Reginald
cyclepoke
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.
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.