Spring suggestion: 92 kat 750 front on 78 1000E

[arcturius]

Hi all,

I just picked up a kat chassis that I'm planning to use to upgrade the GS--initially just the frontend, and the rear wheel/brakes. I'm planning to rebuild the forks, and I'm wondering if it would be a well-advised idea to put in some uprated springs while I'm in there.
Obviously the GS is a fair bit heavier, and I'm no featherweight (about 230lb), but since I'm not planning to push the bike very hard (I've already scraped my exhaust a few times on stock suspension--S&S 4-1--I'll be riding more gently from now on), is it likely to be necessary? Any guesses on what to expect for droop initially, or what spring rates this fork might have or what I might want to upgrade to?

All opinions welcome.

[Billy Ricks]

Just install it first and see where you are at with the sag. You can always shorten the springs to stiffen them up then make up for the difference in spring length with spacers the same length as you cut off.

[arcturius]

Just install it first and see where you are at with the sag. You can always shorten the springs to stiffen them up then make up for the difference in spring length with spacers the same length as you cut off.

Oh yeah. My high school physics is starting to come back.

Couldn't the same thing be accomplished by simply adding the spacers so there is additional tension on the springs, initially? Or are we dealing with spring rates low enough that there is more to be gained by removing load bearing coils than there is in starting partway up the force curve?

Presumably, both methods presumably are going to reduce overall travel and result in bottomed out springs?

[Billy Ricks]

Oh yeah. My high school physics is starting to come back.

Couldn't the same thing be accomplished by simply adding the spacers so there is additional tension on the springs, initially? Or are we dealing with spring rates low enough that there is more to be gained by removing load bearing coils than there is in starting partway up the force curve?

Presumably, both methods presumably are going to reduce overall travel and result in bottomed out springs?

Adding preload spacers will correct sag but do nothing for the spring rate. I'd install the front end first, set up the sag, then see how the fork handles the weight of the bike and yours. Wait to start cutting the springs.

[arcturius]

Adding preload spacers will correct sag but do nothing for the spring rate. I'd install the front end first, set up the sag, then see how the fork handles the weight of the bike and yours. Wait to start cutting the springs.

Thanks! I'll post results when I'm done. Maybe it will be useful to someone else.

[Billy Ricks]

After checking on the weight of the two bikes yours is about 40 lbs heavier than the Kat. You probably will need to take off about 10% or more of the spring and replace that with a spacer.

[80GS1000]

1.0 kg/mm Racetech springs would be a good place to start. That's what I run with a 2004 GSXR 1000 front end on a '80 GS1000E and the sag is perfect.

[kingofvenus]

There is a long mild steel spacer in those stock. It's common practice on them to replace it with one about an inch longer, I did that with the 600 front end (600/750 same parts) on my '83 750, one inch worked fine for me, I'm 225 dressed. Plenty of clearance ground to yosh header also. We even did this to my sons bike, (3/4 on his) an actual '90 600 Kat.

[arcturius]

Thanks!

Thanks for the input everyone.
I suppose I'll just put the forks on before I rebuild them, and just sit on the thing to see what I'm left with before I tear into things.
Found that the stock spring rate is 0.629kg/mm; that should help in calculations later.

Edit:
80GS1000 hit the nail on the head! The racetech site has a spring rate calculator, and with my weight +40lb to account for the GS/GSX difference, it's recommending 1.063kg/mm springs. The highest I can buy is the 1.0, so they'll be just a _little_ soft.
Now to figure how much travel I'll be left with if I cut the stockers down to get a rate of 1.06-1.1...or just spend the $110.