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
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.
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
When they tell you that you need to keep the tires matched, they are talking
aobut left and right, not fore and aft.
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.
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
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.
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
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.).
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.
(Been there; slid that)
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.
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
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.
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 ... ;^)
(Been there; spun that)
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.
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
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 ?
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
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
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:
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:
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.
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
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...
(I got tired of reading that one :) )
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