It’s pretty important when building a frame that you know where where the butts are.

A frame tube normally has a thicker part at each end. Although the outside of the tube looks the same along it’s length, the internal diameter varies. This essentially allows you to make a tube which is very light (thin in the middle) but still maintains strength (thick at the end).

When you join tubes by welding or brazing, the tubes get to an incredibly high temperature. Since the addition of heat will (in most cases) weaken the material, it’s important to minimize as much as possible, the area affected by the heat. The area that gets this hot is sometimes know as the HAZ (Heat Affected Zone).

If we were to weld two tubes together that had very thin walls (0.5mm for example) it would be harder to minimize the distortion that can occur when welding or brazing. To make things easier, we use tubes with thicker walls at the end. In a typical tube with wall thickness in the middle of say 0.5mm, you can expect the thick part at the end (the butted section) to have a wall thickness of about 0.8mm. Although we’re still talking about pretty thin-wall tubing, this additional material makes for a much better weld.

There’s a pretty good description of how butted tubes are made here: Reynolds Butted Tubing and here’s a wee diagram to help.

That’s not why I started writing this though!

The main point of the post was to illustrate how you determine where those butted sections are.

It’s important that you know where the butts are as when you cut the tubes down (not all bikes are the same size!) you want to make sure you still have a butted section at each end. Sometimes you can just trim one end to get your required tube length complete with butted section, other times you might need to trim some off each end. It’s also pretty useful to know where the butted sections are when your adding braze-ons.

One way is to simply read off the dimensions from the supplier spec sheet. Like the wee picture above. Unfortunately the spec sheet might not always be available and in my opinion it’s always better to double check these things anyway.

Another way is to clean out the inside of the tube with a good degreaser and hold it up to the light (a florescent bulb works well). If you squint and peer through the tube while running your hand down the length of the tube, you can usually tell roughly where the butts are. While this works ok, it’s not very accurate and doesn’t tell you what the actual wall thickness is.

My way is to use a little device that allows you to very accurately determine both the wall thickness and the position of the butt.

Its a rigid, U-shaped device that’s long enough to take at least half the length of most tubing we use to build with. One leg of the U-shape has an arm attached to it and attached to the arm is a standard dial indicator. The moving part of dial indicator rests on a small sphere attached near to the end of the other leg of the U-Shape.

The indicator is set to zero while resting on the sphere.

We then slide the tube we’re measuring onto the leg and let it rest on the small sphere. The indicator then rests on the tube and if we take a reading from the indicator, this gives us our first wall thickness measurement.

When we slide the tube further on the leg of the U-shape we notice where the dial changes and gives us a new reading. This gives us the new wall thickness and the exact position of the butted transitions can be marked on the outside of the tube. The process is repeated for the other end of the tube.

This little device took about an hour to make and should have been part of the ‘Just Making Stuff Up Series’!

Job done

I’m sure there was a “Gary’s Bike (part 4)” post. It had a drawing of the frame with wheels and bars etc. It was sort of minty green. It’s not here no more. Weird.

UPDATE: thanks to the google cache it’s back

Got all the parts for Gary’s bike gathered together.

Ready to be cut mitred and prepped.

Here’s a sneak preview!

In my day job (programmer), we’re pretty ok about diving into something and ‘spiking’ something out to see if an idea’s got wings or if it’s just dumb. It’s easy to do this with something like programming as the cost of change is really low. In other words if you spend a couple of hours working on something only to come to the realization that it’s really stoopid, then you can just roll everything back and start again as if it never happened.

All you’ve lost is that two hours. In fact there’s a pretty strong likelihood that you’ll have learned a significant lesson there somewhere so you can put it down to education.

With manufacturing or construction or pretty much anything that requires investment in materials and anything that can’t be easily ‘rolled back’, this concept of ‘spiking out’ is usually called prototyping and far from being being cheap, is usually very expensive. Usually at this stage, techniques, materials and tooling costs more than the production equivalent. This in turn means that without significant investment and without a good deal of risk, it’s hard to just play around and see what comes out.

I’m kind of rambling now but what I’m getting at is that I’m pretty fastidious at producing quality specs and plans before doing any kind of fabrication work. I’m limiting the risk mostly. The negative impact of this is that as well as limiting risk (and cost), I’m also limiting imagination and in turn, limiting creative design.

So recently I’ve decided to go against convention and just make something up as I go along. The something in question is the incredibly cool and inspiration bottle trolley. Normally my oxy/acetylene tanks live inside a wooden frame that’s fixed to the ground. While this is safe, it makes it real pane to move them around. So if I’m welding or brazing, the work comes to the torch and not the other way round.

While I could throw caution to the wind and go out and spend about £20 on a bottle trolley, I thought I’d build one. In order to limit the risk and negate the ‘cost of change’ penalty, the main criteria was that I need to be able to build said trolley out of crap laying around the workshop.

I’ve made a start on this and work will progress over the next few days when I’ve got a couple of odd minutes to spare (I don’t want to invest any quality time in this). Stay tuned for further fascinating updates

Currently playing in iTunes: Cowboy Song by Thin Lizzy

Somebody asked me if I had a way of visualising a full bike when designing a frame. The answer is yes. The drawing I posted earlier was a stripped down drawing that makes it easy to read dimensions. It’s also possible to produce the drawing with more detail. Even including a view with a rider if needed.

Here’s a pic of the frame with more detail:

THIS FRAME IS NOW SOLD

More stuff from the clear out. This time it’s an 18.5″ 2002 Rocky Mountain Hammer frame. Very good condition, again usual scratches and chips you’d expect. No dents or dings. Braze ons for disk or cantis.

You can see it here. You can see all my ebay items here.

Here’s a piccy of the frame. Feel free to ask me any questions if you need more info.

Currently playing in iTunes: Gone by Kelly Hogan & The Pine Valley

THIS FRAME IS NOW SOLD

I’m doing a clear out so I’m putting some stuff up on ebay. First to go is a 58cm(c-t) 2000 Lemond Tourmalet frame. It’s in good condition, usual scratches and chips you’d expect on a used frame but certainly doesn’t look 7 years old.

You can see it here. I’ve got other stuff that wil be appearing over the next wee while. You can see all my ebay items here.

Here’s a piccy of the frame. Feel free to ask me any questions if you need more info.

Currently playing in iTunes: Gouge Away by Pixies

Checking fork crown clearance.

The CAD tool I use for drawing frames lets you enter dimensions of the fork crown itself, and when combined with the stack height of the lower head-set race, lets you check if the adjusters on your fork are likely to ding the down-tube or not.

You can see here that with the forks Gary’s planning to use, there’s no chance of any dingage.

Currently playing in iTunes: Winter by Teenage Fanclub

The seat-tube

Garys bike’s going to have 35mm seat-tube. That’s pretty big. Most steel frames have 28.6 or 31.8 seat-tubes but this is going to be much bigger. There are a few other steel bikes out there that use seat-tubes this big, possibly the most well known is the WTB Phoenix. Gary has a lovely Phoenix already and it’s pretty clear that this frame is inspired to some extent by that frame.

The seat tube for this frame is actually a down-tube.

We’ll take a Columbus down-tube and cut off one butted end. This’ll give us a 35mm tube, butted at the bottom-bracket area to a wall thickness of 0.7mm. The rest of the tube will have a wall thickness of 0.5mm.

Doing this gives us a couple of problems. The first is that tubing with walls this thin is more likely to deform under heat and given that we’ll be joining the top-tube and the two seat-stays to this part, it’ll get pretty hot.

The other problem we’d get leaving it like this is that we’d struggle to get a seat-post to fit. It’d need to be a 34mm post and that would limit our options.

To get round these problems, we’ll make a sleeve to slide inside the seat-tube and we’ll braze this in place. This’ll give us a much thicker wall to work with which will prevent deformation when doing the rest of the torch work and will bring the inside diameter down to 31.6mm. A much more sensible size for sourcing seat-posts.

The sleeve will either just be a constant diameter shim that will slide in and be brazed or it might have a step on it. Still not decided yet.

Here’s a pic of the stepped sleeve:

Currently playing in iTunes: Naked As We Came by Iron & Wine

I’m about to start a new frame for my friend Gary. He’s asked if I can document it’s progress so I figured I’d write about it here. There’s also a flickr set with the images for the build.

The design of the frame is mostly Gary’s with some input from me as we went along. Not sure quite what the inspiration of the design was but it’s meant as a replacement for his current full suspension bike. A Gary Fisher Cake.

One of the main aims of this bike was to produce a riding position as similar to the Fisher as we could get. I think this’ll come pretty close. It’s a fairly standard sloping top-tube frame designed round a 120mm travel fork. Columbus tubing, Ritchey socket style dropouts, s-bend chain-stays and a huge 35mm seat-tube (more on that later!).

Anyway, I’ve not cut or burnt anything in anger yet so I guess we’re still officially at the design stage. Here’s a dimensioned drawing to get started with.

Currently playing in iTunes: Hard Life by Bonnie Prince Billy