Tuesday, February 5, 2013

The Main Parallelogram

Step-Through Experiment, Notched Mockup
I am working on a step-through bicycle frame - a design which is simultaneously common and unusual. It is common if you look around the streets of Boston, which are teeming with vintage step-throughs. And it is unusual considering that no one I know has built this exact style of frame. Mixtes and modified step-throughs yes. But not plain step-throughs where the top and down tubes are parallel. In fact, this bicycle does not even have what can be called a main triangle. It is a main parallelogram. 

The head tube and the seat tube are also parallel lines - each at a 72° angle. This double set of parallels makes for an interesting visual pattern. 

Tubing, Artisan's Asylum
For this frame I used straight gauge tubing, so that I could practice cutting and brazing unsupervised without worrying about butting and thin walls. The hardy tubing should also minimise flex and twist in the step-through design, as well as make it possible to store the finished machine outdoors and generally treat it as a beater bike. 

Step-Through Experiment, Notched Mockup
The joints will be fillet brazed (lugless) - partly because I would like to practice fillet brazing, and partly because there isn't currently a reliable source for a step-through lugset. Fillet brazing requires using brass as the filler material and heating up the joints considerably more than you would with lugged silver brazing. Since I am using straight gauge tubing, this should not be a problem. The only thing I am a little nervous about is the bottom bracket. With a lugged bottom bracket, the tubes are inserted into hollow sleeves, allowing you to look inside after brazing and check whether the filler material has pulled through properly. With a plain shell like the one pictured here, this cannot be done. I've considered using a lugged bottom bracket while fillet brazing the rest of the joints, but ultimately decided against that. I'll just have to be especially diligent in this area. 

Step-Through Experiment, Notched Mockup
My goal in making this frame was to get some practice with basic technique without having to worry about thin wall tubing, unusually wide tires or multitudes of braze-ons. However, it was also crucial to me that I ride the finished bike as much as possible as part of everyday life, and I knew that would not happen with a plain diamond frame. The resulting compromise was a single speed 26" wheel step-through with a raked-out fork. Basically very similar to the prototypical English "Sports Roadster," but with lower trail. 

Step-Through Experiment, Notched Mockup
Getting the slanted top tube was in a sense straightforward, but not without its quirks. To start with I specced out the slope to match the angle of the downtube. Funny thing though: When the angles were mathematically identical they looked off to the human eye (several spectators confirmed this), so in fact the tubes had to be not quite parallel in order for them look right. Paul Carson taught me how to use his notching lathe, and none of the notches were problematic except for the top head joint. That one had to be adjusted repeatedly to make the angle look right - but finally it got there. 

As the mocked up tubes are starting to look bike-like, those who see the beginnings of this frame tend to have an "Aha, I know what kind of bike this is!" type of reaction. It's been nice to get that feedback. While the first frame I made was done in the privacy of a teacher-student environment, now I am working in a shared shop space with loads of people around. Random people passing through will ask what I am working on, and repeatedly I find myself articulating not just the concept of the frame but the step-by-step process of building it. No doubt this recital helps me make more sense of the process myself. 

64 comments:

  1. Er.............



    Everyone pay attention: v has come fill circle.

    This looks almost exactly like your much-reviled mountain bike from days of yore, the one we argued over. I know you're going to say, " but this one has .35mm less trail."

    Anyway the straight gauge straight pipe "TT" should be a little better for power transfer than your other mixte.


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    1. Have you met Lucy 3-speed?

      No trail comments, but the height of the TT & ST joint will prob be the main diff between this thing and my school-days MTB.

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    2. Forgot about her. 27" wheels? This one's 26" just begged for it.

      Incidentally last summer I lifted a Raleigh Sports frame, weighed nothing, super flexy. Just a wild guess I'd say this would weigh about the same or a little more.

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    3. Did I write Lucy was 27"? If so, mistake. Raleigh Sports were 26" (but the old style English 26" not the modern MTB). This new one will be the latter. Yes, she will be heavy. Lots of vintage 3-speeds around, will be fun to compare.

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    4. I don't know if you wrote that - assumption on my part. Learn something new (every other) day!

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    5. This looks nothing like a mountain bike! I've ridden them enough through the 90's and early 2000's. The old 26 inch wheels are actually 650A(590mm) and should be called thus to end the mountain bike wheel size confusion. To be fair, these classic bikes were designed as all around commuters, tourers, and designed for all terrain. They have serious stability and great handling, which means I can indeed take my raleigh sports on mild mountain bike trails and have a blast.

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    6. Re GR Jim's observation.....

      Didn't you reject the Van Nicholas Ti Amazon with step through frame (and the same exact design of the bike you're building) on the basis of its lack of aesthetic appeal?

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    7. I don't like step-through frames where the TT and DT are not (roughly) parallel. Not only aesthetically, but the stepover is considerably higher with designs like the VN. Just a personal preference. On a city bike, I'd rather the stepover be low-low. On a sporty bike, I'd rather just go with a diamond frame.

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    8. You've never explained why you must have a step thru for urban use. One of the great mysteries.

      Maybe you'll tell us when I'm old. Or dead.

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    9. No profound reason, I just like it. There is a certain max stepover height that feels ideal given the way I like to mount & dismount.

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    10. You're an athlete now, with the ability to adapt to the bike.

      Figs you don't even wear minis. I swear were you to get a job in Cali you'd learn how to mount any bike wearing anything within two minutes.

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    11. Oh I can mount any bike wearing anything. And I ride diamond frames in the city sometimes. Just prefer the other thing.

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    12. when I have my bicycle fully loaded front and back, and have to mount stepping over the top tube, I find myself pining for a step-through.

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  2. Does the black tape mark where you will cut the head tube? What is the geo of the frame if you don't mind?

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    1. Yes re the tape. I wanted to give myself some room in case I miscalculated (thankfully, didn't).

      I will post the full geo later, have it at home.

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  3. When I think of older stepthrough frames, I remember a lot of variation in angle between those tubes rather than being dead-on parallel. I had always assumed that the higher the top tube connected to the seat tube, the stronger the general shape of the frame. I'll be curious to hear how tubing and fabrication balance out with geometry.

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    1. It depends on the era and type of bike. The classic English Sport Roadsters had them mostly parallel-ish, within a few degrees.

      "The higher the top tube connected to the seat tube, the stronger the general shape of the frame."

      Yup. And the higher the stepover height. The purpose of the bike (and the tubing used) determines the ideal balance for the end user.

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    2. Thanks for the photo link. If that's close to the target you have in mind, I can understand the appeal. Reminds me of why I enjoy my '51 BSA so much.

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  4. That's really cool.

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  5. I have run across some brief discussions of frame geometry and design in books and magazines - everyone seems to know exactly how these changes influence the ride. Is there a book or technical work that describes these principles? Sometimes I get the impression that everyone is referring to some master text I have not yet found, other times it seems like this know-how is only handed down master to apprentice. Do you have any bicycle design books/articles/encyclicals you could recommend?

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    1. Well we only seem to know. Has to do with riding experience and a little paying attention.

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    2. It can indeed seem that way, but my feeling is that it's largely an illusion. There are lots of areas of disagreement, and there is no one definitive source. Everything I know (or rather believe) so far, I learned piecemeal...

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    3. Areas of disagreement exist but in competition things are minutely codified. Someone's general impressions in a blog? All hell can break loose.

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    4. Mixtes get all the love in the step through department these days (if I went out with a decent chunk of cash to buy one frivolous but useful bike, I'd get a Rivendell Gomez) but all the step-throughs make so much sense for city riders of both sexes, especially with rear loads and baby seats.

      I may have posted a pic of my Bianchi Varazze here previously, but man it's such a cool bike. Labeled in the 1946 catalog as a "26", but that was Italian for 650B.

      What fun to be set up with your own workspace. You've obviously put some thought into picking a good project to start with--something not too complicated, but not a total no-brainer, and something you are excited about creating. Looking forward to seeing your progress.

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  6. Another frame?! Have you built up your first one yet or are you still choosing components? Either way, looking forward to following your journey with this one as well. You'll be taking orders soon...

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    1. The first one is awaiting powdercoat and I am saving up for the components it requires. Initially I planned to move everything over from the Rawland, but on second thought I would like to test ride them side by side.

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    2. I admire your patience and dedication. Looking forward to reading and seeing more.

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  7. "There are lots of areas of disagreement, and there is no one definitive source"

    I'm curious: who are these people and how much weight to you give to their opinions based upon their experience or study?

    Personally I give a lot of weight to those whose life/work depend on it: engineers, successful bike designers, racers. Beyond that if a guy/gal has the legs to truly put the machine to the test I listen. Beyond that...well let's say it's an interesting parlor palaver.

    BTW I'm talking design principles of nth degree efficiency, not riding to the grocery store.

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  8. "Do you have any bicycle design books/articles/encyclicals you could recommend"

    Velonews and cyclingnews tech features. The trick is to know what's marketing what's relevant. You'll learn more there than any hard-bound book.

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  9. So, you are onto a second frame? What are you going to do with the lugged frame you made? Parallelogram sounds nicer than 'step through'. People always lump step throughs and mixte together, but not the same. I love mixtes, but this is my favourite.
    I love that Mercian builds their Miss Mercian this way and offer just as good tubing as their other frames. While there are some production mixtes out there, the tubing is often heavier and it does not need to be.
    I wish all my bikes could be step through or mixte, but lightweight versions are hard to find unless you go to the Uk and people practically give them away. ( boo hooo, tears) Some day I'd like a titanium reproduction of my beloved raleigh sports. I love the geometry for a daily commuter, but it is heavy, and rusting away.

    You are fortunate to have found a place to do some bike building and learning!

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  10. Nice. If the bike won't have a third set of stays from the upper tube to the rear fork ends it should a reinforcement inside the seat tube to prevent it from bending there. You can either shove another tube inside or just use a very strong (large diameter and/or thick walled) seat tube. Amongst Dutch bikes good ones are made this way. Cheap ones are not and break in the middle.

    72 degrees parallel is very steep for such a bike BTW but I imagine you've already done plenty of homework there and have good reasons for that.

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    1. I've been considering that, but then someone told me I was being silly and that the Raleigh Sports were not made this way (which is why they were so relatively light and responsive). I am still not at the brazing stage yet and am continuing to look into this.

      72 ST and HT was apparently pretty standard for the "Sports" models. Short head tubes, too. These were not the laid back DL-1/ Dutch bike models.

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    2. Funny I was going to ask why so shallow.

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    3. 66-69deg angles on Dutch utility bikes, English Roadsters and the like. My Austrian city bike had something like a 68deg HT and 100mm(!) of fork rake with 28" wheels.

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    4. Here's what I'd like: 73 st, perfect saddle with epic length rails, slack ht like 71 or so, lowish trail, long tail, extremely rigid front triangle, absorbant rear triangle.

      Effiency + the ability to take a hit from the front. I'll be dead before that happens.

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    5. Oh and a really low bb. And a double with a bash guard.

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    6. The wall thickness of the bottom few inches of your seat tube has as little influence on the "responsiveness" of your bike as just about anything. But with a short chunk of tubing jammed in there the bike will be many times stronger.

      GR Jim,
      Keep in mind that the more upright you sit on a bike the shallower the most comfortable/efficient seat tube angle will be. My track bike is 74.5 degrees, road bikes 73.5, touring bike 72 and city bike 68. I was assuming that V will set this bike up to sit upright thus 72 is on the steep side. But if V's building a sportier bike it's nothing strange for her size.

      Of course the 73 degree ST you describe above with the saddle slammed back on its "epic rails"... is just a 71 degree seat tube with another name.

      V,
      That 100mm of fork rake is necessary to bring the trail down with the shallow head angle and large diameter wheels. Otherwise the bike will ride very nervously within the range of speeds it's normally ridden at.

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    7. Henry - I think you would agree that there are city bikes, and then there are Dutch bikes as a subset thereof. On a city bike, I prefer swept-back bars level with the saddle, and a ST angle of 72-73deg. This works for me very nicely, but it is not a Dutch bike and my posture is considerably more leaned forward than it would be on one.

      The kind of bike I am making is basically a slightly modified, hack version of this:

      "Sports or "light roadster" bicycles ...feature 590 mm (26 x 1 3/8) wheels, ..."North Road" upright handlebars, and cable-operated brakes. Sports bicycles had rather more nimble frame geometry, typically with 72 degree frame angles. These bicycles were faster and lighter than roadsters."

      I would love to find a scrap frame and cut open the ST, to see whether they used an internal sleeve.

      (And re the 100mm fork rake serving the purpose of lowering trail - yes, that was my point.)

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    8. Henry, exactly!

      With a 73/epic set up (which i have due to two Brompton adapters) I can go back and sit up straight or, with a quick allen key turn or seatpost/saddle swap via qr, go roadie/forward. The cockpit has been adapted for both positions.

      If starting from a native 71 I wouldn't be able to attain the 73 without reversing the seatpost set back, which is doable but aesthetically like fingernails on a chalkboard.

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  11. So you're making a step-through variation on a Retrovelo Paula.
    ;)

    I seem to have seen something about a biplane or triplane fork crown somewhere...

    Going to go the Fat Frank route?
    Can't wait to see the color.

    GR Jim has a point about the fullness of circles. Though your DL-1 may be the closest example for tubing stiffness with this frame design.

    CK

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    1. Nah, really more like a Raleigh Sports with a replacement fork. Tires will be "only" 35mm. Also, the Retrovelo has a taller headtube and slacker angles.

      The fork... If the plan works out, it will be either twin or triple plated, but I don't want to speak too soon. I have instigated Paul to look into getting plates made for oval blades (Mike's are made for round). Still prototyping.

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  12. Regarding not being able to see inside the tubes to look at the internal fillets, you might consider making some practice joints and keep track of how much filler rod you used on the "tinning" pass to get the internal fillet you want. Using a similar amount when brazing the frame should give you confidence in what you cant see.

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  13. Where do you buy your tubing?

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    1. For this project I used scrap tubing which I bought from the guy who runs the shop. If I didn't have that option, I'd get it from Nova Supply.

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  14. I'm not a frame builder but I have built many structures in wood and in metal. The parallelogram with small diameter tubing looks way unstable. What about a small counter-bar (in compression) to prevent folding of the parallelogram?

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    1. This is not a design I invented; have a look at the Raleigh Lady's Sports. A counter bar should not be necessary given my tubing choice and the way this bike will be ridden. However, if it does prove necessary, I want to experience that for myself. This is an experiment, and the question of "how will this feel?" is part of why I want to make such a frame.

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    2. OK, good luck. I admire your pluck in building your own frame. Every success.

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    3. my partner's 1930's Healing - a lightweight step-through sports-roadster has parallel small diameter down + top tubes- the top tube being smaller diameter than the down tube <1" it has survived 80ish years of use and the odd crash and is still intact and straight. It does of course have the lug to provide a little extra support at the joint.

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  15. I can't wait to see the final result!

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  16. Greetings from Brooklyn!

    This is so inspiring! I don't have the courage or the resources - yet - to attempt this. But, having returned to cycling around the time you did (after a multi-decade layoff), I am stunned that you've progressed to the point of building TWO frames since then - not to mention collaborating on multiple bicycle designs, doing brevets and pacelines, etc. Like I said, inspiring stuff.

    A couple of quick suggestions for your Upright consideration, if I may? The current Schwinn Coffee 1, except for the wheel size and the use of a Shimano hub instead of an SA 3-speed, is the spitting image of the old Schwinn Racer/Speedster model. The Torker Graduate is an upright, drum-braked model with an SA-5 speed IGH. And Schwinn also has the Rendezvous 1, a step-through that also uses the Sturmey 5-speed hub that your husband thinks so highly of. All of these bikes are closer to the classic Raleigh Sports than anything Raleigh offers on this side of the Atlantic at the moment.

    I now return to agonizing over which SA hub (3-speed AW, SRF3, freewheel equipped S3X; or 5-speed X-RF5 (W)) to choose in order to have my LBS turn my single-speed into a modern day Raleigh Clubman...

    Rudy

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  17. Another frame!? You might be a frame builder in disguise.

    Is the lugged ANT painted and built up?

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  18. Bobbin Bicycles have a frame like this! The "Bramble". http://www.bobbinbikes.co.uk/wordpress/bobbin-bramble-plum/

    I used to ride around on an old Speedwell like this, it was rusty and creaky but it never gave out on me.

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  19. What are the plans for brakes, particularly the rear brake? That's always the biggest problem I've had with these frames; the cable routing isn't conducive to proper set-up of the rear rim-brake.

    I want to echo what others have said above: this frame isn't designed for 26" wheels. 26" wheels are generally understood to be 559mm wheels, but I'm not too comfortable with that, either. The truth is, if you take a 700x23c, a 650x42b, a 26x1 3/8", 26x1 3/4" or a 26x2.125", the overall diameter will be roughly the same (@26inches), but the wheels will be markedly different. Which is why savvy folks would call them, respectively: 622x23, 584x42, 590x37, 571x48, 559x54. This alleviates confusion.

    If you're really into avoiding concise measurements, and you feel like using old-timey tire taxonomy, you can safely call those ol' Brit sports wheels EA3, or 650a, or 26x1 3/8". The simple designation "26inch" invites questions such as "Ya gonna put Fat Franks on it?", which is a perfectly reasonable question for a 26"-wheeled bike...

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  20. Congratulations! I wish I lived in a city that had a facility like Artisans Asylum around (Maybe it does but I don't know of one in the Washington, DC area). I would love to build a bicycle frame.

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  21. I vote coaster brake. Why track ends instead of forward facing dropouts? Your drawing seems to indicate you intend to install fenders. Might make wheel removal a pain. -Matthew

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  22. I just now measured the space between the two tubes on my wife's Raleigh Sprite step-through frame. They're about 97mm apart just behind the head tube, but flare out to about 130mm apart near the bottom bracket. I had never really noticed that before. Good luck with your fillet brazing. I saw some photos of Tom Ritchey's work on his early hand-built bikes, and it's amazing. I seem to recall that one of his early frames went for several thousand dollars on ebay not too long ago. Perhaps someday your creations will command similar prices.

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    1. Those figures seem about right. The tubes look parallel despite that slight flare. And when they are truly parallel, they appear to flare out at the front end. Not sure why the human eye does this.

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  23. My vote is for a shined up old Bendix or New Departure coaster brake hub, some black painted middle weight 26" Wald fenders, Wald Model 8095 3-speed bars, and a Wald Front basket. Maybe Wald folding rear baskets too. That would make for a great city bike. I have a converted mountainbike that is somewhat similar to that, with a 58" gear, and it works great.

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  24. Re components: It's going to be a single speed, drum brakes, fenders, longish stem, North Roads. Not sure which fenders yet, but the rest of the stuff I have lying around already. I might hand-paint the frame with enamel, or spray paint it. Some puke military colour no doubt, or flat black.

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    1. Yes, hand paint it! I find the most interesting bikes to be those 'claimed' by their owners. Thinking of doing the same to mine but so far have only, timidly, put paint on the fenders. Sorta dipping my toe in the water but why not? Life is short :)

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  25. I am in the process of building/assembling a frame jig.
    1. do you notice some features that would be good to have and were missing on this or other jigs you used before?
    2. do you like using Paul Carson's jig? It looks well thought out, but also heavy and possibly difficult to make and verify fine adjustments.#
    I like Paul Carson's take on the jig subject on his website and know that he is selling it. it would be sweet if he also shared detailed dimensions and how to make some of the parts for someone to copy.

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  26. Where can I get plans/drawings for the frame jig you are using for this project. I'm getting ready to build a frame and need a jig. I have all the other tools.
    Emile

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  27. I find this very interesting and i can't wait to hear how the finished bike rides. As you know a English sports roadster rides very nice.

    Chris531

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