The "preload" adjuster on a cartridge fork tube is a very commonly misunderstood adjuster. I myself was under the impression it adjusted main spring preload exclusively until I really started to delve into fork tuning and what I discovered surprised me a great deal. I use preload in quotations because while it is called the preload adjuster that is only its most periphery function.
The 2 major results of the "preload adjuster are:
1. Fork Length adjustment
2. Top-out/Negative spring compression
I will try not to yammer on needlessly as ill let the pictures do the talking, but the basic sequence goes like this:
1. When the "preload adjuster is 'tightened' it pushes against the main spring which in turn pushes against the negative spring.
2. Since the entire 'system' (the shaft that contains the springs) is a length that never changes, something internally has to give. This is always the weakest link.
3. In a traditional fork that has no topout/negative spring the only thing that can 'give' is the main spring and therefore it compresses giving us main spring preload.
4. In the newer style fork with the topout/negative spring there are 2 springs that can give. Since the main spring has an rate of ~1kg/mm and the topout negative spring has a substantially lower rate, guess which spring compresses first.
5. Proof you say? Here are the measurements taken from the zx6 cartridge pictured:
Full Soft "preload"
Main spring = 255mm
Negative spring = 40mm
Full Hard "preload"
Main spring = 250mm
Negative spring = 32mm
Top cap height growth = 15mm
Even more interesting are the measurements from the cbr1000rr...
Full Soft "preload"
Main spring = 258mm
Negative spring = 42mm
Full Hard "preload"
Main spring = 255mm
Negative spring = 32mm
Top cap height growth = 15mm
From full soft to full hard the main spring compressed 1% while the negative spring compressed 25% which incidentally is as far as it can compress. the negative springs coils are now touching and the spring now serves NO purpose.
This means that the fork will no longer extend past the maximum extension of the main spring.
This means the fork will not extend down into a dip in the pavement.
This means that the front tire is no longer touching the pavement.
This means that if you are in a lean and this happens, you crash.
So why on earth would you WANT to negate your topout/negative spring?
No reason I can think of. I cant answer why RaceTech uses shorter stiffer topout springs. I can say that although im certainly no expert, I know a few who are (AMA suspension techs) and they absolutely use long soft topout springs...
So what does the damn adjuster do?
Well it adjusts ride height, and this has to do with frame geometry, but at the risk of eliminating your topout/negative spring, is it worth it? Some grossly oversimplified trig says that steering head angle goes from 23 to 22.7. Thats it! Of course the 15mm of length will translate into ~14mm of front end height increase but thats only ~0.015%. This does not take into acount change of center of gravity, rear antisquat, etc but you get the point. Its seems a minor dimensional adjustment compared to the additional negative effects brought upon by what we discussed earlier.
So the question becomes, Why is the adjuster even there?
I cant answer that. All I can tell you is what ive found. I did some track testing with the "preload" setting on full hard to see if i noticed a seat of the pants difference. Would it really be that big of a deal to negate the topout/negative spring? We have very bumpy tracks here in Canada and I didnt make it out of the second corner, I lowsided after hitting a bump at lean. I reset the "preload" adjuster to full soft and won my next 2 races the next day. Take what you will from this but I firmly believe in the attributes of the negative spring.
So for setting sag then, what to do?
Well since we have established that the adjuster doesn't really adjust preload, what to do? If all you need is 5mm of preload and you have flat tracks, crank her all the way to hard. For the rest of us, spring changes or changing the preload by way of the spacer on the damping rod itself is the only way.
The 2 major results of the "preload adjuster are:
1. Fork Length adjustment
2. Top-out/Negative spring compression
I will try not to yammer on needlessly as ill let the pictures do the talking, but the basic sequence goes like this:
1. When the "preload adjuster is 'tightened' it pushes against the main spring which in turn pushes against the negative spring.
2. Since the entire 'system' (the shaft that contains the springs) is a length that never changes, something internally has to give. This is always the weakest link.
3. In a traditional fork that has no topout/negative spring the only thing that can 'give' is the main spring and therefore it compresses giving us main spring preload.
4. In the newer style fork with the topout/negative spring there are 2 springs that can give. Since the main spring has an rate of ~1kg/mm and the topout negative spring has a substantially lower rate, guess which spring compresses first.
5. Proof you say? Here are the measurements taken from the zx6 cartridge pictured:
Full Soft "preload"
Main spring = 255mm
Negative spring = 40mm
Full Hard "preload"
Main spring = 250mm
Negative spring = 32mm
Top cap height growth = 15mm
Even more interesting are the measurements from the cbr1000rr...
Full Soft "preload"
Main spring = 258mm
Negative spring = 42mm
Full Hard "preload"
Main spring = 255mm
Negative spring = 32mm
Top cap height growth = 15mm
From full soft to full hard the main spring compressed 1% while the negative spring compressed 25% which incidentally is as far as it can compress. the negative springs coils are now touching and the spring now serves NO purpose.
This means that the fork will no longer extend past the maximum extension of the main spring.
This means the fork will not extend down into a dip in the pavement.
This means that the front tire is no longer touching the pavement.
This means that if you are in a lean and this happens, you crash.
So why on earth would you WANT to negate your topout/negative spring?
No reason I can think of. I cant answer why RaceTech uses shorter stiffer topout springs. I can say that although im certainly no expert, I know a few who are (AMA suspension techs) and they absolutely use long soft topout springs...
So what does the damn adjuster do?
Well it adjusts ride height, and this has to do with frame geometry, but at the risk of eliminating your topout/negative spring, is it worth it? Some grossly oversimplified trig says that steering head angle goes from 23 to 22.7. Thats it! Of course the 15mm of length will translate into ~14mm of front end height increase but thats only ~0.015%. This does not take into acount change of center of gravity, rear antisquat, etc but you get the point. Its seems a minor dimensional adjustment compared to the additional negative effects brought upon by what we discussed earlier.
So the question becomes, Why is the adjuster even there?
I cant answer that. All I can tell you is what ive found. I did some track testing with the "preload" setting on full hard to see if i noticed a seat of the pants difference. Would it really be that big of a deal to negate the topout/negative spring? We have very bumpy tracks here in Canada and I didnt make it out of the second corner, I lowsided after hitting a bump at lean. I reset the "preload" adjuster to full soft and won my next 2 races the next day. Take what you will from this but I firmly believe in the attributes of the negative spring.
So for setting sag then, what to do?
Well since we have established that the adjuster doesn't really adjust preload, what to do? If all you need is 5mm of preload and you have flat tracks, crank her all the way to hard. For the rest of us, spring changes or changing the preload by way of the spacer on the damping rod itself is the only way.
