A discussion on kartbook brought up the question - do kart chassis "go off" when they do a lot of work? Do they just get slower or less responsive as they get used?
Unfortunately, the answer in my opinion is "it's definitely possible".
The process of a kart going off can be confusing, however, even for someone with a lot of experience. The easiest way to end the effective life of a chassis is to have an epic crash in it. Any chassis that is bent will fail to perform, and unfortunately a chassis which is straightened after being bent might come back to life, and it might not. I can think of four or five examples of bent karts that were like new once straightened, and bent karts that never went any good again, once straightened.
The other manner in which kart chassis' come to the end of their useful life is when they "go off". Many karters describe the chassis once this happens as "soft", and they're usually howled down by the geeky engineers (like me) who will tell you that metal gets harder as you work it, not softer.
The engineers are right, by the way, metal definitely work-hardens, but that's on a microscopic scale. If you do a Rockwell Hardness test on metal that's been worked a lot, it will return a higher figure than virgin "soft" material. On a macro-scale, however, the opposite is true. There's one real-world example which surely everyone is familiar with - bending a coat hanger until it breaks.
When you first bend the coat hanger, it feels pretty stiff. Trying to bend it back again, sometimes you've got to really concentrate the force on the area of the first bend to make it bend back in the same place - you can feel that material is slightly harder than before! Once you've bent it back once or twice, though, you can feel it really soften up. As the metal work hardens, it becomes more crystalline and more brittle, and as a result cracks start to appear. The material might be harder, but the coat hanger itself now feels a whole lot softer! Eventually, of course, the cracks get large enough to split the material in two.
Go karts are similar, especially at the welds. In metals, sharp corners create stress concentration risers. Basically, it's like having a slightly soft spot in your coat hanger. Every tube junction has a gigantic stress concentration riser all the way around it! When the chassis gets bent back-and-forth, back-and-forth, the area of metal right around each weld is copping a beating - just like the spot on the coat hanger where you decided to break it. The metal at the welds starts to harden, become brittle, and crack microscopically. (Incidentally, the metal in the free-spanning tube areas of the kart probably never work hardens at all. Chromoly has an exceptionally high yield strength and little-to-no work hardening occurs below that point. Only where there's a significant stress concentration riser will the chromoly start to work harden).
The result from the drivers seat? The kart feels soft.
The result on the stop watch? The kart goes slow.
he only way to fix it? Sorry... the answer is "bin it". You can't rejuvenate that much metal effectively.
So, do all karts eventually go off? Sort of, but not necessarily. Firstly, the less horsepower you've got and the harder the tyres are, the less the chassis seems to rely on "spring" to function, so you'll often see guys running J's with a great deal of success, even in older chassis. Also, for some driving styles a super-springy kart might not suit, so in actual fact a used chassis feels better and goes faster for that particular driver.
So, what's the conclusion to all this? Well, basically this is the reason kart chassis value drops pretty quickly once the kart is more than a year or so old. Guys that are contesting for State and National Championships will often try to keep themselves in new equipment, and this is why. Do you have to replace your kart every year... no. But by the same token, don't waste thousands and thousands of dollars on fuel, tyres, travel and accommodation, while constantly scratching your head and wondering why you're not getting good results. Keep track of the hours (and damage) that your chassis clocks up, and keep a new chassis budget ticking as you race.