Coker Frame Help

I am going to be making a frame for my coker soon, with the help of a good welder. I came up with a design of my own frame. Can those that have the experience of building frames before tell me what they think?



I will be using 4130 chromoly pipes, and I will be getting some bearing holders to weld onto the bottom of the forks. To weld them on I plan on flattening the end of the two pipes, and then welding them on.

Is it possible to weld these bearing holders to 4130 chromoly ? I am concerned that I wont be able to if they are different metals.


Yes, you will be able to weld the steel bearing holders to chromoly tubing. You do not necessarily need to angle the tops of the legs where they attach to the seat tube. Here’s a thread discussing a Coker frame that I built. Hope it helps:

now thats my kinda protoype drawing

lol wtf?

That looks like a nice plane for a frame. How much does it cost to make?

what ? I cant draw any better I am probably worse

I just thought it was funny what you said.

oh hahahaha

That’s a good start for your drawings, but if you give that to the welder he will laugh at you. I went to the machine shop I’m working at with similar plans. They weren’t to scale, had nearly NO dimensions marked, and didn’t really give much of an idea of what was going on. I started actual machining of my frame 2-3 weeks late because I had to continue modifying the drawings until the guy who ran the shop was satisfied. It payed off in the end.

A quick list of things you need to do to the plans before you go to the welder:

  1. Decide on the diameter if your seattube. 22.2mm will work, but basically whatever size you can get should work. The Thomson seatpost website has the full list of sizes, so you can just compare that list to a tubing source list. 1"=25.4mm. It’s actually 25.399…, but that level of precision is unneccessary. Tubin is sold by the OD (outside diameter) and wall thickness. For example, 1.25" tubing with .090" wall thickness. The way to calculate ID, which is the important dimension on a seattube, you do 1.25"-2(0.090). That equals 1.07"ID, which you then multiply by 25.4mm. 1.07x25.4=27.178mm. This is just under 27.2. Sadly this won’t work for you, so you should find something else. Remember OD-2xWT(wall thickness)=ID. Then muntiply by 25.4 to get the ID in mm.

  2. Decide on the diameter and wall thickness of the tubing in the crown. If you are using chromoly, don’t go over 1" diameter, and you don’t need very thick walls.

  3. Consider whether or not you want a reinforcing plate under the tubing, a-la KH frames. This really strengthens the crown, and is worth consideration. 1/16" (0.0625") chromoly plate would be plenty.

  4. Measure the angle of the crown pieces relative to the seattube, so that instead of being a 20 degree angle it will be a 110 degree or 70 degree angle. Mark the axis you are measuring that angle from.

  5. Decide on how wide you want the crown to be. You will need at least 1/8" of clearance on each side, and 1/4"-3/8" would be far better. But on the other end of things, a crown wider than about 4.5-5" will be in the way (I don’t know about cokers, but when riding muni and trials wide crowns piss me off). The clearance, crown width, and tubing OD arte all related, and should be considered with the others in mind.

  6. Give the length of each of the branching crown pieces, and define the angle you want them to interface with the fork leg.

  7. Define the length of the fork leg. This should be at least 18" (36"/2), but 19-20" would be better. I would weld up the crown, cut the angles for the interface on the crown, and then taper the legs with an estimated length. Then weld on the bearing holders. After welding on the bearing holders, put the legs on the frame and see how the crown fits on top of it all and decide if you would like to shorten the legs any.

  8. Define how much taper there will be over the course of the leg. Does it taper from 1" round to .5" wide at the bottom? You will need to know this, and should know if the shop you’ll be at has a press before you buy anything. It isn’t easy to taper steel tubing without a good hydraulic press.

  9. Decids how long you want the seattube to be. Cut it 1-2" longer than what you expect to need, and then cut it if it’s too long. You can’t add tubing, you can always remove it.

The rest of the design looks good. Good luck.

Edit: You shouldn’t decide on the the seatpost size until you see a tubing list. Tubing lists don’t compromize, but you can. I had missed that you wanted a 25.4mm seatpost earlier. Also, as someone else already said (Krashin’ Kenny?), welding the bearing holders to the chromoly will be fine. Just remove the chrom, since it burns up and does nasty vaporizing things during welding and can be toxic.

Final edit: I woudl reccomend drawing the plans on butcher paper with a pencil, compass, and steel ruler. Printers can change scaling. You want the plans full size.

Yeah i know, these plans are really just my idea jotted down, very roughly. I will be working with this welder because he is a friend, and so I wont need as detailed instruction as you suggest. But thanks for a great detailed post! that is going to help me out greatly!

I have a tubing list, I am ordering from Aircraft Spruce, an airplane catalog with millions of different parts, for airplane building. And I am going to order a tube with a little more Inner Diameter than 1", so the seat post can have a little slop. Otherwise, I would not be able to get the seatpost in.

What outer diameter should I use for the fork legs and the crown?

That sounds good, but i am afraid that will add to much height to the frame, because it will have to raise the fork higher.

One more question, is it stronger to have a flat tube crown, than a sloping tube crown?

Any more help is appreciated


I’m not an engineer, but I would say 3/4" ovalized tubing to 3/8" (wide laterally, thus makig the tubing longer) would be enough. Try and look at bike tubing and see how thick the walls are for an idea of the thickness you need. 090" is way too thick.

The flat crown with a small reinforcing plate would be lower profile, easier to weld, and strong enough. 1/8" chromoly should be good, and just make it as wide as the tubing. The reinforcing plate can be as thin at it’s thinnest point (vertically) as 1/2 or 1/4". I would go with 1/2" and the flat plate crown, and have the plate get wider near the legs, to 1.5 or 2".

The crown design is very similar to the DM Ringmaster Advanced and the DM ATU. That design works very well for DM’s high end unicycles. DM’s unicycles using that design are very solid.

The downside is that design requires more difficult welding and difficult cuts and shapes (especially around the seat tube). It would be easier to make a unicycle frame similar to the 2004 KH frame, KH Pro frame, or the GB4 frame. That style of frame would be easier to weld and simpler to cut the tubes for and simpler to jig up to keep everything aligned while welding. Simpler means cheaper when it comes to welding and fabrication.

Using the KH/GB4 design it is also easier to use ovalized tubes so that the frame is narrower. If you use round tubes in your design the frame is going to be wider and that can cause the frame to rub the inside of your legs while pedaling. That can/will get very annoying. It’s best to keep the frame narrow as possible to avoid that potential problem. The leg rubbing issue will be more of a problem with shorter riders on a Coker than taller riders.

Hows this? And where can I get ovalized tubing?

flat frame1.jpg

You don’t want too much slop in the neck. If there is too much slop around the seatpost you will never be able to keep the seatpost tight or keep the seatpost from slipping. Ideally the seatpost should slide in with a tight fit (no slop). Even just a little bit of slop can make for a seatpost that will not stay tight. If you do end up using tubing that has some slop around the seatpost you will end up needing to use a heavy duty two bolt BMX seatpost clamp to keep the seatpost tight and keep the seatpost from twisting.

Finding a tube with a 1" ID that will work for the neck of the frame may be difficult. I know George Barnes has had problems finding that perfect tubing size to use for the neck of the frame. If you can’t find that perfect tubing for a 1" seatpost then you could consider a different seatpost size to match the tubing that you can find. That’s why there are so many different bicycle seatpost diameters.

Pay attention to the OD of the neck too. You want an OD that is going to fit a standard seatpost clamp. Seatpost clamps come in many different sizes but if you have an oddball size then your choices are going to get very limited. The two bolt BMX seatpost clamps come in a very limited number of sizes and are designed to fit the standard tubing dimensions used on chromoly BMX bikes.

Re: Coker Frame Help

On Sat, 19 Feb 2005 18:09:08 -0600, “gerblefranklin” wrote:

>1"=25.4mm. It’s actually 25.399…, but that level
>of precision is unneccessary.

1" = 25.4 mm exactly. Not originally so but by way of a later law. See
e.g. <>.

Similar simplifications have been applied to other units. When I went
to school, a litre was not exactly the same as a cubic decimetre.
They just happened to be ALMOST the same but where defined quite
differently. They too have been equated since.

Klaas Bil - Newsgroup Addict

people who unicycle are shyly exhibitionistic - GILD

Re: Coker Frame Help (Klaas Bil) writes:

> 1" = 25.4 mm exactly. Not originally so but by way of a later law. See
> e.g. <>.

That’s part of the picture. points
out that different parts of the world had different standards for the
term “inch”. In 1958, US and Britain agreed on the modern definition

> Similar simplifications have been applied to other units. When I went
> to school, a litre was not exactly the same as a cubic decimetre.
> They just happened to be ALMOST the same but where defined quite
> differently. They too have been equated since.

You are older than I am! The liter has been defined as 1 cubic
decimeter since 1964. This is no coincidence, though, because the
liter as originally conceived in 1793 was the same. In 1901, the unit
was redefined as the volume of one kg water (1.000028L, under
specified conditions), a bad idea that was rectified in 1964.



I have used 1" OD 4130 tubing for the neck. If the wall thickness is 0.058 inch the the ID comes out as 22.45mm, even though this is nominally 0.25mm larger than the 22.2mm seat post you will be amazed how much the neck will need to be crushed to minimise slipping, this can be resolved by a double clamp.

Alternatively if you go up a wall thickness (0.065inch) the ID = 22.1mm & you will never get the seat post to slide down. I can confirm this because my supplier sent me the wrong wall thickness which caused to panic my maths had been wrong…

The first uni I built (29er) was identical to your first design, it was only afterwards that I compared it to DM’s design. I braze welded with gas & think this design would be better if TIG welded.

For my next uni I made a Nimbus/Onza style frame which was a lot easier to fabricate & weld cleanly. Currently I am trying to decide what design to use for a replacement Coker frame.