Rear Rotor thickness - BMW F800 Riders Forum & Registry



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  1. #1
    Lag's Avatar
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    Had my bike into BMW for some service yesterday,,. They are telling me the rear rotor needs to be changed with the next pad change which are are at 50%, the bike has 40,000 kms and has used up the OEM original pads and now 50% on the second set?

    Rear rotor measurement by them was 4.27mm-spec 4.5mm?

    Brakes are functioning fine?

    I'm wondering what some of you are doing or have done regarding rear rotors, this is on an F700GS,,..

    Thanks
    Lyle

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  3. #2
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    Measure it yourself and if it is out of spec replace it. No need to buy from BMW or pay their labor rates. There are some good, readily available aftermarket disks available. EBC comes to mind. Installation is not particularly complex. Get yourself a Service DVD or a Haynes Manual and you’ll have the guidance you need. 24K km is pretty low mileage to need a disk. Not sure what to say about that.
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  5. #3
    Lag's Avatar
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    Its actually 40,000 Kms,,. Easy enough to verify the measurement, however my question is why do they recommend to change the rotor at there 4.5mm spec when its functioning properly,,. Just had the original OEM pads out and put them back with the same 40,000 Kms and they are still 40% remaining, both front rotors are,. Seems a bit weird, is this spec right & if it is,,,,,,,,,, is this a systemic problem, is the rear rotor system under designed,,.

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  7. #4
    AKBeemer's Avatar
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    I meant 24k miles. There is certainly some padding built into the spec by the engineers, but as the disk thins it is more prone to over heating and warping
    Kevin
    The Outpost, Silver City, Montana
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  8. #5
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    Defo measure yourself, I've had car dealers tell me a disc needs replacing & found it measured well within tolerance. I bought a cheap micrometer on ebay £GBP15
    As AKBeemer says, thin discs can overheat/warp.

    Depending how long you intend to keep the bike, you might consider a used item. UK suppliers advise the thickness & give a 6 month warranty
    Ajay

    F800GT 2018 

  9. #6
    Richard230's Avatar
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    I wouldn't be too concerned about the rear brake overheating through use, unlike the front discs which will warp if stressed. But if they do that, you can replace them yourself. Aftermarket discs are sold by several different firms, such as EBC Braking, and Bike Master. But if you do that, be sure to purchase new fastening hardware, as the old stuff will likely get beat up when being removed. I doubt that the rear brake is overworked based upon my observation that most motorcycle rear brakes, including the one on my GS, don't seem to do all that much unless you really stomp on them.

    I might add that my daughter's 1981 R65LS still has its original front discs (and the original rear brake shoes) after something like 125K kms. They do look kind of beat up and are scored enough to give a certified BMW mechanic fits, but they still work fine and have not warped, although they probably eat up brake pads more than if they were the original thickness and also had a smooth surface.
    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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  11. #7
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    yeah... so new they are 5.0 mm new, and the official wear limit is 4.5 mm.
    While on long trip in MX - Central America I wore my rear down to about 4.25 with no ill effects.

    vernier calipers can give a faulty reading as there is a "lip" that sometimes forms on the outer edge of the rotor where the pads do not touch on the OEM setup.
    So I do recommend an inexpensive micrometer as Ajaay recommended!

    I once ...just once... used a Galfer brand rotor and it wore very quickly compared to the OEM (using same brake pads)

    So I have switched to buying "slighly" used OEM rotors on eBay. With patience I can get ones with very little wear 4.95mm for around $50

    It is true of course that as the rotor wears the pistions must protrude farther from the caliper assby, and that was a concern to one mechanic i talked with...
    since the caliper is only designed for a certain safe travel range for the pistons ... I personally don't know how big the margin of safety is there ....
    Blue 2009 F650GS - H-B crash bars, TT luggage rack, Pelican top case, BMW Vario Panniers & bash plate, 20mm handlebar risers, LED voltmeter, Sargent seat, MudSling, a few other trinkets ... I may be just about at the end of the farkle train  

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  13. #8
    Lag's Avatar
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    I'm betting that if you go around you will find plenty of rear brakes systems that are functioning just fine that are well below spec, I will keep and eye on them thou,,!!
    Thanks for all the feedback?

  14. #9
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    Quote Originally Posted by Lag View Post
    I'm betting that if you go around you will find plenty of rear brakes systems that are functioning just fine that are well below spec, I will keep and eye on them thou,,!!
    Thanks for all the feedback?
    Thin rotors WITH thin pads would be the worry if there is one at all
    Blue 2009 F650GS - H-B crash bars, TT luggage rack, Pelican top case, BMW Vario Panniers & bash plate, 20mm handlebar risers, LED voltmeter, Sargent seat, MudSling, a few other trinkets ... I may be just about at the end of the farkle train  

  15. #10
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    The problem with a thin rotor is that eventually it will be too thin to have the strength to brake the wheel. With any luck this will appear as warping before anything breaks.

    As for measuring with calipers? Go right ahead. Look to see if the jaw fits flush. Use common sense. 4.5mm is not OK and 4.49mm is certain death. A lot of guessing is involved, and overkill in attempt to defeat a few total idiot owners that you know are out there.

    Also if the rotor is too thin and the pads get down to their EOL, one or both pads may slip out of the caliper.
    2016 Yamaha FJR1300A; 2016 Beta 430RS; 2007 BMW F800S; 2009 Husaberg FE450; 2016 Subaru Outback; 2018 F150; 2013 Tesla Model S 85; 1983 Porsche 928S; 9 cats 

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