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From Fellrnr.com, Running tips
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* For most shoes, the limiting factor in shoe life is the foam midsole rather than the harder rubber outsole. The means the foam becomes compressed and offers less cushioning. The reduced cushioning is generally not the problem, but rather it's the uneven reduction across the shoe that starts to cause biomechanical problems. A common situation is that the foam under the ball of the foot wears and so the foot is tilted inward.
* Thinner shoes tend to last longer than thicker shoes, which is perhaps counterintuitive. This is because a shoe with a 40mm midsole that loses 20% of its thickness due to wear would be 8mm thinner, where a shoe with only 10mm would only lose 2mm.
* The life of the hard rubber outsole is rarely an issue, and by the time the outsole is worn through the foam is dead. However , there are shoes that use relatively soft outsole that can wear more quickly. If you're wearing through the outsole before the foam is dead, there's a good chance you're not pawing back correctly and your shoes are scraping as they land. This tends to waste energy and is a sign of poor [[Running Form]].
* Shoes that don't have a hard rubber outsole can suffer from abrasion wearing away the foam. With these shoes the scraping due to lack of paw back has a much bigger impact on shoe life as the foam is much less resistant to being scraped.
* Temperature can play a factor in shoe life. Hot temperatures can soften shoes and cause them to wear quickly. At Badwater I've had shoes wear away more in 30 miles than I would normally see in 300. On the other hand, foam gives far less cushioning in cold weather, so a shoe that might be okay in the summer might feel worn out in winter.
* The running surface changes the rate of abrasion if that's the limiting factor. Rough concrete or asphalt will erode the shoe more quickly than a smooth surface or dirt.
* How much wear is too much also depends on the individual runner. Some runners may tolerate very little wear, but I know runners that can continue to run injury free in shoes that most people would consider destroyed.
{| class="wikitable"
|- valign="top"
|[[File:WornHS6Shoes.jpg|none|thumb|300px|This is a picture of a friend's [[Asics Gel Hyper Speed 6]] showing dramatic outsole wear. These shoes had only completed 230 miles but you can see the outsole is completely gone in places. I suspect that the runner in question is scraping slightly on landing rather than pawing back fully.]]
|[[File: ComparisonHS6Shoes.jpg|none|thumb|300px|By comparison this is my Hyper Speed 6 shoes that have far more miles on them. You can see that most of the outsole still has its texture, and the only real outsole wear is on the extreme heel.]]
|}
==My Experiences with Shoe Life==
Since I started tracking my shoe life more accurately with [[MilestonePod]]s, I've found that there is far more wear on my shoes than I realized. With my [[Foot Strike]] the cushioning degrades so that the sole under the ball of my foot is quite a bit lower than the rest of the forefoot. I calculate that about 2mm of compression under the ball of the foot is like running on the camber of a standard road (~3°), which creates additional stresses and might be a cause of injury. In as little as 100 miles (160Km) I've had shoes compress by 4mm under the ball of my foot! Since I injured my meniscus I've found I can't tolerate shoes with that much wear, as this artificial camber irritates my knee. Prior to my knee injury I could get away with quite massively worn shoes, sometimes putting on as much as 3,000 miles (5,000 Km) before retiring them.
