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Massive Tire Pressure Differences: Two Door vs. Four Door. Why?

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#1
I'm the owner of a new, white, two-door Fiesta ST (I live in New Zealand), and after checking the owner's manual and reading various posts on-line from US owners of the four-door, I've noticed a huge difference in tire pressure recommendations for the two vehicles.

Ford recommends 39F/36R for the four-door, while for the two-door they recommend 36F/26R(!). I know there are some differences between the two (the two door is a hair lighter, and the US suspension was, apparently, tweaked a bit to pass US pot hole handling requirements), but they just don't seem different enough to justify such a massive (10 PSI) difference in pressure.

26 PSI also just seems insanely low to me, even for the rears. I've never run anything below 36 PSI before, and the idea of 26 PSI has me paranoid about pot holes and rim damage. But then surely Ford tested these out and know what they're doing? Anyone have the scoop? Any other two-door owners out there? What pressures are you running?
 


D1JL

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#2
The difference could be because of the design characteristics.
I am sure you can understand that the 4 door is designed to carry passengers (extra weight) in the rear.
The USDM car is rated as a sedan and not a sports car so as to help keep the insurance rates lower in the US.



Dave
 


JBO

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#3
When I took delivery of my 3 door in Australia the dealer had all four tyres at 42psi which was absolutely ridiculous. The ride was terrible.
I now have them set at 36psi front at 27psi back and the ride is way better.
I went for a very spirited drive in the hills yesterday and the lower tyre pressure did not impact the handling one bit. Never had any tyre squeal, grip and turn in was insane.
I was contemplating some Koni STR.T shocks to fix the low speed ride (and I still might get them to remove the low speed jiggle) but low rear tyre pressures on the three door are there for a reason - they work!
 


Rhinopolis

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#4
I have always kept the recommended 39/36 tire pressures in the FiST, and am very happy with how the car handles as I have it setup at present (rear sway bar, rear seats removed, trunk brace, mid brace, 4 point chassis brace, FSTB, lighter weight & wider wheels + tires).

In the mx5 that I had before this FiST I decreased the rear tire pressures by 3 psi and the fronts by 1 psi, because after lightening the car up + adding power and chassis braces, the rear would lose stability and slide around corners too easily. Long end short of my post is that I am very picky about tire pressures and handling feel, but what Ford suggest on the U.S. FiST tire pressure wise, seems perfect to me.
 


D1JL

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#5
I too am a firm believer in using the recommended tire pressures as on the door post.
A lot of factors go into these numbers.
Safety as well as tire longevity and gas mileage.
The last two have a lot to do with the cost of ownership of the car and play a big factor in overall sales.

I do however, alter the pressures on track days so as to compensate for additional temperatures.


Dave
 


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Kansas City
#6
Lower psi in the rear on a FWD...interesting

I run 34 on all 4 corners. Stock Potenzas, stock suspension, all Pierce braces. Any higher than 34psi and it rides like complete crap. Crashing on everything at all speeds.
 


frankiefiesta

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#7
I would play with the pressures to find what suites your tastes best. I prefer 32-34 all around every day use. When I would go to the drag strip I would lower the fronts to 22-25 and inflate the rears to 40
 


dyn085

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#8
I run 44f/40r for increased tire longevity and fuel mileage. The ride from the tires doesn't bother me, the OEM suspension sucks though.
 


no-pistons

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#9
I ran the US spec tire pressures for 25k miles on OEM tires and the centers of the tread wore down a lot more compared to outer portions of the tread. This leads me to believe that it's too much pressure for the weight of the vehicle.
 


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#10
I run 44f/40r for increased tire longevity and fuel mileage. The ride from the tires doesn't bother me, the OEM suspension sucks though.
But that's just it...your tire pressure is a part of the suspension. The OEM suspension can suck less with an adjustment to the pressure. In my case, less than recommended psi helps mine.
 


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#11
This sounds counter intuitive but tire pressure works opposite of the way you think on grip.

In general as long as your tires aren't squishing (too low pressure) or crowning (too much pressure) more pressure equals more grip. So stock pressures listed as they are is done for a reason. More pressure in the fronts reduces understeer while less pressure in the rears promotes oversteer. This can all be altered by going to different tire construction.

My first guess for differences in pressures between Euro/SEA markets and USDM/NA markets would be differences in weight of the vehicle and differences in tire construction between markets. Possibly a difference in desirable handling characteristics across cultures as well.
 


RAAMaudio

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#12
I agree with you AM....so just adding a bit more:)

The US car spec is probably geared to expectations of carrying more weight in the rear with the easier ingress for passengers and 26 PSI could cause overheated tires quite easily.

Then we have to take into consideration fuel mileage requirements fleetwide, higher pressure means less fuel used.

You can buy a digital infrared meter for low cost which can be used to dial in the temps for most driving, even across the tread is the goal.

Until you have one take a drive, put your palm on the outside, middle and then inside of the front then rear tires as quickly as you can and feel the difference.

You can do this with after an autocross run, shorter mountain run, commute drive, etc to get an idea of what is best for each situation.

For a road race track it is best to use a probe that goes deeper into the tire as the heat has more time to soak into it.

As for fuel mileage gains the higher the better but if the tires measure hot in the middle they are going to wear out faster so you have to calculate tires costs into the equation.
 


OP
K
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Thread Starter #13
OK, so it sounds like it would be worth trying Ford's suggested pressures. When I picked the car up last month, the dealer had put in 32f/29r, and I bumped it up to 36f/34r straight away (my last car was always 38f/36r on 050A and S001s, so even this seemed low). I'll try 36/26 for a few days and see how it goes.
 


dyn085

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#14
But that's just it...your tire pressure is a part of the suspension. The OEM suspension can suck less with an adjustment to the pressure. In my case, less than recommended psi helps mine.
You're exactly right, which is why I add pressure. I don't want my tires, the most pliable component in the suspension, absorbing the impact that should be absorbed by the suspension. This is already a soft-sidewall tire, which means that the ride would be just as brutal with a tire that has a stiffer sidewall and lower pressure.

Tl;dr-Just because the suspension sucks doesn't mean that I need to sacrifice tire longevity and fuel mileage, and compensating for the suspension through the use of lower pressures only masks the true problem-and it does it at the added cost of earlier tire replacement and lower fuel mileage.
 


RAAMaudio

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#15
As in all things there is a point where gains become nil or things start to go downhill. Once past a certain point adding air will start to reduce grip do to the tire becoming crowned in the middle or just to stiff for the tire to be compliant enough to maintain contact with the road surface.

Push the car hard and the tires heat up, if starting at a higher pressure they will get to high pretty easily so pay attention to that issue when you go out for a fun run up a canyon, etc.

It is common for autocross and road course use to take tread temps and measure adjust air pressure after each run and usually reduce pressure to maintain the best grip.

You can also use chalk on the sides right by the tread for a visual reference as to how far the tire is rolling over to get the base pressure set just right on the first run or two then adjust as needed.
 


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#16
[MENTION=930]dyn085[/MENTION] - I hear you but I tend to side more with what [MENTION=636]RAAMaudio[/MENTION] is saying due to what I've learned over the years (some Solo II stuff in my early years)
 


dyn085

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#17
To a degree, but plenty of what's being said is wrong. I'm not one to tell others how to set up their cars, nor am I the deciding factor in what others are comfortable running. Run what you want, I only stated why I chose to run what I want.

With that being said, inflating tires doesn't only cause the center to inflate. I've been running those pressures from the beginning out of habit, and my tire wear is even across the tread.

Secondly, starting with a higher pressure is better regarding temperatures because it keeps the tire from flexing as much, keeping temperatures down. The tires that blow on the interstate are tires that are under-inflated, not the opposite. This is covered ad nauseam online.

Finally, many racers due check their pressures religiously and change them often during runs. Most of that is due to maintaining consistency and to help tune the suspension for specific handling characteristics. Very little concern is given to longevity, comfort, or mileage in those scenarios.

When my tires were at OEM pressures the sidewall roll was immediately apparent and not what I wanted to feel when turning-in. That's personal preference. I wouldn't be comfortable running pressure in the 20's on a daily basis, even in the rear, and more especially if I ever had passengers or added weight in the hatch (which happens often due to having kids).
 


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Location
Hastings
#18
When I took delivery of my 3 door in Australia the dealer had all four tyres at 42psi which was absolutely ridiculous. The ride was terrible.
I now have them set at 36psi front at 27psi back and the ride is way better.
I went for a very spirited drive in the hills yesterday and the lower tyre pressure did not impact the handling one bit. Never had any tyre squeal, grip and turn in was insane.
I was contemplating some Koni STR.T shocks to fix the low speed ride (and I still might get them to remove the low speed jiggle) but low rear tyre pressures on the three door are there for a reason - they work!
Just had my tyre pressure warning go off this morning - (2015 3 dr model) - had them checked and, like you, the pressures were stupidly high as set by the dealer. Brought them down to reasonable and the low speed ride is so much better. Tyre wear will be more sensible too.

Wonder if this is an Aus/NZ thing with the dealers and over inflating?
 


JBO

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#19
Just had my tyre pressure warning go off this morning - (2015 3 dr model) - had them checked and, like you, the pressures were stupidly high as set by the dealer. Brought them down to reasonable and the low speed ride is so much better. Tyre wear will be more sensible too.

Wonder if this is an Aus/NZ thing with the dealers and over inflating?
Apparently they do it so they don't get flats during storage/transport or some sh*t like that! Anyway, lower pressures have made a difference. Still considering STR.T shocks when the black friday sales start in the US…
 




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