Modifying R90/6 front forks

[jagarra]

I ordered a set of the BMW HD springs, hope they arrive this week. Checked my notes, looks like I put in 9.6 oz of 15 wt fluid in the forks. That's what my factory manual called for.
Duane, I did look over your article on checking alignment. Have to figure out how to make a holder for the dial indicator so I can check parallelism of the tubes.

[melville]

I ordered a set of the BMW HD springs, hope they arrive this week. Checked my notes, looks like I put in 9.6 oz of 15 wt fluid in the forks. That's what my factory manual called for.
Duane, I did look over your article on checking alignment. Have to figure out how to make a holder for the dial indicator so I can check parallelism of the tubes.

This is a piece of AL angle, drilled and tapped for the pin from my magnetic base:



And here it is in use, checking parallelosity:



I got the AL from the local Boiler Works, who stock all sorts of metal. As a leftover bit, it was just $2 IIRC.

I'm running 7.5 wt Bel Ray oil in my /6 and liking it a lot. Is there a chance the 15 wt is pumping down (not rebounding quickly enough) and that's what's eating your ground clearance? That said, there can be tremendous variation among oils of different brands.

[Mal S7]

15 wt oil is your problem.
Way too viscose for airhead forks.

I have had joy with Sonic springs, apparently a similar rate to the HD BMW springs, but non-progressive.

[chasbmw]

15 wt oil is your problem.
Way too viscose for airhead forks.

I have had joy with Sonic springs, apparently a similar rate to the HD BMW springs, but non-progressive.

+1

[jagarra]

Thanks, never entered my mind that the oil was too thick. This is a new one on me, I always equated stiffer compression with heavier oil. I'll try some 7 1/2 wt .

gg

[boocephus]

On a side note, if the fork is leaking at the bottom, is the copper washer the only thing that seals it? My '71 was not forking right and I found that most of the oil leaked out of one leg. I added some and it leaked out again.

[chasbmw]

The copper washer is on the inside, to replace it you have to drop the slider, replace it with a smear of Hylomar and it should stay dry. The wavey washer goes on the outside.

[Duane Ausherman]

Yep, 15 is way to heavy, even 10 of the brand that we tried was too heavy. Depending upon the brand, either 5 or 71/2 is the way to go if everything is stock. Rider weight and riding style can cause one to want a change. For 95% of the riders the stock stuff is the way to go.

Don't be one of those that is convinced that you know more than the factory. You may, but in my early days I was changing lots of stuff. For most of those, I realized that I was just trying to pump up my ego with "smarter" changes. I wasn't smarter.

At least I didn't go out and swap out the engine for something larger, like a VW. That would have been the height of foolishness. I heard about it though.

I have a few spare lower trees that I could cut up. As soon as I discover which ones are too bent up to use, then I will do some cutting.

[mattcfish]

Luftmeister HD springs (probably the same as OE HD), 5 wt oil, no pre-load spacers, 190Lb rider. Handles great (I really flog this cow), lots of travel, never bottoms out. Heavier oil just increases damping and slows down the suspension.

[Duane Ausherman]

There are two quick tests for fork alignment, static and dynamic. I would always do both before taking it apart.

Static is the measure of how it returns to neutral while bouncing the front end.

The dynamic test is while riding it.

Did you run the "vibration test" of the fork alignment? While riding, reach down with your left hand and grab the fork tube just above the lower casting. You should feel the forks going up and down so fast that you can't count it. The lower legs (castings) will be bumping your hand so fast and never stop doing it. If you feel it move once or twice a second, you have sticktion. Fix it.