Pressure selection for low-profile tires

[dguy]

Are there any guides floating about, or can anyone suggest a "starting" pressure for a low profile tire which is unlikely to wreck the thing until I have the opportunity to do some runs and check tread temperatures?

The originals were OEM-spec 215/60, running at 30psi (cold).

I've moved to 225/45 on the front, and 255/40 on the rear.

They're set to 30psi for the moment; I've put about 30km on them so far.  While they don't feel like as though they're underinflated while driving, every now and then when I look at them something doesn't seem quite right about the shape of the sidewall at the contact patch.  Not sure if I have a valid concern, or I'm just not used to looking at them yet.   


Toyo apparently has a tech line for this sort of stuff, but the contact numbers on their web site don't appear to work outside of the USA.  Their Canadian web site is yet another Flash-driven POS which takes four days to load via dial-up, and neither of my PCs at work have the correct plug-in.   


Help!

[dguy]

Alternatively if someone with high speed wouldn't mind poking about on http://www.toyotires.ca/ for a tech-type contact number or e-mail, I'd appreciate it.

[MacPhail]

What a POS site! 

No phone numbers...only e-mail forms.

Also couldn't find the online tool you were looking for.

If I was shopping around for tires and was considering Toyo's, this site would put me off of them completely.

Cheers, Derek

[dguy]

Thanks Derek.

On a whim, I tried their "east coast" 888 number again; it seemed to work this time in that I at least landed in someone's voicemail.   

I seem to have tomorrow all to myself with not much of a to-do list, so I think I'll just grab my infrared thermometer, cram an air pig in the trunk, and go play.


...oh and don't let the web site turn you off of the product.  My first impression after a short drive on these things leaves only two words:  F'ing sticky.  Even on cold pavement.

[dguy]

I seem to have tomorrow all to myself with not much of a to-do list, so I think I'll just grab my infrared thermometer, cram an air pig in the trunk, and go play.

So much for that idea.  My simple little infrared thermometer, a suitable pyrometer does not make.   

[FieroBUZZ]

I don't know what use this is as it looks like techno-gook to me.  Kind of like asking a politico a question. From the Ottawa Prelude Club:

After talking to the Canadian Yokohama Tire rep and having him put me through to one of their technicians I realized one thing..... Its amazing who gets hired at these places. I was looking for information about proper tire pressures when plus sizing. The answer I got was to keep the same tire pressure and go back to my installer monthly for adjustments and signs of wear. Needless to say that was completely wrong. So I took it upon myself to find the answer through tire research. The bottom is what I had posted on PreludeOnline some time ago but thought I would share. It took me a while to figure out the formula's but I have had them confirmed by a tire tech at 1010tires so I know they are correct.

Quote:
I think I've got it. Here we go.

My Tire
215/35/18 84W - 215/35/18 (size) 84 (load capacity) W (Speed Rating)

Load Index
Looking at the Load Index provided by Discount Tires here you'll see for my specific tire the load max is 1102 pounds per tire.

4th Gen Weight
Weights:
gross vehicle weight rating (kg) 1,719 and curb weight (kg) 1,274
curb wieght is the car from factory
gross weight is the maximum it can carry (you and your buddies inside)

Calculations
1719 Kg = 3,789 pounds
3789 pounds / 4 tires = 947.25 pounds per tire

84 as a load index provides 1102 pounds of support as a maximum @ 50 psi
1102 pounds / 50 psi = 22.04
947.25 / 22.04 = 42

So if I haven't botched up the top you need 42 psi in each of my tires to support the wieght of the car.

The above (assuming correct) should provide the same information for any car as long as you have the vehicles gross weight and tire load number


Also Tiretrends runs a forum which may be helpful. I admit I didn't read thru this one, but the question may well have been answered there already.
http://www.canadiandriver.com/forum/index.php/board,2479.0.html

[dguy]

I don't know what use this is as it looks like techno-gook to me.  Kind of like asking a politico a question. From the Ottawa Prelude Club:

...snip...

Interesting.  Thanks for digging that and the Tiretrends forum link up.

That Prelude Club formula and a number of posts I've reviewed on Tiretrends coincide with each other, in that generally speaking if you switch to a tire with a higher load rating, then with all else being equal the pressure requirements go down.

...then what do I find, but a few vague comments along the lines of not lowering the pressure "too much" on low-profile tires to reduce the possibility of rim damage.   

Using the new tires' load index and an assumed GVWR of 2900lbs, that forumla says that 22psi should be the target.

22psi sounds just plan squishy to me, but what do I know? 


Had all four feet at 32 for the cruise, and it certainly didn't feel significantly different from 30 which is what I had been running previously.  Argh.

[dguy]

Got it... more or less.

To make a long story short, inflation pressure vs. load capacity is not a linear relationship and there are tables which plot load cap. against inflation pressure for a given tire's load index.  I wouldn't mind gettting my hands on a set of these charts for the sake of academia, but for now...


"Doing the math" as described above can get you close in most cases, but it likely won't be dead-to-rights accurate.  One thing which is missing from that formula above, is any difference in the pressure at which max. load is achieved in the old tires vs. the new.

Something like this should get you close most of the time:

Code:

OEM = ~2900lbs, 30psi.
...which is ~725lbs per tire.

Load capacity of old = 1356lbs @ 44psi
Load capacity of new = 1477 @ 50psi

x:725/1477 @ 50
30:725/1356 @ 44

x:0.4909 @ 50
30:0.5347 @ 44

x = 27.543 * 50/44
x ~= 31

[aaron88]

Dudes, you are putting way too much pressure in your tires.

I run 27.5 psi in the front and 28 psi in the rear.  These tire pressures should be good for upto 200kph.  After that it is recommended that you add about 2 psi for better stability in the corners.

Lotus recommends running 26 psi front and rear upto 200 kph track speeds, on their Esprit.

Because we have cars that are under-tired in the rear we have to run slightly higher pressure in the rear.

Ultima GTR runs 19 psi, all the way around, and it holds the world record skid pad numbers.

Basically you want the most tire on the road that you can achieve without sacrificing stability in the corners (which happens when the tire is loaded unevenly).

You can dial in your tire pressure by running it on a track and checking tire temperatures on inside center and outside of your tire.  Even temperature everywhere means your tires are just right.

High temp in the center of the tire combined with even but lower temps outside and in means tire pressure is too high.

High temps on outside and inside with low temp in center means tire pressure is too low.

High temps only inside or out mean that suspension toe in and camber etc must be adjusted.

Aaron

.

[aaron88]

Just one thing to add.

If you are running tire profiles lower than 50 series, add a few pounds for the potholes.

Aaron

.

[dguy]

Because we have cars that are under-tired in the rear we have to run slightly higher pressure in the rear.

I'm actually a hair over-tired in the front now, based on GM's published curb weight for the car.   

46/54 front/rear ratio, with 225/45 on the front ant 255/40 on the rear.

The rears will likely get switched to a 45 profile once the swap is done, as I'll be switching to custom coil-overs at the same time.  I'll get a real world weight for the car some time after that, and adjust the stagger if necessary.

You can dial in your tire pressure by running it on a track and checking tire temperatures on inside center and outside of your tire.  Even temperature everywhere means your tires are just right.

That was what I tried originally, but I can't get a decent temperature reading without a proper tire probe.  The surface of the tread cools down too quickly for an infrared or contact thermometer to be useful.

So for now I have it at 30 psi front and 28 rear, which was determined via industry load tables and an approximate GRVW.  I'm hoping to purchase or borrow a proper pyrometer some time between now & next season, 'cause guess work drives me nuts.

[aaron88]

with those tire size's I'd run 28 front and rear, if not a bit less.