Suspension set up GS 425 for a Noob

[jimfj]

Hello All,

I just finished putting the bike back together after a paint job and and some other goodies, one of which was a new rear tire. (3.50-3.75/18, basically stock size) Chen Sheng. Balanced and new tube.

I have ridden it for about 50 miles after the rework and noticed that at 40+ miles per hour the bike seems to hobby horse. IE, rock for and aft much more than it did with the crappy rear tire I replaced. I noticed the handlebars moving much more than before. Sorry about the description but this is kind of hard to get across.

Anyone got a clue what the motion I'm describing is caused by?

What else can I do to set the bike up for a smooth but stable ride?

I haven't done much riding at over 50 mph for fear of the old rear tire.
I would like to set it up for some short distance highway riding. Most of the riding I do is in the city but I have access to some great twisties that I want to explore and learn from. I just don't feel like the bike is dialed in yet.

I am 6 foot tall and weight about 250.
I have 15w fork oil but haven't had a chance to change it yet. (I have no idea what is in there now or how old it is)

Many thanks for your help and opinions.

Jim

[Ironriot]

For the front, progressive fork springs is what you need. On top of that the 15 weight oil will help. If you're doing things on the cheap you can do what others have done. Chop off an inch from the fork springs and put in a one inch spacer made from pvc pipe. It'll stiffen things up considerably. For the rear seeing as you're a bit on the heavy side you'd probably want to go with some progressive shocks. The stock shocks on GS'es are notorious for being crappy. Seeing as you're 250lbs you can probably go with some cheap aftermarket MIDI shocks. Alot of people have said the spring rate on those badboys is rather stiff. But overall a good deal as they work better then the originals and cost $90

[Nessism]

I concur with what Ironriot suggests. Cut an inch or so off the stock springs and replace the lost stack height with longer spacers. The MIDI shocks would match the bike nicely from what I've read - commonly available on ebay for about $65 + shipping.

Good luck.

[jimfj]

Thank you for the suggestions, I will definately do the fork oil and another question on the springs.

Does stiffening things up translate into more motion and my spine being used to absorb the bumps in the road?

Maybe what I'm asking is how to get a firmer, more dampened, ride and less springyness.

Does the 15wt fork oil help this or should I go with a heavier wt?

Again thank you and this is a first bike so I'm learning and appreciate the help.

Jim

[Steve]

A stiffer spring will simply not compress as easily. With the age of these bikes (25+ years), they have usually lost some of their springiness, so are softer than usual. Mentally straighten out the coiled spring. (Again, mentally) Clamp one end of it, pull on the other, note the effort. Move your pulling point closer to the clamp and pull again. More effort is required due to reduced leverage. Chopping off some of the spring coils accomplishes the same thing. However, that reduced the overall length of the spring, so you need to install a spacer to compensate for that. You might also experiment a bit with a spacer that is longer than the spring section that you cut off, which will either restore lost height or simply increase the preload.

As a rule of thumb, you want to have the front suspension drop about 1 to 1 1/2 inches when you are seated on the bike. One easy way to check this is with the help of a friend and a nylon wire tie. Wrap the nylon wire tie around one of the fork legs tightly enough so it stays in place, but loose enough that it can be slid by hand. Sit on the bike, assume normal riding position. Have the friend slide the wire tie down so it meets the top of the lower tube. Carefully get off the bike and put it on the centerstand, measure the distance from the fork tube to the wire tie. If it is more than 1 1/2 inches, you need to cut the spring, increase the spacer length or both.

Changing the fork oil will also make a difference in the way it handles, but it is best to start with the correct spring rate. A thicker fork oil will slow down the movement of the fork. As long as you ride solo, 15w should be fine. If you plan on doing mostly commuting or highway cruising, rather than carving through the twisties, you might prefer 10w for a smoother ride. (One way to think of the difference is to consider the ride difference between a 1/2 ton and a 3/4 ton pickup.) Considering your stated size and the size of the bike, I would imagine that your two-up miles will be limited. 8-[

Rear suspension should also be set for correct spring rate, but you do not have quite as many choices available. Using your friend again, hand him (her) a tape measure. With the bike on the ground, pull the rear up so the suspension is fully extended. Measure from a point on the bike to the ground. The bottom of the license plate usually makes a good reference point. Sit on the bike, assume a normal riding position, measure again. Once again, you are looking for 1 to 1 1/2 inches of drop or 'sag'. Here, your only recourse is usually just cranking the preload collar on the shocks. Some newer shocks also have damping adjustments, but I don't think they exist on your 425.

Progressive Suspension has fork springs and shocks for your bike. Not exactly cheap, but the results are well worth it.


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[jimfj]

Hello,

I drained the old fork oil and removed the springs. They both looked okay, no damge or wear that i can see. (I know that can't tell me much but lets just say they are in one piece). They old oil was very thick, almost like engine oil (30Wt)

refilled with the 15W to spec and installed a half inch spacer to increase the preload in the springs. Did not cut the springs yet.

result: The 15wt oil is considerably thinner than what came out and the dampening effect is less, IMHO. The front is a little bit stiffer (good) but I'm feeling like the brake dive is increased.

I set the rear preload on the shocks to the middle setting, I like the feedback from the rear.

(I haven't had the chance to do the measurement for lack of a helper but i will.)

Am I nuts or do I want to increase the fork oil weight to 20 or 30Wt to increase the dampening effects? Or am I thinking this through backwards?


Thanks

Jim

[chef1366]

Check the front tire, front and rear wheel bearings, triple tree bearings and the swing arm bearings.