Triaxial tension in a lugged steel bicycle frame
It’s an unexpected marvel of nature: the weakest chain in the link can hold its own with the stronger links. But this blog isn’t about chains; it’s about a lugged steel bicycle frame. In an earlier blog, I described the difference between a lugged frame and a TIG welded frame.
When lugs are used as sleeves to join the tubes of a bicycle, everything is joined with a metal filler, usually brass or silver. When I built frames, I used a silver alloy metal. Silver will flow into the very small gap (from 1 to 5 thousandths of an inch) between the lug and the tube and, like glue, hold everything together.
One way of measuring the strength of a material is its tensile strength. Very loosely speaking, tensile strength refers to how much force can be applied to a material before it starts to deform and “break”. All you need to know for this blog is that higher numbers mean stronger materials. An investment cast lug and steel tubing have a tensile strength of about 120,000 psi (pounds per square inch) and the silver alloy (the “glue”) has a tensile strength of about 70,000 psi.
When the tensile strength of steel joints properly brazed with silver were tested, the joint was able to withstand a stress of 120,000 psi … even though the silver “glue” holding the joint together had a tensile strength of only 70,000 psi. How is this possible?
The science behind this still isn’t completely understood, but it’s been hypothesized that the brazing filler material is so constrained by the lug and tubing around it that it can’t “slip” along the tiny, tiny little planes in its atomic structure.
Another theory has to do with the area of the filler material. Think about pulling on either end of a metal rod. As you pull with more force, the rod stretches before it finally breaks. If you measured the area of the rod before and after stretching, you’ll find it becomes smaller. Perhaps a drawing will help:
Because the brazing material is constrained by the lug and tube around it and has nowhere to go, its area can’t change. Materials engineers refer to it as being in a state of “triaxial tension”. Simply put, the effect of this state is to make the brazing material stronger.
This is one of the many things that makes engineering and bicycles so fascinating.
1 Handy and Harman Brazing Technical Bulletin No. T-3, “Strength of Silver Brazed Alloy Joints”, pp. 1- 2.
Steel Bike Weighing 9.6 lbs More Than Titanium BIke Climbs Just As Well!
Did I get your attention with that headline? It was inspired by an article I read in the most recent issue of Bicycle Quarterly, a fantastic, niche magazine published by Jan Heine.
In this issue, Jan tested a titanium bike against a steel randonneur bike. It was a real world test: two guys racing each other up the same hill, one on the ti bike, one on the steel. They swapped out the bikes several times. Both were evenly matched in terms of strength, endurance and weight.
The weight of a steel bike is always of interest. This steel bike was 9.6 lbs heavier than the ti bike, but it climbed as well. It sounds implausible, but as Jan explained, when the weight of the riders was taken into account, the steel bike plus rider was only 5% heavier than the ti bike plus rider, but the steel bike “planed”, helping the rider generate the extra power needed to overcome the weight difference.
Fans of Jan’s bike testing will know there is an advantage to a bike that “planes”. This is a bike that is in synch with the rider and flexes in a way that “gives back” some of the rider’s energy to the drivetrain.
Just for fun, I backed through Jan’s math to calculate that he weighs about 175 lbs. I used my own 100 lb. weight in his calculations and found that the difference in weight plus rider for me is about 8%. Ah, we smaller riders have a rougher road to ride, do we not? A bike that planes is a must!
The important point is that weight is not a big deal. Choice of tires and construction of the frame is though.
Take the road less traveled — not only on your bike, but in your reading as well.
Lugged Frame or TIG-ed frame?
By default, the bicycles shown on my site are TIG welded. Even though you may see this kind of construction almost exclusively these days on both steel and aluminum frames, you may not have know what it’s called. Here’s a photo of a Coto Doñana Vagabond showing a TIG joint — seat tube, top tube and seat stays.
But another way of “putting the tubes together” is with lugs. Lugs are sleeves that hold the tubes in place. Brazing rod is melted into the sleeve and becomes a very strong “glue” that holds everything together.
Is one method better than the other? It just depends on your preferences. Aesthetically, a lugged frame looks classy and elegant. With some creative filing and cut outs, lugs can become works of art. If you’re thinking about purchasing one of my bikes and want lugs, I’ll be glad to oblige. They are well worth the extra cost.
Structurally, either method is fine. Typically, a TIG-ed frame will weigh a little less because it has no lugs. In theory, the “butts” (more wall thickness at the end than the middle of the tube) can be a little bit shorter since heat from TIG welding is more localized. With a brazed, lugged frame, the heat spreads over a greater length of the tubing, so the butts may be longer. Longer butts add weight.
Lugs typically come in set angles, so this may an issue if the frame design calls for something unusual. Since TIG-ed frames don’t use lugs, any frame angles will work.
In an upcoming post, I’ll talk about the “miracle” of lugs, tubing and brazing rod.