Calculating front spring rate? Valving too.

[Anonymous]

Does anybody know a site with some sort of table or equation used to calculate spring rate? I'm going to get a pair of front springs for my 400 as they are pretty cheap from Progressive at about 80$US and dirt cheap on ebay. Unfortunately they do not spec a spring for my bike so I have to figure it out. Could I just look up another 400 pound bike and steal its number? I'd just have to measure length and diameter then.

The bike weighs about 400 pounds dry.
I weigh about 210 plus another 90 pounds for leather, boots, and school stuff.

So, with some magic I get about 650 pounds total weight on the average day.

Also, what kind of valving is in my bikes forks? Does it wear out? I'm trying to tackle the pogostick feeling I get when riding. With the front break on I can push the bikes fork up and down very easily and quickly. There's a lot of weight shifting when riding too...feels bad at about 120km/h changing lanes.

Thanks, Steve

[Anonymous]

Try:

http://www.traxxion.com/

And look for the charts area. They give fork spring rates based on combined bike/rider weight. Their recommendation will be on the firm side, but you can always go down a bit for a more comfy street ride. Progressive springs will have a variable rate, starting soft and getting stiffer as the springs compress. They generally don't have set made expressly for your bike, but just a generic one that is "about right" for it. It is preferable to have springs set up specifically for your bike/rider weight, but even the progressives will be a serious improvement over sacked out stock springs. With the older bikes, we have to take what we can get a lot of the time for stuff like this...

Your 400 will almost certainly have damper rod forks on it. There is only a couple of moving parts in them and nothing really wears out, except the oil itself. If you are getting pogoing, it is an indication of lack of rebound damping, which means it is time for new fork oil, probably in a heavier grade than stock. To really fix things, put in new springs and add cartridge emulators along with a fork brace. Of course, this may be more than you are interested in. As a minimum, change the springs and oil.

There is more you can do to your forks in the form of blueprinting and lapping things together to improve damping, but they are for someone who has lots of time on their hands and can't simply add newer forks with shim stack damping (say someone racing in a vintage class that prohibits the fork swap).

Mark

[Anonymous]

Now that's dead simple. Very good help indeed.

I'll have to find some 0.9kg/mm+ springs. This is also the spec for GS500s so I know it's right.



I've changed the oil in my forks twice now and the third time will be after 20,000 km of ownership when I go in to measure the springs. I've been using Castrol non-additive 30 oil with spacers of about 1" in length. The spacers did help some. The oil was very black in one of my forks when I first changed it at 25,000km.

Cheers, Steve

[Dink]

Damper rod orks as per most GS's, actually have a couple of bushes in them that are supposed to be replaced every time the forks are disassembled, one is nylon and is on the damper rod itself, the other is metal and goes around the chromed upper tube near the base, BTW later models also have another bush that sits under the fork seal. Replace all these bushes change the oil, after a good flush with kerosene,fit the new springs. You wont recognise the bike as being yours
Dink

[Anonymous]

It's funny, I've had the springs apart but I never saw anything like the damping rods and "free" pistons in the 1100. There is something bolted to the bottoms of the forks though.

I hope the changes are that dramatic. I'm planning on putting 16-20,000km on my bike this summer so I am doing everything I can to make sure its a comfortable journey.

Steve

[Dink]

Steve, you know how you need to take a bolt out of the bottom of the fork leg to get them apart? well the piece that falls out after you do this is the damper rod! it sits inside the upper fork leg and stops it from coming out and also has a top-out spring on it.
Dink

[Anonymous]

Steve, the weight of your fork oil will also effect the damping.