T47 Bottom Bracket: Engineering Guide (Threadfit)

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Assembled T47-68

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”

Hambini T47 Bottom Brackets can be found here

T47Drawing
Original Drawing of 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

Threadfit T47-68

  • 68mm wide shell
  • M47x1.0mm LH on the drive side
  • M47x1.0mm RH on the non-drive side

Threadfit T47-86

  • 86.5mm wide Shell
  • M47x1.0mm LH on the Drive Side
  • M47x1.0mm RH on the Non Drive Side

Disadvantages

Weight

The biggest concern for most bicycle riders will be the weight. A typical T47 bottom bracket will weigh around 180g. The bike will also need aluminum 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, PF30 weighs around 130g.

Tooling

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.

T47 schematic drawing
T47 showing the 68mm and 86.5mm variants with the difference in tooling drive flanges

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 complete 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.

Advantages

An undisputable advantage for 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 exists 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.

Performance

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.

Hambini T47

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.

16 thoughts on “T47 Bottom Bracket: Engineering Guide (Threadfit)

  1. Do you have T47-68 for SRAM DUB (ya… I know, but…)?

    1. I can make you one to order.

  2. Can it be used on an asymmetric T47 frame like the Factor Ostro?

    1. That bike needs a special version of T47. It doesn’t comply with the standards laid out.

      1. Ok, thanks. Is there any technical/functional advantage going for an asymmetric T47 design over the symmetric one?

  3. Chris King (which usually has heavier BBs) T47 30x and T47 24x are 120g and 134g, respectively, so they’re definitely heavy but not the 180g as stated above:
    https://r2-bike.com/CHRIS-KING-Bottom-Bracket-ThreadFit-T47-30X
    https://r2-bike.com/CHRIS-KING-Bottom-Bracket-ThreadFit-T47-24X

    Also Trek has decided to go with 85.5mm wide shells specifically to combat the issue of tool engagement:
    https://cyclingtips.com/2019/06/trek-t47-threaded-bottom-brackets/

    1. On the Trek, The extra 0.5mm is not going to do much for tool engagement. The video shown was on a Trek bike. The engagement is too small.

      Regarding Chris King. The centre section of that is plastic, not metal and the outer has it’s teeth engagement section reduced to a minimum. You can either have the extra material or a stripped BB.

  4. Hambini, would you explain, how the bigger BB leads to a reduction in stiffness? I remember, you were mentioning the Look BB65 being exceptionally stuff due to its large bearing arrangement.

    1. The bearings are the same width. In this case 7mm as a 6805. So the ball diameter is the same. The difference is the distance from the centre of the axis so you end up with a longer distance and thus a longer lever arm. If you apply the same amount of lateral force, the defelction is higher because the torque is higher. Hence it’s less stiff.

  5. Have you considered developing a crankset, given the less than ideal solutions on the market? Yes, Shimano axle / bearing standard is good, but Hollowtech delamination, outright failure and DS PM problems rather less so. Also, potentially, for the unfortunately increasingly popular T47, you could perhaps engineer a way to further ameliorate the inherent increased drag as described in the article, in a T47 version. Maybe moulded carbon arm with 110BCD 4S compatible fitment so that third party spider PMs could be fitted, or collaborate with Sigeyi or Xcadey to offer one as a ‘factory’ solution?

    1. The current Rotor crankset is very good, It doesn’t have hollow arms so that business about delamination has gone. Shimano are by no means infallible but I’d say their engineering was better than the vast majority out there.

      1. Would you say the Rotor crankset is the best you are aware of currently? All Rotor cranksets or a specific one?

        1. The one that was reviewed was very good. If the others in the range are if the same design then I would have no hesitation in getting one.

  6. How important is ovality in a T47 shell? I just received a top-of-the-line custom titanium bike frame, and the bottom bracket minor diameter seems to vary between 45.95mm and 46.61mm. (Official spec is 45.94+0.15mm.) Is this something that would cause issues?

    1. you may find it binds or winds in eccentrically.

      1. Thanks – do you have any plans to make a 30mm T47-86.5mm BB?

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