This thread is started specifically to address whether the design of the BMW center stand on current boxers is responsible for the reported failures noted in another thread. My bike is an 08 RT and is the basis of my points here.
I do believe the stand has compromises/weaknesses as built on the RT because it does not follow well understood principles of ideal fastener use. The choices in its design might be made by many but engineers know this design is in fact not ideal.
A handy reference for good fastener practices is Carroll Smith's text on nuts, bolts, fasteners and plumbing (google will pull it up easily). Carroll Smith was a racing engineer who helped put reliability into today's racing machinery. His texts have been the training bibles for many racers for a long time, though they are now somewhat dated (especially on brakes)- but some basics never change. If you were to compare what's in the text to a NASCAR machine, for example, you would see the vehicle adheres almost entirely to the recommended practices. Those old enough will also note that many of the types of failures that plagued racing machinery years ago no longer happen- much of that progress is attributable to proper fastener selection and installation the Carroll describes at length. The text remains the best general education available on fasteners short of those intended for metallurgy professionals.
The basic weakness on the center stand appears to be the very elementary one of carrying a shear load (a cross bolt load) on the threaded portion of a bolt. This is never a great idea because threaded parts of fasteners are weak in shear and ideally should only be subjected to tension loads. A thread subjected to shear (a crosswise rather than length wise load) is much more likely to break than an unthreaded portion of the same bolt due to impacts of the thread forming process on the material, the handy stress crack starter a thread can provide, reduced cross section compared to the unthreaded portion, etc etc. The center stand bolts (on the RT) are M10X30 and threaded for their entire length, a choice that means the head of the bolt can also shear off at a thread (the stand would detach when this happens). The bolt type used is in fact close to the worst possible choice for something that will see shear and should ideally be used only in tension/compression situations.
Where and what exactly is the shear load? The design (RT) works like this. The threaded bolt goes through the stand and through a threaded, collared bushing that carries much of the shear load and supports the threaded bolt at a lot of its length. That's actually pretty good, so far. Specifically, the collared (outside on the RT) end supports shear impacts at that end.
The head of the bolt is another matter entirely- it sits in a recess in the center stand itself and has no additional support at that end. Basically an unsupported threaded bolt head that can see a shear load- far from ideal.
Where does the load come from? Well, first, by nature the center stand cannot be a zero clearance/compression load only on the bolt. It has to have enough clearance to rotate, for example. The fact that clearances must exist ensures that many types of loads will have some sort of shear component and the larger a clearance becomes, the larger the shear load can get (quickly). Clearly a pair of M10s in good shape can support the entire weight of the bike in static shear so we're talking about impact or temporary shear spikes that can start cracks as being the reason for reported bolt failures.
How? A few ideas for this include riding the bike off the stand, twisting the handlebars when the bike is on the center stand (look at stress on the center stand when you do this some time), actual object impacts from off road riding or jumping over obstacles, etc etc. Enough impacts or a big enough one start a crack at a thread near the head and the bolt breaks.
Another way for this system to fail is for a bolt to back out a little, increasing clearance and the potential for shear cracking. It is certainly not good practice to retain a bolt that might see a twisting load (when the stand is used) with only thread locker. In fairness, BMW guys have used the lubed, O ringed bushing to try to isolate the twisting load so it is not transferred to the bolt BUT if the lubrication fails or corrosion starts, a twisting load can end up being carried by thread locker alone. This is no doubt the primary reason that checking "ease of movement" of the center stand is part of the routine 6K service procedure as stated on my RepROM.
Still, it would b a better idea to ensure bolt retention with a washer/ lock nut. Other ways include a roll pin, circlip, or safety wire and ultimately are not as robust a choice.
What about bolt size? IMO, the M10 size is the minimum OK size for normal operation of the RT, a road bike, though the design itself can ask a lot of them in less than ideal situations. I have not looked at the bolt size for a GS but given the intended usage, M12s, better bolt retention and keeping threads at the head of the bolt out of shear would be a better design, IMO.
It is not at all unusual to find bolts used in a less than ideal manner on street machines of all types. Anyone who uses a modified street car in a track situation will encounter many of these choices that need reworking to increase robustness. The designers generally aren't terminally stupid when they make such choices. They are usually done with full knowledge of the tradeoffs and the expectation of a low to zero failure rate in normal use. The driver for the choice is the usual one- cost reduction in labor, parts or both. There are a lot of parts in a vehicle for which cost based choices are inevitable.
Once one has failures, it becomes time to re-examine choices and it is a fact of life that most automotive firms only do this when lawsuits get pricey or legislatures start leaning on them. BMW engineers have only done what many others have done or would do with a similar choice. But because many BMW bikes and cars are used in a "sporting" manner than can inflict unanticipated loads, some of their choices turn out to be "wrong". (In cars, some recent M3s got a well deserved reputation for shearing body welds when their owners used them for track play- something that ought to have been very predictable to the designers of a pricey performance oriented vehicle)
One can imagine boring or substituting the threaded bushing, using M12s and locknuts in an attempt to improve robustness (I haven't verified clearances in the bike hard parts so am not suggesting you run off and do this without more thorough investigation). Whether this or other improvement would be worthwhile depends mostly on how much abuse you might heap on those bolt heads.
Those who want to learn more about the math of shear loads can start with the SHEAR wiki and look at the formulas for beam, semi-monocoque, and impact shear. Not recommended unless engineering math is your thing and you want to plug in the values for the M10 8.8 bolt and estimate various load situations to see what type of margin might result.
At the very least, it is wise to remember that service inspection is REQUIRED by factory procedures and that you might want to touch those two bolts as part of your pre-ride inspection drill to be sure they haven't backed out or had the head crack off.
I only use my center stand in my garage where the bike sits on a turntable that eliminates stand stress caused by handlebar movement. And I don't have my weight on the bike when it comes off the stand, nor do I ride it off, despite the temptation.
Comments???
I do believe the stand has compromises/weaknesses as built on the RT because it does not follow well understood principles of ideal fastener use. The choices in its design might be made by many but engineers know this design is in fact not ideal.
A handy reference for good fastener practices is Carroll Smith's text on nuts, bolts, fasteners and plumbing (google will pull it up easily). Carroll Smith was a racing engineer who helped put reliability into today's racing machinery. His texts have been the training bibles for many racers for a long time, though they are now somewhat dated (especially on brakes)- but some basics never change. If you were to compare what's in the text to a NASCAR machine, for example, you would see the vehicle adheres almost entirely to the recommended practices. Those old enough will also note that many of the types of failures that plagued racing machinery years ago no longer happen- much of that progress is attributable to proper fastener selection and installation the Carroll describes at length. The text remains the best general education available on fasteners short of those intended for metallurgy professionals.
The basic weakness on the center stand appears to be the very elementary one of carrying a shear load (a cross bolt load) on the threaded portion of a bolt. This is never a great idea because threaded parts of fasteners are weak in shear and ideally should only be subjected to tension loads. A thread subjected to shear (a crosswise rather than length wise load) is much more likely to break than an unthreaded portion of the same bolt due to impacts of the thread forming process on the material, the handy stress crack starter a thread can provide, reduced cross section compared to the unthreaded portion, etc etc. The center stand bolts (on the RT) are M10X30 and threaded for their entire length, a choice that means the head of the bolt can also shear off at a thread (the stand would detach when this happens). The bolt type used is in fact close to the worst possible choice for something that will see shear and should ideally be used only in tension/compression situations.
Where and what exactly is the shear load? The design (RT) works like this. The threaded bolt goes through the stand and through a threaded, collared bushing that carries much of the shear load and supports the threaded bolt at a lot of its length. That's actually pretty good, so far. Specifically, the collared (outside on the RT) end supports shear impacts at that end.
The head of the bolt is another matter entirely- it sits in a recess in the center stand itself and has no additional support at that end. Basically an unsupported threaded bolt head that can see a shear load- far from ideal.
Where does the load come from? Well, first, by nature the center stand cannot be a zero clearance/compression load only on the bolt. It has to have enough clearance to rotate, for example. The fact that clearances must exist ensures that many types of loads will have some sort of shear component and the larger a clearance becomes, the larger the shear load can get (quickly). Clearly a pair of M10s in good shape can support the entire weight of the bike in static shear so we're talking about impact or temporary shear spikes that can start cracks as being the reason for reported bolt failures.
How? A few ideas for this include riding the bike off the stand, twisting the handlebars when the bike is on the center stand (look at stress on the center stand when you do this some time), actual object impacts from off road riding or jumping over obstacles, etc etc. Enough impacts or a big enough one start a crack at a thread near the head and the bolt breaks.
Another way for this system to fail is for a bolt to back out a little, increasing clearance and the potential for shear cracking. It is certainly not good practice to retain a bolt that might see a twisting load (when the stand is used) with only thread locker. In fairness, BMW guys have used the lubed, O ringed bushing to try to isolate the twisting load so it is not transferred to the bolt BUT if the lubrication fails or corrosion starts, a twisting load can end up being carried by thread locker alone. This is no doubt the primary reason that checking "ease of movement" of the center stand is part of the routine 6K service procedure as stated on my RepROM.
Still, it would b a better idea to ensure bolt retention with a washer/ lock nut. Other ways include a roll pin, circlip, or safety wire and ultimately are not as robust a choice.
What about bolt size? IMO, the M10 size is the minimum OK size for normal operation of the RT, a road bike, though the design itself can ask a lot of them in less than ideal situations. I have not looked at the bolt size for a GS but given the intended usage, M12s, better bolt retention and keeping threads at the head of the bolt out of shear would be a better design, IMO.
It is not at all unusual to find bolts used in a less than ideal manner on street machines of all types. Anyone who uses a modified street car in a track situation will encounter many of these choices that need reworking to increase robustness. The designers generally aren't terminally stupid when they make such choices. They are usually done with full knowledge of the tradeoffs and the expectation of a low to zero failure rate in normal use. The driver for the choice is the usual one- cost reduction in labor, parts or both. There are a lot of parts in a vehicle for which cost based choices are inevitable.
Once one has failures, it becomes time to re-examine choices and it is a fact of life that most automotive firms only do this when lawsuits get pricey or legislatures start leaning on them. BMW engineers have only done what many others have done or would do with a similar choice. But because many BMW bikes and cars are used in a "sporting" manner than can inflict unanticipated loads, some of their choices turn out to be "wrong". (In cars, some recent M3s got a well deserved reputation for shearing body welds when their owners used them for track play- something that ought to have been very predictable to the designers of a pricey performance oriented vehicle)
One can imagine boring or substituting the threaded bushing, using M12s and locknuts in an attempt to improve robustness (I haven't verified clearances in the bike hard parts so am not suggesting you run off and do this without more thorough investigation). Whether this or other improvement would be worthwhile depends mostly on how much abuse you might heap on those bolt heads.
Those who want to learn more about the math of shear loads can start with the SHEAR wiki and look at the formulas for beam, semi-monocoque, and impact shear. Not recommended unless engineering math is your thing and you want to plug in the values for the M10 8.8 bolt and estimate various load situations to see what type of margin might result.
At the very least, it is wise to remember that service inspection is REQUIRED by factory procedures and that you might want to touch those two bolts as part of your pre-ride inspection drill to be sure they haven't backed out or had the head crack off.
I only use my center stand in my garage where the bike sits on a turntable that eliminates stand stress caused by handlebar movement. And I don't have my weight on the bike when it comes off the stand, nor do I ride it off, despite the temptation.
Comments???
