Home > Forum > MTB > CRC Wheel builders > Send message
Sunday 22 November 2009 | Personalise | Help  
 FORUM Send to a friend
 
 Cormac Eason
Posted: 06/10/06 13:21:20 20
Message: It depends on the coating. It's a well known problem with anodised aluminium parts as the aluminium oxide is much harder than the Al it's grown on, leading to very high local stress at the interface between materials (This is why anodised aluminium bike frames are so rare - most companies know that they're dodgy so they save the anodising to parts with low stress or keep the anodising as thin as possible).

This is why painting is used to protect aluminium rather than anodising for highly stressed parts - That's why you don't see anodised planes.

For a bike example, look at Mavic's products. They hard anodise their lower end rims, but paint their higher end ones as they are running nearer the limits of the materials being used leaving less of a factor of safety. Any aluminium spoked wheel they make has painted spokes rather than anodised too as the spokes are where the highest varying stress resides.

If you think in terms of the bulk material performance based on the loaded cross section of the spoke you'll never get spoke failure. This doesn't explain how many bike parts fail at low stress levels compared to the peak stress the parts experienced - by that logic if the part survived a certain load once without damage, then it can't fail at lower loads than that.

All that's needed for a fatigue crack to initiate is a cyclic localised high stress as can be created by the stiffness mismatch between the bulk metal and its coating. Once the crack has started, the tip of the crack will create its own high stress allowing the crack to propagate through the material until there isn't enough cross section left in the material to support the load and the part fails visibly. That's why you don't see sharp edges, sudden cross section changes and tight corners on load bearing parts either, they act as stress risers too...

It really depends on what the spokes are coated with - If it's a layer of paint then it's not a problem as the paint won't be stiff enough to change the surface condition...
 Email Address:
Support our sponsors
 
Join BikeMagic login to BikeMagic
Forgotten your password? | Why should I join BikeMagic?

Support our sponsors

Shopping Partners
Kudu Bikes
Wiggle
AW Cycles
Cycle Surgery
Sprockets.uk.com
Chain Reaction Cycles
 
United Kingdom
Bike tours in France & Italy
Endurance Life
Three Peaks Bike & Hike
Explore Cycles
 
Offers, Competitions and Promotions
win free stuff with rcuk

Squirt Competition
Win 1 of 50 bottles of SQUIRT - Long lasting Dry Lube
Ticket2RideBC Adventures
Advertorial: Ticket2RideBC specialises in guided mountain bike adventures
British Heart Foundation
Advertorial: Get cycling for the BHF and raise vital money