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Old February 26th, 2018, 06:30 PM   #1
corksil
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GSXR 750 rear shock mod.... again

Hey I did this modification and installed [what I thought was] an 09 GSXR 750 shock. Used different dog bones to keep the stock ride height.. made a bunch of spacers so it would all bolt up.. drilled out the shock eyelet etc.

I read through quite a few different guides and DIY on the project, which were helpful.

All finished and complete -- except the rear suspension is way too stiff.

I have zero spring preload -- the adjusting collars are all the way on the loose side, barely compressing the spring at all.

When I sit on the bike, the rear end doesn't drop more than a few millimeters. If I have a friend help me push down hard on the tail section, I can get about an inch of rear suspension compression.

I'm 160lb roughly, for what it's worth.

No idea what's going on. The ebay auction was for an 09 GSXR 750 rear shock, and everything I read about this pointed me toward that shock being perfect for the project.

Why the hell is it so stiff? I had plans on rebuilding the shock, changing the oil, and re-charging it with nitrogen... but it seems like waaaay to high of a spring rate.

Not sure what I would say if I called racetech either... can I measure the coils of the spring and the un-sprung length and identify it somehow?
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Old February 26th, 2018, 06:59 PM   #2
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Compare the leverage ratios of dog bones you used vs. factory ones.
Any binding? Remove spring, reinstall shock and verify swingarm moves smoothly.

Ride height should be adjusted by changing shock length or the upper mounting point.
There was guy that made an upper bushing with offset hole. Very innovative.
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Old February 26th, 2018, 08:04 PM   #3
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If you used different dog bones, you probably change the leverage points. I did the 600 shock conversion on mine. I made collars to put on either side of the shot eye to use the factory ninja mount to keep geometry all the same. Works great.
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Old February 27th, 2018, 02:37 PM   #4
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I measured the links that I'm currently using. 158mm

The oem links are 165mm, according to a search.

Shorter links raise the back of the bike.

The shock I'm using looks exactly like the ones pictured in searches for "09 GSXR shock" -- and it has the same yellow spring.

After doing some math -- and figuring that it was indeed the stock 9.5kg/mm spring, with my body weight of 165lb [75kg] -- standing on top of the spring caused it to compress by 10mm.

According to my calculation -- if it were a 9.5kg/mm spring, and 75kg were applied to it, the resulting sag should be be 8mm. I measured 10mm so not sure what that means but it's supporting evidence that I do indeed have a 9.5kg spring.
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Old February 27th, 2018, 02:43 PM   #5
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And yes -- I've removed the spring and reinstalled everything and run the rear tire all the way up and down by hand. [125mm travel]

No binding or sticking, everything moves freely with no spring involved.

As soon as I put that dang spring in there the rear suspension is rock hard.
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Old February 27th, 2018, 03:35 PM   #6
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Shorter dogbone have less leverage, so effective spring-rate at rear-wheel through linkage and swing-arm length is higher than longer dogbone. While dogbones can be used to adjust height, it's not ideal because their length is linked to effective spring-rate as well.

To adjust ride-height without affecting spring-rate or any kind of damping, you want to change upper mounting-point of shock.
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Old February 27th, 2018, 10:52 PM   #7
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Mine is the same way. I have never ridden the bike since doing the shock swap, other than putting around the house. But I can tell the rear is super stiff. I weigh 180 and barely get any movement. No binding, though I did buy dog bones to raise the rear .5 inch over stock. After reading this thread makes me think my lower links are too short, stiffening the spring leverage.
Shock is from a 2006 gsxr600/750.
It's a project bike that got pushed to the side for a few years.
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Old February 28th, 2018, 10:10 AM   #8
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bad geometry

I was about to ride my pregen on the track a few weeks ago and I took it too the suspension guy who told me "there is nothing there" the rebound was off and the shock was not responding properly due to "incorrect geometry" (r6 rear shock) He said I would be better off using the stock shock, which I don't even have. My solution... sold it and got a newgen. Makes sense that the bike is designed for a particular shock and drilling/changing dog bones and putting a shock designed for a different chassis and bike all together alters the geometry of the bike and when that is off, the suspension won't work properly. All tho this work was done by a "pro"... It makes sense to me and he has been doing suspension for 20+ years so I take it he knew what he was talking about...
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Old February 28th, 2018, 12:26 PM   #9
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Quote:
Originally Posted by DannoXYZ View Post
Shorter dogbone have less leverage, so effective spring-rate at rear-wheel through linkage and swing-arm length is higher than longer dogbone. While dogbones can be used to adjust height, it's not ideal because their length is linked to effective spring-rate as well.

To adjust ride-height without affecting spring-rate or any kind of damping, you want to change upper mounting-point of shock.
What do you think about using dogbones to change ride height, and then theoretically changing spring rate to compensate for the difference in geometry?

For what it's worth -- the gsxr shock I have is a whopping 6mm shorter than the ninja 250 original shock. Doesn't seem like a large difference.

Quote:
After reading this thread makes me think my lower links are too short, stiffening the spring leverage.
I'm about to put ninja 250 links back on instead of the custom ones. Will let you know if the rear end feels any softer after reassembly.
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Old February 28th, 2018, 01:29 PM   #10
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Quote:
Originally Posted by corksil View Post
What do you think about using dogbones to change ride height, and then theoretically changing spring rate to compensate for the difference in geometry?

For what it's worth -- the gsxr shock I have is a whopping 6mm shorter than the ninja 250 original shock. Doesn't seem like a large difference.
That should work to get softer spring. How much softer can be calculated by drawing pivots of dogbones, knuckle and swingarm-length. Due to variable-leverage ratios of the linkages, 6mm at shock can translate to 18-24mm change at rear-wheel depending upon which section of stroke.
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Old February 28th, 2018, 10:41 PM   #11
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Originally Posted by DannoXYZ View Post
That should work to get softer spring. How much softer can be calculated by drawing pivots of dogbones, knuckle and swingarm-length. Due to variable-leverage ratios of the linkages, 6mm at shock can translate to 18-24mm change at rear-wheel depending upon which section of stroke.
My calculations have determined that I have a 9.5k spring. Called racetech today and they confirmed lots of my figuring. Softest spring they sell is 8.5k, which seems like my best option with the shorter links [to retain proper ride height].

8.5k spring may still put me out of the dampening/valving sweet spot, but if I have to tear down the shock for purpose of replacing the bump stop anyways -- I could re-do the shim stack to compensate.

Do you know the math on how I could calculate optimal spring rate? Racetech figured an 8.3 or 8.2 spring would be ideal, and the 8.5k spring they want to sell me is much shorter so it would give me better preload options. Best compromise.

How do I calculate the optimal spring rate?

Thank you very much for your help. I mean that.
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Old February 28th, 2018, 10:42 PM   #12
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Not sure if this is relevant but maybe these measurements are a place to start..

http://www.racetech.com/download/SwingarmGeometry.pdf
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Old March 1st, 2018, 10:01 AM   #13
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The RaceTech calculations are for simple twin-shock rear-end, not our more complicated rising-rate linkage. I think we'd need to measure the knuckle's angle relative to the dogbones and use an inverse-sine function to arrive at leverage-ratio. When knuckle's parallel with 90-degrees motion to dogbones, we'd get most leverage (swingarm to shock). Then as it moves away from centre, leverage decreases more and more.

With different shock-length and spring-rate, the resting-position of the knuckle & dogbones are no longer parallel and shock has less leverage. Combined with stiff spring, this adds even more effective spring-rate at rear-wheel.

I found spreadsheet with various GSXR shock options.


I think you want to go with '04-05 GSXR-750 shock with longer length if you wanted to raise rear-end. And the 7.3kg/mm spring is more appropriate for a street bike with your weight. Spring of 8.4kg/mm is what you'd use on race bike. The 9.5kg/mm spring is more fitting for +200-lb rider on race bike.

Last futzed with by DannoXYZ; April 5th, 2018 at 06:46 AM.
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Old March 1st, 2018, 12:55 PM   #14
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When I swapped my swing arm I had to use shorter dog bones which had a beneficial effect of improving the spring rate which was to soft.
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Old March 1st, 2018, 03:08 PM   #15
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SV Spring!!??

I had my stock SV650 shock laying around and discovered the spring would fit the GSXR shock body. I posted pix of mods to spring end and retainer.
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Old March 1st, 2018, 04:55 PM   #16
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Quote:
Originally Posted by DannoXYZ View Post
I think we'd need to measure the knuckle's angle relative to the dogbones and use an inverse-sine function to arrive at leverage-ratio. When knuckle's parallel with 90-degrees motion to dogbones, we'd get most leverage (swingarm to shock). Then as it moves away from centre, leverage decreases more and more.

With different shock-length and spring-rate, the resting-position of the knuckle & dogbones are no longer parallel and shock has less leverage. Combined with stiff spring, this adds even more effective spring-rate at rear-wheel.

I found spreadsheet with various GSXR shock options.


I think you want to go with '04-05 GSXR-750 shock with longer length if you wanted to raise rear-end. And the 7.3kg/mm spring is more appropriate for a street bike with your weight. Spring of 8.4kg/mm is what you'd use on race bike. The 9.5kg/mm spring is more fitting for +200-lb rider on race bike.
I bought a 05 GSXR 750 shock from ebay, should be here in a few days. I verified that it does have the 7.3k spring through other sources and all of your info on length looks correct.

My original 250r shock was 323mm. The '09 750 shock is 317mm. The '04 750 shock is 332mm. Ten milimeters longer than stock.

I have the original 250 linkage (164mm), and the aftermarket (152mm) linkage so I will have options to experiment with.

If it's important -- the '09 GSXR shock gave me 120mm of suspension travel with the aftermarket dog bone links. 127mm of suspension travel with the 250r links.

The 250r shock gave me 130mm of suspension travel with the 250r links. Never measured travel with aftermarket dog bone links and the original 250r shock.

Didn't seem to have any clearance issues with anything -- the rear tire came within 3/4" of the fender when it was all the way up -- and my rear tire is pretty worn. A fresh tire would certainly be taller but I don't think it would rub the fender. I'm also running the 140 tire in the rear instead of the recommended 130 tire. And none of the suspension travel tests were done with bumpstops. None of these shocks have bumpstops. They have all rotted and fallen out. The bumpstop should come into action before the rear wheel gets that high.
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Old March 6th, 2018, 01:23 PM   #17
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I think you want to go with '04-05 GSXR-750 shock with longer length if you wanted to raise rear-end. And the 7.3kg/mm spring is more appropriate for a street bike with your weight.
Bought and paid for, seller said it will be shipped out today.

Fingers crossed!
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Old March 6th, 2018, 06:28 PM   #18
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Is the reservoir going to fit??

The table says no...
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Old March 7th, 2018, 08:23 AM   #19
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Have to trim plastic fender liner I think.
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Old March 7th, 2018, 11:55 AM   #20
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I'm hoping that's what "reservoir doesn't fit" means. Mine has already been trimmed to accommodate the other GSXR shock that I currently have.

Shipped out yesterday, really looking forward to it arriving!
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Old March 16th, 2018, 09:47 PM   #21
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I finally installed the 05 GSXR shock. With the OEM links I measured 6" of suspension travel with no spring. Without a bumpstop, the rear tire came up within an inch of hitting the rear fender, which is good.

With the aftermarket 2" raise links --- surprise! They definitely seemed to raise the rear of the bike 2". I measured 4" of suspension travel, because the wheel hit the ground and I couldn't measure the full range unless I were to jack the bike up higher (which I didn't.) The rear tire only came up high enough to leave a 3" gap between the tire and the fender. Not ideal.

I didn't try riding the bike with the shock/spring installed and the aftermarket links --- seemed as if it would have made the suspension way too nose-heavy with the back end too high for comfort.

With the 05 GSXR shock/spring/oem-links, the ride height seemed good, the rear end was still pretty stiff, but the bike seemed to handle alright.

Will post more once I have a chance to take the bike further out of the stable. A ride around the block isn't much of a true test or assessment.
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Old March 18th, 2018, 08:11 PM   #22
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I've ridden 150 miles with the 05' GSXR rear shock installed. The rear end of the bike definitely feels stiff. There is zero preload on the spring, so I can't back that off.

With the OEM shock, I could hit speed bumps at 50mph and the bike would soak it right up and I wouldn't be jostled from the seat.

With the 05' GSXR shock -- it I hit the same speed bumps at 50mph, I get my ass kicked up and forward out of the seat. Seems like the spring is pretty stiff. Playing with the compression/rebound dampening doesn't seem to affect this.

I also raised the front forks 24mm in the clamps, which may be a factor. Might have to split the difference and try 13mm of fork above the top clamp.
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Old March 19th, 2018, 08:19 AM   #23
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Assuming the spring rate is right for you. Your high/low speed compression is too tight sir. iirc, your shock will have both high and low speed compression adjustments. Typically, the low speed is in the center of the high speed adjust nut. 15mm or 17mm I think.

Turning either of them to the right will make the ride "harder", turning them to the left will "soften" the ride.

Adjusting compression would be typically done after getting sag numbers within range and an acceptable rebound setting that is in harmony with the front.

Start with sag and rebound, then go to compression. Watch some Dave Moss videos if you have no clue how to do this and/or get some experienced help. You can really muck up the readability of your bike by adjusting these settings without a clue of what you are doing and WHY you are doing it. AND.... it will cost in you tires.

I could be convinced to let you know the settings on mine, but the would be most likely too hard for your liking on the street AND will most likely not be close to what you need based on the front setup.
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Old March 19th, 2018, 08:36 AM   #24
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Does that shock have a HS damping adjustment?
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Old March 19th, 2018, 08:45 AM   #25
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Does that shock have a HS damping adjustment?


04/05 600/750 should have rebound at bottom and hi/lo comp at top. HI=big nut, LO=flathead screw in center
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Old March 19th, 2018, 08:51 AM   #26
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Tip: Many of these click. When you read others settings, they are typically in "clicks out" from full tight. The ones that don't click are measured in "complete/half turns out." Also, these are precision adjustments. NO TORQUE!!!! NO TORQUE!!!! NO TORQUE!!!!
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Old March 19th, 2018, 09:28 AM   #27
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Yeah, damping adjustments is new feature for Ninjette. Read the manual, and turn everything full-soft to start.
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Old March 19th, 2018, 04:18 PM   #28
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04/05 600/750 should have rebound at bottom and hi/lo comp at top. HI=big nut, LO=flathead screw in center
Just to be clear --- I don't think that the big nut is an adjustment. I just backed mine out about 5 or 6 turns, and PSSSST!!!!! nitrogen pressure and oil came out. Threaded the nut back in, and it's still leaking oil so I must have buggered the o-ring. Which means now the shock needs to be rebuilt with fresh oil and nitrogen charge.

I guess it would've had to happen anyways to replace the bump stop --- but FU<K!! Now the bike is out of commission again.

And the rear shock spring rate still doesn't feel good. Way too stiff --- the bike sags about 6mm in the rear when I sit on it. That's no where near 30% of the travel (6").

It's also with the original links.

GAAHHHHH!!!!
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Old March 19th, 2018, 04:30 PM   #29
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However on this 05 shock, I can back that big nut out and it doesn't take much force to unthread it but I'm not sure if it's going to come all the way out and have the shock explode in my face.

Just to confirm --- the 05 GSXR shock DOES have high speed compression adjustment, and the big nut IS designed to unthread for the adjustment?
It shouldn't take much to turn the adjuster. As stated above... NO TORQUE! Will the big nut come all the way out? I dunno, never had a need to turn it more then 3.5/4 turns out.

Simply put, it opens and closes a valve inside the shock. Full right to close the valve completely off. With the valve fully closed, nothing can effectly move inside the shock, hence a very HARD ride. To free the shock up to move and actually let the shock do its job, turn left to loosen it.

EVERYONE measures from turned to the right, FULL tight... and then counts turns/clicks "out" how loose it is.

HS Damp (big nut): Start about 2 turns out
LS Damp (little screw in the middle): Start about 3 turns (6 clicks) out

In terms you fully understand; righty tighty (hard ride), lefty losey (softer ride)

Going further;

High speed damping is what you adjust for speed bumps, potholes, 2x4s and other more abrupt shock travel like super hard braking
Low speed damping is for the lesser imperfections on the surface you are riding

It's not really rocket science when you finally figger out what to adjust and why to adjust it.
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Old March 19th, 2018, 04:31 PM   #30
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And may be worth mentioning that when I rode the bike with the 05 shock, hitting a speed bump seemed to buck me up out of the seat and onto the handlebars --- so it seems that the original shock is now going back on the bike.

What's with the super stiff spring rate? It's not a track bike -- and even a street bike is supposed to have ~30% sag in the rear. All of these shocks (09/05) seem waaaay to stiff.

The 09 was a 9.1k spring I believe, and this 05 is a 7.3kg yet both are rock hard.
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Old March 19th, 2018, 04:34 PM   #31
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Dangit you replied to my post before I edited it.
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Old March 19th, 2018, 04:35 PM   #32
corksil
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I threaded the big nut in and out again and it's still leaking oil and tiny bubbles of compressed gas. Looks like i buggered the o-ring and now it does need to be rebuilt.
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Old March 19th, 2018, 07:44 PM   #33
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Quote:
Originally Posted by corksil View Post
And may be worth mentioning that when I rode the bike with the 05 shock, hitting a speed bump seemed to buck me up out of the seat and onto the handlebars --- so it seems that the original shock is now going back on the bike.

What's with the super stiff spring rate? It's not a track bike -- and even a street bike is supposed to have ~30% sag in the rear. All of these shocks (09/05) seem waaaay to stiff.

The 09 was a 9.1k spring I believe, and this 05 is a 7.3kg yet both are rock hard.
Bummer about the o-ring.

It don't matter what the spring rate is if the valves inside the shock are closed. The shock will be all locked up and not want to move, so it will be a hard ride and hitting a speed bump will have your bottom off the seat. Get the o-ring fixed and have the shock serviced, it might be cheaper to just buy another. :\

Next, find the 05 gsxr specs for that shock and return it to factory settings on the adjusters. Next, use the preload to achieve sag numbers. If you can't hit the sag numbers, then that is not the spring for you. Then move to rebound, then h/l damping.

What is with the rock hard spring and settings on these shocks? Simple... they are designed and made for machines that can accelerate harder, brake harder and corner harder than a 250. So the shock is more rigid.
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Old March 20th, 2018, 09:08 PM   #34
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Thank you for the kinds words and keen insight!

I really doubt that I had both of the shocks set to full hard dampening, but that would explain the absolute stiffness.

As stated, the 05 GSXR 750 shock seems to have lost pressure and oil so it's compromised and no longer good for use until a rebuild takes place.

Spent a half hour on the phone with jake at racetech today.. great guy and very helpful. I think the 09 GSXR 750 shock is my best option -- I'll swap it back onto the bike when I have a minute and play with the dampening settings. Again -- I seriously doubt that I had it locked out at full stiffness but I need to rule that out before I can continue with this.

Sounds like a lighter spring is my best option. Along with a shock rebuild/installing-bumpstop, and maybe messing with the shim stack or gold valve doohickey to wrangle the dampening closer to where it should be for such a light bike.

Thanks again, I'll post back once I get that 09 GSXR 750 shock back in there. Race tech may have been mistaken, but he told me that there are substantially lighter spring rates available for that shock. My understanding is primitive, but as far as I know ---- I should have roughly 30% sag. Certainly didn't see that with either GSXR 750 shock. 5% sag at the most, with zero preload on the spring.
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