Subaru Forester Owners Forum banner

1 - 20 of 27 Posts

·
Registered
Joined
·
996 Posts
**** that, id be removing PSI otherwise you get that boaty feeling from the tyres being to hard/over USDM market ;-)
Really? So removing pressure from the 29-30 psi gives you more assurance on the road?
With my Michelins(rated to 44 psi) if I were go with the rec'd Subyrating above, sudden lane changes, or spirited driving in the turns results in boatishhness, squealing tires, and excess tire wear on the shoulders of the tread. There's more tire contact patch on the road at higher psi... more traction.
Yesterday, I readjusted my psi to 37-36 front to back....
 

·
Registered
Joined
·
267 Posts
Discussion Starter #3
Yes, because the correct pressure is 32psi (iirc) for a scooby (im away from the car at the moment but it shoud say on the door jamb area), so by removing a few psi when the tyres are cold will go back to 32psi when hot. Pending on which tyre you are running will determine how quickly they are up to the correct temp.

If i start at a 32 psi, then theyll go to 36 psi which means they are over inflated and therefore you have less contact pressure with the road. When they are underinflated then the sidewalls take more of the strain. Of course, your scooby might need 36 psi in them from cold, i dont know

Just because the tyres are rated to 44psi, doesnt mean you go to them.

im not sure if your just trying to troll or just abit simple but have a read;
https://www.national.co.uk/information/tyre-maintenance.

Then google how tyres react under harsh cornering and then think to yourself why UHP - slicks have curved corners as opposed to squareish corners that youll find on mileage munchers.

But im sure you know all this and just trolling right?
 

·
Registered
Joined
·
996 Posts
No troll. Not simple...my Michelin tires wear evenly across tread face... not worn in the center at all.
Rec'd ratings cause front of car to push and rear to oversteer during cornering... what I call boatish.
 

·
Registered
Joined
·
130 Posts
Really? So removing pressure from the 29-30 psi gives you more assurance on the road?
With my Michelins(rated to 44 psi) if I were go with the rec'd Subyrating above, sudden lane changes, or spirited driving in the turns results in boatishhness, squealing tires, and excess tire wear on the shoulders of the tread. There's more tire contact patch on the road at higher psi... more traction.
Yesterday, I readjusted my psi to 37-36 front to back....
37 PSI?! Thats what I run on a mountain bike mate! 😁

If thats cold pressure and your tyres are rated to 44PSI max then maybe consider checking them when they are hot as you could get close to that rating. Cold pressures at SG Forester weights should be somewhere between 27 and 35 max, and 37 to 40 hot so it doesn't leave you much headroom.

Not sure what you mean by spirited driving as everyone is different but running Yokohama AD48Rs on track recently that were 22 Front 24 psi Rear on a 900kg car (1985lbs), we found after some hot laps at 31C (88F) ambient temps and re-checking they were upto 29 and 32 psi so had to take some pressure out as we were aiming for 27 front 29 rear when hot.

Edit: furthermore increasing hot tyre pressures will change the balance, depending on the car of course, higher pressures are often used to reduce grip at one of the axles (ie. you would increase front tyre pressure to reduce grip at the front) having high pressures front and back will simply decrease your lateral grip overall.
 

·
Registered
Joined
·
267 Posts
Discussion Starter #6
If his signature is correct then hes got a 17 plate and the recommended psi is 32.

So even cold, hes running 36psi which at hot, will be circa 39 psi.

I'll let you explain about why air pressure changes when running and how outdoor temps can affect cold tyre pressures
 

·
Registered
Joined
·
1,068 Posts
TIRE PRESSURE SPECIFICATIONS ARE FOR WHEN A TIRE IS COLD.
If you read the tire article, it even states this, to wit:
"Tyres must be cold when you carry out the check"
Cold is relative. If it gets down to freezing, your tire should be checked at that temperature.
The reason is tires can fail easily when under inflated. Take a look at a tire and you will see it on the sidewall.
The maximum pressure rating is for a cold tire, which includes an intended rise in pressure for when it gets hot.
Releasing air from a properly adjusted cold tire when it gets hot is great way to destroy your tires.
 

·
Registered
Joined
·
130 Posts
… and the thread makes a hard left turn...
Since we are in mid turn...
TIRE PRESSURE SPECIFICATIONS ARE FOR WHEN A TIRE IS COLD.
If you read the tire article, it even states this, to wit:
"Tyres must be cold when you carry out the check"
Cold is relative. If it gets down to freezing, your tire should be checked at that temperature.
The reason is tires can fail easily when under inflated. Take a look at a tire and you will see it on the sidewall.
The maximum pressure rating is for a cold tire, which includes an intended rise in pressure for when it gets hot.
Releasing air from a properly adjusted cold tire when it gets hot is great way to destroy your tires.

So, baack to the thread topic - Pointless/stupid modifications to the Forester.
Removing air from properly inflated hot tires.
Specifications are of course cold, however else could they be? When I mentioned on track (ie. "spirited driving") you may want to balance hot pressures side to side and may need to remove some pressure depending on the car/track/ambient temp, of course, you put this pressure back in when cold (much more important on a heavier car like the Foz).
 

·
Registered
Joined
·
1,068 Posts
@batou - You said "your tyres are rated to 44PSI max then maybe consider checking them when they are hot as you could get close to that rating". Nope. 44PSI max would be the cold, not hot rating.
When a tire is set cold and warms up it isn't a problem.
All ratings on tires are for cold. Not that running tires at maximum pressure is a good idea.
The track example doesn't apply anywhere but the track.
For "spirited driving", recommending release of pressure isn't such a good idea, but adding some extra cold pounds might be...

The factory recommendation is just an average "most case scenario", and is a spec designed for the balance between mileage and a comfortable ride. The factory likely prefers the latter.
You can go over that safely, but unless you are really loaded down, a LOT of extra pressure will downgrade handling and wear out your tires more quickly well before the cold pressure rating of the tire, which is actually a safety warning, not a pressure recommendation.

Primarily, I was referring to Kingstonjames statement "If i start at a 32 psi, then theyll go to 36 psi which means they are over inflated" which is entirely incorrect. Not that the pressure goes up, which it does, but that the pressure rise causes them to be over inflated. All tires warm up when driven, which is why you check them before you drive.
That doesn't make them over inflated, actually it's the opposite, when it's hot you want them at higher pressure so they run cooler, because much of the heat generated in the tire is due to its flexing, and at higher pressure, this occurs to a lesser extent. When set correctly cold, this naturally occurs.
 

·
Registered
Joined
·
267 Posts
Discussion Starter #11
Lets just agree to disagree on tyre pressure, ill continue to run mine slightly under so they are at cold pressure once warm and handling how i want (ignoring the suspension and set-up ive got), you keep running them over the recommended and wonder why your getting under/oversteer and light feeling on the steering wheel when 'pushing' it. Lets not bother with tyre compound or sidewall stiffness either and how that can affect it all.
 

·
Registered
Joined
·
996 Posts
[QUOTE="kingstonjames, post: 7574428, member: ] you keep running them over the recommended and wonder why your getting under/oversteer and light feeling on the steering wheel
[/QUOTE]
To be clear. I get under/ oversteer when running FACTORY rec'd air pressure .
... And a more planted feel(less boaty) at the elevated pressures I use.
Note fwy speeds are in the range of 130- 140kph(70-80 mph give or take)
Agreeing to disagree.
 

·
Registered
Joined
·
259 Posts
Lets just agree to disagree on tyre pressure, ill continue to run mine slightly under so they are at cold pressure once warm and handling how i want (ignoring the suspension and set-up ive got), you keep running them over the recommended and wonder why your getting under/oversteer and light feeling on the steering wheel when 'pushing' it. Lets not bother with tyre compound or sidewall stiffness either and how that can affect it all.
Wow, just wow. The spec is cold. Why is that so hard to understand? The manufacturer expects them to rise in pressure when hot but that's not overinflated, it's actually correctly inflated to manufacture recommendation.
 

·
Registered
Joined
·
130 Posts
@batou - You said "your tyres are rated to 44PSI max then maybe consider checking them when they are hot as you could get close to that rating". Nope. 44PSI max would be the cold, not hot rating.
When a tire is set cold and warms up it isn't a problem.
All ratings on tires are for cold. Not that running tires at maximum pressure is a good idea.
The track example doesn't apply anywhere but the track.
For "spirited driving", recommending release of pressure isn't such a good idea, but adding some extra cold pounds might be...

The factory recommendation is just an average "most case scenario", and is a spec designed for the balance between mileage and a comfortable ride. The factory likely prefers the latter.
You can go over that safely, but unless you are really loaded down, a LOT of extra pressure will downgrade handling and wear out your tires more quickly well before the cold pressure rating of the tire, which is actually a safety warning, not a pressure recommendation.

Primarily, I was referring to Kingstonjames statement "If i start at a 32 psi, then theyll go to 36 psi which means they are over inflated" which is entirely incorrect. Not that the pressure goes up, which it does, but that the pressure rise causes them to be over inflated. All tires warm up when driven, which is why you check them before you drive.
That doesn't make them over inflated, actually it's the opposite, when it's hot you want them at higher pressure so they run cooler, because much of the heat generated in the tire is due to its flexing, and at higher pressure, this occurs to a lesser extent. When set correctly cold, this naturally occurs.
Okay, I don't want to really go to town on this subject as pressures really differ depending on car and the type of driving. Noone in the UK puts 37 PSI in their sub 1500kg Subaru, or even the current 1564kg SK (32psi), too much pressure will impair heavy braking and traction or wear the centre of the tyres out.

As I said, everyones spirited driving is different so I used a track day as example on the further end of that spectrum. Depending on the layout and the car you want to ensure your pressures are at an optimum level and not too uneven, at Donington Park we saw one of the rears had spiked 3-4 psi over the target hot pressure after a cool down lap and coming in to check, this happened to be one of the inside rears on most corners (we could feel it braking loose now and again) and with 300 rwhp and no Limited slip diff you want to balance that one out across the rear axle to prevent excessively wearing that tyre out or overheating it. This is a scenario where bleeding some warm/hot pressure out is a good idea (you'll never see the actual peak hot temps without proper sensors anyway).

This change allowed us to stay out longer and provided more traction out of the old Hairpin and Coppice both which are both right handers the latter of which was on lift/cam (supercharged toyoya 2ZZ) and in peak boost in 3rd gear.

In regards to checking tyre temps after heating them up, so your telling me, if after a few hot laps of a track or your local "touge" run you saw 44PSI in all one or all four wheels, or something over that, you'd be okay with that number? Of course not, yeah its a cold rating but you shouldn't be seeing more than 40PSI unless your riding bike or truck.

Yes, the higher pressure keeping them cool theory works until your losing grip and sliding on that tyre, then you will overheat it and spike the pressure which is why what constitutes as "spirited driving" is important. If your just hitting one or two bends at 5/10ths of the cars grip on the way home from work it won't make much of a difference, spend an hour smashing through the A5, A543 and the B4501 in North Wales or a 20 minute session on your local track at 7/10ths then you may want to manage them a little more and keep the cold pressure lower than 37 if you want to drive home on the same tyre/not end up in a ditch/gravel trap.
 

·
Registered
Joined
·
1,068 Posts
If you are talking about racing, it's just a "slightly" different set of circumstances.
This may come as a shock, but most people don't drive that way.
The pillar plate is 30/29 psi front to rear as was noted above, and is also the recommended setting on my car.
Again this is a factory recommendation based on average use.
Heavy loads approaching the GVWR benefit from additional air.
Some people like a little more air and that's just a personal preference if they like the handling better.
It's a balance, and either too low or too high can cause problems.
I don't know where you are getting 37PSI from, but that's too high of a cold setting for me.
It could be approaching excessive tire wear at that point, not to mention a stiff ride.
Once on the road, I, like most, am not at the track and I wouldn't be checking my tires when hot as it isn't necessary.
It is specifically stated by every tire manufacturer that you don't check tire pressure when hot. Bad idea. Misleading reading.

Typical tire pressure rise is not that high, so no, I wouldn't worry about it, because for me it never happens.
The problem with recommending air release on hot tires is that for the 99% of drivers it's a bad idea and is more misinformation than a helpful hint.

You could start a thread on tire settings for optimal traction on a track, as that seems to be the circumstance where your suggestion might apply.
If you have one tire that is spiking in pressure (assuming all were set cold) how do you predict that it will be encountering the same set of forces that heated it up unless you are driving on an oval... You can't.
So twenty turns later on the road with "spirited driving", now tire pressure on the opposite side is "spiking".. so you release pressure?
Great - Now you have two underinflated tires once you start driving "normally".
BTW - Are you at all concerned with getting hit by a truck while checking your tire pressure on the highway....
It sounds pretty silly to me.
 

·
Registered
Joined
·
996 Posts
..

I don't know where you are getting 37PSI from, but that's too high of a cold setting for me.
From me... and to clarify, I set that hot. I have even wear across the face of the tread and no boatishness that the fact.rec'd psi gives me (along w/ excessive shoulder wear).
 

·
Registered
Joined
·
1,068 Posts
MFGA - The reason manufacturers tell you to set tire pressures cold are primarily...
1 - You want the tires set appropriately for the environmental cold conditions so you don't wreck a severely underinflated tire when you start out in the morning.
2 - Hot is relative and can vary a lot depending on how you drive, how fast you go, how far you went, the ambient temperature, road conditions, etc, etc, etc.
If you like 37 hot - Good for you. How hot is your hot? It can vary significantly.
See what the tires read when cold and you would know what they are actually set to.
Hot readings have too many variables, and don't account for the morning low temperatures, which is why no competent authority will recommend doing that.
 

·
Registered
Joined
·
996 Posts
I know, I had a spurious tire light on the dash, went around and checked, saw that most were set to 32, one at 35(just had a COSTCO tire rotate and they always mess with my higher than rec'd setting), readjusted all to 37, 85 degree day...
Light went away after restart
 

·
Registered
Joined
·
1,068 Posts
That's odd...
Why would TPMS trigger for 32 psi, or 35?..
Normally I don't get a light unless a tire is 4+ pounds down..
37 after driving at 85F is probably close to 33 psi cold, well above that threshold...
 
1 - 20 of 27 Posts
Top