While we were on the topic of What 4130 Chromoly and Heat-Treating Is, we got thinking about a handful of the terms that you might see in the descriptions of BMX frames and parts like forks, bars and cranks that might be leaving some of you young guns not quite sure what those small details really mean. We decided to compile a handful of these terms and explain what it means like butting, tapering, machining, Investment Casting and Swaging. Let’s get into this!
Butted is a term that we see very frequently in BMX products. A butted tube means that the outside of the tube is a constant diameter and the wall thickness varies. For example on BMX frames, top and down tubes are frequently double butted. Double butted means that the thickness of the tubing on the ends is thicker than in the center. So there are two “butted” sides. The reason for doing this is because the ends of the tubes are joined with other tubing and welded. The thicker tubing helps increase the strength and allows for a better weld in high stress points. The tubing can be thinner in the middle because it takes less stress than the ends and typically you will find most breaks or cracks on frames and parts on the ends versus the middle. This helps to reduce weight in low stress points and bulk up where there needs to be more material. A single butted tube would mean that it is only butted on one end and a constant throughout the rest of the tube. A lot of seat-tubes are single butted for example. Some handle bars are 9-butted or even 11-butted because of all of the bends that require extra material. This extra butting puts material only where it’s necessary and reduces material in low stress points similar to tubing for a frame.
Straight Guage tubing is similar to Butted tubing in a sense that it is just a tube that has a constant inside and outside diameter throughout the tube. This means that it is as thick on the ends of the tube as it is in the middle. Straight Gauge tubing is stronger because of its thicker tubing, but typically a double butted tube is just as strong on a BMX frame as a straight gauge tube because the most stress is put on the ends where the extra material is necessary. Straight Gauge tubing is however stiffer which helps reduce flex and is more resistant to denting.
Tapered tubing is another common term we find on a lot of BMX frames and forks these days. Tapered tubing is when a tubes overall diameter decreases (or increases) from one end of the tube to the other. A lot of chain and seat stays utilize tapered tubing to give extra clearance for tires and give a cleaner look. A tighter overall diameter can also make tubing stronger which is important in high stress points like the stays that take the majority of impacts from landing.
Ovalized tubing is a technique that we have seen make appearances on BMX frames quite frequently over the years. Ovalized tubing is traditionally utilized on down tubes and chain stays which are more likely to take an impact from grinding or landing on something and helps to resist denting because the surface area is more likely to deflect to a side than to take a direct hit from it like wider round tubing can take. A dent in the down tube can structurally damage tubing to the point of cracking and / or breaking. Since most frames use butted tubing, the middle of the down tube is easier to dent than the thicker ends. This is a reason as to why straight gauge tubing is sometimes utilized on tubing as well.
Sunday Bikes also utilize their “Wave” tubing which also helps resist denting. This tubing essentially takes a round tube and adds multiple ovalized lines down the tube which is stronger than a round tube. Although it’s still possible to dent the wave tubing, in most cases the dent only applies to one or two of the waves versus a traditional tube that would have the dent spread across the whole tube. Back in 2009, Sunday explained exactly how the wave tubing works. Check that out right here.
The gusset is a staple in BMX frames and is very commonly found on the top and down tubes of most BMX frames. Gussets are essentially extra material that is added to high stress junctions to increase the overall strength to resist cracking and breaking. Gussets traditionally are welded on connecting the head tube to the top and down tubes making for a strong joint. Some frames also utilize Internal Integrated gussets which are similar to butted tubing except they have thicker tubing similar to what an additional external gusset would add to the junction between the top or down tube and the head tube. If you look at the graphic above tapered tubing showing the different features of the Flybikes Fuego frame, you can see they utilize Internal Integrated gussets and an offset thickness head tube giving plenty of material to create a strong junction. It’s rare to find an aftermarket BMX frame that doesn’t have a gusset of some sort on the top or down tubes.
Flybikes put out these graphics showing you how their internal integrated gussets and multi-butted and tapered tubing looks cut in half.
Investment Casting has quickly become a common addition to most BMX brands frames and forks. Investment Casting essentially takes a cast molded piece and joins it with other tubing through a weld. The reason why Investment Casting is so beneficial is because it allows you to more the weld to a point where there is less stress than at the junction where tubing traditionally meets which creates a stronger product. Another positive to Investment Cast tubing is that it allows brands to be more creative with their tubing since it’s molded versus CNC machined. Above is a look at Wethepeople’s Scorpio fork where you can clearly see where the weld has been moved to and below is an example of casted drop outs from Division Brand. Most products utilizing Investment Casting are able to completely hide their welds giving it a smooth and flush look from top to bottom which is a great aesthetic touch. We have found Investment Casting all over frames and forks from drop outs to seat and chain stay bridges, seat post clamps and head tube junctions. Wethepeople actually released Andrew Jackson’s C.R.E.A.M frame with Investment Casting in almost every place possible which was interesting, but not ideal.
Internally and Externally Machined
Internally and Externally Machining is a process that is done through CNC which allows tubing shave off additional material where it is unnecessary to help reduce weight and can give tubing a butted look externally. They also use Internal and External machining post heat-treatment to ensure that tubing like the Integrated Head Tube or Bottom Bracket has not been warped from the process making sure that bearings fit properly.
Internally and Externally Relieved
Internally and Externally Relieved is another term that is used in reference to tubing. This is the same as Internal and External Machining which helps to reduce excess material to help save weight.
Extruded tubing is a term we don’t see as frequently, but it’s still a process that is utilized on some BMX products. Extruding is a process that takes a block of material and presses it through a die to give the material a specific shape. This process can be done hot or cold and essentially forces the material to take the shape of the die through pressure. We found a video on Youtube that can give you a better idea of the extrusion process. We find the extruded process used on parts like stems, seat posts and pedals.
Not to be confused with “swag” or swagger… Swaging is a process that is similar to Extrusion where a die is used to form the shape of the tubing. Swaging expands tubing to a wider size through pressure that can give it a tapered look without the machining. We find Swaging commonly on tubular chromoly BMX cranks because they’re often a tapered shape that are wider where the arm meets the spindle boss and smaller where the arm meets the pedal boss.
Alright, so that just about wraps up different terminology used on BMX tubing. Let us know if we missed anything in the comments below or if you have any questions!
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