that state until the top flange-securing nut has been fully screwed down.

A simple question arises from this, purely out of academic interest:

Why is a compression coil spring in a strut-type suspension unit compressed prior to any of the vehicle mass being placed upon it?

 

[desertfox]

Hi nenesteamer

Have a look at page 39 of this book ref

http://books.google.co.uk/books?id=...&q=how to assemble a macpherson strut&f=false

desertfox

 

[NormPeterson]

Chances are that the suspension linkage does not provide enough suspension droop travel to completely unload the OE spring. This is not necessarily the case with short, stiff springs such as might be used in competition (and some of these may actually be "loose" with the suspension at full droop).


Norm

 

[nenesteamer]

desertfox - Interesting article but I can't see any explanation in the article as to 'why' the MacPherson strut is designed to have the spring compressed prior to assembly to the car.

NormPeterson - Good point about competition springs often being shorter, but surely the car makers wouldn't add to the production costs of millions of everyday cars (by requiring the standard spring to be compressed before fitting) simply because a few of them may be modified for competition work at a later date?

I hope I haven't stumped everyone with this question. I need to sleep at night!

David W.
Mechanical Engineering Group
Nene Valley Railway

 

[desertfox]

Hi nenesteamer

I thought it had to be compressed so it could be assembled and disassembled.

desertfox

 

[nenesteamer]

True, the spring does have to be compressed for assembly and disassembly, but that wasn't the question I was trying to ask.

I'm trying to find out why the spring isn't made to a length that would allow it to be dropped into position (uncompressed) and the top flange pulled down to the stop, at which point the spring would be just lightly clamped.

That would remove the need for pre-assembly compression and would avoid the danger of the top flange and spring flying off and causing injury if the spring clamp slips or breaks during assembly and disassembly, this not being unknown.

In view of the production line and after-service drawbacks, there must be a very sound technical reason for the spring needing to be compressed in its fitted position before any vehicle load is placed upon it.

It has been suggested that it could be so that the spring is held firmly to its seat at all times, with no sideways displacement in service. But the lip on the top and bottom flanges should serve that purpose anyway, or the top & bottom rubber insulation seats could be formed as simple dual-O/D items so as to positively locate the spring ends.

David W.
Mechanical Engineering Group
Nene Valley Railway

 

[DennisP]

I would think it's due to the fact that the spring in it's full extension position is somewhat nonlinear regarding rate. But if you start with some preload the rate is more nearly linear.

 

[nenesteamer]

DennisP,

Thanks for your very feasible suggestion. That will do for me unless someone comes along with a better answer.

David W.
Mechanical Engineering Group
Nene Valley Railway

 

[BrianPetersen]

Requiring the spring to be somewhat preloaded reduces the total vertical travel that the suspension system has to accommodate. Or, it allows a lower spring rate (softer ride) to be used within the same amount of travel.

As an aside, some lowering springs have no preload when the suspension is at full droop travel.

 

[desertfox]

Hi nenesteamer

A compression spring is non -linear for the first and last 10% of its compression so between these figures its linear and I doubt its anything to do wth non linearity.
If you alter the spring length you might find that for a given wire size and diameter you can no longer have the same number of active turns, if thats the case then the spring stiffness changes and the amount you compress the spring also changes, finally these changes will impinge on the stresses in the spring and if the stresses go unduly high may cause the spring to yield or fail from fatigue premerturely so I would think its more to do with getting the right spring to absorb the energy and keeping the stresses in the spring to a reasonable level.
Have a look at these sites:-

http://www.roymech.co.uk/Useful_Tables/Cams_Springs/Shock_absorbers.html

http://www.roymech.co.uk/Useful_Tables/Springs/Springs_helical.html

desertfox

 

[patprimmer]

I expect the spring performance requirements will determine the spring geometry.

The available space required installed loaded height and cost of struts will determine installed height on the unloaded strut.

Sometimes the former will exceed the latter, especially for soft suspensions with long travel.


Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[GregLocock]

I suggest you look at the cross section through a shock absorber in its fully extended and fully compressed conditions, and consider the implications.

The non linearity argument is a red herring.





Cheers

Greg Locock

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

 

[nenesteamer]

I have received a most interesting view on the matter by email from Roger Cooke:

"My thoughts are that, whatever the spring rate, if the strut was assembled uncompressed, fitted to the vehicle, and the vehicle lowered onto its wheels, the strut would immediately compress until the weight carried by the strut and the restorative force (=kx according to Hooke, where k is the spring constant and x the compression) provided by the spring are in equilibrium. This initial compression would probably exceed the travel required for unloading conditions during normal operation of the suspension (the droop distance) and the difference between the two would represent wasted travel in the strut. The softer the spring, the greater the waste. By precompression of the strut, the wasted travel could be eliminated.

"If the initial compression provided preload equal to the static weight carried by the strut, there would be no initial deflection but roadholding would be impaired since as soon as the vehicle reached the crest of a hill or bump the contact force on the road would be reduced, which is probably not ideal."


David W.
Mechanical Engineering Group
Nene Valley Railway

 

[LionelHutz]

A soft or low rate spring has to be compressed a lot before it will carry the weight of the vehicle. It's just not practical to allow enough suspension droop to accomodate fully unloading the spring.

It's not unique to struts either. Lots of the SLA suspensions and multi-link rear axle suspensions required the springs to be compressed to get the suspension pieces put together.

In some cases of suspension systems other than struts it becomes necessary to fasten a high rate spring into place to keep it from falling out when the suspension is unloaded.

 

[gt6racer2]

The simple answer it that we design in a load on the spring so that it stays together at fill droop. Otherwise we can have bits moving around ( spring rotating, isolators out of place ) and making noise or wearing out. In production use, if the spring rate is increased, for example for a sporty version, the strut extended length might be reduced to keep the spring in some compression at all times.
However, as noted by others, in some cases, due to low spring rates, and/or limited available droop travel (due to driveshaft joint angles etc...) the spring force at fill droop may be significantly bigger than it needs to be for the above purpose. Be careful.