Fork tube Diameters list


"Just tryin' to make a living"
Just joined. Rider for 49 yrs.Retired, doing a build for one of my kids. This site, in particular the fork tube diameter list is outfuckingstanding!. Thank you soooo much. Not even factory tech manuals have this info. I'm in for this site, and will contribute my expertise when I can.


Been Around the Block
I think it's worth adding that the '96+ ST1100-ABS "Pan-European" was a 43mm - if only because it's the biggest diameter of TRAC ("Torque Reactive Anti-dive Control") forks that they ever put on a street-bike. It SEEMS like this model might not have wound up on North American shores, but rather in Europe where the Sport-Touring thing was more of a ... thing. The later ST1300 was also a 43mm, but no TRAC anti-dive.

And it's the TRAC anti-dive which I'm interested in here. The only "problem" being - or rather, MY only hang-up, is that they only put the TRAC mechanism on the one side.

As your table shows, the other ST1100 models had the 41mm tubes. (Some of which had an extra thick wall and a narrower bore for a smaller spring coil, from the POLICE models. Presumably so they'd be more stable if you pushed the thing harder - IMHO that sounds like a potential upgrade for a 41mm dual-TRAC fork from GL1500, which is the go-to fork for an authentic "Freddie Spencer Replica" DOHC-4 Honda, CB750F/CB900F/CB1100F etc.)

You can tell them apart by the 41mm version having the TRAC on the left leg and the 43mm TRAC being on the right.

Those 43mm TRAC legs look somewhat "ambidextrous" though. If I can find a pair without too prohibitive of a shipping bill, I'd really like to throw together a double-sided TRAC fork using two right-hand-side legs. You'd either have to whip up an off-side caliper hanger to match the good one, IF you were to keep the original calipers and discs (which are 316mmm, from what I gather - all of the other ST1100 discs being 296mm) Or perhaps it might be better to use some alternate calipers.

'Cause the GOAL here would be to whip up a replica of the NS500/NSR500 front end, for use on a DOHC-4 or VF-series/VFR-based RACE REPLICA. On the V-four series 'cause they'd be a decent mimic of the real racers, or on the DOHC-4 because they're the thickest TRAC forks and the AHRMA "Forgotten Era" allows for 43mm as the biggest fork allowable, but most DOHC-4 racers (Well, those which DO share my fetish for the TRAC & COMSTAR wheels & other cool '80s-Honda tech!) are stuck using the 41mm GL1500 front end - One might just as well pull off the same Spencer-Replica STYLE of TRAC-fork, it's just that HERE you'd have that max-diameter 43mm - and so many "Spencer Replica" style bikes jump straight to the CBR/VFR stuff 'cause it's 43mm, and they lose a whole huge chunk of authenticity, pick up this whole '90s CROTCH-ROCKET (slash "Starbucks Racer") vibe, whereas a full-on TRAC fork gives it that much more '80s Authenticity. (If you're a huge fan of '80s Honda stuff, you'd know where I'm coming from.) I'd be curious about the weight, but they're a late-era model and the 43mm probably weighs the same as the earlier 41mm anyhow.

Of course I'm not suggesting the whole ST1100-ABS front end, with the Gold-Wing style yokes etc, but rather some other 43mm yokes from this or that CBR/VFR model.

Plenty of options to choose from there - which reminds me, there should probably be a category on this list for 43mm forks being that it's such a popular thing for the "STARBUCKS RACER" crowd, as I've dubbed 'em, to stick the later model CROTCH ROCKET shite all over the hacked-down torso of their once beautiful classic machines.....) Those NS/NSR racers had some interesting calipers on 'em. They were of course a true opposed FOUR-pot caliper

(As were the grabbers on those original AHM "CB750F" racers, Spencer's & Wayne Gardner's AMA Daytona champion Superbike, & their brethren - they LOOKED like the ordinary sliding calipers from CB1100F etc but they were a bolt-up assembly with an external hard-line connecting hydraulics from one side to the other, rather than an O-ring seal as with Grimeca/Lockheed/AP-Racing "Scarab" calipers, or Brembo calipers from that era - the safest easiest race-replica brake set-up for a DOHC-4 would have to be the '76 RCB 750, 'cause they used off-the-shelf BREMBO calipers, nothing fancy. While the late RCB & RS1000 were some truly rare & unusual works-spec bits, of which there are fantastic pics on FACEBOOK of all places, from the Caillou-Bedier RCB restoration! I wonder though - those '82 Honda Calipers, were they the first ever FOUR-pot calipers on a motorcycle? And if not, then what other bikes might have preceded 'em? I'm curious about the precedents, just to know what's "Period-Correct" to this or that era. See, I'm all for using the later-era parts, but I'd just like to be able to fit them in a way that they fly under the radar as authentic, period-correct originals - via re-finishing, retro style hardware, rounding down all of the sharp corners etc.)

But yeah - those NS500/NSR500 four-pot calipers also had the outer faces cast with a (+)(+) motif over the piston bores, just like the early VFR750F calipers. One might well use the VFR grabbers, if it weren't such a pain in the ass to make a custom caliper hanger for a sliding, single-sided caliper - AND that this caliper-hanger should be a swinging TRAC-type hanger as well. One or the other MIGHT be feasible - but both? Sheesh.

Furthermore, on those works-spec calipers from NS500 & early NSR500, they also had mis-matched bores. For a staggered pressure on the back-side of the pads, to create something like the "Twin Leading Shoe" action on a drum brake. Only in the instance of the huge over-sized disc brakes they're set up as a twin TRAILING action. This is so they avoid a "self energizing" principle, which smooths out the brake function rendering the upper range of braking force more USEFUL 'cause it isn't gonna lock up and get away from you.

I gather this was the big complaint you'd hear from users of the biggest 4LS drum brakes back in the day. And if you look at some of the pics of the late-mid '60s Honda racers such as RC-181, you can clearly see drums which are set up with either 2xSLS "Doppel-Simplex" or even Twin-TRAILING cams and links. It's kind of counter-intuitive, when you think about how much goes into making the brakes BIGGER, giving 'em more "OOMPH" as it were. But it's all about the CONTROL of those brakes.

Well I guess there are plenty of calipers available NOW, for the Y2K+ machinery. Four-pot opposed-piston calipers of the modern variety, but which ALSO have the NS500/NSR500-style mis-matched piston bores, and the uneven pressure they impart to the back side of the caliper pads.

So one COULD conceivably throw together some decent NS500/NSR500 style calipers. The only question being just how accurate of a copy. Hekc, maybe you could even get away with carving the "(+)" pattern into the outer face of each piston bore. Or maybe you'd be happy just using some of the matched-bore sliding calipers from the first-gen VFR750F Interceptors? Looking at the works-spec brakes from the '82 Spencer/Gardner bike, it might even be possible to slice the sliding-pin type calipers in half and bodge 'em together into an opposed-piston style, with an external hard-line just like the original. A ridiculously difficult project, to be sure - but if you were shooting for an authentic race-replica build and you don't want to wait for another $20,000 original works fork to come up on the market - for probably double that figure next time - It might be well worth the trouble AND the expense? I mean, people are ALREADY paying through the nose, or working their nose to the grind-stone of their Bridgestone Mill, for that '82 41mm Billet yoke to pair up with GL1500 legs.

If you look at the NS/NSR the yokes are much more conventional. So, being that there's already a decent 43mm CBR/VFR yoke which could be sanded & re-finished as a decent replica NS500/NSR500 triple-tree, you're already ahead on THAT aspect. I figure if there's enough material on the outer faces of some modern 4-pot calipers, with the two big pots and two small pots, to carve in a light "(+)(+)" motif, without gouging into the cavity and leaking hydraulic fluid all over the place, then that's just about the simplest way to do it. And the caliper hangers would be sooooo much easier without sliding pins and the seals which keep 'em greased - Unless of course you could somehow incorporate the original sliding-pin calipers INTO the new hangers? At which point you'd be talking about not only machine-shop time, but WELDING - I guess it all depends on whether you're throwing a DYMAG type rim up front, or a COMSTAR wheel - 'Cause the Comstar wheels are much like a wire-spoke wheel, in that their spokes can be too wide for an opposed-piston caliper to CLEAR on the inside of the disc. There are modern calipers of the "flat-back" type, which have shorter pistons and a thinner body on the inner side. But they're hideous chachi crotch-rocket billet shit. I wouldn't want that stuff anywhere NEAR a classic '80s Honda Superbike! Of course, one more way to clear the inner caliper faces would be to use a larger rotor and a smaller wheel. Which is what you'd want on this style of build anyhow. Like a 16" rim with 296mm rotors minimum, more like 310mm/316mm/320mm/330mm IF you could squeeze 'em in there. And heck, with a really big rotor in a tight rim like that, there might be no other way to put it all together, but to remove the calipers at the same time as the wheel - Some of those old racers had a QUICK-CHANGE HUB, where the discs and calipers stay assembled on the axle stubs, while the wheel pops straight out. Wouldn't want to mess with building a replica of THAT type of shit. At least, not a FUNCTIONAL replica anyhow. Might be cool to have a Comstar hub with a raised lip shaped in a lopsided pentagon, just to give it that whole RS1000 vibe, but that's about as far as it goes..... Well, at least in the context of my CURRENT project, it's good to know that whatever worked with a Comstar SHOULD at least work with a wire-spoke wheel.

IMHO, the NSR500 replica front end might be the more reasonable option. Heck I'd take a stab at it right now, if I could find a decent machinist to whip up some caliper hangers for it. The regular "half-Track" ST1100-ABS 43mm fork would probably be the better option, but for now I've found a good price on a GL1500 front end, just gotta figure out a decent set of yokes for it. Ain't gonna be able to afford the billet replica version, that's for sure. Or rather, I guess it's more like "I'm not that into it". 'Cause the same $$$'s to pull off the NS500/NSR500 front end, I'd definitely find an even cheaper brand of ramen noodles, maybe eat 'em for an extra ten years, to pull THAT one off.

Just picture it with the soft powdery looking paint to emulate the original Magnesium-passivate finish, and those huge orange "SHOWA" stickers running down each leg. The right type of ROTORS would be the truly difficult part - they didn't use buttons, but rather some type of flat-sided sliding tabs, with riveted sheet retaining 'em in place. I've seen it done on the VF-orums, with either an NS400R or VF1000R based replica, I don't recall which. They rebuilt some VF1000R front and NS400R rear hubs in 3.50x17" front (from the REAR wheel off the VF1000R Comstar pair) and a 4.0x18" or 4.5x18" rear, which was sourced from some ridiculous Unobtainium refinery somewhere. I've been digging up my OWN sources of odd-ball Comstar rim sizes though, with the ideal being the Akront "NERVI" type, in either NOS stocks or better still if MORAD could bring their old tooling out of mothballs - But there's also some potential with using chopped-up "BILLET" wheels, machined to the same specs. Of course, the "NERVI" type is probably a hell of a lot lighter, and well proven on everything from the Bimota Tesi 1D, to Ducati F1 Montjuich/SantaMonica/LagunaSeca, TONS of awesome bikes they were used on. Guess it depends on where those BILLET wheels come from, and how much they emphasize cheesy ORANGE-COUNTY-CHOPPER vibes, versus weight & performance. It's a big "IF" but then again, some obviously huge rewards - not just the SOHC CB750F2 & RCB/RS1000 replicas, the CB1100R, the CBX six, and Gold-Wing heck even the CX, but ALSO the later VF-series applications whether they all be practical street-bike applications OR race-replica stuff.

But whatever - it all starts with the FORK, with the FORK! Probably doesn't mean shit to the rest of you. Ha-ha. But if you're into the '80s Superbike thing, maybe you'll get where I'm coming from. It's kinda like another "Retro-Futurism" thing, where the "Inboard Disc" and TRAC forks etc, are analogous to the FINS on autos of the 1950s.....


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