Get The Sandwiches Ready - GSA Rear Wheel Collapse

[Mr Bean]

And it's a caliper in contact with a spoked wheel, not cast. I get that contact with a spoked wheel is possibly more likely to be catastrophic, but no reports (I've seen) of a caliper coming in to contact with a cast wheel

 

[Mr Bean]

I've another thought on this....

What if its a combination of things.

A) Starts off with a loose spoke

But would the spokes rip a caliper off?

 

[Deleted account 191119001]

And it's a caliper in contact with a spoked wheel, not cast. I get that contact with a spoked wheel is possibly more likely to be catastrophic, but no reports (I've seen) of a caliper coming in to contact with a cast wheel

This is the valid point, that you correctly point out, would be really interesting if somebody says that their caliper fell off or broke off with cast wheels, at the moment as you rightly say only spoke wheels are talked about.

 

[Deleted account 191119001]

But would the spokes rip a caliper off?

Maybe not the spokes, but the actual wheel at collapse, when it twists at a right angle, think Wapping mentioned that.

 

[GrinningGSer]

This has developed into a horrendous ongoing thread fueled by lack of forensic info ;
We only have three crash scene photos and all taken in the dark with use of flash.
The rim appears scored, so possible that the rim has fractured the caliper mounting. Paramount is the knowledge of why all of those spokes are broken and a metallurgist could easily determine if it’s physical trauma ( from the caliper or other object ) or spoke stretch failure .
Can we have more detailed info from the OP ? PLEASE

 

[Malvern_Beemer]

I think we're gonna need more sandwiches

 

[Andi_archer]

And none of them Cucumber and Cress nonsense

 

[knutk]

The whole scenario is weird.
On one side, it should not happen, and yet, it has.

Spoked wheels have been around for nearly a 100 years and has proven its reliability. The spokes in my -75 90S are still as tight as when the bike was new, and it has been driven beyond 100K km. The spokes on BMW bikes have no bends and are usually considered as no need of maintenance. But all of a sudden a few people experience several loose spokes for seemingly no reason.


Brake calipers seems to unscrew them selves. When ever we remove the front wheel the calipers have to come off. I have yet to find one caliper with lose bolts.


What if the problem lays elsewhere?


When we change the tires the wheel is mounted in a machine that grips the rim at three or four points. Is there a remote possibility that this grip in some cases will put excessive strain on the rim, and since the spokes on affected bikes are fastened at the outer edge, this strain is transferd to the spokes? And the failure of the strained spokes and the subsequent disaster rips the caliper of? (as already suggested)

 

[Engineer]

It’s a mystery.

 

[fred_jb]

I've another thought on this....

What if its a combination of things.

A) Starts off with a loose spoke
B) Rider style uses a 'snappy' throttle for low revs excelleration, using the bikes torque rather than high revs.
C) Large torque force is applied to spoked wheel on and already overloaded heavy bike
D) This overstresses other spokes around loose one, stretching them? to become lose themselves.
E) Which, over time continues until total collapse.

If so, not checking your spokes will leave you unaware of this.

Remapping your bike with higher torque levels, and using a lazy accelleration style might impact this more

Not sure what you are suggesting here? Is it that "low revs acceleration using bike's torque rather than high revs" is somehow a lazy style of riding which could damage your bike? How is using high revs any better as the maximum engine torque comes at about 6500 revs, and in any case you have to go through the low revs area to get to the higher revs. Is there a non-lazy style of accelerating which would reduce stress on the wheel?

I suppose what you might be getting at is a style of short shifting, particularly in the lower gears, since the torque applied to the wheel depends not only on the torque being produced by the engine, but also what gear you are in. The lower the gear then the bigger the multiplying effect on the turning force at the wheel compared to the crank. In that case maybe the advice should be not to hold onto the revs in the lower gears so as to avoid hitting max torque revs. Short shifting in the lower gears would avoid hitting the max torque output in gears which would exert the maximum turning force on the wheel, but you are then losing most of the bike's ability to accelerate quickly.

If the bike's spoked wheels can't withstand using all of the bike's engine performance, then BMW have a big problem. Maybe the rumoured VVT engine will have to be restricted to cast wheels only! Spokes may well have been around for 100 years, but they have not generally been asked to take such high power outputs in the past. For anyone who is not a serious off-road rider or is purely a road rider I think they are just a fashion item anyway. I suppose they add to the adventure bike image, but cast wheels will be a more logical choice for most people.

 

[Deleted account 220214001]

I agree with most of what yu say, Fred. Especially the spoked wheels being a fashion item mostly. I have no intention of using mine for enduro riding. I might use it on a green lane occasionally but as I doubt I shall be buying any offroad tyres, I doubt much more than that.

I think there is some merit in what Warlord says. Firstly, the clearance between spokes and caliper is very tight; it's not hard to see how a loose or broken spoke (or two) might set off a chain reaction. Excuse the pun.

I had a crap physics teacher at school and I'm not an engineer but torque is a very flat curve; on the GS 80% of it is available from very much lower down than the max RPM. Inertia will also play it's part; torque will be gentler on the spokes at higher speeds than from a standing start under hard acceleration (within reason - most us enjoy the acceleration between 0-60 but rarely use the same rate between 60-100). I'm not sure there is really much difference between "lazy" riding and "short shifting" as far as the effect goes.

To conclude, then, yes, BMW may well have a problem. Owners also have a problem because spokes need to be checked on a fairly regular basis. That has always been the case, it's just that not many bother.

 

[fred_jb]

I agree with most of what yu say, Fred. Especially the spoked wheels being a fashion item mostly. I have no intention of using mine for enduro riding. I might use it on a green lane occasionally but as I doubt I shall be buying any offroad tyres, I doubt much more than that.

I think there is some merit in what Warlord says. Firstly, the clearance between spokes and caliper is very tight; it's not hard to see how a loose or broken spoke (or two) might set off a chain reaction. Excuse the pun.

I had a crap physics teacher at school and I'm not an engineer but torque is a very flat curve; on the GS 80% of it is available from very much lower down than the max RPM. Inertia will also play it's part; torque will be gentler on the spokes at higher speeds than from a standing start under hard acceleration (within reason - most us enjoy the acceleration between 0-60 but rarely use the same rate between 60-100). I'm not sure there is really much difference between "lazy" riding and "short shifting" as far as the effect goes.

To conclude, then, yes, BMW may well have a problem. Owners also have a problem because spokes need to be checked on a fairly regular basis. That has always been the case, it's just that not many bother.

By inertia I guess we are talking about the effect of how sudden is the application or for that matter removal of turning force on the wheel, and I can see that this could be an issue. Possibly exacerbated on the GS by the fact that it has shaft drive so maybe not the same amount of give in the system as chain drive with cush rubbers in the hub, despite the recent introduction of the judder damper. Probably use of GSAP doesn't help either.

 

[fred_jb]

It does seem particularly difficult to separate cause and effect in these incidents. While a loose caliper could be the cause, equally if the wheel has multiple spoke failures and starts to distort and the rim move off-centre, then the spokes could start to make contact with a perfectly innocent caliper. The caliper could then hit the ground and be dislodged that way.

 

[Warlord]

Yeah sorry, I've not explained myself very well as I'm not an engineer or physicist, but you've got the gist of it.

During 'snappy' hard acceleration (as most of these bikes are on road tyres with good grip levels), the energy is being forced through the spokes to the tyre and road surface. In off-road scenario's the tyre just spins in the dirt, but on the road it grips and doesn't spin.

Gear Shift Assist Pro (GSAP) has been reported 'harsh' for some riders causing a jolt in between gear changes. BMW then added this cush-drive on 2017+ models. But I've got a 2017 model and there can still be a jolt occasionally.

This sudden application of torque and jolting is being transferred to the rear wheel and a huge amount of torque being applied (as the bike is usually under acceleration).

Could this be stretching spokes or over-stressing spokes around a loose one? Over time, being unchecked, causing failures.

BMW Recommend you check the spokes after EVERY off road session, according to my dealer. But maybe there is another danger for road users also?

Which would also make sense why only rear wheels are failing and not front ones !

 

[Warlord]

Just as a matter of theory. Please don't try this at home

If you were to slacken all the spokes on your rear wheel. So they were all loose.

Put the wheel back on your bike, add a load of luggage weight, and ride off down the motorway giving it a handful of throttle every now and then.

What would happen to the wheel? would it start to buckle or twist? Maybe enough to hit the caliper?

Would it resemble the OP Scenario?

 

[oddshapedballs]

I can't be bothered to read 13 pages, just want to ask, has the owner admitted that despite the fact he is a competent home mechanic he just forgot to do the bolts up? Maybe he was thinking about the trip and made an honest mistake?

 

[Warlord]

I can't be bothered to read 13 pages, just want to ask, has the owner admitted that despite the fact he is a competent home mechanic he just forgot to do the bolts up? Maybe he was thinking about the trip and made an honest mistake?

He says he never touched the caliper bolts

 

[knutk]

I don't think the torque from the engine is even close to causing damage to the spoked rear wheel. If such was the case we would have numerous front wheels disintegrating as the force from heavy brakeing is way beyond the acceleration rate and to a slimmer wheel with a larger diameter.

 

[Warlord]

I don't think the torque from the engine is even close to causing damage to the spoked rear wheel. If such was the case we would have numerous front wheels disintegrating as the force from heavy brakeing is way beyond the acceleration rate and to a slimmer wheel with a larger diameter.

Under normal conditions thats correct.

But we're suggesting torque on damaged, loose spokes which then escalates

 

[Deleted account 220214001]

I don't think the torque from the engine is even close to causing damage to the spoked rear wheel. If such was the case we would have numerous front wheels disintegrating as the force from heavy brakeing is way beyond the acceleration rate and to a slimmer wheel with a larger diameter.

I disagree, Knutk. The front wheel only has forward braking forces to deal with (Ok and turning and centrifugal but they aren't extreme enough, I don't think, and part of the design anyway). Under acceleration the rear wheel carries most of the weight and can increase it by quite some margin) as it does when you use engine braking and normal braking. Under normal braking the rear wheel wants to sink but is mitigated by the ESA and also carries a fair bit of weight.

The front wheel only really experiences extreme forces under hard braking. It also experiences stresses offroad but, again, designed for that with the caveat that the spokes should be checked. Offroaders (scramblers, enduro, pure ones, not big KTMs and the like) do not tend to be fast or used on motorways, much less with any luggage or pillion loads and they also tend to be much, much lighter.

I agree, the forces on a gravel or offroad surface are not the same as on tarmac where the traction is far greater.

Warlord, I don't think it would take much more than a couple of loose spokes at most to get the effect you are after in your experiment.