Fork Stanchion Issue... some perspective?

[fred_jb]

Just been out to the garage to take a look at the front shock on my LC, it does have a hard rubber ‘stop’ fitted - does this limit the travel sufficiently?

I guess that depends on whether anything else reaches its limits before the shock can move no more! And that is only in the compression direction - what limits movement in extension?

 

[Engineer]

Well my background is electronics and IT, so I'm not qualified to say anything definitive about this either - this is just speculation on my part, though hopefully logical. It would really need someone to remove the spring and then manually move the suspension to its extremes in order to determine whether the bushes are providing a hard stop or not. However if they are not, and there is still compliance in them at the extremes of suspension deflection, then I don't see how the crimps could be sufficiently loaded to fail, at least not in normal operation.

The other possibility, as you suggest, is abnormal loads, such as hitting kerbs, etc, or maybe just extreme braking loads, but at the end of the day even these should generally get translated into either up or down deflection, or if head on, taken by the lower suspension arm, so I still think such loads should be absorbed without putting bending stresses into what are essentially just a pair of telescopic steering arms. Because it is free to shorten or lengthen as required, I find it hard to see how the stanchion and slider assembly could be stressed other than through excessive angular deflection.

Electronics is my profession so I too am just trying to understand this logically .

Imagine the front wheel taking a large horizontal hit - as the Telelever has a ball joint at its apex then I can imagine the forks trying to rotate forwards, only stopped by the top yoke, this would stress the crimps. The collars would guard against this.

The Telever migh separate the suspension movement from the steering but the top joint surely must resist braking forces????

 

[Engineer]

I guess that depends on whether anything else reaches its limits before the shock can move no more! And that is only in the compression direction - what limits movement in extension?

In extension I guess there is an internal stop in the shock otherwise it would fall apart ?

 

[fred_jb]

Electronics is my profession so I too am just trying to understand this logically .

Imagine the front wheel taking a large horizontal hit - as the Telelever has a ball joint at its apex then I can imagine the forks trying to rotate forwards, only stopped by the top yoke, this would stress the crimps. The collars would guard against this.

The Telever migh separate the suspension movement from the steering but the top joint surely must resist braking forces????

You could be right - unlike a hub centre arrangement the suspension arm is not at the level of the wheel centre, but above it, so you could get some rotation of the lower arms, leading to stress in the stanchion top mounts, under braking or from a horizontal hit high up on the wheel. However, I think rotation about the ball joint would result in more of a front to back sideways pressure on the plug/stanchion interface, rather than the rocking you would get from excessive angle changes in extreme suspension deflections, so I'm not sure if that would be so damaging, but definitely a possibility. Thinking about it I think we may have reached the same conclusion in an earlier discussion on this, but I had forgotten.

Anyway, I guess we will never know, but overall I am now feeling that Telelever is a rather flawed concept, and may well not carry on in the next generation of GS.

Regarding bump stops and extension of the damper, I guess the shock can only extend so far, but I guess the big forces will be in compression not extension anyway, so probably an extension stop is not such an issue, and as you say, the damper rod is unlikely to be able to be pulled right out!

 

[Engineer]

You could be right - unlike a hub centre arrangement the suspension arm is not at the level of the wheel centre, but above it, so you could get some rotation of the lower arms, leading to stress in the stanchion top mounts, under braking or from a horizontal hit high up on the wheel. However, I think rotation about the ball joint would result in more of a front to back sideways pressure on the plug/stanchion interface, rather than the rocking you would get from excessive angle changes in extreme suspension deflections, so I'm not sure if that would be so damaging, but definitely a possibility. Thinking about it I think we may have reached the same conclusion in an earlier discussion on this, but I had forgotten.

Anyway, I guess we will never know, but overall I am now feeling that Telelever is a rather flawed concept, and may well not carry on in the next generation of GS.

Regarding bump stops and extension of the damper, I guess the shock can only extend so far, but I guess the big forces will be in compression not extension anyway, so probably an extension stop is not such an issue, and as you say, the damper rod is unlikely to be able to be pulled right out!

I think that between us we have explained it without being mechanical engineers

I do disagree with you about the Telever being fundamentally flawed though as the pre LC bikes didn’t show this problem - something about the Marzocchi re-design for the LC meant it wasn’t quite as robust or their quality control was not as good - that however has to remain speculation without inside information.

 

[Karlos]

I found a reference somewhere to the fact that the HP2 had spherical bearings, but I'm pretty sure the current bikes don't. When I looked at this before I'm sure that on the parts diagram it is shown as a bush which looks like it includes both metal and rubber. I'm not sure if there is a rubber layer between the inner and outer tubular sections, but it looks like it from the following video. I used to look at this sort of stuff on Rainbow's very useful parts ordering website, which shows diagrams and part numbers, but can't seem to get to it anymore. Maybe they are revamping their website. However, I found this on a USA website, and I think the part which provides some limited movement is called a joint link, part no 2 in this diagram:

http://parts.bmwmotorcycles.com/a/B...6_5831558/Upper-fork-cross-brace/31_1035.html

The plug which goes into the top of stanchion seems to have a rounded section below the retaining screw/bolt that goes up through the joint link, but I don't think this is a spherical joint. You can see that part here:

http://parts.bmwmotorcycles.com/a/BMW_2017_R1200GS/_51506_5831297/STANCHION/31_1038.html

It seems that the crimped stanchion is made up from three parts, the stanchion, something described as an ISA screw, and an O-ring.

The following picture of this is included in a useful article here: https://www.bennetts.co.uk/bikesoci...7/july/bmw-r1200gs-and-gsa-fork-safety-recall

This is a difficult one to solve without affecting steering precision as I think you need to allow movement within the joint in a wide range of directions dependent on how much steering lock is available. This is because as far as I can imagine it, when the steering is not in the straight ahead position the angular movement of the stanchion under suspension deflection is nevertheless still going to be aligned with the longitudinal axis of the bike, so is effectively requiring a sideways freedom of movement with respect to the top mount fixing.

Fred

Why are you talking about a bush that isn’t part of the recall? The part (number 2 in the diagram) has cock all to do with the recall. The bush you are talking about is a compliance bush, the reason is has rubber to to dampen the vibrations to the top yolk and handle bars.


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[Karlos]

I think that between us we have explained it without being mechanical engineers

I do disagree with you about the Telever being fundamentally flawed though as the pre LC bikes didn’t show this problem - something about the Marzocchi re-design for the LC meant it wasn’t quite as robust or their quality control was not as good - that however has to remain speculation without inside information.

I think between you, you have come up with a load of bollocks.


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[Engineer]

I think between you, you have come up with a load of bollocks.


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We are not blaming the bush - we were trying explain the fork crimp fails and the movement and forces that might cause the crimp to fail - the compliance bush is just part of the puzzle, as is the Telelever ball joint and the suspension travel extremes.

What do you think causes the crimp joint to fail? What forces do you thing are significant? If you have another theory I would be interested to hear it.

 

[fred_jb]

Why are you talking about a bush that isn’t part of the recall? The part (number 2 in the diagram) has cock all to do with the recall. The bush you are talking about is a compliance bush, the reason is has rubber to to dampen the vibrations to the top yolk and handle bars.


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No you certainly wouldn't want vibrations to the top yolk - you might end up with egg on your face!

 

[Karlos]

We are not blaming the bush - we were trying explain the fork crimp fails and the movement and forces that might cause the crimp to fail - the compliance bush is just part of the puzzle, as is the Telelever ball joint and the suspension travel extremes.

What do you think causes the crimp joint to fail? What forces do you thing are significant? If you have another theory I would be interested to hear it.

It’s no puzzle, the ball joint takes care of the slight angle variation as the forks compress. The reason for the failure of the peening is due to it being a peen. Peening is a me mechanical locking method that relies on an interference to maintain the ‘lock’.

I am sure BMW used this method as there is very little suspension force at that point, however it appears that on some GSs the force has been more that anticipated, probably why it only happened on the bikes used off road.

Your looking to deep into this and coming up with all sorts of spurious answers. The answer is simple, all metal flexes, in this case it just happened that the flexing was more than the peening could cope with. Peening is generally a crap way of securing things.



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[XPilot]

From Wikipedia - Peening is the process of working a metal's surface to improve its material properties, usually by mechanical means, such as hammer blows, by blasting with shot (shot peening) or blasts of light beams with laser peening. Peening is normally a cold work process (laser peening being a notable exception[citation needed]). It tends to expand the surface of the cold metal, thereby inducing compressive stresses or relieving tensile stresses already present. Peening can also encourage strain hardening of the surface metal.
Nothing to do with mechanical locking - or do you know something different ?

 

[roamer]

Maybe with the tubes not having a thread as per earlier forks, they are thinner wall and flaring , placing a sleeve over the thinner tube resists this,
Adding the saved weight but probably still quicker and cheaper to make,

 

[fred_jb]

It’s no puzzle, the ball joint takes care of the slight angle variation as the forks compress. The reason for the failure of the peening is due to it being a peen. Peening is a me mechanical locking method that relies on an interference to maintain the ‘lock’.

I am sure BMW used this method as there is very little suspension force at that point, however it appears that on some GSs the force has been more that anticipated, probably why it only happened on the bikes used off road.

Your looking to deep into this and coming up with all sorts of spurious answers. The answer is simple, all metal flexes, in this case it just happened that the flexing was more than the peening could cope with. Peening is generally a crap way of securing things.



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Yes the ball joint on the suspension arm allows for a change in angle, but there is also a change in angle at the top of the stanchions where they are secured to the yoke. This would ideally be a rigid joint because it is where steering inputs from the yoke are transmitted to the stanchions and thence to the wheel. However, it cannot be totally rigid because it also has to accommodate the change in angle between the yoke and stanchions as the suspension arm moves up and down following an arc - hence the semi-rigid fixing in the shape of the compliance bush, or joint link as BMW call it. This is why I now regard this as a compromised design.

The change in angle at the top of the stanchions is why we have been discussing the role of the joint link, through which the tops of the stanchions are secured to the yoke. It it were not able to accommodate the full range of movement required at extremes of suspension movement, or under other loads such as heavy braking or impacts to the wheel, then it could be the reason that the crimped joint comes under pressure. You say metal flexes, but it doesn't flex if it is free to move in response to the geometry changes in the suspension. It will only flex, putting pressure on the crimped joints, if it runs out of allowable movement in the joint and becomes rigidly held at one end.

 

[Victor c]

I remember reading sometime ago, that BMW had put the issue down to resonance transmitting up through the fork tubes during a/or series of, heavy impacts. This then causes the crimps to loosen and the tube to flare out. To me this makes sense, as the later LC bikes rely on less crimps than the earlier ones (less stability in the tube metal) and the fitting of the collar will eliminate it by keeping the top of the tube in shape.

Just my two penny worth as a fellow engineer

 

[Engineer]

I remember reading sometime ago, that BMW had put the issue down to resonance transmitting up through the fork tubes during a/or series of, heavy impacts. This then causes the crimps to loosen and the tube to flare out. To me this makes sense, as the later LC bikes rely on less crimps than the earlier ones (less stability in the tube metal) and the fitting of the collar will eliminate it by keeping the top of the tube in shape.

Just my two penny worth as a fellow engineer

You could well be spot on there.

It’s an interesting discussion If I had the time and the inclination I measure all the relevant dimensions on my bike and do the geometry to see if there is an angular movement at the top yoke as the suspension travels up and down - I don’t think there should be any but it’s difficult to visualise qualitatively so only an accurate geometric drawing and calculation would convince me one way or another.

The collars would certainly stop any resonant flexing from weakening the joint.

 

[fred_jb]

You could well be spot on there.

It’s an interesting discussion If I had the time and the inclination I measure all the relevant dimensions on my bike and do the geometry to see if there is an angular movement at the top yoke as the suspension travels up and down - I don’t think there should be any but it’s difficult to visualise qualitatively so only an accurate geometric drawing and calculation would convince me one way or another.

The collars would certainly stop any resonant flexing from weakening the joint.

I think the geometry makes some angle change at the top of the stanchions unavoidable - see this, which shows the principle, but is not necessarily a realistic representation of the actual change in angle on today's GS design.

I would say there are three ways in which the crimps could come under pressure.

The first is longitudinal. However, I find it hard to believe that the simple tromboning of the telescopic parts would be able to exert sufficient longitudinal pushing or pulling to put pressure on the crimps.

The second is where hard braking would cause the lower slider to try to rotate around the ball joint on the suspension arm which carries the damper, and this movement would be transmitted to the top of the stanchion, and opposed by the joint links in the top yoke. In this case the pressure on the joint would be exerted perpendicular to the direction of the stanchion, and I would think the joint link is fairly well designed to resist this. It also puts pressure on the crimped section, but not, IMO, in a direction which could easily distort and loosen it, though it is definitely a possibility.

A similar rotation effect, possibly of higher magnitude than braking, could be caused by an impact high up on the front of the wheel, but I think this is more in the realms of a crash impact than normal use, in which case I think all bets are off anyway.

This leaves the third form of motion where the angle of the stanchion with respect to the yoke changes due to the geometry of the suspension changing as the suspension arm moves up and down. The ball joint attachment to the forks moves through an arc which has the effect of pushing and pulling on the forks which slightly changes the angle at the point where they attach to the yoke.

The pressure from this angular movement at the top of the stanchion tends to tilt the inner part of the joint link with respect to the outer, and there is a very limited amount of movement available. If this movement is fully used up in extreme suspension deflections, then it could lock the top mount, meaning that any further change of angle can only be accommodated by flexing in the stanchion/slider assembly. In my opinion this sort of angular flexing is the most likely sort of stress to do damage to the crimps by rocking the joint back and forth and gradually deforming the top of the stanchion.

Fred

 

[Victor c]

Need to check with Chester Motorrad in the new-year, to see if they have any collars for my bike yet. But to be honest, i’m in no rush, as not concerned about the very slim chance of failure. There are far worst things to look out for when riding a bike.

 

[fred_jb]

I remember reading sometime ago, that BMW had put the issue down to resonance transmitting up through the fork tubes during a/or series of, heavy impacts. This then causes the crimps to loosen and the tube to flare out. To me this makes sense, as the later LC bikes rely on less crimps than the earlier ones (less stability in the tube metal) and the fitting of the collar will eliminate it by keeping the top of the tube in shape.

Just my two penny worth as a fellow engineer

By blaming some mysterious "resonance" I suspect BMW is just using a bit of disinformation to mask the fact that they don't know, or more likely don't want to admit, what is really causing the problem. Resonance is a specific condition whereby a part or assembly will preferentially vibrate at its resonant frequency, but to do so it must firstly be triggered by some suitable external force, and secondly must be free to vibrate at that frequency, in this case to the extent that it can cause damage. Of course I believe that external forces are causing the damage, but I very much doubt that this is through triggering some uncontrolled resonance, and if movement of the suspension was properly controlled by design, then it should not be susceptible to resonance effects in the first place, so this is hardly a valid excuse.

 

[Neil W]

F*ck mysterious resonance, older series bikes with the same design suspension ie r1100, r1150, r1200 pre lc and the very early gs lc models never had a problem as they used a mechanically sound design ie the tops of the stanchions were threaded and the top cap was screwed in.

BMW by accepting a no doubt cheaper to produce and buy design with a push fit and crimped fixing have caused the problem by penny pinching and trying to increase their profit margins.

 

[fred_jb]

F*ck mysterious resonance, older series bikes with the same design suspension ie r1100, r1150, r1200 pre lc and the very early gs lc models never had a problem as they used a mechanically sound design ie the tops of the stanchions were threaded and the top cap was screwed in.

BMW by accepting a no doubt cheaper to produce and buy design with a push fit and crimped fixing have caused the problem by penny pinching and trying to increase their profit margins.

Totally agree. The same forces have always come into play with this design, but the current version seems inadequate to stand up to them. Possibly the problem has also been exacerbated by providing greater suspension travel on the later bikes - though I'm not sure if that is actually the case. If so, then a weaker design, combined with greater demands on the design, is a potentially fatal combination.