Does using post length affect stability?

[Jess Haas SOC]

I was not making a generalization and saying that non rotational inertia doesn't matter I was saying that the difference in inertia between a long post with less weight at the bottom and a short post with more weight at the bottom doesn't concern me nearly as much as the rotational inertia because the difference in mass is so small compared to the total mass of the rig. Sorry if that wasn't clear enough.

 

Heavy cameras are a whole different ball game. As you said they are super smooth as the extra mass adds to all types of inertia.

 

~Jess

[Eric Fletcher S.O.C.]

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.

 

Well that in it's self is incorrect and a very gross generalization. Reducing the Polar Moment of Inertia makes the rig easier to move in space. Weight has absolutely NOTHING to do with it. Lengthening the post has NOTHING to do with pan inertia (All movement with respect to a central pivot point is rotation, it just depends on the reference plane that you choose to use. And incase it's not totally clear for you the gimbal is the central pivot point) Moving the Battery mass and monitor mass outwards in relation to the pivot centeriod changes the pan moment of inertia and thereby effecting pan acceleration. As for not caring about Lateral and vertical movements well that's a choice and it most certainly does have an 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.

 

I know that and have already said that in my last post.

 

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.

 

Already said that also also.

 

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.

 

Okay now you're just repeating info that's already been posted many times

 

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.

 

Then why make the post if you are going to let the mistake stand. It devalues the currency of anything you post. Let me state this again Rig LENGTH (height) has NOTHING to do with Pan rate. it WILL affect Roll and Tilt.

[Eric Fletcher S.O.C.]

I was not making a generalization and saying that non rotational inertia doesn't matter I was saying that the difference in inertia between a long post with less weight at the bottom and a short post with more weight at the bottom doesn't concern me nearly as much as the rotational inertia because the difference in mass is so small compared to the total mass of the rig. Sorry if that wasn't clear enough.

 

Heavy cameras are a whole different ball game. As you said they are super smooth as the extra mass adds to all types of inertia.

 

~Jess

 

What??? What is Non rotational inertia, in steadicam EVERYTHING is Rotational, Remember that three axis gimbal and it's purpose? To access the cameras center of gravity. What is CG? the mass's rotation point.

 

Since you believe that mass doesn't really matter I suggest reading about a gentleman named Archimedes and his lever.

 

Heavy cameras abide by the same laws of physics that light cameras do so it's not really a "Whole Different ball game"

[Jess Haas SOC]

What??? What is Non rotational inertia, in steadicam EVERYTHING is Rotational, Remember that three axis gimbal and it's purpose? To access the cameras center of gravity. What is CG? the mass's rotation point.

Non rotational inertia would be the inertia of moving the entire rig in the X Y or Z axis. As in the inertia that is effected by the change in mass besides rotational inertia. As in what Afton and I were talking about. Did you even read his post or did you just read mine and look for something to criticize me for?

 

Since you believe that mass doesn't really matter I suggest reading about a gentleman named Archimedes and his lever.

 

Heavy cameras abide by the same laws of physics that light cameras do so it's not really a "Whole Different ball game"

I never said mass doesn't matter or that the laws of physics are different. I said there is a difference between a 1 or 2 pound weight difference and 10-30+ pound weight difference. Try to read and understand the post before criticizing it.

 

~Jess

[Jess Haas SOC]

Well that in it's self is incorrect and a very gross generalization. Reducing the Polar Moment of Inertia makes the rig easier to move in space. Weight has absolutely NOTHING to do with it.

What is incorrect? Mass has everything to do with inertia. Besides we already discussed the generalization of my statement and it was not a generalization it was a miss understanding. Read the posts.

 

Lengthening the post has NOTHING to do with pan inertia

I never even mentioned pan inertia. I only mentioned the possible need to move the batteries out because when you remove weight from the bottom of a rig and lengthen a post to maintain the same rotational inertia you may need to move the masses at the bottom of the post out slightly in order to make up for the removed mass as it may be necessary to maintain the same pan inertia. I didn't go that in to detail because it wasn't necessary for the point and I probably wouldn't even have mentioned it but I was being thorough so that there wouldn't be any holes in my statements for someone to jump down my throat about.

 

(All movement with respect to a central pivot point is rotation, it just depends on the reference plane that you choose to use. And incase it's not totally clear for you the gimbal is the central pivot point) Moving the Battery mass and monitor mass outwards in relation to the pivot centeriod changes the pan moment of inertia and thereby effecting pan acceleration.

No one is questioning that and I think we all already knew it.

 

As for not caring about Lateral and vertical movements well that's a choice and it most certainly does have an impact on the frame

I never said I don't care about them just that small changes in non rotational inertia are not as important as large changes in rotational inertia when it comes to the smoothness of the shot. Hence the whole design of the steadicam to minimize unwanted rotational movement.

 

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.

 

I know that and have already said that in my last post.

I guess in the future I will stop illustrating my points and just leave that up to you.

 

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.

 

Already said that also also.

Actually I already said that although you may have repeated it. I was reiterating as there was some questioning of my statements.

 

 

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.

 

Then why make the post if you are going to let the mistake stand. It devalues the currency of anything you post. Let me state this again Rig LENGTH (height) has NOTHING to do with Pan rate. it WILL affect Roll and Tilt.

I didn't let it stand I pointed out the mistake. Besides it is a minor error that doesn't effect the point of the statement. And again who said rig length has anything to do with pan rate?

 

~Jess

[Ken Nguyen]

Wow!

How the "post" evolves to!

Very confusing gent.!

I love this forum.

 

Advantage and disadvantage of sled length:

Short: good bottom clearance, boom down lower, but the sled weight is heavier (the shorter the sled the heavier the sled bottom to counter-balance the weight camera);

Long sled: lighter in weight, but less bottom clearance and limited boom down.

 

The sled length won't affect the stability of the steadicam (assumed that the post is rigid).

The drop time is the one that will, but can be controlled through practice.

 

About the use of LEAD dead weight, I think it is better than using the 3 batteries option.

In the 3 batteries configuration, you need 3 batteries at a time.

That means, 3 on the sled, 3 freshly charged standing by, and 3 on the charger(s).

The total is at least 9 batteries to travel with you in a bigger case.

With the dead weight, you just need a minimum of 6 batteries for the job (using 2 batteries configuration) or only 3 batteries. No case is needed for the dead weight. For Steadicam branded sled, you can stand the sled down on the ground with the dead weight. But, not with the 3rd battery in the bottom.

Dead weight is also needed on the top stage for lighter camera.

Unnecessary accessory added into the sled will consume more power; draw more current through the wiring; require more time to hook up and secure to the sled.... too complicated ... too luxury ... too NOT practical.

 

Peace!

Ken.

[Eric Fletcher S.O.C.]

Advantage and disadvantage of sled length:

Short: good bottom clearance, boom down lower, but the sled weight is heavier (the shorter the sled the heavier the sled bottom to counter-balance the weight camera);

Long sled: lighter in weight, but less bottom clearance and limited boom down.

 

The sled length won't affect the stability of the steadicam (assumed that the post is rigid).

The drop time is the one that will, but can be controlled through practice.

 

 

Just because the sled is short doesn't mean that it has to be heavier to counterbalance the camera. That is determined by the Gimbal to camera distance. There's where that Lever comes into the equation. Sled length does affect the stability, but only in two axis.

[Ken Nguyen]

Advantage and disadvantage of sled length:

Short: good bottom clearance, boom down lower, but the sled weight is heavier (the shorter the sled the heavier the sled bottom to counter-balance the weight camera);

Long sled: lighter in weight, but less bottom clearance and limited boom down.

 

The sled length won't affect the stability of the steadicam (assumed that the post is rigid).

The drop time is the one that will, but can be controlled through practice.

 

 

Just because the sled is short doesn't mean that it has to be heavier to counterbalance the camera. That is determined by the Gimbal to camera distance. There's where that Lever comes into the equation. Sled length does affect the stability, but only in two axis.

I stay firm what I stated.

The length of the sled doesn't affect the stability of the sled.

It's the ability of the operator to handle the sled.

For the long sled you need more force from your fingers to dampen the pendulum effect. Yes, it's a little harder than the short sled. But, can be easily done through practicing.

Solid example of long sled are: (PRO & XCS) super post, long mode Steadicam sled, MK-V AR sled. Are any of you having stability problem flying those?!?

If YES, need to practice more or retrained!

 

Shorter sled means more weight. Period.

Eric, you are very proud to have the XCS sled to keep the camera CG as close to the gimbal as possible (from the other "war waging" post about the best sled).

And, you love to fly with your sled short.

How can you achieve your goal without adding more weight into the bottom of the sled?

With the same camera weight, shorten the sled will need more weight at the bottom; this is also based on the LEVER or simple machine equation you stated.

As you shorten the sled length, where can the gimbal goes when it is already at the edge of the top stage? Remember, the camera weight is much heavier than the bottom stage. Well, sorry, this is not true if you fly DV camcorder.

Ken.

[RonBaldwin]

Shorter sled means more weight. Period.

 

...How can you achieve your goal without adding more weight into the bottom of the sled?

 

what you say is true...but I think Eric is referring to is the weight distribution advantage in his particular sled. An xcs sled is not necessarily any heavier than other sleds but it can be shorter because has better (if one happens to think a shorter sled is better) placement of mass as well as the ability to get the gimbal higher.

 

An Ultimate sled with 2 dionics will be much shorter (if the op desires) than a U2 and be approx the same weight. I have used my Pro 1 side by side with a U2 and a Nexus with identical camera packages and there was a huge difference in sled length. The Nexus was at a minimum 6" longer than my Pro 1 because of the 4 stage post and perhaps a bit less mass at the bottom. The U2 can collapse to the same length as my sled but you'll need to add a good deal of weight to use it like that (as you said). Not dissing the U2 or the Nexus here, nor am I saying one is better than another. Just different. I only use these sleds as examples as I've geeked out with them a bit. They all have their own set of advantages and disadvantages, though it's good to know what you are getting into.

[Charles King]

I've been reading this post diligently. It is interesting to see how certain sleds differ to one another and their behavior. It's nice to know the feedback of these rigs so as one become more experience as time goes by( like I'm growing) it would be nice to look back at this thread and see the choices one have that will fit ones needs. :) Great thread people. Also, I'm getting deja vu from my physics lessons.. Ha Ha Ha :)

[Alec Jarnagin SOC]

"Just because the sled is short doesn't mean that it has to be heavier to counterbalance the camera. "

 

Yes. Weight distribution is the key here. With heavy cameras (or more specifically, cameras with a high CG), I place as many AKS on the bottom of the sled as possible but with low CG cameras (or light ones) I place them up top. CG of camera is a big one. For example, I've been flying the Panavision F35 a lot lately with their fiber system. Great weight, but the CG is higher than the normal F35 because of the Panavision shoe on the bottom. It actually places my gimbal and post in the places I'm used to for normal 35mm work, but its less weight overall.

[Eric Fletcher S.O.C.]

Shorter sled means more weight. Period.

Eric, you are very proud to have the XCS sled to keep the camera CG as close to the gimbal as possible (from the other "war waging" post about the best sled).

And, you love to fly with your sled short.

How can you achieve your goal without adding more weight into the bottom of the sled?

With the same camera weight, shorten the sled will need more weight at the bottom; this is also based on the LEVER or simple machine equation you stated.

As you shorten the sled length, where can the gimbal goes when it is already at the edge of the top stage? Remember, the camera weight is much heavier than the bottom stage. Well, sorry, this is not true if you fly DV camcorder.

Ken.

 

 

No it's doesn't. Sled length is controlled by five things:

 

1) Weight of the camera

2) Height of the cameras CG

3) weight of the lower section of the sled

4) CG of the lower section of the sled

5) position of the gimbal

 

Some rigs have a very large gimbal to top stage distance (Ultra and MK-V) and some don't (XCS Ultima) The higher the gimbal the shorter the sled can be all because of the lever ratio.

[RonBaldwin]

an additional geeky note on all this hubbub (no pun intended) many of us deem important about the ability/advantage/option -- whatever you want to call it -- of keeping the gimbal as high as possible and it's distance to the camera. I've been wanting to upgrade sleds for almost 2 years now so I've been gathering a bunch of useless info as I came across it!

 

Here are some measurements from the bottom of the upper dock ring to the top of the camera plate:

 

-Pro1 with Pro camera plate (original non quick release post) -- 2 3/4"

-Pro2 with current Pro post design/Pro plate -- 3 3/8" (adding about 5/8" to the height)

-Ultimate with Greg's plate -- 2 5/8" (about 1/8" less than the Pro1 and 3/4" less than a Pro2)

-Ultra2 -- 5"? (a guess)

-Nexus ?

 

please chime in with additional info as well as corrections if I just plain screwed up!

 

using the Pro2 vs Pro1 as an example, that extra 5/8" might not seem like a big deal but it will require almost two extra inches of post length to balance the same camera -- obviously more with lighter battery/monitor combos.

 

The XCS I test flew was with (I believe) an older dock ring which was thicker, so the distance might be a bit less on current rigs? And I know several Pro2 ops who use the top knurled ring as the dock ring, which would keep it similar the Pro1

[Colin Donahue]

No it's doesn't. Sled length is controlled by five things:

 

1) Weight of the camera

2) Height of the cameras CG

3) weight of the lower section of the sled

4) CG of the lower section of the sled

5) position of the gimbal

 

Some rigs have a very large gimbal to top stage distance (Ultra and MK-V) and some don't (XCS Ultima) The higher the gimbal the shorter the sled can be all because of the lever ratio.

 

I thought I was following this, but Eric's post made me think of something. If you want the shortest post possible, aren't you also trying to achieve the shortest distance between the top (camera) cg and the base cg? If a sled has a base that is larger (Pro) and extends several inches below the post, the distance of top cg to bottom cg might be similar to a sled that has a smaller base (Ultra 2) even though the post is longer. Is this correct?

 

Colin Donahue

[Eric Fletcher S.O.C.]

No it's doesn't. Sled length is controlled by five things:

 

1) Weight of the camera

2) Height of the cameras CG

3) weight of the lower section of the sled

4) CG of the lower section of the sled

5) position of the gimbal

 

Some rigs have a very large gimbal to top stage distance (Ultra and MK-V) and some don't (XCS Ultima) The higher the gimbal the shorter the sled can be all because of the lever ratio.

 

I thought I was following this, but Eric's post made me think of something. If you want the shortest post possible, aren't you also trying to achieve the shortest distance between the top (camera) cg and the base cg? If a sled has a base that is larger (Pro) and extends several inches below the post, the distance of top cg to bottom cg might be similar to a sled that has a smaller base (Ultra 2) even though the post is longer. Is this correct?

 

 

Now you're asking me to go all math on you...

 

THIS IS A HYPOTHETICAL RIG

 

30lbs camera

6" high CG from camera baseplate (Film Camera)

5" to the gimbal centeroid

330 in-lbs of moment

 

For the rig to have neutral balance you need to have 330 in-lbs of Moment on the bottom of the rig

15 lbs base

330/15= 22" under the gimbal centeroid to the base CG

 

Now try that with the

 

30lbs camera

6" high CG from camera baseplate (Film Camera)

3" to the gimbal centeroid

270 in-lbs of moment

 

270/15=18" under the the gimbal centeroid to the base CG

 

Where you're getting confused is you're looking at the physical size of the base without taking mass and CG into account. Build the ultra's base out of Depleted Uranium or Tungsten and it will have a shorter post even though the base is "Smaller". Tabling the Monitor and running it at the base of the rig helps to get the CG lower, as does running the batteries under the base.

 

Did I answer your question?