More modern cartridge fork options - retaining GS wheels & proper/revised geometry

[John Kat]

So if you don't make it through that ranting and rambling, here's conventional RSU cartridge fork breakdown:

Ultimate swap for slightly lower front, best dampening, fully adjustable rebound&compression, opposed piston 4/6 pots:
*CBR600F4/F4i 43mm forks 755mm height and modded VMX12 93-05 50mm offset or custom billet triples


Excellent dampening and stock ride height, 310mm brakes and 4/6 pots, 775mm length:
*Bandit 1200 43mm forks (no external dampening adjust, need longer shocks and slight drop in front height to get respectable stable trail with 34mm offset GSXR 89-90 triples or other similar triples as discussed by other members - Posplayr etc - see their thread for gen 1 vs gen 2 b12 fork details, different triple width I believe)(custom billet triples will make this a GREAT geometry upgrade/converson)(Suzuki models seem to run the calipers inwards more, need triples spaced 2.5+mm wider than Honda options if brake/spoke clearance issues)


41mm cartridge fork with external rebound and preload adjustment, near stock ride height, 4/6 pot brakes:
*Honda VTR1000F SuperHawk (Firestorm non-USA) 775mm tall forks, PERFECT with the scarce GS1100GK triples, may be able to adapt the stem on VTR-width late model 90's/00's CB750 Nighthawk triples, slightly clearance calipers inboard side for extra assurance), same possibility on 1984 Honda VF1000F Interceptor triples but they are 190 spacing, must adapt to CBR600F2/F3/VFR750 non-abs calipers - great swap if you go to that length) (dampening could use some mods on these or Racetech gold valve upgrade, same as most of the other forks here really to get dampening perfection) $325-$700 custom billet triples will solve all research/modding hassles.
Honda PC800 triples are in the mail, I think these will be very similar to the GS1100GK triples, 55mm offset 204mm width was my Photoshop guesstimation. Great trail number with a tall 110/90-18 & minimal rear ride height increase, could be the biggest winner for geometry & cheap availability running the VTR1000F Superhawk forks & 599 Hornet spec rotors (or non-USA CB400SF Super Four, all have greater 23mm offset in 296mm diameter).


41mm cartridge forks near stock ride height, much easier brake to wheel clearance with sliding 2 piston calipers:
*CBR600F2 41mm forks
*CBR600F3 41mm forks
*VFR750 41mm forks non-ABS '94-'97
same triple options as the VTR1000F Superhawk although the 1984 VF1000F Interceptor triple, IF the stem can be adapted, would be a great candidate, as well as the newer retro CB750 Nighthawk triples IF stem height can be adapted. $325-$700 custom billet triples will solve all research/modding hassles


43mm cartridge forks external rebound/preload, A++ factory dampening, shorter than stock @727mm for GS550/650/400-425-450-500/GR650:
*94-97 RF900R 43mm forks in custom triples, 727mm tall - shortest you can run on a GS without offset triples.


43mm cartridge forks, fully adjustable rebound/compression/preload, shorter than stock longer than RF's @ 745mm:
*89-90 GSXR1100K forks and triples, 1"+ taller rear shocks as typical to help reduce rake and trail to an appropriate improved geometry
(These are a substantial upgrade to any GS, but Racetech says that the dampening is rather crude for a cartridge fork, but they can custom modify their parts to vastly improve the dampening for a reasonable labor charge)



So far that covers most of the models that I am aware of, with emphasis on RSU (conventional appearance) forks closer to 775mm height as to retain ground clearance.


I have also read multiple mentions that the Bandit 1200 and RF900 forks, as well as the CBR600F4/F4i forks are really just about the ultimate RSU fork swap because their stock factory dampening is superior to most counterparts & has a very high level of tunability, and they have 4 pot/6 pot caliper compatibility with mounting configurations used on many highly regarded models for lots of caliper swap options. The Suzuki and Honda run similar calipers in Nissin and Tokico, but the Honda's (vtr1000, f4, etc) run a closer 62mm mount spacing with the lower mount sticking much further out behind the fork legs, Bandit GSF1100 & RF900R run 100mm caliper mounting with both mounts same distance right off the fork lower.
The RF900R / GSF1200 forks run the calipers more inboard on the forks, where the Honda F4/VTR1000F run the calipers more outboard with the rotors nearly touching the forks. So on the same width triples, the RF900R/GSF1200 forks will put the calipers several millimeters closer to the wheel spokes, where the Honda brake mounts put the calipers further away from the wheel spokes.

In the RF900R /GSF1200 Bandit fork brake options, the aftermarket $$$ Shindy Nissin 6 pot calipers give 3mm additional brake clearance @ 80mm width vs 86mm on the Tokico OEM Hayabusa 6 pots (& the VTR1000F/CBR600F4/F4i brakes & the 6 pot GSXR1000K2 01-02) calipers.







I have also read multiple mentions that the Bandit 1200 and RF900 forks, as well as the CBR600F4/F4i forks are really just about the ultimate RSU fork swap because their stock factory dampening is superior to most counterparts, and they have 4 pot/6 pot caliper compatibility with mounting configurations used on many highly regarded models for lots of caliper swap options. The Suzuki and Honda run similar calipers in Nissin and Tokico, but the Honda's run a closer mounting spacing with the lower mount sticking much further out behind the fork leg.

I was also checking out some ZRX1100 & ZRX1100 RSU forks which were a great height (765mm? Have to look up my notes), have adjustable preload, rebound,&compression, & run the same 6 pot Tokico's as the first 10 years or so of Hayabusa, & for 310mm rotors (98-99 CBR900RR rotors are a shoe-in for bolting to GS hubs!!!). These were VERY NICE RSU fork swap candidates!
Yamaha R6 RSU forks were the lightest I've had my hands on, 4lbs lighter than the cbr600f4i w/vmax 1200 43mm triples w/steel stem, 6 lbs lighter than the zrx1100 forks with a very long steel stem. R6 had alloy stem like the gsxr/Busa stem I'm using. I've also seen a crashed R6 fork split in two almost it creased so hard, the upper fork tubes are MUCH thinner than my GS750's... The R6 runs an excessive 320mm rotor, no easy bolt ons for a GS 6 bolt 78mm pcd hub. 296mm is plenty, 310mm duals is ALOTTA BRAKE, 320mm probably needs ABS to not tuck the front!

I confirm that the Bandit 1200 phase 1 forks offer a far better damping than the fully adjustable GSXR 1100 K ( 1989) forks.
Beware that the disk spacing is slightly different between the GSXR 1100 K and the Bandit 1200!
On top as the Bandit forks are much longer they will fit on the 1100 Kat's.
Here with GSXR 1100 triples and clip-ons:

 

[Chuck78]

I confirm that the Bandit 1200 phase 1 forks offer a far better damping...Bandit forks are much longer they will fit on the 1100 Kat's. Here with GSXR 1100 triples and clip-ons:

FYI, my good buddies here in Columbus at http://www.RicePaddy.com are blowing the dust and mothballs off of ^^this same model GS1100 Katana (same color even)^^ you have here that has been stored for ages. 500 original miles, owner bought it only to race, 1 owner. Has a Falicon crank supposedly, and other racing goodies - including a silver painted disc over top of the front wheel spokes with racing logos and lettering painted on it. Carbs and brakes in pieces in a box, but they are likely going to resurrect it from it's slumber and get it in in ridable shape and ready to turn over to a new owner as a running project bike, if anyone is looking for one, call them on the phone (closed Wed&Sat) and ask for Grant.

 

[Chuck78]

GS750+ w/VTR1000F forks in PC800 triples - revised geometry

GS750+ w/VTR1000F forks in PC800 triples - revised geometry

Alrighty well I did some rough approximations of geometries based on crude manipulation of the RB Racing online rake and trail calculator (it doesn't allow any input of rear ride height changes and resulting rake and trail changes, only fork length and front tire size changes - I am assuming raising the rear or dropping the front equal amounts will give you the same rake and trail changes, which may not be true from what I've read).

So far in my rough estimations, using this as the baseline:
*Stock 77-79 GS750 - 58.7" wheelbase 27 degree rake 4.21" trail 4.00H18 26.4" diameter rear tire 3.25H19 26.1" diameter front tire

*swapping to modern tire sizes of 110/90-18 (26.1" diam per Avon spec) and 90/90-19 (25.5" diam per Avon spec) on stock 2.15/1.85 rims gets you 26.87 degrees rake and 4.03" trail using the same ride height front end and the almost exact same offset in the 55mm offset PC800 triples and Superhawk VTR1000F forks (a good increase in steering response already! But at 4.03" trail or whatever that gets you, the steering is still a tad bit slow but very stable as we have witnessed in real world GS road testing).

*take the above modern tire size swap, 41mm VTR1000F cartridge forks in 55mm offset Honda PC800 triples, and add 20mm to your rear shock length in a nicely sprung/dampened shock upgrade = approximately 26.2 degrees rake and 3.87" trail (A NICE GEOMETRY UPGRADE!)

*take the above and make that about 25mm longer than stock rear shocks and 4mm drop in your fork ride height on the triples = approximately 26 degrees rake and 3.81" trail (A VERY NICE SPORT GEOMETRY UPGRADE!)

*take the above and make that about 30mm longer than stock rear shocks and 4mm drop in your fork ride height on the triples = approximately 25.75 degrees rake and 3.76" trail (A VERY EXCELLENT SPORT/RACE GEOMETRY UPGRADE!)

 

[Chuck78]

Yamaha R6 forks + EBC MD729LD/RD CBR600/900 oversized race rotors 320mm

Yamaha R6 forks + EBC MD729LD/RD CBR600/900 oversized race rotors 320mm

well... now after spotting and looking up the specs of Ducati 748R/998R 6 bolt rotors that I spotted (320mm 15mm offset 6 bolt 80mm pcd, 2mm larger diameter than what we need, could redrill new pattern in between for GS hubs).... I noticed a 78mm PCD 320mm rotor, turns out EBC makes oversized rotors for CBR's at 320mm with 17.5mm offset! They are quite spendy, but this makes the Yamaha RS RSU fork - fully adjustable compression/rebound, lightest fork I have had in my hands at 3.5lbs less than the CBR600F4i, a good possibility for those looking to get the lightest and most ultimate setup out of their bike upgrade... the set I had in hand looked to be 43mm. I did not measure the width of the triples or the width between the brake mounts to see how they'd work in VMAX 1200 triples, but they were somewhere in the 755mm length vicinity, close to the CBR600F4 from memory.

03-04 Yamaha R6, 06-07 R6S Forks is what the pair in this ad fits, $600 fully serviced with Ohlins cartridges added:
https://www.ridemerchant.com/items/...orks-ohlins-cartridges-straight-just-serviced

EBC Race Rotors (May need to have them make you a set with steel floating rivets instead of aluminum if using on the street)
MD729LD left MD729RD right 21.1mm offset 78mm pcd 6 bolt:

 

[John Kat]

well... now after spotting nad looking up the specs of Ducati 748R/998R 6 bolt rotors that I spotted (320mm 15mm offset 6 bolt 80mm pcd, 2mm larger diameter than what we need, could redrill new pattern in between for GS hubs).... I noticed a 78mm PCD 320mm rotor, turns out EBC makes oversized rotors for CBR's at 320mm with 17.5mm offset! They are quite spendy, but this makes the Yamaha RS RSU fork - fully adjustable compression/rebound, lightest fork I have had in my hands at 3.5lbs less than the CBR600F4i, a good possibility for those looking to get the lightest and most ultimate setup out of their bike upgrade... the set I had in hand looked to be 43mm. I did not measure the width of the triples or the width between the brake mounts to see how they'd work in VMAX 1200 triples, but they were somewhere in the 755mm length vicinity, close to the CBR600F4 from memory.

03-04 Yamaha R6, 06-07 R6S Forks is what the pair in this ad fits, $600 fully serviced with Ohlins cartridges added:
https://www.ridemerchant.com/items/...orks-ohlins-cartridges-straight-just-serviced

EBC Race Rotors (May need to have them make you a set with steel floating rivets instead of aluminum if using on the street)
MD729LD left MD729RD right 21.1mm offset 78mm pcd 6 bolt:

A quick comparaison of fork lengths can de done by looking at the dimensions of the bare fork tubes.
The Tarozzi site provides this kind of information:
http://www.tarozzipaolo.com/inglese/home.htm
Among the fork tubes you have mentionned with a 43 mm outside diameter the lengths are:
R6 ( 01 to 04) 585 mm
GSXR 1100 K ( 1989) 588 mm
Bandit 1200 ( up to 2002) 614 mm
CBR 600 F1 to F4 587 mm
While the GS 1000 in 37 mm OD are 595 mm
I'm personally keen to try eitheir the CBR or R6 fork tubes in place of my current GSXR fitting but I wonder if I will be able to adapt them to my GSXR 1100 first or 2nd gen wheels?
See page 3: http://www.thegsresources.com/_foru...rk-upgrade-for-17-18-and-19-quot-wheels/page3
In any case we will have to buy stronger springs to cater for the extra weight of a GS 1000/1100.
As a side point I don't understand why you recommend Vmax triples as they won't take 43 tubes?

 

[Chuck78]

As a side point I don't understand why you recommend Vmax triples as they won't take 43 tubes?

93-05 vmax 1200 are 43mm fork tubes, up to 92 are 40mm I believe. I have a set of 93-05 vmx12 triples with rf900r forks mounted in them.

Cbr600f2-F3 are 41mm, you quoted tarozzi saying they are 43mm. The last year of the F2 are cartridge forks, all f3+ are cartridge. Early F2 & all F1 are damper rod.

 

[John Kat]

93-05 vmax 1200 are 43mm fork tubes, up to 92 are 40mm I believe. I have a set of 93-05 vmx12 triples with rf900r forks mounted in them.

Cbr600f2-F3 are 41mm, you quoted tarozzi saying they are 43mm. The last year of the F2 are cartridge forks, all f3+ are cartridge. Early F2 & all F1 are damper rod.

Good to know!
Tomorrow I'm going to look at an R6 fork that's for sale...

 

[Chuck78]

I believe the R6 conventional right side up fork was 43mm but I didn't put my calipers on it. Very nice fork indeed, but 320mm rotors makes GS wheels require greater lengths to mount. Adapters on the 15mm offset Ducati rotors could be made and could even just re-drill the bolt pattern possibly, it's 80mm circle, shift it between the holes and drill ours 78mm circle. Adapter would just be a top hat spacer to locate it better on the bore than bolt-centric, & spaced out approximately 16.25 mm off hub approximately.

I did not measure the distance between the R6 caliper mounts, so I am not sure the clearance to wire spokes. With mag wheels I think you'd be good in vmx12 triples. Youll need to slightly mod steering stem to drop upper bearing surface several millimeters, or machine a gsxr/Busa aluminum stem to dimensions of vmx12 triple, use m14x1.0 top bolt instead of busa/gixxer alloy nut, remove retaining roll pin & press out vmx12 stem press in shorter aluminum busa/gixxer stem turned in lathe to dimensions of vmx12 stem diameters.

Of course, if you go your typical route, maybe you'd just get a fancy adapter made to run stock GS calipers? Going to these lengths, I'd definitely look into swapping to the twinpot calipers or just using the R6 and the Ducati rotors or custom ordering those EBC Racelite rotors but with steel floating rivet buttons not aluminum as they come. Don't get too spongy on master cylinder (bore to piston ratios) as you'll end up with immensely powerful brakes that you could lock up too easily in a panic scenario @ 320mm !

 

[Chuck78]

By the way the vmx12 stem is slightly too long but can be even hand filed down to allow the bearing to run slightly lower. Swapping the Busa/gixxer aluminum stem will allow you greater ride height as you will now be able to run the upper triple lower, & correspondingly the forks can slide down lower for closer to stock ride height.

Matt slikwilli420 on DoTheTon.com is making me custom upper & lower offset triples for my Franken-Rickman/GS project, he could mill a new VMX12 spec upper triple from 2" billet with a 20mm-25mm downward offset on the fork tubes so that you could run the fork tubes even lower up front for near stock ride height. 1" billet is $175/ea, 2" billet would probably drive cost upwards of $225 per triple but allows use of those really nice and light fully adjustable forks with a near stock ride height.

Weiss Racing also makes semi-custom billet triples and billet stems for $325, up to 43mm.

 

[Chuck78]

*running the 15mm offset Ducati rotors may cause issues with speedo drive, may need to machine an adapter to space it out further if cable connection interferes with rotor/rotor bolts being too close

 

[John Kat]

I believe the R6 conventional right side up fork was 43mm but I didn't put my calipers on it. Very nice fork indeed, but 320mm rotors makes GS wheels require greater lengths to mount. Adapters on the 15mm offset Ducati rotors could be made and could even just re-drill the bolt pattern possibly, it's 80mm circle, shift it between the holes and drill ours 78mm circle. Adapter would just be a top hat spacer to locate it better on the bore than bolt-centric, & spaced out approximately 16.25 mm off hub approximately.

I did not measure the distance between the R6 caliper mounts, so I am not sure the clearance to wire spokes. With mag wheels I think you'd be good in vmx12 triples. Youll need to slightly mod steering stem to drop upper bearing surface several millimeters, or machine a gsxr/Busa aluminum stem to dimensions of vmx12 triple, use m14x1.0 top bolt instead of busa/gixxer alloy nut, remove retaining roll pin & press out vmx12 stem press in shorter aluminum busa/gixxer stem turned in lathe to dimensions of vmx12 stem diameters.

Of course, if you go your typical route, maybe you'd just get a fancy adapter made to run stock GS calipers? Going to these lengths, I'd definitely look into swapping to the twinpot calipers or just using the R6 and the Ducati rotors or custom ordering those EBC Racelite rotors but with steel floating rivet buttons not aluminum as they come. Don't get too spongy on master cylinder (bore to piston ratios) as you'll end up with immensely powerful brakes that you could lock up too easily in a panic scenario @ 320mm !

I need to absorb all this information but the plan is to use my GSXR 1100 K triples and just slide the R6 stanchions in them.
As for the front wheel I would be using the 18" GSXR 1100 H 3 spoke model.
The calipers are currently the GSXR 1100 2nd gen calipers.
This project is described here: http://www.thegsresources.com/_foru...ngine-in-GSXR-1100-first-gen-frame&highlight=

 

[GSYOSHI]

I am looking into a local front end from a 2007 SV1000. Comes with brakes in good shape, he's asking 250(Can). Just waiting to hear back from the seller about length and offset. Found a length on a forum of 735mm. I am hoping to put Slabside GSXR 18" wheels on it and install risers for superbike bars. I think they are a pretty basic (not cartridge) fork but 43mm and dual disk. Any thoughts on these for a 78 GS750?

 

[Chuck78]

Why bother with all that work to retrofit if they are not cartridge????

 

[GSYOSHI]

Why bother with all that work to retrofit if they are not cartridge????

Sorry I should have put a question mark at the end of my statement that they weren't cartridge. These are the things I am trying to figure out. Anyway they are fully adjustable cartridge forks as far as I can tell from the internet. I found someone on another site that put SV650 forks on his Katana. Still digging stuff up. Anyway Seller said they are 736MM long and the offset is 36mm. Trying to get more info from the Kat swap.

 

[Chuck78]

36mm offset doesn't seem like anywhere near enough to run a slabside gixxer 18" front wheel, even if you raise the rear drastically, you'll still have a large amount of trail. Trail is what makes the steering stable, but too much and it will feel slow like a cruiser hardly davidson, not sporty and fun. Not much point to a swap of a sporty high tech front end to give it even more trail than stock, unless you don't ride it aggressively or have any twisty fun roads around. Stock forks with emulators, sonic springs, and 20mm-25mm longer upgraded $300-$450 rear shocks will make a nice sporty geometry and upgraded damping.
The SV1000 front on 18's will have much more sophisticated damping and better roadholding, but is not going to steer terribly well (slow).
1" taller rear ride height and a lower profile front like a 100/80-18 combined with a taller 140/80-18 rear or 150/70-18 (not as tall but taller than stock on most gs's) will get you closer to a sporting geometry. I'd speculate that'd get you back to stock GS trail specs but with the rake steepened by about 1.85 degrees.

Look at the SV1000 rake and trail, and SV1000 front tire outside diameter. Compare that to the GS spec.
Then figure that every 1" change in ride height changes the rake roughly 1 degree (raising the rear +++ combined with dropping front ---- , adding both numbers no negative/positive addition/subtraction)

That will give you an approximation of your trail . You can mock up your bike ride heights by dropping g the forks in the triples and shimming the rear tire up for simulation of taller rear and shorter front (use HALF the diameter of the tire spec as your shim height, the radius), then measure the head tube rake. Smartphone angle finders can be pretty helpful if calibrated.

Then you can use an online rake and trail calculator to plug in proposed front tire diameter, triple offset, and your measured rake simulation figure, and see where you are at with trail. 4" is good, 3.9" is a bit more sporting, 3.8" is great quick sporty steering, 3.75" is about the limit for a very sporty steering geometry before you start to compromise stability. On our old frames, it's bordering on getting twitchy there.

 

[GSYOSHI]

36mm offset doesn't seem like anywhere near enough to run a slabside gixxer 18" front wheel, even if you raise the rear drastically, you'll still have a large amount of trail."

Thanks allot for your reply. I have been following this thread for a while trying to figure out the best way to do this. Also John Kats posts... I have punched the numbers into rake and trail calculators that you have given here and I understand that I want just under 4" of trail to be an improvement. I have seen recommended and have been recommended myself the B12 forks but being less offset and longer than the SV1000's wouldn't they be even worse? I am getting no better than 4.4" trail with the SV's and so it would make sense that they are not what I am looking for but with the bandits I am getting 4.75" trail so if these are considered to be a working swap than either I am doing something wrong or they just are not a good option either. These are rough and dirty calculations of course but I want to get allot closer before I crunch down to fine numbers.

Thanks again. I was following the thread hopping a plug and play option would show it's head. I appreciate the work you have put into this. I am likely to just run what I have this summer and look into it further next winter. The sun is shinning and my bike is in pieces.

 

[Chuck78]

17" front wheel will cure it, or custom triples. I can point you to 2 affordable custom triple options.

 

[Chuck78]

I just got this crazy idea due to WERA vintage racing rules that I could swap GS1000 37mm dual disc fork lowers onto GS500E stanchions to have a GS dual disc fork under the 38mm max. fork diameter regulation for Formula 500 (building a 489cc GS425 to fit in that class).
Then I recalled how some Triumph Thruxton owners swap 41mm cbr600f3 fork stanchions/cartridges/etc into the triumph lowers...
Then I thought I could go as far as to swap these VTR1000F Superhawk stanchions and cartridges into some GS1100GK lowers... This would preserve the GS's 1970's classic appearance, & I could still run my salty_monk custom twinpot brake setup, & I wouldn't have to worry about adapting my GS hub to the big Honda axle (although it would be much stiffer), and no worries on the street about having to run a speedometer drive off of a 17" wheel bike (best solution I found so far was get custom gauge faces printed that have the speed markings corrected)

If this or even the cb600f3 (same as last year of F2) guts would swap into the 41mm GK sliders, that would be fantastic.
Then the only problem one would have is getting replacement bushings for the GK forks, as they are obsolete, & racetech does not list them in their catalog.
Lastly, I could make some custom 41mm triples with 194mm spacing or refer back to my notes to see if a late model nighthawk or VF1000F triple had that same width. Narrowing it would mean I could run the salty brake mod without any hub spacers.

 

[TiiMuch]

Ok. Maybe got quite the treasure trove of restomod parts tonight.
Two 85-87 GSXR 18x3.5 rear wheels and one 85-87 18" front wheel.
And a 1989 1100 GSXR front end. Complete.
Is that angels I hear singing?!.

Ive got a Bandit rear end too.
Whats the smartest/simplest way to upgrade my 83 GS1100E suspension and especially the front brakes without losing the ground clearance for the 4-1 Supertrapp?.
And radial tires.

 

[limeex2]

I used a 95-97 Triumph Sprint 900 front end. It bolted right up with only a upper bearing and a thicker washer on top of the bearing. Harder to find though, but I have one I use for spare parts after the motor went away. 97's are fully adjustable. They use above the triple clip-ons and have 3 different height bars, 4 with a spacer. The project is stalled for now due to summer riding season. I have a Bandit rear end I will be using. With Chucks advice on this thread I am going to figure out how to use 14" ZRX shocks. Then I can swap shocks around with my ZRX as needed, and I have 3 nice sets.