Rickman CR900 "GS" engine choice poll - big bore GS550-741/789cc vs GS750-894cc

[Chuck78]

I know the 750's quite well and have lots of spares, and the 900CC 750 would be extremely powerful, more than enough torque on a solo seat performance built non-production model based build... the 789cc hemi 550 would likely be quite powerful enough. Really the ultimate would be habe some 67mm gs650 custom pistons made, which if that were more readily available, would really be a serious contender for horsepower vs the 894 750... the 67mm hemi 750 798cc pistons are readily available here tho, so it's a no brainer for now, could look into the gs650-789cc slugs in the future if I wanted just a head&piston swap.

You are exactly right about the engineering challenge there! Honestly I think I will make a long motor mount plate for the lower left side that would accomodated the 750, 1000, 1100, 1150 engines with multiple bolt on mounts for each middle (lower) mount differences between all those models. The 750/1000/1100/1150 front and rear mounts seem to be all the same. The 550 would require a bolt on adapter plate on the rear mounts to come 1" forward.

If I wasn't so familar with & stocked up on 750 engines, the tscc 1100/1150 would be the no brainer, but honestly the lighter GS550 with pods & pipe is sufficiently fast up to 95mph or so. Revving up the 750 above 4500/5500 rpm, pods & pipe, is just simply a rocket, plenty of too end. What a 900/1000/1100 build gets you is far more low/mid torque without having to attract attention revving it. When you do rev those engines under heavy throttle, they clearly are every bit of a vintage superbike power and some of the fastest vehicles on the road.

[Chuck78]

35 lbs less & 2" narrower on the 550-789cc prospect is the biggest plus that makes it quite a contender for this build, however... but the 894cc is still alluring. 35 lbs is A LOT, that's like 5hp for free in weight savings, plus LOTS of added cornering ability and more agile handling... I think I may go for both engines since I want to set up the frame to accept a multitude of various GS engine mount locations...

The VTR1000F Superhawk forks run common 296mm Honda rotors that are available in the same bolt pattern as GS hubs, if wanting to run stock GS wire spoke hubs c this is a huge plus. 94-97 vfr750's are similar, better calipers for wire spoke clearance. Both aRe cartridge 41mm forks, and are taller than most modern forks, nearly the same as gs1000 forks. Steering stem is nearly as tall as a GS as well the superhawk/Firestorm has adjustable rebound and preload. Dampening stock is so-so, Racetech and Daugherty both offer full performance cartridge setups for them. 20mm axle requires machined adapters to run gs wheels, or custom spacers and larger i.d. hub bearings with a 17" wheel gsxr speedo adapter if not adapting the GS axle.

[Chuck78]

"The VTR1000 forks work nicely in terms of length on a GS1000. I used those forks with the stock calipers and a CBR600F2 wheel. Triples are CBR600 lower with a custom aluminum top clamp. In retrospect I could have probably bolted on a Bandit 600 triple clamp assembly"

-Jim aka DiamondJ's bike, superhawk forks/brakes.

Mine with gsxr1000 6 piston calipers on bench and Rickman fender mocked up:



OhioCafeRacers Greg's Dunstall GS1000 with VTR1000F Superhawk forks (mild rebuild thread on here, more info on dotheton.com or caferacers.net):


Superhawk owners complain about the stock springs and cartridge dampening, but there are a few homebrew mods to improve them. The cartridge kits available from Daugherty and Racetech fully transform them to the most compliant and awesome dampening available.

These vtr1000f forks are near stock GS1000 height with a useable steering stem length (a hair shorter that a GS750 8v stem) in a nice quality triple clamp yoke set, and the VTR1000F has externally adjustable preload and rebound, and 296mm rotor spec (many many honda floating rotors available that fit GS hubs) with a fairly universal 62mm spacing modern caliper mounting setup!

The Bandit 1200 forks are 775mm same as a GS1000 height. I looked up two of the older gsf1200 Bandit specs 95/96-2000, & they run dual 310mm rotors, so 2 years of CBR900RR 310mm 6 bolt 78mm bolt circle rotors (98-99???) can be run on GS hubs and spaced appropriately to work with those years of Bandit brakes. The Bandit rotors are 5 bolt, did not look up center diameter specs.

Bandit steering stems are too long and need a gs1100e stem pressed in place, or need machined down in height. 89 gsxr1000 k triples work with them, or one GSR member is running ghe lower triple clamp from an RF900R and the upper triple clamp from the Bandit 1200 with a very minor bit of fab work to get them working great. Other 80's gsxr triples may also work.


Aside from the early bandit 1200 forks, 94-97 vfr750 (sliding 2 piston calipers, more clearance for GS wire spokes if used), all vtr1000f superhawk/firestorms, and 93-94 gsxr1000 are the good right side up modern forks to swap onto vintage bikes if you care about ground clearance, geometry, and proper handling/suspension setup, they are the tallest of the options for modern fork conversions. Reading vfr750 forum threads about fork upgrades is a good way to dig up information on which modern forks are taller to fit our vintage bikes, since the vfr's run the same height range of forks...

Edit - also, the last year of CBR600F2 and all years of CBR600F3 have another excellent 41mm cartridge fork option with sliding 2 piston calipers (more clearance for GS wire spokes if used). CBR600F4 & CBR600F4i (fuel injected and different wheels/rotors) are an EXCELLENT 43mm choice, and they are one of only 4 forks that I have considered viable that have adjustable compression dampening. Usually, rebound is far more critical to have adjustability on, but the compression is another bonus.

Also, as I have learned, the Honda forks seem to generally run the calipers more outboard, so if you are running wire spokes or wider spoke mag wheels, you don;t need as wide of triples to clear opposed piston calipers far enough from the spokes. 6 piston calipers have smaller piston bores, so they have more clearance due to the spokes angling inward toward the rim, but 6 pistons require more frequent maintenance (cleaning the exposed portion of the pistons).

The Kawasaki ZRX forks that I looked at (1100 and 1200) both had adjustable compression and 310mm rotors, nice forks indeed. the heaviest of the 4 I compared, but a lot of that was in the heavy steel triples. the Yamaha R6 conventional RSU forks were the lightest, adjustable compression and 320mm rotors (have to mod some Ducati rotors to fit), but I saw a pair of those crashed in the salvage yard, and the stanchions were very thin...


The F4, VTR1000, Bandit, and RF900R forks all run opposed piston calipers. The Suzuki's use more period correct looking brake mounting points, but you either need some modern mags ( some of those honda's look fairly period appropriate in design) or custom wider billet triples (contact Weiss Racing - $425) to clear wire spokes and other wider spoked wheels.

[Chuck78]

By the way, I think the style of Honda 17" mags running on both those GS1000's are simply some of THE BEST looking modern wheels I have seen in terms of looking APPROPRIATE on a vintage bike, big bonus.

[Buffalo Bill]

I think that's probably the first time we have ever agreed on anything.

Nah, many more times.

[Buffalo Bill]

Just one engineering thing I suggest, cut and weld the frame so the forks have a steep rake like a GSXR (24º-26º maybe?). That easy rider GS rake is so lame.

[Chuck78]

Just one engineering thing I suggest, cut and weld the frame so the forks have a steep rake like a GSXR (24º-26º maybe?). That easy rider GS rake is so lame.

Rickman CR900 (Betor Fork):.............Suzuki GS750:
Wheelbase .......59.8"............................58.7"
Caster ...............62°...............................6 3°
Trail .................. 2.67" (68mm).............4.21" (107mm)
Steering Angle ..___°..............................40°

Another spec from a CB750 website, likely this uses the other of the two Rickman forks available:

Chassis Dimensions
Wheelbase: 57.1-59.1" / 1450-1500mm (depending on swing arm)
Rake: 62°
Trail: 3.54" / 90mm


So I am assuming that stating 62° caster would be the reference from horizontal as bicycle head tube angle is referenced, making the Rickman 28° rake and the GS750 27°. Not sure what the 40° steering angle spec listed for the 750 is.

It looks as if the lazier steering on a stock GS may be due to the large amount of trail...??? It is always said that an 18" front wheel swap makes them handle much better, less trail, slight bit steeper rake, right? The Rickman's are criticized as not having steep enough rake, but with less trail

Looking at the swingarm angle on a cr900 with the original shocks, it's not near steep enough to give enough antisquat on comfortable sporty springs, but due to the short swingarm, you can't make the swingarm angle too steep, as that will cause acceleration to try and lift the rear end more under acceleration out of turns. I'd like to put approximately 1" longer shocks on it to help with antisquat and also as a side effect increasing rake slightly, and also drop the triples on the forks slightly to gain about a degree more steeper rake. That should put it slightly closer to modern spec.
Once the bike is set up better and tested, I can have custom Billet triples made in the offset that I need for the best trail.

Sounds like I'll want to take that 28° rake and steepen it back to 27° or 26° if possible by raising the rear end primarily, dropping the front slightly. If I dropped the front too much, I would kill the swingarm angle. With this in mind, it seems that if I consider adding an additional box tube for a more forward swingarm pivot to run a longer zr550 or gs1100e swingarm, I might consider raising the swingarm pivot a slight bit higher, or in tuner fashion with a cue from Rickman chain adjusters and some fancy modern Supersport bikes with eccentric adjustable swingarm pivot geometries, I could make a slotted vertical track pivot mount with aluminum block spacers that lock in the up&down position of the swingarm pivot bolt....

Chopping the frame up front is beyond the limit of my tooling and extent of work I want to do, especially considering the amount of additional bracing that the previous builder has brazed onto the front end after the frame was repaired.... it'd be a MAJOR undertaking at this point. If I received an all stock frame with the head tube joint cracked and unrepaired, it'd be more likely.

I supposed I could unbraze the head tube only (major job) and jig the bike up extensively in front of my drill press, and shave off material from everything angling in more by 2mm or so by the bottom, and bronze weld it back in again after extensive measuring and jigging... LOTSA WORK, holy smokes...

Elevated/forward relocated gs1100e swingarm pivot, longer shocks, dropped front end height, less offset in triples to correct the trail sounds like the easier plan...

[limeex2]

Now you really have my attention! Oh wait its the coffee. Thanks for the info on the forks. What is the braced swing arm off of? Those CBR wheels do look great. I could be talked into trading my spokes for better brakes and 17" wheels
Imo the 6-pot calipers look nice but 4-pots grab better,less caliper flex. I just went through this on my ZRX. With modern forks and a wheel or disc set-up that has more spoke to caliper distance,that would open up choices for calipers,like new radials. There are brackets available on e-bay for this. This is the direction I will go on my next project,an ES. I want modern suspension,brakes,tires/wheels which I am starting to accumulate. Trippivot,its on to the next dream...

[Chuck78]

Not sure where Jim's braced swinger is from, metmachex or calfab etc? OhioCafeRacers Greg later welded the bracing from a CBR900RR swingarm onto the bottom of that GS1100E swingarm, however:


And I stumbled on another photo of Jim's bike in Greg G's photo album, as well as another GS1000 that is running GS forks with the salty_monk twinpot mod and the same swingarm:

[limeex2]

Pulled the forks of my 97 Triumph Sprint this morning for service. Hum...these look long enough for a GS. Yes,length no problem,other rough measurements close enough to spark my curiosity. I have a spare Sprint with a complete front end.Race Tech,braided lines and straight. Best of all its my garage not ebay. What measurements or specs would you need to see if this worth pursuing? That last pic of the red GS looks like a ZRX swingarm? Sorry if I am hijacking your thread.

[Chuck78]

No problem on the hijack, got me looking more & realizing from another member that a 730mm tall RF900R 43mm cartridge fork with dual 310mm discs would be an EXCELLENT alternate to me running a more budget oriented GS500E fork on my GS425-475cc project. $100 racetech emulators for the GS500 fork puts it up there in price, may as well just spend a little more coin & get full on cartridge forks that will dampen as good as emulators, stock! And come stock with adjustable rebound dampening in the latter years, and (?) adjustable preload. Nice. Just abandon the caliper mount on one side and run a single 310mm brake on this featherlight twin cylinder GS barebones racer. 43mm is a bit overkill on this light little bike though. I wonder if the rf600 or rf400 were available in the US?

[limeex2]

Not overkill if they work better. Maybe if they are heavier. I would take a little weight for a better fork. I believe they made a RF600 stateside. I pulled the triple's on the sprint. Back to back the triples are almost identical to a GS1000/1100. Bearings are the same so I threw the Sprint triple into a GS1000 bare frame. Fit perfect other than about a 3mm gap under the stem adjusting nut and the steering stops. I may have to call or pm you to understand what is involved in setting up the chassis other than switching parts. Thanks

[Chuck78]

As with a lot of newer bikes, the flagship model RF900R gets the awesome forks, and the RF600 & RF400 get damper rod forks that only look the same externally...

No cartridge forks & Yoshi cams this paycheck, wife's got expensive tastes, her January birthday just drained me... very excited about the RF900R fork prospect though. AND the custom CNC 1/2" Billet Al fork brace I pre-emptively got for gs500 forks that I didn't even own yet is the same style (oem steel plate w/ears on fork lowers ) as what the RF900R uses, makes this a total no-brainer!

[limeex2]

These are off of my Triumph Sprint. Handlebars are clip on style above the upper clamp. The triple trees went into a GS1000 chassis with no issues. I have an entire front end off of a spare Sprint. Where does one start for proper set-up? I will be using a Bandit rear end.

[Chuck78]

I think I'm going to start another thread solely about upgrades regarding revised frame and suspension geometry. rake and trail, ride height and suspension height changes front and rear, and overall effects on bike handling.

Limeex2, from what I've taken in so far on my quest for learning best modded setups, bolt the swingarm into the frame with the wheel in it. Jack the back end up by the frame, with no shocks until the swingarm is as close to a 12.5° angle to the ground as the chain line will allow without rubbing the swingarm pivot tube (gearing size - bigger front sprocket - allows closer to 12.5 degrees w/o chain rub on swingarm). You might only get to 9° or so with a vintage frame/engine.

Then with the front end and wheel installed, measure the rake angle of the head tube of the frame. Get the front tire diameter handy. Measure the offset of the triples precisely. Go to a rake& trail calculator online, or use the math formulas, and determine your trail.

You can also measure it manually with an L shaped framing square or plumb bob off the axle center, and a yardstick or straightedge and a very good eye and steady hand to simulate the steering stem centerline projected to the ground in front of the mark on the ground vertically under the axle. The amount the axle vertical centerline is from the line that the center of the steering stem points to is the amount the axle/tire contact patch "trails behind" the steering stem centerline.

Stock GS750 is around 4.65" trail (118mm?), quite a lot, very stable but slower turning than a sport bike. A stock 69-78 CB750 has about 5" of trail - cruiser territory, hence they steer like trucks even with a "fast for a cruiser" 28° rake. GS's are around 27°. Very stable, decent turn in but no sport bike. 4" is a safe trail # for more sport. stiffer than stock GS fork springs are needed to make sure you don't go into sketchy low rake/trail territory on front end brake dive, nasty head shake will ensue. Most modern sport bikes will be around 3.7"-3.9" trail ( which requires firmer aftermarket springs as you get closer to 3.7"), and 24.5-25.5° rake. Newer race oriented bikes are as steep of rake as 23.5. Going from 27° on a GS down to 26° or 25.5° shouldn't be too much trouble, but you need to make sure to watch the trail number. 3.7" may be a good idea to run a steering dampener for bumpy roads under braking & hard braking into corners (may upset the trail).

Bigger diameter front tire will make the trail # difficult to get down closer to 102mm/4", so a 19" will not work well here at all with a shorter fork than stock and modern triples that fit it. A 17" front wheel and a slightly shorter than GS length modern fork won't be much trouble at all to arrive at a good trail number. With 18" wheels, a 100/90 or 100/80 or 110/80-18 is helpful if you are running 35-45mm offset on the triples and trying to make the trail work.
More offset= less trail. Less offset = more trail. Modern triples have less offset generally, so you are likely to have too much trail with most 18" setups. With 17", should be closer. Taller tire or lessened rake (raise front/lower rear) will get your trail number higher if needed with the 17" front wheel.

Get all that figured out, then pick rear shock length. Easy if welding twin shock mounts to the Bandit swinger.a lot involved if monoshocking. Copy the linkage angles and positions of the donor bike if monoshocking, adjust shock position to get to the swingarm angle you settled at after getting the trail figured.

Trail is more important to get in the ballpark than rake, trail makes it stable/unstable and a good balance will be quicker to turn in than stock.

That's the result of my lunchtime reading the past few weeks...