mikeread

The top of most seat tube lugs seem to be contoured to some extent and come up to a point at the front.

Shims are straight across at the top and have a lip to stop them going into the tube.

So if these are used together, the shim will not sit flush and will look odd sticking out the top of the lug.

I can see two options, either file the top of the lug off square or extend the seat tube half an inch or so beyond the lug.

The second option seems the best to me but I would be interested to know what others think or have done.

There is a third option, to sleeve the inside of the tube, but it would subject the tube to another heating cycle which, as a novice I would prefer to avoid. Also with a 31.7 ST the wall thickness of the tube + sleeve + lug would be significant 2.25mm + lug thickness. So alot of clamping force will be needed.

unterhausen

there have been some square-topped lugs that were meant to use with shims. Personally, I would do anything else other than require a shim. If I was shimming a lugged bike, I probably would just cut most of the lip off the shim so that it is mostly hidden

duanedr 

First, I try to avoid shims. For exactly the reason you bring up. I just think on a custom bike there should be a contour to the top of the seattube and that looks inelegant with a shim. Just my opinion.

Are you trying to use a 27.2mm seatpost?

If you're using 31.7mm 9/6 butted tube and lug with a cast in binder, I would use a 30.4mm post - which are a bit uncommon but are available. Then you can contour the top and thin the lug as you see fit. You will need to find an uncommon sized reamer as well. But the results will look much nicer.

Andrew R Stewart 

Some bond their shims in the seat tube tops, no heat. But I completely agree that choosing to design for a shim is a path I avoid at all costs. Andy

taras0000

Or you can file the shim to leave little "wings" on the sides to catch the lug.

Davet

I’ve done something like this on several lugged bikes and it works surprisingly well. If you’re clever, the shim will be barely noticeable.

fietsbob

If you are not needing perfect bike points at the hand made bike show you just do with them as they come..

I did so twice , moving the 25.0 seat posts from my AlAns to 2 steel bikes, with USE sizing Shims ..

One of which later migrated to use that post again on a bike [Friday] which already had a seat post shim to 27.2

So now it has a double concentric shim , with the 25.0, 1 bolt SR Campag seat post within..

25.0 > 27.2 inside a 27.2 sleeve inside the seat mast on my BF Tikit..








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mikeread

Some good ideas thankyou which I will have to think about.

I am also now considering making my own seatpost or if I can find a suitable tube, extending the seat tube right up to the saddle and making a suitable seat rail clamp to fit.

So many things to think about :-)

duanedr 

Yes, there are!

mikeread

I am thinking about a straight through seat tube - taking it right up to the saddle. (I am not a traditionalist)

It is not likely to be a practical solution as I would need a 725mm long seat tube which as far as I know- doesnt exist, also it would stop the frame fitting in my bike box so I wouldnt be able to take it on a plane.

But I like to consider the idea anyway :-)

Assuming I could get a long enough seat tube and it were 0.5mm wall thickness, would there be sufficient strength at the ST / TT lug?

I have 280mm from the top of the saddle to the centre of the TT and it has to take my 168lbs. The tube will be cantilevered which is not how any other tube on a frame is loaded and the stress would be concentrated at the top of the seat lug - where the lug re-inforces the tube.

Also I suspect that the top end of the seat tube is often un-butted as the seat post acts as a reinforcement, with a straight through seat tube you would loose this reinforcement at the point of max stress.

Anyone ever tried something like this or have an opinion?

Thanks

Mike

fietsbob

1/2mm wall tube on its own seems excessively thin, but yes a 28.6 OD seat tube, to fit a 27.2mm seat post in ,

[math] would have an 0.7mm tube wall..

my BiFri uses a 1.5mm wall (32-29) seat mast.. 12" up 305mm, from its lower bracing portion..

going oversize OD on the seat tube? it gains in stiffness, through increased diameter ..

you could sleeve inside the steel tube with carbon , epoxy wet wrap cloth , inflate a balloon. to expand inside it, let it cure..

or buy composite tube and slip it in..






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unterhausen

I have seen seat masts on steel bikes. I think they just use the seat tube full length and then the topper is longer. Not sure if you have to reinforce the seat tube though

mikeread

I guess it does need a little thought ......


The seat mast situation ? Big Bike Little Bike

taras0000

What if you made a seatpost "topper" to match the frame? Basically sleeve a seatpost (without the head) into a tube that has the head attached. Insert said topper post into your regular seat lug and tighten as normal. Full seat mast look, but you can take it apart for travel.

Or do a seatmast "cap" over the above mentioned post.

79pmooney

Put the shim, in with some sticking out. Insert post to hold it. Etch or magic marker the portion sticking out to the tops of the lugs. Remove, cut/file excess away and reinsert, perhaps with glue. Done neatly, there will be very little to see and what you do see will look OK.

Ben

duanedr 

A standard 28.6mm seattube is 0.6mm thick walls in that TT/SS/ST joint area. Even when using a seatpost, you need a sleeve at that junction (according to everyone I've ever talked to about it) to avoid cracks due to the stress. It's a highly stressed joint.

ISP's have been done since the '40's that i know of. They aren't new. They just have to be designed for the purpose. The trade off might be weight.

Lots of folks currently build with ISP's. Rob English is probably the one that comes to mind first and his are pretty extreme. Most of his that I have seen use shaped tubing that would support the load better than a thin round tube. In his description of a standard bike, he mentions "A custom blend of tubing to suit the design and intended purpose, ranging from True Temper S3 to hand machined/formed 4130 cromoly" This makes me thing he may custom turn/form his seattubes to deal with the stress.

The one way I can imagine doing an ISP would be with straight gauge tube for the seattube. Using a bigger tube (1.250" OD) with thicker walls (0.045-0.058" or 1.14-1.47mm) might provide the strength - but at a weight cost. And one problem with this route is if the tube bends, you can't really cut it off and slide in a seatpost without some serious metalwork.

I guess after all of that, I'm confused as to what problem we're trying to solve. The OP was about a shim in a seattube but now we're thinking about ISP's. My solution to the original question would be to buy the right sized seatpost for the seattube.

mikeread

Thanks duanedr

I am not really trying to solve a problem, just considering all the options.

I enjoy the design and learning process as much as the building and riding and since I cannot start the build for a good few months I might as well investigate different ideas.

For reasons already stated I am not that serious about going the ISP route, but neither do I want to just follow what everyone else does.

If I were to go the ISP route I would (at the moment) go for a 1.25 inch double butted tube .8/.5/.8 with the .8 section starting below the TT/ST join and going right to the top. However the longest tube I can find is way too short whiich would mean having both an ISP and a seatpost, which sort of defeats the object. :-)

CliffordK

I'm a little lost on what you're wanting to do. If this is a new build, then I'd encourage you to find tubing that either matches what you can find for seat posts, or can be reamed to match the seat post offerings.

Some seat tubes are listed as "externally butted". In theory, you could thin tubing in the middle before assembly with a good lathe.

I'm not sure how deep a seatpost must be supported, but I'm not a big fan of simple 2-3" "shims". If I was going to do it, I'd consider taking a 6" or so tube, and braze it in place, then machine as needed.

You may also be able to make your own custom chromoly or stainless seatpost to your own specs.

duanedr 

Ah, got it.