Introduction to T47
T47 was touted as a solution to a problem that should never have really existed, that of the creaking bottom bracket.
T47 was originally developed by Chris King with the assistance of some others. In short, it is a threaded version of PF30. Pressfit bottom brackets have such a bad name that some clever individual in the wankateering department decided to call it “Threadfit T47”
Creaking bottom brackets are fundamentally the result of bike manufacturers reducing their costs to the absolute minimum and neglecting some geometric locating features during the manufacture of their frames. The finished products rarely have round holes that are aligned with each other. That statement is somewhat caviler as a certain amount of ovality and eccentricity (the engineering word for out of alignment) is permitted but these manufacturers generally fail to meet these basic engineering tolerances which you would find in a typical food blender. As an example an injection molded Lego brick has tolerances that are 5 times tighter than modern bikes.
The consumer has long lived with this because they did not have the ability to check these things or were aware of them. Many bike shops also did not have the tooling or measuring equipment to check for these abnormalities. The cycling media who are effectively an extension of the big bike supplier’s marketing departments also skipped over this troublesome issue.
Fundamentally, any type of modern bottom bracket is a press fit, the bearings always press into the housing (the majority of cranksets) or press onto the crank axle (Campagnolo).
Shimano designed their Hollowtech 2 system to use 6805 bearings, this is a popular size and had a load rating that is ample for most bottom bracket bike usage. This standard was heavily patent protected and to this date the Shimano crankset system is probably the best. It uses a steel axle with sacrificial parts within the bottom bracket to prevent under rotation and corresponding grinding of the crank axle. The current crop of 30mm and 29mm DUB/AXS axles exists to try and circumvent Shimano’s patents.
The usual marketing claim of a larger diameter axle is stiffer neglects the material and bearing spacing. Almost all 30mm and 29mm (SRAM) axles are aluminum. This aluminum interfaces directly onto a hardened bearing seat which if you follow the Hambini YouTube channel will know, leads to grinding of the axle. Aluminum is much softer than steel. Aluminum is also much less stiff. There are only a few grams in it between a 24mm steel axle and a 30mm aluminum axle.
The chosen size of 30mm for aluminum axles is not a sweetspot for any engineering. It is because the next bearing size up from 6805 that will remotely take the load happens to be a 6806. The 06 indicating a 30mm bore.
In threaded applications, BSA has for a long time been the standard for bottom brackets. The geometry of this shell (1.375″ x 24TPI, approx. 35mm diameter) results in a system that is ideal for a 24mm axle – there is around 5.5mm of wall thickness but severely restricts a 30mm axle – only 2.5mm of wall thickness. Hence to allow for enough wiggle room, the T47 standard was born.
T47 allows 6mm of wall thickness which is almost exactly the same as the original 24mm/BSA combination.
However, the engineering on T47 is not without compromise.
T47 Bottom Bracket Geometry
T47 comes in a number of variants
|Designation||Drive Side||Non-Drive Side||Shell Width(mm)||Notes|
|T47-68 (T47 or T47 External)||M47x1.0||M47x1.0||68||This is a very popular standard on custom bike frames that are mainly made of metal.|
|T47-86 (T47i or T47 Internal)||M47x1.0||M47x1.0||86.5|
|T47-85.5 (T47i)||M47x1.0||M47x1.0||85.5||This standard is unique to Trek. In most applications, it is interchangeable with T47-86.|
|T47A-77||M47x1.0||M47x1.0||77.5 (See Note)||This shell is Asymmetrical, The drive side is identical to a T47-68, and the non-drive side is identical to a T47-86. This is used almost exclusively on Felt and Factor bikes|
The biggest concern for most bicycle riders will be weight. A typical T47 bottom bracket will weigh around 180g. The bike will also need aluminium inserts or a sleeve inside the frame to accept T47, the typical weight of the sleeve is about 100g. The sum of those is approaching 300g. In comparison. BB86 weighs around 95g, and PF30 weighs around 130g.
A lot has already been mentioned by other sources regarding the tooling to take the bottom brackets in and out. A particular variant of T47 has a wide 86.5mm shell (mirroring BB86) and correspondingly thin flanges for the tool to grip on to. 2mm is simply insufficient, the risk of it slipping or stripping is significant. Trek, Felt and Factor have indicated they will adopt this variant of T47.
For comparative purposes, a Chris King Threadfit T47 to Shimano is pictured below vs a Hambini T47 to Shimano. The narrow tool flanges are not required to maintain clearance to the crank, they are there to make it easier and cheaper to manufacture.
It may not be completely obvious how much of a problem tooth slippage is until it is experienced. This video from YouTuber, Rides of Japan shows the slippage problems with a Park Tool.
An undisputable advantage of threaded bottom bracket systems is their ease of use and installation, especially for the end user. Providing the individual has the correct drive tool, it is straightforward for them to install and remove a threaded bottom bracket. They do not have to contend with out of round holes because the threaded system will not permit it. The issues around misaligned threads between sides still exist but these can be mitigated with an alignment sleeve (Hambini Bottom Brackets have this feature as standard).
In metal framed bicycles which are often found at lower price points and as custom builds, a T47 bottom bracket is ideal. The 68mm variant lends itself well to the material and construction techniques.
The frictional loss in a T47 Shimano setup is worse than a BSA Shimano setup. The main loss is through the bearings. The T47 uses the 6806 whilst the BSA uses 6805. The difference is small but measurable.
Stiffness is also a characteristic that is desirable. The BSA Shimano Setup is stiffer owing to the smaller size and more accurate absolute manufacturing tolerances. Ultimately, a pressfit solution such as BB86 is superior because some of the load is transmitted directly into the frame.
A number of people have asked for a T47 bottom bracket, mainly for custom builds. At the time of writing, the 68mm version of this bottom bracket was prototyped and released after extensive testing. This bottom bracket has many features carried over from the highly successful BSA Shimano bottom brackets including the tight-fitting center sleeve, tier one bearings and the precision engineering that has become synonymous with the brand.
Watch on YouTube
You can watch the full video on this subject on youtube