H0gr1der, if you are happy with your front tires at 16 psi and they are only increasing pressure by 2psi, you can certainly continue to ryde on them like that, just be aware that you might be compromising some aspects of your tires performance & ability to do their job properly. Tread wear, puncture resistance, overall traction, and likely hydro-planing resistance on wet roads are just some things that come to mind which may be less than ideal, but how measureable that difference will be, I don't think anyone could tell you with any degree of certainty, and regardless, the 16 psi in that tire on your Spyder for the way you ryde and in the conditions you are riding in is going to be better than any pressure that's a 'one size suits most' pressure on a placard that was intended for an entirely different tire anyway, or than a pressure that increases by less than 4psi after an hours ryding. That said, I am currently running slightly larger tires on the front of my RT at 14 to 16psi depending upon the conditions, load, or ryding I'm about to do and I pretty reliably get the 4psi increase that the '4psi rule of thumb' calls for. But you might be ryding in colder weather and on colder roads, you might ryde slower or with less stress on your tires than I do, as others have mentioned, there are hundreds of variables that may make a difference, but the rule of thumb is simply a repeatable & measureable way of taking into account all those things and getting your tires as close to their ideal pressure as any 'Joe Public type rider can manage readily - and all you need is a reliably consistent tire pressure gauge & the ability to use it! Still, I don't think there are many tires or riders out there that should be running at anything much lower than about 14psi - that's certainly as low as I'm prepared to go for normal highway ryding, altho there have been and likely still will be the odd occasion/conditions where I'll drop my front tires down to 8 or 10psi for a bit, but it will only be if the surface I'm ryding on is soft or slippery enough to warrant it and even then I'll certainly be keeping my speed down and all control inputs gentle - and the pressures that low will only be as long as is absolutely necessary!
When it comes to your rear tire, yes, it is going to be working harder cos it's the single drive tire, but it's also a tire with somewhat more volume of air inside it as well as being proprtionally larger & more heavily constructed than your front tires, so while it will need more air inside to do what's being asked of it, it's not usually a much greater amount. I generally run something between 14 to 16 psi in my fronts, and usually something between 16 to 18 in my rear tire, altho on hot road surfaces &/or when we are ryding loaded and 2 up, I might go as high as 20 or possibly even 22 if the roads have been baking in the sun for a few weeks. But the 4psi rule still applies - the greater volume of air & the heavier tire construction as a result of the bigger tire make it still applicable - and remember, this 4psi rule may be something you've only heard about recently, but it's not something I just plucked out of the air, it's something that has been around for a long time now, helping a whole lot of people get the best from their tires over many millions of miles!
As for Physics always working, that's absolutely correct, and this '4psi rule, a 'rule of thumb' always takes all the rules of physics and all the many variables you've just encountered over the last hour of your riding into account, because they are what makes the pressure in your tire go up by 4psi or 2 psi or 26 psi! You can't escape the physics of it all, it applies all the time (unless you know something I don't yet!) Sure, if you want to measure all the variables that might make a difference, take all the readings and then do all the calculations that result in a more exact pressure, you could do that, altho it might take you a day or more plus a shed load of expensive gear too, each & every time you want to go ryding! But remember, this '4psi rule' is a 'rule of thumb'; and as such it is something that has proven over many years and millions of miles to get pretty damn close to the optimum pressure each & every time, and a whole lot easier too! For those who may not understand the term 'rule of thumb', here it is as defined by google seach:
rule of thumb
/ˌruːl əv ˈθʌm/
phrase of rule
a broadly accurate guide or principle, based on practice rather than theory.
"a useful rule of thumb is that about 10 hours will be needed to analyse each hour of recorded data"
I've found over quite a few years of using this rule & doing this, that most driver/riders who practise & use the 4psi rule over a few weeks to get the hang of it properly can then just check their pressures & the increase after an hours drive once a week or so, or if they don't ride that often, maybe once a month or so if that suits them better. And once they get a little practiced with it thru use, most get pretty good at estimating what pressures will work for todays weather & road conditions & loads & their intended ryding style. Yes, you will probably want to use a (slightly) different pressure for summertime or wintertime; you'll probably want to use a different cold start pressure if you are heading out on a 500 mile ryde on the Freeway to what you'd use on a 200 mile ryde on the back roads & byways; and you'll almost certainly want to use a different pressure if it's pouring with rain or hotter & dryer than a dead dingo's donga - but for most, it doesn't take a heap of practice to start getting better at recognising what is likely to make a difference to them and their best pressure, nor does it take that long to check their tire pressures, and the more you do it the better you'll get at working out what's right for today's drive/ride! And you'll get better at recognising the different feel & handling you get from a tire that's not anywhere near the right pressure too! Besides, if you aren't changing your ryding style or load all that much, the difference in pressure between road surfaces or weather conditions probably isn't likely to be all that much - maybe a couple of psi. However, sustained high speed driving/ryding is likely to make your tires run hot, so you'll probably want to go for a bit more of a higher pressure for that than you'd use just tooling around the back blocks - maybe 4psi or so, possibly even more if you are fully loaded/ryding flat chat!
So NO, it's not a do once & forget thing! Just like the weather changes & we all get older, you'll probably want to check your tires fairly regularly for a while, at least until you are comfortable with the range of pressures that's working and close to right for you, but like I said, most who do bother using this rule of thumb fairly quickly learn what pressure's going to be pretty close for the ryding they are facing today! And once you know what cold start pressures you are using for this season/load/particular trip, you probably only need to check them in an hour if there's a significant change in one or more of those many variables that do make a difference! Btw, the more effort & care you put in to looking after your tires & running the right pressures, the longer your tires will last and the better they can do their job of looking after you!
Sorry about the epic saga again, but you did ask, and it's a complex thing, even if the 4psi rule of thumb tries to make easier for the average driver/rider to use - and it's been proven to work over many years & for many millions of miles thru it's practical application so that YOU don't hafta bother with all those variables and rules of physics or calculations etc etc etc...... but if you don't want to bother - it's your ryde, your life, & your tires; no-one else is ryding, living, or wearing them out for you! :thumbup:
hyea:
