Internal frame – using tent poles?

[Huzefa @ Blue Bolt Gear]

I have been giving some serious thoughts to pack designing. I want to make a large volume ski/mountaineering/ice climbing/winter hiking/do-it-all pack that can be compressed like MLD Zip/Pinnacle for day hikes or just carrying skis. I will use 500D Spectra Gridstop for the back and bottom and 330D cordura for the body with pocket made of tough mesh on the front/sides. So its not exactly SUL but it will be very light for its intended application.

In another thread Rog suggested use of pole sections as pack stays. I have been working on that idea. My idea is to use a two sheet of foam with a verticle pocket at each side end between them. Tent poles will be go into these pockets to make a frame.

Some questions:
Is there any better way to do this?

My tent uses 12 x 18inch sections. I am 5 feet 8in – so will 18inch stays be ok?

I am open to any suggestions.
Thanks!

[Richard Matthews]

Using tent poles or even the rolled pad tube works when I keep my pack weight below 20 pounds.

Over 20 pounds I need curved pack stays.

Because my back is very curved the poles or pad tube pulls away from my back and the torque makes my shoulder straps uncomfortable.

Below 20 pounds I do not try to shift weight to my waist belt and simply carry the pack with the shoulder straps.

It depends on the pack weight and how you are built.

[Derek Goffin]

Can you make some of your tent poles back shaped in any way? If you do can you stop the poles rotating in their sack pockets?

[Huzefa @ Blue Bolt Gear]

Richard, I certainly expect my load to go over 20 pounds when skiing or mountaineering but not more then 35 pounds.

>Can you make some of your tent poles back shaped in any way? If you do can you stop the poles rotating in their sack pockets?

Yes I think I can. But I can make a curve only of which I have a precise mathematical equation. So the question is how much curvature should be optimal? It should also work with the tent. I have no idea how to go about finding that.

I can stop pole from rotating by using grommets.

[David Wills]

When I made my internal frame pack, I used 1"x1/8" aluminum that are about 20-22" and I'm 6'1, so you should be fine. They only weigh 6 oz for 2 stays though, so it may be a decent alternative if the poles feel awkward. Make sure whatever you use, you cover the metal ends with something smooth to avoid abrading the sleeve material.

[David Erekson]

Have you thought of an external frame like Bill Fornshell has made with arrow shafts. This will keep the poles straight and allow you to carry more weight. Just a thought.

[Huzefa @ Blue Bolt Gear]

David, I found your post 'Framed backpack 18.5 oz'. That pack looks awesome. Really good work.

I will think about aluminium frame. Actually my tent pole has 2 sections with inserts and (10 without). The inserts will protrude about 3". So I do have 2 x 21" poles. My guess is that Easton .344 diam poles will easily curve to my spine by tightening the waistbelt. So I may not even need to prebend them. As I said this is just my guess.

I hope someone with exprience will prove me right or wrong.

[Huzefa @ Blue Bolt Gear]

>Have you thought of an external frame like Bill Fornshell has made with arrow shafts. This will keep the poles straight and allow you to carry more weight.

Yes I have thought about it. The main problem I have read is that the pack sits away from the back, so it is prone to bounce around, which could throw you off balance. That why external frame packs are rarely used these days for mountaineering/skiing/climbing.

[Roger Caffin]

> My guess is that Easton .344 diam poles will easily curve to my spine by tightening the waistbelt.
I will bet they will not curve that much! Not a chance!

Cheers

[Huzefa @ Blue Bolt Gear]

Roger, thanks for showing me the right path.

Now I have two options. Buy closed foam pad and make a suspension based on pinnacle (I was surprised to find out that it can carry upto 40 lb weight).

Or, use relatively flexible (aluminium?) bars that can contour to my spine.

[Jason Brinkman]

I made curved stays for my GG Mariposa+ from Easton alluminum arrow shafts. I cut them to length with a small diameter tube cutter like plumbers use, deburred, pounded in threaded female inserts for arrowheads into the ends, and bent them to the desired shape. They take considerable effort to bend, but they also require some care as not to overbend them (which would buckle the shaft).

I was advised that I would have to take some precaution against them rotating in use. However, this has not proven to be the case. The bent shafts are S-shaped, and the backpanel between them is a uniform width, so if one tries to rotate it is restrained by it's opposite end and by the other shaft (essentially diaphragm action). You have to load the pack to make this work.

Now if I were to sew a pack from scratch, I would make sure that I planned for the curved stays. Meaning that I would reflect the curved shape in the side panels of the pack where they meet the backpanel. Otherwise, you are forcing the entire pack to assume the bent shape of the stays or stressing the fabric or both.

My Mariposa+ also has a slot for using a pad as a backpanel/frame between the stays. I keep a single section of egg crate style closed cell pad in there, but primarily for comfort and air circulation.

[Huzefa @ Blue Bolt Gear]

Jason, Mariposa comes with CF stays(?) I guess you are using an older version.

Thanks for the all the tips.

I want to multi use the pole so another option I have is to tweak my tent design so as to make poles into an half ellipse instead using three bends.

Here is what I came up with:
A

This what prebend sections will be like.

B

This is the actual model of the poles in the sleeve. The tension will help to make the fabric taut. But as you can see the top has a lot of curve so I will have to prebend the top section as shown in B.

I will share my prebending idea. Hopefully it will help someone else. The idea is to make a model using a diagram software (I use Dia). Then take a print out and glue it too a wood ply. Then ask a exprienced carpenter to cut out the ply exactly along the lines of the ellipse. Then you assemble the poles and bend it along the this ellipse model. This ply model can also help to make measurements for the tent fabric.

I will use the sections that fits best along my spine for the pack stays.

[Jason Brinkman]

No, it's a modern Mariposa – I simply replaced the CF stays so I could have curved ones. The CF rods are useful for other things.

Pack stays are S-shaped in most designs, but if you took 2 appropriately curved sections with a joint in between to make up each stay, you might be on to something.

The combination of both of my arrow shaft stays weigh about an ounce. Is that amount of savings worth the fuss of dual-use tent poles?

Ellipses are easy to generate graphically, but tent poles may be stronger if pre-bent to a parabola shape, or would probably shed wind best if fit to a catenary curve.

[Roger Caffin]

If you want your Easton poles to take anything tighter than about a 2m radius of curvature, you must give them a pre-bend. In fact the limit for the 344 tubing is about 1m or 40" radius of curvature, but this is pushing the material too far.

Translation: I pushed a set of Easton 7178-T9 poles in my orange tent too far, and they ended up shattering in the Pyrenees near the end of the trip. Oops!

However, do not prebend the poles to the full curvature you want: that way lies failure as well. Only go about half way, so that when the tent is erected there is still some tension in the poles. Um – the exact details may vary, depending on exactly what final curvature you want.

Now, how to curve the poles? Yes, a board cut to the shape you want is not a bad idea, but the problem is how to roll the tube down onto it. This is not so simple, especially when you want a parabola rather than a circle.

The alternative is a tubing roller, like a 'Jenny'. You will need to look this up on Google to see what it is. The idea with one of these is to make several passes along the tube, increasing the amount of curvature each time. But you need to use rollers carefully shaped to fit the tube or it will buckle. Again, a bit complex.

Cheers
Roger

[Huzefa @ Blue Bolt Gear]

This has turned into a tent pole discussion but very interesting ideas have come up and I am enjoying it.

I can intuitively understand why parabola will shed wind better then a ellipse. But then why arent any manufactures making tent with parabolic poles? If they can make geodesic cross poles tents using CAD then surely parabolic tent are not that complex?

After a lot of googling I have learnt how to to make a parabola. It turned to be much more difficult to find proper information on finding the pole length on a parabola then a ellipse. My software has something called bezierline which is I found a parabolic segment using which I made this parabola.

After considering different pole configurations I came up with this parabola. It has a 38in height and 54in diameter. The groundsheet width is 46in. It requires a 97.5 inch pole. Instead of 12 x 18" sections and 6 bends arches, I now require just 8 x 18" sections and 2 x 26" sections. That turns out to be quite a bit of weight saving.

Roger, thanks for tips.

>Only go about half way, so that when the tent is erected there is still some tension in the poles. Um – the exact details may vary, depending on exactly what final curvature you want.

I think I will prebend only the top 26" section of the pole. Will the rest of the sections automatically follow the parabola shape?

By jenny do you mean a textile spining wheel? If so, I get the idea. Ok, my ply parbola section may not have some cavity(?) but using help from some strong hands I may be able to bend it. I havent tried this before; it is just an idea. Roger, is it worth a try?

[Mike Hinsley]

Huzefa:

Some of my early pack designs tried to use tent poles as an internal frame. It didn't work very well. The problem is that the tent poles are always ramrod straight and a pack frame works best when it is not straight – in fact has a definite 15degree kink in it to put load onto the shoulder blades.

For your parabola tent poles it's a bit of overengineering. The sides can be straight and you could just have the curve at the top. The curve is so minor that it will be difficult/impossible to force any fabric to follow the curve for the length of the tent – wind and anything else will flatten it.

You will also have to be careful wtih pole weights. The weights of the poles can be significant when compared with alternative single-pole designs that compensate for inefficiency by using extra fabric.

If you are carrying trekking poles anyway then an A frame either with or without a parabola at the top would produce a strong front for the tent – The rear of the tent could then have either one or two short poles – two 40cm carbon struts (like in Henry Shire's tarptents) would be very efficient.

If you made an A-frame out of carbon fibre tubes then with connectors in aluminium it would weigh about the same as a single-pole Aluminum since you are using something like 2.5x as much tubing.

A-frames and parabolas and other variations always tend to use a strip of nylon or equivalent to prevent spread to in effect create a load-bearing trapeziod.

One final tweak that vargo use on their tunnel tents is to use two reinforcing tapes that cross-brace the members so that strong side-winds don't collapse the tent.

[Huzefa @ Blue Bolt Gear]

oops. The above curve is not a parabola. Not sure what kind of curve it is. The actual parabola, generated by parabola calculator program, is this:

>You will also have to be careful wtih pole weights. The weights of the poles can be significant when compared with alternative single-pole designs that compensate for inefficiency by using extra fabric.

Funny. Here I am trying to make a four seoson tent using two poles and you are advising me to be careful about pole weight.

Mike, thanks for all the suggestions.

________________________________________

Comparing an ellipse with parabola, ellipse gives better headroom and space for the same length of pole. But the problem is flat top. My plan is to prebend the top 26" pole into ellipse section with a distinct peak.

[Mike Hinsley]

Huzefa

Pre-bending will be simpler to do – you only need the top pole to have a few bends in it to give a good approximation of the curve that you need. It means that you can use cheaper aluminum (not heat-treated) if you want since stuff that will hold a bend without distorting tends to be more expensive since it almost always comes from Easton these days.

I know you are making a 4-season tent but I also know you are in the process of designing all sorts of things and weight is a factor, as no doubt is snow-shedding.

All I'm really suggesting is that you keep an eye on things.

The old Vango Force 10 tents were very simple bomb-proof tents that just had a single heavyweight pole at each end.

I'm assuming the fly is going to be made out of SilNylon which will probably weigh around 50-60gsm.

The thing to bear in mind is that whilst curves like this will give you good height for a particular volume of fabric it's worth considering the weight of the poles vs. the weight of the extra fabric for a sub-optimal design.

I've overlaid your image with an 'inefficient' A-frame that provides similar peak-headroom and width at the base.

Hopefully, you can see that the amount of fabric used is approximately the same – if you straightened the curve you'd see very little or no difference.

Since a pole's stiffness/strength is proportional to the square of the radius one large pole can be stronger and lighter than two small ones.

Let's assume that your curved poles are made out of 10mm tubing with a 1mm sidewall. The alternative single-pole design would require at least 12mm and maybe even 14mm tubing with a 1mm sidewall to match strength.

Cheating a little the weight-per-length of the poles will be in proportion the tube diameter and but their strength is in proportion to the square of the tube diameter and so a 14mm pole will weigh something like 1.5x as much as a 10mm pole but be about 2x stronger. So a 10mm parabola pole (that say uses 2-2.5x as much tubing) will end up being heavier for no strength-gain.

Any saving in tube weight may then be offset by the extra silnylon required. If SilNylon weighs in around 60gsm then a 25cm strip down each side of the length of a 3m tent would be 90g. What's the weight differential between the big pole and one bendy pole?

For an A-frame to be really wind-efficient it has to be well pegged an gueyd out since if the fabric pockets its wind resistance increases greatly.

Airflow over an A-frame may be better since wedges are relatively efficient. You'd have to do the Cd calcs to be sure though.

I'm not suggesting one idea is better than the other, just throwing some ideas around for you to consider.

I'm in the process of beefing up a tent for more space and mountain use and I keep coming back to the fact that an A-frame tent is a good design for weight and simple to build….

[Huzefa @ Blue Bolt Gear]

Hi Mike.

>Hopefully, you can see that the amount of fabric used is approximately the same – if you straightened the curve you'd see very little or no difference.

I made this in my diagram software and calculated the pole lenght using pythogorean theorem. The pole lenght of the a-frame is 120inch vs the 98inch pole lenght of the ellipse. Obviously then a-frame requires much more fabric and is heavier.

Another thing to consider is that the tension in a ellipse makes the fabric tout and improves wind resistance.

I want to make a tent which can be easily setup. My plan is to make use of strong lateral tension at the ends (like stpehnsons) so I only need to stake four pegs.

[Mike Hinsley]

Sorry, perhaps I've not made it clear. I'm talking about a single central pole not two poles – the big grey lump in the centre of the picture. The trinangle is the fabric lines.

Let's say that based on your numbers it's using 20% more fabric – what's the actual weight?

Just four pegs is not a lot – unless they are amazingly meaty pegs or strong anchor points.

If you go for a curve or approximation shape then please consider some sort of cross-bracing even if it is only something like the Vargo TBS (Tension band system). A cross-wind onto a tunnel tent can be an exceedingly interesting experience…

[Roger Caffin]

Your strange curve looks like a hyperbola with a tilted Y axis. Not what I would use.

Have a look at these two photos of a two-man single skin tent. With correct guy ropes it can take snow conditions, although in a strong wind you may find spindrift coming in at a great rate.

The inside of the tent looks like this:

Now this looks large (and is), and in fact is a little too large or high for good alpine use. But it was meant to be a summer tent, not a winter tent. It shows what you can do with corners and straight carbon fibre tubing – actually CF arrow shafts with a 7.5 mm outside diameter. The total weight of this tent including poles is 1.26 kg. That's 630 grams per person. Try getting a tarp and a bivy sack into that weight per person. Then try cooking in a storm, as we frequently have – in comfort in the vestibule.

Please note: since taking this photo I added another guy rope attachment at the corners on each pole, above the visible guy rope anchors. This improved stability greatly in bad weather. If you want to use this sort of design as a winter tent you need to be able to add an INTERNAL guy rope horizontally between the corners on the center pole as well. That way the corners are locked in space, as it were.

It uses silnylon for the fabric, at 48 gsm. It has full insect netting as well.

Cheers

[Huzefa @ Blue Bolt Gear]

Mike, your comments have made me seriously consider straight pole design. Not a single pole in the middle but two pole design based on MSR twin sisters/shangri la 2.

I only have to find how to calculate a curve between two poles. Then it could be seen as two pyramids connected together. I believe the lower profile and steeper sides gives it a better wind/snow shedding ability . Also it doesnt require a large clearing and 11-14 stakes like a single pole requires.

Roger, the pics are looking great. Thanks for sharing. I am sure its a great tent but I am looking for lighter shelter. There is a limit to how light a good tunnel can be.

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