rear rack seat post mount

[nathanccoste]

So I am new to touring but not new to road biking and am gearing up for a trip across the country in the spring and am trying to get the bike setup nailed down. I'm working through a couple of different options in terms of the bike and the rear rack. One option would mean taking a bike that has the lower rack mounts down near to wheel hub but not the upper ones near the breaks. The guy at the shop says this isn't a problem and showed me these pieces that attach to the seat post. hear's my question. I know that my rack is rated up to 150 pounds or so (not that i ever really plan on carrying that much) but will this piece be able to handle that kind of weight? Will it fail on me eventually? or is there an alternative way to attach the rack? Thanks

[waynemyer]

I do not like the seat post mounted racks. Their capacity, in both weight and volume, is low and will definitely be too low for a cross-country trip. My major complaint with them is that, in my experience, they introduce an odd wobble in handling of the bike. Basically, the rack is a large moment arm attached near the center of gravity of the bike. And usually, if you're using a seat post rack, it's because the bike does not have otherwise suitable rack mounting points and short seat stays. This can put a good amount of weight out past the rear axle. When I have used these racks, even with a moderate load, the bicycle handling was analogous to driving a muscle car in the rain.

If you don't have upper rear braze-ons, you can either use p-clamps or rack clamps (http://www.universalcycles.com/search.php?q=rack+clamp). This way you can have a proper rack with decent capacity.

[nathanccoste]

thank you waynemyer,
Yeah i was never considering the racks that only attach to the seat post alone. What he showed me in the store was what you had in your link. my concern was that that piece would fail on me. Or that that puts slot of strain on the seat post. but if you think that they are good i guess thats my best option.
thanks again for your help.

[Pat Lamb]

The seatpost clamp does look like an interesting possibility.  I think I'd stick with p-clamps if possible, if only because they could mount down lower.  However, if you were trying to put a rack on a full-carbon bike, where clamping onto the seat stays was strongly discouraged, it might be possible to use a skewer-mounted rack at the bottom, and clamp to the seat post on top.

18-pound bike with a rear rack?  Hmm...

[staehpj1]

I think I'd stick with p-clamps if possible, if only because they could mount down lower.

That could depend on frame size.  I'd think that on smaller frames higher might be better.  I know that on my bike the brackets have to be long and bent down a lot.  They connect to the frame much lower than the rack platform and I am 5'10".  These seat post brackets might actually offer a better mounting location.  For shorter folks it might be even more the case.

I do prefer smaller frames for my size than some folks do.  I typically pick a frame two sizes (not 2 cm, 2 frame sizes) smaller than Rivendell would recommend.  I generally wind up with enough seat post showing that the warning mark typically shows or is close to showing.  So it would be a bit different for someone my size who likes big frames a la Rivendell.

[cgarch]

I've installed a seat-post clamp like the first image on the link as I didn't have mounts on my carbon frame bike. I had to severely modify the portion that hung down to clear the seat tube structure. The Salsa would be a better choice, had I known about it. I've also used the p-clamps - actually the Tubus clamps - mounted at a large bulge on the seat stays where the stays are beefed up for the dropouts, for the lower rack mount. Been this way for a long time and no issues to date. But now that I've seen the Salsa version I'm ordering that asap.

Craig

[waynemyer]

What he showed me in the store was what you had in your link. my concern was that that piece would fail on me. Or that that puts slot of strain on the seat post.

Nah... I have never heard of p-clamps failing when used as a rack mounting point. I'm sure it's in the realm of possible, but steel has tensile strength. The p-clamp is pre-loaded by the bolts holding the rack and that's that. In order to make the p-clamp fail, you would need to continually unload/reload or flex it.

I recently hosted a couple that was riding the northern tier. They only had rear racks, held in place with p-clamps and loaded at or over the capacity of their racks. They were just fine for their entire trip.

As far as straining the seatpost, this part of the bike is designed to take loading. Riders apply all manner of directional forces to the saddle, which is attached to a nice, long moment arm so you can have extra leverage against the frame. You'll be fine as long as your seatpost is not carbon fiber.

[DaveB]

The seatpost clamps referred to in waynemyer's posting actually put no load on the seatpost itself.  They replace the frame's seattube clamping collar and do double duty as the seatpost clamp and as a rack mounting point.  They will replace the seatpost clamp on frames with separate clamping collars but won't work on frames with integral welded or braze-on clamping "ears".