Seat Rack Question?

[cfi on the fly]

New to the forums and trying to figure out what to do with about a seat rack.  I see that most people use a pack specifically designed to attach directly to the seat like the Viscacha from Revelate Designs, or they use a rack that attaches to the rear axle.

I recently purchased a frame that attaches to the seat post and then lashed a dry bag to it- see pic.  Why dont I see more of these setups?  Is it inheritantly unstable?  Is it a weight thing? 

Newb that wants to learn...

Thanks

[sigma7]

I've never used those. They are probably not reliable enough for off road usage. I saw some broken racks used by Brevet riders (on road).

What's the size of the StS dry sack?


-- sigma7

[cfi on the fly]

Its a 13L STS Big River bag.  Im not worried about that rack, its definately strong.  But it probably is a bit heavy for what it does.

[trebor]

I started that way on the firs trips I made; with a beam rack.

The number one thing that I didn't like is how it affected handling. You can feel that puppy back there! It is like a rudder on a boat.

The other issue I always faced is it would twist around the seat post. Especially in the singletrack. Never could seem to get it tight enough.

Seat bags seem to have little to no perceptible affect on bike hadling. Your results may very.

[DaveC]

To be blunt, no one uses them because they suck.

Ride rough stuff enough and you will break it.  You can't get behind your saddle at all on tech stuff.  Getting beam racks to not swing sideways occasionally is tough.

[rocky rode]

I'm on the GDMBR and we are a stop for riders.  I've seen or heard of about a dozen, or so, of these breaking.  I have one in the shop that a rider left here that was broken and I repaired it but have never used it.  A real rack is a much better option.

Gary

[cfi on the fly]

Ill take you guys word that this thing would break...but from looking at the welding done on this thing, I don't know if I could purposely break it.

Just wondering what makes the  framed rack that much better?  Is it further behind and just more out of the way?  To me those things look more flemsy than this.  I can see how they would have a harder time swaying laterally, but at the same time I cant get this thing to budge laterally either.

Again, Im just trying to figure this out.

[Done]

cfi, maybe you should give it a try to see if it works for you?

[jhl99]

Just wondering what makes the  framed rack that much better?  .

1.  The stayed rack will be lower, lowerering the load's center of gravity for better handling
2.  4 point attachment (2 @ seatstay, 2 @ dropout) vs 1 @ seat post.  No chance for the rack slip on seat tube and be skewed to the bike centerline. 
3.  The stayed rack structure is triangulated-- a truss in combination with the bike frame, which is structurally effecient and strong, there are supports in direct alignment with the load path while the beam rack is cantilevered.
4.  The beam rack is single point of failure...it breaks you are done.  Stayed racks can be still work even if a stay gets breaks (the stays can  be field splinted).
5.  Seatpost height can be independently adjusted from rack height. (If you want to drop the post for technical downhill).

[HOser]

I have no experience with that rack, but I believe all the reports of failures.  One of the most basic principles of mechanical design is that "cantilevers are bad".

But to answer your question, the rack approach can work for bikepacking, I'm a fan of the Salsa Minimalist myself.  Here's this rack on my Fargo carrying a 15 L Outdoor Research dry sack:

[Rob Colliver]

Hi all - didn't dare post this until after the Tour Divide - I carried sleeping bag, thermorest, pillow, long sleeping trousers and top in a 12 liter drybag on a seat post rack from Topeak - it was perfect!

I did modify the rack slightly and also linked the straps to the seat rails, but this system got all camping kit from one end of the Tour to the other with no glitches.

I rode a Tallboy and that bag was solid even on the washboard and CDT in NM - it can be done with a seatpost rack and a little effort in the preparation - I may post some pics when I get the bike back together, but just at the moment I don't feel the urge to ride - I feel like eating!

[jon_baler]

In my limited experience, I have found seat post racks to NOT be very good.

An acquaintance of mine just raced the Tour divide, and he broke 1 or 2 seat post racks.

This looks promising though:  http://www.adventurecycling.org/store/index.cfm/product/579_19/arkel-randonneur-rack.cfm

[AZTtripper]

Cfi this set up might work fine for you as Toby said maybe you should just try it.

On the other hand they do have a tendency to break. Looks like you could use those two straps and just strap the bag to the seat.

If everybody is doing something the same way there is likely a good reason. If you want to ride advanced single track there are several factors. Getting behind the seat has been pointed out even if you drop the rack and strap you pack to the seat it still looks a little big. Weight is the next big factor most of the riders I see are shooting for a minimal weight as this allows for more riding and less walking.

The seatpost bags carry gear well with very little added weight even if the rack doesn't break its extra weight that you don't really need to carry.

Hope these post have helped.

Tim