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Discussion Starter · #1 ·
So I installed Ikon progressive fork springs at the very end of last riding season and followed Ikon's instructions - which meant I put in the recommended amount of 10w fork oil and a 12mm spacer. At the time it felt far too stiff to me so I removed the spacer and immediately realized that was not going to work. So I thought maybe I might be overdamped and removed 10mL of oil from each fork and put the spacers back in. That was far worse - the front end would bounce after every bump. I wasn't taking a very scientific approach, and was irritated that I couldn't adjust the preload, but I was getting the idea that the front end was *UNDERDAMPED*, not overdamped. I thought maybe it might be oversprung as well, and since my fork caps weren't in the best shape anyway, got a set of Thruxton adjustable preload caps (and washers) today.

So with the new adjustable preload caps in hand I thought I'd add 30 mL of oil back to each fork; figuring that I may need more damping than I can get with 10W oil in any quantity, I got 15W, under the idea that for my initial addition it won't hurt to use a little bit of different-weight oil.

I was surprised to find out that the Thruxton adjustable-preload caps have a *MINIMUM* preload equivalent to about a 13mm spacer on our Bonneville caps - clearly an indicator that I was not oversprung before.

Anyway, I added 30 mL of 15W oil to the existing (roughly 460mL, IIRC) 10W oil and put the adjustable-preload caps on at their lowest setting and took off for a test ride.

It feels substantially better - surprisingly to me (and clearly I don't understand the way preload and damping affect each other), it actually feels *less* sprung even though it's slightly *higher* preload, and so the initial attack on bumps is smoother and there's no hop afterward. The initial attack is still a bit more unsettling to the bike's geometry than I'd like though.

I turned up the preload two turns and took another ride and noticed that the initial attack feels better at this setting but that the hop returns. So I'm assuming that means I need more damping so I can set the higher preload without getting the after-hop, right? Am I beginning to understand this? If I add more damping (i.e., more or heavier oil) I can have more preload which should allow for a smoother ride until it exceeds the springs' capacity, right?

Anyway, assuming I'm right that at this point I need more damping still, I gather that I'm best to remove some of the existing 10W oil and replace it with 15W rather than adding even more oil to the forks, right? I weigh about 210 lbs (I'm 6'2" and I know that doesn't matter but if I mention the weight I have to say it!) and so I hadn't assumed I'd need 15W oil back when I added the springs, but now I'm thinking I do. Opinions?

I'm finding the front shocks harder to dial in than I'd expected. But these Thruxton caps are fabulous, and I am already seeing that tuning the suspension will be *much* easier with fine, precise control over preload...
 

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If you run too high of an oil height then you will increase the risk of a blown fork seal, as there is less of an air gap due to the higher oil height & you get a rising internal pressure inside the fork from the air spring as the fork compresses.

You can mix (blend) different oil weights, no problem. Just siphon out some fork oil (a turkey baster will work, but don't tell my wife!) & replace with heavier weight to the correct height, & repeat until you get it the way you want it.
 

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I did this mod on my T100 after reading this post
http://www.triumphrat.net/twins-tec...k-springs-preload-adjusters-on-the-scram.html

I used the Hagon progressive fork springs, they did not come with any additional spacers. If you have fitted the preload caps and the spacers you are compressing the springs about 35mm more than the standard caps. I cut 21mm off the top of the original spacer tube to allow for the additional length of the preload cap, therefore the compression of the spring was the same at that point, before adjusting the preload caps.
It was not clear whether you had reduced the length of the spacer tube?
 

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Preload, underdamped, over sprung, oil weights...

...I'm going to understand this suspension business someday or melt my brain trying.
 

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Discussion Starter · #6 ·
I used the Hagon progressive fork springs, they did not come with any additional spacers. If you have fitted the preload caps and the spacers you are compressing the springs about 35mm more than the standard caps. I cut 21mm off the top of the original spacer tube to allow for the additional length of the preload cap, therefore the compression of the spring was the same at that point, before adjusting the preload caps.
It was not clear whether you had reduced the length of the spacer tube?
I am not using any spacer. I was using a 12mm spacer (the minimum recommended by Ikon) with the regular caps but with the adjustable-preload caps I'm using nothing. My very rough estimate showed the adjustable-preload caps to add more like 13mm or so at their lowest setting.

pokeyjoe said:
Too many variables. Set your sag first, then play with the oil weight and amount.
I suspect you're right. Propforward, if you read this, let's find a time and measure sag on our bikes. I'm probably free this Saturday late AM / early PM...
 

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Duly read and acknowledged. I'm sure we can work something out. I am installing clutch springs and new cam cover this weekend - hopefully the majority of that friday night, finishing it up saturday morning. That makes sat morning a good time for it if your up for riding out my way. I want to measure my sag too :eek: so that will be good.

Lemme know. Any time saturday will be OK actually.

Let a 4 variable full factorial designed experiment commence!
 

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Discussion Starter · #8 ·
Sounds good! I'm helping with a political convention in the morning, but since we have nobody to endorse it should be pretty quick. Want to say some time between noon and 1 on Saturday? Out at your place? PM me your address since I don't remember it.

It will be great to get real, empirical measurements! Oh, and to see your totally-reworked bike!
 

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Discussion Starter · #9 ·
Sag Results - SHOCKING!

Well, thanks to propforward for helping me get the sag measurements - I think we were both surprised with the results, but for my bike especially, it totally changed the way I think about what's going on.

This is a important point for anybody who's tuning their suspension - remember that FRONT AND REAR SHOCKS WORK TOGETHER! It turns out that my front shocks, which I had been fiddling with and was sure were the problem, had a static sag of about 40mm - a little more than ideal, but in the right ballpark. The rear shocks (the stockers) had a mind-blowing number of 50-55mm! I was on the second preload setting; by dialing it all the way up to the highest preload I got them to about 40mm (38? I don't remember exactly). With that adjusted, I turned my front preload adjusters to 4 turns (roughly 4mm additional preload over the minimum on the thruxton adjusters, or about 20mm) and got that static sag measurement down to ~35mm.

On a ride this evening I discovered that the front feels best for me at about 6 turns up from minimum; I am slightly underdamped so I think I'll suck out about 150mL of (10W) fork oil from each fork and replace it with 15W, which I'm guessing will take care of it.

The rear is definitely the weak point now; I think I might try to pick up some Ikon rears. It sucks to already have your rear shocks on *MAXIMUM* preload just to get them close to the right static sag measurement. I can feel that they're still not quite there, though they're certainly passable for now.

The big thing I learned, which in retrospect should have been obvious, is that the front and rear should have about the same static sag measurements, or expressed more generically, they should be about the same. With both at 50, it just felt soft; with the front at 35, a 50 rear makes the bike handle strangely. And since your hands are on the handlebars, problems in the rear (at least for me) expressed themselves in the front, which made it hard to determine what was going on.

I was fighting in the dark without a static sag measurement, stabbing aimlessly based on hunches. With the static sag measurement, everything makes sense.
 

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I'd better email you the measurements like I promised to!

Here is the static sag measurement procedure we used. I like it, because it uses a simple "averaging" approach for the suspension once loaded, borrowed from Sport Rider magazine 1995:

REAR END
Step 1: Extend the suspension completely by getting the wheel off the ground. It helps to have a few friends around. On bikes with sidestands the bike can usually be carefully rocked up on the stand to unload the suspension. Most race stands will not work because the suspension will still be loaded by resting on the swingarm rather than the wheel. Measure the distance from the axle vertically to some point on the chassis (metric figures are easiest and more precise; Figure 1). Mark this reference point because you'll need to refer to it again. This measurement is L1. If the measurement is not exactly vertical the sag numbers will be inaccurate (too low).

Step 2: Take the bike off the stand and put the rider on board in riding position. Have a third person balance the bike from the front. If accuracy is important to you, you must take friction of the linkage into account. This is where our procedure is different: We take two additional measurements. First, push down on the rear end about 25mm (1") and let it extend very slowly.
Where it stops, measure the distance between the axle and the mark on chassis again. If there were no drag in the linkage the bike would come up a little further. It's important that you do not bounce! This measurement is L2.

Step 3: Have your assistant lift up on the rear of the bike about 25mm and let it down very slowly. Where it stops, measure it. If there were no drag it would drop a little further. Remember, don't bounce! This measurement it L3.

Step 4: The spring sag is in the middle of these two measurements. In fact, if there were no drag in the linkage, L2 and L3 would be the same. To get the actual sag figure you find the midpoint by averaging the two numbers and subtracting them from the fully extended measurement L1: static spring sag = L1 -[(L2 + L3) / 2].

Step 5: Adjust the preload with whatever method applies to your bike. Spring collars are common, and some benefit from the use of special tools. In a pinch you can use a blunt chisel to unlock the collars and turn the main adjusting collar. If you have too much sag you need more preload; if you have too little sag you need less preload. For road race bikes, rear sag is typically 25 to 30mm. Street riders usually use 30 to 35mm. Bikes set up for the track are compromise when ridden on the street. The firmer settings commonly used on the tract are generally not recommended (or desirable) for road work.
You might notice the Sag Master measuring tool (available from Race Tech) in the pictures. It's a special tool made to assist you in measuring sag by allowing you to read sag directly without subtracting. It can also be used as a standard tape measure.
Measuring front-end sag is very similar to the rear. However, it' much more critical to take seal drag into account on the front end because it is more pronounced.

FRONT END
Step 1: Extend the fork completely and measure from the wiper (the dust seal atop the slider) to the bottom of the triple clamp (or lower fork casting on inverted forks; Figure 2). This measurement is L1.

Step 2: Take the bike off the sidestand, and put the rider on board in riding position. Get and assistant to balance the bike from the rear, then push down on the front end and let it extend very slowly.
Where it stops, measure the distance between the wiper and the bottom of the triple clamp again. Do not bounce. This measurement is L2.

Step 3: Lift up on the front end and let it drop very slowly. Where it stops, measure again. Don't bounce. This measurement is L3. Once again, L2 and L3 are different due to stiction or drag in the seals and bushings, which is particularly high for telescopic front ends.

Step 4: Just as with the front, halfway between L2 and L3 is where the sag would be with no drag or stiction. Therefore L2 and L3 must be averaged and subtracted from L1 to calculate true spring sag: static spring sag = L1 - [l2 + l3) / 2].
Step 5: To adjust sag use the preload adjusters, if available, or vary the length of the preload spaces inside the fork.
Street bikes run between 25 and 33 percent of their total travel, which equates to 30 to 35mm. Roadrace bikes usually run between 25 and 30mm.
This method of checking sag and taking stiction into account also allows you to check the drag of the linkage and seals. It follows that the greater the difference between the measurements (pushing down and pulling up), the worse the stiction. A good linkage (rear sag) has less than 3mm (0.12") difference, and a bad one has more than 10mm (0.39"). Good forks have less than 15mm difference, and we've seen forks with more than 50mm. (Gee, I wonder why they're harsh?)
It's important to stress that there is no magic number. If you like the feel of the bike with less or more sag than these guidelines, great. Your personal sag and front-to-rear sag bias will depend on chassis geometry, track or road conditions, tire selection and rider weight and riding preference.
Using different sag front and rear will have huge effect on steering characteristics. More sag on the front or less sag on the rear will make the bike turn more slowly. Increasing sag will also decrease bottoming resistance, though spring rate has a bigger effect than sag. Racers often use less sag to keep the bike clearance, and since roadraces work greater than we see on the street, they require a stiffer setup.
 

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Very interesting thread!
Once I get my carbs dialed in, this will be the next project.

Last summer I bought Ikon progressive springs but didn't do any sag measurements or any experimentation with spacers or oil weight/volume.
I did notice an immediate improvement over the stock springs but I'm sure I can make it even better with suspension tuning.

I still have the stock rear shocks, I suspect not for more than another season though.

Thanks for the detailed instructions and reporting on results!
-K
 

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The measurements are very revealing. I too need to adjust my preload, but have decided to wait until I get some riding miles in this season, so that I can "feel" the benefit after making the adjustments.

Taking this approach certainly helps shed light on what is going on.
 

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Discussion Starter · #13 ·
Thought I'd update this thread. Last night I dropped the forks and dumped the 10w oil, and refilled with 15w to 125mm from the top (forks compressed). I put the preload back to 6 turns, which seemed a good compromise when we measured static sag last weekend.

It rained all day today so my ~12 mile test ride last night is all I've done so far, but it felt *NICE* last night - absorbs the littlest bumps, dampens the big bumps, doesn't wallow in corners (very, very confidence-inspiring in my favorite hairpin) and actually seems to have improved braking considerably.

Only problem now? The rear was way undersprung so to get the static sag close to right in the rear, I raised the stock springs to the highest preload setting - which is fine except that they're significantly underdamped now. So I really will be ordering those Ikons some time soon...
 

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Ikon Progressive Springs

So I installed Ikon progressive fork springs at the very end of last riding season and followed Ikon's instructions - which meant I put in the recommended amount of 10w fork oil and a 12mm spacer. At the time it felt far too stiff to me so I removed the spacer and immediately realized that was not going to work. So I thought maybe I might be overdamped and removed 10mL of oil from each fork and put the spacers back in. That was far worse - the front end would bounce after every bump. I wasn't taking a very scientific approach, and was irritated that I couldn't adjust the preload, but I was getting the idea that the front end was *UNDERDAMPED*, not overdamped. I thought maybe it might be oversprung as well, and since my fork caps weren't in the best shape anyway, got a set of Thruxton adjustable preload caps (and washers) today.

So with the new adjustable preload caps in hand I thought I'd add 30 mL of oil back to each fork; figuring that I may need more damping than I can get with 10W oil in any quantity, I got 15W, under the idea that for my initial addition it won't hurt to use a little bit of different-weight oil.

I was surprised to find out that the Thruxton adjustable-preload caps have a *MINIMUM* preload equivalent to about a 13mm spacer on our Bonneville caps - clearly an indicator that I was not oversprung before.

Anyway, I added 30 mL of 15W oil to the existing (roughly 460mL, IIRC) 10W oil and put the adjustable-preload caps on at their lowest setting and took off for a test ride.

It feels substantially better - surprisingly to me (and clearly I don't understand the way preload and damping affect each other), it actually feels *less* sprung even though it's slightly *higher* preload, and so the initial attack on bumps is smoother and there's no hop afterward. The initial attack is still a bit more unsettling to the bike's geometry than I'd like though.

I turned up the preload two turns and took another ride and noticed that the initial attack feels better at this setting but that the hop returns. So I'm assuming that means I need more damping so I can set the higher preload without getting the after-hop, right? Am I beginning to understand this? If I add more damping (i.e., more or heavier oil) I can have more preload which should allow for a smoother ride until it exceeds the springs' capacity, right?

Anyway, assuming I'm right that at this point I need more damping still, I gather that I'm best to remove some of the existing 10W oil and replace it with 15W rather than adding even more oil to the forks, right? I weigh about 210 lbs (I'm 6'2" and I know that doesn't matter but if I mention the weight I have to say it!) and so I hadn't assumed I'd need 15W oil back when I added the springs, but now I'm thinking I do. Opinions?

I'm finding the front shocks harder to dial in than I'd expected. But these Thruxton caps are fabulous, and I am already seeing that tuning the suspension will be *much* easier with fine, precise control over preload...

I have a 2013 Thruxton fitted with a few of BC tuning mods & Ikon
Rear Shockers & was thinking of trying to improve the front
end/forks with Ikon Progressive Springs, but after reading all this on here my head is Spinning :goofy, So I think rather than have all the hassle like the poster above & others on here & chance of cocking it up & making it worse, I would rather put up with the Stock set-up..:ride
 

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I've personally have found that adjustable preload caps do not work as well with Ikon fork springs as they do with many other springs. It's because the Ikon springs are so long with an equally long 'ramp-up' before the spring rate rises sharply. In other words, you'd need preload caps with twice or more range of adjustment.

Don't get me wrong: Good springs those Ikons. But adjustable preload caps will have minimal effect.

/M
 
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