The main benefits of alloy nipples are weight saving. However, commonly they are made of the 2000 series aluminium which are comparatively weak. They can deform during the build, they can corrode with the elements and seize up to prevent re-truing. In contrast, Sapim’s alloy nipples are made out of 7075 aluminium which is stronger and more fatigue resistent. It also has a special anodized coating which resists corrosion. A problem that has plagued alloy nipples in the past. Why use alloy over brass, you might ask? Well, aluminium nipples are roughly 1/3 of the weight, which over a 32h wheelset will save around 40g. Brass can be used where weight is not a concern and ultimate strength is the priority.
‘Double Square’ nipples are advantageous over slotted nipples. They have longer threaded sections which means more threads on the spoke can engage, spreading the load better. They can be adjusted with a square driver as well as a conventional spoke key which reduces the chance of the colourful anodising and protective coating being damaged during building or truing. However the main advantage is the increased strength when compared to nipples of a slotted design – which can collapse in at the top if the spoke doesn’t reach the very top of the nipple. The double square nipples to not have a weak point like this and therefor are not vulnerable to the same types of failures sometimes seen with slotted nipples.
Brass nipples, overall, are stronger and should be used when strength is the top priority.