Recommended: Tyre Sizes and Pressure values for E39 skid

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Ok, after having a hair raising skid (no damage! thank goodness), and then experiencing a certain feeling of instability with my E39. I have been trawling through this board for some answers and would like a few
recommendations
I've read that the most common cause of BMW E39 feeling unstable is mismatched tyres or incorrect pressure first. So I need to be sure of this before turning my attention to other parts of the car.
My E39 523 has 225/40/R18 on Front 265/35/R18 on Rear
My problem seems to be from the rear, which I lost on the skid in wet condition I might add..
A poster recently asked if these weren't 235s on the front. Is there a problem with 225s?
Should I be running with the same size tyres all round? I've read differing opinions as to which set should be bigger front/rear. Considering the wet conditions that would be encountered soon, what would be best??
What tyre profiles does BMW recommend as standard for R18's on E39?? And what pressures are best for all round driving. If not whats the dry and wet condition values.
Thanks
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There is no need to run the same size tire front and rear. The tires you have appear to be the standard tire for the car, but in any case they appear to be appropriate if they are an optional fitment.
You said your skid was on wet pavement. The wide tires will work like snow shoes to hold the car above the road on whatever covering there is at the time -- this is called hydroplaning if the tires are floating on water. Think of water skiing, if your skis are wide, you get up easily, if narrow then it's a bit more work. Of course, narrow skis on water are a good thing, but wide tires on a car are generally considered to be the preferred fitment. Having said that, wide tires have overhead that you found out about.
When they tell you that you need to keep the tires matched, they are talking aobut left and right, not fore and aft.

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By the way, your tires are within a quarter inch of being the same size, with the rear tires being larger than the front.
Beyond the fitment issues of stuffing that much rubber under the fenders, I don't see that your tire choice is a problem from a pure mechanical perspective. You might have serious issues with it comes to wet pavement because -- as has been said by many -- tires that wide are akin to strapping on show shoes then wondering why you walk on top of the snow instead of sinking into it. Your tires ride on top of the water that is on the street, and this presents all sorts of issues relative to traction. After all, water doesn't provide much traction, which is why we can drag people around the lake while they stand on what is essentially a stick. If we drug people around the street while they had sticks strapped to their feet, I'm not sure they would enjoy the sport as much as the spectators.

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ChrisJ wrote:

Are the skids power dependant? Does the balance of the car change dramatically as you go on and off the throttle?

Worn out suspension bushes are another suspect. Are your tyres wearing evenly?
My car has 340bhp and needs deliberate brutality and the traction switched off to unstick it, even in the wet, you should not be going sideways in a 523 unless you want to and really try.

I'd not run tyres that wide with less than 3mm tread and I'd always replace both tyres on an axle at the same time.

That was me. :)
My car is an ALPINA B10 V8 and is fitted with:
235/40 R18 front @ 2.7bar/39psi 265/35 R18 rear @ 3.0bar/43psi
AFAIK the sixes use the same.

If you have wider rear rims you need wider rear tyres if all your rims are the same width ditto the tyres. The wider ones always go on the rear with a RWD car. IIRC ALPINA fit 9.5" front, 10.5" rear.
Having 225s on the front should reduce oversteer / increase understeer compared to 235s - i.e. make the car *less* tail-happy.

First fix the tyre pressures and inspect your tyres. If your pressures were far out and the tyres are evenly worn you may well have a fix, if not you need to get chassis alignment and suspension bushes checked out.

I think BMW use mostly R16 and R17 on the E39.

ALPINA specifies only one setting which is suitable for high speeds and loads. You /may/ improve wet grip slightly by dropping the pressures a couple of notches but I think you need to get the basic handling of the car sorted and *understood* before attempting any fine tuning.
A
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"ChrisJ" wrote

Well, as others pointed out, wide tires aren't necessarily your friend when it's wet. Wider tires are more likely to hydroplane sooner because the pressure that the car's weight exerts onto the pavement gets reduced per unit of area (weight stays the same, but contact patch is increased) and hence the tire will float on top of water instead of cutting through it like a narrow tire would. Also, the more tread is worn, the worse it's going to get.
Alas, we don't know much about the conditions during which the skid occured (speed, how tight the turn, on gas/off gas/on brakes, etc.), so it's hard to say whether the tires are to blame or whether there's something else (suspension, alignment, etc.).
Cheers,
Pete
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Pete wrote:

Ummm, not exactly. The contact patch area is determined by the weight divided by the pressure/sq. in. of the tire. Assuming a 3600-lb. car with equal front-rear and side-side weight distribution on four tires inflated to 36 psi each, the total contact patch would be 100 square inches, or 25 sq. in./tire. One dimension will be determined by the tread width and the length of the patch will be close to the width dimension divided into 25. To a certain extent, the contact patch *length* can be varied with inflation pressures, but the width remains essentially the same.

Nonetheless, this is essentially correct. -- C.R. Krieger (Been there; slid that)
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"C.R. Krieger" wrote

Yeah, as you change tire widths, the contact patch area may remain the same, but its shape changes. So a narrow tire will have a more elongated patch shape whereas a wide tire will have a wider but shorter patch. Maybe like a difference between a ski and a snowboard.

LOL!
Pete
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Not when the tire size changes. Wide tires will hydroplane easier than a narrower tire on the same car.
The guy you are replying to simply said that all things remaining equal -- weight of the car -- the wider tires are more prone to hydroplaning than the narrow tires. This is essentially correct, and the things that make it not correct are also things that are not within the realm of "all things remaining equal."
Assuming a 3600-lb. car

If the comparison is the same car with different size tires -- width -- then all other things being equal, the wider tires will float sooner.
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J Strickland wrote:

With all due respect, that's a non sequitur as a reply to what I wrote. Read it again, stated more precisely: The contact patch *area* is *equal to* the weight on that tire divided by the pounds per square inch of inflation of the tire. Tire size has *absolutely no bearing* on that statement.
Tire width will affect the *shape* of the contact area, but not the size, other conditions remaining equal, but I didn't refer to width *at all* in my statement (unless you consider refuting a false statement to be a reference).

You misunderstand. I did not disagree with the basic statement. I simply disagreed with the theory [that contact area is tire width-related] upon which it was based.

I think you're oversimplifying here ... ;^) -- C.R. Krieger (Been there; spun that)
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I gotta think about that one.
Surely an F1 car with 18" rims (or whatever) has more contact area than another car with the same 18" rims -- where "same" means diameter only because the width is obviously going to be different on the F1. The contact patch is a combination of diameter and width, where inflation is a variable that affects both length and width of the contact patch.
Two tires that are roughly 25" in diameter will have the same length of contact patch, but different widths correlating to the width -- 225 or 265 -- of the tire. Assuming the air pressure isn't an issue, the width plays a dramatic role in the contact patch, contrary to what you said.
I don't mean to argue with you, but I can't resolve your statement with the logic that I have. If two tires are 25" in diameter (this is roughly the diameter of the tires the OP brought to the table) then the length of the patch is a function of tire pressure and the tire's circumference. Assuming proper inflation, the length of the patch on these two tires should be pretty close to identical. This leave the width of the patch, which is more a function of the 225 and 265 dimensions he gave us to work with. Again, assuming the air pressure is correct in both of these tires, the 265 will be about 1.5 inches wider than the 225. The size of the tire in this instance plays a significant role in the contact patch.

That's where I became confused. I think the contact patch is a function of the tire size specification. It has to be.

I'm not meaning to, but I don't see it as a complicated issue. If a car carries narrow tires, then it behaves one way, if the tires are exchanged for wider tires it will behave another. The width of the tires being the only significant change is a pretty simple concept.
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A rather significant size stagger ... normal staggered size would be 235 front, 255 rear and that's typically for the 540 sport package (with heaps more power than your 523).
The larger rear tires offer greater grip, primarily for acceleration, but also to generate more understeer than same size tires all around (typically 235 on both ends). So, your experience is counter to what should be happening ... all things being equal.
BUT, given that you're running wheels and tires that are not OEM for the E39 six cylinder, anything is possible. You might have a larger rear anti-roll bar. Springs/shocks may have been changed. You might be running different tires front and rear (you didn't say). You may have suspension issues unrelated to the tires.
Typical setup for E39 is to run about 6 psi less pressure in front than in back. Check the owners manual (you can download them from several sites) for specifics. So, I'd start with pressures close to OEM and go from there.
While the stagger setup you have makes the car look racier, it doesn't really improve the handling. If the road is really slick, even a small displacement six can allow you to steer with the throttle. Of course, DSC will do its best to prevent that from occuring, but it can't rewrite the laws of physics.
R / John

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But, he has control issues when driving in water, and we should expect there to be problems because the wide tires will hydroplane easier.
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heaps
but
there
I'm not fully convinced ! Let assume that for a dedicated car, you change the tyre width, the pressure remaining the same. The surface of the rubber in contact with the road will remain the same. While the contact width (*) is increased and the length is shorter.
(*) When I say "width", I'm referring to the tyre width which is almost a constant for one tyre whatever the pressure is (reasonably !)
The grooves are also shorter, leading in a better avacuation of the water. So, in my opinion, larger tyres should be more water pumping efficient and less prone to hydroplaning.
Now, decrease the pressure, the contact surface will be greater and for the same reasons as above, the groove length will proportionnaly increase. The consequence is that under water, the traction would be degraded because of the accumulation of water, especially at high speeds. Am I missing something ?
Another question: the friction losses are proportional to the tangent of the angle of friction. So nothing to deal with the contact surface. Correct ? Hence, why do large tyres, everything remaining equal, do drip more than narrower ones ? Is it a urban legend or not ?
Regards
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Why does the contact patch remain the same with wider tires?
If the DIAMETER remains the same, approx. 25" in our example, then the Length of the contact area will be something that equates to the radius and is affected by the inflation pressure. For the sake of argument, let's assume the length of the patch is 5 inches -- I have no clue about how accurate that might be, but it feels good pulling it out of my ass, so I'm going with it for now.
Now, let's say the tires are 225 mm wide, or 265 mm wide. The difference in these two widths is 40 mm, and this equates to tad under 1.6 inches. So, one tire has a contact patch of 5 x 8.85 (44.25 sq. in.) while the other has a contact patch of 5 x 10.45 (52.25 sq. in.). The difference in the contact patch of these two tires is 8 sq. in. That's nearly 20% greater contact patch, all of which is width and none of which comes from air pressure.
What am I missing?

Yes, the width is a constant. It will change a bit as the air pressure changes, but it's reasonable for this discussion to say it is a constant. I see that the constant will be changing later, and the constant at this point is to establish a baseline for the discussion.

Well, the tread design certainly plays a significant role, and some tires will hydroplane more easily than others. With a very poor tread design on a narrow tire, you could have hydroplaning more easily than a good tread design on a wide tire.
I'm not sure what you mean by "shorter" grooves.

Yes, reduce the tire pressure and the contact patch will increase, and add this to the tread design issues, the hydroplaning can be serious.
I think the greater increase in the contact patch will come from installing wider tires though. As I pointed out earlier, the OP is asking about a 265mm wide tire on the rear of his car, and he has 225mm wide tires on the front. His concern is that the rear stepped out in water, and my point is that this is a function of hydroplaning, a reasonably light car, and a very wide tire. All of these things come together in water to create a very exciting driving experience. ;-)

"Drip" more???
On a dry surface, I think there is little dispute that wider tires offer more grip, but the OP asked about a wet surface. Wet and Dry are worlds apart.
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"Jeff Strickland" wrote

Jeff, I think the idea is that the contact patch AREA remains the same, only its shape changes. I tried to illustrate this for you here:
http://www.zie.neostrada.pl/contact-patches.gif
I believe this has to do with the way the rubber stretches on a pavement under the weight of the car which remains unchanged. I don't exactly understand the physics behind it, but maybe someone else (C.R. Krieger?) can explain. Also, check this page from TireRack:
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid 
Regards,
Pete
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But the comparison there is with a very wide 18"-rim tire, and a relatively narrow 14"-rim tire. The contact patch in one example is 4x3, and 3x4 in the other. Both have a patch of 12 -- again, removing numbers from my ass to illustrate the point, not define the precise size of the contact area.
Our discussion is about two 18"-rim tires, one that is 225mm and another that is 265mm. The Length of the contact patch in these two tires should remain pretty much constant, while the Width of the patch presents us with a variation of 40mm.
In the poor illustration below, we see that the length of the patch is the same for each, but the width of the patch varies soley because the width of the tire has changed.
|\/\/\/\/| |\/\/\/\/\/\/\| |/\/\/\/\| |/\/\/\/\/\/\/| |\/\/\/\/| |\/\/\/\/\/\/\| |/\/\/\/\| |/\/\/\/\/\/\/|
225x18 265x18
The TireRack link discusses different aspect ratios as well, one tire is a 35% ratio, the other is a 70% ratio, but the tires the OP has are only different in the aspect ratio by about 5% in raw number, and much less as a variance from one to the other. The TireRack tire example has tires that have double the aspect ratio, or half - depending on which you use for the baseline.

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"Jeff Strickland" wrote

That is your school of thought. And another school of thought claims that as as you increase the width, the length of the contact patch decreases. I believe this has to do with the fact that you have the same amount of car's weight to work with and it is not able to "flatten" the rubber enough to increase the footprint area. Again, I'm not claiming this is correct. I'm just trying to present the other side of the discussion.

Because that's how you drew it. :) And as you saw, I drew it that the length was different. Who's right? Who's wrong? I don't know. :)

Yes, but the overall diameter of the two tires in their example is practically the same.
The topic of contact patch areah has been debated on various other boards/forums when trying to decide if wider is better, alas there seems to be too many variables to get a definite answer...
http://www.bikeforums.net/archive/index.php/t-77451-p-2
http://home.att.net/~ferrari/tires2.htm
http://forums.bimmerforums.com/forum/showthread.php?t $5422 (I got tired of reading that one :) )
Cheers,
Pete
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relatively
the
a
Wrong, Think about p=F/S !

Please, let me correct your drawing ( congrats, it's better than mine !)

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Jeff Strickland wrote:

You ain't gettin it, Jeff. This is n't somethin that we're making up. It's more like this:
|\/\/\/\/| |/\/\/\/\| |/\/\/\/\/\/\/| |\/\/\/\/| |\/\/\/\/\/\/\| |/\/\/\/\|
get it?
-Fred W
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Jeff Strickland wrote:

It stays the same because as you increase the width you now have the weight of the car spread across more tire so the tire would not compress as much as with the narrower tire
--
-Fred W
Toys for sale, Hey get your toys here:
  Click to see the full signature.
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