Setting sag and preload on new fork springs?

[Oso]

I recently changed the oil and installed new Racetech springs in my front forks. The installation called for cutting the length of the stock spacers. I shortened them by about 50mm. I went with the manual's oil level of 92mm. When I checked the sag with rider on the bike (195lbs) it was only 21mm. Sag with no rider was only about 3mm. Articles I've read call for sag with rider to be about 30% of total shock travel. This would be about 45 - 50mm on my bike. After contacting Racetech it appears I may have made several mistakes. They recommend an oil level of 110mm. So, a lot less oil. Their instructions also call for subtracting the height of the top cap assembly from the length of the fork tube from top of spring to top of tube, then adding 20mm for preload. I don't think I added the 20mm of preload to the spacer length before cutting it.. Coincidently this is close to the amount of additional sag that I'm trying to get.

End result two potential issues affecting the sag, preload and ultimately the stiffness of the forks:

1 incorrect spacer length

2. incorrect oil level ( what I understand is this may have more to do with damping than preload)

What I am having trouble grasping is how changing the spacer length effects preload and sag. Does making the spacer longer increase or decrease preload?

I will probably start from scratch and rebuild the forks again but not before knowing which direction to go here.

Thanks for any advice.

 

[FJRLoonie]

I had Racetech springs installed by a dealer. They were unaware of the spring guides that are necessary`and had to remove and reinstall the springs. Did you install the spring guides as stated below from their website.

FRSP S4031 WASHER WARNING. Requires new spring guide FPSG 1027127 P to use RT springs. Install 24mm ID washer on top of cartridge. Washer must locate on shoulder. Spring must be installed with small tapered end down. Install new RT spring guide (long taper down) then RT spring collar. Next install preload spacer tube. (RT spring is longer than stock-check preload instruction for proper setup.) Install nut then fork cap. Gold Valve Kit comes with preassembled Compression & Rebound Valves. TFSH 20 Shaft Holding Tool required for GV Kit. ``

 

[RaYzerman19]

Yes, redo your spacer length calculations, that is the key. Go with the RT recommended fork oil level (in the past they have recommended 130mm), although it shouldn't have caused your issue.

 

[Full House]

Maybe RaYzerman19 will chime in. He installed Race Tech springs/guides in my forks at WNYFJR's tech day in June. He seemed to understand this very well.

I see he just did! Way to go Ray

 

[Oso]

Yes, I did install the spring guides.

 

[Oso]

RaYzerman19, If I were to follow Racetech's instructions I would than be adding 20mm (recommended preload) to the spacer. That would make the spacer longer than it is now, intuitively at least for me, this is going to exacerbate the problem. I am trying to get the recommended from Racetech sag of 40mm.

 

[sath182]

End result two potential issues affecting the sag, preload and ultimately the stiffness of the forks:1 incorrect spacer length

2. incorrect oil level ( what I understand is this may have more to do with damping than preload)

What I am having trouble grasping is how changing the spacer length effects preload and sag. Does making the spacer longer increase or decrease preload?

To answer your question directly, adding spacer length increases preload. That's why RaYzerman19's insistence on getting the calculations correct is so important. Did you start by measuring the depth of the fork cap with the preload adjuster in the middle of it's range? Walk us through your calculations.

Also, Oil level affects the air pressure inside the tube. As the suspension is compressed the air acts like a spring. More oil = Less air = Stiffer spring. If you have too much oil then the system can hydro lock before you reach full travel. The after market springs probably have more submerged volume as well as being stiffer so you need more room and less assistance from the air spring.

 

[MCRIDER007]

Measure the sag with half of the ride height/preload rings showing. If you want more sag then shorten the internal spacer in a 1:1 ratio (to increase sag by 20 mm, shorten the spacer by 20mm). You can then use the external ride height/preload adjustment to fine tune the sag.

Oil height is used to adjust the amount of air in the forks and to use compressed air to increase resistance at the end of the fork's compression stroke to prevent bottoming, usually comes into play during heavy braking.

Opps....looks like Sath182 posted while I was typing and I said the same thing he did.

 

[Oso]

I measured the depth of the fork cap with the preload flush with the bottom of the fork cap. I subtracted that from the fork length from top of spring to top of tube. I don't believe I added the recommended 20mm of preload back to that number. It was a few weeks ago so my memory has faded.

But, let's say I didn't add that 20mm, if I had wouldn't my sag be even less than it is now?

 

[sath182]

Hmm, sure sounds like you were doing the right thing. If you had, say, 200mm from the uncompressed spring to the edge of the fully extended fork tube, and the depth of your cap from the top of the threads to the bottom of the preload adjuster is 50mm, then adding 20mm of static preload would give a spacer length of 170mm. It sounds to me like that's the process that you've followed. Did you already set your preload adjusters after installing, or leave them all the way out like you measured them? That can make a pretty big difference. BTW, chopping 20mm off the spacer will probably give more than 20mm increased sag depending on the spring rate.

 

[Oso]

The 21mm of static sag is with the preload adjustment all the way out (softest). Again adding that recommended 20mm of preload to the spacer would make my sag even less, right?

This is how Racetech instructions read:

"Use a tape measure to record the distance from the top of the fork tube down to the top of the spring washer. Set the preload adjuster to minimum. Measure the fork cap height from the bottom of the sealing lip to the point on the bottom of the cap where the spring touches. Subtract this distance from the first measurement. This would be the required length of the spacer for zero preload. Add the amount of preload required to determine the length of your spacers."

I may be over analyzing this but could they mean add the preload number to the fork cap number before subtracting from the fork tube length? Which in sath 182s example above would give me a spacer length of 130mm.

 

[MCRIDER007]

You are over analyzing because the RT instructions are an attempt to get the sag right on the first attempt...which can be tricky because aftermarket springs are rarely the same length as the OEM springs. The RT instructions also assume you know what internal preload would give you the correct sag. However, once you have installed the springs and measured the sag it's time to forget the RT instructions and simply change the spacer length to give you the sag you want....and you are in luck because it's easier to shorten a spacer than make it longer.

 

[sath182]

Yeah, I'd say you're reading it correctly. I think you're gonna have to pull it apart and re-measure everything. I expect that your measurement from the spring to the top of the tube was with the spring guide and washer in place as well?

And no, if you had a 130mm spacer in my example you'd end up with a 20mm unsprung gap when you top out the fork and WAY more sag than you want.

BTW, I found this while cruising the web just now, maybe it will help. I'm hazy, as it's been a long time, but doesn't the spring not get compressed until you push the cap down to screw it on? Maybe you can measure how much preload you have by just popping the cap and measuring from the rim to the sealing lip. (see the External Top-out figure at the end)

https://www.racetech.com/download/InstructPDF/IP%20FRSP%20S%20Street%20Fork%20Springs.pdf

Edit: I've also come across other instructions that recommend setting the caps with +10mm preload so you can back it off if needed once assembled. There were also recommendations of 25-33% total travel length for rider sag. That equals roughly 34-45mm total sag for the FJR. Which ideally would be balanced with the sag percentage you have in the rear.

 

[RaYzerman19]

I'm missing something, dunno what it is.... we don't know what your RT spring length is, but if typical of the length of the ones I've done, with 15mm of preload, for 40mm of sag (for your weight depending on spring rate you chose), I cut about 33-ish mm off the spacers. Calcs done with preload flush to bottom of fork cap. You cut more and I'm not quite getting why you have LESS sag. Crank your preload all the way down to simulate a longer spacer, see what sag you get.

 

[Oso]

Okay, I cranked the preload all the way down and checked sag via zip tie method. I only get 23mm. Cranking the preload down extends it about 14mm from being flush with the bottom of the top cap. Barely any change at all. What am I missing here? By the weekend I will remove the forks and start over but the conundrum of already cutting the spacer too short yet I need to increase sag. I have a long email thread with RT support but it is like pulling teeth to get clear information.

 

[RaYzerman19]

Time to ask what spring rate you are using and what is your weight.......

 

[escapefjrtist]

Ray might be on to something. RT is notorious for recommending very heavy spring rates for FJRs.

When I installed RT springs in my '06, I wanna say I ended up with a ~4" spacer length...basically removed the difference due to the longer RT springs. IIRC, stock FJRs have ~15 mm preload on the OE spring.

There's an old thread detailing my install, I'll try to find it for specific details.

--G

Edit: Take a look at this thread for more RT spring information. My spacer ended up at 4-1/8".

 

[Oso]

Spring rate = .95/kg, weight 195lbs. I've got the forks off and will start rebuilding in the next day or two.

 

[RaYzerman19]

Sounds like you're in the ballpark....... interested in your new spacer calculation and sag.