Author Topic: Why do fork seals fail?  (Read 1249 times)

0 Members and 1 Guest are viewing this topic.

  • Offline wethogs   scotland

    • MTS 950 Member  ‐    90
    • **
    • Topic Author

    Offline wethogs

    • MTS 950 Member
    • **
    • Topic Author
    • Posts: 90
    • Town / City: Stirling
    • Country: scotland
    Why do fork seals fail?
    on: January 04, 2022, 04:19:29 pm
    January 04, 2022, 04:19:29 pm

    In another thread about how reliable the 950(S) has been some folk are talking about fork seals. I ask then why do fork seals fail? Is it because of dirt, seal manufacturing defect or a fault with the installation of the fork seal? If some folk feel dirt is the answer then I ask how can this be the case with upside down folks being used on lots of bikes now.
    Kind Regards,

    (A transplanted Canadian in Scotland)

  • Offline Pegasus   gb

    • MTS 950 Member  ‐    71
    • **

    Offline Pegasus

    • MTS 950 Member
    • **
    • Posts: 71
    • Bike: 950 Touring
    • Town / City: Cotswolds
    • Country: gb
    Re: Why do fork seals fail?
    Reply #1 on: January 06, 2022, 09:48:57 pm
    January 06, 2022, 09:48:57 pm
    My fork seal on one side was leaking before I first picked it up back in 2019. This is for a 2018 950 with 6k on the clock. That was corrected by the dealer under warranty. I then had a big leak from the other seal last year. Frustrating as I'm always maticulously cleaning the thing so little chance of mud getting in. I can understand if your pulling wheelies all the time and the front end takes the grunt of the landing, but I think it must be down to fitment at the factory. As I don't hear of it alot.
    Sometimes you need a really crooked road to get your head straight

  • Offline Paul_Smith   ie

    • MTS 950 Pro  ‐    239
    • ***

    Offline Paul_Smith

    • MTS 950 Pro
    • ***
    • Posts: 239
    • Bike: MTS V2S
    • Town / City: Roscommon
    • Country: ie
    Re: Why do fork seals fail?
    Reply #2 on: January 08, 2022, 12:24:53 am
    January 08, 2022, 12:24:53 am
    Manufacturing defect is far and away the least common form of failure, after all, it is just a rubber ring, how hard can it be to make it properly? Fitting errors on new forks are just as rare, after all, you are just putting a rubber ring on a clean metal tube in a factory, not many things to get wrong?

    So why do they fail? First off, there are two very different seals doing two very different jobs. The outer one, the one you can see, is more correctly called a dust seal. It's job is to stop dust, grit, water and any other contaminants that are on the lower fork leg (the bit exposed to the elements) from getting past the dust seal so they can't do damage further up. Unfortunately, anything that does get past the dust seal will become trapped between the dust seal and the oil seal above, and with the fork leg moving up and down - a lot! - over time it will cause damage to both seals. You might have looked after your bike, but if the previous owner didn't and sh#t got stuck up there, then the damage would continue to accrue until a failure occurred. 
    The upper seal has a much harder job. It has to keep the suspension oil above it from leaking down along the fork leg that is moving through it. As long as the fork leg is absolutely smooth, this is not a major problem. Of course, when you land a wheelie badly (or hit a pothole), the fork leg moves Very fast and the oil pressure can get Very high, but as long as the fork leg is perfectly smooth, this should not be a problem as the seal is designed to handle it. Of course, if there is any dirt on the fork leg then it stops being perfectly smooth and if the dirt gets lodged in the rubber of the seal then the seal stops being perfectly round, in either case, oil will get past.

    With this in mind, fork seal scrappers will clean out anything caught in the dust seal that is a) allowing dirt from the fork leg up and/or b) oil from the inner fork down. In other words, they stop the symptoms of the problem (fork oil on the fork leg), but they don't address the cause of the problem which is how the fork oil got past the oil seal. If you use them before you have a problem, then they can help stop debris getting caught between the seals which can prevent the damage from accruing in the first place.
    Last Edit: January 08, 2022, 12:28:13 am by Paul_Smith