I have a 25 year old eighteen speed. In order to get the seat high enough to keep my legs comfortable, I need to raise it so the "Max Limit" line on the post is about a half centimeter above the seat post.

A bike mechanic said that it was okay, as long as the bottom of the post was at or under the bottom lug on the seat tube, which I'm pretty sure it is.

Do you agree with this guy, or am I flirting with disaster? What could go wrong?

I'd get a longer post, or a larger frame - but then I'm allergic to broken bones.

Well, I took it out for a 20 mile ride today, and nothing happened.

Can you explain what exactly could go wrong with my set-up?

I know that old bikes usually were designed for the seat to sit really low toward the top tube. The mechanic seemed to be referencing this fact when he said the height limit was "conservative".

At least 2 things could go wrong: first, the seat post could break suddenly, causing you to sit down, hard, on a jagged piece of broken off seat post projecting out of your frame. Or the top of the seat tube could fail (though probably not as suddenly as a post failure, at least if your frame is steel), ruining the frame.

The the pre-CPSC "rule of thumb" for safely clamping seat posts was to have at least twice the diameter of the post inserted into the seat tube. Thus, for a 27.2mm diameter seat post typical for a 25 year old steel frame you would want about 54mm of post inside the seat tube. That's a little over 2 inches. But if your post was marked by the manufacturer with a specific limit, you should not go above that limit. Buy a longer post or you may find yourself singing soprano later...

well bikeslob, we have no way of knowing how long you post has been like that however, on just a short 20 mile trip nothing is likely to happen. however bicycle parts are designed to work in certain ways. a min insert mark on a seat post is to insure that there is enough seatpost in the seattube to disperse the forces over a larger area and prevent frame damage. perhaps someone can explain it better than I. I do think that mechanic was dumb and misinformed you. he should never allow a customer to walk out with a seat or stem raised past the max mark.

owowowowow. You've convinced me. Are there any good cheap seatposts I could buy? Is a cheap post just as likely to fail?

Kalloy makes very inexpensive seat posts. Just make sure that you get the correct diameter. Also make sure you get one that works with your saddle, either with the rails or one of those seat clamps.

Kalloy makes just about every size possible. They are inexpensive as a rule, and most look ok on an older bike.

I tend to agree with everyone who says it's a bad idea to ride with the post above the limit line, for the reasons already mentioned. On the other hand, 5 mm--half a centimeter--isn't much. Relative to the other dangers of riding a bike on a road among cars, the danger of the post breaking is probably slight.
The bigger issue, I think, is that the fact that you have to raise the post that high indicates that your frame is too small. You don't need a new seatpost, you probably need a new bike. How much clearance do you have between your crotch and the top tube? If it's more than two inches, I'd advise you to trade in your current ride for one with a larger frame. You'll be happier and more comfortable in the long run--possibly even a little safer, too.

Don't know what sort of saddle you are using. But you will find that some saddles have a much bigger distance between the top where your butt rests, and the rails. Try maybe looking in that direction. Worst comes to worst, put on a horrible leather saddle of the older generation, that could give you a few cm's diference.

The frame may be a bit small, but it's fun to ride, and I haven't experienced any significant discomfort either during or after my rides on it. Maybe that will change as time goes on. I hope not.

I haven't measured the stand-over clearance, but it feels pretty similar to the last bike I rode.

I don't have a whole lot of spending money now, and it apparently has some pretty good components, so if I can make this bike work just by purchasing a 15 dollar seat post, I'd like to do that.

Should I worry about the frame, even if I get a new, longer seat post? Could a taller post give too much leverage to my weight or something, and damage the seat tube? I'm about 190.

Sure, if you like the way the bike fits just scrape up the 15 bucks for a longer seatpost and be done with it. You're probably okay with the one you have, but a new longer one will quiet the in-the-back-of-your-mind anxieties that we've raised here. That's worth the 15 bucks.

I would tend to agree with Caterham that as a rule you are well within reason, but I would sure as heck not accept any liability for having told you so. The limit lines are defined to cover manufacturers from liability and do not necessarily reflect what is truly required. More often than not, they are overly cautious and are marked in such a way to cover all eventualities. Much depends on how much offset you have, how much you weigh, how you are seated on the bike (weight distribution) and the construction methodolgy and materials of the frame.

it's fine.

since you are bringing this question up now, i'm assuming you just raised it. you might even find that in a week or two it is too high...or maybe even too low. or maybe it's just right and you'll have the confidence of riding on it for two weeks.

don't sweat it.

Well, it's your rectum and other meaninful parts.

I'd have no problem at all with using a seatpost 5 mm (about 3/16") past the limit, if its bottom is past the seatlug. But that's just me.

I think 1/2 centimeter is closer to 1/4" but otherwise this makes sense to me, especially the part I bolded. I can't imagine how a shorter length inserted could make the post more likely to break above the lug, unless that length becomes so short that the post can use the lug as a fulcrum. Mind you, I'm no metallurgist; maybe someone can school us.

The top of the seat post lug becomes the fulcrum. The more seat post that sticks into the seat tube, the lower the mechanical advantage and the less stress placed on the post or tube. However, the shallow inserting increased mechanical advantage, dramatically as the lever to fulcrum length shortens. I have seen at least one high end bike with a slightly stretched seat tube lug and a small bump just under the front of the seat tube lug.

My recommendation is minimum insertion line. There is an alternative, actually. Thinner bike shoe soles

Or a longer seat post. Failing that, a larger frame.

Yes and no. The post needs to be further down than the bottom of the seatlug. Half a cm under the minimum insertion line should be ok. No way the physics is calculated so precisely to determine the exactly ideal minimum. Its really just a ballpark anyway. Use your discretion. I wouldn't be worried about .5cm.

Pics!

But whatever you do, do NOT remove the tag from your mattress!!

Like I said, I'm pretty heavy, and my seat is set back a ways. What are the chances the seat tube could be damaged? How do those chances change with the current seatpost versus a longer seat post?

You've hashed over just about everything.

Nobody can state for certain if you will damage your frame, or how long it will take, not even the frame manufacturer.

If you are heavy and your saddle is set back, you are creating a greater bending stress where the seatpost enters the frame, that would a 120 pounder with a forward saddle position. Are you close to a stress limit inherent in the frame? Who knows?

The stress tends to torque the seat lug open, and it's worse if the post doesn't extend all the way through the seat post. Whether having say, 5 inches extra is better than say, just the minimum insertion, is a matter of seat TUBE design. If it's not a buttted tube then the seat tube contacts the seat post all the way down the enclosed length of the post. This decreases the tendency to torque out the seat lug. Are you heavy enough to damage your frame even if you do get a 330 millimeter seat post? We can't know.

So there are risks and uncertainties. Considering the downside risk is a destroyed frame and possible personal injury, and the upside risk is a cost of $15 to $100 dollars for a new longer seatpost that fits the seat tube properly, you are better off spending these few bucks to get a longer seat post. The probability of the downside is unknown but not high (I can't remember seeing such a frame failure). Have I not seen such a failure because frames are stronger than we think? Is it because the scare stories are yet another load of crap? Is it because I don't have enough exposure to frame failures?

I don't know that either. But the thing you can do to minimize your risk at low $$ cost is to get a longer seatpost. If you don't and you have no problems, great! If you don't and you do end up needing a new frame, well, you've been informed by many people.

Put in a longer seatpost and you should be fine. I think that's been said already.