So that news is different to the, "It's bikes that are being used off-road that may have a problem", message from my local BMW.
Fork Stanchion recall?
- Thread starter gerarddwatts
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So that news is different to the, "It's bikes that are being used off-road that may have a problem", message from my local BMW.
Cotswoldshire
Registered user
fred_jb
Registered user
I don't think the plug in the top of the stanchion which is held into the top of the stanchion by the crimps has any rubber component. It just has an integral bolt coming out of the top which together with a matching nut is what holds the stanchion into the top yoke (well at least so long as plug and stanchion don't separate!).
The part which appears to allow the necessary angular movement of the stanchion seems to be called a joint link. The bolt in the top of each stanchion plug passes through this and is held in to it with a nut at the top, and these nuts can be seen under the plastic top covers in each side of the top yoke.
Item 02 here: http://www.rainbowbmw.com/BMWETK/Products/0A01_31052.php
A film of the part is here: https://www.youtube.com/watch?v=DKmhowy1yq0
It looks like the central metal part of this so-called joint link, which the bolt coming out of the stanchion plug bolts through, has a rubber surround, so I think it is this which allows the slight angular movement normally required. I don't think this is a particularly good design as it looks like it has a very limited range of movement after which any further angular movement of the stanchion under heavy loads is going to try to flex the stanchion itself, and I suspect it is this which is putting a strain on the crimps and causing them to loosen. If this is the case, then the current "fix" is really tackling the symptoms rather than the cause. Apparently the HP2 had a proper spherical bearing at the top of each stanchion to take care of this movement, which I suspect would have a bigger range of movement, but it seems that a re-design/cost reduction has resulted in the current, possibly inadequate, solution.
I think the only other relevant rubber part is the rubber sleeve which covers the crimps in the top of the stanchion and has to be slid down to inspect the join between stanchion and plug.
You can see all these parts in this video from a dealer posted earlier in the thread: https://youtu.be/EXGJkOG-5P0
Fred
Looking at the drawings on Rainbow's site, BMW have been the same "upper fork brace" joint link arrangement since the R1100. The rubber sleeve over the crimping on the LC stanchions is cosmetic, IMHO.
fred_jb
Registered user
It seems that this problem has only become apparent on bikes built since the style of crimp was changed shortly after the start of LC production, and it is only these which are being recalled. I think this proves that the plug-stanchion interface is coming under great strain, which is why a small change in the crimp style has been enough to expose this problem. The crimps should only really be needed to resist longitudinal movement of the stanchions as the forks shorten and lengthen to follow the movement of the suspension arm. As the stanchions don't take any of the suspension loads, just the small drag from the bearing and oil seal surfaces inside the lower leg, plus the steering input, then this should be fine.
However, with the limited angular movement available at the top mount, the forward push and rearward pull on the bottom yoke, which will be more extreme with the long travel suspension on the GS/GSA than other models, is possibly using up all the limited movement available, even with moderate suspension deflections, so with more extreme suspension movement the stanchions could be getting a forward and backward flex loading. If this happens, then as well as the moderate longitudinal stresses the crimps are also having to resist a forceful lateral rocking movement, and this seems to be the cause of the problem.
In my opinion, this would not happen with a better design. If the design allowed for adequate, but controlled angular movement at the top of the stanchion to accommodate even extremes of suspension movement, then I don't think the crimps would be so critical.
Victor c
Registered user
There is a guy call Shawn Smoak, who is a BMW master tech that does vlogs on YouTube, explains how the stanchions flex and how the modification collars work in one of his resent vlogs. Just stick his name in the search box
fred_jb
Registered user
I did find one video by this guy where at about one third of the way into a long video record of his whole day - shopping, putting gas in his car, talking to his wife and kids about domestic matters, etc, etc, (boring, boring) - there is a short section where he rambles on about the fork issue in what seemed to me to be a pretty ill-informed way. I would therefore be surprised if he is a "BMW master tech" and a bit alarmed if he really is! He seemed to think it was only an issue if you hit something at 40 mph! Maybe he has done another better video just devoted to the subject, but if so I didn't find it.
According to the dealer today this issue will now become a full DVSA recall issue, all bikes with one or more fork leg with a 2mm gap will be getting brand new modified fork legs, all bikes showing no sign of movement on the joint or with gaps of below 2mm will be getting the modification collar fitted.
they took delivery of a new Rallye while i was there and it already had the collars fitted on the forks .
Mine has a 1mm gap on one fork so will be sorted next month at service .
they took delivery of a new Rallye while i was there and it already had the collars fitted on the forks .
Mine has a 1mm gap on one fork so will be sorted next month at service .
fred_jb
Registered user
That's interesting, and shows that it is now seen as a serious issue. Incidentally, I believe the figure for acceptable gap is up to 0.2 mm not 2 mm, otherwise they might be measuring it with a ruler not a feeler gauge! 
Mine had no discernable gaps so got the collars fitted yesterday.
Mine had no discernable gaps so got the collars fitted yesterday.
fred_jb
Registered user
The drawings of the joint link parts look different on the K50 (LC) and previous K25 versions of the bike and have different part numbers, so have evidently changed. It occurred to me that the more compliant these parts are, which would reduce strain on the crimps under extreme suspension movements, then the more they will affect steering feel as you have a squidgy rubber layer between the bars and the parts that transmit steering movements to the wheel, which is the stanchions.
Could be that these were tightened up a bit for the LC to improve steering feel, but this might have made them a bit more marginal as regards allowing the top of the stanchion to move in response to the lower suspension arm pushing and pulling on the forks, so could be putting more strain on the crimps. However, I'm still a bit puzzled as to why the early LC production run using round crimps are apparently OK and later rectangular ones aren't. Maybe it is not because they used round crimps, but because these early versions also used the earlier K25 joint link?
RVJM
Registered user
Better video including replacement process
Here is video from a dealer in Cardiff showing the entire process.
https://youtu.be/TaGaR9Fn4WE
or search youtube for Stanchion Recall BMW GS/GSA by pistoleharely
Here is video from a dealer in Cardiff showing the entire process.
https://youtu.be/TaGaR9Fn4WE
or search youtube for Stanchion Recall BMW GS/GSA by pistoleharely
fred_jb
Registered user
I found this nifty animation while trying to improve my understanding of the Telelever system. This clearly illustrates what I think is the major weakness of this system, namely that the angle of fork (brown line) changes with deflection of the suspension, and the greater the suspension travel available, then the bigger the change in angle can be. To me the problem is not so much the geometry changes this causes, but the fact that this precludes the use of a solid fixing to the top yoke, given that it is the top yoke which transmits steering input into the system. This means there will be a degree of give in the steering, which might explain the complaints of poor steering feel with this system. This also means that any undue resistance or limit to this angle change could put a strain on the stanchion and cause flexing at its weakest point.
It is clear from this that the connection to the top yoke must provide some freedom of movement - it cannot be solid. Furthermore, this freedom to move is always required to be along the longitudinal axis of the bike, even when the steering is not in the straight ahead position. That means that movement cannot be limited to one direction within the fixing to the top yoke, as when the steering is turned to the side the top of the fork will still flex back and forth along the axis of the bike, so with respect to the yoke fixing, movement must be available through a range of angles. This appears to be catered for by the bolts on the top of the stanchions being held in a metal bush with a concentric rubber layer, which BMW call a joint link.
My suspicion is that for the LC model BMW stiffened up this bush and/or limited its range of movement in order to improve steering feel, but with the side effect that the greater resistance to movement and possibly more limited range would have put more strain on the crimped fixing between the stanchion tube and the aluminium plug which provides the connection to the top yoke, especially when the suspension reaches its extremes of travel. Possibly the early LC production used up stock of the system from the previous model which could explain why these bikes are not subject to the recall
Fred
It is clear from this that the connection to the top yoke must provide some freedom of movement - it cannot be solid. Furthermore, this freedom to move is always required to be along the longitudinal axis of the bike, even when the steering is not in the straight ahead position. That means that movement cannot be limited to one direction within the fixing to the top yoke, as when the steering is turned to the side the top of the fork will still flex back and forth along the axis of the bike, so with respect to the yoke fixing, movement must be available through a range of angles. This appears to be catered for by the bolts on the top of the stanchions being held in a metal bush with a concentric rubber layer, which BMW call a joint link.
My suspicion is that for the LC model BMW stiffened up this bush and/or limited its range of movement in order to improve steering feel, but with the side effect that the greater resistance to movement and possibly more limited range would have put more strain on the crimped fixing between the stanchion tube and the aluminium plug which provides the connection to the top yoke, especially when the suspension reaches its extremes of travel. Possibly the early LC production used up stock of the system from the previous model which could explain why these bikes are not subject to the recall
Fred
I found this nifty animation while trying to improve my understanding of the Telelever system. This clearly illustrates what I think is the major weakness of this system, namely that the angle of fork (brown line) changes with deflection of the suspension, and the greater the suspension travel available, then the bigger the change in angle can be. To me the problem is not so much the geometry changes this causes, but the fact that this precludes the use of a solid fixing to the top yoke, given that it is the top yoke which transmits steering input into the system. This means there will be a degree of give in the steering, which might explain the complaints of poor steering feel with this system. This also means that any undue resistance or limit to this angle change could put a strain on the stanchion and cause flexing at its weakest point.
It is clear from this that the connection to the top yoke must provide some freedom of movement - it cannot be solid. Furthermore, this freedom to move is always required to be along the longitudinal axis of the bike, even when the steering is not in the straight ahead position. That means that movement cannot be limited to one direction within the fixing to the top yoke, as when the steering is turned to the side the top of the fork will still flex back and forth along the axis of the bike, so with respect to the yoke fixing, movement must be available through a range of angles. This appears to be catered for by the bolts on the top of the stanchions being held in a metal bush with a concentric rubber layer, which BMW call a joint link.
My suspicion is that for the LC model BMW stiffened up this bush and/or limited its range of movement in order to improve steering feel, but with the side effect that the greater resistance to movement and possibly more limited range would have put more strain on the crimped fixing between the stanchion tube and the aluminium plug which provides the connection to the top yoke, especially when the suspension reaches its extremes of travel. Possibly the early LC production used up stock of the system from the previous model which could explain why these bikes are not subject to the recall
Fred
Very interesting and informative - thanks, but is that animation a true reflection of the actual GS geometry? can't do it in my head but is there a geometry that give littles or zero fork angle change?
alp
Registered user
Engineer, I feel it's ok to show the principle of the movement but more detail would show that the suspension angle does increase with compression UNTIL the blue link goes past the 90 degrees relative to the stanchion tube as compression increases at which point it reverses and decreases the suspension angle. I don't have actual numbers for this but it certainly looks that way on the bike that the 90 degrees link position is max suspension angle but not yet at max compression.
Ref. the damage to the stanchion tube fixing at the top, I don't think it has been mentioned but when the front wheel lifts off the ground (offroad obviously Officer) and the suspension fully extends there is likely to be a force linear to the stanchion tube trying to draw the tube out of the top fixing....hence the gap that gradually appears.
Ref. the damage to the stanchion tube fixing at the top, I don't think it has been mentioned but when the front wheel lifts off the ground (offroad obviously Officer) and the suspension fully extends there is likely to be a force linear to the stanchion tube trying to draw the tube out of the top fixing....hence the gap that gradually appears.
It would be good to see the real geometry and also the effect of the telelever ball joint, what freedom of movement is there with that joint?
The extension force that you mention is real but should be lowish and easily resisted by the interfence fit and crimp at the top - like all these things the devil is in the detail and unfortunately we don't have all that detail.
fred_jb
Registered user
Glad you found it useful, I certainly did, though I doubt it is a true reflection of relative angles, link lengths, etc, so I was just regarding it as an example of the general principle.
The bottom link, the blue line, is a simple fixed length link on the GS in the form of the lower wishbone, so as it moves the point where the fork carrier bolts to it is going to describe an arc. That means the fork legs will be pulled back and pushed forward in following that arc, so I can't see any way that the resulting angle change can be avoided without some sort of non-linear linkages somewhere in the system. The amount of angle change I guess will depend on the shallowness of the arc which will depend on the length of the lower arm, and also on how far along the arc it goes, which in turn will depend on the amount of suspension travel available, so I think these two factors plus the length of the forks are the only factors which determine the angle, so I can't imagine any way that it could be eliminated with the current construction.
I believe BMW have adjusted the point where the bike settles on that arc under a nominal expected amount of front suspension sag in order to set the lower link at an angle that gives just a very small amount of dive under braking. However, I guess you also want to ensure that you minimise geometry changes and also make best use of the movement available in the joint link by ensuring that the range of normal suspension movements gives a roughly equal backwards and forwards deflection of the forks, though I don't know if BMW have done this, and I don't think this particular animation would be very representative of that. I suspect best use of the joint link could also be optimised to some extent by making it asymmetrical, or by altering the angle of the joint link within the top yoke, or even the top link itself, at the expense of possible effects on steering, but it doesn't look to me like BMW have done this.
Fred
The bottom link, the blue line, is a simple fixed length link on the GS in the form of the lower wishbone, so as it moves the point where the fork carrier bolts to it is going to describe an arc. That means the fork legs will be pulled back and pushed forward in following that arc, so I can't see any way that the resulting angle change can be avoided without some sort of non-linear linkages somewhere in the system. The amount of angle change I guess will depend on the shallowness of the arc which will depend on the length of the lower arm, and also on how far along the arc it goes, which in turn will depend on the amount of suspension travel available, so I think these two factors plus the length of the forks are the only factors which determine the angle, so I can't imagine any way that it could be eliminated with the current construction.
I believe BMW have adjusted the point where the bike settles on that arc under a nominal expected amount of front suspension sag in order to set the lower link at an angle that gives just a very small amount of dive under braking. However, I guess you also want to ensure that you minimise geometry changes and also make best use of the movement available in the joint link by ensuring that the range of normal suspension movements gives a roughly equal backwards and forwards deflection of the forks, though I don't know if BMW have done this, and I don't think this particular animation would be very representative of that. I suspect best use of the joint link could also be optimised to some extent by making it asymmetrical, or by altering the angle of the joint link within the top yoke, or even the top link itself, at the expense of possible effects on steering, but it doesn't look to me like BMW have done this.
Fred
fred_jb
Registered user
I suspect that the damage to the crimps which allows the joint to loosen is caused by a rocking motion, not a simple linear pull. This would happen if there is insufficient angular movement available at the top of the stanchion to accommodate the angle change caused by full compression of the suspension, in extreme or in some cases it seems, not particularly extreme usage. In that case something would have to give and the stanchion would need to flex, so once any play in the bearings in the lower leg is used up, the next weakest point may well be the crimped joint rather than the stanchion tube itself bending.
EDIT: I suspect the damage is done on compression, not extension, as the forces in the former will be higher.
It's quite complicated when one starts to get into the detail
if I had the time and the inclination I do some measurements on my bike but that could be quite tricky, I will continue to search for some more pictures on the internet firstRe. Alps point earlier about a linear force on the stanchion during full extension: As far as I am aware, the stanchions are not attached to the lowers as per a conventional fork, for example when you do a fork seal change the stanchion simply pulls out of the lower. (does on an 1150 anyway!). As such, full extension will not apply any force to the stanchion. I'm sure the problems are caused by the angular movement at the top yoke being too much for a crimped joint. The earlier threaded forks never had a problem.
Chris
Chris
alp
Registered user
Thanks Chris, I assumed that the full extension position was controlled by the stanchion tube within the sliding lower section, but it looks like it is actually controlled by the full extension of the front shock. Thanks for clarifying that.
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