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      03-06-2011, 08:17 AM   #1
tavoex29
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X3 Recommended Tire Pressure

Hi there. I just got my new X35i and have a question on the 'recommended' tire pressure.

Mine came with Pirelli's all weather P7 245/50/R18's. The manual and door sticker recommend 32 front/35 rear PSI for normal load. However when I picked up the car at the dealer it had 39F/46R. It rode fine but when I deflated the tires later to the recommended pressure I noticed the tires produced a bit more noise.

Wanted to hear other X3 owner's experience with their PSI

Thanks!
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      03-06-2011, 08:29 AM   #2
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I run mine at 4-5 psi over. I prefer the bit firmer set up that seems just a bit better absorbing edges and things during hard cornering. Or I like imagining that it makes that difference, either way it's good.
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      03-06-2011, 09:05 AM   #3
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keep in mind
higher air pressure = less grip
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      03-06-2011, 09:52 AM   #4
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Quote:
Originally Posted by kmarei View Post
keep in mind
higher air pressure = less grip
Whew! Now you've done it! What a can of worms you've opened!!!

My only comments on what always turns into a long and opinionated debate on every car forum:

IT DEPENDS!

It depends on the actual pressure: Too low pressure reduces grip, increasing it increases grip and reduces slip-angle up to the point of carcass deformation when it then begins to reduce grip (usually at very high pressures).

It depends on the road surface; optimum "grip" occurs at a different pressure on snow or sand than on asphalt.

It depends on the static loading of the vehicle and will typically be different for the front and rear tires.

It depends on the driver's preference for a car that has some understeer "push" or oversteer "loose"

It depends on the vehicle's polar moment of inertia

It depends on suspension geometry

It depends on damper dynamics

It depends on specific tire internal construction

It depends on tire temperature

It depends on road surface temperature

It depends on many other factors.

IT DEPENDS!
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      03-06-2011, 10:12 AM   #5
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LoL sorry about the question but I 'get it'

Thanks for your replies
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      03-06-2011, 10:36 AM   #6
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Quote:
Originally Posted by Lotus7 View Post
Whew! Now you've done it! What a can of worms you've opened!!!

My only comments on what always turns into a long and opinionated debate on every car forum:

IT DEPENDS!

It depends on the actual pressure: Too low pressure reduces grip, increasing it increases grip and reduces slip-angle up to the point of carcass deformation when it then begins to reduce grip (usually at very high pressures).

It depends on the road surface; optimum "grip" occurs at a different pressure on snow or sand than on asphalt.

It depends on the static loading of the vehicle and will typically be different for the front and rear tires.

It depends on the driver's preference for a car that has some understeer "push" or oversteer "loose"

It depends on the vehicle's polar moment of inertia

It depends on suspension geometry

It depends on damper dynamics

It depends on specific tire internal construction

It depends on tire temperature

It depends on road surface temperature

It depends on many other factors.

IT DEPENDS!


You strike me as a bit of a track rat.
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      03-06-2011, 10:41 AM   #7
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Quote:
Originally Posted by MA78 View Post


You strike me as a bit of a track rat.
Track rat, Who me?

I've never spent more than (3) hours in practice sessions tuning tire pressures. - I swear!
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      03-06-2011, 11:51 AM   #8
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Quote:
Originally Posted by Lotus7 View Post
Track rat, Who me?

I've never spent more than (3) hours in practice sessions tuning tire pressures. - I swear!
Hahaha

I thought as much.

I'll be buying a Caterham Roadsport next year. So I guess I'll be a track rat too.
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      03-06-2011, 12:22 PM   #9
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Quote:
Originally Posted by MA78 View Post
Hahaha

I thought as much.

I'll be buying a Caterham Roadsport next year. So I guess I'll be a track rat too.
You will either love it or hate it, which ever way it takes you, your perspective on what it means to DRIVE a car will forever be altered. You've been warned.

Didn't mean to hijack the thread. Sorry
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      03-06-2011, 12:31 PM   #10
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Quote:
Originally Posted by Lotus7 View Post
You will either love it or hate it, which ever way it takes you, your perspective on what it means to DRIVE a car will forever be altered. You've been warned.

Didn't mean to hijack the thread. Sorry

Already driven one many many times. Love it.
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      03-06-2011, 02:13 PM   #11
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Quote:
Originally Posted by Lotus7 View Post
IT DEPENDS!
well that arguement relates to every discussion on this forum
but in regards to this question
on the X3 if you should put the stock air pressure or more
more pressure means less grip
since more pressure means smaller contact patch
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      03-06-2011, 02:55 PM   #12
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Quote:
Originally Posted by kmarei View Post
well that arguement relates to every discussion on this forum
but in regards to this question
on the X3 if you should put the stock air pressure or more
more pressure means less grip
since more pressure means smaller contact patch
Sorry - Don't want to be argumentative, but your logic is just too simplistic.

Grip depends on the uniformity of pressure across the contact patch (as measured by spot tire surface temperature measurements) and shape of the contact patch as much or more than the final size of the patch. At low pressures, which usually include the factory recommended pressure, which is picked for the best ride/ handling compromise the ultimate grip (or cornering power, or lateral acceleration-however you want to define it) is usually not achieved. Most passenger cars will increase cornering grip as pressure is increased above the factory rec. setting for a while and then loose grip as the contact patch deforms and becomes smaller.

It's not just the number of square inches of contact, but the amount of contact pressure the vehicle is able to apply to each of those square inches of contact patch. the idea is to get the most uniform pressure on the most square inches. Having a slightly larger contact patch does not help if the pressure on EACH part of it is not the maximum possible. At stock or lower pressures that's what happens during moderate to hard cornering.

Anyone who has ever raced on a track or participated in an autocross will usually wind up with pressures substantially above the factory rec. settings if they want any chance of being competitive.

This is well documented in race oriented magazines such as "Grassroots Motorsports" where they routinely do tire tests on various chassis, as well as every basic book on how to improve your cars handling for competitive purposes. "Grassroots Motorsports" last comprehensive test on a Miata wound up using pressures in the 41 to 46 psi range for a car that has a factory rec. pressure of 32 psi. This was for a autocross at the TireRack's test track in Indiana.

Personally, EVERY street car and race car I’ve driven, road raced or autocrossed from my full race SCCA A-Modified Corvette and Lotus S-1 to my wife’s Subaru wagon has had significantly higher grip as well as better transient response at a tire pressure higher than the factory recommended settings. Sometimes it’s only a few psi more and sometimes it’s as much as 10 psi. I’m speaking about ultimate dry road grip independent of ride quality.

Plus, I'm not even getting into what happens to the tire patch shape at stock or lower pressures because of sidewall deflection lifting the tread and reducing the uniformity of pressure distribution across the patch.

Last edited by Lotus7; 03-06-2011 at 03:10 PM.
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      03-06-2011, 04:23 PM   #13
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Lotus you are amazing. So let me ask a simple question in hope of a technical answer. Convenience an blow out safety aside, what does the above mean for run flats and would you advocate them for all X3 rims? Issues for me are mainly ride, handling & mpg. Thanks.
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      03-06-2011, 06:30 PM   #14
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Quote:
Originally Posted by X3buyerScotland View Post
Lotus you are amazing. So let me ask a simple question in hope of a technical answer. Convenience an blow out safety aside, what does the above mean for run flats and would you advocate them for all X3 rims? Issues for me are mainly ride, handling & mpg. Thanks.
35 years of trying to find optimum tire pressures on racecars has taken its toll. Sometimes the setting that makes the car handle the way you want or the way it handled last week is not possible to find. Sometimes it depends on the phase of the moon. Hence my cop-out: "It Depends".

Am a novice re: Runflats. This is the first car that I've had with them. All I know is that the first and second generation runflats had sidewalls that transmitted a lot of road shock. The 2010-2011 versions (Gen. 3) are supposed to be more "shock-absorbing". Personally, I don't really like the idea of driving on a uninflated tire, so I've already equipped our 35i with a "on-the -rim" tire repair kit. A factory BMW toolkit (BMW Europe calls it "The European Driver's Utility Set" that fits perfectly in the left rear cubby hole under the rear side panel), and a small, but reliable 12volt air compressor. If I get a flat while driving, the plan is to pull off the road as soon as possible, find the nail or other puncturing object, remove it, install a fiber/rubber plug, and inflate the tire and drive off.

I find the X3 ride (with P7 tires) to be excellent, in fact it rides better than our Subaru LGT with Goodyear GSD3's or our old Audi A4 with Dunlop summer tires, so I have no complaints regarding the X3 ride, even in the VDC Sport Mode. BMW also claims that all of the run-flats fitted to X3's are designed for extra low rolling resistance (for better fuel efficiency). I suspect that is a function of the sidewall hysteresis coefficient (which should be low) and the tire rubber compound Shore Durometer hardness which is higher for a low-resistance tire. The ďSummer P7Ē has a DOT wear rating of around 200 and "All-Seasons P7" has a rating of 500 implying harder, slower wearing rubber, lower rolling resistance, with less ultimate grip. High performance summer tires will undoubtedly give slightly more cornering grip, but on a high center of gravity SUV like a X3, that's not always something you really want or need.

Sometimes it's better to let the back end drift out or the front end slide a little wide (and let the stability control take care of it) than to have enough cornering grip that the inside tires lift and the car gets tippy. The X3 is not a M3 and it's a mistake to try to make it into one. I have a 5-point harness in the Lotus and a pair of rollover hoops. No such protection in the X3. ..........................................yet
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      03-06-2011, 08:42 PM   #15
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Quote:
Originally Posted by Lotus7 View Post
Anyone who has ever raced on a track or participated in an autocross will usually wind up with pressures substantially above the factory rec. settings if they want any chance of being competitive.

This is well documented in race oriented magazines such as "Grassroots Motorsports" where they routinely do tire tests on various chassis, as well as every basic book on how to improve your cars handling for competitive purposes. "Grassroots Motorsports" last comprehensive test on a Miata wound up using pressures in the 41 to 46 psi range for a car that has a factory rec. pressure of 32 psi. This was for a autocross at the TireRack's test track in Indiana.

Personally, EVERY street car and race car Iíve driven, road raced or autocrossed from my full race SCCA A-Modified Corvette and Lotus S-1 to my wifeís Subaru wagon has had significantly higher grip as well as better transient response at a tire pressure higher than the factory recommended settings. Sometimes itís only a few psi more and sometimes itís as much as 10 psi. Iím speaking about ultimate dry road grip independent of ride quality.
not sure how anything to do with taking a car on a track has any relevance a SUV driven on the road
plus 10psi more means if you are driving on a hot day on hot asphalt, your tires could increase in psi due to heat, above the 51max psi
i've seen an 8psi increase from the start of a journey and 30 minutes later after driving around 65-70mph on a highway
8psi increase if you set your tires at 46 means you are at 54psi
that's 3psi more than the maximum tire pressure set by the manufacturer

i don't feel i can trust your judgement in a car that carries my entire family
you can put what makes you feel more racey (in an suv)
but the recommended psi is put there for a reason
you wanna go higher be my guest

and that's not even taking into account the shape of the contact patch or how uneven the wear is going to be on your 46psi tires.

not everything learnt on the track is relevant in the real world.
we don't change our tires after 1 or 2 track events
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      03-06-2011, 09:40 PM   #16
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All of my comments were in response to your statement "higher pressure = less grip".

The point was ONLY that increasing the internal pressure can, and usually does increase grip. Which is in direct opposition to your statement. Nothing more and nothing less.

The issue here is only about making a blanket statement that is simply not correct. The examples mentioned were documented, provable facts that contradict your blanket statement. I'm truly sorry that I am not able to convey this concept in a manner you can understand.

Obviously running 46 psi in a X3, for most regular driving conditions would probably be a bad idea. The example was for maximum grip for a Miata on a racetrack.

However, for a driver who wanted to trade a little ride comfort for increased transient capability (which could result in a car that was actually safer to drive in typical urban traffic.) adding a few psi of pressure could be an appropriate solution, and that driver would not loose grip, but could actually gain some. That is an easily documented fact.

For YOUR X3, I would absolutely stick with the factory recommended pressures, which incidentally should be increased (for the stock 245/50-R18 tires) to as much as 44 psi (cold) depending on load and expected speed.*

*Reference: X3 Owners Manual pp. 209

This is my last contribution to this “can of worms” thread.
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      03-06-2011, 09:45 PM   #17
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Quote:
Originally Posted by Lotus7 View Post

However, for a driver who wanted to trade a little ride comfort for increased transient capability (which could result in a car that was actually safer to drive in typical urban traffic.) adding a few psi of pressure could be an appropriate solution, and that driver would not loose grip, but could actually gain some. That is an easily documented fact.
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      03-08-2011, 07:54 AM   #18
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Not to be redundant to points above, and I concede that IT DEPENDS is the best answer really. A couple of points I consider when setting my pressure on my non-track drivers:
Higher pressure = better gas mileage. This is quite noticable too. Higher pressure equals a firmer tire, less rubber on the road and less friction. These days, not a bad consideration.
Higher pressure = better and more even tire wear generally. Also less stress on the corners and sidewalls.
The reason for the 32/35 typical recommendation is to provide a certain amount of understeer. I inflate to about +3 PSI most of the time and that is my personal preference. Some people like to run even pressures all around... depends on the handling you like.

I would say experiment and see what you prefer in terms of handling and mileage. But I would never recommend running lower than prescribed numbers.
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      08-15-2012, 12:49 PM   #19
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X35 tire pressure

Quote:
Originally Posted by tavoex29 View Post
Hi there. I just got my new X35i and have a question on the 'recommended' tire pressure.

Mine came with Pirelli's all weather P7 245/50/R18's. The manual and door sticker recommend 32 front/35 rear PSI for normal load. However when I picked up the car at the dealer it had 39F/46R. It rode fine but when I deflated the tires later to the recommended pressure I noticed the tires produced a bit more noise.

Wanted to hear other X3 owner's experience with their PSI

Thanks!
I just picked up my new X35 at the factory (great experience) and checked the tire pressures the next day. Same Pirelli P7 tires as on your vehicle. Found them to be higher than the 32 F, 35 R recommended in the manual for speeds under 100 mph so I reset them to those pressures. Found the handling was not as good, so I contacted my service advisor and he recommended I inflate them to the over 100 mph pressures, 38 F and 44 R. I did that and handling has improved, but I can now feel the imperfections in the road more, unless I am on a smooth surface.
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      09-21-2012, 09:55 AM   #20
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Time to check the pressure in those tires. It has cooled down considerably here and when I checked my tires last night they were all down by 4 pounds.
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