I have an 89 cbr600f street fighter (lost maybe 15-20 pounds off the front, it should be around 400 pounds now) that is too soft in the front. The other day I was doing circles in a parking lot trying to get rid of the chicken strips on my new back tire. The front end slipped pretty good and gave a nice shot of adrenaline.

Anyway, the front end feels like it floats, and I don't feel like I am getting enough feedback. Its like an old Cadillac or something. She dives quite a bit under heavy braking, and refuses to hold a line under hard acceleration ( maybe thats another problem?). Also when I push down on the front end it springs back up and bounces a couple times before it settles.

Under the bikes own weight it sags 22mm, and with me (150 w/ gear) it sags 35mm. The bike is my commuter, but I ride it pretty hard. I also ride twisties and tour on it too. Mostly I like to go fast, and get sideways.

I was planning on getting racetech springs for it (current are stock), but I want to try putting in some spacers first as I'm not sure about doing a fork swap yet and dont want to waste my money. I am putting in new oil seals in a couple days and figured now would be a great time to try to tune the forks a bit.

My hopes are that some of you guys can help me answer two questions:

1) How big of a pcv spacer should I start with? I am pretty sure I'd like to the sag down to 30mm...right?
2) What weight fork oil should I use? I am using ATF right now, and I dont think its enough. Would 10 weight be enough for a guy my size?

Any input would be greatly appreciated. Ride safe

 

Do you have sufficent oil in the tubes? It sounds like a dampner issue in reference to the bounce before settling.
New springs aren't a bad idea. You really shouldn't need spacers, in your weight class.
You might try upping the pre-load (sort of the same as adding spacers).

Sag is actually the difference from static load (the bikes own weight) to rider-load
(with you). so you're actually talking 13mm of sag (with your numbers).

By the way, all of these comments are based on my understanding, as a
decent, "shade-tree" mechanic. I am not a trained suspension tuner.

See my sig, lol, Ern

 

Have you checked to make sure you don't have slop in your wheels or swing arm from worn out bearings. In regards to the forks, it sounds like there might be a couple things going on. Has the bike been lowered? If it has, then your geometry can be off. It wouldn't surprise me if you have too much friction in the front and rear due several things (worn bushing, bearings, seals, bent forks, or forks misaligned), so make sure to inspect your suspension front and rear for worn components. The sag is with in the range for street sag, but you may want to increase it to 25mm to 30mm with your riding to help with the dive.

Here is a good method to determine static sag {Sag = L1 - ((L2 + L3) / 2)}. You can also determine your front and rear friction {L3 - L2 = friction}. If this value is equal to or greater than 25mm, you have too much friction. At this point, you're going to have to go through your suspension looking for worn components. When measuring front, measure from bottom of triple to dust seal. For shock, measure from fix point on frame to rear axle. Be sure to use this spot for all front and rear measurements. You may need three or four buddies around to help with getting measurements.
L1 = Lift bike from frame to extend forks/shock. Make sure rear wheel is off the ground. No rider.
L2 = With gear on and rider saddled in riding position and friends holding bike up, push down front and 25mm/1" and let suspension return. Take measurement of returned position.
L3 = With gear on and rider saddled and friends holding bike up, lift front/rear 25mm/1" and let settle slowly. Take measurement of settled position.

However, there are more suspension settings than just sag. One has to also tune compression dampening and rebound dampening. With rebound and compression dampening, there is high speed and low speed. So the goal with these circuits is to have the an ideal amount of dampening to allow the wheel(s) track perfectly over both leading and trailing edges of a bump.
I don't know if your bike has the adjustments for these in different circuits (e.g. 600/1k RR forks and shock), so you may not be able to adjust some of these circuits without changing the shim stack. If I remember correctly, all you have is rebound adjustment at the top of the forks. If Race Tech (RT) Gold Valves are available, they will definitely improve the performance because you can dial in your rebound and compression shim stacks.
Shimming your spring isn't going to fix the problem because you don't have enough compression and rebound dampening. Your bike is diving and maybe bottoming out with hard braking (compression) and you're getting a "Cadi" feeling when returning to the static position (rebound). Even if RT Gold Valves are not available for your bike, you can still change the compression and rebound shim stacks. If you do this, you can purchase additional shims from RT. Just an FYI, one can't just go adding more shims here and there in the stacks. It's a calculated to determine which thicknesses to use and how many to use. This calculation takes in elements such as riding style, spring rate, oil, weight of rider. Hopefully RT Gold Valves are available so that you can get access to their matrix of shim stacks. Here is a nice visual of front suspension at work (:28). Watch how the fork tubes are stationary and the wheel is moving to the tubes.