Hi, Cna anyone let me know the correct tyre pressures for an '84 512 BBi? The owners manual indicates 43 PSI front and rear, the DOT/EPA sticker on the car indicates 36: which is right? Thanks, Robert
I would go by the epa/dot sticker saying 36. Always best to go what is stamped on the car. 43 seems too high. Regards, Jim
The original window sticker states 3.0 kg/cm2.........that equates to 43 psi. The tires are labeled with max pressure at 51 psi (in English), yet total load pressures are metric.......???? I keep mine at 43 psi. Mybe DOT doesn't want you to go too fast!!!!!!!haha Doesn't "spirited" driving call for higher tire pressures?
Spirted driving as you say will tend to increase the tire temp and thus increase pressure. so if you set your pressure too high, the contact patch on the tire may not be enough for good grip. best regards, jim
I have heard that the tire pressures are raised on purpose........this is to minimize sidewall flexing, while taking fast, sharp turns.
If you increase tire pressure to stiffen the sidewalls, you will also change how the tire contacts the road surface. When checking tire temps on the race track, too high a pressure will cause extreme temperatures to the center of the tread and a loss of grip while cornering. best regards, jim
Grip is what is most important, if the sidewall flex is too much, the tire must be replaced/upgraded. I would still recommend for the BBI to set at cold pressure of 36 PSI. regards, jim
Preventing sidewall flexing was more of a problem in the early 80's......when there where no low profile tires available. Too much flex could cause the tire to unseat itself from the rim........very dangerous. This all may just be a mute point. I would drive the car at the two pressures and see where it feels better. I have noticed better handling, and even tire wear at the 43 psi, though. Just my 2 cents!!!!!!!!
true, maybe then the only option. and with the tire options at the time I can see the concern. Tire ratings have come a long way. regards, jim
How true! I remember, taking a Skip Barber 2-day high performance driving school, back in the 80's, and they told me that they would raise the street track cars (BMWs at the time) tire pressures to 50 psi, for the reason I gave. It all seems to be a compromise.......with these older cars. I agree that higher pressure will decrease grip. My BBi is easier to handle at 43 psi.......more fun for me, but I am rather careful, and I feel I don't do anything stupid. Steering is quicker. The fact that the tires wear evenly across the threads, suggests to me, that 43 psi is OK........if so, then at 36 psi, I would suspect the edges will wear faster. But, again, one must be aware of the compromises. If I could only find a set of low profile tires for my Boxer!!!!!!!!!!!!!!!!!
probably with the older tires you get a good contact with the 43 psi tire pressure, what you say does make sense. Now that I think about it the pressures were high with factory michelin tires on the daytonas as well. Maybe a upgrade to a BBLM setup with avons?... regards, jim
Quote: "Maybe a upgrade to a BBLM setup with avons?..." Sounds great.......can I send you the bill?..........hahaha
I believe there is a different explanation for variations in tire pressure for high speed driving. I do not believe it has anything to do with tire contact area (footprint). Heat is the enemy of rubber tires, as it leads to separation of cord layers and possibly tread. Everytime the tire turns there is a bulge at the bottom (we have all seen this when riding behind a car with low air pressure). Thus, for each revolution there is a flexing of the sidewall. The higher the speed, the more flexes per minute, and the more heat generated. High speed cruising calls for increased tire pressure to minimize this bulge (flex). Nothing more, nothing less. A slight stiffening of the sidewall through increased air pressure makes a significant difference in sidewall temperature. Over many miles, overinflation will lead to central tread wear, underinflation to lateral tread wear. But for the slide through a turn, the contact area is somewhat independent of air pressure. The side wall roll-under will be minimized with increased pressure, and therefore the tire will slide rather than scrub, but this is a sidewall event. Thus, oversteer and understeer can be compensated for with manipulation of sidewall stiffness through pressure variations. My thoughts. Jim S.
41 PSI is the pressure we like to run at road courses after they get up to full operating temperature. The cold in garage starting pressure is 35 PSI for my F355 and the between session pressures are 37 PSI. Note: I use nitrogen as teh inflation pressure in my Bridgestone S03S so I get less pressure rise from the same temperature rise. Therefore; 43 is probably a good number for high speed driving--after the tire gets hot. The trick then becomes, what are the starting pressures? (cold in garage)
I had the same question a few months back. The OM for my 1982 BB512i indicates to use 43psi with the TRX window sticker indicating 3.0 kg/cm2 a/k/a 3.0 Bar which is around 42.9psi. . The tires themselves are stamped "36psi max" and the sticker in the glove box says 32psi front and 34psi rear. FNA told me to use the 43psi figure whereas Michelin UK told me to use the 36psi figure. So, since Michelin made the TRX tires, I am currently using 36psi and the car drives fine.
Mitch - I believe that when the manufacturer (tire or car) calls for a specificied pressure when cold, that they have done the testing to determine that the pressue will be appropriate when hot. The higher the cold pressure, the lower the hot pressure. This results from reduced friction at the side wall with each revolution of the tire. Jim S.
I too was confused about all the different specs for tires pressures on my Boxer. I ended up calling Dick Fritz (owner of AmeriSpec) to ask. He said to run 38psi all the way around on the TRX 415mm tires. He said that the Ferrari window sticker and owner's manual listing of 43psi was for very HIGH speed driving only.
Frank, YES, that is true. But, Dick said that the maximum tire pressures listed on the sides of those tires are determined for vehicles which weigh -much more- than a Boxer does. If you look at your sidewalls they list: Max 36psi at 1,710 pounds per tire. That would equate to a vehicle that weighs 6,800 pounds. He said it was indeed okay to run 38-43psi with those TRX tires and recommended 38psi for the street. Which is what my DOT Federalized sticker lists as the pressure to use.
The load rating is designed to incorporate cornering and braking forces. It is quite conceivable to dramatically raise your cornerweight when hitting bumps and/or on track. For example, and in theory....with a 200lb spring on the front, under 3' of static preload, it's already supporting 600lbs of weight....now brake heavily on bumps and you may easily get 4 more inches of spring compression....that means the tyre is now supporting 1400lbs of weight or more allowing for more travel/severe bump/sidewall deflection....... Are you sure the TRX sidewall printing says 36psi?? That seems very low.
We are obviously talking of different tires!!!!!.....?????????? Mine are Michelin TRX 240/55 VR 415........all 4. It then reads: "MAX LOAD 580 kg (1279 lbs)"....and...."MAX PRESS 350 kPo (51 PSI)" What's up? Also, what does the DOT/EPA conversion have to do with tire pressures, anyway? My guess is that; since the car is "de-tuned" due to the conversion, they are opting for the more grip compromise on the car......hence the 36 psi. Just a thought.
My tires are also Michelin TRX 240/55 VR 415 and they all all stamped "36psi maximum" . When I called Michelin USA and then Michelin UK I was told that the 36psi rating was correct.
