Results 1 to 31 of 31

Thread: Nitrogen filled tires; Anyone ...? Opinions..?

  1. #1
    Team Z8 RRZ8's Avatar
    Join Date
    Nov 2006
    Posts
    417

    Nitrogen filled tires; Anyone ...? Opinions..?

    I was wondering if some of you guys have their tires filled with Nitrogen.
    My Z8 had it when I bought it, did some reading on the Internet, but am not sure if I will fill them with Nitrogen if they need to be inflated. What do you guys think ?
    (by the way; I drive with the Z8 original RFC Bridgestones).

  2. #2
    The biggest reason to use nitrogen is because it doesn't expands/contract in fast heat cycling at the track, where tire temps can go from ambient to 220º+ in just a couple of laps, and thus increase the air pressure exponentially.

    I can tell you from my own experience that my tires at the track can pick up 10psi in 10 flying laps, and the first two sessions of the day are always spent getting the hot temps bled down to about right, but you spend all day bleeding off a pound here and there.

    Here is some info I found on the web, can't vouch for it, but it does have a lot of info in it.

    Selling Nitrogen In Tires - Is It All A Lot Of Hot Air?
    By James Burchill

    Actually... no. But it will require a little explanation and some basic chemistry discussion so here goes -

    The earths atmosphere is composed primarily of Nitrogen (78%), the remaining balance is a mixture of Oxygen (21%) and a small percentage of Ozone, Argon and Carbon gases. Isn’t it ironic that the very gas (oxygen) that sustains life also causes organic materials to decompose and metals to rust?

    Remove The Oxygen And Stop The Rot. To stop this decomposition you simply need to remove the oxygen from the ‘air’. (Incidentally, removing oxygen also means that you remove water because water is two parts Hydrogen gas, and one part Oxygen gas.) Removing Oxygen from products is not new, we do it all the time with our food and drink. In fact, if we did not remove the oxygen our food would not likely last long enough to make it into the hands of the consumers – or it would taste stale and unappealing.

    Nitrogen In Tires
    Here are a few other benefits of using Nitrogen in tires:

    [1] Nitrogen is denser than Oxygen: This means the larger molecules escape less easily from tires resulting in a more gradual loss of pressure over time. According to the Michelin Tire Manual, a tire that is inflated with Nitrogen loses its pressure 3 times slower than if it were inflated with air.

    [2] Nitrogen is moisture free: Pure Nitrogen inflated tires experience less steel belt and rubber degradation. Nitrogen use also reduces valve and wheel corrosion.

    [3] Nitrogen provides longer tire life: Nitrogen inflated tire run cooler and require less maintenance according to the Goodyear application bulletin.

    [4] Nitrogen is non-flammable: Nitrogen technology has been used in aircraft, military and race car technology for over thirty years.

    Should we start paying for this new ‘air’ anytime soon? As always, the answer is “it depends.” If enough businesses and corporations get behind the idea and promote it steadily, uniformly and explain the numerous safety and cost savings, then eventually the message should make it through.

    The other point to consider here is ‘selling Nitrogen’ is still science – and most people are not that comfortable with the whole science thing. And if you recall I mentioned that regular everyday air is 78% nitrogen anyway – so what would they be paying for? Are we to believe that an extra 20% makes all the difference? Ironically we now know that the answer is yes.
    Andrew Macpherson

    Expert Z8 Inspections, with full support for both Z8 sale and purchases.

  3. #3
    My recollection is that that Nitrogen bleeds off more slowly, because the particles are larger.
    This is a good site for information and where to get Nitrogen: http://www.getnitrogen.org/

  4. #4
    Team Z8 RRZ8's Avatar
    Join Date
    Nov 2006
    Posts
    417
    So, Andrew you don't use N2 filling in your Elise (at the track) ?
    As the article said; 'normal' air is already almost 80% Nitrogen, I am starting to think that the 'extra 20%' N2 is not really worth the extra effort (and money).
    Where I live (The Hague area, The Netherlands) there are about 6 stations that can fill with N2, costing about 10 euro's per tire.

  5. #5
    At the track the Elise likes hot 25psi front & 26psi rear, but for the drive home you want at least 32-36 hot, so you have to top up with regular air at the end of each track day, so it all just seems like another thing to deal with. I'm pretty happy with air because I know what to expect, and it is easy to deal with. On the Elise my tires only last two or three days max, so there is no long term benefit to it, but if I was racing and had a team to deal with it I'd go with nitrogen. .
    Andrew Macpherson

    Expert Z8 Inspections, with full support for both Z8 sale and purchases.

  6. #6

    My own experience . .

    Even with Nitrogen-filled tires on my 330xi - when I track it, the tire pressure still goes up by a good 5 or 6 psi. Cold temp is 37 psi & it always goes up to 42 or 43, (depending on sun or rain & pavement temperature). I bleed it down to 40ish after the first session, which is still a bit high, but there is less sidewall use this way. I suppose that if it were just a regular air mixture, the pressure would be even higher.
    I swap wheels/tires at a local tire garage who store the extra set & keep them inflated w/nitrogen free-of-charge.
    Evie

    Andrew - You are right as usual, & I'll have to decrease the pressure. I'll start by 2 psi the first time out, then decrease by 1 each time after that & see how it goes. I'll try to finish with ~ 36 or 37 hot. I'm not as knowledgeable as you are on this subject so I'll take your lead & ask my racing guru next time out (since vehicle weight might be a factor).
    Last edited by fun2drive; March 14th 2007 at 15:42. Reason: updating

  7. #7
    Z8 Novice
    Join Date
    Nov 2006
    Posts
    22
    I'm running on Nitrogen with the Bridgestone Pole Position and very happy about it.
    The car is more silent, don't know much about the rest.
    Kind regards.
    Stephane

  8. #8
    Team Z8 RRZ8's Avatar
    Join Date
    Nov 2006
    Posts
    417
    Quote Originally Posted by forstephan View Post
    I'm running on Nitrogen with the Bridgestone Pole Position and very happy about it.
    The car is more silent, don't know much about the rest.
    Kind regards.
    Stephane

    Stephane, I personally think that the Bridgestones PP do 99% of the job.

  9. #9
    Z8Mania
    Guest
    I agree. I find the benefit of the nitrogen is in the more extreme situation. Our RR dealer put nitrogen in my wifes tires and there is no difference in terms of ride quality or noise but I have checked the psi a few times now and they are holding nicely.

  10. #10
    Sport Button On - DSC Off
    Join Date
    Aug 2007
    Posts
    182
    The secret to getting the full benefits of using nitrogen to fill your tires is making sure they are filled with pure nitrogen and nothing else. Nitrogen is non-hygroscopic which means it does not absorb moisture. So if your tires are filled with nothing but nitrogen, there won't be any moisture in them. Since it is moisture that causes most of the pressure rise when tires get hot, running pure nitrogen keeps tire pressures relatively stable. The trick is to get all of the old air out and use lab grade nitrogen which is the only kind that does not have any moisture contamination. Top race teams vacuum all of the air out of their tires before filling them up again with pure nitrogen. Another benefit to running nitrogen (pure or not) is that the moisture content inside the tires will definitely be lower than if they were filled with air and since moisture promotes oxidation within the tires, they will last longer. Also, nitrogen's molecules are larger than oxygen's molecules, so a tire filled with nitrogen is less likely to lose pressure over time. Frankly, for most street applications, nitrogen filling is probably not worth the effort. Done properly, it can make a very big difference under race and track day conditions.

  11. #11
    Team Z8 RRZ8's Avatar
    Join Date
    Nov 2006
    Posts
    417
    Right, so in conclusion we can state that in our Z8's Nitrogen filling is obsolete ?

  12. #12
    Sport Button On - DSC Off
    Join Date
    Aug 2007
    Posts
    182
    That depends. If you plan to track your Z8, I think filling the tires with nitrogen is worth considering. Also, if you are trying to preserve your Z8 in its original configuration, i.e., keeping its original tires in place for as long as possible, then filling them with nitrogen is probably a good idea to help prevent deterioration. For typical street use where you drive your Z8 fairly regularly and plan to replace the tires on a periodic basis, I can't see any real benefit to using nitrogen except for the lower pressure maintenance requirements it will provide. Having said all that, I use lab grade nitrogen in my Z8's tires because I want to maintain the most consistent tire pressures possible during spirited driving. I spent a great deal of time and effort to determine the best tire pressures for my particular set-up and keeping those pressures as close as possible to that level insures maximum handling performance. Lunatic fringe, no doubt, but I like chasing perfection.

  13. #13
    Team Z8 RRZ8's Avatar
    Join Date
    Nov 2006
    Posts
    417
    Thanks Bruce ! I will keep it in mind

  14. #14
    Z8Mania
    Guest
    Great points Bruce- but lets face it- if you want to keep the original parts pristine but still drive your Z8- the best advice is to swap out those heavy run flats for something that doesnt put as much stress on the vehicle. maybe shrink wrap the original tires for that day in 50 years at Pebble...........

  15. #15
    Sport Button On - DSC Off FWK-Z8's Avatar
    Join Date
    Jan 2006
    Posts
    229
    There is a lot of science here being bandied about that simply is not true.

    Away from their critical temperatures and pressures (126.2 DegK/-147 DegC/33.5 atm for N2 and 154.4 DegK/-119 DegC/49.7 atm for O2). gasses obey the ideal gas law PV=nRT. This negates the statement made by the "expert" Mr> Burchill that "Nitrogen is denser than Oxygen" in fact with a moecular weight of 28.02 for N2 and 32.00 for Oxygen, at the same pressure and temperature O2 is 14% denser than N2. Also clear form this is that the rise in pressure is linear with temperature, not exponential as stated by Andrew. BTW - are you sure you don't mean 220 DegF - 220 DegC is much too high for rubbers other than Kalrez.

    As far as nitrogen being moisture free, it has nothing to do with the affinity of either O2 or N2 for water, but rather in the way N2 is produced from air. N2 has a lower boiling point than does O2 (77.3 DegK vs 90.2 DegK). Air is the source for both N2 and O2, as it is roughly 80% N2 and 20% O2. By compressing and cooling air, you can reach the point where O2 liquifies, leaving gaseous N2 - of course this occurs at a temperature by which point all of the water vapor has frozen out. Compare this to compressing air to fill tires where compression lowers the saturation point of water, and liquid water spits out of a compressed air line unless a dryer is added (and maintained) in the system. In any case, at 30 PSI half of the gas in your tires will still be air - the air that was in your tire when you first mounted it on the wheel. Figure that each time you inflate your tires to 30 PSI and let them deflate to ambient, you will be changing half the gas in the tire. You would have to repeat this sequence five times to get the amount of oxygen below 1%, and you would still not be able to get the liquid water out, which is the real culprit

  16. #16
    Fred, I didn't see any mention of centigrade, and I believe that my use of the word exponentially is correct in this context.

    exponentially.
    a. the constant e raised to the power equal to a given expression, as e3x, which is the exponential of 3x.
    b. any positive constant raised to a power.
    Andrew Macpherson

    Expert Z8 Inspections, with full support for both Z8 sale and purchases.

  17. #17
    Z8Mania
    Guest
    You guys lost me (I'm not that smart ). Our local Land Rover dealer put N2 in my wifes tires and gave us all the lines about how great it is and I just bit my tongue and went about my business...

  18. #18
    Sport Button On - DSC Off
    Join Date
    Aug 2007
    Posts
    182
    The trouble with relying on basic scientific equations/laws to explain why nitrogen in tires wouldn't leak out more slowly is that the process is far more complicated than you might think. While the diffusivity of O2 and H2 are essentially the same, we must consider their solubility relative to the ingredients in a tire before we can determine the actual permeability of the gases within that tire. For most materials, solubility of the gas is more important than diffusivity in determining the rate of permeability. While diffusivity is relatively constant (given equal pressure and temperature), solubility is affected by the polarity of the molecules in both the gas and the tire materials. This attraction/repulsion plays a major role in determining the rate at which a given gas will travel through the matrix of a given material. Another factor we must consider is the effect of pressure on the gas molecules, with higher pressures resulting in higher mobility. Given that tires filled with air, which contains water, will run at higher pressures than tires filled with nitrogen, this too contributes to the difference in observed leakage rates. Obviously, the explanation relying on molecular size is inaccurate but may be forgiven given the complexities involved in offering a technical explanation to lay readers. I can offer an example which might help illustrate the point. Using relative numbers, the diffusivity of helium is 43, oxygen is 16 and nitrogen is 15. For a given material, in this case dimethylsilicone rubber, logically we would expect the helium to leak through its membrane much faster than oxygen or nitrogen. However, because of the complex relationships mentioned above, it turns out that the permeability of helium through silicone is 65, for oxygen it is 62, and for nitrogen it is only 28.

    As far as water content in the gases is concerned, the reason race teams use pure nitrogen to fill their tires is because it is non-hygroscopic and can be injected in a pure form. Air, because of its oxygen content, is hygroscopic and will contain water molecules which increase the pressure within the tire to a higher level than tires filled with pure nitrogen, given similar temperatures. That rise is also non-linear in that the major pressure increase comes from the water reaching its boiling point, not just heating up. Filling their tires with pure nitrogen (no moisture content) means the teams can start their cars out with higher cold pressure settings because the overall rise in pressure will be less and that rise will be more consistent. Trust me, if this weren't the case, the F1 teams, who are known for their engineering prowess, would not use it. This is a real advantage on a race track for those early laps and it is that same relative consistency of pressure that makes using pure nitrogen in my street car attractive to me.

    As I stated earlier, the secret to getting the full benefits of filling your tires with nitrogen is to purge all of the air from them before refilling with pure (lab grade) nitrogen. This can be done inexpensively at home by using a shop vac with a bleeder valve and a piece of rubber vacuum hose.

    While the benefits of nitrogen vs. air may not be intuitive on the surface, when you factor in the actual process involved in maintaining pressure in automobile tires, it turns out that nitrogen does have its advantages, including a lower leakage rate and a lower pressure rise with temperature increase. When you add in a reduction in wheel and tire degradation due to a lack of oxidation, perhaps filling tires with nitrogen isn't such a bad idea after all.

  19. #19
    Team Z8 RRZ8's Avatar
    Join Date
    Nov 2006
    Posts
    417
    Bruce, are all () your Z8's filled with N2 ?

  20. #20
    Z8Mania
    Guest
    My F dealer just did the nitrogen fill on my 430 tires and they vaccu sucked out all the air first and then shot the N2 in there.... I thought the vaccu suck thing was cool!!! (No dirty comments please!!! )

    Hope everyone celebrating it had a great 4th of July!

  21. #21
    Sport Button On - DSC Off
    Join Date
    Aug 2007
    Posts
    182
    Yes, I use lab grade Nitrogen in all of my Z8s' tires. I have a large bottle of it in my shop and use it whenever I need to adjust the pressures. And, like Z8Mania's Ferrari dealer, I do the initial fill after vacuuming out all of the air.

  22. #22
    Team Z8 RRZ8's Avatar
    Join Date
    Nov 2006
    Posts
    417
    Update: I checked my tire pressure (since december 2006, when they were filled with N2...) and was suprised to see that the pressure had dropped from 2.4 bar tot 2.0 bar....Or is a drop of about 0,5 bar in 1,5 years quite normal ? (even with N2?)

  23. #23
    Sport Button On - DSC Off
    Join Date
    Aug 2007
    Posts
    182
    For those of us who haven't converted to the metric system yet, that's a drop from app. 35psi to 29psi in 1.5 years. In my experience, that would be typical for air filled tires and a little high for Nitrogen filled tires, but certainly not unexpected. Keep in mind, whatever you fill your tires with, the leakage rate will be directly influenced by the materials used in the tires and those materials vary from one type/brand of tire to another. There can also be leaks due to deterioration/improper seating of the valve stem seals and a history of extreme temps causing excessive pressure. Since your tire pressure monitoring system did not warn you of a problem during that time, it is safe to assume that the pressure drop you experienced was consistent throughout all 4 tires therefor just normal leakage. I would recommend checking those pressures more often since you really can't rely on the Z8's tire pressure monitoring system to warn you about unsafe pressures due to even leakage and 2.0 bar, in my opinion, is getting close to that level. Which brings up another subject near and dear to my heart: tire pressure gauges.

    Over the years, I've amassed a collection of differing types of air pressure gauges (and I suspect I'm not alone). Everything from old VDO analog style to hi-tech digital. All worked reasonably well but none ever agreed on the pressure being measured making me highly suspicious of their accuracy.
    As I became more sophisticated in my chassis tuning, and in particular began using a tire pyrometer to measure the temperature of the tire's working surface, I really wanted a gauge I could trust and that would provide repeatable, accurate results, down to 1/10 of a lb.

    Well, I finally found it! Made by Intercomp, this digital air pressure gauge is the very best available. It reads in PSI, BAR and Kg/CM2. Range in PSI is 0-100 with a readout of 0.1 PSI. It has a push button to zero the display which compensates for temperature and altitude variations, peak hold function, and a 1/2" tall backlit display. Here's the really trick part: accuracy is guaranteed to be within 0.5% of the applied pressure! Most other high quality (read expensive) 0-100 PSI gauges have a top accuracy rating of 1% of full scale which means they can be off by up to 1 lb. at any pressure reading. In other words, where the other gauges could read anywhere from 34-36 PSI when measuring a 35 PSI pressure, the Intercomp would read 34.8-35.2 PSI. In my own collection, some of the gauges were off by up to 5 PSI! And the Intercomp goes one big step further: every time you turn it on, the gauge goes through a self-check program which verifys its accuracy and if the gauge ever needs recalibration, it will alert you!

    To make it easy to use, the Intercomp gauge has a 20" hose with a 45 degree swivel nozzle and a thumb operated bleed-off valve which makes adjusting pressures much simpler. Plus, there are 2 versions available. #360045 Digital Air Pressure Gauge has all of the features described above. #360094 adds fill capability which I really like. That means you can hook your air supply right to the gauge and fill, bleed and read air pressure without removing the nozzle from the valve stem. Pretty slick! You can also use this gauge without an air supply so it covers all the bases.

    Now,all this accuracy and convenience doesn't come cheap. The #360045 gauge runs $295 and the #360094 gauge runs $395. One way to justify the expense involved, aside from peace of mind, is to use the Intercomp to calibrate the rest of your tire pressure gauges. Just note the difference in readings (you'll be amazed at the variations) and use that knowledge to compensate for the inaccuracy of your other gauges. Now you can have truly accurate gauges in all your cars without having to spend a fortune! Plus, your Intercomp gauge can be returned to the factory for recalibration if it ever needs it so it should be the last one you'll ever have to buy.

    Lunatic fringe? Yes, but life is short and then you die!

    Intercomp: 800-328-3336. www.intercomp-racing.com (Check out their weight scales while you're there; really nice!)

  24. #24
    Z8 Novice
    Join Date
    Jun 2008
    Posts
    7
    Here's the TSB from BMW regarding nitrogen;

    SI B 36 04 06
    Wheels and Tires

    April 2006
    Technical Service

    SUBJECT
    Nitrogen Inflated Tires

    MODEL
    All

    SITUATION
    Many customers have inquired about the use of Nitrogen, instead of air, to inflate tires on their BMW vehicles. Please review the information below to help answer some of the customer's inquiries.

    INFORMATION
    ^ Nitrogen, an inert gas, is nothing more than dry air with the Oxygen removed. The majority of "air" already consists of Nitrogen (78% by volume). The other elements of air by volume are 21% Oxygen and 1% other gases.

    ^ All BMW approved tires have been designed and tested to deliver their expected performance when inflated with "air" and the correct tire inflation pressure is maintained on a regular basis.

    ^ The use of Nitrogen to inflate tires is not recommended for normal use of BMW vehicles; however, the use of Nitrogen is also not prohibited.

    ^ There are certain applications where Nitrogen inflation is advisable, i.e. in abnormal environment where wheel and tire overheating may occur such as in aircrafts, space shuttles, military vehicles, off-road trucks, and race cars. In these applications, the non-flammable Nitrogen can reduce the risk of fire, because Nitrogen does not support combustion and does not add fuel to the fire.

    ^ The physical properties of Nitrogen may reduce the inflation pressure loss only from the tire material natural permeability (diffusion through rubber). However, similar to air, Nitrogen can still escape from other sources of inflation pressure loss such as from wheel, wheel/tire interface, valve, and valve/wheel interface.

    ^ The most important thing for the customers is to maintain the correct tire inflation pressure on a regular basis as recommended in the Owner's Manual. Under inflated tires can reduce road holding, increase hydroplaning risk, increase road hazards sensitivity, reduce tire life, increase fuel consumption, etc.

    ^ If equipped on the vehicle, always reinitialize the Flat Tire Monitor (FTM) or reset the Tire Pressure Monitor (TPM) after the tire inflation pressure has been corrected.

  25. #25
    Z8Mania
    Guest
    Great info Bruce- how do you vacuum out the tires at home? Any suggestions for a home N2 rig? Im going to check out that tire pressure gauge. I have no problem spending money for something that is worth while and having an accurate read of tire pressure is IMHO very important. Bruce, do you prefer the ball or angle chuck? In my experience I find I prefer the concept of the angle chuck but it sometimes will stick more to the valve stem and cause more air leakage when being removed. Some of these new car tires are truly rubber band- an excessive amount of air release upon removal will show up if you do a follow up reading. But the ball chuck doesnt seat as well and is sometimes more difficult to maneuver around. Thoughts?

    Steve,
    Thanks for that TSB from BMW. The translation is- you can use N2 if you want but we are not bothering with it and don't want to be bothered with questions and won't fill your tires up with N2 since we don't have the equipment.

  26. #26
    Sport Button On - DSC Off
    Join Date
    Aug 2007
    Posts
    182
    I let out as much of the air as possible by attaching a Griot's Garage Professional Tire Inflating Gun #92559 to the valve stem (it clips on so you don't have to hold it in place) and then squeeze the trigger to allow the air in the tire to escape. If you wrap a piece of wire around the trigger to keep it depressed, this becomes a hands-free operation. You can certainly use a different model tire inflation device as long as it allows you to release air pressure from the tire but I really like the Griot's unit and you can use it for normal air filling off a compressor so it's not a limited purpose purchase. Next I push a small rubber hose over the quick disconnect fitting (you must supply) on the Griot's unit and hook the other end to one of those "micro" attachments that come with Shop Vacs. Then I just let the Shop Vac suck out the remaining air until the tire's sidewall collapses. Now I release the trigger so the Griot's unit will maintain the vacuum within the tire and remove the chuck from the valve stem. Then I attach my trick Intercomp pressure gauge, which also has the ability to add or subtract gases, and is hooked up to my lab grade Nitrogen bottle and just fill'er up with the good stuff. One point: I like to release some Nitrogen from the Intercomp before attaching it to the valve stem to make sure that there isn't any air in the hose. This is essentially the same procedure used by the F1 teams and it provides a virtually pure Nitrogen fill. If you are really obsessive, you could repeat the procedure one more time and reduce the level of contaminants to a miniscule level.

    Nitrogen is available in large steel cylinders from "gas" supply companies. They carry gas bottles for welders, etc. They will probably have Nitrogen in stock but you want to specify "lab grade nitrogen" or "dry nitrogen" and they may have to order it for you. Standard nitrogen can contain some H2O which negates one of the primary benefits of running nitrogen. Lab grade nitrogen does not have any H2O in it. Some places let you put a deposit down on the cylinder and just pay for the nitrogen inside. Then you can exchange the cylinder for a new one when you need more nitrogen. Others make you buy the cylinder and just swap you for a new one when you need more nitrogen. They also come in various sizes so you'll have to decide how much you might need. You'll also need to purchase a set of gauges and valves which mount on top of the cylinder and allow you to monitor and control the gas inside. Never transport the cylinder with these attached and make sure you secure the cylinder at home so it can't tip over. Given the pressure inside, if the valves break off, the cylinder becomes a missile and can do considerable damage. The cylinders are very robust and can handle lots of abuse; it's just the valve mechanism that is somewhat delicate.

    I definitely prefer the angle chuck because it makes it easier to get a proper fit on the valve stem. Yes, there will be some pressure released when you disconnect the chuck, so I usually overfill just slightly to compensate for this problem. A little experimentation will allow you to establish the proper amount. In my experience, we're talking about 1/10 psi.

    The translation of BMW's Nitrogen white paper was right on!

  27. #27
    Z8Mania
    Guest
    Thank you Bruce. Do you have any sources for what I will call a wheel peg? I have one of these things- it looks like a metal peg and its threaded on one end and you can screw it into where the BMW lug nuts go to help mounting new wheels on a vehicle- you can use this as a guide to make sure you dont bang the rotors, other things.

  28. #28
    Sport Button On - DSC Off
    Join Date
    Aug 2007
    Posts
    182
    You can find those pins at www.thextontools.com or call 800-328-6277. An alternative that I use is to install wheel studs on your hubs and use lug nuts instead of bolts. This allows you to experiment with spacers without the need for various length lug bolts and makes mounting a wheel very simple. They are available from www.bmpdesign.com.

  29. #29
    Sport Button On - DSC Off
    Join Date
    Aug 2007
    Posts
    182
    While the Z8 does have a tire pressure monitoring system, it is important for owners to understand how it works since the system will not offer a low pressure warning under certain circumstances. The Tire Pressure Warning (TPW) system on the Z8 does not use internal sensors to measure the pressure within each tire. Instead, the 4-channel ABS system, which monitors each wheel's rotating speed when the car is traveling above 10 mph, compares those speeds to a calculated average speed and if it finds a discrepancy of app. 30% (see footnote below) during a 1-3 minute period, it activates the TPW indicator which is located on the face of the tachometer. The TPW system needs a set of base values to undertake its calculations and these are initially set at the factory. However, whenever tires are changed or tire pressures are altered substantially, it is necessary to re-calibrate the TPW system's base values by pressing the TPW button, which is located next to the radio, long enough for the indicator lamp on the tach to light up. You should then drive the car above 10 mph for app. 10 minutes while the TPW system gathers information about the average speed of each wheel's rotation relative to the others. This process is necessary because each wheel's rotating speed is directly related to its overall circumference, which can vary if you change to a different size tire or if the inflation pressure is altered.

    In operation, the TPW system checks to see if any of the tires is rotating at a significantly faster speed**(see footnote below) than the other tires which could indicate that it has lost pressure (smaller rolling circumference). It is important to remember, however, that the TPW system needs at least 1-3 minutes of comparison to detect a rotating speed differential. The reason for this slow response is because wheels will turn faster or slower than others while the car is going around a corner or when there is wheel spin due to slippery surfaces or hard acceleration. If the TPW system reacted immediately to these discrepancies, it would send false warnings on a regular basis, so it is designed to spread out its analysis over a 1-3 minute period to eliminate discrepancies related to normal driving circumstances. The problem with this delay is that it makes it impossible for the system to react in a timely fashion to a major loss of tire pressure. That is the reason this type of system, as compared to pressure sensors inside the tires, is only used on cars which are equipped with run flat tires. Since run flat tires should not suffer catastrophic failure despite a sudden major pressure loss, the Z8's TPW system was deemed adequate. The obvious problem for many owners today is that they have switched to non run flat tires which are subject to catastrophic failure as a result of a sudden major pressure loss and they may not receive a TPW warning under those circumstances. It is also important to remember that the Z8's TPW system cannot detect a flat tire at start up which means you could drive for 1-3 minutes before the system detects the problem. Again, this is not a problem when run flats are installed but could easily result in a ruined tire (sidewall failure) when non run flats are in use. Another problem, for both types of tires, is that the system cannot detect slow pressure loss in all 4 tires at once, which is a normal occurrence over extended periods of time. This means you could be driving in a spirited manner around sharp turns with dangerously under inflated tires with no warning from the TPW system. The TPW system in the Z8 is also known to give false warnings on an annoyingly frequent basis and as a result, may be ignored when most needed.

    The TPW system in the Z8 is an old design, rarely used any more, having been replaced by pressure sensors in the tires which can monitor actual pressures in each tire under any and all circumstances without relying on rotational speed as the determining factor. If you are using run flat tires on your Z8, the TPW system is adequate as long as you keep in mind the above mentioned limitations and occasionally check your tire pressures manually. If you have switched to non run flats, it is imperative that you manually check the pressure in your tires on a regular basis because the TPW system is not designed to provide adequate warning with those tires. Hope this helps.

    *The 30% pressure reduction figure is supplied by BMW but in my experience, the system reacts to much smaller pressure reductions.
    **I believe the TPW system also reacts to wheels which are rotating slower than the others indicating an increase in tire pressure and thus increased rolling circumference but have no idea why this would be of significance.

  30. #30
    That is great info, I never understood how the system worked on our cars, thanks for the very clear explanation.
    Andrew Macpherson

    Expert Z8 Inspections, with full support for both Z8 sale and purchases.

  31. #31
    Z8 Ate My Homework! Norcal's Avatar
    Join Date
    Nov 2002
    Posts
    2,471
    This may or may not be appropriate but.........who cares if we have TPS? Most of us survived just fine all these years with no seat belts, not ABS, no TC etc. One of the reasons I love the Z8 is because it doesn't seem to have as many electro-nannies as most contemporary cars. What did I do in the old days when a tire went down without an immediate warning? I noticed I had a problem, and brought it to a gentle rest at the side of the road, where I had no AAA and no cell phone...where I got out the jack and the spare and fixed it!

    In all seriousness, I don't worry about not having TPS save my ass since I'm using non-run-flats (double negative?). Is it safer to have all this computer stuff? Yah, I guess. But I still enjoy driving, and the thrill of what's over the horizon...even if it is a cow in the road.


    But thanks anyway for the tech specs GM, I'm just not so sure I wouldn't turn it off if I could.