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Does using post length affect stability?


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#1 John E Fry

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Posted 05 December 2009 - 02:07 AM

Hi all,

This might seem like a fairly basic question, but one I have not seen answered on here elsewhere. (this may be due to my inability to use the Search thingy properly of course!)

It seems that most operators prefer to shoot with the shortest post possible and add weight to the bottom of the rig, rather than extending the post. Apart from the obvious clearance & knee-knocking considerations, is their any other reason why keeping your post very short is a good idea?

In addition to that, what difference does post length have on the rigs stability? Antlers extend mass & therefore increase stability on that axis, so does a longer post do the same vertically?

I have been trying to answer this myself but even with the windy weather I have had trouble determining if any increased resistance to that wind (when on the move) is due to my deliberate lenghthening of the post or faster drop time.

I have done a few really precise jobs needing ultimate stability & I'm just wondering if lenghthening the post in these situations actually makes much of a difference?

All input greatfully recieved, thank you!

John

using an EFP, Master and Flyer
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#2 Eric Fletcher S.O.C.

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Posted 05 December 2009 - 02:34 AM

It seems that most operators prefer to shoot with the shortest post possible and add weight to the bottom of the rig, rather than extending the post. Apart from the obvious clearance & knee-knocking considerations, is their any other reason why keeping your post very short is a good idea?

In addition to that, what difference does post length have on the rigs stability? Antlers extend mass & therefore increase stability on that axis, so does a longer post do the same vertically?

I have been trying to answer this myself but even with the windy weather I have had trouble determining if any increased resistance to that wind (when on the move) is due to my deliberate lenghthening of the post or faster drop time.



I like a short post because I like a "Fast" rig, that doesn't mean that my drop time is fast, (it's not I like to operate close to neutral) I just like the rig to be very responsive. Lengthening the post increases the inertia in both tilt and roll but doesn't affect pan. Just like if you run the antlers for and aft you affect the Tilt and pan inertia and if you fun the antlers at 90 degrees you affect roll and tilt.

I also like a short post to keep the damn thing out of my legs. The only viable option to add weight at the base of the rig is with power or required accessories, never lead.
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#3 Rob Vuona SOC

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Posted 05 December 2009 - 05:57 AM

Eric,

Why never lead?

Personal preference or technical preference?

Peace Out
Rob "Machined too much weight out of my rig" Vuona
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#4 Rob Vuona SOC

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Posted 05 December 2009 - 06:02 AM

Hi all,

This might seem like a fairly basic question, but one I have not seen answered on here elsewhere. (this may be due to my inability to use the Search thingy properly of course!)

It seems that most operators prefer to shoot with the shortest post possible and add weight to the bottom of the rig, rather than extending the post. Apart from the obvious clearance & knee-knocking considerations, is their any other reason why keeping your post very short is a good idea?

In addition to that, what difference does post length have on the rigs stability? Antlers extend mass & therefore increase stability on that axis, so does a longer post do the same vertically?

I have been trying to answer this myself but even with the windy weather I have had trouble determining if any increased resistance to that wind (when on the move) is due to my deliberate lenghthening of the post or faster drop time.

I have done a few really precise jobs needing ultimate stability & I'm just wondering if lenghthening the post in these situations actually makes much of a difference?

All input greatfully recieved, thank you!

John

using an EFP, Master and Flyer


Oh John,
I wondered this myself and did a test on a live to tape show I was working on.

First show I put the post as short as possible and second show as long as possible and the stability seemed to be the same

I am height deficient at 5' 9" which made the post length a pain in the knee at the longest setting so I always have it as short as possible and do fly with lead on the bottom of the rig, especially with the new Lithium 190 batteries being so much lighter than my AB Hytron 120s
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#5 John E Fry

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Posted 05 December 2009 - 06:49 AM

Thank you Eric & Rob,

That's what I expected, but was just wondering if anyone else had any directly comparative knowledge.

According to the Dynamic Balance Primer equations the post length shouldn't make much difference overall (sorry Jerry, I really did try to read all of it!) but was curious to hear if this worked out in practice.

Great, I can happily go off with a nice short post without worrying that I may be compromising the rigs stability unduely.

Cheers guys,

John
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#6 Jason Torbitt

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Posted 05 December 2009 - 08:32 AM

I agree with Eric with his statement - why add dead weight to the base of the rig? Add stuff to your sled that will be useful and do something - i.e. larger batteries, more accessories etc. Make the weight functional. The latest sleds out there are as lightweight as they can possibly be...but one problem is that they're getting to the stage where extra weight needs to be added for ballast...which is surely fundamentally wrong. Dead weight makes no sense. Useful weight makes a little bit more sense...
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#7 Jess Haas SOC

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Posted 05 December 2009 - 11:26 AM

If you extend your post but don't change the weight of the components it will make the rig more inert in certain axes. This will also result in more room between the topstage and the gimbal for a given drop time. If you lengthen the sled and remove weight from the bottom and retain the same drop time and top stage to gimbal distance the rig should have the same inertia as the shorter and heavier setup.

The practical effect of this is that a longer 1 or 2 battery setup and a shorter 3 battery setup are probably normally going to be just about as inert. The design of the XCS sled allows for a much shorter post than the Ulra2 which in effect should allow you to create a less inert rig than most other sleds will allow. You of course can set it up with whatever length and in effect intertial dampening you like. While for many things we want a more inert rig for some things such as covering fast action you are going to want as quick(less inert) of a rig as you can setup.

I hope that was all as clear as mud and I didn't make any mistakes. I just got home after being up all night and I may not be all that coherent. I am sure someone will tell me if I said something that is incorrect.

~Jess
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#8 RonBaldwin

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Posted 05 December 2009 - 11:26 AM

Why never lead?


because we are out of the 80's (and we've learned you can't turn it to gold)

I'm amazed that people do this...I remember doing this with my model 2 for gawd's sake (yes, I'm old). Add a battery (ding ding), a recorder, a gyro, a rear-view mirror, a squirt gun, a fart machine, a dvd player to watch movies during boring shots, a sign-up sheet and pen for the cute extras, a business card dispenser, a printer to spit out invoices while you walk by the upm, a spare pair of shoes, deoderant, a back-up gimbal...something useful -- not dead weight!

rb
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#9 Jerry Holway

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Posted 05 December 2009 - 11:37 AM

A good reason to add pure weight to the bottom (if you don't need more power or accessories) is the compactness of a weight vs. an accessory. The volume is less, so it is less likely to hit you or extend the sled as much. More importantly, you can also position a weight creatively for inertia (where it goes). Wind shadowing might also be a reason. Weights are easily stacked, generally have sturdier mounting than batteries and accessories.

Yes, I'm from the old school and adding pure weight just fundamentally cuts across the grain; but with arms carrying a bigger to smaller loads, making the sled light in the first place (with those lighter batteries and monitors) and adding weights is quite useful.

Jerry
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#10 chris fawcett

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Posted 05 December 2009 - 12:41 PM

Yes, I'm from the old school and adding pure weight just fundamentally cuts across the grain; but with arms carrying a bigger to smaller loads, making the sled light in the first place (with those lighter batteries and monitors) and adding weights is quite useful.

I'm a new convert to this heretical practice too. All it takes is a leap of faith ;)

Chris
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#11 Alfeo Dixon SOC

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Posted 05 December 2009 - 01:28 PM

never lead.

I have to go with Eric on this one...

Now IRON!!! That's a different story!
Lead can kill you if it gets in your blood vs if your feeling a bit drained from your workday, just scrape off a bit of your iron and give yourself a little boost.
no super hero named Lead Man... unless your an under achieving art director and your Leadman is always bailing your ass out.

But seriously... Dead weight can be useful, but with power hungry cameras sucking the life from your rig like vampires, a nice hunky third batt sounds good about now in the bottom of the rig.

-Alfeo "AB Hytron 120's as a 3rd?!?!" Dixon
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#12 Alfeo Dixon SOC

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Posted 05 December 2009 - 01:52 PM

I am sure someone will tell me if I said something that is incorrect.

:ph34r:

If you extend your post but don't change the weight of the components it will make the rig more inert in certain axes. This will also result in more room between the topstage and the gimbal for a given drop time. If you lengthen the sled and remove weight from the bottom and retain the same drop time and top stage to gimbal distance the rig should have the same inertia as the shorter and heavier setup.

Close. Inertia is directly related to an objects mass. So removing the weight and lengthening the sled changes the physical dynamics of the sled, there for not retaining the same inert qualities. By only lengthening and not changing the weight, your changing the Moment of inertia, just like an ice skater in a spin changing the length of her arms.

-Alfeo "Ouch, that hurts my head when I do that" Dixon
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#13 Eric Fletcher S.O.C.

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Posted 05 December 2009 - 02:20 PM

I am sure someone will tell me if I said something that is incorrect.

:ph34r:

If you extend your post but don't change the weight of the components it will make the rig more inert in certain axes. This will also result in more room between the topstage and the gimbal for a given drop time. If you lengthen the sled and remove weight from the bottom and retain the same drop time and top stage to gimbal distance the rig should have the same inertia as the shorter and heavier setup.

Close. Inertia is directly related to an objects mass. So removing the weight and lengthening the sled changes the physical dynamics of the sled, there for not retaining the same inert qualities. By only lengthening and not changing the weight, your changing the Moment of inertia, just like an ice skater in a spin changing the length of her arms.



Well again, close but not entirely correct. Inertia is an object's ability to resist change. IE if at rest it will want to remain at rest and if in motion it will want to remain in motion. Moment is leverage or force multiplier. Double the moment and you double the force required to change the acceleration of an object, and in the end all we are really concerned with is Acceleration
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#14 Jess Haas SOC

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Posted 05 December 2009 - 08:55 PM

I am sure someone will tell me if I said something that is incorrect.

:ph34r:

If you extend your post but don't change the weight of the components it will make the rig more inert in certain axes. This will also result in more room between the topstage and the gimbal for a given drop time. If you lengthen the sled and remove weight from the bottom and retain the same drop time and top stage to gimbal distance the rig should have the same inertia as the shorter and heavier setup.

Close. Inertia is directly related to an objects mass. So removing the weight and lengthening the sled changes the physical dynamics of the sled, there for not retaining the same inert qualities. By only lengthening and not changing the weight, your changing the Moment of inertia, just like an ice skater in a spin changing the length of her arms.

Removing weight is going to make it easier to move the rig around in space as there will be less mass but it is still possible to retain the same rotational inertia by lengthening the post and the distance between the batteries. That rotational inertia is what we want. Personally I don't care nearly as much about the changes in inertia for lateral and vertical movements as that has minimal impact on the frame.

Rotational Inertia(mass moment of inertia with respect to a given axis of rotation) equals mass times the distance to the center of rotation squared.
I=mr^2
I=object's resistance to changes in its rotational rate
r=distance to the axis of rotation
^2 means r is squared.

So when you change the mass there is a value you can change the distance of that mass from the axis of rotation that will retain the exact same rotational inertia.

For example we have a short sled that we want to lengthen and lighten while retaining the same inertial feel. We remove one of 3 batteries from the base changing the mass. There is a length we can then set the sled at that will have the exact same rotational inertia. In order to also retain the exact same rotational inertia in the pan axis we may need to also lengthen the distances of the remaining batteries from the post.

I just noticed I may have made a mistake in stating that the gimbal to stop stage distance would be the same with the longer rig when retaining rotational inertia but that doesn't really change the rest of it.

~Jess
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#15 Afton Grant

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Posted 05 December 2009 - 11:00 PM

Removing weight is going to make it easier to move the rig around in space as there will be less mass but it is still possible to retain the same rotational inertia by lengthening the post and the distance between the batteries. That rotational inertia is what we want. Personally I don't care nearly a much about the changes in inertia for lateral and vertical movements as that has minimal impact on the frame.


Rotational inertia is what "we" want... when "we're" looking to stabilize rotation. What if the shot calls for an ultra slow creep in/out/around an actor or object? Rotational stability now takes a back seat to simple, glass smooth stability, especially in the vertical axis. Larry McConkey has said about the IMAX camera, it is heavy as all sin, but the thing flies like a dream thanks to its sheer mass. Be careful with your generalizations, Jess.

Eric's point is a good one. Acceleration is the key factor in all of this. It measures the rate of change in velocity of an object. In our case, it's another way of referring to how shaky the rig is. Obviously, we're looking to reduce acceleration as much as possible in most shots - the exceptions being the odd shots where they're looking for something in between handheld and steadi. There are a hundred ways to affect acceleration and increase stability in our rigs. No single one of them is the right way.
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