“Do big alloy wheels crack more easily?” Asked by an anonymous Google searcher, August 2012.
For the purposes of this question, we need to assume we are talking about different size alloy wheels on the same car, as you can’t compare smaller wheels on one car with larger wheels on a completely different car. If you choose larger alloy wheels for your car (for example, going from 16-inch wheels to 18-inch wheels), you need new tyres to fit them, and this can potentially cause cracking of your wheels.
The picture above shows a 15″ alloy wheel (left), 16″ wheel (middle) and 17″ wheel (right). Despite different size wheels, the overall rolling height (wheel + tyre) is the same.
The law says that, when changing the wheels on your car, the rolling height must stay basically the same (although there is a small tolerance allowed), so fitting larger-diameter wheels to your car require tyres with lower-profile sidewalls, as shown here, to maintain the same overall rolling height.
The tyre sidewall flexes to absorb bumps and impacts, so a tyre with less sidewall (a low profile tyre, like the one on the right) has less ability to flex than a tyre with more sidewall (a high profile tyre, like the one on the left).
The reduced flex on a lower profile tyre means it is less able to absorb shocks and impacts from potholes, kerbs and speed humps, which means that if you give a kerb a big whack, you are more likely to damage the tyres and/or the alloy wheels. It would take a fair old hit, though, so it may not ever happen to you.
So yes, bigger alloy wheels will potentially crack more easily than a smaller one on the same car. Or, of course, you could just avoid kerbing your wheels… ;)
Further reading on alloy wheels
The Wheel Deal: Why do some drivers spend thousands of pounds upgrading the wheels on their cars?