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flatline
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Tyre Pressure and Performance

Postby flatline » July 15th, 2006, 10:12 am

Nice read, with the pics.
http://www.tirerack.com/tires/tiretech/ ... p?techid=3

but they recommend 35psi as ideal pressure!

Most drivers realize that tire load capacity is determined by tire size and inflation pressure. Larger tires and higher inflation pressures provide more load capacity, while smaller tires and lower tire pressures provide less.

An underinflated tire will tend to wear the shoulder areas of the tread faster than the center. This is because there is insufficient air pressure to allow the center of the tread to carry its fair share of the weight. A correctly inflated tire receives appropriate support from the contained air pressure to provide an even distribution of load across the footprint. And while most drivers recognize that this has a significant impact on tire wear, rolling resistance and durability, only a few realize it also has a noticeable influence on how effectively the tires can resist hydroplaning to maintain wet traction.

As they taught us in physics class, you can compress and move a gas quite easily, but you cannot compress liquids and it requires significant energy to move them. Our tires easily push air around and through their tread designs as they roll. However, when water pools in highway ruts and builds up on the road surface during rainstorms, the vehicle's speed and weight, as well as the tires' tread designs, tread depths and evenness of their footprint pressures determines if and when the tires will be forced to hydroplane.

One of the ways tire manufacturers evaluate their products' hydroplaning and wet traction effectiveness is by driving them over a glass plate covered with a specific depth of water. The water is dyed for better visibility and to allow high-speed cameras in underground rooms to photograph the tires from below. Michelin has provided several photographs of its HydroEdge premium All-Season tire to help illustrate this tech feature.

Image
The first photograph shows a tire properly inflated to 35 psi sitting still in the water on the glass plate. This provides an accurate idea of the tire's footprint size and shape.

The black area is where the tire's rubber compound is pressed on the glass, and the green areas identify water in the tire's circumferential and high-angle lateral grooves, and on the remainder of the glass plate.

A properly inflated tire will have enough pressure in the center of its tread to resist collapsing.


Image
The next picture is of a tire properly inflated to 35 psi, driving across the glass at 60 miles per hour. If the glass plate were dry, the footprint size would be virtually identical to the first picture because air does not prevent the tread from contacting the plate. However with standing water on the plate, the tire's tread depth and tread design must evacuate the water as the tire rolls across the plate at 88 feet per second. You will notice that the footprint still shows good contact with the plate, but is slightly smaller than the static tire's footprint.
A tire that is slightly underinflated will apply less pressure to the center of the tread and it will become slightly concave.

Image
The next picture is of a tire inflated to only 30 psi, again driving across the glass at 60 miles per hour. With the same amount of standing water on the plate, the center of the tire's tread is lifted as the tread design unsuccessfully attempts to evacuate water as the tire rolls across the plate. You will notice that the actual footprint shows poor contact with the plate and is significantly smaller than the footprint in the photograph of the properly inflated tire.
A tire that is significantly underinflated will allow the center of the tread to collapse and become very concave, trapping water rather than flowing it through the tread design.

Image
The final picture is of a tire inflated to only 25 psi, driving across the glass plate at 60 miles per hour. With the same amount of standing water, the water lifts the center of the tire's tread as its footprint rolls across the plate. You will notice that the actual footprint shows little contact with the plate and has been virtually reduced to the shoulder areas

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Postby Sanctifier » July 15th, 2006, 10:21 am

Thanks nice info. :wink:

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Postby Val » July 15th, 2006, 10:59 am

I'll try out the 35 psi.

But what does this say about Manufacturer's Recommendations for Tyre pressures for particular cars? Are they wrong? Or do they have reasons for such low pressures?

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Postby Sanctifier » July 15th, 2006, 11:43 am

^ ^ ^ Not many people drive at 60 mph in heavy rain.
Also comfort is important to most drivers.

With lower profile tyres (like '45' series) I run 32 lbs. front / 29 lbs rear to allow the car to
'rotate' easier (oversteer). I'll try 35 psi front / 31 psi rear next week to check it in the rain. :wink:
Last edited by Sanctifier on July 15th, 2006, 10:57 pm, edited 1 time in total.

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Postby ~Vēġó~ » July 15th, 2006, 12:44 pm

makes sense.........but as Sanctifier pointed out, the comfort of the ride is important to many.......I for one have been running 28psi on all 4 of meh 225/60/15's ever since, giving me a comfortably soft ride, but as supported in the article the shoulder areas of the front tyres especially tend to wear much faster...never had any problems running through water with the t/p.......but maybe I could look at 30psi x 28psi.....
Last edited by ~Vēġó~ on July 15th, 2006, 12:47 pm, edited 1 time in total.

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Postby DIESELTRINI » July 15th, 2006, 12:44 pm

If you all read the last paragraph of that article, it will answer your questions.

Adjust your tire pressures as indicated on the vehicle tire placard or in the owner's manual. Check you inflation pressures at least once a month and before highway trips.

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Postby cacasplat3 » July 15th, 2006, 1:00 pm

Adjust your tire pressures as indicated on the vehicle tire placard or in the owner's manual. Check you inflation pressures at least once a month and before highway trips.

i was told that these pressures on the vehicle are the min. pressure and should never go below this, above it is still recomended at a 4psi tolerance. very good post flatline.

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Postby VexXx Dogg » July 15th, 2006, 1:35 pm

i think i goin to try 35 psi too..

excellent info there flatline!
good looking out.

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Postby belalegosi » July 15th, 2006, 7:42 pm

everyone goin to try 35 psi... but no one realises that the 35psi stated is variant on the tyre :|
for all we know that could be a 255/30/18 tyre and some fella wit a 175/90/13 tyre gone try n put 35psi n feel he go get same effect?
lol
but good article though i always though underinflating the tyre is better for wet condition thoughs :|

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Postby Rx » July 15th, 2006, 8:14 pm

running 28psi ..... but willing to try 30 psi :wink:


great post girl- tuner :D

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Postby RYO3149 » July 15th, 2006, 9:43 pm

i was running about 28psi as well.. but will try that out when i get back meh ride.

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Postby plex » July 15th, 2006, 10:27 pm

i have been using 32psi on 205/40/17 works fine for me.

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Postby cacasplat3 » July 16th, 2006, 12:07 am

for all we know that could be a 255/30/18 tyre and some fella wit a 175/90/13 tyre gone try n put 35psi n feel he go get same effect?

90 series. hahahahahahahahahaha
most likely that tire is anywhere between 50~35 series

oh ppl remember to check tire pressures when tire is cold. :wink:

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Postby redsupra101 » July 16th, 2006, 12:38 am

i been using 35psi for the past month and had no problems besides the harder ride..
slightly less dry 'launch' traction but wet traction is notably improved

on 28psi, 3rd gear at 3krpm constant and then a sudden WOT... wheelspin
on 35psi, 3rd gear at 3krpm constant and then a sudden WOT... controlled accelleration

same road.. tested several times..

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Postby wagonrunner » July 16th, 2006, 12:43 am

interesting i running meh 205/60/14's at 30 psi for a while. wheelspin when moving off in 1st, and shifting to 2nd in the extreme wet. but in every other aspect i prefer this pressure to 28psi. 35's been tried but has the car feeling like im riding on balls. not tires.

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Postby Sanctifier » July 16th, 2006, 4:29 am

^ ^ ^ Yup I know what you mean. With the rwd GSR on Goodyear Eagle F1 195/60 x 14"
I couldn't run 32 ft / 29 rr. Adhesion at that pressure for those tyres was terrible.
It behaved a lot better at 30 ft / 28 rr but it was still terrible in the wet.

On Yokohama 195/55 x 14" it was better with 32 ft / 29 rr and wet adhesion was great.
Lower profile allowed more adhesion at higher pressure.
Better compound gave more wet adhesion I suppose.

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Postby redsupra101 » July 16th, 2006, 4:46 am

wagonrunner wrote:interesting i running meh 205/60/14's at 30 psi for a while. wheelspin when moving off in 1st, and shifting to 2nd in the extreme wet. but in every other aspect i prefer this pressure to 28psi. 35's been tried but has the car feeling like im riding on balls. not tires.


me im sticking to 30 as well.. 35 is harsh.. i just had mine there cuz i left dex at 35 and never dropped it back down
doh matter what tire pressure i at i just gonna have to learn to drive with it and make sure i comfortable with it.. i think that's the important part! :lol:

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Postby Rx » July 16th, 2006, 9:57 am

I just pressurised my tyres to 31 psi ........


is traction in allyuh ar$e :twisted:


This thread should be stickified :)

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Postby cacasplat3 » July 16th, 2006, 12:14 pm

This thread should be stickified

yep i agree.

right now i have 34psi fr. and 30 rear, i think i go push up the front to 35 and leave the rear as is. the engine in the car heavy and the tires need more air to hold the weight, and i have 225/45ZR17 so i think the extra psi could make a difference,

although ppl tell me "why your tires have soo much air in it? u working taxi?"
i never understand that though. :|

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Postby InDeForest » July 16th, 2006, 7:57 pm

this thread is great, ive never seen those images to display wet traction, and its quite frightening considering how I had been running my last set of tyres!
I was running something around 28fr 24r, low threads, I guess I was just saved by having AWD.
On reccomendation that I should be running my forester around 34-36 front, 32 rear, ive been increasing pressure gradually, im up to 38 front, 34 rear, with no real loss in comfort since they are largish 215/60/16, a little loss in traction in the wet when going WOT, i used to be able to run WOT in the wet with no traction loss at the lower pressure, ironically.
I now keep a dial pressure gauge in the car and pay alot of attention to tyre pressue.

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Postby bleedingfreak » July 17th, 2006, 8:34 am

Those photos really show alot there!

I was wondering why I was hydroplaning with brand new tyres :oops:

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Postby Hook » July 17th, 2006, 11:49 am

so what's the relationship between hot/cold tyre pressures for the same tyre?

for example...if a tyre reads 35psi cold, after say 15mins of 60mph driving, doesn't the hot pressure rise out of the "acceptable" range for maximum tyre performance?

and what if u check at home and find ur pressures kinda low (let's say by 4psi or so), and by the time u get to a gas station or tyre shop to top up, ur gauge reads correct (cuz now iz hot pressure u measuring) what's a safe value to inflate by to make up the difference?

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Postby cacasplat3 » July 17th, 2006, 8:34 pm

^^^^ i eh too sure why they say check pressures cold, but i guessing here, but if tires that are 30psi(cold), are heated up to lets say 32psi, the air in the tire is actually less dense, and now supports less weight, so i think thats why they say check tires cold, so that u could get the best pressure, cuz normally a cold tire looks a lot harder than it should be. :?

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Postby flatline » July 17th, 2006, 8:51 pm

...is the same volume of air in the tyre when u heat it up from cold. Same volume with more kinetic energy, so it will give u a higher pressure reading than when cold. Density is mass per unit volume, and pressure is force per unit area.

As to why it's better to measure cold than hot? I think it would have to do to variable heating conditions i.e. driving speed, air temperature, tarmac temperature, braking style. These would cause a different temperature depending on conditions applied, thus changing the tyre pressure reading. Measuring for a given psi when cold or after slowly driving a couple minutes to a gas station would be more standard and consistent. Most tyres are good for up to ~50psi, so there is room for expansion

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Postby Hook » July 18th, 2006, 11:19 am

^^^ tnx for the response :wink: thas what I was lookin for

cuz I don't have a pump at home and the nearest gas station is abt 7-10mins drive from home...so what I do is measure at home and measure when I get there (rises by between 2-3psi, but it's unreliable cuz it varies according to the time of day I go and between tyres due to the higher number of right turns it takes to get there)
so that when I inflate (34F, 32R according to the pump) I measure again (this checks if the gauge at the station is correct as well..highly improbable that their gauge AND my gauge are off by the same amount)

so my cold pressures are actually about 32-33F and 30-31R the next morning when I check again and then I equalize the pressures by letting some out the higher tyres on the front and rear respectively to give me 32F/30R...anyone know where I can get a pump to keep at home so checking my pressures isn't a two day process?

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Postby seanf3000 » July 18th, 2006, 1:38 pm

some of those at home electric pumps really are poor. And wasn't there some discussion about the water vapour content in at home pumps. And if there actually are dehydrators in those gas station pumps?

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Postby Hook » July 19th, 2006, 8:32 am

mmm....I guess the only real way to do it properly is to go to a tyre shop and deflate completely and pump back up to spec then?

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Postby 3stagevtec » July 19th, 2006, 2:39 pm

i've always ran my civic at 28 front and 26 rear with 205/50R15. i might move them up by 2psi each. they should have stated the tread width and profile of the tyres though...

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Postby bleedingfreak » July 20th, 2006, 9:08 am

But I thought too much pressure in the tyres was dangerous wrt stopping distance on dry road?

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Postby VexXx Dogg » July 21st, 2006, 1:16 pm

*update*

car: eg8/9 auto.
rim /tyre size: 195/50 R15 82V Dunlop SP Sport FM901.
previous pressure 28 F/R cold.

ran it at 32 psi all round and got a noticeable difference in handling on road.
didnt test wet conditions, but at 80Km/h the car didnt 'wobble' on bad roads, eg sinks @ Kay Donna Westbound lane.

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