Seat post pack

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Paul A
Posts: 98
Joined: 5 Feb 2007, 11:43am
Location: Chester UK

Seat post pack

Postby Paul A » 31 Aug 2012, 2:45pm

I'm sick of riding around with a sweaty back caused by carrying a rucksack on day rides.
Though I've got lugs for a rack on my bike a '2010 Specialized Tricross Comp' I don't really want to fit a rack to it.
I've seen a seat post pack 'Altura Aero Seat Post Pack' that looks to be perfect for my needs.
It's removeable and fits to the seatpost by means of a quick release Rixen & Kaul clamp.

However, the seatpost on my bike is made of carbon with a 'Zertz' (rubber like vibration damper) insert in it.
I obviously don't want to damage the seat post by either over-tightening the clamp or by the weight of the bag causing stress to it.

Has anyone got one of these seat post bags? Would it be wise to use one with a carbon seat post?


http://www.amazon.co.uk/gp/product/B004 ... d_i=468294

Thanks, Paul

http://www.francecoast2coast.co.uk

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NUKe
Posts: 3706
Joined: 23 Apr 2007, 11:07pm
Location: Suffolk

Re: Seat post pack

Postby NUKe » 31 Aug 2012, 3:00pm

I personally wouldn't put this type of rack onto a carbon Seat post, too much stress in the wrong direction. And to be honest a rack is probably lighter and more versatile
NUKe
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cycleruk
Posts: 5201
Joined: 17 Jan 2009, 9:30pm
Location: Lancashire

Re: Seat post pack

Postby cycleruk » 31 Aug 2012, 4:16pm

What do you carry in the rucksack?
Do you have to have a rack.?
Would just a large seat-pack be big enough?
There's no such thing as a tailwind.
It's either a headwind, or you're going well.

Brucey
Posts: 35528
Joined: 4 Jan 2012, 6:25pm

Re: Seat post pack

Postby Brucey » 31 Aug 2012, 4:18pm

suppose you put ~5Kg into it. When you go over a bump that turns (briefly) into 15Kg. The load is cantilevered outwards about x10 further than the vertical distance between the lowest part of the clamp and the band that goes round the seat post.

To a first approximation you will see repeated loads of 150kg on the lowest part of the clamp, and a corresponding 150kg load through the (rather narrow) clamp band as well. These loads in some cases add to the clamping loads that stop it from moving or swaying.

There will additionally be increased cantilever loads near the 'seat lug' (for want of a better term).

This might just fine. Or not.

I would not be too concerned by the additional cantilever loads near the seat lug, unless you are already close to the weight limit for the components. Although there is a stress concentration at the binder (and this can and does occasionally cause carbon seat posts to fail at this point), these loads are to some extent spread over the insertion length of the seatpost, and will -or should- have been designed for.

However it strikes me that the loads from the rack clamp are potentially large, possibly concentrated, not very well controlled, and are in a place where the seat post designer may not have anticipated them to occur. If the loads were better spread over the length of the seatpost , or the seatpost were relatively thick-walled aluminium it would be less of a concern I suppose.

I you want clear guidance you ought to be able to consult the rack or the seatpost manufacturers. But I bet you don't get a straight answer out of them. A while back I tried to find a recommended torque setting for bolts on an E****n stem when they were used to secure an E****n carbon handlebar. Simple enough, I thought. Ha.... I could not get a straight answer out of the so-and-sos, even though they had designed both parts to work together in exactly the fashion that I was using them (obviously).

If it were the only way of carrying luggage on your bike it might be worth a go, perhaps by getting a seat post with better known properties to take the carrier. But it isn't the only way; you have a lovely set of carrier mounts. I'd use those.

A bar bag, big jersey pockets, or a seat pack might be good alternatives, too.

It might be worth asking the question 'who carries the can if it breaks?' If it is you (because the other parties will weasel out of any 'express or implied warranties' because it was 'not used in the way intended' ), then you may come out of it feeling royally rogered; possibly literally, by a jaggy length of broken carbon tubing.

Sorry if this isn't very helpful.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

pete75
Posts: 11704
Joined: 24 Jul 2007, 2:37pm

Re: Seat post pack

Postby pete75 » 31 Aug 2012, 4:28pm

Why not use a saddle bag? Carradice do loads in various sizes.

David Cox
Posts: 192
Joined: 14 Jan 2008, 9:15pm
Location: Birmingham

Re: Seat post pack

Postby David Cox » 31 Aug 2012, 4:54pm

I am coming round to Pete75's view and its back to the future where I started. It's just that a saddle bag does look a bit like an air brake for a bike where a rack bag looks more streamlined. I had the same dilemma and stop using a cantilevered bag, attractive though it was, when I switched to carbon seat posts including one like yours on a Tricross with a Zertz insert. The single fixing bag did sway a bit and always ended up slightly out of alignment. It went to the last Cycle Jumble. Seat packs (Carradice do quite a large one) work well, I often use a Decathalon bar bag (bought for the fresh aspargus season) and I have a Nelson saddle bag with a bagman support fitted to the saddle rails when I want more capacity. I did fit the pannier rack for a while but for club runs it's a bit excessive and adds weight when everyone else is on very light minimalist bikes with everthing they need in jersey pockets. On this year's CTC Birthday Rides he weather alternated wet and dry, I managed to be underdressed when it was cold and wet and then overdressed the next with a bulky waterproof stuck in stretched jersey pocket - a bit more capacity would even these everyday variations of UK weather and riding. Carradice do a neat little saddle bag reviewed in the last edition of Cycle and its going on my Xmas list.

But once you've got the bags sorted you have to find somewhere to attach the lights which I am using more and more when the weather closes in. When Arrive arrives from AudaxUK I scan the photos to see what people are doing about bags and lights! A full touring or a racing set up is easy but nearly all my mileage is between.

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Swallow
Posts: 887
Joined: 4 Feb 2010, 10:13am
Location: Cornwall

Re: Seat post pack

Postby Swallow » 31 Aug 2012, 6:25pm

Is the seat post carbon? I have 2009 Tricross Sport and the seat post is carbon wrap not carbon and was advised that it would be fine to use the Altura Aero Seat Post Pack, which I do, great bit of kit.
Last edited by Swallow on 31 Aug 2012, 9:53pm, edited 1 time in total.
'Kernow bys Vyken'

Brucey
Posts: 35528
Joined: 4 Jan 2012, 6:25pm

Re: Seat post pack

Postby Brucey » 31 Aug 2012, 6:48pm

is it one of these?

http://t1.gstatic.com/images?q=tbn:ANd9GcTNP-YzBy-vJteayrCd1o5ZONefybjeI0Jx72yt_Hs1tzkDwC_uyQ&t=1

or one of the several other 'zertz' equipped seat posts?

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

Paul A
Posts: 98
Joined: 5 Feb 2007, 11:43am
Location: Chester UK

Re: Seat post pack

Postby Paul A » 31 Aug 2012, 9:57pm

Brucey wrote:is it one of these?

http://t1.gstatic.com/images?q=tbn:ANd9GcTNP-YzBy-vJteayrCd1o5ZONefybjeI0Jx72yt_Hs1tzkDwC_uyQ&t=1

or one of the several other 'zertz' equipped seat posts?

cheers


No, it's one of these;
http://www.evanscycles.com/products/spe ... e-ec020082

I don't believe it's a carbon wrap post, I think it's solid (hollow :-) ) carbon.

I don't want a 'sideways saddle bag' as it reminds me too much of 60s/70s teenage days and a bar bag is too small.
I need a bag for days out and for my regular Sunday jaunts to the pub. I ride 25 miles or so to my mother-in-law's house, shower and get changed then spend a couple of hours in the nearby pub with some mates before Mrs A arrives in the car to take me and the bike home. So, the tailpack idealy has to hold a full change of clothes - both summer and winter.

I could always put a rack on the bike but surely, as someone else said, there must be a half way house between minimalist jersey pocket and full rack and pannier?

Cheers, Paul.

http://www.francecoast2coast.co.uk

Brucey
Posts: 35528
Joined: 4 Jan 2012, 6:25pm

Re: Seat post pack

Postby Brucey » 31 Aug 2012, 10:30pm

well you certainly wouldn't want to put a clamp round the zertz bit.

I wonder; if you could bear going to the pub in your bike kit, couldn't Mrs A take the clothes, then its job done? Or maybe leave a change of clothes at the in-laws?

just a thought...

cheers
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Swallow
Posts: 887
Joined: 4 Feb 2010, 10:13am
Location: Cornwall

Re: Seat post pack

Postby Swallow » 31 Aug 2012, 11:30pm

'Kernow bys Vyken'