The aerodynamic performance of wheels often grabs the headlines and the marketing budget but the reality is the hubs and the bearings within them will have more of a performance differential for the average cyclist. As an example, the power differential between wheels of an equivalent depth at speeds of less than 35km/h will only be 1-2W, the difference in bearing friction can easily exceed that.
Wheel hubs and freehubs account for around 60% of the rotational friction in a bicycle drivetrain. The remaining friction is from the pedal bearings, bottom bracket and pulley wheels. The total amount of friction is small but a cost effective marginal gain.
The term rotational drag has been used of late to describe the amount of power required to spin a wheel up to speed. This loss is a significant aerodynamic loss of about 25 to 30 percent. Rotational drag is not the same as frictional drag which comes from the mechanical components.
Wheel Hub Design methodology
There are a several engineering considerations to be made when designing hubs which have been discussed below
- The most obvious is the choice of bearing type. The vast majority of hubs use 6000 series ball bearings which are deep groove. A notable exception are Shimano and some Campagnolo/Fulcrum wheels which use cup and cone bearings. A cup and cone bearing is a type of angular contact bearing. A cup and cone bearing will always have more friction than a radial bearing because of an axially oriented contact angle. A loose cup and cone bearing allows for fairly slack manufacturing tolerances.
- The bearing material (ceramic or steel) is often credited with lowering the friction in a hubset. The reality is the seal and choice of grease will have more effect.
- The axle size determines the friction torque from the bearings. Axle sizes of between 10mm and 17mm are common on bicycle hubs. The smaller axles will always have lower frictional torque because the moment lever is shorter, the negative effects are the wheel hub will have more flex and the bearings will not last as long.
- Machining accuracy, a lot of hubs are manufactured in Taiwan by forging aluminium 7075 alloy and then machining bearing landings afterwards. This is a standard process but it can be riddled with quality control issues. Having bearings that are misaligned due to slack jigs and holes that are too tight due to worn cutting tools will severely hamper friction levels.
The chart below shows the friction loss through hubs which were loaded. These tests results have some limitations
- They do not take into account Road Vibration
- A sample size of one for each hub set does not take into account any natural variation
- Some of the hubs had completed an unknown mileage, they were checked for vibration to ascertain any bearing fault frequencies.
- The power figure has some factors for calculation and it’s advisable to use this as a relative reference rather than an absolute figure
Upgrading Wheel Bearings
It is not cost effective to replace OE bearings straight from the factory unless the rider requires the ultimate in marginal gains. It is more advisable to wait until the bearings have worn out first and then retrofit.
Generally speaking, the Hambini recommended hubset to use would be the Carbon-TI. These have low levels of friction, are well made and use decent quality bearings from the factory. The Miche Primato hubs are a good budget option but the small axle diameter is not favourable in the longer term despite having the overall lowest friction, it should be noted that the test example had retrofitted bearings so the exact figure from the factory may be higher or lower. The angular contact Shimano hubs do quite badly in this test because the contact angle lends them to high levels of axial load, these wheels will be extremely stiff and predictable when cornering but the trade off is the higher linear friction.
In the longer term, bearings will wear out and for this reason, Shimano hubs are not recommended unless the cyclist is willing and able to carry out regular maintenance. The bearing surfaces are costly to replace and are a proprietary component.