More Fork Swap Related Questions

[jwilliams]

Hello,

Again I know there are tons of posts on this topic but there are a few things I can't find. Mainly I was curious if people who swapped their front ends all got them revalved of if the valving is OK for the Kat and you just change the springs (if that).

I have 2 options I am looking at:
1. 2003 GSXR 600 complete front end
2. 1994 GSXR 750 forks and triples (then source wheel/brakes etc)

The 2003 is probably too short but I have access to the complete front end. With the custom steering bearing from the SV guys at http://www.twfracing.com/ it becomes a direct swap (maybe some thread length work on the stem).

I am pretty sure the '03 front end would need revalve work since the is close to 100lb difference BUT if the '94 generally needs a revalve to work with the Kat as well then it's a wash and the '03 is considered to be one of the best stock suspensions put on a bike (though not USD *shrug*).

But from the measurements I could tally (would like to know R1 length) the '95 GSXR 750 might be the best option anyhow due to length and weight of doner bike. I found lots of how to type info but not much results (re-spring, re-valve, general opinion) related info when I searched.

Thanks, Jeff

Lengths etc of interest (maybe):
97 Katana 750
- measured with measuring tape
- 780mm approx axle centre to top of fork extended (on bike)
- From http://www.robwilton.com/tech.html
- GS 1100 Kat Fork Length 812mm
- '91/'95 gsxr 750 Fork 750mm
- '91/'95 gsxr 1100 fork 775mm
- from http://www.tlzone.net/forums/tl1000s...rk-length.html
97' TLS forks - - - - - - - 720.5mm
01' gsxr 1000 forks - - - - 716 mm
03' gsxr 1000 forks - - - - 710 mm
Note: the above 03' gsxr forks were revalved & lengthened by Jim Lindemann & then I took the measurements.

- from a SV racer (supplier of custom bearings)
K series GSXR 600 (2003) - 724mm (about 28.5")

[Black_peter]

Read this before you get all hung up on springs and weight:

http://katriders.com/vb/showthread.php?t=67723

[jwilliams]

Hello,

I am aware of a lot of this information, I actually would also recommend anyone who wants to know more about this to read John Bradley's books (http://www.broadlandleisure.com/), mainly Volume 1 for suspension workings. I also recommend Tony Foale's stuff (http://www.tonyfoale.com/)! The motorcycle setup software is awesome if you are willing to measure your bike.

The issue I have is that simply preloading a spring and/or cutting some off does change the effects of the spring. If you continue to preload a spring to get the support you require you actually start to remove the compliance of the spring (depending how much you crank up the preload). For small preloads it's OK but for larger you are better off with a stiffer spring to begin with.
More importantly the spring (changed via preload or by different spring) effects the valving of the suspension. A good example is fitting a R1 shock to a '97 Katana 750 (I did it - Penske shock). The link/leverage setup is drastically different and to get proper sag numbers I had to go up significantly in the spring strength (beyond preload alone). Well in doing so to get any rebound dampening at all close to usable I had to crank the rebound setting to the max.
On the other hand a '95 ZX6R (Ohlins shock this time) needed approx the same spring weight as the final R1 config but it wasn't significantly over the stock spring strength. Therefore the damping circuits in the shock allowed me to play with the settings without getting to the outer edges of the adjustment.

Maybe I am looking too deeply at the numbers but if I am going to add a $300-500 front end to the bike I'd prefer to know if I should expect a revalve and spring before.

From your link and comments I get the feeling that most suspension upgraded are so much better for the Kat that valve work is usually not done and the springs are just preloaded to set sag numbers.

Thanks, Jeff

Read this before you get all hung up on springs and weight:
http://katriders.com/vb/showthread.php?t=67723

[Black_peter]

Well shocks and forks are different animals,
But you must know that. Link suspensions can go all wrong easily because of the leverage changes possible. I don't think the same is true for forks. Getting new springs and valves is fine but also keep in mind the diminishing returns of tuning the nuts off a Katana..

[jwilliams]

Hello,

Link suspensions can go all wrong easily because of the leverage changes possible.

This is true as my example shows. The ZX6R was too long (about 2.5" too long) but the damping was close. The R1 was about the right length once you change the clevis but the damping is way off all because of the link setup. Also if you aren't careful you can go from a anti-sqau to pro-sqat characteristics as well, not that the squat characteristics matter huge on a Kat unlike a 150hp+ light bike.

I don't think the same is true for forks.

The leverage issue isn't really there but the damping characteristics vs spring needed to support the bike is still problem. If the orig spring was (for example) .6kg/mm but your need a 1.1kg/mm to get correct sag without excessive preload (that's the spring in my Kat right now - I was over 260lb in leathers, now 225 in leathers) then you have a significant difference in how the rebound will happen. The spring will want to rebound quick due to the 'stffness' but the rebound circuit will calm that down, but if the rebound circuit is overloaded by the spring you get your harsh ride (or one version of it).

Getting new springs and valves is fine but also keep in mind the diminishing returns of tuning the nuts off a Katana..

I agree with this. I have the emulators and stiffer spring in the current forks but with a brake and fork rebuild needed I figured I would play with upgrading both with a new front end. Then maybe do something else on the rear as well to get the better damping etc.

Thanks for the comments, Jeff