Seat frame construction

This is a pictoral walk through the construction of the Barnett Williams seat.    The drawing  for it is here Mach 1 DRAWINGS, page down to sheet #8.   The patterns for the seat rails and stretchers are here LINK TO FILES, if you print all six of the seat files and tape them together you will have full scale drawings to work from.    I deliberately keep this construction primitive, done by eye and feel, rather than bringing in precision jigs and measurements.    This is both by inclination for me and by intent for the first time builder.   It IS possible to do this with very little experience and with nothing but your eyes and my advice as a guide.    It isn’t that hard.   In this set of pictures, I am making 5 seats.   You might be making only one.   I suggest you cut 3 ‘blanks’ at least from ½” EMT conduit – bend all three and you can pick the best two later for the seat you are making. 

 The overall measurement for a  seat rail should be 36 ¼” unbent (thus you get three pieces from one 10’ length of conduit).      Use the drawing to mark off the bending points on each rail – I suggest you label where each bend is supposed to go – the 45, the 90, the 15, the 20, etc.   You can get lost figuring out which mark is which.

 

 

 

 

Here is the conduit bender.   I mount my ½” bender into my bench vice – this is not how your neighborhood electrician bends conduit for the construction site.   Bring the pipe to the stationary bender – much easier this way.

 



 Because I make this seat regularly, I have a dusty sample seat rail laying around – it’s sturdier than a paper pattern, and I use this one to compare all my initial bends to.   It is good to check every bend you make with a pattern.    You can do using the pattern made from the PDF files.  Heck, you can ALTER the patterns to your own idea of what makes a good seat for you.   Higher lumbar Curve?   Lower?   Deeper seat ‘pan’?   You decide.   

 

Depending on the conduit bender you use (the radii are not standard on these tools) your bends  may not line up exactly.   But, eyes on the prize, if your overall bends follow the pattern, and YOUR seat rails are consistent one with the other, you will be fine.

 


The first bend to make is for the 45 degree sweep at the very bottom of the seat.   Set this in the bender’s clamp, mark on the bender’s start point, most of the pipe extended out over the tool.  

 

Bend to the guessed-at 45 degree point – remember that you have to bend a skosh past the mark to achieve the bend- steel ‘springs back’ a little after you release pressure.  In this 'jig' I use a level, placed where the glove is now, to tell me I bent a true (or at least consistent) 45 degree first bend for the 'skirt' of the seat.


Now check your work.   Using either a paper pattern or a template, see if you ‘like’ the bend.   Spot on?   Too much?   Not enough to get the desired 45 degrees?   Put it in the bender and adjust – just a little.   Not too much, you get a feel for how to correct.   If you bent it too far reverse the bend in the bender and pull a little.   You will find (with ½” conduit) this is fairly easy.

 

Bend the 45 degree bend in your second seat rail.   Now the pattern doesn't matter - do the two rails match each other?   Here is an easy test -
lay the two pieces of conduit on a flat surface, press the flat part of the two bent ends together.  Do the long sections line up parallel?  Fix it now, while it's simple.    
 

I put the template aside, and bend each of the remaining rails (be it two more pieces for you or eleven pieces for me) to 45 degrees.   I check each of these by the first piece I bent today.   Ultimately, it doesn’t matter that these pieces all match the template – they mush match each other!   

 


With all pieces done at the first bend, time to turn to the ‘bucket’, or main seat bend in the rail.    Position the rail at the next mark (after the flat section) like this –





Before you bend, eyeball along the end of the pipe, viewing the alignment in plane of the first bend, the rest of the rail, the bender, everything.    Satisfy yourself that the first and second bends will be in the same plane when done.****       

****Slight wrinkle.   I like to ‘splay’ the 45 degree bends out a little – keeps them from jabbing you in the thighs later.   Like this –  the camera doesn’t see it quite as well as your eye, but in this picture the farrrrrrr end of the 45 degree bend is just visible to the right of the seat rail’s bending plane.   If you do this, make one offset to the left, one offset to the right.  

 

You don’t have to do this.    The seat works just fine built with completely parallel rails.  

 

 

OK, bend that bucket.    All the way to the end of the bending tool.  

Check you bucket bend.  Use the pattern.   You can also check it with a framing square or a large tri-square if you have one.   It’s a 90 degree bend, easy to see.     Some people like 85 degrees, some laid back souls even love 80 degree seats.   I have found after many miles in the seat that I prefer the 90 degree bucket.    When you  check this bend with your template, do the bottom, the straight seat back, AND the 45 degree sweep all line up?   The measuring marks make this easier to line up.    Correct and get satisfied on the first pipe with the bucket bend.   Use that to match all the others you bend today.   


Here I have a batch of rails, all with the seat bucket bend completed.    I get to practice this a lot.










Now we turn to lumbar bends – a pair of spaced, 15 degree bends that create the very comfortable arch in the seat back. You might experiment with less or more lumbar arch – I have, and I keep coming back to this design.  





Continue setting the pipe into the bender in the same direction – the ‘bucket is behind the clamp and arched down like this.

 

    

BEFORE YOU BEND, eyeball this from the end of the pipe – now it is REALLY critical that you see that these next four bends align as well as you can with the bucket bend – everything must bend in the same plane.



 

OK, a 15 degree bend.   Not very much at all, just a little.   Make a guess, eyeball and remember how far you went moving the pipe in the bender.    Before checking things, go ahead and move to the second 15 degree bend as marked on your pipe.    Eyeball from the end before bending.   And make the same bend as the first one.    Take this out of the bender, and check by your drawing or template.  

You should be aligned from that first sweep through the top straight part above our two lumbar curve bends.

 

   

This is my pile of rails with all the lumbar curves done now.    See the lefty-righty out-of-plane alignment of those bottom sweeps?   

 

 

 

 

 

 

 

 

 


Moving on to the two bends at the top of the seat rail.   These curve in the opposite direction to the lumbar curves – sort of a shoulder blade cup in the seat back.   




Set the rail in the bender backwards now – can’t keep going in the same direction (nothing to hold onto to bend if you try that).   See that the seat bucket is now past the bending jig and arched UP?

 




OK, head out to the end of the pipe and check that alignment before bending.   See something like this – close one eye.

 

 





Bend again, one after the other of these two 20 degree bends – just a little more than the 15 degree lumbar bends earlier.    Do the first one, move to the second mark, check the alignment, do the second bend. 





 




Now pull it out and check by template/pattern.    Remember, it is more important that the new rails match each other – the template is, uh, “more of a guideline, really.”

 



Here you go – all bends done!

 



Now cut off the excess down at the end of the bottom 45 degree bend – cut straight over that first mark.   

 

 

 

 

 

 

 

 At this point, I rack all the rails together.   Despite the best of care, there will be small differences between the rails.   It is time to pick the best two.  Or in my case, to match the best pairs – this lefty most closely matches with that righty, and so   on.

SEAT STRETCHERS

Now you need to make the stretchers – the two cross pieces that hold the rails/seat together and connect the seat to the bike later.   These take more strength to bend – I make them from ¾” conduit.   The layout (also on the drawing) is much simpler that the seat rails – a 24” piece marked as shown on the layout (remember “Sheet 8” on the website?).   You can get 5 'stretchers from one piece of conduit – make 3 and you still have spare conduit for a stem riser later…..

 

Here is a conduit ‘blank’, ready to bend, and the full sized template I use to check things as I bend.

  

 THe patterns are on the FILES page here as PDF's you can download.   Mark the 24" piece of conduit - the blank, per the drawing in the EZ Clone DRAWINGS on Sheet 8.











And here it is set in the conduit bender – this is a different bender die from the ½” bender used earlier.   But it is mounted the same way in the workbench vice.    (I took down the red bedsheet, sorry for the busier photograph.)

 

 

 

Set it on the mark, and MAKE the 60 degree bend….

Whoa!   That’s harder!    If you did that without any help I am proud of you.  

I find I need an extender bar to make this bend work this way.   Here is my extender bar – a simple thing courtesy of Schwinn Bicycles……

(It’s a 24” piece of bike tubing left over from who knows what frame.   This sleeves nicely over the ¾” conduit to give me the extra leverage *I* need to make this bend.)    Like this:

 



Now make that 60 degree bend.    Bring it back to the pattern, get happy with that first bend.    Your radius may not match the radius of the pattern – that’s OK.   See that circle on the ‘end of the bend in the pattern?   That is where you want your rail to bend to – that’s the cut-off point after we finish  bending.   

 

 

 

 

Now bend the other end.   Just put the extender bar on the bent part – you really need the extra leverage now.     Go out the end of the bender/pipe set up and eyeball the alignment – these two bends need to be in the same plane.    Ready?  

WONK that thing down there, make that second bend.    On my bender, after releasing tension, the finished bend looks like this.

 

 

 

 





Back to the template – adjust either bend if needed to suit the pattern.   The secret for me is that line at 60 degrees.   Look directly over that line up, and mark that 60 degree line onto your pipe.   You will find this is on the curved part of the pipe, maybe ¾” inside your bending mark, like this – and not necessarily radial to the end of the bend.   

 

 

 

That mark is your cut-off for the seat stretcher.   Cut the piece off here and discard the excess little bits.   You are done bending stuff now!

Now to finish the piece.   The seat stretcher must mate to the side of the seat rail.   In the drawing, one stretcher goes on the flat spot between the seat rail’s ‘bucket’ and that front, 45 degree sweep.   The other goes on the back – I place it midway between the  lumbar curves and the top curves.    Mark a center position for the stretchers on both rails side by side before starting an assembly.  

The end of that seat stretcher needs a curved, cupped end, so that it nests snugly onto the seat rail.    Filing this shape onto the end of the tube is called ‘fish-mouthing’ the tube.   You did  this before when we made the bike frame  (first assembly), and now we use the same technique for the seat stretcher.   Four times (sigh) – each end of each stretcher.    Finished, the end of the ¾” conduit looks like this –

Let me tell you how I do it (you may want to do something different).    Put the trimmed stretcher piece back in the vice – I level it in the vice



-         Across the tube…..



 

-         And on each end…..

 

I like the stretcher  level in the vice – this helps me keep the fishmouth joint level – As I cut the end, a sense of what is plumb helps guide the grinder for me.. 





I use the edge of a grinding wheel in a 4 ½” motorized grinder to carve this shape(you can use a file, it just takes longer.   You can use a mill and a round cutter of the proper diameter – but that is a steep investment). 

 

BY THE WAY, PLEASE WEAR EYE PROTECTION.   I do this a lot.   I DO wear wrap-around safety glasses.    I’ve even taken to wearing a full face shield for these operatons where I have to peer down directly over the working grinder.

 

 

 

Step by step with the grinder:

 

 







Focus in the top-center of the tube.   For this joint (half-inch conduit fitting onto ¾” conduit), cut a notch straight in  about 3/8” into the top center surface of the tube.    Hold your eye directly over the end to judge this – I do not mark the distance – the cut is approximate.   

Then ‘match this notch in the bottom center of the end of the tube.   Visually, I hold my eye directly over the top notch - my ‘sense’ of what is plumb– which way is straight down is all visual.  I try to notch the bottom center of the end of the tube exactly as deep as the  top notch already done. 

These two notches are my reference – as deep as the final fish mouth cut needs to go.   I never (until I correct things later) go back and deepen these reference points – the rest of the fishmouth is carved away to the outside from this reference point. 

From the top notch (this is the third picture above), using the edge and surface of the grinder’s disk, begin to carve material away in an arch, feathering towards the side of the tube.  You will gain with experience a sense of the diameter of the tubing you are mating with (the half inch conduit in this case) – try to create that arc as you carve.   Create the ‘nest’ for the seat rail – working for now just in the top, visible surface of the tube in the vice.   Copy the curve you cut in the top of the tube visually on the bottom surface.

Check your work.   Put a scrap of half inch conduit in  the fishmouth notch, and be satisfied that it seats nicely onto the stretcher, is straight up and down in all directions.   You will have some corrections to make.  I do.   Take the time to make this fishmouth fit well and tightly.   A tight fit is the strength of the eventual fillet braze here.  

Since the 3/4" conduit is wider than the 1/2" conduit that seats in the joint, I use pliers to 'crush' the edges of the joint in a little bit.   This makes the mating of the two pieces a lot smoother - you will have to carve it a little more to make this work.   Go on with the grinder and clean up the edges.    Check to see the scrap tube fit again.   Do both ends the same, and move to the next one.    Four ends for one seat – this is more of a chore than the seat cover!   

GALVANIZED STEEL.   Standard EMT has a zinc galvanized finish.  I use a knotted wire brush/wheel on the 4 1/2" grinder to remove the galvanizing on the four points where I will make a brazed joint - each end of the stretchers and at the contact points on the seat rails.    I brush as much of it out of the way as I can, no more.   There are other recipes for removing galvanizing – I  have been told that a Draino paste will do it nicely – I have not tried that.   Reading the MSDS information on galvanizing, what you create with heating the tubing (with whatever galvanizing is still there) is zinc oxide - the same stuff you rub on your nose to prevent sunburn.   It is a white powder, and a slight irritant if you inhale it.   I suggest you don't - use a combination of cleaning and good ventilation when you weld.  

ASSEMBLY   - You have the parts, now make a seat frame!


The top of my old rolling tool box is just big enough to hold a complete seat frame ‘face down’.   I use a home made plywood square –  6x6 and 6x8 bits of scrap ply built into an “L shape - to hold the two seat rails straight down on the table.   An old drill press vice holds one rail, while I set the other with the plywood 'square.'  The rail is held to the ‘square’ with a spring clamp – pretty rudimentary.   









I stand the two rails on the table – the ends of the rail at the top and the bottom of the sweep are touching the table.  I use the table itself to square the seat rails relative to each other.    Use the first stretcher, placed on the seat back (this part of the rail is relatively level on the table top) to separate the two rails the proper distance. 

 

Don’t set the bottom rail yet – just check the distance to make sure it will work a few minutes from now.   Happy?   Squared up?   Bottom dimension checked?   










Fire up and braze that joint!   I do the parts I can reach right now – the top, bottom, and outside of the fishmouth joint on both ends of the rear stretcher.   

 



Now you can release the jig pieces – the three parts of the seat so far will hold together.  


Set the bottom stretcher in place – I use one of the jig pieces and a clamp to make sure it won’t move. 

 






Braze what you can reach/see here – the top (in this position), the outside edge.    With that done, you can feel free to move the seat frame around, turn it over, etc.  to braze the remainder of the fillet around each joint.   

 

 

 

Clean it up, and there you go!    This is where you go to the seat cover page -    to move forward!

 

 

 

Um, you might want to paint it before you put a cover on it.  Cheers!   ADC


Seat Covering is on a separate page, here -    Seat Covering

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