I am a large rider ~300lbs. I have a Novarra Ponderosa with an aluminum seatpost. With about sixty miles so far, the bike seems to be holding up fine to my weight with one exception, the seatpost.

Shortly after I start riding the seatpost slides down about two inches or so. I have the clamp as tight as I am comfortable with. My idea is to machine a piece of aluminum tubing 27mm outside diameter with an inside diameter of 19mm and a length of approximate 210mm as a shim to prevent the seatpost from sliding below my preferred height.

Does anyone see a problem with this solution to the problem?

what kind of clamp is it? you could possibly just go with a double bolt clamp for 5 bucks, but it depends on how deep the cut is on your frame.

Your idea of a stop collar on the post isn't a bad one at all if the other solutions don't pan out.

But before you go with that option try replacing the seatpost clamp with one that uses a bolt instead of a quick action lever. It's MUCH easier to generate serious clamping forces with a bolt as opposed to the lever style and that may be all you need. For longevity of the clamp threads try to get one with a bigger bolt size instead of the small road style clamp bolts. And be sure to grease the threads so you can achieve the clamping tension with lower torque values.

It has a quick release clamp, which I have set pretty tight. I'm hesitant to increase the torque for fear of damaging components.

I have a sherline lathe, so the support tube only costs about $5 in materials and maybe 2-3 hours to machine. I was just concerned about the pressure on the bottom tube portion of the frame. If it works than I can have a much lower torque on the seat tube clamp.

I'm not talking about a stop collar, but inserting a length of tubing into the frame for the seat post to rest against. This would allow the downward force from the seat post to rest against the bottom bracket portion of the frame. The tube will be a nice sliding fit, with a cross pin that would allow it to be tugged out if needed, though it should just slide out. Since the tube and frame is aluminum I don't need to worry about rust freezing them together, but the insert would be greased to help make it easier to remove.

why dont you try degreasing the inside of the seat tube, and reinstalling the post using carbon assembly compound instead (grease with a bit of grit to it).

+1

Can't believe it took 5 posts to recommend this.

There's far too much risk of an internal spacer tube causing troubles later on. I'd give that option a pass. And yes you sure do need to worry about alloy and steel corroding together. The two metals set up a galvanic action that corrodes the aluminum and it'll lock that puppy in there like a bank vault door if it happens. This is the primary reason all alloy seat post tubes MUST be greased on steel frames and it's still a really good idea on alloy frames with alloy posts.

As I mentioned before it is VERY hard with some designs of lever clamps to achieve the best clamping force. The Shimano style internal clamps are better at this but they still are not great. The easy solution to your trouble is to just swap the lever clamp out for a bolt style. The torque needed to easily hold your seat post can be achieved with only two fingers on an allen key in the clamping bolt.

If it bothers you to have to carry an allen key set or combo bike tool it shouldn't. Every bike should be set up with a little seat bag or other way to hold a spare tube, tire levers, a universal bike tool that has the common allen key sizes and a hand rag to clean up after. Also every frame should have a tire pump on it. That's an absolute must on all these items if you venture more than about 4 blocks from home. Otherwise you're looking at a long walk somewhere along the way.

Degrease first, and +1 for a bolt for the seat post. Is the seat post the right size? If not try a shim cut from a soda can.

Definetly NOT unless you want that seat post to act like it has been welded in place within one to two years. Leebo, do a search on "stuck seat posts" and see all of the recomendations to ensure that the posts are greased and that they are checked, cleaned and RE-greased at least every second year.

I ran up to the LBS and picked up some Finish Line Fiber Grip Carbon Fiber Assembly Gel and degreased the seatpost and applied the gel. I should know tomorrow morning (when I ride) if it works. Since this stuff increases friction, is there any concern about the seat post freezing to the frame as there would be if no grease was used?

Ok. I have tried the carbon fiber assembly gel instead of grease and have ordered a bolt style seat clamp. Hopefully between the two the problem will be solved. If not I may have no choice but to resort to the internal spacer tube. The frame and seat post are both aluminum. The space will be made from aircraft aluminum as well, if needed, so I don't see what would cause the corrosion? Also the spacer would be greased and could be removed regularly for cleaning and regreasing... Hopefully it will not be nescessary!

Thanks for the help!

I already carry a multi-tool with allen keys, patch kit, spare tube, tire levers and a pump--so I'm good to go for any adjustments needed to the seat on the trail. Thanks for the help.

If the cf gel and new clamp don't work you should try a stop collar on the seatpost. That would be better than the internal spacer tube. You would have to get the tube past the bottle cage riv-nuts. And if the seattube is butted there would be enough play for it to rattle in there.

Just thought I'd let you'll know that the carbon fiber gel seems to have worked. I rode this morning with no noticeable change in seat height! I suspect that the double clamp will make this an issue of the past once it arrives.

Thanks for the help!

have you tried a flex hone to burnish the inside of the seattube to remove paint? also removing the grease is a good idea.

as for your 'shim' or slide stop, it will get hungup on the bottle brazeon before it hits the bottom of the tube.

While an alloy to alloy tube joint won't suffer from a galvanic corrosion issue aluminium does still corrode. Just in a different manner than steel. But left out it will develop an aluminium oxide that packs the joint and locks things together over time. It is definetly still wise to ensure that the metal is greased or use that carbon gel stuff to act as an oxygen and moisture inhibitor.

The carbon assembly paste is good stuff. Most of them have tiny plastic bits that act as grit.

My LBS added a seatpost collar on my carbon seatpost when they sold me the bike. It was insurance against slipping down. It attaches to the post, above the clamp on the bike. I didn't have slipping problems, but it's helpful to see if the seatpost has moved at all, and I can put the post back exactly when I remove it.

Examples here