Here are two tubing size questions we get quite often: Can You Use 1.25" Tubing Rather Thank 1.5" Tubing? Can you Use Square Tubing?
For example, here is a question from Shane, one of our subscribers: "I have seen on the features page of the 250 rigid Sporster the frame tubing is 1.5". I would like to purchase frame plans with dimensions using 1.25" tubing as I have already purchased the tubing (1.25"), can you tell me if you supply plans for a rigid Sporster frame which is built using 1.25" tubing.
You can replace the 1.5” tubing size with 1.25” tubing. Just make sure that the wall thickness is adequate so that the frame is very rigid. You may need to tweak the design slightly with the smaller tube diameter but we are only talking about ½” change or so. I recommend going through the plans in detail and marking it up where you’ll see the difference in dimensions.
Keep in mind that the side views show the tubes as 1.5” and when you replace them with 1.25”, you’ll need to make the center slightly wider to make up that gap and maintain the width between the frame for your wheel thickness. Good luck with the build and feel free to let us know if you have any other questions or concerns.
Using smaller diameter tubing size is perfectly acceptable. I recommend using a little bit thicker gauge wall thickness to make sure the frame is strong enough. Steel is pretty durable and some standard DOM tubing should be pretty sturdy unless the design is very extreme.
The size of tubing is an aesthetic detail. Larger diameter tubing size really makes the bike look beefy and sturdy. If the builder is looking to show off something else on the bike like the engine or some unique artwork on the gas tank, or something like that, then a smaller tube will be less visible and may hide behind the “art” to make it more visible. Again aesthetics is important when picking a tube size.
Square tubing is very sturdy and can be used, but there is an inherent issue that you’ll constantly be fighting with. The two sides of the square tube really will make the tube rigid when trying to bend. This will result in the top and bottom of the tube bending and creating a concave region where the bend is. This is difficult to avoid when making bends with square tubing. It potentially can create pockets or depressions where rain could collect if bends are on the down rails near the bottom of the frame.
If you are able to cut and weld instead of bend, you can avoid this issue. Also keep in mind that a flat surface will catch light differently than a round tube. It may seem like a great idea, but once you are done it really will give the bike a completely different look.