PSA: Check your tire pressures

Why do people over-inflate tires?
>blog incoming
>2 months after getting my first sports car from a dealership, my dad was riding with me and told me to check the tire pressure with the ones in my car manual, said it was bumpier than it should be, and handles strangely.
>He hadn't noticed previously because he thought I was still learning proper clutch control.
>I had noticed it was a twitchy motherfucker to drive, and the slightest bump would move my feet around on the pedals and cause more bumps, but I assumed that's just how sports cars drive, and pegged it down as the car needing next level clutch control.

Turns out the dealer inflated the rear tires 35kPa higher than it should be, and the front 30kPa higher than it should be. Why? What's the benefit? Does it make people think the car is more bite-y and powerful than it really is?

Other urls found in this thread:

kwik-fit.com/blog/10-things-that-can-affect-your-stopping-distance
twitter.com/NSFWRedditVideo

The higher the pressure the lower the rolling resistance, thus better fuel economy. I pump my tires up to 3 bars which is about 40 PSI, it really doesn't bother the tire.

They probably read "MAX PSI" on the side of the tires and ignored what the car asks for. Basically they're retarded

>Doesn't bother the tire
Really now?

That might be ok in an auto, but the way the cars weight shifted made it difficult to change gears and drive around smoothly.

I was trying to teach myself to be as smooth as fuck with the car, and couldn't figure out why my feet were being thrown around so much during turns, takeoffs and slight bumps.

For my car (manuel) is about 0.5 bar over the recommended pressure. Smooth gear changes is more in the play with clutch / throttle than the actual contact between road and tire.

I two years I haven't seen any noticeable wear on the threads. You need a very light car and very over inflated tires to have problems. My car is almost 1.3 tons, most of which in the heavy diesel engine under the hood.

My gear changes weren't great to begin with, but me dad eventually noticed the bumps because the shifts were smooth and the car was still being thrown around

gotta keep dat dere high psi for stretched lyfe

i once drove my nans car with a massive tyre leak (loses all pressure in 24 hrs)
>driving my nan to some place
>i get out of the car and notice the front right wheel is down to the steel belts
>nearly shit my pants
>go to get the spare
>spare is down to the threads
>look at other tyres
>all with 2+mm tread
>yell at my nan for letting her car get in such a shit state

From 1st to 2nd it might be tricky to do it smootly but you learn on. Upshifting in general doesn't take much effort as the RPM will almost always fall in the sweet spot while the clutch is in.
Downshifting can be tricky, I still haven't learn how to do it properly, so as workaround I downshift just before the engine starts to cough.

Does overinflation cause longer stopping distances? My car says to inflate to 38 psi. I once inflated to 43 psi to see if it would increase the range, and it did. I have had to do a panic stop before when the tires were at 38 psi, and the stopping power impressed me. After I overinflated the tires to 43 psi, I had to do another panic stop for a pedestrian who ran out in front of my car. The car felt like it was dragging itself. The abs was making a bunch of noise, and it felt like I couldn't stop as well as I should have. I missed him by a little less than a car length. Both instances were in similar weather. I realize there could have been oil on the road, or something, but I since dropped the psi back to 38, and it never happened again.

It's a well known fact that under inflated tires have more grip, so yes over inflating will longer the braking distance.

Yeah, I had previously driven a manual for about 2 years, then drove an auto for 3 and 1/2 years, needed to relearn shifting techniques.

I'm back to full proficiency, I just was unsure how sports cars are supposed to react compared to 'normal' and didn't think to check the pressure. 1st gear threw me for a loop under preasure, but I'm now shifting better than ever, having to learn and compensate for the excessive bumpiness. On the proper pressure it feels too damn stable, doesn't feel twitchy and powerful.

my tires say maximum psi is 51. 46 seems to be the sweet spot? under 35 and the tires look flat and steering is heavy.

forgot to actually ask the question, what psi should i run on my tires. my back 2 are on 36psi cause i have 2 different tires on the back and thats the max for the right side. and both the fronts are running 46 on same tires.

>Yelling at your grandmother

Most old people don't have a lot of disposable income. Maybe she couldn't afford new tires?

Coulda just helped her.

Does your car the have ranges written somewhere? Some manuals or instructions on the inside of the driver door will give you values for various performance characteristics, like load bearing, economy, comfort etc.
If not, figure out the minimum and work between the ranges to decide on what you would prefer.

What car?

the dude who owned it before me put on bigger aftermarket wheels with canadian tires. im running 205/45/17 fronts and 215/45/17 rear on 195/205. The oem wheels on my teggy is 195/55/15

205/40/17 and 215/40/17 not 45

2015 wrx sti

>Does overinflation cause longer stopping distances?

No, changing the tire pressure doesn't change the grip you get from a tire unless you are running race rubber at racing temperatures. Grip is the product of two factors the normal (vertical) force through the tire and the coefficient of friction between the rubber and road.

38F 40R

That isn't a sports car kid.

Ok

under braking (and any situation where their under stress) your tires deform so having higher pressure will prevent that and that will increase stopping distances
under-inflated tires have more grip hence why you have a worse fuel economy and better stopping distance just because they can deform more and fit the road better

but by changing the surface area of the tire on the road, can't you change the total amount of friction between the tires and the road?

you're a dumb tit

he's also right

and yet i'm right
but maybe you want to explain why i am wrong
if overinflating your tires have an effect on fuel consumption it's because it has an effect on grip
you can try that with your bike

No.

No, you gain no grip from the deformation of the tire other than at the microscopic level and that happens within the tread depth, tire pressure doesn't change that. You get worse economy and identical stopping distances because you are transforming more of the energy of moving the car into heat in the rubber by deflecting it more.

kwik-fit.com/blog/10-things-that-can-affect-your-stopping-distance
TLDR: Underinflated tyres will make more contact with the road on the outer edges of the tyre whereas overinflated tyres make more contact in the centre. Both are bad news for your tyres and stopping distance. Not only does this cause irregular wear of the tyres, but traction will also be reduced meaning your tyres are less effective at biting into the road surface and bringing the car to a halt.
Braking Distance (in feet) from NHTSA testing
Stopping Distance from 60 mph
Surface 15 psi 35 psi
Wet Concrete 148.8 146.5
Dry Concrete 142.0 139.8
Wet Asphalt 158.5 158.0
Dry Asphalt 144.0 144.0

>what is contact patch
Retard.

>grip isn't a function of contact area
Get your stupid ass the fuck out of here.

Retard, go look up the equation for friction, then look up the equation for pressure. You then do some simple canceling out and guess what it isn't but you are an idiot.

He's right, it's a common misconception that more contact area gives more grip but it's not true. Sadly the ignorant have perpetuated this myth despite the fact it has been repeatedly disproven by tyre manufactures and car companies.

i always inflate 2 psi higher than what's stated for my tires.

>Grip is the product of two factors, vertical force through the tire and the coefficient of friction between the rubber and the road

Jesus christ, you're retarded. The coefficient of friction is multiplied by the surface area in contact with the ground and the vertical force.

Think about what you are trying to say the formula means:
If it's just the two factors, its downward force, and how much friction the rubber can generate. You're completely overlooking how much of the tire is even touching the ground.

What you are saying is pic related will have the same tire grip as a car scaled with proportionate weight on the tires on all 4 wheels

i was talking about light under inflation but you are right the studies i looked at
got better stopping distance with a tire 5 psi below the recommendation

>The coefficient of friction is multiplied by the surface area in contact with the ground and the vertical force.

No go back and read in you moron. Grip is the coefficient of friction times the area times the surface pressure. The Pressure is the normal force divided by the area. so you have

g=c*p*a
where p = f/a so you have
g = c*(f/a)*a
or (c*f*a)/a
cancel out the a and you have
g=c*f

go back to school.

>Proxies
Shittest tyres I've ever had

>He thinks pressure means tyre pressure and not object load

Mate you're wrong, as you decrease pressure you increase the contact patch which increase the grip, until you pass the point where it build up so much heat it will fail.

How do you know what to inflate your tires to, or do you just wing it?

I got new fronts and they were inflated to the rating inside my cars door, but they were visibly under inflated at that pressure so I added 5psi or so for the time being. My door says fronts 32psi, rear 29, and my front tires are max 44 and rears max 52. I know to keep them under the max though of course.

I have them on my car and they were great. Really sticky and rode well.

>Grip = coefficient*area*surface pressure

That's a different story from vertical force*coefficient of friction, it's still wrong.

Your proposal (with less contact area somehow not affecting friction and therefore grip, despite them being factors in your new made-up formula)
is so fucking stupid.

You're saying the end result of G, the whole basis of your equation, doesn't matter. It could be a higher number or a lower number due to changes in how much surface area is in contact with the tires.

Race car drivers change the pressures depending on car handling aspects; engine configuration and drive, weight distribution, conditions of the road and what performance they need out of the tires etc.

In my plebby daily driver experience though,
>If you add more, it's bumpy but feels more tactile on the road, you're able to distinguish the different textures of the road.
>If you keep it at the suggested values you get the safest rated stopping distance and it's more comfortable.

>tire factory spec is 33 psi
>inflate to 38~ psi
>max is 45 psi

Not really sure why anyone would inflate tires to anything other than the recommended tire pressure from the tire manufacturer. At worst, it is a safety hazard and, at best, it's just causing premature and uneven wear of your tires (i.e. it is wasting your money).

If you want better mileage, buy tires with a higher mileage rating. If you want high performance tires buy performance tires. You can't just over/under inflate and expect the tire to change how you want it. Companies spend tons of money developing tires, it goes into tread patterns, rubber compounds, ply geometry, etc. If you think you know better than the manufacturer what pressure to inflate to you are a fool.

Depends. Factory spec on my car is 30 front 29 rear, but I have them at 44 psi and they're wearing evenly. Probably depends more on driving style, I drive it pretty hard.

Just drive on a long straight somewhere at 30-35 mph practicing rev matching from third to second gear. It really shouldn't take long to learn at all, it's super easy in a modern car with ok synchros.

>Probably depends more on driving style, I drive it pretty hard
Fucking how? Are you implying that driving hard make your car weigh more? Because otherwise the graphic is correct. Why not just use the tire manufacturers recommendations?

i can gander a guess. They may over inflate them to shush idiots coming back saying their tires look low (having the slight bulge on the bottom) when set to factory pressures.

>come home to see parents
>"user your tires are low, you should fill them"
>"your tires shouldn't have that bulge on the bottom, user"
>it was the slightest bulge but i checked with my gauge to be safe
>32psi F, 30psi R. over inflated on the rear according to the doorjamb by 1 psi
>"wow user that's really low. you're going to wear out you tires"
>tell him that they're factory spec.
>"nah, that number is just a scam. you're supposed to fill them till they don't have that bulge. They actually want you to wear out your tires faster so you buy more"
>points to mom's honda for example of how a tire "should" look.
>check with gauge
>45 all around and starting to show center wear
>doorjamb denotes 33 all around

I'm not that guy, but I've found sliding through a lot of corners in my shitbox, has definitely started wearing tread on the sides of my front tires faster.

It's not that I'm adding weight, cornering hard shifts the weight from the center of the tread (which would wear faster overinflated) over to the shoulders, which normally wouldn't contact the road as much at a higher pressure.

I seriously doubt this. Most driving is done straight and not turning. Even if you're going into corners extremely hard all day every day, you aren't putting thousands of miles on the tires through turns.

Maybe this would make an appreciable difference if it's a track car, but not in daily driver.

Well, my tires are wearing evenly, so if what I'm doing works I'll keep doing it. Maybe the manufacturer's recommendation is low for ride comfort over tire wear.

Out of curiosity, what tires are they and how long have you been on them?

ur nan drifts her car on the mountain pass every weekend, that's why all the tires were down to the belts

205/55R16 Eagle RS-A 91H (most are 89H load rating), since January 2015

2016*

>mfw I know a bunch of idiots who think this is right too

gross

Pro tip:
When you do a fair amount of hard cornering as well it helps even it out on the sides. Think of it as further economical gains. You're maximising the use of the gas you burned by not using the brakes and maintaining that momentum through corners

she gambles it so i have no sympathy

Heres another tip: stop driving on low profiles

>but they were visibly under inflated at that pressure
radial tyres squat slightly at proper pressures. the deformation wasnt due to underinflation. go let some air out of your tyres right now

Holy fug

> TFW I didn't know not to be so close to the psi limit

>Hear someone say you can be 3 psi below the max
> Not know they meant the factory recommendation not the tires psi
> 51 max psi
>Inflate to 48 psi
> Drove to Miami for job interview
> Drove from Wellington 1 1/2 hours to and from
> Didn't get the job
> Tires are perfectly fine
>TFW I could've had a blowout in 95 degree Florida weather

I figure the max psi is what it can take under ideal conditions when new
then above that is factor of safety
older things could take 3x peak load vs expected load
now days expected load and peak load are much closer together
so when you hit a bump the tire can not deal with the over pressure

>TFW I could've had a blowout in 95 degree Florida weather
you have spare tire ???

...

my car on has r17 tires on it side doors rated at 39/36psi, im using r16 tires, what psi should i be running?

im running right now 35/33psi

car is audi a3 sportback quattro 2009