Proprietary spokes, internal nipples, frame-specific seatposts….everyone in this industry wants to build that better mousetrap. Yet most of these “innovations” fail to deliver on their marketing promise, and no mechanic is excited to know that he’ll need to source more sku#s (or needs more tools to service them). So I had my doubts for the new T47 bottom bracket “standard”, but at least for the bikes we make at DKCB it has so far proven to be an advantage.
Above is a recent Davidson titanium with a T47 bottom bracket shell, 1-3/8” seat tube, and 1” chain stays. Below is an older Davidson titanium frame with BSA bottom bracket shell, 1-3/8” seat tube, and 7/8” chain stays.
For those unaware, T47 is an open standard that was developed by a handful of US companies, largely as response to issues with the BB30 and PF30 bottom brackets. Specifically, within the industry BB30/PF30 is commonly thought to be prone to creaking, though there is some debate as whether this is due to poor tolerances endemic to mass scale frame manufacturing or if it is inherent to the design. But at DKCB, Bill Davidson developed a method to attain a very high tolerance in PF30 bottom brackets for titanium frames.
Starting with a custom PF30 bottom bracket shell with an undersized internal diameter, he would later bore (rather than ream) to the correct diameter to accept PF30 cups. This not only insured a proper ID and superior roundness, but the bore axis could be precisely placed so as to be perfectly perpendicular to the plane of the frame’s centerline. In other words, the frame alignment is machined in. With such tight tolerances in a metal frame, theoretically a PF30 would have the best chance for giving a long service life without creaking. However, the boring process require very careful (and consequently slow) labour. One doesn’t simply slam a nearly finished titanium frame, which already represents hundreds of dollars of raw materials and hours of skilled welding, onto the jig and let the Bridgeport mill rip. So T47, with it’s relatively shallow threads that don’t require really massive force to chase after post-weld distortion, represents a more practical production method.
From the perspective of choosing cranksets, the T47 is a great choice because its dimensions allow just about anything under the sun to be used (though the availability of suitable bottom bracket units is a different issue). For some 30mm-spindle cranksets like Rotor’s 3D Plus or FSA’s EVO386-type cranks this isn’t that much of an issue, because the length of their spindles allows them to work with PF30/BB30 shells (with spacers on the spindle) as well as external bearing cups that thread into the venerable 68mm BSA (English-threaded) standard shell. But the majority of BB30 crankset designs have narrow width 30mm spindles that require the bearings sit approximately 68mm apart. External bearing cups on a BSA shell put the bearings about 86.5mm apart. The oversized nature of T47 can seat the bearings inside the shell width, so it’s the natural choice if the client specifies a BB30 crankset for their custom titanium frame. But there is a reason aside from crankset choice to make titanium frames with T47 shells.
At DKCB, both the 333fab and Davidson frames use mostly 35mm seat tubes. Additionally, most of the big frames get 1” chain stays since about a year ago when we found a steady supply of suitable titanium tubing stock. That’s a lot of weld line to lay down, particularly on the rear wall of the shell where the seat tube and chain stays crowd together, even if you offset the centerline of the chain stay to below the center of the BB shell. By increasing the diameter of the shell, the weld lines for the chain stays and seat tube are spaced a little further apart, allowing for stronger welds that are less troublesome for the welder to perform. All of that for a shell that in titanium weighs 5-10grams more than what we would usually use for BSA. When you consider that the larger diameter means that a few millimeters less tubing is used for each tube that connects at the bottom bracket, there is no practical weight penalty for the frame.
Currently the only problem for the system as a whole is the availability and price of the bottom bracket units. Chris King Precision Components was a company closely associated with the origins of the T47 standard, but their engineering capacity doesn’t currently allow them to produce units for the market beyond the couple frame manufacturers who partnered in the initial T47 development. We have mainly been using White Industries bottom brackets, which is natural given that I already so often source their hubsets for the custom wheels that accompany the DKCB framesets. Recently White Industries were temporarily sold out, so I had to order bottom brackets from Enduro/ABI so as not to hold up custom bike deliveries. Thankfully the Enduro bottom brackets are pretty nice pieces rivaling White Industries, but all the T47 choices are in the $100 plus range, whereas a Shimano bottom bracket is $30-45 and a SRAM PF30 about the same. On a $7000-10000 custom titanium bicycle, this doesn’t mean much though.
All of these reasons are most relevant to the titanium frame manufacturing at DKCB. As a small frame studio, we can adapt much quicker than a large manufacturer. Once we could get the titanium T47 shells and taps from our suppliers, Bill simply machined new fixtures for the mills and welding jig. It is my feeling that aluminium frames could really benefit from T47 too, but I believe that for premium carbon fibre frames that PF30 should offer a lighter, better setup. Yet since the vast majority of carbon frame manufacturing just isn’t going to be able to achieve the necessary tolerances, perhaps T47 might offer advantages there too.…
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