USD fork conversion advice

[FJ-Heretic]

There seems to be several options for upside down fork conversions (YZF, GSXR, ZX14). Which seems to be the least
troublesome to do? Are there certain ones that are a real migraine? Do all conversions require pressing
out the pivot tubes, modifying the FJ tube to fit the grafted forks triple clamps and reassemble?
Which forks of a particular front end assembly use the same diameter axle as the FJ1200 ('86)?
Do all of these conversions require using extensions on top of the fork tubes? Thanks for your info.

[Pat Conlon]

Barry Edwards has a great site you should review: http://www.fjmods.co.uk/

In a nut shell...what I've learned is that the '91-93 FZR1000 USD front end is plug and play.
The YZF750 USD has better forks with external rebound and compression adjustment, but needs the FJ steering stem welded and pressed in the lower YZF triple clamp.
No extension tubes needed on the FZR or YZF forks, they are long enough.
Both the YZF750 and FZR front ends use a mechanical speedo drive which is compatable the the FJ's mechanical speedometer.

The GSXR USD's do need extension tubes and you will lose the mechanical FJ speedometer.

These are entire front end conversions meaning forks, triple clamps, wheel, fender and axle. If you are going the USD route, use the entire front end and don't bother with the skinny FJ 15mm axle or FJ 3" wide front wheel.

Cheers Pat

[FJ-Heretic]

Hey Pat, thanks for the info. Have there seen any others like R1, RC51, Ducati etc. or is it easiest to
convert using Japanese forks? So, let me see if I'm thinking right. Whatever forks I choose, I need to
match the FJ steering stem diameter to the lower triple clamp so that I can use the factory style tapered
roller bearings? Is that correct?

[Pat Conlon]

Yes, now that you mentioned it, there is an R-1 front end conversion I forgot to list (sorry Tim) http://www.fjowners.com/index.php?topic=3148.0

The steering stem needs to be matched in diameter and length to the FJ's neck, thus it's easiest just to use the FJ steering stem and modify it to fit into the donor lower triple. (excepting the FZR stem which is a direct match to the FJ)

Clear as mud?

IOW, most (if not all) of the modern USD steering stems are larger in diameter (and a different length) and thus won't fit the FJ's steering neck, so the FJ's steering stem is utilized. It's simply pressed out of the FJ lower triple, modified, and pressed into the donor lower triple.
....and yes, with the FJ steering stem, you retain the use of the FJ's oem tapered neck bearings.

If you wanted to use the USD steering stem (and not the FJ's) you would need to do some significant surgery to the FJ frame to weld in a new neck.

BTW, what's your name? We are on a first name basis here....

[fj1289]

I'll add another option - but it takes a little work but it does retain the mechanical speedo drive.

Early R1 forks and triples with a ThunderAce front rim.  Need to address the FJ stem which is slightly smaller diameter than the R1 stem.  If you do a tube handle bar conversion, you don't need extensions.  The lower left fork leg has a large lug cast into it (like a built in soccer).   The lug is large enough to machine most of it off to make room for the speedo drive, and the ThunderAce rim accepts the speedo drive - but retains the R1 style brake rotors.  None of it is bolt in, but it s not extensive.  Also, no "recipe" measurements, but it is not difficult to figure out. 

This is the setup I've run on the street bike and the race bike for several years. 

[FJ-Heretic]

Thanks for the clarification. Since I have CAD and my next door neighbor has A CNC machine shop I will
probably draw and have a new steering stem turned on a lathe. I think when you put that much heat in the steering
stem (with several passes of welding) you alter the structure of the steel stem. It will probably be cheaper too.

[aviationfred]

I think when you put that much heat in the steering
stem (with several passes of welding) you alter the structure of the steel stem. 

Here is my write up on the GSX-R1000 USD fork conversion. http://www.fjowners.com/index.php?topic=16432.0

IMO, My concerns of altering the structural integrity of the steering stem shaft after welding and turning to fit into the Suzuki lower triple is not very high. Almost all of the load that the steering stem is carrying is in the near vertical plane....We are talking 8 inches or so of steel. I am not concerned at all.

Fred

[brentjgordon]

hey thanks now i know what to do with that pesky 89 fzr1000 sell it! seriously though this sounds like a good way to go but still begs the damper vs cartridge question. from everything ive heard on this forum the cartridge is the way to go my question as one that never used them are emulators good enough to warrant the older tech?

[Pat Conlon]

The best I've found on conventional FJ forks are the RPM valves.
I've had Racetech Cartridge emulators and they are much much better than the oem damper rod forks. No question. The Racetech emulators allow you to use a much heavier fork spring, in the range of .95-1.05 kg/mm spring.
IMHO The RPM valves are a cut above the Racetech valves.
But remember, you still are dealing with the idiosyncrasies and limitations of a conventional fork design. Small 15mm dia. axle and 41mm stanchion tubes on a 550 lb bike with no provisions for rebound or compression adjustment.

For the majority, the RPM valves, .85kg/mm springs and a fork brace are just fine, and truth be told, is a very nice setup.

Although it's always good to remember, the USD forks gets the chicks.

[jvb_ca]

Yes, good advise from Pat.
I too went the route from stock setup (sucked) to racetech cartridge emulators with 1.0 straight rate springs (big improvement)
To Randy's RPM fork valves with recommended springs and fork oil. Actually he sells them all as a package (plug and play). Another huge improvement in handling. I can't compare to the uber sexy USDs (that the chicks really dig) but the RPM system paired with a quality rear shock and fork brace, 3.5" wide front wheel (89 fzr1000) and 5" wide rear wheel (yzf600r) with modern radials definitely brings the FJ into modern day handling.
HTH's... Cheers Jake.

[Russfjr1]

There seems to be several options for upside down fork conversions (YZF, GSXR, ZX14). Which seems to be the least
troublesome to do? Are there certain ones that are a real migraine? Do all conversions require pressing
out the pivot tubes, modifying the FJ tube to fit the grafted forks triple clamps and reassemble?
Which forks of a particular front end assembly use the same diameter axle as the FJ1200 ('86)?
Do all of these conversions require using extensions on top of the fork tubes? Thanks for your info.

Take a look at my project write up   'FJ1200 with a bit of Exup ru and a bit of R1 big bang'  The exup 1000 almost falls in and with good brakes.   Russ