# Maximum fork oil level?

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The bike specs say there is up to 130 mm of travel in the forks. Since the inside diameter of the forks is about 36 mm (or a radius of 18 mm), we must have at least

$\pi \times \left(1.8\,\text{cm}\right)^2 \times 13.0\,\text{cm} \approx 132\,\text{cc}$

of "air" inside of the forks to prevent hydraulic lock (aka, "hydro-lock").

So, with that in mind, the manual states the oil level in the forks should be 162 mm as measured with the forks fully compressed and no springs/spacers/washers inside. But, does that mean that all those components (spring, washer, spacer, plus fork cap!) really take up

$\dpi{120} \bg_white \pi \times \left(1.8\,\text{cm}\right)^2 \times 16.2\,\text{cm} \approx 165\,\text{cc}$

of space inside of the fork?

This seems rather much, and I was also thinking of raising the fork oil level to get less nose-dive during braking (an effect similar perhaps to getting stiffer springs, for example). But, do all those components really take up 165 cc of space inside the forks?

What's the highest fork oil level that the FZ-07 forks can safely (i.e., hydrolock-free) handle?

Edited by maz20
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2 hours ago, maz20 said:

The bike specs say there is up to 130 mm of travel in the forks. Since the inside diameter of the forks is about 36 mm (or a radius of 18 mm), we must have at least

$\pi \times \left(1.8\,\text{cm}\right)^2 \times 13.0\,\text{cm} \approx 132\,\text{cc}$

of "air" inside of the forks to prevent hydraulic lock (aka, "hydro-lock").

So, with that in mind, the manual states the oil level in the forks should be 162 mm as measured with the forks fully compressed and no springs/spacers/washers inside. But, does that mean that all those components (spring, washer, spacer, plus fork cap!) really take up

$\dpi{120} \bg_white \pi \times \left(1.8\,\text{cm}\right)^2 \times 16.2\,\text{cm} \approx 165\,\text{cc}$

of space inside of the fork?

This seems rather much, and I was also thinking of raising the fork oil level to get less nose-dive during braking (an effect similar perhaps to getting stiffer springs, for example). But, do all those components really take up 165 cc of air inside the forks?

What's the highest fork oil level that the FZ-07 forks can safely (i.e., hydrolock-free) handle?

You've discovered the best kept secret in the world. Those parts do not equal the vacant space in the forks. That 'variable' is what gives garage tuners some wiggle room. You are correct, more oil (within reason) generally means more resistance to bottoming and a more linear feel. Less oil - softer, more progressive front due to the progressive compression rate of a gas (air). We're only talking a couple mm either way from factory specs. You can easily experiment with it yourself going up and down some with a turkey baster type syringe or suction device. You'll be surprised how different a couple mm feels.

You can also experiment with oil viscosity too. This will alter the damping rates and between the two most people can find a combination that better suits their riding style in short order.

I don't know what the 'highest oil level' is.

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Two Dave Moss videos on fork oil level.

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7 hours ago, mossrider said:

You've discovered the best kept secret in the world. Those parts do not equal the vacant space in the forks. That 'variable' is what gives garage tuners some wiggle room. You are correct, more oil (within reason) generally means more resistance to bottoming and a more linear feel. Less oil - softer, more progressive front due to the progressive compression rate of a gas (air). We're only talking a couple mm either way from factory specs. You can easily experiment with it yourself going up and down some with a turkey baster type syringe or suction device. You'll be surprised how different a couple mm feels.

You can also experiment with oil viscosity too. This will alter the damping rates and between the two most people can find a combination that better suits their riding style in short order.

I don't know what the 'highest oil level' is.

I see, so to figure out what the "safe range" or the highest level would be, I'd need to actually measure the volumes of those components. It seems simple enough -- the thin washer's volume can probably be ignored, and the spacer and fork cap "volumes" can be estimated by just measuring their dimensions.

The spring, on the other hand, is a little more involved. I'd probably need to actually dunk it into a graduated cylinder somewhat filled with oil and get the difference in the level with the component fully submerged. That should give me a pretty accurate "volume" of the spring, and I already have the cylinder https://amzn.to/3ie5zaH from my last fork oil change.

Edited by maz20

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3 minutes ago, maz20 said:

I see, so to figure out what the "safe range" or the highest level would be, I'd need to actually measure the volumes of those components. It seems simple enough -- the thin washer's volume can probably be ignored, and the spacer and fork cap "volumes" can be estimated by just measuring their dimensions.

The spring, on the other hand, is a little more involved. I'd probably need to actually dunk it into a graduated cylinder somewhat filled with oil and get the difference in the level with the component fully submerged. That should give me a pretty accurate "volume" of the spring, and I already have the cylinder from https://amzn.to/3ie5zaH (from my last fork oil change).

In practice you'll never get anywhere near the max level. You'll end up somewhere 10mm's more or less from your starting point, the recommended level.

Or if you must, then yes, measure everything and add that figure to the manuals volume specifications. Just remember you'll never get an absolute, dry volume figure as it's virtually impossible to accurately disassemble, clean and measure everything in the forks.

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I recently just tried 150mm fork oil level. I might be experiencing a bit of hydro lock...

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