Fork Preload Questions

RobbieAG · Jan 3, 2017

It?s time for my ST1300?s annual winter maintenance. This year it includes new tires (Bridgestone T30 GTs ordered). While I have the front wheel off, I want to give the forks some attention. I checked the sag and it was well above the recommended 35 ? 40 mm. It?s about 51 mm. I weigh 180. The rear was okay. I was thinking of getting new springs, but after doing some reading on here, I?ve decided to go with new spacers to increase the preload.

Is there a way to calculate how much to increase the spacer length for my scenario? I?m hoping I can get this right the first time without too much trial and error. Is 1.5 inch PVC what I need for replacement spacers? Also, I gathered from my reading, that some change the fork oil level from factory spec. Are you raising it or lowering it? I?ve never had an issue with the forks bottoming out. I understand that since the air has a progressive effect, the less air there is, the more progressive (harder) it is as it compresses (especially past midpoint). Am I correct to say that raising the level in effect increases the spring rate? According to the RaceTech site, the stock spring rate is .86 and my recommended rate is .90, so a bit of an increase might be a good thing. So far, I?m planning to use 10 weight fork oil (since I have a bottle sitting on my shelf). Any reason to use something else?

I?ll inspect the seals and bushings to ensure they look good. The bike has 65k miles. Thanks in advance.

dwalby · Jan 3, 2017

the additional spacer length would just be subtracted from your sag measurement. If you want to go from 51mm to 35mm, add 16mm to the spacer. Now when the fork compresses the spring 35mm from the bottom, its also compressed 16mm more from the top than before, so the same effective spring compression as 51mm with the shorter spacer.

the air inside the fork will act as you say, less air, higher compression rate. Your fork travel determines the compression rate. Say you have 150mm of air above the oil, if the fork compresses 75mm, you double the pressure. Raise the oil level to 100mm (of air), now you get a pressure doubling at 50mm fork compression, and pressure is quadrupled at 75mm fork compression.

dduelin · Jan 3, 2017

Is your weight of 180 in riding gear? I'm 172-175 lbs geared up and 16 mm of extra preload gave me 11 mm less sag (47 to target of 36). OEM springs are rising rate so the amount of preload is not 1:1 for sag reduction. Adding preload works on the softest coils first not the higher rate further up the spring.

1.5" OD fits closely in the fork tubes. I used PVC sink tailpieces for an exact fit.

Air gap does affect progression of spring rate in the last third of fork compression. I stayed with the stock level and retain about 10 mm of travel under threshold braking.

Oil weight by brand does matter. There are "5 to 15wt" fork oils that have similar viscosity as 10 wts and vice versa. If I did not use Honda branded Showa oil I'd try and match whatever to a viscosity in centistokes that approximates Honda SS7 5wt or slightly thicker. The stock set up is under spring and overdamped. Getting the sag right and slightly reducing damping worked really well for me. I mix my own oil with a 2:1 ratio of SS7 & SS8.

dwalby · Jan 3, 2017

dduelin said:
Is your weight of 180 in riding gear? I'm 172-175 lbs geared up and 16 mm of extra preload gave me 11 mm less sag (47 to target of 36). OEM springs are rising rate so the amount of preload is not 1:1 for sag reduction. Adding preload works on the softest coils first not the higher rate further up the spring.

preload/sag calculation should be the same regardless of spring type. If you compress the spring by any given number of mm, it doesn't matter if you compress it from one end or the other, you compress the coils the same amount either way. You are correct in saying that the spacer compresses the lighter rate coils first, but it also shortens how far the fork has to travel before the rate gets stiffer, so its all the same in the end.

Kevcules · Jan 3, 2017

I asked a very similar question ........ click link below.

1.5" PVC isn't quite the proper diameter and won't go into the fork caps properly I found. I ended up buying some aluminum pipe and having it cut down a few mm in diameter to 39mm on a lathe.

https://www.st-owners.com/forums/showthread.php?155408-Front-shocks-removal&highlight=

RobbieAG · Jan 3, 2017

dduelin said:
Is your weight of 180 in riding gear?

No, that's 180 without gear - not sure how much the gear would add as it varies depending on weather and how many layers I have on (maybe 5 - 10 pounds?). Generally I'm an ATGATT rider.

dduelin said:
1.5" OD fits closely in the fork tubes. I used PVC sink tailpieces for an exact fit.

Got it - I see a lot of ones that are plastic rather than PVC. I don't know if that would work or I'd need strictly PVC.

dduelin said:
The stock set up is under spring and overdamped. Getting the sag right and slightly reducing damping worked really well for me. I mix my own oil with a 2:1 ratio of SS7 & SS8.

How would you describe "overdamped"? What is the net result of your mixture - is it around 7?

Generally, heavier oil slows down compression and rebound correct?

RobbieAG · Jan 3, 2017

Kevcules said:
I asked a very similar question ........ click link below.

Your thread is the one I was reading. Have you had a chance to try it out? How do you like the changes so far?

dwalby · Jan 3, 2017

one more comment on sag measurement. The friction of the oil seal (generally called stiction) affects the sag measurement with rider on the bike. To compensate for this you take two readings, one after pushing down on the forks and slowly letting it rise, and a second after pulling up on the forks and slowly letting it sink. You will get two different measurements, take the average of the two. Its been a long time since I did any adjusting, but its not exactly a precision operation with exact repeatable results, there's usually a bit of variation on multiple trials even with the same spacer length because of the stiction effects.

dduelin · Jan 3, 2017

dwalby said:
preload/sag calculation should be the same regardless of spring type. If you compress the spring by any given number of mm, it doesn't matter if you compress it from one end or the other, you compress the coils the same amount either way. You are correct in saying that the spacer compresses the lighter rate coils first, but it also shortens how far the fork has to travel before the rate gets stiffer, so its all the same in the end.

You are correct that it doesn't matter if you compress the spring from one end or the other - the force applied is the same. However, with a rising rate spring the amount the spring compresses under a given force changes as the tighter coils of the "soft end" become coilbound. That is how a progressive spring changes spring rate from its initial softer rate to a stiffer rate during spring compression. The relationship of preload to sag reduction is not linear as it would be with a straight rate spring.

DbleDutch · Jan 3, 2017

RobbieAG said:
I checked the sag and it was well above the recommended 35 – 40 mm. It’s about 51 mm. I weigh 180.

My suggestion is like the "carpenters rule - measure twice, cut once". Be certain of your sag measurements and your goal figure. I agree with you your weight and oem spring rate merit a simple addition of pre-load. Possibly take the stock preload washer to the hardware store for a washer match rather than the PVC. Nothing wrong with the PVC, I used the high temp thick walled PVC when I added my RaceTech springs. I also lowered the stock Honda fork oil level to relax the damping.

I am pleased with my adjustments which were done primarily due to Two up touring.

Best to you in your project, ride far and safe. :03biker:

dduelin · Jan 3, 2017

RobbieAG said:
No, that's 180 without gear - not sure how much the gear would add as it varies depending on weather and how many layers I have on (maybe 5 - 10 pounds?). Generally I'm an ATGATT rider.

Got it - I see a lot of ones that are plastic rather than PVC. I don't know if that would work or I'd need strictly PVC.

How would you describe "overdamped"? What is the net result of your mixture - is it around 7?

Generally, heavier oil slows down compression and rebound correct?

I think that you will find that using more than 16-18mm of preload it becomes difficult to compress the assembly and thread the fork cap on. This limits your target sag at 180 to a value greater than 36 mm - the spring calculator suggests a rate of about .92 for your weight and the stock spring is about .86. A rider of about 150-155 lbs can hit 36 mm with 15-16 mm longer spacers. 20 or 25 lbs more and you will have a bit more sag. Preload is a not a cure for springs with too low a rate.

Sink tailpieces are PVC. The OD of stock sizes of thin wall or thick wall PVC don't give the 1.5" or 38-39 mm OD of the stock 200 mm long spacers, or at least I could not find it.

Yes, heavier oil flows slower and slows rebound and compression damping. On the ST1300 overdamping makes itself known with a plush ride over smooth pavement and small bumps then harsher on larger or sharp edged bumps that cause faster fork movements. I'm using a mix that is 6.7 "weight".

RobbieAG · Jan 3, 2017

dwalby said:
If you want to go from 51mm to 35mm, add 16mm to the spacer.

dduelin said:
16 mm of extra preload gave me 11 mm less sag (47 to target of 36). OEM springs are rising rate so the amount of preload is not 1:1 for sag reduction. Adding preload works on the softest coils first not the higher rate further up the spring.

So it sounds like I'll need a minimum of at least an additional 16 mm of spacer. Maybe I should go 20 or more to be safe.

dwalby · Jan 3, 2017

dduelin said:
You are correct that it doesn't matter if you compress the spring from one end or the other - the force applied is the same. However, with a rising rate spring the amount the spring compresses under a given force changes as the tighter coils of the "soft end" become coilbound. That is how a progressive spring changes spring rate from its initial softer rate to a stiffer rate during spring compression. The relationship of preload to sag reduction is not linear as it would be with a straight rate spring.

With a constant rate spring its easy to figure how much force (weight) the spring is supporting for a given length spacer and given fork sag measurement. Let's use the variable D to represent the total amount the spring is compressed by the spacer plus the fork sag. It doesn't really matter how much was due to the spacer, and how much was due to the fork sag, its only the sum of the two that matters. So the force exerted by the spring is D mm * X kg/mm, which we will call Y kg.

A progressive spring compresses by whatever distance it needs to balance out the same Y kg force from the front wheel. We don't need to know exactly how it changes as a function of distance, we just need to know the total distance required to balance out the force Y at the front wheel. Let's call that distance D2, to differentiate it from the distance D of the constant rate spring.

The distance D2 for the progressive spring is directly related to the force on the front wheel from the weight of the bike. If that weight doesn't change, which it doesn't, the distance D2 that we need to compress the spring also will not change. Since D2 = the sum of the spacer and fork sag, if we add distance to the spacer, the fork sag will have to be reduced by the same amount to maintain the force balance associated with the spring being compressed D2 mm to counteract the Y kg force on the wheel.

Your argument is saying that with the original spacer the distance D2 is required to balance the bike, and with a longer spacer the distance D2 + 5mm is required to balance the bike (16mm longer spacer, with only 11mm less sag means the spring was compressed by 5mm more than originally). The math just doesn't add up. Regardless of the spring rate, constant or variable, the force Y from the spring being compressed by D2 mm, is not the same as the force of the spring being compressed by D2 + 5 mm. So you will have more force from the spring than weight force on the wheel, it can't compress another 5mm and maintain a balance of forces.

dduelin · Jan 4, 2017

dwalby said:
With a constant rate spring its easy to figure how much force (weight) the spring is supporting for a given length spacer and given fork sag measurement. Let's use the variable D to represent the total amount the spring is compressed by the spacer plus the fork sag. It doesn't really matter how much was due to the spacer, and how much was due to the fork sag, its only the sum of the two that matters. So the force exerted by the spring is D mm * X kg/mm, which we will call Y kg.

A progressive spring compresses by whatever distance it needs to balance out the same Y kg force from the front wheel. We don't need to know exactly how it changes as a function of distance, we just need to know the total distance required to balance out the force Y at the front wheel. Let's call that distance D2, to differentiate it from the distance D of the constant rate spring.

The distance D2 for the progressive spring is directly related to the force on the front wheel from the weight of the bike. If that weight doesn't change, which it doesn't, the distance D2 that we need to compress the spring also will not change. Since D2 = the sum of the spacer and fork sag, if we add distance to the spacer, the fork sag will have to be reduced by the same amount to maintain the force balance associated with the spring being compressed D2 mm to counteract the Y kg force on the wheel.

Your argument is saying that with the original spacer the distance D2 is required to balance the bike, and with a longer spacer the distance D2 + 5mm is required to balance the bike (16mm longer spacer, with only 11mm less sag means the spring was compressed by 5mm more than originally). The math just doesn't add up. Regardless of the spring rate, constant or variable, the force Y from the spring being compressed by D2 mm, is not the same as the force of the spring being compressed by D2 + 5 mm. So you will have more force from the spring than weight force on the wheel, it can't compress another 5mm and maintain a balance of forces.

I'm not going to derail this thread beyond this point but this I know from experience with the ST1300 and other bikes in my garage. I've set sag on three of my bikes that have progressive springs and one with straight rate springs and the amount of extra preload in mm is not the amount of sag reduction on progressive springs.

RobbieAG · Jan 4, 2017

dduelin said:
I think that you will find that using more than 16-18mm of preload it becomes difficult to compress the assembly and thread the fork cap on. This limits your target sag at 180 to a value greater than 36 mm - the spring calculator suggests a rate of about .92 for your weight and the stock spring is about .86. A rider of about 150-155 lbs can hit 36 mm with 15-16 mm longer spacers. 20 or 25 lbs more and you will have a bit more sag. Preload is a not a cure for springs with too low a rate.

I've been wondering how much you could increase it and still get it back together. I'm reconsidering using the OEM springs. It sounds like the stock springs are not strong enough for my weight to get enough preload.

I may do the following and see what happens. Then if I'm not happy with that, get the new springs.

1) increase spacer length by 16 mm
2) raise the oil level by a small amount (like 5 mm) to increase the spring rate
3) use a lighter oil like 7 weight

I found this link that should help comparing the weights of different manufacturer's fluids

http://www.peterverdone.com/wiki/index.php?title=Suspension_Fluid

dduelin · Jan 4, 2017

That sounds like a good plan. Start with the easy inexpensive things first and if the results are not what you desire you can go from there.

The fork fluid chart is a good point of reference.

SMSW · Jan 4, 2017

dduelin said:
Is your weight of 180 in riding gear? I'm 172-175 lbs geared up and 16 mm of extra preload gave me 11 mm less sag (47 to target of 36). OEM springs are rising rate so the amount of preload is not 1:1 for sag reduction. Adding preload works on the softest coils first not the higher rate further up the spring.

1.5" OD fits closely in the fork tubes. I used PVC sink tailpieces for an exact fit.

Air gap does affect progression of spring rate in the last third of fork compression. I stayed with the stock level and retain about 10 mm of travel under threshold braking.

Oil weight by brand does matter. There are "5 to 15wt" fork oils that have similar viscosity as 10 wts and vice versa. If I did not use Honda branded Showa oil I'd try and match whatever to a viscosity in centistokes that approximates Honda SS7 5wt or slightly thicker. The stock set up is under spring and overdamped. Getting the sag right and slightly reducing damping worked really well for me. I mix my own oil with a 2:1 ratio of SS7 & SS8.

Kevcules said:
I asked a very similar question ........ click link below.

1.5" PVC isn't quite the proper diameter and won't go into the fork caps properly I found. I ended up buying some aluminum pipe and having it cut down a few mm in diameter to 39mm on a lathe.

https://www.st-owners.com/forums/showthread.php?155408-Front-shocks-removal&highlight=

Ok, I'm confused. Does a 1 1/2 OD pvc tailpiece fit inside the fork tube as a spacer or does it not? We have two opinions here. Thanks to both of you.

dduelin · Jan 4, 2017

SMSW said:
Ok, I'm confused. Does a 1 1/2 OD pvc tailpiece fit inside the fork tube as a spacer or does it not? We have two opinions here. Thanks to both of you.

1.5" OD of the tail piece I used is a very close fit, virtually identical to the stock spacer OD. 38-39 mm is 1.496" to 1.535".

Kevcules · Jan 4, 2017

SMSW said:
Ok, I'm confused. Does a 1 1/2 OD pvc tailpiece fit inside the fork tube as a spacer or does it not? We have two opinions here. Thanks to both of you.

Sorry to confuse you.

Dduelin knows his stuff ,he's played around with these springs a lot where it's my first time. I used his advice and the others here to change my spring pre-load and sag. He says that a sink tailpiece is the perfect diameter for the shock. I didn't use the sink tailpiece because I couldn't find a piece long enough. I was afraid of the thin wall too,but he says he's used it for some time and I believe him.
I checked a piece of 1.5" grey PVC and I found that it wouldn't go into the fork cap very well. (try a piece and see what I mean) I forget if it was the OD or the ID that was interfering with it seating into the fork cap. I went ahead and bought some aluminum pipe. 1.5" i believe. I had my caliper right there measuring the OD too. I measured 42 mm OD and I knew I needed around 38mm to fit into the shock. Since the wall of the aluminum pipe was fairly thick, I was confident I could have someone turn it down on a lathe at work a few mm and then try it. I had it shaved to 39mm , tried it and it fit perfectly.
I don't know if the changes I made to my front suspension will be perfect for me, but I know it will be better. I'm in the right "sag" range thanks to the guys here. I won't be able to try my bike for at least another few months. It's probably hard to drive these heavy bikes in the snow.

JimGregory · Jan 5, 2017

If you go to home depot or the like you can find a grab rail for the bathroom with a 1.5" OD in stainless or aluminum (I forget which). You cut off what you like. I think they are 10 to 15 bucks for a lifetime supply. Or you can get the PVC tail piece.

Fork Preload Questions

RobbieAG

Robert

dwalby

dduelin

Tune my heart to sing Thy grace

dwalby

Kevcules

RobbieAG

Robert

RobbieAG

Robert

dwalby

dduelin

Tune my heart to sing Thy grace

DbleDutch

dduelin

Tune my heart to sing Thy grace

RobbieAG

Robert

dwalby

dduelin

Tune my heart to sing Thy grace

RobbieAG

Robert

dduelin

Tune my heart to sing Thy grace

SMSW

dduelin

Tune my heart to sing Thy grace

Kevcules

JimGregory