Saturday 28 July 2012

Lightweight shelter supports

It's becoming increasingly common for the lightest shelters to use trekking poles for support - and why not? Why should you bother carrying extra poles, that take up space and add weight to the pack, when you are already carrying something suitable? Perhaps, there is a very good reason to carry dedicated poles. Some of the lightest poles, particularly those around 200g/pr, might not be suitable for all shelters. The load on a shelter in a storm can be high and not all lightweight trekking poles are not designed for this. Just how much pressure is on the poles? Next time you are out camping in a 30kph wind let down the support pole and try holding the shelter yourself, you will be surprised at the force in even this modest wind. Now, lets up it to 60kph...

I've not owned a shelter which used dedicated poles since 2006 and this was also the time when I got my first really light trekking poles, the fixed length Gossamer Gear TL's. In 2009 they were joined by the adjustable LT4's and in 2011 by the Black Diamond Z-Poles. All these poles have been used year-round for shelter supports, and in all weathers, and so far I've not had one break. I like to think that has not just been luck but because I have thought about, and overcome, the limitations of light trekking poles.

Lets think about what's happening to these poles while keeping it very simple and maths free! Lightweight poles, when loaded, will generally flex more than a heavier set. That's hardly surprising, less material can mean less strength and resistance to bending. Material selection and diameter is also important here but is way beyond the level of this post! In general trekking pole terms, lighter, normally means weaker and with more flex. Flex is a problem with shelter supports, as the load increases the more your centre pole bends and the more likely it is to snap. Flexing can be increased not only by the load exerted on it but also its angle to the load, confusing? Not really, in layman's terms keep it vertical! Flexing of a material increases with its length. Take an adjustable pole and set it to its maximum height and try to bend the pole, then try the same thing with the pole set to 75% and then 50% of its height. Again, not surprisingly, the poles resistance to bending increases as its length is reduced. All very simple and obvious! The important points to take from this is that in a storm you want the pole vertical, and as short, and therefore, stiff as possible.

Lets look at some different shelter types

A conventional tarp works well with even the lightest trekking poles. You have total freedom over the pitch height and as the wind increases you can lower the attachment point on the pole. As we know, lower height increases stiffness and resistance to bending.

But what happens if you don't want reduced height? After all, reduced height normally = less comfort. No problem, if you can't reduce the height then increase the stiffness. That's possible by using both poles upfront to share the load. You will have to find a method of supporting the rear and I sometimes carry a very light tarp pole if I'm expecting to use this method. You could find a suitable stick but even a hiking shoe standing on its end has been used before. As a last resort the rear could be pegged directly to the ground.

The MLD Trailstar is a shaped tarp and my favourite bad weather shelter. For its weight I know of no other shelter that resists wind as well. One of the key features that makes it so resistant is its variable pitching height, I pitch at either 100cm or 120cm depending on the conditions. A fixed length pole doesn't work well with the Trailstar so I use my adjustable Gossamer Gear LT4's. With the centre pole set to 100cm I've never experienced any flexing but at 120cm some flexing occurs in high winds. Want a higher Trailstar pitch in a storm? I've not tried this but doubling up would work, but again, you would have to find something to support the door if you are a solo hiker. Couples with extra hiking poles will not have a problem here.

The MLD Cricket is an excellent fair weather shaped tarp but has a fixed pitch height. My 130cm Z-Poles are just long enough but can flex at that height in a storm. They need to be doubled up when it gets nasty. Luckily it's possible to pitch the Cricket without using a front pole if you pull the front corners out as far as possible. This lowers the porch and is only possible with the silnylon version, the cuben version is less adapable and needs a front support.

The MLD Duomid is more of a problem and not ideal for pitching with the lightest trekking poles. It's a high shelter and I don't have any poles long enough without some sort of extension. MLD know this and supply a handy pole extender - just use that right? NO! Extending a lightweight hiking pole this way is asking for breakage! Extra length increases flexing as we know, but also the supplied extender does not fit lightweight pole tips snugly and will cause it to sit at an angle when loaded, thus further increasing the stresses on the pole. You might get away with this in fine weather but DO NOT USE A POLE EXTENDER WITH ANY LIGHTWEIGHT TREKKING POLE IN A STORM. You could try the doubling-up method, as used with the Cricket, if you have some exceptionally long poles but for the rest of us we need to use a different method. My method is a compromise and if you are intending to use a Duomid in really stormy weather then more substantial poles ARE recommended. However, this method has been successful in some very nasty weather without problems.

This is how I pitch my Duomid. It's light, simple and strong. It is a bit fiddly and adds a couple of minutes to the overall pitching time, only you can decide if you can be bothered with this. The idea is to double-up the poles for as much of their length as possible only leaving the short ends single thickness. My method uses a linloc mid-cord to adjust the overall length of the pole, not only does this guarantee a tight initial pitch but as silnylon stretches and slackens overnight it's easy to re-tension the shelter from inside.