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Front suspension oil height effect

Started by h106frp, May 03, 2016, 10:00:28 am

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h106frp

Just for anyone who wanted to know what the typical curve looks like, the effect of oil heights on front suspension. Wish the garage screen would generate plots like this for the actual set up;


CapeDoctor

for the slightly technically challenged, how does this translate into the real world?
with a higher oil level (more mm's), the forks will get stiffer, right?
how would this serve me (or not) when trying to find an optimum setup?
any chance of providing a real-world reference, i.e: on which track/s a higher oil level will be better or worse?
or is this ultimately defined by several other factors?
just trying to learn a little more about how to improve my setups.....  ;D

h106frp

Not an expert and i am sure i will be corrected but possibly demonstrates that;

..Until you are using >50% front travel the oil level really does not make much difference to stiffness, so it really ties in with selecting a spring/preload/sag that maximizes fork travel and position for a given track

..For a linear change in oil level the stiffness rises exponentially

..its probably most noticeable under braking conditions for typically smooth track riding

..having 25 to 30% static sag gets you close to the start of the active part of the curve so the level then has some impact on smaller suspension travels. Excessive sag should probably be avoided.

I guess the real life racers here have more experience with this  :)

HornetMaX

The annoying thing is that 80mm means more oil than 100mm. Oil level is measured from the top of the fork: this means that in fact 80mm is the "air height".
From the top of the fork you have 80mm of air, then oil down to the bottom: so if you have 100mm, you have less oil (and more air).

Air in the fork acts as a spring, sort of in parallel to the (metal) spring of the suspension.
However the "air" spring is non-linear: simplifying a fair bit, for small displacements (stroke), it's soft and hence the suspension's spring will dominate (no effect from oil/air level). But at higher displacements the air spring could become important and add stiffness to the suspension.


BOBR6 84

Yep all this would be great in the garage.. to properly see what changes we make. iv'e read these types of suggestions many times because things are not clear.. mainly trial and error and a whole lot of head scratching, and placebo effect..

like the swingarm pivot.. finally it works in beta8! but to me it worked all along lol  ;D ::)


passerBy

Then, basically, the air acts as a sort of a bump stop, I guess? And the less air there is, the more it will resemble a rubber bump stop in its effect.

HornetMaX

April 20, 2017, 11:01:48 am #6 Last Edit: April 20, 2017, 11:19:52 am by HornetMaX
Quote from: passerBy on April 20, 2017, 02:26:25 am
Then, basically, the air acts as a sort of a bump stop, I guess? And the less air there is, the more it will resemble a rubber bump stop in its effect.

http://motocrossactionmag.com/home-page/mx-education-you-can-fix-your-own-forks-with-some-basic-fork-oil-height-tuning-tricks

passerBy

Quote from: HornetMaX on April 20, 2017, 11:01:48 am
Quote from: passerBy on April 20, 2017, 02:26:25 am
Then, basically, the air acts as a sort of a bump stop, I guess? And the less air there is, the more it will resemble a rubber bump stop in its effect.

http://motocrossactionmag.com/home-page/mx-education-you-can-fix-your-own-forks-with-some-basic-fork-oil-height-tuning-tricks

Pretty much what I was talking about: less air equals less chance of bottoming out hard.

HornetMaX

Quote from: passerBy on April 20, 2017, 11:12:47 am
Quote from: HornetMaX on April 20, 2017, 11:01:48 am
Quote from: passerBy on April 20, 2017, 02:26:25 am
Then, basically, the air acts as a sort of a bump stop, I guess? And the less air there is, the more it will resemble a rubber bump stop in its effect.

http://motocrossactionmag.com/home-page/mx-education-you-can-fix-your-own-forks-with-some-basic-fork-oil-height-tuning-tricks

Pretty much what I was talking about: less air equals less chance of bottoming out hard.

It's not the only effect. With less air you have a fork becoming stiffer at high compressions (i.e. the suspension is "more progressive" than when it has more air).

You can probably read part of the book here: https://books.google.fr/books?id=rJTQxITnkbgC&printsec=frontcover&hl=fr#v=onepage&q&f=false
(Page 155 and subsequent is where suspension stuff is).

passerBy

Quote from: HornetMaX on April 20, 2017, 11:27:31 am
Quote from: passerBy on April 20, 2017, 11:12:47 am
Quote from: HornetMaX on April 20, 2017, 11:01:48 am
Quote from: passerBy on April 20, 2017, 02:26:25 am
Then, basically, the air acts as a sort of a bump stop, I guess? And the less air there is, the more it will resemble a rubber bump stop in its effect.

http://motocrossactionmag.com/home-page/mx-education-you-can-fix-your-own-forks-with-some-basic-fork-oil-height-tuning-tricks

Pretty much what I was talking about: less air equals less chance of bottoming out hard.

It's not the only effect. With less air you have a fork becoming stiffer at high compressions (i.e. the suspension is "more progressive" than when it has more air).

You can probably read part of the book here: https://books.google.fr/books?id=rJTQxITnkbgC&printsec=frontcover&hl=fr#v=onepage&q&f=false
(Page 155 and subsequent is where suspension stuff is).

That's why I said "the less air there is, the more it resembles". As I see it, it's like having rubber (or more like silicone?) bump stops in conjunction with progressively-wound springs: more air makes the setup more focused on the springs, whereas more oil turns the thing into bump stops.

Thanks for the links, but I would prefer to read the whole book once I have a copy on my hands and feel like reading it.

davidboda46

Quote from: HornetMaX on May 26, 2016, 09:40:57 am
The annoying thing is that 80mm means more oil than 100mm. Oil level is measured from the top of the fork: this means that in fact 80mm is the "air height".
From the top of the fork you have 80mm of air, then oil down to the bottom: so if you have 100mm, you have less oil (and more air).

Air in the fork acts as a spring, sort of in parallel to the (metal) spring of the suspension.
However the "air" spring is non-linear: simplifying a fair bit, for small displacements (stroke), it's soft and hence the suspension's spring will dominate (no effect from oil/air level). But at higher displacements the air spring could become important and add stiffness to the suspension.


Yeah, sometimes it's hard to know if you're actually increasing or decreasing just based on the numbers in the garage. More engine brake = lower number. Confusing. Making the wheelbase longer by changing rake angle, more rake in numbers or less rake in numbers? And so on... It would be nice with some popups, like the ones in the control settings menu, that explains these basic things at least.

Cheers,

/David "Gonzo" Boda #46
"THE EDGE... THERE IS NO HONEST WAY TO EXPLAIN IT BECAUSE THE ONLY PEOPLE WHO REALLY KNOW WHERE IT IS ARE THE ONES WHO HAVE GONE OVER"

Hawk

Quote from: davidboda46 on April 20, 2017, 04:34:54 pm
Quote from: HornetMaX on May 26, 2016, 09:40:57 am
The annoying thing is that 80mm means more oil than 100mm. Oil level is measured from the top of the fork: this means that in fact 80mm is the "air height".
From the top of the fork you have 80mm of air, then oil down to the bottom: so if you have 100mm, you have less oil (and more air).

Air in the fork acts as a spring, sort of in parallel to the (metal) spring of the suspension.
However the "air" spring is non-linear: simplifying a fair bit, for small displacements (stroke), it's soft and hence the suspension's spring will dominate (no effect from oil/air level). But at higher displacements the air spring could become important and add stiffness to the suspension.


Yeah, sometimes it's hard to know if you're actually increasing or decreasing just based on the numbers in the garage. More engine brake = lower number. Confusing. Making the wheelbase longer by changing rake angle, more rake in numbers or less rake in numbers? And so on... It would be nice with some popups, like the ones in the control settings menu, that explains these basic things at least.

Cheers,

/David "Gonzo" Boda #46


Totally agree David. +1 mate!  ;)

Hawk.

HornetMaX

Quote from: davidboda46 on April 20, 2017, 04:34:54 pm
Yeah, sometimes it's hard to know if you're actually increasing or decreasing just based on the numbers in the garage. More engine brake = lower number. Confusing. Making the wheelbase longer by changing rake angle, more rake in numbers or less rake in numbers? And so on... It would be nice with some popups, like the ones in the control settings menu, that explains these basic things at least.

Agree, some pop-ups would be useful and not hard to implement (perfect solution would be some sort of drawing/diagram moving when you change the value, but that's quite more work than a pop-up).

For the rake/wheelbase it's quite easy though: rake angle is measured from the vertical (whatever that means) so bigger rake -> bigger wheelbase.

And the oil level is what is used in the pits in reality so, even if it's confusing, it's just GPB sticking to the real deal :)