LarrySellerz

Today I stopped at the bike shop because I had a broken spoke and wanted to know if my low spoke count wheels (20 up front, 24 in back) were not suitable for large aggressive riders. The lady helping me was insisting that my seat was too high, its over the maximum line by over an inch. The seatpost is very long and there is plenty to clamp on to, but she claimed that the lever arm could crack the frame. I haven't ever heard of an aluminum frame cracking from this, and she is very conservative/safety oriented. Always telling me that my wheel is too out of true or something or other. Have you ever heard of a metal frame cracking from this?

base2 

Absolutely.
Levers & fulcrums are indeed a thing.

koala logs

She's right. I think you need to get a bike with bigger frame size or swap the frame you're using for something bigger and right for your size to reduce the amount of seatpost exposed.

Too much exposed seatpost is bad especially for big riders. Pro racers can afford to use undersized frames by several centimeters because they're light.

You can never destroy a seatpost or a seat tube by power. In fact, the more you make power or w/kg, the more you unload the saddle, unless something's terribly wrong with your pedaling technique. It's mostly the rider's weight, the severity of the road bumps, and how much seatpost is exposed.

LarrySellerz

Yeah ive been sticking with this tiny bike because its very fast and its an interesting experience, figured riding such a twitchy bike could improve my handling skills. I normally ride a 57-62 and this is a 47, I haven't ridden a bike this small since elementary school. I think ill try to fix my road bike but honestly its kind of cursed/possessed (wants to turn left, broke a tire iron, pinch flat at 125 psi on 25mm tires) so maybe I should just get another road bike that doesn't give me such an uneasy feeling. Nothing about that road bike makes sense.

Rolla

I broke an aluminum bmx frame by running too much seatpost above the collar. Snapped the part of the seat tube that extended above the top tube right off. I had it re-welded by the manufacturer, but it broke again within the year. That was the last aluminum frame I ever owned, or will.

phughes

Why did I click? Why did I click?

79pmooney

How much seatpost do you have inside the seat tube? This is important. The seatpost acts as a lever every time you hit a bump. Your weight times a distance related to the seatpost showing. Below the top of the seat tube, the post is now acting as a lever again; resisting exactly the lever force from your weight and the length of the post showing. Now that resisting lever consists of the length of the buried seatpost times a force.

So, if you halve the seatpost inside the seat tube, you double the force on the wall of the seat tube. The minimum insertion point limits that force to something that seat tubes can handle without damage. Do this with a steel frame and there is a very good chance the tubing will bulge forward at the bottom of the post. Perhaps to the point of a crack starting. If aluminum, you will see no change until a crack starts and that could be quite sudden.

ThermionicScott 

How can you really know what the maximum seat height was until you break something, though?

beng1

I would not trust aluminum for that situation. The only way I would feel safe is if I had a steel seatpost that was long enough to go down to the bottom-bracket, that way when the frame cracks it might not be severe enough to make you crash.

The other fix would be to have a solid-steel clamp around the top of the frame-tube, that would keep the frame from cracking, but still get a steel seatpost. A bit smaller frame is better than one that is too big for sure.

Outrider1

Can you post a picture of you on the 47 so we can see how much seat post we're talking about?

LarrySellerz

I did in another thread and people roasted me because I am fat without a shirt, haters... Honestly the seatpost isn't that high

base2 

It's not necessarily the height of the seat post (thought that is a big part of the lever) as much as it is the amount of post inside the frame to counter (be equal & opposite) the forces of you sitting on the post levering it around.

A foot between the saddle & seatpost clamp with 4 inches of post in the frame equals a 3:1 multiplication. Ergo 200 pounds of you equals 600 pounds trying to blow apart the seattube/top tube junction. Mentally multiply to that 2,3,4g transient loads from potholes etc...You'll see we are dealing with appreciably large forces on relatively thin walled tubing. An inch of post inside the frame with a 200 pound you a foot above is 2400 pounds of bending force. Well over a ton...& that's just a static value. A simple unexpected bump could easily make momentarily well over 10,000 pounds or more lever force trying to pry your bike apart.

Don't be stupid. Get a longer post that at least extends down to below the lower top tube weld.

LarrySellerz

I have a hacksaw, many old seatposts, and a master welder who is my mentor at work. Could easily make my seatpost longer if need be, ill try to get a picture once I fix my phone and find a metric allen key to change the seat height. This post is pretty long as is, the rando who helped me raise my seat happened to do bike fits in the past, and he didn't see anything wrong with this setup (he commented on how long the seatpost was for being such a small bike.)

MattTheHat 

You mean when you ride a 47cm frame instead of a 62cm? I’m not sure which frame material I’d trust for that situation.

J.Higgins 

I tried for one summer to ride a Crux that was a size too small for me. Got in a trade, and took a shine to it. No matter what I did, it just wasn't good. When I made an adjustment or added a component that would make it fit better, it threw something else - some other dimension off. So why can't you simply get a bike that is light and fast and actually your size, Larry? I know you like that bike, but I guarantee you'll love a better, lighter bike, that fits you properly. Best of all, derision from your contemporaries will change into envy.

Herzlos

If you can see the maximum line on the seatpost, then it's out too far. It might not fail, but you'd be stupid to ride it aggressively because the manufacturer have said it's a bad idea.

beng1

If you look up the tensile and shear strength of steel vs. aluminum you would be more sure that steel is a better choice, especially when you add the fact that steel can bend like a spring an almost limited number of times, this is why it has been used as the valve-springs in internal combustion engines for over a century. There are very few aluminum applications for springs because it quickly fails when allowed to flex like a spring does. This is why aluminum bike frames have to have larger diameter tubes and thicker walled tubes to get reliability, if they are not massive enough to prevent flex, then they crack.

Aged and hardened aluminum can be very strong, but it will still crack if allowed to flex. The best bet for a long seat-post in a small alloy frame is to have a tough steel clamp around the top of the seat-tube so the alloy seat-tube can not deform and flex and crack, and also to have the seat-tube be steel and long so that if it bends or something does crack it will not be a sudden catastrophic failure, but a slower noticeable one that would give the rider a chance to stop riding the bike by choice.

The best option is of course to have a seat-tube the correct size, that will allow a standard length seatpost, but variety is the spice of life, and it is fun to look at the best engineering for extreme applications.

I am not a master welder, but have been welding for a half-century or more, gas, mig, and tig, have had schooling, and have read books on it, and if anyone is going to butt-weld two steel tubes together then make sure the wire or filler rod is not coated with anything like copper as it often is, to get a weld that is not contaminated and weakened. If you are talking about an aluminum seatpost then it is going to be weakened by welding, it will destroy the heat-treat and hardening process it has gone through if any, and unless the filler for welding the aluminum will take the same heat-treat and hardening as the original alloy, the part will never be able to have it's original strength restored, not to mention that almost nobody has the knowledge or equipment at home to properly heat-treat aluminum for extra strength, whereas steel is easy to heat-treat and anneal to different hardnesses in a small shop using either a torch or some other way to bring the steel up to a heat where it glows red before proper quenching and/or annealing.

Reflector Guy

Maybe one of those other old seatposts is longer than the one you're currently using? It seems to me the effort to weld on a piece of seatpost to make your current one longer is more trouble than it's worth. Just to make it fit in the tube, you'd have to make sure the alignment is perfect, and you'd have to grind off all the extra weld at the splice. The slightest imperfection will make it so it doesn't fit.

koala logs

My 6061 aluminum seatpost was cleanly bent permanently when I rode up a kerb sitted. Not a single crack!

On my long 7075 aluminum seatpost, it was flexible enough to smoothen the ride on bumpy roads. It flexes during the ride and I can flex it with my arms. I've used it for thousands of miles without damage before I gave that bike away.

MattTheHat 

Aluminum is great. My point is that I wouldn't trust *any* material not to crack when riding a bike many sizes too small and making up for it with a long seat post. Others' mileage might vary. Personally, I'd just find the right size bike.

ClydeClydeson

Top of the frame can absolutely be broken off from an overextended seatpost.

Kapusta

You can always just get a longer seatpost so that there is more post in the frame.

genejockey 

Jesus, Dude! You can get a seatpost the right length for under $30! Get an actual seatpost, fer chrissakes!

EDIT: It doesn't matter how long the seapost is overall if there's not enough of it under the clamp. In fact, the MORE of it above the clamp, the worse the problem!

livedarklions

I vote you keep the seat post and replace the frame with a larger one.

tomato coupe

Simple solution: Using the hacksaw, cut out a 1/2" section of the seat post that has the insertion mark. Have your friend weld the seat post back together without the insertion mark. You're good to go at that point.