suspension travel

[blackie1]

1991 K75  BMW front forks (not Showa)


having recently bought this bike I am trying to iron out a few gremlins.
mid corner at speed if I hit any decent size bump I can sometimes get a fairly decent head shake


my front tyre is just only legal so will be getting a new one soon. which may fix it


also according to the previous owners records the fork oil was last changed 7 years ago!!!  so it maybe as simple as changing the oil.


so before I do that I wanted to compare the fork travel now (old oil) to the fork travel later (with the new oil)


somewhere on this forum I read about the simple method of putting a zip tie around the fork tube down at the rubber dust boot then going for a ride. the travel of the forks during the ride pushes the zip tie up and leaves it as an indicator as to the maximum fork travel during that ride.


i measured it when i got home from the top of the dust boot to the bottom of the zip tie ,
 112 mm (4 7/16") right stanchion
 114 mm (4 1/2")  left stanchion


then i thought I wonder how this compares to other members K75's


next time you go for a blast, whack a couple of zip ties round your stanchions, measure up and post below.

[Martin]

You might want to check your steering head bearings. Also the K75 has a unique steering damper built in, it consists of a tube which has waffle shaped indentations, these indents are filled with a special heavy silicone grease. As the dampers are no longer available some Bricks have had this feature removed. There is speculation on whether these dampers actually work. Some riders have reported feeling no difference on it's removal. I actually serviced mine and redid the grease when I changed my steering head bearings. I have never experienced head shake on my Brick, and whether my functioning damper has had any bearing on this who knows.  :dunno I have now opened a can of worms stand by.  :hehehe
Regards Martin.

[blackie1]

thx for the info martin
steering head bearing doesn't indicate any sign of being "notchy"
and yes I had read a little bit about the dubious in built damper.
i will search for some more info about that.
 like i said earlier i will change the fork oil (who knows if the previous owner carried out this operation correctly when he did it)
and its due a new front tyre and then i will reassess it after that.

i would be interested to hear how much other peoples  suspension travels, if only to give me some indication how tired my springs are


I came across these aftermarket progressive springs made by some of your aussie bro's
http://store.suspensionshop.org/bmw/k-75/spfkit-bm83-k75-83-91-k75-brembo-fork-41-4-mm-hyperpro-front-fork-kit.html
they would have to be good wouldn't they?  after all they do have a spring loaded kangaroo as their company logo.
i will jumping fences before you know it

[Martin]

Without getting carried away you can fine tune the suspension by playing with the fork oil levels and weights. Adding more oil will firm up the suspension reducing the oil will soften up the suspension. Be careful as adding to much oil can firm it up to the point of blowing the seals. If it helps I run 10w oil in mine at the moment and previously have run 8w with slightly less oil but found it too soft for our bumpy back roads. I have Showa front suspension with OEM rear shock rebuilt and modified by RAD a local shock rebuilder.
Regards Martin.

[Laitch]

my front tyre is just only legal so will be getting a new one soon.
also according to the previous owners records the fork oil was last changed 7 years ago!!!  so it maybe as simple as changing the oil.

A new tire will help. Poor tire condition, incorrect inflation and excessive speed are contributors to head shake, but your fork assemblies need to be tuned up anyway because of the previous owner's neglect. 

Remove the fork from the braces then remove the components each tube. All components should be thoroughly cleaned, seals replaced and probably the bushings, too. Add 7.5 fork oil, ride the bike then determine whether just a change in the oil weight, as Martin suggests, improves the performance enough. If not, different springs might be the next choice

You might even have progressively wound springs already. Until the assemblies are inspected and defective parts are replaced, purchasing new springs is premature. The work is overdue, regardless of the zip-tie travels on other rider's fork tubes.

[The Mighty Gryphon]

Get a new tire.  Drain and flush your forks with ATF.  Then refill them with fresh oil in the proper amount and report back for the brain trust.

How tight are you holding the bars in the turn?  Do you have a loose grip and let the bike take it's line through the turn, or are you forcing it to fight you?  When I got my first K75 after a long absence from riding I had some issues with the weight and handling in turns.  When I  finally relaxed, it was amazing how the handling of the bike improved.

[blackie1]

good advice Laitch, I might replace the  tyre and fork oil 1st . I have a new bottle of 7.5w  "Golden Spectro very light Shock fluid" on my shelf ready to go. 
if no improvement will pull things apart as you advise


Gryphon, I have just come out of riding a K1100 for the past  3 years and believe I ride with a relaxed grip but will certainly put that thought to the forefront of my mind.
 when you suggest flushing the forks with ATF do you mean just pour it in the top and catch it out the bottom as it flows thru?

[The Mighty Gryphon]

I would fill the forks to the proper level with the ATF and ride the bike for a couple days and then drain them.  Let them sit overnight so as much as possible drains out.  Then refill.  If you want to save some money refill with ATF.  It works as well as more expensive fork oil. 

[blackie1]

thanks for that. much appreciated

[blackie1]

further to my head shake issue mentioned above I have been reading up about the fluid block damper and think i have identified an adjustment screw on each fork that I could perhaps tighten  to increase the damping.


the hex screw pictured has an internal tapered end that when the hex screw is wound in the tapered end protrudes further into a rubber block , expanding it slightly and this increases the damping effect.


I believe I have that correct, please advise me if I have not.


as you can see in the pictures attached the hex screw is packed out with 2 x brass colored washers. would I be correct in assuming to gain some adjustment I should remove the hex screw, then remove 1 x of these brass colored washers and the re-insert hex screw and and tighten up.

this would  allow the internal tapered end of the screw to penetrate further the rubber block inside by the distance equaling the thickness of the washer i have removed? and this should increase the damping effect.

[blackie1]

also just to cover off a suggestion by Martin to check for any play in the steering head bearing. with front tyre off the ground there is no discernible play fore or aft and the handle bars fall from the center position  smoothly to both left and right lock , so I reckon head bearing is ok and requires no adjustment.

[Martin]

Blackie the bolts are just to hold the fluid block in position. The actual damping effect is by the heavy silicone grease. Do not remove the washers as this could deform the fluid block and could cause it to bind. The grease is available as Dow Corning Hi Vacuum Grease.
Regards Martin.

[blackie1]

ok. thanks Martin. i won't touch them.


I see in my Haynes manual, in the routine maintenance, section 16, under "check the steering head bearings" that prior to pulling / pushing the lower fork legs to check for any play, these fluidbloc retaining screws must be removed 1st.
it does not explain why.
is it necessary to remove them?  I guess if its in the book its in there for a reason

[Martin]

I believe that the fluid block could mask wear. When checking the bearings removal of the screws takes the damping out of the equation. It allows the steering to turn without any friction from the grease. As you turn the whole fluid bloc turns rather than turning through the grease.
Regards Martin.

[blackie1]

thanks for that Martin