I edited the above post a couple times for clarity.

 

awesome description professor! you are the teacher. As a rider, i push a little harder than most and so appreciate the enlightment. I knew little about suspension other than crank it up when you haul someone and vice versa.

relative to mine, I'm close so I'll try it out and see what happens. Might have to crank it up a little when I put all my gear on.

thanks, nathan

 

Greg, can you do me a favor? I'll explain how I've approached setting up forks and let me know where we are differing in logic:

VTR weight = ~200kg
Stock spring rate = ~0.59kg/mm

In order to achieve static sag you need to compress the springs enough to support the load. Assuming 50/50 distribution and taking into account there are two springs I get a total spring compression of about 85mm. My experiences are pretty close to this, I've observed an installed compression of at least 50mm, usually more like 60-70mm. That would kind of support your comment about 15mm static sag. Add that to 70mm installed compression and you get 85mm. Of course, higher spring rates are different numbers but the logic is the same. I actually use a spreadsheet that calculates the spacer length, etc. and it's been pretty much right on (so far).

Maybe I'm misunderstanding what you are calling "internal preload"?


Another side note: I do use the total mechanical travel of the forks. You eluded to it in your post, during braking you will see the most forces that act to compress the forks. The length of travel within the range of the oil lock will be used up during this time. The oil lock is not a perfect seal, just a restriction. During braking the forks can pass into this range pretty easily. I suppose it's a moot point because the generally accepted range is 25-30% of fork travel is to be used for rider sag. Your approach with 30% is about the same number as mine with 25%.

 

Well explained, G.

I agree the topout spring raises the spring rate. When the topout spring engages, the main spring force drops off at a higher rate.

I shortened my top out springs (talking RC51-SP2 here, not stock VTR), so that I would have a constant spring rate at the top of travel, and better compliance when lightly loaded (acceleration off corners).

I like your explanation of setting sag using the correct range of travel. 100% right on. A more clear picture than I had before.

Initial preload is the amount of spring compression when the fork is fully extended agianst the top out spring. My shortened topout springs calculate to about 0.5 kg/mm, about 1/2 the main spring rate, so they compress about twice as much under initial preload. I know I will need to add spacers to get my sag correct, but I have to have the bike track-ready, before I can measure the required spacer length.

Sag is basically how far below the topout springs the bike sits, or how much upper travel you have when the front end is light. Once the suspension compresses off the top, the main spring length does not change with preload, since the bike and rider are supported by the springs. Adjusting preload changes the sag, not the load on the springs.

 

Ha.

 

Just another VTR, eh?

Hehehe. Good ****, there!

 

Based on the depth of some of these explanations, I think some of you guys belong at MIT or NASA. Hell, we would have likely been in and out of black holes, cured cancer and ended greed and all other injustices by now.

thank god for superhawks and their nerds!

nathan

 

You mean like this? Hey, what about Princeton? HeHeHe.

 

OK. Enough goofing around. Seriously, is your front end OK and working good now?

 

front end seems okay so far. i straightened the triple trees, but not quite enough yet. They're almost there:not pulling to the right nearly as much and alignment mark of top clamp is almost at center. I'll have to take them off again and either replace or bend again.

I'm happy with the suspension so far with less than 100 miles on it. The rebound is something I'm going to work on a bit. I adjusted it by setting it to full on and then backed off till it felt right. I'm going to go the other way and go from full off to where i can feel it and see how it comes out comparatively.


I'm happy with the ground clearance front and back in that it's closely proportional to OEM; i.e., 2/3" for both pipe clearance and and seat height measurements.

I haven't gotten over to the edge of the tires in a controlled way yet because of so much invisible dust that precipitated two scary slides I didn't see coming yesterday.

will post additional feedback.

thanks, nathan

 

I'm glad you've got it sorted. Don't slide into oblivion. But you've been doing this long enough, you don't need a lecture.

I don't mean to get so carried away talking about this stuff. I'm obsessed with these things and live in a motorcycle vacuum, most of the time. Not many sport riders in my town. My friends and family think i'm knucking futz.

 

I think many of us are are the same. I call it passion, intensity, love. It has to be serious when you continue to ride despite the inherent risks. I like to slide a bit, but hopefiully not into oblivion.

It's great to be able to find others who share your enthusiam without having to leave you living room. unbelieveable!!!

thanks, Nathan

 

Yeah, pretty cool.

 

Over the weekend I fabricated a 5mm shim similar to the one Greg has on his site and got it installed, took it for a spin this afternoon and holy s*&$ what a difference. The bike rolls into turns effortlessly. Before I would have to nudge it into turns, now I lean and the bike goes that direction.

 

Yeah, i didn't realize this myself because I hadn't ridden the bike much during the winter. But I switched bikes with a riding buddy so he could check out the hawk and the twin vs in line four and i noticed when riding his 600 honda that it took more to initiate and hold your line. feels pretty good huh?

did you notice your sidestand being short?

thanks Nathan

 

I define internal preload as the distance you need to compress the fork cap until it's fully seated in the tube. This is with the adjuster right in the middle of the range.

How I check this measurement is with the fork fully assembled but with the fork cap not threaded in, I measure the distance between the lip of the fork cap and the top of the fork tube.

On your measurements in the first paragraph, are you doing a little trigonometry taking into consideration the fork angle?

 

I think what you are calling internal preload doesn't really tell the whole story. You need to measure this and add it to the installed compression to get the total spring preload before you get a meaningful number. Even so, if you look at the data you'll see that you can't really get 15mm of static sag with a properly setup front suspension unless your rider weight is around 125lb. Normally it's going to be more like 5mm (a range of 3-8mm is typical for what I seen and calculated). For example, for a 200lb rider I'd suggest 0.90kg/mm springs which would give you 6mm of static sag (range of 2-11mm to be exact).

No trig at all in any of my fork calculations! These are straight numbers inline with fork travel.

 

Now you know why we do it! In fact, I think I'm running something closer to +8mm shock length on my bike. You can to match your front setup as well and there is a limit to how far your should go, but comparing to stock rear ride height more is definitely better.

 

I haven't done VTR forks in a while, but with RC51 forks, when you install the springs, with a spring compressor, you can measure how much spring is exposed above the stanchion tube before you compress it, then compress and screw the top cap on. Once you remove the spring compressor the spring pushes against the top cap and compresses the top-out spring a bit. Take another measurement and the difference is your initial preload. Then you pull the stanchion up and screw the top cap in. No more spring compression occurs.

If you then measure unladen sag, at the middle of the preload range and it's not about 15 mm, you adjust your spacer. Then you measure laden sag. If the laden sag is not about right, you need to think about a different spring rate. It seems to me, if you want to run a soft spring and want your laden sag to be right, you will end up running less unladen sag. For a stiffer spring, you'll have more, because the difference between laden and unladen sag depends on both rider weight and spring rate.

 

If you think about it, it's really the other way around. You always need the same load (the bike itself) at the static sag point. You also want the same rider sag value. If your rider weight is less the spring rate should be less. This would equate to more static sag. The lower the spring rate the more the static sag.

 

I think we're saying the same thing from opposite directions, so it looks like it's the other way around.

There is a single spring rate that will give you both the correct unladen and laden sag numbers. If you deviate from that spring rate and set it up for the same laden sag, the unladen sag will be different. A lower spring rate will have less unladen sag, because it will compress more under rider weight.

 

considering that the springs are mathced to the bike and rider weights, as you increase compression adjustment, unladen or static sag decreases and vice versa. And if you can achieve the appropriate number here, then rider sag should be automatic. is this generally true????

thanks, Nathan

ps: then spacer length and oil weight and air gap come into the equation too.

 

The laden and unladen sag numbers change together when you change the preload.

Oil weight does not affect it. Oil is only effective when the suspension is moving.

Spacer length sets the initial preload, so that you have the corect sag #s. If the spring rate is correct, you'll have both laden and unladen sag #s you are shooting for.

Air gap will have a small effect. It adds effective spring rate. As I understand it, air gap is adjusted to allow adjustment of the effective spring rate, so the fork almost bottoms under hard braking (maximum load), but uses all of the available travel.

 

what I was trying to say is that given the appropriate spring/spacer weight ratio, all you need to do is set the compression to get the desired static/unalden sag and you there. Rider sag should be where you want it. Is that correct?

thanks Nathan

 

Here's a graph of "air spring" vs. fork stroke...