Sunday 22 September 2019

Fitting K75 Showa forks to an 8v K100

There is an unspeakable truth about the Sachs/Brembo forks that come on the 8v K100s - they're really badly made. They have no steel bushes and instead have the stanchion (chromed bit) slide directly inside the leg/slider (bulky aluminium bit). I've even seen it said on K100 forums that this is good because it means the weight of the leg is less.. What nonsense! Anyway, the upshot is that the aluminium of the leg starts to gall and this munches great big scores into the stanchion and then you have a leaky fork - mine were leaky when I bought it and I've seen a few over the years that also have leaky forks, most owners insisting that they just need new oil seals which of course won't help the fact that the two bits don't fit together very well any more. There was even a guy on the old gbbikers forum (SAS Tom if he ever reads this!) who bought new stanchions in the hope it would stop his forks leaking.. Of course they didn't, he just wasted his money. Eventually I got tired of having a saggy, springy front end with forks that would just puke out any oil I put in and something had to be done. A front end swap to the 16v set up with Marzocchi cartridge forks (that kinda work) and proper brakes was incredibly tempting but also very difficult due to the current ABS system and throttle cables not being a straight swap. Instead I decided to fit some Showa forks off a late K75 (and the Mystic R850, I think) which don't come up for sale very often but if you're patient you'll get some eventually - I paid £106.60 after a few months of waiting. The Showas are a very basic damper rod design but in their favour they hold oil and are almost a straight swap save that the stanchions are 41mm diameter instead of the Sachs/Brembo's 41.3mm. No one on the K100 forum could say for definite if the old yokes would clamp the new forks but I gave it a go anyway. So..

Crappy Brembo forks. These are identifiable from the rib that runs down the legs and from the oil that covers the travelling section of the stanchion..
The later K100s and K75s have a fork brace that BMW said wouldn't be necessary because of the oversized front axle (K fun fact) and this means they also use a two piece mudguard, one piece either side of the brace. They are connected by a long bolt that fits a square nut in the front half. This gets sprayed with literally ALL the muck and salt and water and promptly corrodes into place, another fantastic design decision. Bring on a tool of destruction.
A worn down cutting circle fitted in the gaps and only chopped the mudguard and brace a little bit before doing its job:
I'll worry about that later, for now I have some exciting new forks to fit! Old ones on the right, new ones on the left.
Ahh Showa! And smooth stanchions!
And a pic of the Brembo codes, in case it's any help to anyone?
Right, slap them in!
It is true, they are indeed a straight swap - the calipers bolt right up, the original axle fits and after a whole lot of persuasion the yokes will clamp the forks in place.. Just about. I had to tighten the bolts 3 or 4 times to stop the forks working their way up past the yokes but did get there in the end. I still plan to fit some late K75 yokes for peace of mind though.

With that done and the concept proven it was time to get back to that blasted mudguard. My first attempt was to use some mole grips (a real professional's tool, I think you'll agree) to squeeze a nut onto the square nut in the mudguard to stop it spinning and then turn the remains of the bolt out.
This didn't work.

After literally seconds of thinking I decided that there was only one way to go, it may be destructive but the mudguard is no good as it is. Where's my soldering iron and plastic welding tip..
The slot actually works quite well and it gave me an opportunity to close up the hole underneath that both lets all the muck get to the nut but also allows the washer to fall out when I'm putting it back together. I hate this mudguard so much.

With that little fun problem solved I could finish the job..
..And go for a test ride!

First impressions were that I was riding uphill everywhere, as I was so used to the super saggy forks that were on there before. The Showas barely sag when the bike is taken off the stand and sag only slightly more when I sit on it, personally I prefer more sag than that but I guess it's better than it was.

Second impressions were that the ride is very harsh and crashy, obviously this is to be expected with damper rod forks but I didn't expect it to be quite so hard. However it is a definite improvement over the old forks and damper rod forks can always be modified with less/thinner oil or even cartridge emulators so I'll probably end up doing that one day. But for now it's nice to have forks with damping!

4 comments:

  1. Do you still have the bike and did this work long term

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    1. I do still have the bike, but it didn't quite work long term - I eventually fitted YSS PD Valves and some springs from an unknown Harley that also had 41mm Showas. I also had to fit some yokes from a K75 that were meant for the Showas, the original grey yokes just couldn't clamp the slightly smaller diameter reliably enough for me. You can find posts about both jobs on here, just click the K100 button up top!

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  2. Thank you for the info. I'm thinking about buying a complete set of showa forks on ebay. Will the k100 brakes and front wheel bolt on or do I need them as well.

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    Replies
    1. The K100 8V brakes and front wheel and spindle all fit straight in, the front of a K75 is the same as an 8V K100 except the handlebars are solidly mounted instead of using rubber inserts and some later K75s came with these 41mm Showa forks as opposed to the usual 41.3mm Sachs/Brembo forks. The 41mm yokes are black and have SHOWA written on the bottom, happy hunting!

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