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Fork upgrades for 1970's superbikes that will fit within vintage racing rules?

This is a discussion on Fork upgrades for 1970's superbikes that will fit within vintage racing rules? within the Vintage Motorcycle Racing forums, part of the Caferacer.net Forums category; I just started reading the WERA rulebook because I am gearing up to start focusing on acquiring more parts to do a late fall or ...

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Thread: Fork upgrades for 1970's superbikes that will fit within vintage racing rules?

  1. #11
    Senior Member Chuck78's Avatar
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    I just started reading the WERA rulebook because I am gearing up to start focusing on acquiring more parts to do a late fall or winter buildup on 3 bikes, the Rickman and the GS425 being two of them. As it sits now, the GS425 is bone stock.


    Plans now are:
    GS450 or GS850 sleeves & 72mm pistons - 489cc & probably 10.7:1 compression (haven't had JE, Wiseco, or CP pistons made yet but running same pistons from MTC in my 920cc gs750)
    Megacycle cams - probably the .380" lift profile maybe mix a .354" exhaust cam in with that. Or Web .395" lift intake and .365"(?) Exhaust cam.
    Send head to Rapid Ray's for oversized valve installs at least of GS1000 size or larger. May drop a lot of money on fancy alloy valves to drop weight. Titanium hardware up top of the valves. Might have to go shim under bucket to save weight although I despise adjusting shim under bucket valve clearances...
    Head ported a good bit but still trying to retain some low rpm performance
    I was going to run some custom twin-racked Mikuni RS34 carbs, far easier to make into a custom 2cyl rack than a set of vm29 smoothbores. I may, however, bite the bullet and go straight for Keihin CR31 carbs since they were very much period correct, come in adjustable width twin cylinder racks already, & I don't think RS34 were available pre-'83 - so they would likely not be legal in the formula 500 class
    2 into 1 exhaust
    Kerker System K muffler?
    Modified clutch basket & extra clutch plates

    Chassis:
    Additional frame bracing around spine, headstock, & airbox areas
    GS1100E aluminum swingarm narrowed to fit
    GS wire spoke wheels 2.50x18 front 3.00x18 rear (all 3 sets of my wheels are 3.50x18 rear,have to lace another to fit in the rules, can't do 17" as I planned either, 18" min.)
    Possibly do GS500 37mm forks (38mm max allowed in Formula 500), but the 12"/304.8mm rotor limit stomps out the single disc 310mm rotor plan using stock GS500 parts. Custom bracket mods and custom 12" rotor may be necesary, or more likely just go dual disc & try to combine the lighter/shorter gs500 inner/upper stanchions with GS850/1000/1100G dual disc lowers and gs500 2 piston sliding calipers with modded brackets and dual 296mm rotors
    I may even go as far as to do a custom cartridge front end conversion to these forks from Maxton just to have a fork as good as the 43mm rf900r cartridge fork I was going to run.
    Rear will be vintage Fox Factory Shox. I have 2 sets of piggybacks, 1 set of emulsions. Great vintage shocks.
    Pirelli Sport Demon tires 130 or 140/7-18 rear 110/80-18 front.
    Last edited by Chuck78; 09-06-2016 at 09:26 AM.
    dirkchecken likes this.

    '77 GS750 920cc, 4-1, GS1100E swinger, 18" Sun rims, Fox Shox, twinpot dual disc, CR31, Yosh cams
    '74 Rickman VR250MX
    '99 Kawasaki KDX220 rugged terrain ripper
    PROJECTS:
    '77 Suzuki PE250B "Pure Enduro" trail beast, ported, Wiseco, TüBliss, Fox Shox/RaceTech
    '76 Rickman CR GS1000-1120cc roadracer, Yosh cams&4-1, RF900R fork, Works piggybacks
    '79 GS425 489cc Formula 500 racer, Tempter cams, GS650 forks, twinpot brakes, DID rims, GS1100E swinger, Fox Shox
    '77 GS550's 650-740cc susp&brake mods

  2. #12
    Senior Member kenessex's Avatar
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    Looks like a good plan, except for the tires. I would go with some race compound Avons, Continentals or Heidenau. I think the Pirelli Sport Demons, Bridgestone BT45 and the Dunlop GT501, just don't offer enough grip for hard racing use. I run the Avon AM22 front in a 110-18 and the AM23 rear in the 130/65 vb 18. It is a special race profile 130 designed to work ok on a narrower rim.
    I also think that bike will fit in V4 and clubman and be competitive, you can run 17" wheels in those classes. When I had my Ascot set up for V2 I had an extra set of 17" wheels I could swap ouy for V4 and Clubman. Especially since the vintage classes run on Saturday and Clubman and DSB run on Sunday.
    pwalo likes this.
    WERA 119
    CCS 119
    CRA 119
    AHRMA not anymore
    \"Think twice before you speak, and then you may be able to say something more insulting than if you spoke right out at once.\"
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  3. #13
    Senior Member Chuck78's Avatar
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    Thanks for the advice and encouragement, Ken!
    I will definitely consider race compound Avon or Conti rubber. Since the bike will be ridden occasionally on the street, I was only thinking street tires at this point. I may just end up lacing up another front and rear set entirely so that I could run race tires exclusively on those.
    As far as having 2 sets of wheels, one 17" & one 18", I'd be concerned with the 1/2" radius difference when dropping to 17" would substantially reduce the trail geometry and make the bike very twitchy if I had the triple offset and rear ride height set up for optimized geometry running the 18" rubber.
    How much of an advantage do you see in running 17" rubber? Lighter weight (more free hp) is what I see it as primarily. Slightly less contact patch however. A very miniscule gain in needing smaller triple clamps as you will need less offset in the triples, very tiny weight savings.

    '77 GS750 920cc, 4-1, GS1100E swinger, 18" Sun rims, Fox Shox, twinpot dual disc, CR31, Yosh cams
    '74 Rickman VR250MX
    '99 Kawasaki KDX220 rugged terrain ripper
    PROJECTS:
    '77 Suzuki PE250B "Pure Enduro" trail beast, ported, Wiseco, TüBliss, Fox Shox/RaceTech
    '76 Rickman CR GS1000-1120cc roadracer, Yosh cams&4-1, RF900R fork, Works piggybacks
    '79 GS425 489cc Formula 500 racer, Tempter cams, GS650 forks, twinpot brakes, DID rims, GS1100E swinger, Fox Shox
    '77 GS550's 650-740cc susp&brake mods

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  5. #14
    Senior Member Chuck78's Avatar
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    Nice pic! I wonder what class that bike was running in? It has GS500 forks and front brake as I had hoped to run. And 17" wheels. CR carbs, same as I plan to run.
    Interesting...the rear set positiin. It took me noticing the extremely long shifter linkage to realize the rearsets are in the passenger footpeg position, quite the stretch! They are fairly short bikes, however...
    I am very excited about this project, although a 740cc GS550 comes first most likely. 650 head and cylinders, custom run of Wiseco pistons for the closed chamber 650 head. Fox Street Shox. GS500 front brake setup basically (using ninja caliper sane but opposite side as gs500, & 310mm cbr900rr rotor). 3.50x18 rear rim. I have a very important customer to please (wifey). And also needed to really out-do her latest clunker, a 13000 mile $5000 1986 FX Hardly-Ableson that left her stranded on the west coast. 1337cc pig that only has about 7 more horsepower than a bone stock gs550, can't even do the ton (99 mph top speed), & is slower accelerating than a stock gs550... her wall of death coworker buds have infected her with the harley plague... I need to give her a serious dose of wrist twisting to burning rubber to cure her of that slow poking grandpa & grandma hauling sickness....


    Once I get that thing built up with some kz1000 vm28 carbs and megacycle cams and properly jetted, then I can switch back to my projects. I'm building a basement shop at the new house now, so hopefully I can ramp/hoist bikes down there in the winter to wrench on...
    I'll post a build thread of sorts when I have actually started on it (wrenching will start on the 3 bikes after Halloween - I hang it up fir the year then after hitting a deer early November a few years back on my gs750. Mating season. Beware.)

    '77 GS750 920cc, 4-1, GS1100E swinger, 18" Sun rims, Fox Shox, twinpot dual disc, CR31, Yosh cams
    '74 Rickman VR250MX
    '99 Kawasaki KDX220 rugged terrain ripper
    PROJECTS:
    '77 Suzuki PE250B "Pure Enduro" trail beast, ported, Wiseco, TüBliss, Fox Shox/RaceTech
    '76 Rickman CR GS1000-1120cc roadracer, Yosh cams&4-1, RF900R fork, Works piggybacks
    '79 GS425 489cc Formula 500 racer, Tempter cams, GS650 forks, twinpot brakes, DID rims, GS1100E swinger, Fox Shox
    '77 GS550's 650-740cc susp&brake mods

  6. #15
    Senior Member Chuck78's Avatar
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    Quote Originally Posted by dirkchecken View Post
    been looking for a gs400 for a few months. Attachment 28010
    Did you know a Wiseco GS750-844cc piston will bore right into a gs400 block to make it a 449cc 10.55:1 or so? Longer stroke than the 750 but same piston. 750 stock is 8.8:1 iirc. 400 is 9.1:1 iirc.
    If you are looking to go 489 or even 475cc (894cc gs750 piston), let me know. I want at least 1 set of 489cc pistons 72mm, might get a group purchase going through Wiseco for the 71mm 475cc/894cc version.

    '77 GS750 920cc, 4-1, GS1100E swinger, 18" Sun rims, Fox Shox, twinpot dual disc, CR31, Yosh cams
    '74 Rickman VR250MX
    '99 Kawasaki KDX220 rugged terrain ripper
    PROJECTS:
    '77 Suzuki PE250B "Pure Enduro" trail beast, ported, Wiseco, TüBliss, Fox Shox/RaceTech
    '76 Rickman CR GS1000-1120cc roadracer, Yosh cams&4-1, RF900R fork, Works piggybacks
    '79 GS425 489cc Formula 500 racer, Tempter cams, GS650 forks, twinpot brakes, DID rims, GS1100E swinger, Fox Shox
    '77 GS550's 650-740cc susp&brake mods

  7. #16
    Senior Member hillsy's Avatar
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    There's a few thread over on GS Resources about mix and matching GS450/500 and GR650 top ends to make a 500+cc engine. I think the 650 crank, 500 barrels / pistons with the 450 head make the easiest combo (??)

  8. #17
    Senior Member Chuck78's Avatar
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    The compression ratio is fairly low with mist of those combinations. The way to go is 1980+ 450 bottom, 1989-2010 GS500 cylinders, GR650 Tempter cylinder head and cams (biggest of all oem gs twin cams), and custom pistons to correct the compression ratio back up to compensate for the substantially shorter stroke of the gs450 crank vs the GR crank. Maybe it could be possible to mill a lot of material off of the head to bump compression up, but it would be best to just get JE or Carillo to make custom pistons in the desired Compression ratio / dome size.
    The GR650 head, much like the GS650 head, is a 2 valve variant on the TSCC 4 valve technology. The early 2v engines were all hemi heads/pistons. The 4 valvers used Twin Swirl Combustion Chambers, TSCC. They called the GR650 a Twin Dome Combustion Chamber head. It is a more advanced closed chamber raised runner swirl port head and piston design.
    Rensdw on GSR had Carillo make a custom set of CP Pistons in the mid 10:1 range based on the GR650 head/piston and the GS400 2v crank. He did a serious retrofit running a roller bearing early 2v bottom end and changing the head stud spacing pattern to that of the GSX400/GS450/GS500/GR650... quite a feat to get the earlier longer stroke roller bearing crank. The 500 cylinders are slightly shorter as well, and require some base gasket spacing I believe.

    The early gs400/425 had the longer stroke (60mm) and smaller pistons (65/67mm), whereas the GS450 had the shorter stroke (56.6mm) and larger pistons (71mm). The GS500 shared the same stroke (56.6mm) & head stud pattern, but had much larger pistons (74mm).

    The GR650 was a big twin at 77mm bore x 70mm stroke.
    I suppose one could contact Carillo Rods/CP Pistons and use the info Rensdw posted on his GR650 hybrid pistons, & recalculate the chamber volume, and use his compression ratio and piston dome cc size, and then reverse engineer the amount of piston dome volume that would need to be added to make up for the 60mm stroke vs the 56.6mm stroke. Double check the needed compression height of the piston with the gs500 base gasket, cylinders, and check against a GS500 piston. Then double check the wrist pin diameter vs what Rensdw ordered. There you have it, 10.3:1 or so 527cc closed chamber head GS450 or 500!

    The GS500 head does not work with the 450 frame rails, by the way. Major exhaust interference. The GR650 head is the best but most complicated route. I think Rens spent over $800 on those CP pistons. I don't think that was with his custom Carillo rods but it may have been.
    Running a GS500 bottom end or at least all internals if they swap to the 450 crankcase would be best, so you are running a crankshaft and balance shaft that was designed for a 74mm flat top piston. Then bore the GS500 cylinders to their max of the GR650 standard size of 77mm.

    I think I will stick with a resleeved 425 cylinder block and hemi head and go the easier route to attain 489cc...

    '77 GS750 920cc, 4-1, GS1100E swinger, 18" Sun rims, Fox Shox, twinpot dual disc, CR31, Yosh cams
    '74 Rickman VR250MX
    '99 Kawasaki KDX220 rugged terrain ripper
    PROJECTS:
    '77 Suzuki PE250B "Pure Enduro" trail beast, ported, Wiseco, TüBliss, Fox Shox/RaceTech
    '76 Rickman CR GS1000-1120cc roadracer, Yosh cams&4-1, RF900R fork, Works piggybacks
    '79 GS425 489cc Formula 500 racer, Tempter cams, GS650 forks, twinpot brakes, DID rims, GS1100E swinger, Fox Shox
    '77 GS550's 650-740cc susp&brake mods

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