There are actually 3 generations of the RC4, at least the way we break it down. The first gen (5/8″ shaft and Boost Valve) which had some issues with the forged eyelet cracking around the HSC/LSC adjusters, the second gen (same thing but fixed that breakage issue), then the third gen which had the 1/2″ shaft. The 1/2″ shaft variant was a bit of a hackjob to be honest - it had better rebound control but the oversize reservoir which was necessary for the 5/8″ shaft became redundant, you lost the ability to control ending stroke ramp up (the "air assist" thing didn't do a whole lot - it was just left there so they didn't have to change any parts) and they went back to the 1/2″ shafts which occasionally cracked at the eyelet threads. This was basically Fox copying what backyard mechanics had been doing using DHX parts (the shaft assembly and seal head) and removing the boost valve. The 2nd gen ones were the most bulletproof and a great performing shock, but a bit confusing to set up. The 3rd gen ones were a little easier to set up because they weren't very sensitive to air pressure and the air assist thing was pretty useless, so it cut the number of actual adjustments down a bit.
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Budget limitations aren't really a big deal on a $2500 DH fork, but as Tehllama42 pointed out, they're trying to make things work for everyone. Development is an iterative process, and we have the advantage of being able to look at what these companies do and analyse it before we develop targeted improvements. Each individual part is only as good as the level of thinking and research that went into it - it's not like any one person is "smarter" than Fox or Rockshox as a whole, but we get to pick and choose what we want to work on. They don't have that same luxury - they have to design every part of the fork and every part of the shock.
It usually just comes down to firmer spring and damper settings to deal with faster riding and bigger hits. As you move further in that direction it gets harder and harder to keep bump compliance and comfort, because not only do you have stiffer springs and firmer damping, you're also hitting those bumps at higher speeds as well, and tyre pressures have to be a bit firmer to resist squirming in corners. The net result is an exponential increase in the harshness the rider experiences as abilities/speeds pick up and the setups shift correspondingly. We set up a top 20 WC rider not long ago and data logging showed that the maximum fork travel he was using on the test track - which was not as rough as some WC tracks - was consistently around 75%. Below that it was too soft and unsupportive. That setup would have SUCKED for anyone who wasn't as fast or as physically strong as him, unless you were twice his weight.
There's always room for improvement, though there are practical limitations too. No point doing it if it ends up weighing as much as a coil spring, for example.
It's basically because tyres are the first and most effective part of your suspension - if they're at too high a pressure, they can really mess around with the rest of your setup, because it's very easy to chase your tail trying to solve harshness and grip issues by adjusting the suspension only to later find out your tyres were at 40psi and no amount of suspension was going to be able to compensate for that, at least not without severe compromise elsewhere. Tyre pressure should basically be set as low as you can go before you get noticeable tyre squirm when cornering, or start flatting more often than you're willing to tolerate. There isn't really any benefit to running it much higher than that unless maybe you're on a race course where rolling resistance is a big deal.
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