Broken ST rear drive sprocket - BMW F800 Riders Forum & Registry



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  1. #1
    Richard230's Avatar
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    I have a friend who had his rear sprocket break and drop off of his 2008 ST while he was riding along yesterday. Fortunately another friend was nearby who was able to haul is bike to an independent shop to be repaired and he loaned him one of his 24 motorcycles to ride the 50 miles back home. Photo attached of the damage.
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    Richard - Current bikes: 2016 BMW R1200RS, 2018 16.6 kWh Zero S, 2011 Royal Enfield Bullet 500 Classic, 2009 BMW F650GS, 2020 KTM 390 Duke, 2002 Yamaha FZ1 (FZS1000N) and a 1978 Honda Kick 'N Go Senior. 

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    Wow! Never heard of that happening. Glad he was okay and that the damage wasn't worse.

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    It's taken me a minute to figure out what I'm looking at. Is that the area where the rubber absorbers go? Crazy!
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    Melbourne, Australia IanA is offline F800Riders.org Supporter
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    Never seen that one before! Faulty/damaged casting maybe?

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    Richard230's Avatar
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    Yes, it does look like the cog wheel sprocket broke into pieces. I can't imagine why unless it had a casting defect that took years and thousands of miles to show up. I don't know what other damage there might have been, but just looking at that photo it seems to me that maybe the wheel bearing is OK and all he might need is a new sprocket. I will report back once I find out more after the repairs have been made and he gets the repair bill.

    His bike is now in the hands of a local independent BMW mechanic that started his own small business repairing bikes when he got laid off during the start of the pandemic by the BMW dealership where he had been working for several years. I might add that the mechanic was available to receive the ST into his shop on a Sunday. I might add that if he had been still working for a franchised BMW deakersguo and had not started his own business then the ST would have had to be stored somewhere until the BMW shop opened up on Tuesday.
    Richard - Current bikes: 2016 BMW R1200RS, 2018 16.6 kWh Zero S, 2011 Royal Enfield Bullet 500 Classic, 2009 BMW F650GS, 2020 KTM 390 Duke, 2002 Yamaha FZ1 (FZS1000N) and a 1978 Honda Kick 'N Go Senior. 

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    So lucky that didn't cause an accident or jam up and damage the engine/transmission (maybe a belt drive plus). Hard to imagine how the pulley hub could fail symmetrically and instantly like that. The mechanic will figure it out and seems to have treated your friend well. The photo brings up a good question/debate. It shows the tape weights on his rim that offset the pressure sensor. That's more than I've ever needed on my 800GT and some guys say they don't bother to balance motorcycle tires on modern cast/machined rims because they are so good now. I've seen them very close on my other bikes, but the F800 with pressure sensor almost always takes a some weight. But does it matter? Maybe not at normal street speeds. Anyway glad your friend is getting it worked out and is okay.

  9. #7
    notacop is offline The original Schwartz Wald Troll
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    I really enjoyed my ST but wonder if it would have made 100K. I know there have been several that did get high miles but still. Some failures are scary.

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    It could be the sign of running a belt too tight on it. That would be 'pulling' on the pulley and causing stress around the central section where it bolts on to the hub. Looking at the central screw which is the final adjuster for the hub, that looks to be quite far in suggesting the belt is towards the tighter end of the scale.
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    The “belt wheel” bolts to a hub which is rubber isolated from the axle hub. Looks to be held on with a circlip, and that appears to be where the separation occurred. The belt wheel hub has to be able to float for the cush drive to function.

    This is a task for the salvage market. Lost parts new from BMW would be in excess of $1500. Another $900 if the axle hub is damaged.
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    Melbourne, Australia IanA is offline F800Riders.org Supporter
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    Notice the scar on the swingarm face about where the clearance of the belt wheel face is. Is it from the bits of the disintegrating wheel or from some road debris in there. A chunk of, say, metal jamming in there would surely damage the toothed wheel explosively if underway at any speed. Agree that the wheel balance weights are unusually large.
    Lots of food for thought here, and a bit scary,,,,

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    This damage is interesting. Did it occur prior and maybe caused fatigue or stress, or triggered the failure immediately, or was caused by the failure?

    It seems to have similar colouration as the surrounding material which makes me think it's not new.


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    Quote Originally Posted by XR1000 View Post
    The photo brings up a good question/debate. It shows the tape weights on his rim that offset the pressure sensor. That's more than I've ever needed on my 800GT and some guys say they don't bother to balance motorcycle tires on modern cast/machined rims because they are so good now. I've seen them very close on my other bikes, but the F800 with pressure sensor almost always takes a some weight. But does it matter? Maybe not at normal street speeds. Anyway glad your friend is getting it worked out and is okay.
    Not so sure the TPMS is the culprit for those weights. (8) 7g weights? Or 10g, they look longer than MotionPro 7g weights.

    (8) 7g weights is 2 oz is not good news. Worse than usual. The last 20 or so I mounted averaged 3 but for one. Couldn’t believe it. Tried to talk my friend into letting me mount my waiting tire and return his. He bought an unmarked tire from low price seller on eBay. It took (19) 7g weights. 4.75 oz.

    A month later the tire skidded on a wet turn at low speed. He ran 100 yards in grass downhill, could not stop. Hit a deep ditch. Crashed, totaled a Kawasaki Ninja 1000. Broke arm and leg. Caught COVID-19 in rehab. Never got home.
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  16. #13
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    I should mention that the ST does not have a TPMS. Also that is a relatively new tire that was mounted and balanced by a nearby small independent shop at a very good price, according to my friend. He is going to tell me the entire story on Sunday morning, including the diagnosis from the independent shop that will be repairing the final drive. Here is a photo showing the bike being loaded onto a 2020 Ford Ranger.
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    Richard - Current bikes: 2016 BMW R1200RS, 2018 16.6 kWh Zero S, 2011 Royal Enfield Bullet 500 Classic, 2009 BMW F650GS, 2020 KTM 390 Duke, 2002 Yamaha FZ1 (FZS1000N) and a 1978 Honda Kick 'N Go Senior. 

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    While not a controlled experiment (same wheel/tire/tire clocking), my wife's GT w/TPMs and Bridgestone Battlax S22 took 40g (8x5g) while my ST w/o TPMs and Angel GT2 took 7g. A prior set of BS S21 on my ST were very well balanced too. I mount and balance my own tires with a NoMar mounter and balancer so I often play with tire clocking to minimize weight.

  19. #15
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    Quote Originally Posted by andysrage View Post
    While not a controlled experiment (same wheel/tire/tire clocking), my wife's GT w/TPMs and Bridgestone Battlax S22 took 40g (8x5g) while my ST w/o TPMs and Angel GT2 took 7g. A prior set of BS S21 on my ST were very well balanced too. I mount and balance my own tires with a NoMar mounter and balancer so I often play with tire clocking to minimize weight.
    I do the same with all of my bikes, including my R12RS with its large TPM lump sitting in the middle of the "drop" rim. Except I change my tires on my hands and knees while sitting on a piece of scrap cardboard on my garage floor using 2-foot long tire spoons. That is the way I have been changing tires for over 50 years and old habits are hard to break. Using a NoMar tire changer would be really nice, but I am too cheap to buy one - although I do wish I had bought one many years ago.
    Richard - Current bikes: 2016 BMW R1200RS, 2018 16.6 kWh Zero S, 2011 Royal Enfield Bullet 500 Classic, 2009 BMW F650GS, 2020 KTM 390 Duke, 2002 Yamaha FZ1 (FZS1000N) and a 1978 Honda Kick 'N Go Senior. 

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    I wasn't aware the ST didn't have TPM sensors , thanks. Out of curiosity I checked my GT yesterday and it now has three 7g (1/4oz) weights, with Bridgestone T31s mounted. I've mostly used Bridgestone with a set or two of Dunlop on the GT and three 1/4oz weights seems about normal (as I remember) for the rear tire. I use a nomar balance stand so there's a little fudge factor involved. N4HHE sorry to hear about your friend, that sucks.

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    Quote Originally Posted by XR1000 View Post
    I wasn't aware the ST didn't have TPM sensors , thanks. Out of curiosity I checked my GT yesterday and it now has three 7g (1/4oz) weights, with Bridgestone T31s mounted. I've mostly used Bridgestone with a set or two of Dunlop on the GT and three 1/4oz weights seems about normal (as I remember) for the rear tire. I use a nomar balance stand so there's a little fudge factor involved. N4HHE sorry to hear about your friend, that sucks.
    That particular one didn't, is what i am assuming he meant. Mine certainly does have it as an option.
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    Here is the story: While the ST rider was waiting for a friend to pick up his bike, he looked around and found the intact $618 "belt wheel" by the side of the road. It had apparently come detached from the $300 "housing", which looks to be missing to me. The sprocket hit a metal guard rail and was damaged so it is no longer usable. I can't see the "housing" in the photo, so I am not sure what is going on there. Interestingly, the independent BMW mechanic has told him that he has ordered the necessary parts to repair the bike from Germany and the replacement parts will only cost around $300, but will take at least three weeks to arrive here. If that is what the parts cost when ordered from Germany, BMWNA must really be marking them up when they are sold in the U.S.
    Richard - Current bikes: 2016 BMW R1200RS, 2018 16.6 kWh Zero S, 2011 Royal Enfield Bullet 500 Classic, 2009 BMW F650GS, 2020 KTM 390 Duke, 2002 Yamaha FZ1 (FZS1000N) and a 1978 Honda Kick 'N Go Senior. 

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  26. #19
    Richard230's Avatar
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    My friend got his ST back yesterday. The work was performed by an independent BMW mechanic who sourced the replacement parts directly from Europe at decent prices. The edges of the $650 belt sprocket were bent a bit, but the mechanic was able to beat it back into shape so that it looks like new. The sprocket carrier and big retaining nut (or whatever it is) was replaced. The total cost of the parts came to $800 and the labor was an additional $300. I have no doubt that it would have been a lot more if the work was performed by a franchised BMW shop using parts sourced from BMWNA and I am sure that the sprocket and belt would have been replaced and not repaired. I might add that the belt was reused as it appeared to not have been damaged. However, my friend says that he plans to replace the belt himself soon using a $150 belt sourced from Europe. Not surprisingly, the mechanic said that he had never seen a failure like that before.
    Richard - Current bikes: 2016 BMW R1200RS, 2018 16.6 kWh Zero S, 2011 Royal Enfield Bullet 500 Classic, 2009 BMW F650GS, 2020 KTM 390 Duke, 2002 Yamaha FZ1 (FZS1000N) and a 1978 Honda Kick 'N Go Senior. 

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