bicycle seat post?

I have a friend who is about 6'5" tall and weighs maybe 240 pounds, a big solid blue-collar type, very strong. He has recently been involved in a divorce and to lower his stress he has taken to riding his bicycle a lot. He called me last night with a problem. His bicycle seat post - raised way up due to his height - is bending on him. He bought a replacement and it too is bending. He asked me to consider designing and fabricating him a suitable replacement.

Of course, for a piece maybe 18" long the cost of the material isn't going to bankrupt anyone. I was thinking chrome-moly tubing if I can find it in the right diameter, and forging a swaged-down neck area. He is thinking solid bar of aluminum. Minimum weight and cost with maximum strength is the design goal. (A google for some exotic turnkey solution has quickly ruled out such due to really high cost - I had no idea you could spend $200 plus on a bicycle seat post!)

Anyone "been there, done that" or have any good ideas or input?

Thanks,

Grant Erwin Kirkland, Washington

Reply to
Grant Erwin
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Grant Erwin wrote: (clip)Anyone "been there, done that" or have any good ideas or input? ^^^^^^^^^^^^^^^ For minimum weight and maximum strength, he should check out titanium seatposts. Very costly, but I believe there is no better. Does his bicycle fit him? For a road bike, the top tube should be an inch or so below his crotch as he stands flat-footed over the bike. If the frame is too small for him, and he is compensating by raising the seat, that's asking for trouble.

Two other "kluge" (sp?) fixes may be possible. One would be a sleeve that slides over his existing post, to increase stiffness. Unfortunately, if the post is bending just above the frame clamp (very likely,) a sleeve may not do much good. The other possibility would be an insert to go into the post, increasing the stiffness. Possibly a close-fitting insert at the bottom, supplemented by a sleeve over the outside, would solve the problem.

If his bicycle frame is too small, and he doesn't want to buy another bike, possibly a tube could be welded above the clamp, part-way up the seat post, to add stiffness. Welding bicycle frames can be very touchy, however, and should be done by someone who understands it.

This might look a little wierd, but would he consider a diagonal brace from the top tube to a point somewhere below the seat (clamped at both ends--not welded)?

Reply to
Leo Lichtman

Slip a piece of tubing up inside the original post to double it

Reply to
Chief McGee

Heh - that problem happened to friend of mine years ago.

There are two solutions: 1) larger frame, 2) replace the seatpost with solid steel, turn down the end that the seat clamp attaches to.

Avoid aluminum in this case.

Jim

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Reply to
jim rozen

Note the common thread: "get a bigger bike!"

At 6'5" he's not in the exotic-frame catagory (quite). He should see if he can trade his good used bike for a bigger good used bike -- using a bike that's the wrong size is just a bummer. There should be plenty of shops in the Seattle area that have used bikes and will do some sort of a trade.

Reply to
Tim Wescott

I would suggest getting one of his damaged posts and measuring the wall thickness. Hit it with a grinder and a file to at least estimate what sort of steel and hardness you are dealing with. (The strength of steels is very closely related to hardness.)

If it's fairly thin wall mild steel, you might just try using tube with double the wall thickness. This will almost double the strength. I don't think solid aluminum is likely to be very good since once you are past a certain wall thickness you get into a region of diminishing returns.

Don't use too hard material! This is the sort of part where failure in bending is *much* less serious than failure by breaking.

Ted

Reply to
Ted Edwards

I have solid aluminum handlebars on my offroad motorcycle. They are 7/8 inch diameter and virtually indestructable. If you go this route, I recommend rounding the edges of the hole that the seatpost slides into with a file. Probably should do that anyway, it prevents a stress riser.

Fred

Reply to
ff

Most good posts have a marking for maximum height, dont go above it. If neccesary get a longer post. I have a kalloy uno one one of my bikes, its very long. Is it a mountain bike or road? If the problem comes from a small frame, have a look at the handlebarstem. Make absolutely sutre that its not overraised above the max marking. Failure on the seatpost is nothing compared to a failed stem (ive seen it happen) Henning

Reply to
henning

Ouch.

Jim

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Reply to
jim rozen

If you go with aluminum and the frame is steel be sure to use some grease on the portion of the seatpost inserted into the frame. Bimetal fusion as it is called in bike shop lingo corrodes the two metals over time making it difficult to impossible to separate the two. Anyone remember the Bicology Catalogs?

Kent

Reply to
K Frazier

I would go for a solid bar of 7075 aluminum machined to fit. He will also need a beefy seat bracket to grab the seat itself.

Reply to
Ernie Leimkuhler

How about thickening it by pressing a tube of smaller diameter into the seatpost?

Reply to
Robert Haston

REC.BICYCLES.TECH

They would agree with get a bigger bike for starters.

Reply to
Robert Haston

The guy must be jumping tall buildings or something to bend his seat post like that. Up until 1999 I was riding my mountain bike to and from (10 miles daily) work plus pleasure riding any day there wasn't snow over 4 inches deep and I NEVER had a bent seatpost. Unless he had a really cheap bike I don't see how he could bend a post. Oh I weighed about 260 give or take ten pounds at the time. Dennis

Reply to
Gunluvver2

If it were me, solid metal. Tubing is to save cost and weight. solid steel.

Overseas - in the Marshall Islands - we had Goose neck steering and high seats. It was great on the back and such. Sadly they, the high handlebars and high seats are not allowed in the states.

Some extremes were inverted frames and weird stuff - nothing to do, just dream and do.

Mart> I have a friend who is about 6'5" tall and weighs maybe 240 pounds, a big

Reply to
Martin H. Eastburn

If it were me, I'd make it *solid*. It won't add *that* much weight, and he sounds like he's big enough to peddle the little bit of extra weight that it does add to the bike... but it won't break. Ken.

Reply to
Anonymous

Carbon fiber. Might be worth going to a good bike shop, and asking for a new one to fit.

Reply to
Ian Stirling

Actually, there is no law preventing any choice of steering or seats, with the exception of the fact that most states and cities do require the use of both hands on a handlebar of some type. It's probably just the case that whatever you rode over there isn't popular here. Folks here have been building some pretty shoddy examples of "tall bikes" for some years now. You can start here to get an idea - no regulations against wierd designs (or shoddyness!)

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Brian

Reply to
Brian Smith

Maybe it was were I lived in Texas. Here in Ca. - skate boards are banned most places as no one wants a kid to fall in there parking lot.

The basic one is a long Bolt - and steel (blake pipe IIRC) - The bolt was typically a thin rebar or other steel rod, the handle bar bolt cut off and welded on the ends of the rebar - long bolt! - cut the bottom of the pipe and- hum forget about the handle bars - were they welded or drawn by the pipe. hum. Someone will figure -

Maybe I can dig up a picture and scan it in. :-) It was only 40 years ago.

Martin

Reply to
Martin H. Eastburn

On Mon, 16 Feb 2004 09:36:32 -0800, Grant Erwin vaguely proposed a theory ......and in reply I say!:

At 240 lb, why bother about light weight parts? Solid steel or very thick pipe would make little difference. Aluminium would be useless IMO, solid or not. It would bend easier than a steel pipe.

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Reply to
Old Nick

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