-
Enjoy XS650.com? Consider making a donation to help support the site.
XS650.com receives a small share of sales from some links on this page, but direct donations have a much greater impact on keeping this site going.
Progressive Fork Springs, XS1B
- Thread starter Ipa62
- Start date
Great! Please let us know how the task goes once the parts are in.
I will, but it could be a couple weeks before I get back as I am also doing lots of other work on the bike. As for ride comparison, I will have some what of a hard time judging whether it is better or worse, last time I rode an XS1B (or any XS for that matter) was over 40 years ago, and this bike has not been operational for over 30 years. It's all fun and I am looking forward to the first kick.
I've never fooled with progressive springs, but real curious about their setup.
Griz and 5twins, please correct my assumptions here.
The springs in MaxPete's pic appear to show a dual pitch spring, not a progressive wind. The fine pitch portion would have a lower spring rate, coarse pitch with higher spring rate. The overall spring rate would be a linear composite of the two, until coilbind of the finer coils, at which point the spring rate would be higher, from the coarse winds.
The question is, where do you want that coilbind, if at all, relative to the laden sag point?
My thinking is:
If finepitch coilbind occurs AT the laden sag, then you get the stiffer spring rate during compression, and the unbound weaker rate during rebound, possibly assisting rebound damping.
If the finepitch coilbind were set somewhere above laden sag, say about 1" to 1-1/2", then you get the benefit of a smooth ride during normal, minor, road undulations. AND get the stiffer spring during harsher road conditions. This is similar to the dual-rate shocks I've run on my cars, glide on the highways, stiffer suspension on rough roads.
Is this thinking right? Any docs on this?
Griz and 5twins, please correct my assumptions here.
The springs in MaxPete's pic appear to show a dual pitch spring, not a progressive wind. The fine pitch portion would have a lower spring rate, coarse pitch with higher spring rate. The overall spring rate would be a linear composite of the two, until coilbind of the finer coils, at which point the spring rate would be higher, from the coarse winds.
The question is, where do you want that coilbind, if at all, relative to the laden sag point?
My thinking is:
If finepitch coilbind occurs AT the laden sag, then you get the stiffer spring rate during compression, and the unbound weaker rate during rebound, possibly assisting rebound damping.
If the finepitch coilbind were set somewhere above laden sag, say about 1" to 1-1/2", then you get the benefit of a smooth ride during normal, minor, road undulations. AND get the stiffer spring during harsher road conditions. This is similar to the dual-rate shocks I've run on my cars, glide on the highways, stiffer suspension on rough roads.
Is this thinking right? Any docs on this?
Last edited:
If I may....you are correct - the wire from which the new spring has been wound is of uniform cross-section, but there are two distinct coil spacing or "pitches":
Anyhow, the tightly spaced coils form a relatively "soft" spring - until the coils are compressed or collapsed or "bunched" at which point that shorter portion becomes very stiff. That bunching would occur over a fairly short fork travel (not sure how much - maybe...1/2 inch or less). Once the tight coils have bunched, the widely spaced coils really start to deflect until all of the available fork travel has been consumed. This layout allows the fork to absorb small bumps very easily, but ensures that the fork can still deal with a really big bump.
Basically, the dual rate spring is a compromise and like all engineering compromises, there are advantages and disadvantages in the design. The advantage is that on a smooth road, the bike should ride very softly with little energy (ie. bumpiness) being transmitted to the rider. However, on a bumpy road, the bike has less travel available to absorb the larger deflections and bottoming may occur more easily than if a full length linear spring were used. Also, the XS650 has a relatively primitive damping mechanism which cannot adapt to the two spring rates available from the dual rate springs. The damping therefore, is also a compromise.
Anyhow, as I have said, the original spring were badly fatigued and had lost a good deal of their original stiffness. Wither new dual rate springs, the bike now rides very nicely and I am happy with it.
Pete
- a short section with closely spaced coils at the upper end of the fork;
- a much longer section with wider spaced coils.
Anyhow, the tightly spaced coils form a relatively "soft" spring - until the coils are compressed or collapsed or "bunched" at which point that shorter portion becomes very stiff. That bunching would occur over a fairly short fork travel (not sure how much - maybe...1/2 inch or less). Once the tight coils have bunched, the widely spaced coils really start to deflect until all of the available fork travel has been consumed. This layout allows the fork to absorb small bumps very easily, but ensures that the fork can still deal with a really big bump.
Basically, the dual rate spring is a compromise and like all engineering compromises, there are advantages and disadvantages in the design. The advantage is that on a smooth road, the bike should ride very softly with little energy (ie. bumpiness) being transmitted to the rider. However, on a bumpy road, the bike has less travel available to absorb the larger deflections and bottoming may occur more easily than if a full length linear spring were used. Also, the XS650 has a relatively primitive damping mechanism which cannot adapt to the two spring rates available from the dual rate springs. The damping therefore, is also a compromise.
Anyhow, as I have said, the original spring were badly fatigued and had lost a good deal of their original stiffness. Wither new dual rate springs, the bike now rides very nicely and I am happy with it.
Pete
Hmmmm...I just read your post 2M and realized that I haven't really responded to your query.
I'll have to have a good think about those questions you've posed.
I'll have to have a good think about those questions you've posed.
Thanx, Pete. I was hoping the thinking was on track.
I'm thinking there's several events in the spring's compression:
- Free length
- Installed length
- Preload length (bike & rider weight)
- Laden Height (set with spacer, or cut)
- Coilbind of the finer winding
- Fully compressed fork
- Fully compressed (coilbound) spring
And, pondering the use of a length of allthread, with nuts and washers, run thru the spring. Run the nuts down to the aforementioned lengths, observe spring coils.
Then, figure out some scheme of coil cutting, and/or with appropriate spacer, to get the magic combination of laden sag and fine-pitch coilbind. (*gasp*)...
I'm thinking there's several events in the spring's compression:
- Free length
- Installed length
- Preload length (bike & rider weight)
- Laden Height (set with spacer, or cut)
- Coilbind of the finer winding
- Fully compressed fork
- Fully compressed (coilbound) spring
And, pondering the use of a length of allthread, with nuts and washers, run thru the spring. Run the nuts down to the aforementioned lengths, observe spring coils.
Then, figure out some scheme of coil cutting, and/or with appropriate spacer, to get the magic combination of laden sag and fine-pitch coilbind. (*gasp*)...
Last edited:
Yup, I'd say you're on the right track but my poor little 1-hp brain is too tired right now.....
Haha, yeah, ending 2nd shift.
We'll let this cook awhile...
We'll let this cook awhile...
Here's some more interesting info. 2M, because of your question above, I started poking around a bit more for information and found Race Tech (Paul Thede) who seem to be experts, aren't we all. They do sell fork springs for '71 and up XS650, spring selection is based on rider weight and all are 410mm in length for '71, their springs are what they called "straight rate" or wound at the same pitch the full length. For damping and rebound they recommend an emulator valve, which they also sell. the emulator valve sits on top of the dampening rod. Which brings up another question, how does one remove the dampening rod on an XS1b, there's no external fastener holding it and it is not shown on the parts breakdown??? For fluid they recommend filling the fork 150mm (6") from the top with the spring removed and the fork tubes fully compressed together.
Back to the so called progressive spring. for the set I ordered, I could measure the gap between coils on the fine pitch end and then multiply this by the number of winds, this would give an approximate dimension for how much the soft end of the spring would move before fully collapsed. It seems to me that you wouldn't want the soft end fully collapsed from the laden sag, what then would be the point of having this section, ok, maybe for rebound, but this would be totally dependent on rider weight??? Anyway, I'm starting t think maybe cutting the spring a bit may be needed, certainly won't do it until I think this through a bit more and study the springs.
Back to the so called progressive spring. for the set I ordered, I could measure the gap between coils on the fine pitch end and then multiply this by the number of winds, this would give an approximate dimension for how much the soft end of the spring would move before fully collapsed. It seems to me that you wouldn't want the soft end fully collapsed from the laden sag, what then would be the point of having this section, ok, maybe for rebound, but this would be totally dependent on rider weight??? Anyway, I'm starting t think maybe cutting the spring a bit may be needed, certainly won't do it until I think this through a bit more and study the springs.
Once you've got the fork seals replaced and the damper rod installed, removing the spring and changing the fork oil is a really easy job (just be sure to loosen the top pinch bolt BEFORE removing the fork cap). Once the cap is off - you simply lift the spring out (have a rag handy) and after doing whatever to it, you can just drop it back in and re-install the cap).
I'm still suggesting you try the spring as-is before cutting it. It's a lot easy to try it and cut - than cut it and try it.....
I'm still suggesting you try the spring as-is before cutting it. It's a lot easy to try it and cut - than cut it and try it.....
Our early 70-71 steel fork lowers are really quite primitive compared to the later alloy fork lowers. The so-called damper is simply a steel rod, welded to the bottom, and a washer welded on top, which acts as the spring seat. No provision for servicing, no emulator option.
Pete, we don't have a top pinch bolt. These use a simple top cap that also holds down the top tree.
Been doing a lot of studying about progressive springs. A lot has happened in the last 40 years, bit of catching up.
My springs are sacked out. I made appropriate spacers to get my laden sag to around 1" - 1.5". Recommended sag is 1/3 of fork travel. Ours is 130mm travel, making recommended sag about 1.7".
Wish I was there to play with those new springs. Curious if they could be retrofitted to these early forks.
The way I'd approach this is to first get a few dims of the new springs, unmodified, as installed as is.
1- Free length
2- Fork fully extended, spring slid down fully, measure how much sticks out.
3- Length of the fork cap.
4- The exposed spring, plus the cap length, gives the initial compression.
5- Free length minus the initial compression gives the installed length.
6- Install the springs, no spacers.
7- With rider on bike, get the current sag, if any.
8- Installed length minus this sag gives sag length.
9- Remove springs.
10- Using the allthread, compress the spring down to the sag length.
11- Is the fine pitch spring coilbound?
12- If not, continue compressing until fine coilbind. Take measurement.
Grind thru this to see if the springs will work, or can be made to work...
Pete, we don't have a top pinch bolt. These use a simple top cap that also holds down the top tree.
Been doing a lot of studying about progressive springs. A lot has happened in the last 40 years, bit of catching up.
My springs are sacked out. I made appropriate spacers to get my laden sag to around 1" - 1.5". Recommended sag is 1/3 of fork travel. Ours is 130mm travel, making recommended sag about 1.7".
Wish I was there to play with those new springs. Curious if they could be retrofitted to these early forks.
The way I'd approach this is to first get a few dims of the new springs, unmodified, as installed as is.
1- Free length
2- Fork fully extended, spring slid down fully, measure how much sticks out.
3- Length of the fork cap.
4- The exposed spring, plus the cap length, gives the initial compression.
5- Free length minus the initial compression gives the installed length.
6- Install the springs, no spacers.
7- With rider on bike, get the current sag, if any.
8- Installed length minus this sag gives sag length.
9- Remove springs.
10- Using the allthread, compress the spring down to the sag length.
11- Is the fine pitch spring coilbound?
12- If not, continue compressing until fine coilbind. Take measurement.
Grind thru this to see if the springs will work, or can be made to work...
Interesting - and apologies. I hadn't realized how different the early fork seals were from the mid-70s units.
Watching
with interest.
Watching
with interest.Hi, received the springs a couple days ago, thought I would pass on a little info about them.
Length = 495mm
Wire diameter = 4mm
Fine pitch length = 155mm (approximate)
Coarse pitch length = 340mm (approximate)
Fine pitch spacing = 1.85mm with 26 spaces (space between coils, average)
Coarse pitch spacing = 4.4mm with 40 spaces (space between coils, average)
Yes, I know the numbers don't exactly add up, but this is a spring after all.
Measurements were taken with a digital vernier caliper.
There is 434mm space in the fork tube for the new spring (with the cap in place).
Next I put the spring on a piece of 1/2" all thread and compressed it down to 434mm and measured the following).
Fine pitch length = 130mm (approximate)
Coarse pitch length = 302mm (approximate)
Fine pitch spacing = .9mm with 26 spaces (measured with a feeler gauge, space between coils, average)
Coarse pitch spacing = 3.55mm with 40 spaces (space between coils, average)
Again, the numbers don't exactly add up, but I think this gives a good representation on what is happening with the spring under compression. The fine pitched end is not fully collapsed (coilbound as 2M says) and the coarse end does have some compression.
Next step.... try to wrestle the springs down into the fork tubes and get the caps on. I don't see this as an easy task, as the fork tubes have to be installed in the triple clamps first and then I have to compress the springs 60mm. Another question would be oil, anyone have any recommendations? The manual calls for 223cc's of 30wt. Once installed I'll get the all important sag length.
Length = 495mm
Wire diameter = 4mm
Fine pitch length = 155mm (approximate)
Coarse pitch length = 340mm (approximate)
Fine pitch spacing = 1.85mm with 26 spaces (space between coils, average)
Coarse pitch spacing = 4.4mm with 40 spaces (space between coils, average)
Yes, I know the numbers don't exactly add up, but this is a spring after all.
Measurements were taken with a digital vernier caliper.
There is 434mm space in the fork tube for the new spring (with the cap in place).
Next I put the spring on a piece of 1/2" all thread and compressed it down to 434mm and measured the following).
Fine pitch length = 130mm (approximate)
Coarse pitch length = 302mm (approximate)
Fine pitch spacing = .9mm with 26 spaces (measured with a feeler gauge, space between coils, average)
Coarse pitch spacing = 3.55mm with 40 spaces (space between coils, average)
Again, the numbers don't exactly add up, but I think this gives a good representation on what is happening with the spring under compression. The fine pitched end is not fully collapsed (coilbound as 2M says) and the coarse end does have some compression.
Next step.... try to wrestle the springs down into the fork tubes and get the caps on. I don't see this as an easy task, as the fork tubes have to be installed in the triple clamps first and then I have to compress the springs 60mm. Another question would be oil, anyone have any recommendations? The manual calls for 223cc's of 30wt. Once installed I'll get the all important sag length.
30 wt. or 20wt. non-detergent oil is available at some farm/rural stores. Non-detergent because this is a hydraulic application and you don't want the foam that detergent oil has. This non-detergent oil is also used in compressors. I just don't like to pay the extra for oil with fork, compressor, hydraulic on the label, when it is all the same thing and more than likely from the same manufacturer, no matter what the label says.
Scott
Scott
Great numbers, lpa62!
From your preliminary measurements, I was able to derive a predictive guesstimation of your spring lengths, for the various compressions:
495mm - Free length
434mm - Installed length
391mm - Sag, idealized at 1/3 of 130mm fork travel
370mm - Coilbind of fine pitch coils
304mm - Fork bottomed out
264mm - Coilbind of coarse pitch coils, fully compressed
This prediction shows that the transition from light to heavy spring rate occurs after another 21mm of fork travel from ideal sag position, with 66mm travel on the coarse/heavier springrate. Implying that your fork spring setup as-is would probably be good.
Anxious to see if these values come close to your measurements...
From your preliminary measurements, I was able to derive a predictive guesstimation of your spring lengths, for the various compressions:
495mm - Free length
434mm - Installed length
391mm - Sag, idealized at 1/3 of 130mm fork travel
370mm - Coilbind of fine pitch coils
304mm - Fork bottomed out
264mm - Coilbind of coarse pitch coils, fully compressed
This prediction shows that the transition from light to heavy spring rate occurs after another 21mm of fork travel from ideal sag position, with 66mm travel on the coarse/heavier springrate. Implying that your fork spring setup as-is would probably be good.
Anxious to see if these values come close to your measurements...
Cool 2M, but can he get the caps on without cutting them? 61mm, just a hair over 2-3/8", that looks close to the maximum push.
Scott
Scott
Scott, you are correct, it will be a heavy push and difficult to hold and turn wrench to get threads of cap started. The good news is that I don't have to push it in all the 2.4 inches to engage the threads, maybe 2 inches or a little less. Will keep you posted on how it goes. Oh, and BTW, what if I have to take it apart, that should be interesting.
I don't push the spring and start the cap with the fork tube clamped in the tree. I usually have the fork on the floor, someone holding and pulling up on the upper tube while I push the spring and start the cap without a wrench. Hey, what ever works. If it's easy, then there's not enough spring or spacer.
Scott
Scott
