rear tyre pressure

[Peter Aawen]

Oh come on!! Have you even read what I've written PMK, not just here but earlier too?

I repeatedly say that the 'one size fits all conditions/users' thing DOES NOT TRULY WORK, and that the 4psi rule or whatever other repeatable & responsive to condition/load changes method you prefer is better than the 'thumb in bum, mind in neutral, follow the one-size-fits-all-loads-&-conditions idiot guide pressure' shown on your tire placard; and that every rider should check and adjust their tires to suit their particular/current conditions/load/variables more than just every now & then, and adjust their pressures accordingly - it's just that I've not yet met anyone who can load a Spyder up enough to put a load of 1000kgs or more on EACH tire of their Spyder, so the 'usual range' that fairly closely works within the bounds of the 4psi rule's indicated pressures on most Spyders is 16-18 psi, with some (few) who impose lighter loads going as low as 14 psi, and some (few) with heavier loads needing to go up as high as about 20 psi, especially on the rear; and that you shouldn't be fooled by the 'look' of the tire, or by the 'feel' of the tire, because it's very easy to be fooled into thinking the 'look' or 'feel' might be correct instead of the measurable, repeatable, and verifiable results that repeatedly show that 'look' and 'feel' are extremely subjective and quite frequently incorrect, if not dangerously so! And the info re the relationship between pressures and loading is generally printed on the sidewalls of most tires too, so you can weigh your Spyder and using that info do the rough calcs yourself to arrive at a good 'starting pressure' for you to work with when using the 4psi rule, THEN ADJUST YOUR TIRE PRESSURE UP &/OR DOWN TO SUIT WHATEVER YOUR PREVAILING &/OR PARTICULAR NEEDS/WANTS/CONDITIONS!! Ps: those conditions may change daily, or even more often, and in an ideal world, you'd change your cold start pressures, or maybe even your operating pressures to suit those changes, but there's frequently a (justifiable & acceptable) trade off in how often you feel you can do that vs simply remaining 'reasonably close' rather than 'exact' - and besides, the 4psi 'rule of thumb' is a GUIDE, not a 'set in stone' PRESCRIPTION!

The '4psi rule' is just one 'rule of thumb' that allows people to repeatedly and consistently set their tire pressures to suit THE CONDITIONS THEY ARE RIDING OR DRIVING IN!! It takes into account ALL of the variables effecting their tires, including road temps, ambient temps, AND the load on your tires, and it reacts accordingly BECAUSE YOU MUST RIDE IN THOSE CONDITIONS in order to achieve any increase, let alone the 4psi increase that's the 'initial target'. You can choose to accept a little over 4psi if you like, you can even purposefully choose to aim for something a bit higher if you are chasing better traction in marginal conditions; just as you can choose to accept a little less than 4psi if you like, or purposefully choose to aim for something lower because you want to spin the rear tire at every set of traffic lights and be damned with the excessive wear; but at least you should be able to do that knowing that varying too much outside of that 3psi tp 6psi range previously mentioned is very likely to bring compromises that may not be beneficial (or safe) into how your tires are able to work for you.

PeteOz's 3.5 psi increase is PERFECTLY acceptable FOR HIM/His Use/His Conditions and it conforms very well with the 4psi rule; it could be fine for you too, or not!! But the pressure he uses in the height of our summertimes is almost certainly not going to be the ideal pressure Joe Bloggs needs to run in his Spyder in the depths of an Ontario winter!! Altho, unless there's some other factor peculiar to either case (Pete's size & weight for instance!?) it's become apparent from the hundreds of thousands of miles ridden by a significant number of Spyder Ryders under a wide range of conditions, that it's very likely the 'ideal' pressure for both sets of 'normal auto tires' will be fairly close to that 16-18 psi range that gets tossed around a lot!! Pete is quite tall, and heavier than most, so HIS particular 'ideal' is a touch higher, but it's still very close to the recommended range for auto tires, rather than the 'over 30 psi' recommendation for that same tire if it's carrying the load of a full sized car!!

As for running 30+psi in a tire that needs that much air in it to carry maybe 1000kgs under a full sized auto when it's only carrying maybe a third of that when it's fitted to a Spyder, yeah, FFS do that if you want to, but don't come crying to me when your tires hydroplane whenever a gnat spits on the road in front of you; if your steering is so precisely direct that you almost understeer off the road on every tight corner that you try to take at more than 40mph; or if you drive the shadow of a matchstick straight thru the tread cos your tire's blown up sooo drum tight that it can't conform over anything; or when you only get 5 or 6,000 miles out of a tire that those people running a 'more reasonable for the Spyder's load' pressure of 16-18 psi manage to run safely for over 30,000 miles....

There IS a lot of science that goes into working out the 'correct' pressure for any given set of conditions, use, load, etc, and IF you can get enough of a handle on all the variables that impact, one can actually work out fairly closely what the 'optimal' pressure for a given tire and a given set of all those variables is; you can do a rough calculation given some of those variables; or you can use a WAG if you prefer....... or maybe you can use one of the 'measurable & repeatable' tools that have proven to be very successful at getting pretty close to the 'optimal' pressure for any pnuematic tire over the last 120 or so years and the billions of miles travelled that we've been using them - the 4psi rule being just one of those tools, a tool that gets better and closer when it's used often and properly, even more so when your pressures are adjusted accordingly. Or you could simply set and forget, using the pressures recommended on your tire placard, despite the obvious differences in the tires now fitted; or possibly you could use the recommendations given by people who've done/used one or more of those 'non-WAG' methods and arrived at a 'generally useful' range that's proven to be fairly close & useful for most & probably millions of miles now?? Hell, you could even set your Spyder tire pressures at 30-35 psi cos that's what's recommended by the idiot guide when they're fitted to your car - but if that's what you want to do, why not use the 45 psi you'd possibly use in a heavier/higher loaded radial LT tire fitted to your pick-up; or better yet, the 60 psi or so you might put in the really Heavy Truck Radials that most (in Oz, anyway) Mack Trucks & other semi's & heavy transport vehicles run?? (pnuematic tires btw - there's a reason no-one much runs solids anymore - the soft & grippy compounds wear too quick while the harder stuff won't flex enough to get their tread compound up to operating temps so they've got good enough traction! No 4psi increase. )

I try to help people based upon MY knowledge, skills, experience, and learning - hopefully, some will benefit from it, even if just a little. If anyone wants to ignore what I've put out there, that's fine by me too - go right ahead; ignore it, but don't argue over things you don't want to use &/or even try to understand! Genuine questions, I'll certainly try to answer, but maybe we should take them off-line - or hasn't this epic been long enough? I got more - I've done a number of studies, dissertations, a thesis or two, and a whole bunch of essays... but I'm sorry, none of them have been 'peer reviewed' on facebook! .