Need to replace front fork springs on my '79 550E. Stock spring rate/Recommendations?

[roeme]

Prelude:
So I've got to get my 550E pass MOT/DOT/whatever it's called in your country inspection. The bike failed once already due to multiple issues - most of which I rectified since -, but mainly because of the inspector's pedantry...

Anyway, one of complaints was a cracking/snapping sound/miniscule jolt at the end of a very hard/emergency brake cycle. I noticed this already when I got the bike, and to me, it felt like the steering head was loose and shifted slightly when forces got high enough. However, the shop I trust found no fault with it, but tightened it up to upper spec limit as a precaution. Still, the issue persisted.

It never posed a problem - the steering head moved freely, no friction, smoothly, not a single tangible detent anywhere. At the same time, the sound/jolt in my opinion appeared too sudden, was too weak, to correspond to the fork bottoming out.

Recent discussion with the shop owner however leads me too believe that it indeed could be that. By his opinion, the springs are quite tired (likely still factory), so replacing them won't be a waste in any case. He cannot order them from his suppliers anymore however, so it's up to me.

(End prelude)

So I'm going to replace the springs, and reading up this GSR thread, Sonic Springs seem to be a good choice.

However, reading the manual and fork parts fiche (here's the version of cmsnl.com), I a) can't find stock spring rates in kg/mm (or simply am too dumb in interpret the diagrams correctly), and b) there are two springs in each tube? :-k

Per Sonic Springs' calculator, 0.9 kg/mm should be a good fit for my weight+the bike. Would like to know stock though.

Also, anything to watch out for or tricks in addition to the procedure given in the manual?

 

[Guest]

So I'm going to replace the springs, and reading up this GSR thread, Sonic Springs seem to be a good choice.

Sonic springs are an excellent choice.

However, reading the manual and fork parts fiche (here's the version of cmsnl.com), I a) can't find stock spring rates in kg/mm (or simply am too dumb in interpret the diagrams correctly), and b) there are two springs in each tube? :-k

(a) I don't think they give the spring rate(s) anywhere. I never found that info in my 1100E service manual, anyway. I calculated the rate for my bike and it was 0.47kg/mm at the start of compression, which is stupid soft. In the 80's the theory was to use very soft springs and then add air pressure to augment them. That works poorly at best and we now use a stiff enough spring and avoid the air pressure.

(b) That diagram does indeed show two springs per side, which was sometimes used as a way of getting a progressive/stepped rate.

Also, anything to watch out for or tricks in addition to the procedure given in the manual?

Yes, use real fork oil instead of whatever the manual says and set the oil height as per Sonic's recommendations. Typically people use 10W or 15W fork oil these days for our bikes. Since you are looking at 0.9kg/mm springs I would recommend the 10W as a starting point. It might be worthwhile to change the internal fork bushings while you have it apart and it is always a good idea to have a new set of fork seals handy in case you mangle one of the existing ones while pulling the forks apart. Having an impact wrench to remove the damper rod bolt on the bottom of the forks is very handy, it can sometimes be a bear to break loose.


Mark