INTRODUCTION:
There are a variety of places where you can find composite bushings in a bicycle, including the headset, pedals, brake lever, front and back derailleurs, and the rear suspension, just to name a few. In this article, we are going to look at the bushings used in rear hubs.
Many people wrongly assume that a polymer rear hub bushing, instead of a metal one, is a cheap substitution made by hub manufacturers. Nothing could be further from the truth! Lets talk about how to select a proper material for a rear hub polymer bushing that will far outshine its metallic counterparts.
Here are some other great articles on polymer bushiings from the Advanced EMC Technologies Blog:
- Fluorolon Polymer Materials for Off-Road Bikes: Carbon Fiber Reinforced PPS
- Four Reasons Molded Plastic is Replacing Powdered Metal in Bushings and Thrust Washers
- Polymer Bushings Taking a Beating: Two Important Concepts behind the Wear and Tear
Friction Considerations
The appropriate polymer selection for a rear hub will involve a material that is self-lubricating. This makes the bushing maintenance free because no grease or other lubricant is required. In addition, self-lubricating polymers have an extremely low coefficient of friction and almost no issues with stick-slip.
Strength
A rear hub needs to be impact and shock resistant as it stands up to the pedaling force of the rider. It must stand up to that load without deflection, too, requiring a stiff material. In fact, many polymer bushings have a higher load capacity than metal bushings. Polymers can also exhibit superior wear properties, and are not likely to score the mating surface.
Performance to an Aggressive Environment
The cassette on the rear hub, in the opinion of bicycle enthusiasts, seems to attract the most dirt and grime. That means the bushing in the rear hub really should be a material that is resistant to dust and dirt, as well as abrasion. There is a high likelihood that the hub may encounter water, meaning that it should be resistant to corrosion and moisture absorption.
Conclusion
Acetal is one of the four materials we are highlighting in this series of articles regarding polymer bushings for the bicycle industry. Acetal is known for its natural lubricity and low friction. When PTFE is added, it becomes fully self-lubricating with even lower friction and better wear characteristics. It has excellent strength and stiffness, and low wear in fact, under certain circumstances is outperforms Nylon 4 to 1. Acetal, as well as PTFE, is resistant to a wide range of chemicals and also has an extremely low moisture absorption rate, on the order of 0.2%.
In the previous article we discussed PPS composite bushings and their advantages.
Get you copy of the Polymer Bearing Comparison Charts from Advanced EMC Technologies.
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