Seat post weight limit?
 

For a steel monstrosity like this in 27.2 OD, what's the likely weight limit before bending?

http://bikeforums.net/attachment.php...hmentid=423571

The post is not the fragile piece here its the seat post tube.The post is now a leaver with a load.The top of the seat post tube may need reinforcement as that area will see the most stress. The post itself is as strong as one of similar dimension to bending force.With that said keep the clyde's off it.A straight post will distribute the weight better to the frame and resist bending as its now in compression.

The pictured post looks to be a decades old design often found of BMX/Freestyle (does any one still call them that?) bikes, we sold and serviced hundreds in the 1980s with this type of post. Many were bent back from the stress of the rider doing tricks/jumps (or actually landings) while still sitting. But they were usually easy to remove and replace if needed. I have seen my fair share of bent/broken frames with these posts. When the post was raised too high, and the extended section of the seat tube top is the only portion of the ST that contains the post, the stresses can be greater then the ST top can handle. Of course removing and replacing the frame :) isn't quite as easy or as cheap. Andy.

Well, I'm an UBER Clyde @ 265# currently (down from 285, thanks to my biking). I don't plan on doing any BMX tricks or off-road riding that would cause excessive post stress. The saddle will be a Brooks B135 that will, itself distribute stress (trading high impacts for distributed stress over time) via its springs. Yes, the bending moment will be extreme, but the frame is an MTB one made of steel. It was originally intended for a 400mm seat post with a significant bending moment anyway. Taking these issues into account (no hard impacts, sprung saddle to distribute stress, and frame originally intended for off-road use), I think that this should be safe. I'll be watching the seat post, and the seat tube of the frame CLOSELY for bending or cracking. If they occur, then the stressed part will be retired immediately. Thanks for the feedback.

The seatpost itself is pretty strong. I'd be confident in its ability to support a rider's weight, excluding sudden forces like jumps (landings) as mentioned above.

BUT... If I remember your bike from pics in another thread, you have a LOT of exposed seatpost. The long post combined with the extra setback is going to be a pretty effective lever, applying way more force to the seatpost/frame junction than is typical, which may put you at high risk of damaging the frame.

Except they are for BMX, and 7/8" 22.2mm Not 27,2.. anything thicker diameter will be custom So.. a Design and Material, specification TBA.

still suggest getting onto a beefier bigger frame... with a Level top tube .. perhaps Hire one built

Perhaps a cargo bike builder who works in thicker wall 4130 (many are used to light spec materials since people spend more to have it weigh less)

a post with a triangulated strut set, to transfer the bending load instead, as compression , straight down to the rear axle ..



I was just thinking of Dursley Pedersen bike a very triangulated old Design http://www.dursley-pedersen.net/
http://www.oldbike.eu/museum/wp-cont...n-1024x840.jpg

This will not protect you in the case of a sudden failure, such as a tube collapse under bending stress, which would seem to be a likely failure mode given the weight you are hanging off of it.

It seems to me that you are trying to get a too-small frame to fit, which may not necessarily be possible, at least not safely; I wouldn't risk injury in the process.

I have gone to a mountain bike as well for the strength.You need to add street tires and heavy tubes with high pressure.Do you have a reason for not going with a standard post the risks are very high including more weight on rear tire and less on front.That could cause a washout!

Consider finding a bike that fits properly while waiting for this post to break itself or your frame.

I'd only use a post like that if it extended into the seat tube well past where the top tube and chainstay join the seat tube.

I've busted rails on a saddle just from landing on the saddle when I missed a pedal clip in - and that didn't have the lever arm that monster setback is going to have.

If it's a hardtail MTB, even hitting a bump hard could easily be enough to bend the seat tube with a lever arm like that if the post extends into just the very top of the seat tube.

The post will extend into the seat tube well past where the other frame tubes join. The bike is designed for very long seat posts, and is also designed for off-road jumping, landing, and general abuse. The post I'm considering will have a greater bending moment due to more offset, but not significantly greater because it won't extend as far from the frame (in other words, greater horizontal offset, but less height than with the 400mm straight post that the manufacturer supplied). Steel frames and posts, unlike carbon (or, to a lesser extent, aluminum) are not prone to catastrophic failure without previous signs of bending or stress cracking. This is also NOT a 22.2mm post. The frame-insertion diameter and the diameter at the bend are both 27.2mm. The post and frame are both steel.

Yes, I will be looking for a bike that fits better, but for the moment, WAF demands that I use this one for awhile. With the straight post supplied with the bike, my knee is forward of the pedal spindle when at the 3-o-clock position. This is, indeed, indicative that the frame is too small, but for the moment, it's what I've got. The only way to get comfy on this frame is to use a seat post with SIGNIFICANT setback (such as this BMX post). This bike has been a learning experience for me. Next time, I'll pay far more attention to things OTHER than stand-over height.

22.2 ID tube That thing * with a 27.2 OD, would have a 2.5mm wall thickness **,, a Machine shop could take an even thicker wall 4130tube down to a 27.2 OD

Sleeve 6" of it Inside the frame **, and the exposed part left full wall thickness ..

with the top at 7/8" a common saddle Clip mounts right onto it. that is what you need to get with that* anyhow..

[SBS, 'Redline' also makes a chromed straight 22.2 and 25.4 chromoly Post]..



** the shim sleeve you need to put a 22.2 post in a 27.2 frame would have those dimensions.

Thanks again [MENTION=197614]fietsbob[/MENTION] - Rather than have custom machining done for this bike (that doesn't seem to fit me anyway), I think what I'll do is wait until after Christmas and cruise CraigsList for the cast-offs of those who got new bikes from Santa. Surely something will fit better than this! LOL

OTOH, I've got almost a $1K honorarium floating my way in Euros, and that might be the bulk of a new Clydemobile in my size. I'm looking hard at Surly Disc Truckers, Electra Amsterdams, and whatever else I can shoehorn a 2" wide tire onto. Any suggestions? Actually, I'd be just as happy with a coaster brake rather than a multi-speed. I don't need the gears because the bike is primarily for exercise and short commuting & my state is FLAT. I'm looking at Electra Loft bikes at the moment. Any other good coaster brake models out there that might tolerate super-wide tires?

PS - I'm not (repeat - NOT) interested in fixies - I'm a platform pedal guy - even back in the old days when I rode half-centuries. So the inexpensive flip-flop-hub bikes with their narrow road tires don't interest me.

If you have a mostly complete and functional good quality bike, then a replacement frame from a larger, possible lower quality but correct size, donor bike could be perfect. Getting a high-tensile frame that fits properly would be far superior to a little bit 'nicer' frame that is too small.

The actual Made in NL bikes may be worth looking at, Electra is An Asian Import ,, the traditional Dutch Opa. (grand dad) bikes are made to last generations .

now that they have become cool & trendy, for the young urbanites,
there are US importing Dealers , such as In San Francisco http://www.mydutchbike.com/bicycles/...-and-opafiets/

then theres the venerable Worksman Bikes Old american cruiser style .. http://www.worksmancycles.com/

A comment was made about a seat with springs and the load the seat post or seat tube top will see. IIRC stress passed through a spring isn't reduced in level of force. The duration of the force, time wise, does change. Now if the spring were to increase it's temp with the force applied... Maybe a rear engineer will chime in.

Any way, the designing of the seat supporting system to have a cheap and easily replaceable "link" be the failure point makes sense to me. Andy.

Hi [MENTION=391685]Andrew R Stewart[/MENTION] -

You're right - the springs don't destroy energy - the overall force on the post doesn't change whether or not springs are present. But by dispersing the force over time, the instantaneous stress IS reduced.

To use an analogy, it's the difference between hitting a rod with a hammer or hitting the same rod with a heavy spring in between the hammer and the rod. The force of the hammer (swung at the same velocity) is identical, but the spring makes the rod harder to bend due to the temporary absorption of instantaneous impact (and then release over time).