Why a Backmounted Harnass works.....

[Jerry Holway]

Interesting stuff. Some thoughts and a question:

 

I'm very happy with the new Ultra vest which has a 100 percent rigid and very efficient front spar/over shoulder attachment and really good "cross-back" straps. I also prefer (and have) the front spar inside the waist band, like the old IIIA, II, and I vests.

 

I only used to experience aches in my right shoulder from using a IIIA type arm when I had to lift up the sled up or push it down with much more force than the iso-elastic arm. New iso arm, no aches. Still didn't like to run with the MS vest.

 

With both the DSD vest and the traditional vest, your body has to lean back in the vest to counter act the force of gravity on the sled. It's the same force, and the same leaning back regardless of the type of vest. Everyone who has taken a workshop with GB can remember his drawings and the "free" force required for this by leaning back.

 

With a front mounted vest, a less than rigid design in the over-shoulder to the "Y" piece (like the Master Series vests), made for a lot more work. In a back mounted vest, the rigidity has to be in the arm attachment to the back, and from the lower back to the upper/center back). In either vest, the key is how efficient and postive your leaning is.

 

I can only saw now that the new Ultra vest is much much better than the old MS vest, and I only get fatigued in the legs... never in the lower back, shoulders, etc. I also have much more control, less fatigue overall, can run flat out...

 

I'd like to know how other ops with the new Ultra vest feel about it.

 

By the way, I have been in about six different DSD vests, and some fit me well and worked well.

 

Jerry

[RobVanGelder]

Frankly the arm mount could be on the sides, swing up over top or rise up between you legs for all it matters, because the static weight still comes from the center of gravity point and not the contact point of the arm mount to the vest.

Hi Mitch,

This is where I disagree with you and with the theory of Jerry. But at the same time you have made it clear that it is possible to see the whole construction outside the body, be it front, back, side or wherever, can be seen as a load in one point. e.g. center of gravity.Our system works because of our body, one piece of mass with it´s own center of gravity, which is exactly between our feet, underneath our spine is in balance with the other one, the rig+ parts. Now, placing the point of attack in line with your spine, looking from the front or behind, does make a difference than placing the same weight on one side and from that point on divert it into the harnass, thus creating a sideways lever on the spine and hips. Again, my comparison with the fire truck stands and you have to find me a good example to make me understand otherwise.

 

Rob van Gelder, Amsterdam, Bangkok

[PaulEdwards]

Again, my comparison with the fire truck stands and you have to find me a good example to make me understand otherwise.

Hi Rob,

 

I've just sent you an e-mail with a word attachment sketch using your "firetruck" example. I think it shows why the arm attachment point is not the critical thing with the DSD vest.

 

I would have posted it up here if I'd cracked the technology!

 

Let me know what you think.

 

Paul

[RobVanGelder]

Hi Paul,

 

That´s funny, I could not sleep so I sat down and did a similar thing , trying to prove why I think it does make a difference, I try to post it here and I will email it to you.

 

Rob van Gelder, Bangkok, Thailand

[PaulEdwards]

Hi Rob,

 

I think I've found the way to post the pics here and keep this discussion on the board- just in case anyone else is interested!

 

In the meantime here's my reply

 

"..........It also shows that when you swivel the ladder to the other side (= the front-mounted harnass) , the only force that prevents the truck from tipping over is its weight, since the pivoting point is on the same wheel the ladder is above.

 

True, but this is always the case wherever the ladder (or steadicam arm) is attached.

 

"............ That is a less safe situation than your first drawing where the pivoting point is the opposite side of the truck giving a lever to the ground on one side over the full length between the wheels + the weight from the truck + theweight of ladder on the other side."

 

Not true. The drawings of the two trucks are exactly the same in terms of forces (I've assumed that the ladder weight is not significant). It might be clearer if you look at the drawing of the two trucks again but imagine that the truck on the top drawing is on the very edge of tipping over forwards, so that the rear wheels are just a milimetre off the ground. All the forces are pivoting about the front wheels. Putting in some figures;

 

M2=100 kg

M1=400 kg

L1=5 m

L2=20 m

 

The pivot force trying to pull the truck over (clockwise rotation) is;

 

M2(kg) x L2(m) = 100 x 20 (Kg.m) = 2000 kg.m

 

The pivot force trying to stop the truck falling forward (anticlockwise rotation) is;

 

M1(kg) x L1(m) = 400 x 5 (Kg.m) = 2000 kg.m

 

and the two forces balance perfectly.

 

These are the only forces that can act on the ground because this is the only point of contact.

 

 

If you now calculate these forces for the bottom drawing they will be exactly the same because the masses are the same and they are in the same points in space. The ladder point of attachment makes no difference.

 

To get a clear view about what is going on in terms of forces you should forget about "vertical forces". Gravity tries to pull each mass (operator's body, sled etc.) downwards but the important thing is how these forces pivot about the point of contact with the ground (ie. wheels or feet).

 

It is wrong to get tied up with the position of the ladder on the truck (or the arm mounting position with the steadicam). The only things which are relevant are the position of the masses and their distances from the point at which these masses act ie. their distances from the point where the wheels on the firetruck touch the road (or where the operator's feet contact the ground).

 

Let me know what you think.

 

Regards,

 

Paul

[PaulEdwards]

.....and for completeness here's my latest painting entitled "Operator as a Firetruck"......

[Mitch Gross]

Paul--

 

Thanks for your excellently simple and clear diagrams to explain why it makes no difference for the mounting point in vest design. I was dreading having to remember the finer points of my physics and applied mathematics courses from 20 years ago but you summed it up quite nicely. Vest mounting points are not the issue, arm to socket block as fulcrum and center of gravity are.

 

Mitch "cosine theta" Gross

[guillermo nespolo]

and one last thing fisics are ok with firetruck and the sled ...but here its more i think a physicist what realy can help

if u look at your drawing the point of the union of the 2 mases ist on the waist....and firetruck do not vent ont the midle?

 

 

sorry but my english its no good

eso es para vos grimmet la reputa madre que te pario

[Larry McConkey]

That's exactly right, but unfortunately I would then be in a foam sandwich and that would be very, very hot. I used to sweat profusely whenever wearing my front mount vest, partially because of the larger structure and padding that I went to in order to get more contact with the vest. The DSD is by comparison, far cooler and I love that aspect as much as any. I have finally recovered almost all the finesse I had with the front mount, so I don't think I will ever use the hybrid, unless I decide it would be the perfect solution for the tight spaces that interfere with the DSD arm - I could mount the front to the back mount... I continue to have conversations with Walter about reducing the obstruction issue, however, and as I said before, I don't think he will ever stop improving his designs.

 

Larry

[LeighWanstead]

Hello everyone,

 

May I ask if back mount vest has patent on it?

 

TIA

Leigh

[DavidWest]

Paul....

 

nice drawings.....

 

an older thread, so maybe no response, BUT.....

 

 

 

the first drawing, ladder from rear.... has a downward force shared between the two wheels.... the second drawing with the ladder on the front has all the force concentrated upon the front tire while the rest of the vehicle is being "pryed upwared"....

 

in terms of cranes, the first drawing could lift closer to the point of structural failure while the second would be more tip prone.... but most importanly VERY different forces are being applied to the truck and ie your anology, the body....

[RobVanGelder]

Please go on..... this thread is still interesting and I am sure that there are some odds and ends that need to be looked into from another perspective.

 

My original pictures have been replaced for thumbnails that don´t show the drawings I made.

I will try to upload them again.

 

Edit: the links are working again!

[PaulEdwards]

Hi Dave,

 

The point at which the fire trucks are analagous with steadicam is when the system is in balance. ie. as on the drawings, M1 x L1 =M2 x L2. Therefore in the first (and second) drawing, there is no force on the back wheels.

 

Imagine that we have parked the back wheels of the truck on bathroom weighing scales to measure the force of the back wheels on the road. Then we slowly increase the mass (M2) at the end of the ladder. As M2 increases, the reading on the weighing scales will drop until we reach the point at which the truck is just about to (but not quite) tip over and we have reached the point at which the system is in balance. The reading on the scales will be zero value ie there is no force applied between the road and the back wheels.

 

If you're still not convinced, imagine the trucks with no back wheels and the back of the truck hovering balanced in the air.

 

By the way did the drawings in Word!

 

Paul

[DavidWest]

hmmmm....

 

i think i understand what you are trying to convey.......

 

 

my experience is more with cranes than firetrucks... but the standard truck crane is still similar to your drawing.....

 

i guess what i am trying to say is that the tipping point (although i have done my share) is not where the crane is supposed to be operated...

 

unless im wrong, we dont really want to operate a steadicam at the tipping point either.

 

 

I assume that you should be able to extend the boom of your steadi straight out in front of you without tipping over whether mounted from the front or the back??

 

 

likewise there is a shift in the forces from the rear outriggers of a crane (or firetruck) to the front as you extend the load away from the center of gravity until

you reach the point of tipping. ----until you reach that point of tipping the loads are shared with all contact points to the ground.... not equally shared, but shared.

 

 

--- there are two main types of load failure here (or in a load chart for a crane). one would be tipping and one would be mechanical failure.... the cranes that i have run seemed to be more prone to mechanical failure at certian angles and more prone to tipping failures at other angles relative to the undercarriage.

 

 

im probably not making any sense....

 

your physics seem right to determine the tip point, but do not seem to reflect what happens the rest of the time...

 

if you move the ladder to the front of the fire engine the CG changes..

(but it is still inside the body of the truck)

if you move the steadicam from the front to the rear i think the cg changes too..

is this shift enough to be concerned about? i dont know...

 

i think the main effect that back mounted vest has is that it crushes different parts of the body... but, in the words of seargent schultz, "i know nothing..." as i have never worn a back mounted vest....

 

 

 

good or bad? depends upon what feels good to the operator i would guess as long as no long term health effects show up years from now....

[PaulEdwards]

Dave,

 

"unless im wrong, we don't really want to operate a steadicam at the tipping point either"

 

We do actually operate at the tipping point all the time. I don't recommend it, but if you want to prove it then stand wearing your rig, perfectly balanced, and without doing anything else to compensate, either lean forward very slightly or get someone to pull the sled slightly away from you, without you compensating by leaning back. You will fall over!!

 

This is why the drawing of the truck at the point at which it's just about to tip over is so relevant.

 

Paul