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Old June 11th, 2020, 10:17 PM   #1
nocturncal
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What's a Rear Shock's Lifespan? Setting Up Rear Sag...

So I'm setting up my sag on my stock rear shock and planning on buying a GSXR shock 03-04 with a spring rate of 8.6 kg/mm, which from another thread is appropriate for a 172-181 lb rider (which happens to be my weight range, depending whether I pooped or not and what I ate the night prior lol)

I set up my rear sag with to 37mm, preload setting 4 with me sitting on the bike + gear, a SW Motech rear rack, and a small tool tube I have on my bike. Then remeasured my static sag, which was 15mm. Reading online it sounds like generally, you want static sag to be between 5-10mm for street riding. 15mm static would mean my springs are too soft?

If I buy the 03-04 GSXR shock, it would mean going to a softer spring vs the stock 250s 9.3 kg/mm. Which would increase sag even more

I'm wondering how much of this excessive static sag on the stock shock is due to the shock just being old? It's got 37,000mi on it and and many of those miles are from city riding on crap roads filled with potholes and uneven pavement. Is it possible that the shock spring isn't 9.3 kg/mm anymore and much less, causing more sag than if my shock had say, 10,000mi on it?

I'm also assuming it may be partly due to the weight of the top case rack + tooltube. Not sure how much it weights, maybe 10lbs total? I plan on keeping the rack on when i ride in the twisties, as it's a pain to take off. I'll take it off when hopefully later this year, I can get a weekend off and take the bike to the track
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Old June 12th, 2020, 02:00 PM   #2
Ceeloo Yello
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Not sure about life of shock but you could always use some custom dogbones to adjust the final height. Also note that when I put in my gixxer shock the overall height in the back ended up being lower.
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Old June 12th, 2020, 03:28 PM   #3
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Depending upon how smooth you are, those settings should be fine.

Sag is determined by spring-rate & preload. Those don't wear out on shock. It's rare, but if you're an abusive rider that frequently bottoms out shock and coil-binds spring, there may be possibility it's weakened, but mostly in preload; rate should stay constant.

It's damper that wears out inside shock. Seals and valving wears out over time, leading to insufficient velocity damping. But those won't affect sag. Some mid-range shocks use nitrogen pressure to supplement spring and worn seals may cause those to leak and change sag. However, that's over time and you can always re-adjust sag to compensate for wear on those types of shocks.

So your sag should be OK for now. It should also be a function of stroke-length/operating-range of shock as well and that will vary with spring-rate. Proper way to adjust ride-height is to adjust shock-length, independently from sag or spring-rate. Can be done with adjustable clevis. This is preferred over changing dog-bones as that changes leverage-ratio and changes effective spring & damping rates at wheel.

Don't focus too much on equipment at track, best value is getting more time on track itself for practice. Spend money on buying trackdays and coaching sessions. Looking forward to seeing you out there!

Last futzed with by DannoXYZ; June 12th, 2020 at 06:27 PM.
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Old June 13th, 2020, 12:58 AM   #4
nocturncal
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Quote:
Originally Posted by DannoXYZ View Post
Depending upon how smooth you are, those settings should be fine.

Sag is determined by spring-rate & preload. Those don't wear out on shock. It's rare, but if you're an abusive rider that frequently bottoms out shock and coil-binds spring, there may be possibility it's weakened, but mostly in preload; rate should stay constant.

It's damper that wears out inside shock. Seals and valving wears out over time, leading to insufficient velocity damping. But those won't affect sag. Some mid-range shocks use nitrogen pressure to supplement spring and worn seals may cause those to leak and change sag. However, that's over time and you can always re-adjust sag to compensate for wear on those types of shocks.

So your sag should be OK for now. It should also be a function of stroke-length/operating-range of shock as well and that will vary with spring-rate. Proper way to adjust ride-height is to adjust shock-length, independently from sag or spring-rate. Can be done with adjustable clevis. This is preferred over changing dog-bones as that changes leverage-ratio and changes effective spring & damping rates at wheel.

Don't focus too much on equipment at track, best value is getting more time on track itself for practice. Spend money on buying trackdays and coaching sessions. Looking forward to seeing you out there!
Awesome man, thanks for the info! In that case, is it worth it to add a GSXR shock or just go to the track with my stock rear shock? I originally thought it would be a cheap upgrade to the rear to spring the shock for my weight, but now I'm questioning whether I'll actually appreciate the upgrade.

Sidenote: I'm 180lbs without gear.
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Old June 15th, 2020, 01:16 AM   #5
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Quote:
Originally Posted by nocturncal View Post
Awesome man, thanks for the info! In that case, is it worth it to add a GSXR shock or just go to the track with my stock rear shock? I originally thought it would be a cheap upgrade to the rear to spring the shock for my weight, but now I'm questioning whether I'll actually appreciate the upgrade.

Sidenote: I'm 180lbs without gear.
For street use, you won't really gain any benefits from GSX-R shock. I started out at 195-lbs with factory shock at full-preload. I'm at 170-lbs now and ride my bone-stock street bike one down from full-preload.

On track, main benefit is being able to adjust compression and rebound damping independently (some have separate low vs. high-speed compression). For example, when playing with different stiffness springs, you'll need to change damping. Often in opposite directions.

This is really only noticeable when you're going +90% of limit. My 1st season on track, I did +40 trackdays on factory shock. I had chopped off over 30-sec/lap at Thunderhill by end of season.

Going to K-Tech Razor then Ohlins shock was an improvement next season, but I'd have to really be pushing to limit to notice difference (still slightly faster on Razor). I'd say variations between all these shocks over stock is 2-5sec/lap at most. If you're pro-racer and your paycheque depends upon results, yeah, shock-upgrade may be difference between 1st and 10th place. For amateurs, it's nice to have, but not vitally important.

Personally I notice bigger improvements in control and lap-times with front-end upgrade with stiffer springs, thicker oil and gold-valve emulators. If you're going to upgrade suspension, I'd do front-end first. Along with SS brake-line upgrade, grippy pads and DOT-4 fluid. Also as you improve, major impediment to going faster will be footpegs grinding, then exhaust and kickstand-bracket. This is where an adjustable clevis to increase shock-length without affecting spring-preload or damping comes in handy.
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