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  1. #1
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    Default Can I determine optimal tire pressure using differential between hot and cold psi?

    I can't find it now (yes, I've done several searches), but not too long ago on one of the tire pressure threads, someone suggested using a method that involved measuring the difference in psi when hot vs cold. I think they said it shouldn't exceed 15% differential; more than that, it's under-inflated and less than that it's over-inflated.

    Does anyone else remember that? What was that method exactly? I'd like to try it out on my new rear tire. Thanks.
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    Very Active Member BLUEKNIGHT911's Avatar
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    Quote Originally Posted by UtahPete View Post
    I can't find it now (yes, I've done several searches), but not too long ago on one of the tire pressure threads, someone suggested using a method that involved measuring the difference in psi when hot vs cold. I think they said it shouldn't exceed 15% differential; more than that, it's under-inflated and less than that it's over-inflated.

    Does anyone else remember that? What was that method exactly? I'd like to try it out on my new rear tire. Thanks.
    Your Spyder is the same as mine 17-18 psi is darn near perfect...……...good luck in your research ……… I've already done mine …….. Mike

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    Very Active Member Peteoz's Avatar
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    Pete,
    Peter Aawen is usually the one who posts that formula. To the best of my knowledge it’s as simple as a 3-4psi increase in PSI from cold to hot. I started at 18psi on my F3-L in 85f temps the other day, and after an hours riding, my FOBO was showing 21.5psi.
    If you PM Peter, I sure he’ll be able to send you the formula.

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    Thanks Peter. I did track down Peter Aawen's post in a tire recommendations thread. There was another post on a different thread where someone made a specific recommendation of staying within a percentage range (rather than an absolute range of 3-4psi). Still looking for that one.
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    Very Active Member pegasus1300's Avatar
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    I have posted the 10% rule several times. Measure cold ride until tires are at temp. check. If less then 10% pressure too high lower a little. If pressure is more then 10% pressure is too low so add a little.might take two or three tries but works pretty good. We used this method back in my racing days (daze) and so did the other racers.

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  6. #6
    Ozzie Ozzie Ozzie Peter Aawen's Avatar
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    Search the Internet for 'the 4psi rule' - it's been used (& published) by all sorts of people including tire manufacturers & tire users of all types; it's been around almost as long as pneumatic tires have been around; it works for any road surfaces & under any conditions for any type of vehicle (that's running on pneumatic tires!); and it's a whole lot simpler to use than it is to explain! While some use 10% & some use 'footprint area' and others use tread temperatures, done properly they all end up with close to the same pressures, but the easiest & simplest 'rule of thumb' that I was taught as a very young fella in the tire design & test world & the method I've seen used most often & most widely, including by some very well established & respected tire manufacturers as well as widely across the whole spectrum of activities & industries that use pneumatic tires, is to aim for a 4psi increase after an hour's driving, hence 'the 4psi rule'.

    To achieve this, check the cold tire pressures before you start; then drive for an hour (ish!) Check the tire pressures again as soon as you stop! Ideally, the 'hot' pressure will be around 4psi above whatever you used & checked as your cold start pressure, because the tire should have warmed up thru use and that'll be reflected in a pressure increase of the air inside the tire. If your tire pressures have gone up by MORE than 4psi, your cold start pressure was too LOW, cos the tires have been too soft for what your doing & have flexed too much; while if the pressure has gone up by LESS than 4psi, your cold start pressure was too HIGH and your tires haven't flexed enough to warm them properly.... which means you'll need to adjust your cold start pressures as indicated by the pressure increase.

    To allow for ambient temp air going into the already hot air inside the tire, you'll need to adjust the air pressure in your tires up or down as indicated by the 'rule' using only ONE HALF of the pressure difference between what your hot pressure reading IS & what it SHOULD HAVE BEEN if it had gone up by 4psi. Use the nearest whole number with this - there's no point in trying to adjust by 1/2 a psi, the tires on most 'passenger' or recreational vehicles are too small to make an appreciable difference with such a small change & the pressure gauges we tend to use are rarely accurate enough or graduated finely enough to make that small a difference usable anyway!

    So, if your tire pressure started out at 18psi and after an hour or so's riding they only reached 19psi, then the cold start pressure was too high and you need to drop your pressure by 1/2 the difference between 19 (what it IS) and 22 (what it SHOULD have been if it had gone up by 4psi)... 22 minus 19 = 3, so you need to drop 1/2 that, which is 1.5 psi - except you probably won't be able to read that accurately on your gauge or be able to achieve that drop, so I'd aim for dropping 1 or 2 psi rather than 1.5psi. But when you start riding on cold tires next time, remember that 18 was too high, so maybe start out on 16 or 17 psi for your next ride & repeat the check after an hour's ride to see how close they are this time.

    However, if you started at 18 psi & when you checked again after an hour or so's riding, your pressures had increased to 28 psi, then your cold start presssure was too LOW & you need to ADD air now! One half of the difference between 28 (what the pressure IS now) & 22 (what it SHOULD have been) works out at 3 psi ie 28 minus 22 = 6, & 1/2 of 6 = 3..... so you should ADD 3psi before you ride on & for your next ride you should remember that 18 was too low so you need to start your next ride at saay 21 psi cold & then check the pressure in your tires again after an hour's riding.

    Do that 'Check, ride, check, adjust' thing often enough & follow the 4psi rule, then every time you'll be getting closer to YOUR optimal pressure/s for the given tires under the given load, driving style, & conditions. Sure, changes in any of those variables like ambient & road surface temps, the load on your bike, or how fast & where you are actually riding will probably cause some small variations in the results for a given ride, but you should fairly quickly learn to recognise the different pressures that will best suit a given ride subject to the changes in any of those variables that you might reasonably expect. Hot days &/or high speeds generally need a little more pressure than normal; while cold days &/or slow speeds and especially wet roads generally need a little less pressure to get that ideal pressure increase & the best from your tires. But, however long you take to learn or how often you actually apply the 'rule', every time you do apply it you'll be helping yourself to run the ideal pressure for what you are doing every ride and that ultimately means you that are more likely to get the best from your tires!

    Sorry about the length of this post, but I did say it's harder to explain than it is to actually do!!
    Last edited by Peter Aawen; 02-17-2019 at 07:55 PM.
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    Very Active Member Peteoz's Avatar
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    You don’t need to do all that stopping and starting if you have something like FOBO, Peter. As I said above, after about an hour of riding on a 90f day, my pressures had moved from 18psi cold to 21.5. Having read up on the validity of the 4psi rule, I am quite happy to use it as my guideline, so 18psi is my go to psi on my F3-L, and I’m very happy ��

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    Very Active Member jcthorne's Avatar
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    Or you could just set the front tires at 18psi cold as recommended and ride more and quit worrying over it.

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    Quote Originally Posted by jcthorne View Post
    Or you could just set the front tires at 18psi cold as recommended and ride more and quit worrying over it.
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    Quote Originally Posted by jcthorne View Post
    Or you could just set the front tires at 18psi cold as recommended and ride more and quit worrying over it.
    I think we're talking about rear tires mostly, where car tires are being used instead of the 2-ply Kendas.
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    Quote Originally Posted by Peteoz View Post
    You don’t need to do all that stopping and starting if you have something like FOBO, Peter. As I said above, after about an hour of riding on a 90f day, my pressures had moved from 18psi cold to 21.5. Having read up on the validity of the 4psi rule, I am quite happy to use it as my guideline, so 18psi is my go to psi on my F3-L, and I’m very happy ��

    Pete
    No FOBO, unfortunately.
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    Very Active Member Peteoz's Avatar
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    Quote Originally Posted by UtahPete View Post
    No FOBO, unfortunately.
    Bugger, Pete.....
    p.s. Just for info, I had the same results (+4psi) for both the front AND rear car tyres when set to 18psi.

    Pete
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    Ozzie Ozzie Ozzie Peter Aawen's Avatar
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    Quote Originally Posted by jcthorne View Post
    Or you could just set the front tires at 18psi cold as recommended and ride more and quit worrying over it.
    You could... but then again, that 18psi is intended as a 'one pressure covers ALL situations' catch all compromise for those still running/using the OE Kendas who are within & ryde within the expected parameters.... If you change out the tires or think you or your ryding might be somewhere other than within those parameters, or maybe you just want to get the 'best' from your tires - doesn't matter WHAT best, best ryde, best handling, best tire life, whatever, then maybe that 18 psi isn't quite right for you or maybe you just want to confirm for yourself..... then the 4psi rule (or the 10% variance method, or chalking the tread, or measuring the footprint area, yada yada yada) simply provides a consistent & repeatable tool for you (or anyone) to use in order to satisfy yourself that you are doing what's best for you & your tires. In the case of the 4psi rule & a couple of the others, they've been around for about 100 years and have been proven to work for & by those using them over millions of miles & tires.

    If you don't want to change your tire pressures to suit you, or you don't want to use anything but a WAG, then that's fine, no-one's gonna come around & twist your arm or beat you to a pulp if you choose not to bother doing anything at all or even if you want to go by the 'one size fits all' idiot guide the manufacturer chose to stick on the machine in order to satisfy the legislation &/or convince most people who are looking at buying one that it rides nice! Your choice; but others might like to do otherwise....



    Quote Originally Posted by Peteoz View Post
    Bugger, Pete.....
    p.s. Just for info, I had the same results (+4psi) for both the front AND rear car tyres when set to 18psi.

    Pete
    In your case Pete, it's probably just that your size & weight on the seat, which is well towards the rear end of the Spyders' weight bias, just neatly balances out the front/rear weight balance on the vehicle... Or if you like, your weight over the back end of the bike counterbalances the weight of the engine over the front wheels! And I'm not just saying that you've got a heavy arse end at all! (Well, not really, anyway! )
    Last edited by Peter Aawen; 02-18-2019 at 07:16 PM.
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    When you toss in inconsistencies such as changing ambient temps and altitude changes, trying to follow the + or - 4 psi rule is enough to drive a person to drinking. :-)

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    Very Active Member BLUEKNIGHT911's Avatar
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    Quote Originally Posted by 2dogs View Post
    When you toss in inconsistencies such as changing ambient temps and altitude changes, trying to follow the + or - 4 psi rule is enough to drive a person to drinking. :-)
    ……….. This isn't about what can be done - this isn't Formula One race machines …. The best psi for Auto tires ( any brand ) has been decided a long time ago …. is 17 or 18 psi for the rear tire PERFECT for everyone ALL the time - NO, the EXACT circumstances of using those PSI's vary so much in even one day, it's not reasonable to think it can be achieved ……………. That being said the difference between 18 psi and 28 psi is significant and that is why 18 psi is more than a REASONABLE alternative...…….OR you can make achieving the PSI's perfect and spend the rest of your life trying to achieve it.....annnnnnnnnd never succeeding. …… have fun …… Mike

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