# Question about tire swing weight



## obelix (Dec 12, 2012)

Have a look at the photo. I want to suspend a tire swing at the end of a beam so I don't have to put a post in the middle of my yard. My concerns will this hold the weight at the end of the 4x6x16?


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## 65BAJA (May 27, 2012)

I would think a 4x8 would be better. How would you attach it to the trees?


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## obelix (Dec 12, 2012)

I'm thinking bolting them to the trees.. Maybe two lag bolts for each tree. The tree closest to the swing has a small branch (maybe 2 inches in diameter) that I can set the beam on as well.


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## obelix (Dec 12, 2012)

there must be some calculation to figure out how much weight or force it can older over a span away from a post.


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## woodnthings (Jan 24, 2009)

*there is but it's a bit complicated by the second tree*

Weight of the tire plus weight of the person(s) X the distance from the tree = the weight (force) supported by the first tree and the resultant lever ( weight X distance) resisted by the second tree. The first tree is a pivot for the seccnd tree to resist or balance. The sum of all vertical loads must = zero or it's in motion.... :thumbdown: Probably some higher math involved?

Just "overbuild" it .... :yes:

The problem I see it that you may possibly kill the tree in a few years, I donno? A 3/4" or 1" all thread rod passing all the way through the tree may be a better solution than several lag bolts. A forester or arborist may have some advice on the tree's health. We have one here as a member. 
http://www.woodworkingtalk.com/members/forester01-34923/


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## obelix (Dec 12, 2012)

I was actually thinking of cutting them down, I've never liked them, they kill my grass and I have other trees in the back of my properly that I like better, so they arn't really necessary. My kids really want a swing, so I'm only going to keep them around if I actually do this. I think if I accidentally kill them then I'll just cut them down anyways.. What is the main differences in structural stability between a long agressively threaded lag bolt and a threaded rod that goes through? 

The one thing I fear, which is why I'm trying to figure out if this as good idea, is if the weight out at 8ft beyond the pivot that you mentioned will break a 4x6. I wonder how much weight a 4x6 will take before it snaps in half. I image if it were to break, it would likey be at the pivot point where it bolts onto the tree closest to the swing. One person mentioned to use a 4x8,which may be better, but if a 4x6 can hold up say, 1000 lbs at 8ft out without support, then that would be more than enought. I think it can hold 500, that would still be plenty.


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## nblasa (Nov 29, 2011)

Is there any reason why you can't just put the swing between the trees? 8' seems far enough apart to avoid a good whack into the trees


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## 65BAJA (May 27, 2012)

If you do use lags, remember galvanized lags will kill the tree.


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## obelix (Dec 12, 2012)

nblasa said:


> Is there any reason why you can't just put the swing between the trees? 8' seems far enough apart to avoid a good whack into the trees


I guess you'd have to see it, but even at 8ft, it just doesn't look like much.. you have to figure the tire is about 2 ft wide, so that would leave 3 ft on each side between the 3, and the trees are 8ft on center, and are 10" thick, so really it's closer to 2 1/2 - 2ft on each side of the tire.. Just cramped.


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## rrbrown (Feb 15, 2009)

I'm not sure about a 16' span will need at least a 6x8. There will be a lot more twisting and flexing then you think. I'm basing my opinion from the one my BIL built using a 4x8. Although the 4x8 has a safe amount of movement I think a 6x8 would have reduced it a lot since the twisting and flexing is coming from the width. I think your span is longer which will make the effects of the swing moving worse.

He used a wheel hub to mount the swing to the timber. It definitely helps with the swing operation.

Just my .02


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## WoodWorkinRI (Aug 17, 2011)

Woodnthings is correct, threaded rod all the way through with large washers and nuts is the best way to do it. Same with hanging a swing from a limb, it is better to put a rod vertically through the limb than it is to tie a rope around the limb.


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## cabinetman (Jul 5, 2007)

I'm not a structural engineer, nor is anyone else here I don't think. But off the top of my head, I think a truss assembly like below, or some derivative of the sketch below would provide extra strength.
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## woodnthings (Jan 24, 2009)

*If you don't care about the trees...*

Then that's fine. If you allow the trees to leaf out, the wind will want them to bend and shear off the lags. A truss only adds to this issue since there are more points of contact and the trusses will resist any movement.

You could just whack off the top and be done with them using them as embedded posts. A bolt or hole through the center of a beam does not essentially weaken it. You could rest the beam on the top of one of the trees stumps and just keep it in place with plates. By increasing the height of the beam you increase the strength greatly. going from a 4 x 6 to a 4 x 8 would allow you to at least double the load at the end...if I recall from my engineering strength classes. Like I said go bigger or higher/taller in this case, if in doubt.

BTW, yoiur sketch show a 2/3rds to 1/3 relationship to the support. I assume that's just artistic license.... 

You can determine the deflection or load on a cantilevered beam if you know calculus:


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## Shop Dad (May 3, 2011)

Couple of additional things to keep in mind; your force will not just be downward. Swinging to the sides could put enough force on lag bolts to pull them out. Especially when you take into account tree growth. The tree will be moving over time and will put pressure on, or even snap bolts and threaded rod.


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## cabinetman (Jul 5, 2007)

woodnthings said:


> Then that's fine. If you allow the trees to leaf out, the wind will want them to bend and shear off the lags. A truss only adds to this issue since there are more points of contact and the trusses will resist any movement.


A truss will support the beam. This is the point of the post. It's about using a beam in the trees in the first place.









 







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## cabinetman (Jul 5, 2007)

Forget through bolts or lags. If you are into rope, knots and lashing, that truss can be secured that way. 









 







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## frankp (Oct 29, 2007)

obelix said:


> I was actually thinking of cutting them down, I've never liked them, they kill my grass and I have other trees in the back of my properly that I like better, so they arn't really necessary. My kids really want a swing, so I'm only going to keep them around if I actually do this. I think if I accidentally kill them then I'll just cut them down anyways.. What is the main differences in structural stability between a long agressively threaded lag bolt and a threaded rod that goes through?
> 
> The one thing I fear, which is why I'm trying to figure out if this as good idea, is if the weight out at 8ft beyond the pivot that you mentioned will break a 4x6. I wonder how much weight a 4x6 will take before it snaps in half. I image if it were to break, it would likey be at the pivot point where it bolts onto the tree closest to the swing. One person mentioned to use a 4x8,which may be better, but if a 4x6 can hold up say, 1000 lbs at 8ft out without support, then that would be more than enought. I think it can hold 500, that would still be plenty.


It's not so much the weight but the dynamic force that will likely cause a problem. This force will be a vertical force as well as a torque about the beam. while swinging you can generate a lot of rotational torque. The vertical dynamic force will be less but not insignificant. That said, the rest of this response is based upon a vertical force because that calculation is easier to model.

The following is not professional advice but was calculated by using a structural engineering load/design calculator: 

Assuming pressure treated pine lumber (#2 like you buy at big box stores) and a 200 pound load at the tire, you will need at least a 4 x 10. You can also use 2 2x10s (better and easier to find) or a 6x8, or a 3x12. The program says a 4x6 fails by bending, as does a 6x6. 

We calculated based on a 200 pound load because we assume at some point multiple children or an adult will sit on this swing. 

Now, another issue I see is if you kill the trees, they're likely to be dead before you realize and then you're creating a "widow maker" hazard for your kids. I'd probably brace differently than just drilling into or even through the tree. In fact, I'd just build a non-cantilevered swing and get rid of the trees if you don't like them.


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## woodnthings (Jan 24, 2009)

*really?*

Quote:

Assuming pressure treated pine lumber (#2 like you buy at big box stores) and a 200 pound load at the tire, you will need at least a 4 x 10. You can also use 2 2x10s (better and easier to find) or a 6x8, or a 3x12. The program says a 4x6 fails by bending, as does a 6x6. 

Failure by bending? Of course it's going to "bend" How Much?
an inch..... or 6" at the end? Are you saying a 4 x 6 won't support a 200 lb load at a distance of 8 ft from the support? My street sense says that aint so. I will test it when I can get around to it. :blink:

Here a thinner beam 5cm thick X 10 cm wide on it's side, with a 175 lb roughly, man standing on it at roughly 7 or 8 ft. 






http://wn.com/cantilever


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## frankp (Oct 29, 2007)

woodnthings said:


> Quote:
> 
> Assuming pressure treated pine lumber (#2 like you buy at big box stores) and a 200 pound load at the tire, you will need at least a 4 x 10. You can also use 2 2x10s (better and easier to find) or a 6x8, or a 3x12. The program says a 4x6 fails by bending, as does a 6x6.
> 
> ...


No, Bill, your street sense is fully intact. Like you said, "just overbuild it."

The program assumes structural build integrity to match code for buildings that don't fail. I don't know what the tolerance is for "failure by bending." 

I was going to put deflection distances in my original post but left them out. The 4x6 deflects on the order of several inches but with a >90% stress on the beam (with 200 pound load) the whole time. The 6x6 says it only deflects 1.5 inches or something, but with the same stress. I figured err on the side of caution with those kinds of stresses and the fact that it's easier/cheaper to get 2 2x10s. The 2 2x10s will deflect less than a half inch, if I remember correctly. Also my original post didn't consider the torques involved, which are going to play havoc on a 4x6, especially if it's right at the edge of what it can reasonably handle.


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## GeorgeC (Jul 30, 2008)

rrbrown said:


> I'm not sure about a 16' span will need at least a 6x8. There will be a lot more twisting and flexing then you think. I'm basing my opinion from the one my BIL built using a 4x8. Although the 4x8 has a safe amount of movement I think a 6x8 would have reduced it a lot since the twisting and flexing is coming from the width. I think your span is longer which will make the effects of the swing moving worse.
> 
> He used a wheel hub to mount the swing to the timber. It definitely helps with the swing operation.
> 
> Just my .02


It is not a 16" span. There is only 8 ft unsupported.

George


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## woodnthings (Jan 24, 2009)

*Exactly!*



GeorgeC said:


> It is not a 16" span. There is only 8 ft unsupported.
> 
> George


Unless the drawing is not correctly dimensioned, the length of the beam in total is 16 ft, eight feet cantilevered out from the tree. The drawing itself is not showing the proportions correctly as I said it looks to be 2/3 cantilevered, 1/3 supported. If so that would mean a 24 ft long beam....


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## obelix (Dec 12, 2012)

I've put it up like in the drawing. with a few exceptions. LIke one person said, it looked like a lot of force at the end, so I put a 2x6 on the end, that goes back to the tree at a 45 deg angle, (like a triangle or a truss) to enforce the end. I've also put an eye bolt about 1 ft away from the tree on the 4x6 that jets out with the swing with the a chain loop around the tree so that if the worst happens and the end breaks off for some reason due to force, it will be suspended by the chain rather than falling down to the ground should the worst case scenario of the bolts sheering off or the 4x6 snapping happens. The same on the other other tree even though there shouldn't be much force over there.


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## Futurepast (Jun 18, 2012)

frankp said:


> No, Bill, your street sense is fully intact. Like you said, "just overbuild it."
> 
> The program assumes structural build integrity to match code for buildings that don't fail. I don't know what the tolerance is for "failure by bending."
> 
> I was going to put deflection distances in my original post but left them out. The 4x6 deflects on the order of several inches but with a >90% stress on the beam (with 200 pound load) the whole time. The 6x6 says it only deflects 1.5 inches or something, but with the same stress. I figured err on the side of caution with those kinds of stresses and the fact that it's easier/cheaper to get 2 2x10s. The 2 2x10s will deflect less than a half inch, if I remember correctly. Also my original post didn't consider the torques involved, which are going to play havoc on a 4x6, especially if it's right at the edge of what it can reasonably handle.


I agree but the 2 2x10s have to be "stitched" together properly (16 penny nails top and bottom staggered 6" O.C.) and I would add 2 2x8 flat top and bottom as stiffeners for the side sway


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## TomC (Oct 27, 2008)

How about a flitch beam? I don't believe I would trust a load that far out on the 4x6.
Tom


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