I take a close look at the T47 standard including a variety of models brands can choose from.
2021 is the year the cycling world gets serious about the T47 bottom bracket standard.
Specialized have released their S-Works Tarmac SL7 2021 ; Trek, their Emonda SLR and SL. Trek will also use T47 more widely across their road range in the future.
Introduced as ‘yet another standard’ in 2015, T47 will likely become an increasingly important bottom bracket component in the years ahead.
WHAT IS A T47 BOTTOM BRACKET?
It’s a PF30 BB shell with threads.
A PF30 bottom bracket shell’s internal diameter is 46mm. Cutting threads into it gives you an official specification of M47 x 1.0mm.
The drive-side cup is a left-hand thread; the non-drive side is a right-hand thread—an English-threaded setup in other words.
Threaded Bottom Brackets are Rock-Solid
I argued elsewhere that threaded bottom brackets (mainly BSA/English at 34.8mm diameter BB shell) are the best bottom brackets.
The extended helical ridge that defines the thread of a screw or bolt is one of the most important advances in the history of mechanics.
A threaded screw or bolt stays where it is put until you remove it or where extreme stress strips away the ridges.
Stripping thread is extremely unlikely when you are installing a nut onto a bolt, but more likely when faced with a bottom bracket’s broader surfaces (shell and BB unit) in the absence of a little extra care.
T47’s taking of the largest press fit bottom bracket and turning it into a threaded model may turn out to be a defining moment in bottom bracket design.
Nightmare of Proliferating Standards
Press fit bottom bracket design copies integrated headset design: bearings inserted directly into the frame.
The original BB30 was a direct translation of this concept from the head tube into the bottom bracket shell.
When problems with BB30 emerged, a solution was to place the bearings in cups and press these into the shell—just as cups were pressed into head tubes prior to the integrated headset revolution.
Thus we’ve ended up with 41, 42, and 46mm bottom bracket shells, an evolution which has taken us from a 34.8mm ID BB shell to a 46mm shell.
If anyone had set out to design a new standard prior to this evolution taking place, it’s doubtful that 46mm would have been the size selected. The ideal diameter would have had to be larger—but not this large
Boutique, custom frame builders, notably those using titanium and steel (see the extended section on installation below on a steel frame) are the chief users of T47 BBs
Although there has been limited adoption of T47 aluminum shells into carbon frames to date, bonding alloy shells into carbon frames is a mature process, so expect to see more in the near future.
CAN YOU CUT THREADS into a 46mm SHELLto CREATE a T47 BB?
Much T47 commentary claims you can “simply” or “easily” cut threads into a PF30 shell into which you can then install a T47 BB.
Firstly, carrying out the procedure will cancel out your warranty in most cases.
Secondly, altering the BB shell in this way is a significant alteration of the frame. Few if any manufacturers would continue to honor warranty under those conditions.
This suggestion is usually delivered in the context of fixing ill-fitting press fit bottom brackets and the oft-reported creaking problem.
Commentators lay the blame on sloppy manufacturing tolerances—if manufacturers would produce bottom brackets with the standardized precision the bring to headset manufacturing, there would be no problem.
However, you can’t equate headsets in head tubes to bottom brackets in bottom bracket shells. An integrated headset’s sealed bearing drops into place on top and snugs into position at the fork crown—hardly precise.
In contrast, a press fit bottom bracket by definition must be “pressed” into the shell. Tolerances are tight, although not tight enough for the critics.
The force exerted on the bottom bracket/frame interface is extreme, and must endure stresses that exceed the strain exerted on headsets many times over.
Threaded bottom brackets were a non-problem solved with the introduction of press fit. T47 improves on press fit …. by eliminating press fit.
The return of threaded BBs via T47 improves on the BSA standard by giving us larger bearings and a beefier connection amongst the shell and the chain stays, seat tube, and down tube.
The larger surface area at this junction also cancels out a possible weight gain as it allows tubing with thinner walls, opening up a new frontier of frame design possibilities.
T47 BOTTOM BRACKET SURVEY
In this section I cover different types of T47 Bottom Brackets.
FIRST make designs for both traditional spindles—24mm—and 30mm spindle cranks, for either internal or external installation.
Each model comes with variously sized spacers in order to accommodate a wider range of cranks.
The 24mm crank range pictured is divided into the BBs with aluminum sleeves at the top, and plastic sleeves below.
Plastic sleeves are for drilling enabling internal cables to be more easily routed through the BB shell.
The 30mm units are for Shimano cranks at the top and SRAM cranks below. Each range includes the option for a plastic sleeve.
Internal vs External (Outboard) Installation
Each T47 bottom bracket bearings are either installed into the the bottom bracket shell or located externally as “outboard” bearings (the unit on the right).
Which is better comes down to personal preference.
The advent of integrated cranks, 24mm spindles, and bearings mounted in externally located cups allowed the use of larger bearings in the BB and thus greater bearing longevity. The spindle was also stiffer due to the hollow design’s larger diameter.
The only upgrade to make for a 34.8 (“BSA”) BB shell was from a solid-axle cartridge to an integrated spindle with external bearings; it gives you a noticeable performance boost.
But the 46mm T47 internal diameter gives you an extra 11.2mm of diameter and an externally mounted bearing already larger than the external bearing.
A cartridge BB sealed bearing’s outer diameter is 30mm; an outboard bearing’s OD is 40mm; a T47 internal BB sealed bearing’s OD is 42mm giving a 2mm advantage over the traditional outboard bearing.
Thus, although there’s no clear advantage to installing the external unit compared with the internal unit, you have the option nonetheless.
The aluminum sleeve on these internal cups screws into the drive side.
If weight is an issue, you could do away with the sleeve, although losing the protection against the entry of water and grit is not a good trade off for the minimal weight saving.
T47 BOTTOM BRACKET INSTALLATION
The great news is you no longer need one of the often complicated press-fit toolkits to seat the bearing cups in the BB shell.
Beware Cross Threading
Provided your frame maker’s bottom bracket shell is precisely machined to spec, you screw the T47 BB cups into the shell, then tighten with the correctly fitting wrench.
The danger with threaded bottom brackets is cross-threading the cup into the shell. You need to seat the bearing cup exactly into the shell’s threads before screwing the cup very far into the shell.
Even if a threaded bottom bracket cup appears to be perfectly aligned with the BB shell and turns with just a little resistance, it could still be cross-threaded.
T47 bottom brackets, much larger than their threaded cartridge cousins and noticeably shift onto an angle, out of square to the BB shell.
One technique that works well to seat the BB cup in the BB shell making sure the threads are aligned on the drive side is to turn the cup to the left, first of all, then to the right, against the thread.
You will feel, as well as hear, a ‘click’ or a ‘clunk’ as the cup threads align with the BB shell threads.
It often takes several turns to the left and right before you feel the alignment; then you proceed to turn the cup smoothly to the left all the way into the shell.
With the non-drive side turn the cup to the right first, then the left.
Once you feel that the alignment is right, complete the installation.
These are the wrenches FIRST supplies with T47 bottom brackets, and any of the non-standard designs in our range for that matter.
What About Torque?
For the internal T47 BB we use the tool on the far right; the tool (Y30) to the left fits the external BB cups which I’ll look at below.
The flange offers 2mm of surface which means you must hold the tool square to it to gain maximum leverage.
The recommended torque is standard for threaded bottom brackets: a minimum of 35 Nm, maximum of 45 Nm.
How do you know you’ve reached tolerance? A person of average strength exerting full force will reach a torque of 35+ Nm.
You will never over-tighten with this tool. Using a glove or soft cloth on the handle and a good pull, the cup will be tight enough.
An example of the plastic sleeve available in place of the aluminum sleeve. Plastic is lighter if weight really is a concern.
Plastic enable drilling a precise hole to allow cable routing to the derailleurs and brakes.
24mm External T47 BB
The external or outboard bearing version has two cups and an alloy sleeve.
The sleeve screws into the drive side. Or you could opt to just install the the cups.
Unlike the internal T47 BB’s 1mm for the tool to slot into, a 5mm surface is available on the external T47 cup.
With a 30mm spindle fitted.
T47 24mm SPINDLE
The examples so far are relevant to T47 bottom brackets taking 30mm spindles.
The same design features apply to the range of 24mm, with a few differences.
The option exists to add 24mm adapters to one 30mm spindle unit. Each adapter has a 30mm outer diameter, and 24mm inner diameter.
Applying even pressure on alternate sides of an adapter maneuvers the piece into the 30mm ID cup.
The fit is tight, but not too tight. To remove you need a vice to hold the cup, then lightly tap the inside face from the opposite side evenly the way round.
An alloy sleeve either screws into, or is permanently attached to the drive-side cup.
An important part of the sleeve-to-cup connection on the non-drive side is the rubber seal which fits into the end-groove. Without the seal, the connection is too loose and would rattle in use.
The seals are durable, but can be easily stretched into position should one perish.
T47 ECCENTRIC BOTTOM BRACKET
An eccentric bottom bracket enables you to raise and lower the spindle height, and thus the crank and pedal height.
Raising and lowering in this way also changes the chain tension. So an EBB can be used for either purpose.
T47 bottom brackets will pose a serious challenge to press fit BB dominance. T47’s large size makes it perfect for a wide range of niche bikes such as tandems.
This design mimics the popular BSA EBB.
An off-center core housing the sealed bearings rotates within the main, hexagonal shaped, shell.
Rotated almost to the top.
The technique is to rotate both sides together as one so both ends of the spindle remain parallel. If not, one side gets ahead of the other: neither will turn.
Rotated almost to the bottom.
2mm Allen bolts, equidistant on three of the hexagon’s six vertices, prevent the inner housing from rotating.
The large holes on the hexagon’s alternate vertices are for the tool used to tighten the cups into the BB shell—the curved end of the tool hooks into them providing leverage against the flatened edges.
In 2015 when T47 Bottom Brackets were first launched as ‘yet another standard’, plenty of people dismissed the return of threaded bottom brackets in this form.
Five years later with a cycling public fed up with the shortcomings of press fit BBs, the new standard is set to take off, simplifying and rationalizing bottom bracket design, making life easier for cyclists, brands, and manufacturers.