Fork Stanchion Issue... some perspective?

[fred_jb]

I've just been out to my freezing garage in the interests of science! From the very approximate measurements I have been able to make it looks like the angle change of the stanchion between extremes of suspension extension and compression is around 7 degrees. This is for my low chassis GS which has suspension travel of 170 mm. I would expect a bigger angle for the standard GS which has 190 mm of suspension travel, and bigger again for the GSA which has 210 mm of front suspension travel.

Because the full compression position should be the same for all three bikes it is possible to just add 20 mm and 40 mm to the fully extended measurement I took on my bike to see what the angle would be on the standard GS and standard GSA. These come out to approximately 8 degrees and 9 degrees of movement respectively.

What this of course does not tell us is whether the full range of angle change is centred within the maximum back and forth movement allowed by the joint link. For example if the fully extended position coincided with the stanchions being centred within the joint link rather than angled downwards by the maximum amount the joint link would allow, then the whole angle change due to compression would have to be accommodated within only half of the allowable movement within the joint link.

It is very difficult to judge this by eye, but I would expect all three bikes to be set up so that the stanchion is central within the joint link when 105 mm from full compression. The GSA would move 4.5 degrees in either direction, whereas the other two would move the same on compression but probably use less of the available angle for extension. In this case all three bikes would be equally vulnerable to damage to the crimps on full compression if the joint link cannot accommodate this amount of movement.

Fred

 

[fred_jb]

Here is a picture showing the sort of movement I would expect to happen with the inner part of the joint link at the top of the stanchion.

Diagram A shows what happens when the stanchion is in a neutral position halfway through the full range of suspension movement (the green position) which would be 105 mm from full compression in the case of the GSA, compared with on full compression (the red position). The angle is not exact but is meant to be 4.5 degrees which is half the full angular movement of the stanchion which I have calculated for the GSA.

Diagram B show what would happen in the worst case if the stanchion was not centred in the yoke at half deflection, but instead was centred at full extension. In that case the whole angular movement of about 9 degrees due to the full range of compression would have to be accommodated one one side of the joint link so the inner part would have to tilt twice as much, and would be much more likely run out of movement and put strain on the crimps.

This shows how important it is that the geometry of the suspension is so arranged as to allow the available range of movement within the joint link to be fully utilised.

 

[tanneman]

You have to take into account that with the suspension fully extended it will rest against the stop or the resistance of the elastomer in the fitting. Hence it will not be 4.5 deg either side of natural.

 

[fred_jb]

You have to take into account that with the suspension fully extended it will rest against the stop or the resistance of the elastomer in the fitting. Hence it will not be 4.5 deg either side of natural.

So are you saying that when BMW say there is, for example, 190 mm of suspension travel, that it is actually less? I would have expected this to be the true figure taking into account any mechanical stops at both full extension and full compression. I measured the full extension position on mine with the bike jacked up to take the front wheel off the ground, and would have thought there was enough weight there to take it to full extension.

 

[tanneman]

You don't understand, at full extension the trail is reduced and the suspension (forks) rests against the resistance offered by the elastomer fitting in the top yoke and the stop in the shock. Compare that with the movement of the suspension from normal position to fully compressed ie from static sag to fully compressed. Hence the deflection is less in one direction (the rear) than the other (the front). So the max angle will be achieved with the forks moving to the front ie under compression. The static sag will also depend on the sprung weight so for each bike there will be a different result, hence why it is not really relevant (too many variables and different users) and we cannot accurately calculate/measure it but your figure is in the ball park. I got 5 deg for a GS. That is a lot of trail added to the wheelbase.

To me the conclusion is and always have been that BMW have a telelever front end because in the good old days it had several advantages over conventional suspension for touring application and to a degree sporty riding to offer a more relaxed steering that sacrificed a bit of feel. Only when you really push the front do you get a feel (tracking the R1100S made me appreciate that) and it ads stability when the front is near the limit. Thus the description as an adventure bike or dual sport doesn't really suit the GS moniker. Those times are well and truly behind the GS now as it is more road bike (in the western world) and leave many current owners and prospective customers speculating weather a GS or RT would be best suitable. Even more apparent in the off road scenario where the GS is suppose to be a gravel tourer rather than a green lane machine. Used in the extreme of its abilities the design will show up its shortcomings as the debacle with the front suspension has shown. However brilliant many claim the bike is it is nowhere near as good as it can be thanks to the limitations imposed by financial margins when it is designed. Conventional suspension technology has moved on massively with BPF (big piston fork) and electronics that I question the need for the telelever. Which it all comes down to and can be summed up as: If you want fast, buy fast. If you want an offroad, buy a light bike. If you want to tour, buy something with proper seats. The GS is none of that but there are worse.

 

[fred_jb]

You don't understand, at full extension the trail is reduced and the suspension (forks) rests against the resistance offered by the elastomer fitting in the top yoke and the stop in the shock. Compare that with the movement of the suspension from normal position to fully compressed ie from static sag to fully compressed. Hence the deflection is less in one direction (the rear) than the other (the front). So the max angle will be achieved with the forks moving to the front ie under compression. The static sag will also depend on the sprung weight so for each bike there will be a different result, hence why it is not really relevant (too many variables and different users) and we cannot accurately calculate/measure it but your figure is in the ball park. I got 5 deg for a GS. That is a lot of trail added to the wheelbase.

OK - I think I see what you are getting at, but from what point you regard the suspension as extending rather than compressing is just relative, and as you say, depends on the sag for a particular bike and rider, though I guess BMW will be working with what they regard as an average rider weight. If the static (bike with average rider) sag settles at 1/3 rd compression then you can regard that as the normal position, with 2/3rds of the available travel remaining for compression, and 1/3rd for extension. I think we are coming at this from different directions, so I guess this will be relevant to your concerns regarding change in trail, but is not really relevant to my concern regarding angular movement of the stanchions within the joint links putting strain on the crimps, as I am only concerned with the total range of movement. Whether this movement is centred on that available within the joint links is something which BMW could and should ensure when setting the angle of the yoke assembly, so I will assume that is optimal.

However I don't think we can make too many assumptions about what place on the arc of movement of the wheel is the neutral position as far as sag goes. One could assume that to minimise the total movement and change in wheelbase BMW would have the centre of the arc correspond to half compression of the suspension, so as the suspension moves the wheel starts off close the the bike, moves out to its furthest at half travel, and then goes back to the minimum at full travel, but I suspect this is not necessarily what happens, even though the animated diagram suggests this.

I think a lot depends on the angle of the wishbone which will be chosen for the desired anti-dive characteristics. If you wanted this completely neutral with no dive, then I assume you would arrange for the arm to be horizontal at the typical rider sag deflection - say 1/3rd compressed. I believe BMW have programmed in a little dive, so this would imply the neutral position of the arm would be pointing upwards slightly. It is making my brain hurt now thinking about this, but I'm pretty sure this will affect the arc of movement of the wheel axle.

 

[tanneman]

Yes, I hear you and correct in the assumptions. The wishbone to front axle is a constant. The preload on the front shock determines the angle of the wishbone. Just to add to the hurting That sums it up then.

 

[fred_jb]

Yes, I hear you and correct in the assumptions. The wishbone to front axle is a constant. The preload on the front shock determines the angle of the wishbone. Just to add to the hurting That sums it up then.

Thanks - good discussion, though still not certain exactly how the crimps get sufficiently loaded to be damaged!

 

[Bem]

Almost afraid to make further contribution here as the Flames of Hades burn hot!
Bearing in mind that my mechanical qualifications ended with an O level in Metalwork (albeit grade 1) I have been following the details of the thread with some difficulty, but do have some thoughts which might be right or wrong.

The role of the Part no 2 (Bush) has not been fully explored here as yet. It has been dismissed as being outside the recall (true) but I still think that it has some significance. I think that when the steering is turned, whether at slow speed or high, there is some additional flex applied at the top yoke area which the Bush allows for. I think this force would be particularly significant when banked or turning as there will then be asymmetric forces applied to each fork leg. Normally this would only be a momentary force, but surely off road will apply continuous left/right forces at the bottom of the wheel, providing maximum leverage back to the top yoke.
As discussed elsewhere, the crimp is the "weakest link" in the whole assembly, but is not normally continuously over-stressed in everyday activity sufficient to overwhelm the crimp.

Also not fully explored is the difference in detail before and after the change in 2012 (or whenever it was) - I do not believe that there has been any change in geometry but rather the changes have been about the overall surface contact areas in the top of the fork leg - the actual mechanical joining method used being less important than the area over which the joint was interfaced.

Whether the parts fiche would be enough to show this difference is unlikely.

And, Happy New Year to you all.

 

[fred_jb]

The role of the Part no 2 (Bush) has not been fully explored here as yet. It has been dismissed as being outside the recall (true) but I still think that it has some significance. I think that when the steering is turned, whether at slow speed or high, there is some additional flex applied at the top yoke area which the Bush allows for. I think this force would be particularly significant when banked or turning as there will then be asymmetric forces applied to each fork leg. Normally this would only be a momentary force, but surely off road will apply continuous left/right forces at the bottom of the wheel, providing maximum leverage back to the top yoke.
As discussed elsewhere, the crimp is the "weakest link" in the whole assembly, but is not normally continuously over-stressed in everyday activity sufficient to overwhelm the crimp.

Also not fully explored is the difference in detail before and after the change in 2012 (or whenever it was) - I do not believe that there has been any change in geometry but rather the changes have been about the overall surface contact areas in the top of the fork leg - the actual mechanical joining method used being less important than the area over which the joint was interfaced.

Whether the parts fiche would be enough to show this difference is unlikely.

And, Happy New Year to you all.

I have been strongly focused on the bush, called joint link by BMW, as a potential culprit, though I haven't considered that it is especially an issue when steering, other than possibly being responsible for some give and vagueness in response to movement of the bars. My concern with the joint link is primarily that it may not give enough flex to accommodate the full range of movement needed when the suspension compresses and the angle of the stanchion has to change in response to the lower wishbone joint moving through an arc.

If the joint link ran out of "give" before the suspension reached maximum compression and before the stanchion reached its maximum angle change, then it would effectively become a solid fixing at extreme compressions and so tend to cause flex in the stanchions, with the weak point in these being the crimped joint which attaches them to the fixing plug in the top. In this scenario, i.e. the joint link running out of movement and locking, repeated maximum suspension compressions would tend to rock the crimped joint and over time deform and loosen the top of the stanchion. I have calculated that the maximum range of movement of the stanchion is about 9 degrees, so the question is whether the joint link can accommodate this without running out of movement and locking. This of course could be made worse if the yoke angle was not optimised to ensure the mid-range of the stanchion movement coincides with zero deflection within the joint link.

I suspect that extremes of suspension compression are not likely to be happening combined with heavy steering lock, but even if this happens the stanchions will still deflect in the fore and aft direction, i.e. along the longitudinal axis of the bike, even when the bars are turned, but as the joint links give flex in all directions, I'm not sure that this is really a problem, and most of the forces, such as side loading when banked are going to be resisted by the attachment to the lower wishbone. However, I find it impossible to mentally model the geometry when suspension compression is combined with significant steering lock, so you may be right that this introduces additional stresses.

 

[fred_jb]

PS: I think the consensus view is that just after the early production runs of the LC in 2013 the method of attachment of the stanchion to the fixing plug that goes into its top changed from a threaded attachment to a crimped one. The forces that cause the current problem have probably always been there, whether they are caused by the joint link or some other factor, but it seems that the crimp method of attachment is susceptible to damage from these forces, whereas the previous attachment method apparently wasn't.