30mm cranks into BB86 bike frames - Avoid if you can...

A common question I am often emailed about is asking if I can supply a bottom bracket that will allow a 30mm crank to be fitted into a BB86 bike frame. This post will talk through the many problems associated with this.

BB86 uses a bottom bracket with a diameter of 41mm and a width of 86.5mm (Drawing shown below). In practice these are nominal dimensions and have a tolerance on them that makes them slightly smaller. BB86 is a Shimano standard and is used by lots of frame manufacturers (BMC, Canyon, Look etc). BB86 is effectively the pressfit version of an ISO threaded bottom bracket.

BB86 Shimano OEM Shell

The size of Shimano and it's market share allows them to dictate standards and as such to an external engineer looking in, it appears that Shimano designed BB86 to try and exclude (or make it very difficult) to run a 30mm crank. Shimano uses a 24mm crank as standard which is a combination of 6805N bearings and nylon inserts to make up the difference in the bore.

A lot of people have 30mm cranks and a large number of those individuals have power meters attached to them and understandably don't want to purchase a new power meter, they would rather find a solution to allow fitting in a BB86 shell. As a result a number of companies have produced bottom bracket solutions. I use this term loosely because there are some severe performance limitations that they omit from their product pages.

There are two popular methods of BB86 30mm bottom bracket design. I will cover them both

BB86 30mm Drawing

From left to right. A 6805, 6706 and 6806 bearing. Note how slender the 6706 bearing is in comparison to the others

6805 6706 6806 Comparison 6805 6706 6806 Comparison

Method 1 - Modified 6806 bearing

The original BB30 specification and 30mm cranks as a whole, are designed to run inside 6806 bearings. Irrespective of BB30, PF30, BBright or BB386EVO etc - there may be a cup between the bearing and the frame but all of these standards use 6806 as a design basis. Unfortunately this bearing is fractionally too large to fit into a BB86 shell. A BB86 shell is 41mm and a 6806 bearing is 42mm on the outside diameter. Hence a rather crude solution is to machine or grind 1mm from the outer race of the bearing.

This might sound like an easy and perfect solution but it has some implications for performance. Grinding down the outside of a 6806 bearing by 1mm reduces the strength of the outer race and it is much more likely to deform. Any rider that is putting a large amount of torque through the cranks - typically going up hill, will be deforming the bearing outer race significantly.

At a torque of 50Nm (~500W at 100RPM) the deformation is over 9 times worse than an unmodified 6806 bearing.

Correspondingly the bearing life is also reduced.

Method 2 - Use 6706 bearings or a Double row bearing

BB86 30mm Drawing

6706 bearings have an outside diameter of 37mm so they will comfortably fit inside a BB86 bottom bracket shell. They are however quite difficult to obtain and of the tier 1 manufacturers, only SKF have them readily available. Several bottom bracket suppliers use these bearings inside machined cups.

These bearings have an EXTREMELY low load rating, it is about 25% of the load of a 6806 bearing which is the next size up (used in BB30). Realistically, a rider would expect to achieve only 25% of the life or could only put through 25% of the peak torque before the bearing would fail.

A minor modification to this method is by using a double row ball bearing. This increases the life marginally but it is still far short of either a 6805 or 6806 bearing.

Performance

In every criteria of performance, running a 30mm crank in a BB86 shell is suboptimal. Friction, bearing life, stiffness and load rating are all significantly worse than running a 24mm crank or using a bike frame with a larger diameter bottom bracket

The graph below shows the relative friction against distance travelled for each type of bearing. Having had a discussion with SKF technical support, their conclusion was the 6706 and ground down 6806 were in an overloaded state due to the sinusoidal loading. Their firm recommendation was to avoid using these bearings in this application.

BB86 30mm Friction Graph

The Life chart speaks for itself

BB86 30mm Life Chart

Load rating can be obtained from bearing handbooks. They are an effective way of predicting bearing life using an L10 calculation. The slender 6706 and ground down 6806 bearings have a substantially reduced load capacity in comparison to the 6805 and 6806 bearings.

BB86 30mm Load Rating

An often overlooked parameter is bearing stiffness. When using 6806 and 6805 bearings, this is not usually a problem because the bearings are robust and stiffness can be ignored until very large torque is encountered at which point they flex and deform. In a 30mm to BB86 application this deformation limit occurs much earlier and even a modest rider will be able to feel spongeyness at power levels of ~250W.

BB86 30mm Stiffness

Conclusion

Using a 30mm crankset in a BB86 should be avoided. The likelihood of a failure is very high. I receive countless emails every week about people having problems with their 30mm cranksets in BB86 bike frames. My usual recommendation is to try a different crankset with a smaller diameter axle.

There will inevitably be anomalies in any data analysis and plenty of people will be able to use their 30mm crankset without having seen any problems however the engineering analysis clearly demonstrates this practice should be avoided.