Technical Koni FSD rear shock rattle.. Shock!

[The Twins]

I find it hard to believe that these KONI FSD shocks are unreliable.

Regarding the bolts issue I have some doubts too. Maybe I'm wrong, but this is what I remember from my years at school some ages ago: The shaft of the bolt itself isn't supposed to keep the bush of the shock in place. That's the job of the head of the bolt. The bush of the shock should be squeezed so firmly between the head of the bolt and the chassis that no movement can occur. If bolts with fatter shafts fix the problem, then this means that the bolts aren't thightened enough to prevent movement of the bushes. Then you reduce the harm done by the symptoms without removing the cause.

Yes exactly

 

[jrkitching]

Maybe I'm wrong, but this is what I remember from my years at school some ages ago: The shaft of the bolt itself isn't supposed to keep the bush of the shock in place. That's the job of the head of the bolt. The bush of the shock should be squeezed so firmly between the head of the bolt and the chassis that no movement can occur. If bolts with fatter shafts fix the problem, then this means that the bolts aren't thightened enough to prevent movement of the bushes.

You're wrong; bolts work best in tension and are not intended to absorb free play in shear.

"A bolt is primarily designed to withstand tensile loading while clamping components together. Any forces tending to slide the clamped components laterally should be withstood by separate means."

The correct fit between the shank of the bolt and the bush is essential in preventing unwanted movement in the shock absorber mountings and tightening the bolts beyond their recommended torque settings is extremely poor practice which should be deprecated. The shock absorbers should first be assembled with the bolts just loosely tightened; if there is any noticeable play in the connection then the bolts/bushes are either of an incorrect size, or there is wear in the bolts or bushes, and the worn component(s) must be replaced. The bolts should be tightened to their recommended torque only after it has been established that there is no unwanted free play in the joint.

 

[AGH1965]

"A bolt is primarily designed to withstand tensile loading while clamping components together. Any forces tending to slide the clamped components laterally should be withstood by separate means."

That's exactly what I meant to write, although you have another conclusion. The shaft of the bolt isn't meant to prevent lateral sliding of the clamped components. The sliding has to be withstood by the friction that is a result of the force with which the bush is squeezed between the head of the bolt and the chassis. Therefore the shaft of the bolt doesn't even have to touch the inside of the bush of the shock. The play you mention doesn't have to be a problem as long as the tightening torque is high enough to generate enough friction between the head of the bolt, the bush and the chassis. If this play would be a problem, then every single 500 would have rattling shocks.

 

[jrkitching]

The shaft of the bolt isn't meant to prevent lateral sliding of the clamped components. The sliding has to be withstood by the friction that is a result of the force with which the bush is squeezed between the head of the bolt and the chassis. Therefore the shaft of the bolt doesn't even have to touch the inside of the bush of the shock. The play you mention doesn't have to be a problem as long as the tightening torque is high enough to generate enough friction between the head of the bolt, the bush and the chassis. If this play would be a problem, then every single 500 would have rattling shocks.

I'm sorry, but what you have posted is just 100% wrong. You have completely misunderstood the basic engineering concepts behind how bolts are designed to work. It is essential that the shaft of the bolt is a good fit with inside of the suspension bush and it is vital that you do not attempt to correct for any significant slack by overtightening the bolts.

Once again, I'm sorry I have to be so forthright about this, but the suspension is an important safety feature of the car and I don't want anyone to be misinformed about how these parts should be assembled and checked.

 

[306maxi]

jrkitching is right as usual.....

 

[The Twins]

I'm sorry, but what you have posted is just 100% wrong. You have completely misunderstood the basic engineering concepts behind how bolts are designed to work. It is essential that the shaft of the bolt is a good fit with inside of the suspension bush and it is vital that you do not attempt to correct for any significant slack by overtightening the bolts.

Once again, I'm sorry I have to be so forthright about this, but the suspension is an important safety feature of the car and I don't want anyone to be misinformed about how these parts should be assembled and checked.

The bush sleeve is clamped in a fixed position by the bolt and even if there is some 'slop' there would be no rotational let alone 'rattling' / movement unless the bolt was loose which would allow rotation and would wear the sleeve out in about 10 minutes. It's not a bolt sleeve issue JRK. Sorry to be forthright ; )

 

[The Twins]

That's exactly what I meant to write, although you have another conclusion. The shaft of the bolt isn't meant to prevent lateral sliding of the clamped components. The sliding has to be withstood by the friction that is a result of the force with which the bush is squeezed between the head of the bolt and the chassis. Therefore the shaft of the bolt doesn't even have to touch the inside of the bush of the shock. The play you mention doesn't have to be a problem as long as the tightening torque is high enough to generate enough friction between the head of the bolt, the bush and the chassis. If this play would be a problem, then every single 500 would have rattling shocks.

Exactly right, some others don't get it though ; )

 

[jrkitching]

The bush sleeve is clamped in a fixed position by the bolt

Indeed it is, and the clamping force provided by the bolt will by design stop the bush from rotating. However, the rotational force on the bush in normal service is minimal; the principal load on the connection is perpendicular to the major axis of the bush and must be taken by the close fit of the bolt shank/bush; the clamping force provided by the bolt is not designed nor intended to carry the primary loading of the connection.

Attempting to argue otherwise demonstrates a profound lack of basic engineering knowledge and I'd recommend anyone who has difficulty in following this argument to search the internet for further information about how bolts work.

 

[The Twins]

That's exactly what I meant to write, although you have another conclusion. The shaft of the bolt isn't meant to prevent lateral sliding of the clamped components. The sliding has to be withstood by the friction that is a result of the force with which the bush is squeezed between the head of the bolt and the chassis. Therefore the shaft of the bolt doesn't even have to touch the inside of the bush of the shock. The play you mention doesn't have to be a problem as long as the tightening torque is high enough to generate enough friction between the head of the bolt, the bush and the chassis. If this play would be a problem, then every single 500 would have rattling shocks.

Indeed it is, and the clamping force provided by the bolt will by design stop the bush from rotating. However, the rotational force on the bush in normal service is minimal; the principal load on the connection is perpendicular to the major axis of the bush and must be taken by the close fit of the bolt shank/bush; the clamping force provided by the bolt is not designed nor intended to carry the primary loading of the connection.

Attempting to argue otherwise demonstrates a profound lack of basic engineering knowledge and I'd recommend anyone who has difficulty in following this argument to search the internet for further information about how bolts work.

Your right JRK but this isn't what causes rattling. If you think about it, rattling would mean that there is movement between the sleeve and bolt including movement at the bolt head which would subsequently wear the faces of the sleeve causing the bolt to loosen - which doesn't happen. You see where I'm coming from?

 

[jrkitching]

Your right JRK but this isn't what causes rattling. If you think about it, rattling would mean that there is movement between the sleeve and bolt including movement at the bolt head which would subsequently wear the faces of the sleeve causing the bolt to loosen - which doesn't happen. You see where I'm coming from?

Yes - I'd agree that it may not be the reason for the rattling, in which case overtightening the bolt will not help anyway, nor will fitting bolts with a closer fit between the shank and bush. The theory that overtightening appeared to provide a temporary fix because it disturbed the configuration is sound, if you'll pardon the pun, which fits with the observation that just jacking up the car sometimes provides a brief respite.

If it's been rattling for an appreciable length of time, there should be a tell tale sign somewhere on the metal, although that just might be inside a part of the shock absorber that you can't easily dismantle. Tracing the root cause of rattles can sometimes be surprisingly difficult and frustrating.

 

[The Twins]

I'm pretty sure the rattle is to do with the 'FSD' damping system inside the shocks. Seems too widespread and suffered by too many 500's to be anything else outside the shock itself.

 

[jrkitching]

I'm pretty sure the rattle is to do with the 'FSD' damping system inside the shocks. Seems too widespread and suffered by too many 500's to be anything else outside the shock itself.

That certainly makes sense on the basis of what's being posted both on this forum and elsewhere.

A quick google suggests the problem with FSD's isn't confined to the 500. This and this are examples of relevant posts.

That same quick google search also suggests Koni's warranty returns policy is far from straightforward .

 

[AGH1965]

Since some might have misinterpreted what I wrote earlier:
I DON’T ENCOURAGE ANYONE TO EXCEED THE TIGHTENING TORQUE SPECIFIED BY FIAT!

Nevertheless I have a question about the following:

the principal load on the connection is perpendicular to the major axis of the bush and must be taken by the close fit of the bolt shank/bush

if there is any noticeable play in the connection then the bolts/bushes are either of an incorrect size, or there is wear in the bolts or bushes

This all seems logical, but how does this match with the fact that FIAT deliberately uses bolts with shank diameter smaller than the thread diameter?

With the inner diameter of the bush being larger than the thread diameter (otherwise the bolt won’t go through the bush) and the shank diameter clearly smaller than the thread diameter, it’s absolutely impossible to have no noticeable play, not even with brand new standard shocks and bolts.

According to the following statement by Gregi there will be a play of 2 mm at the upper bush and of 1.5 mm at te lower bush. Both are large enough to be noticeable.

I have standard rear shocks lying around as they are left after having Esseesse Koni kit fitted lately. Upper shock bush has 11 mm diameter, lower shock bush has 12,5 mm diameter. Original upper shocks bolts have 9 mm shaft diameter, lower shocks bolts have 11 mm shaft diameter.

(Previously I used the word "shaft" for the unthreaded part of the bolt, but apparently the word "shank" is a better translation. Sorry for that.)

 

[jrkitching]

Nevertheless I have a question about the following:
This all seems logical, but how does this match with the fact that FIAT deliberately uses bolts with shank diameter smaller than the thread diameter?

If it's intentional, it's just a very poorly engineered design. Car makers design stuff badly more often than they should; Fiat don't have a monpoly on this.

With the inner diameter of the bush being larger than the thread diameter (otherwise the bolt won’t go through the bush) and the shank diameter clearly smaller than the thread diameter, it’s absolutely impossible to have no noticeable play, not even with brand new standard shocks and bolts.

If the manufacturing tolerances are sufficiently tight, you can certainly have a fit between the shank of the bolt and inner surface of the bush which is both easily assembled and has no appreciable play. It should also be designed in such a way that no part of the threaded portion of the shank lies inside the bush when the joint is assembled.

Think about it - every shaft/bearing connection in the car is designed this way, and shafts generally don't flop about in their bearings (unless you have an M32 gearbox - sorry Mel ). But if they do wear to the point where there is noticeable play, it's not long before the bearing fails. If you think about the places where a shaft is running in a plain bush, the comparison is even more relevant.

Starting out with a design where there is substantial free play in a connection which has to carry significant loads is just asking for trouble - and it sounds like that's exactly what owners are getting.

However, whilst this is all good engineering theory, I'm still minded to think that the root cause of the rattling could be wear inside the shock absorber itself.

 

[Murano16v]

Before I had my Koni's changed last year I was all for getting some bolts made. I did have a quick look under the car to check the tightness of the bolts and concluded that in no way could there be any movement between the bolt, bush, and mounting. The car was quieter with the new shocks, though maybe not 100%. It has got no no worse since. I would however say it's not as quiet as my son's five year old Abarth on standard shocks.
It almost seems that the Koni's do not cope too well with situations where there are several undulations in the road surface close together at slow speeds.

 

[AGH1965]

Are there any forum members who mounted the adjustable yellow KONI SPORT rear shocks? If these have the same bush construction as the FSDs but no rattling problems, then it's clear that the FSD valves are to blame.

 

[The Twins]

Are there any forum members who mounted the adjustable yellow KONI SPORT rear shocks? If these have the same bush construction as the FSDs but no rattling problems, then it's clear that the FSD valves are to blame.

Good point you make. I'd replace mine with Koni sport if I had the ££

 

[AGH1965]

Then you must be lucky. KONI is Dutch and the € is getting cheaper and cheaper for you.

 

[Gregi]

Is there an update? Are there any new experiences with or without custom made bolts worth mentioning?

Since 22438 km (14023 miles) made on different quality roads and custom bolts fitted - still no problems with knocking Konis

In terms of bolts tightening - mine were primarily tightened on loaded suspension (while the car was on column lift) and later on once more while the car was already standing.

 

[The Twins]

Since 22438 km (14023 miles) made on different quality roads and custom bolts fitted - still no problems with knocking Konis

In terms of bolts tightening - mine were primarily tightened on loaded suspension (while the car was on column lift) and later on once more while the car was already standing.

Good that they are silent now.
I still cannot understand how this can be the fix though as the bolt head / face of bush sleeve interface would be worn fast if there was movement which would make the bolts come loose. The fact that the bolt heads of face of the sleeves do not show any wear tells me there is no movement here.