Setting Race Tech Fork Spring Static Sag

Reginald

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Reading the threads on adjusting fork spring for pre-load is rather haphazard with little diamonds of knowledge here and there. So I got methodical and this was my approach.

I weight 210 lbs and ordered .95 spring rate Race Tech fork springs. My ST1300 has 31,000 miles.

Measurements between bikes and riders may differ do to wear an tear. Use you own measurements. I rounded all my measurements to the nearest millimeter.

My original OEM sag was 38 mm. Oooops. I want something between 33 and 36 mm.

I removed all cowls from the sides of the ST for ease of disassembly, installation, pre-load adjustments, and sag measurements that followed.

The stack is the spring, spring seat, any spacer used, plus Spring collar.
Pre-load reduces sag.

My OEM Stack Measurements: 451 mm total length
Spring 249 mm
Spring seat 2 mm
Spring collar 200 mm

Race Tech Stack Measurements: TBD total length
Spring 276 mm
Spring seat 2 mm
Spacer(s) if used 3 mm each
Spring collar TBD mm

Length of Cap threads 15 mm (adds pre-load)

Determine zero pre-load Spring collar measurement as a reference point. Use this figure to determine how many millimetres of pre-load are added to achieve desired sag.
Measure zero sag for Spring collar:
With fork tubes, seal, and damper assembled, insert spring then spring seat washer into fork tube. Extend the fork tube to its limit. Measure from top of washer to top of fork tube from inside the fork tube. Subtract cap length from this measurement. If you wanted zero pre-load this is the length of the Spring collar you would use. I don't recommend doing this, you won't have enough spacers to set pre-load.
Measurement from top of spring seat washer to top of fork tube: 168 mm
Zero sag spring collar length: 168 - 15 = 153 mm

For my starting pre-load I cut my Race Tech stack to match the same length as the OEM stack.
Rach Tech Spring collar length: 451 - 276 - 2 = 173 mm
(OEM stack length minus Race Tech spring length and minus spring seat washer)
Starting Race Tech Stack length (same as OEM): 451 mm
Starting pre-load: 173 mm - 153 mm = 20 mm
(Race Tech Spring collar length minus zero sag collar length)

I cut the new collar from PVC tail pipe and assembled the forks with the 451 mm Race Tech stack. You can use the original OEM spacer if you can cut it accurately. With every thing back on the bike I measured my squishy new sag. Sag: 46 mm. Augh, got to take it all apart and add Race Tech spacers.

Took forks off, braced forks up right, took cap off, took spring collar out, and added two Race Tech spacers. You don't need to take anything else (including oil) out of the the forks. The cap has become much harder to push in. (Just to check what it would feel like, I added 4 spacers to one fork stack and barely got the cap on)
Total Stack Length 451 + 3 + 3 = 457 mm
Total Pre-load 20 + 3 + 3 = 26 mm

With everything back on the bike I measured the sag. Sag: 38 mm. Augh, got to take it all apart and add another Race Tech spacer. Note: 6 mm of spacers where added and sag decreased by 8 mm. I have no way of determining the geometric rate of decreased sag to increased pre-load, so this becomes trial and terror,... meaning I gotta do it again.

Took forks off, braced forks up right, took caps off, took Spring collars out and added one more Race Tech spacer for a total of 3 spacers. The stack was much harder to push the cap down to screw it in.
Total Stack Length 457 + 3 = 460 mm
Total Pre-load 26 + 3 = 29 mm

With everything back on the bike, I measured the sag. Sag: 33 mm. Note: An additional 3 mm spacer reduced sag by 6 mm.

With my tupperware back on and me in my riding gear, I measured my sag at 35mm.

What about the safety of using a sink PVC tail piece? I am not an engineer nor is this a Honda approved solution for spring collars. This was something I did to put myself at ease about using PCV pipe for spring collars. This is a decision you need to make for yourself if you use PVC.
I found a discussion thread on a physics forum that checked the PVC Pipe Buckling load. See
http://www.physicsforums.com/showthread.php?t=411953
You can find an approximate number for PVC buckling load using the buckling formula from Euler.
Buckling force F = (pi^2)*(E)*(I)/(L^2)

I= width inches
This was a cross section which the engineer roughed out to the width of 3" PVC pipe. The wall width of a 3" PVC pipe is 7mm while the wall width of 1 1/2
PVC tail pipe is 2mm. I used the ratio of 2/7 to use as the loaded cross section for the tail pipe. Being conservative here. Since it's a percentage of the 3"
cross section there's no need to convert to inches.
E = 400000 lbs / in^2 (Young's modulus of Elasticity: PVC Tensile Modulus of Elasticity, psi @ 73?F is 400,000 to 420,000 depending on manufacturer)
L = length inches
I used the 6.8 inch length of my collar spacer converted to inches from mm.
F = result is pounds pressure to buckle PVC spacer
Engineer's recommended safety buffer; back off 1/3

3.14^2 * 400,000 * .286 / 6.8^2 (formula with numbers plugged in)

9.891 * 400,000 * .286 / 46.24 = 24470.813 * .66 = 16150.737 lbs
Yup, looks like it'll be plenty strong; especially, considering there's two of them.


The easy way to measure Sag:
I found an easier method of measuring. I used the L1 - (L2-L3)/2 method but drew a line on the fork with a fine tipped Sharpie where the fork dust seal is.
I only had to make two measurements. The lines wipes off with alcohol when you're done.
- Measure L2 by first pushing down on handle bars and then seated on bike. Draw a line at the fork dust seal with the Sharpie.
- Measure L3 Second pulling up on the handle bars and then seated on bike. Draw a line at the fork dust seal with the Sharpie.
- Lift front wheel off ground and measure from the fork dust cover to the lines on the fork. Add both together and divided by 2. That's your sag.
This method allows the tape measurer to lie flat against the points being measured. Best for accuracy.
 

dduelin

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TMI

I'm amazed you had 38 mm sag with OEM springs. I weigh 155 in street clothes and my sag with OEM springs was 47 mm. Did you find the original springs and 200mm spacers in there?

RT springs are straight rate so there is no geometric rate of change. Adding 8 mm of spacers should raise ride height by 8 mm. If the rate was progressive like OEM springs the relationship of spacer increase is not linear.
 
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Reginald

Reginald

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I'm amazed you had 38 mm sag with OEM springs. I weigh 155 in street clothes and my sag with OEM springs was 47 mm. Did you find the original springs and 200mm spacers in there?
My rear shock was already replaced by a Race Tech rebuild. This took 55% of the weight and static sag. The front fork static sag was close to what I expected.

What I didn't expect was the number of shims I had to add; hence, all the write up. For a newbie at setting the sag I didn't have a good perspective on how the springs would react. For other newbies this gives an explanation of what to expect. For experienced mechanics and suspension experts it's definitely TMI.
 

dduelin

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My rear shock was already replaced by a Race Tech rebuild. This took 55% of the weight and static sag. The front fork static sag was close to what I expected.

.
That makes no sense. The F/R weight balance is not affected by changing the rear shock....... at least not as much to reduce 11 mm of fork sag from stock. If anything, jacking up the rear end with the new RT shock would shift a little weight forward and increase fork sag. Just measure the length of the springs and spacers you found in the forks - the length will tell us if they were OEM.
 

dduelin

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That makes no sense. The F/R weight balance is not affected by changing the rear shock....... at least not as much to reduce 11 mm of fork sag from stock. If anything, jacking up the rear end with the new RT shock would shift a little weight forward and increase fork sag. Just measure the length of the springs and spacers you found in the forks - the length will tell us if they were OEM.
I'm sorry, you did give the length of the springs and spacers you found in there (249 and 200 mm). The spring length doesn't sound like OEM but I can't reference the SM right now.
 
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Reginald

Reginald

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That makes no sense. The F/R weight balance is not affected by changing the rear shock....... at least not as much to reduce 11 mm of fork sag from stock. If anything, jacking up the rear end with the new RT shock would shift a little weight forward and increase fork sag. Just measure the length of the springs and spacers you found in the forks - the length will tell us if they were OEM.
What's important is the starting sag was not good. My wife took the measurements for the sag before I started. Was it exactly 38 mm? That's why I started having her draw the line on the fork as I upgraded; way better accuracy. By the way, I'm grateful for my wife's help. You're going to have to get me a time machine to get a more accurate measurement for the original sag. ;)

The OEM measurements are in the write-up.
 

dduelin

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Hey no worries I just noted to myself that over the years at various tech events where we fooled around with checking or setting sag the stock riders sag (forks) with average ST owners is close to 50 mm or more. If yours started at 38 I thought the springs had already been replaced with aftermarket or the measuring methodology used was incorrect and that same methodology might explain why you had to add more spacer length than originally though in order to hit your target sag. What's important is that you like he changes.
 
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When I replaced my front springs with .95 race-tech I had the almost the same outcome as you did. I started out with the same combined length as the stock stack giving me about 15mm preload as racetech recommended. But I soon had to add about 9mm spacers to get the proper sag. This required a little more effort to replace the cap then before. I ended up with around 35 mm rider sag.
 
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Reginald

Reginald

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There also is some help there in figuring out a good place to start with preload
Yup, I got this with my springs plus other explanations for determining zero preload length. None of this helped me with determining final pre-load in one go. They said as much in their documentation.

It'd be interesting to hear from somebody who got it right on the first shot.
 
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I have always thought of Race-Tech's spring calculator as decent for front springs, and perhaps on the stiff side for rears, but in this case for a heavier machine like the ST, and given the extra preload you needed, in hindsight, it indicates the .95 springs are a little light. Sounds like one weighing 210 might require a 1.0 or 1.1 spring for this bike. I weigh similar and used 1.0 on the FJR, and still found I had to dial in the preload to half way (approx. another 10 mm over initial) to get 35mm of sag. However, I think the additional preload you had to add is quite OK..... too bad there wasn't adjustable fork caps......
 
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Here is an amusing GoPro vid of the forks - not meant for serious consideration (arguments). Zero sag(#1 see below) is when the bottom of the zip tie lines up with the bottom of the black fork protector. Black lines on the zip tie are one inch apart. My sag is about 30 mm on level ground, steady speed, or 1.2 inches.

Bill

http://youtu.be/VVE1sAQSGbs

#1 - Forks fully extended by lifting the front wheel off the ground. Does not account for anti top-out springs keeping the forks from fully extending.
 
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mlheck

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While the 1300 doesn't come with adjustable preload forks, Racetech does make a conversion kit. It does cost a little more, but makes setting sag much easier. I also like being to add some preload to the front and rear when riding 2 up.

Sent from my XT907 using Tapatalk
 

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dduelin

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Umm.. how do we calculate in top-out spring loading if we don't have one? Just re-did my forks and didn;t see anything that would correspond to a spring like that.
Did I miss it?
There is a short stiff spring on the exterior of the damping cartridge but I thought it functioned as a bottoming spring because it is fixed on the upper end and free on the bottom end.
 

T_C

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There is a short stiff spring on the exterior of the damping cartridge but I thought it functioned as a bottoming spring because it is fixed on the upper end and free on the bottom end.
I thought so too.. that is why I was wondering how it applied to the pre-load.
 

ST Gui

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too bad there wasn't adjustable fork caps......
+1 I had some on my '77 GL and they were great. An air valve on one cap a balance tube and a fitting on the other cap for the tube. A small tire-pump syringe fit on the valve to pump up the forks. It held the air during removal until the valve closed so there was no air loss as when filling tires.

I used this in conduction with a fork spring kit from some popular manufacturer at the time. Added some PVC spacers and the ride was great.
 
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I'm sorry, you did give the length of the springs and spacers you found in there (249 and 200 mm). The spring length doesn't sound like OEM but I can't reference the SM right now.
249.6 new, 244.6 sack limit. I'd comment that shimming for preload is not shimming height, it is adjusting fully extended for preload (and you are also modifying frame geometry slightly). If you are running the same oil level (same initial air volume), the air pressure (fork force) stays on the same progressive curve, while the spring force is higher for a given fork compression amount. The RaceTech suspension bible is a good book to have on your shelf, even if you are not an active tuner. Also give procedure for doing what you developed that sounds simpler.

I would suggest you look at chemical compatibility of PVC with oil - could be an issue. Also, the extra thin-wall sink tailpiece tubing might not be the best bet. What yield strength did you use for the PVC tube in the buckling formula? Different PVC formulations have different mechanical properties.

Me personally - I'm not relying on plumbing plastic (never meant to be pressurized or load bearing) for a MC load-bearing suspension part. If that piece collapses on hard braking or a big bump/pothole, the front end will collapse, and that is NEVER a good thing. My free opinion, FWIW.
 

dduelin

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249.6 new, 244.6 sack limit. I'd comment that shimming for preload is not shimming height, it is adjusting fully extended for preload (and you are also modifying frame geometry slightly). If you are running the same oil level (same initial air volume), the air pressure (fork force) stays on the same progressive curve, while the spring force is higher for a given fork compression amount. The RaceTech suspension bible is a good book to have on your shelf, even if you are not an active tuner. Also give procedure for doing what you developed that sounds simpler.

I would suggest you look at chemical compatibility of PVC with oil - could be an issue. Also, the extra thin-wall sink tailpiece tubing might not be the best bet. What yield strength did you use for the PVC tube in the buckling formula? Different PVC formulations have different mechanical properties.

Me personally - I'm not relying on plumbing plastic (never meant to be pressurized or load bearing) for a MC load-bearing suspension part. If that piece collapses on hard braking or a big bump/pothole, the front end will collapse, and that is NEVER a good thing. My free opinion, FWIW.
My book shelf go-to reference is Trevett's Sportbike Suspension Tuning. The founder of Race Tech, Paul Thede, is used as a reference for this book's measurement methods. Many tuning guides refer to Thede.

I hope the PVC holds up...........its been in there 85,000 miles and PVC spacers are used by aftermarket spring suppliers and tuners. I see them them every 15 to 20,000 miles when I change the fork oil, they are plenty strong. Have you measured the load bearing capability of a 45 mm OD steel tube with .5 mm wall thickness?
 

T_C

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I would suggest you look at chemical compatibility of PVC with oil - could be an issue.
Nope... not an issue at all.


Also, the extra thin-wall sink tailpiece tubing might not be the best bet.
Not sure how tight it fits to the outside wall, but if it is a close fit the tubing can't buckle till you get a few tons applied.
 
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