Questions regarding BMW wheels and tyres can be
found on every Forum, with the same questions
being asked time and time again, usually 'Do these
wheels fit my car?' But that is not surprising really,
as the way wheels and tyres are classified is
extremely confusing. Measurements are made in
Imperial (inches) and Metric (millimetres) on the
same tyre, and although the width of the tyre is
specified, the wall of the tyre is only inferred by the
aspect ratio!
WHEELS
The problem of 'will these wheels fit my car' is really
two questions, although the wheel might fit, the
tyres may be the wrong size for the car (their
diameter may be wrong). Let's start with the real
basics, for the alloy to fit your car the following
measurement must be correct
CENTRE BORE = 72.6mm
Alloy wheels are centred on the hub by the centre
bore, the bolts do not centralise the wheel. For the
wheel to be centred correctly it must be a perfect
match with the hub diameter. The E31, E32 and E38
all have a centre bore of 72.6mm.
Some BMW models have a different centre bore, for
instance, the E39 has a centre bore of 74.1mm. It is
possible to fit hub-centric rings (also know as
spigot-rings) to increase the diameter of the hub.
However, it is imperative that these are constructed
of the best quality materials and machined for a
tight fit on the hub and inside the centre bore of the
alloy.
The weight of the car is suspended on the hubs, if
the hub-centric rings are of poor quality they will
wear and distort and this may cause steering
problems such as shimmy.
Most replica alloys have a 74.1mm centre-bore,
this allows them to be fitted to all BMW hubs but
does mean that spigot rings or hub-centric spacers
must be used when fitted to the E31, E32 and E38.
Click here for more information on fitting replica
alloys to the 7 and 8-Series models.
PCD = 5 x 120mm
This strange measurement (pitch circle diameter)
specifies where the fixing bolts are located. The
measuring specifies that there are 5 bolts spaced
equally on a 120mm diameter.
Nearly all BMW's have this arrangement except the
E46, 1.8Ltr Z3 and the new Mini.
The only other consideration is the length of the fixing
bolts which have to be long enough for safely fixing the alloy to the hub. Strangely, this measurement is rarely mentioned, in practice the bolts
should have at least 5-turns before they become tight
Right, that's sorted out the real basics, most BMW alloys and replicas are supplied as 72.6mm centre-bore and 5 x 120mm PCD so there is
usually no problem with these two measurements. The most confusing measurement is the offset of the alloy.
OFFSET = 15 - 25mm
If an alloy has an offset of zero then the mounting pad (the face that is bolted to the hub) would be exactly on
the centre-line of the alloy. This would mean that the face of the hub is equally spaced between the inner face
and outer face of the alloy.
BMW alloys usually have a positive offset (+20mm) which means that the mounting pad is further towards the
outer face as shown to the right. This means that the centre of the alloy (and hence the weight of the car) is
inboard of the outer wheel-bearing. With an offset of zero the weight of the car would be outside the wheel
bearing and this would increase bearing wear significantly as the majority of the weight would be applied to
the outer bearing rather than being shared between the inner and outer races.
The other important point about offset is that it determines the clearance the wheel has in respect to the
suspension and outer wing. With a greater offset the alloy moves towards the suspension, with a lesser (or
negative) offset the alloy moves towards the outer wing. So, the correct offset of the alloy determines two
things. It ensures the correct balance of weight on the wheel bearings and it ensures that the alloy sits centrally
in the space between the suspension and the outer wing. However, having said all that blurb about offsets it is
not uncommon that an alloy is fitted that has the wrong offset. Where the offset is too positive (say an offset of
30mm) then a 10mm spacer can be fitted. This reinstates the combined offset to be 20mm (30mm - 10mm =
20mm). There is always a downside though, and that is the reduction of hub-to-bore
contact which will stop the alloy being rotationally centred correctly.
For wider spacers to be effective they must be machined so that they have a bore for the
hub and a spigot for the alloy. The maximum width for a plain spacer is around 10mm.
These are only used where the clearance between the tyre and suspension is too close
for comfort. Where a spacer is used it is imperative that longer bolts are also employed.
One important point to note is the offset for the car is expressed as a RANGE of
offsets (usually specified as 15mm - 25mm for the E31, E32 and E38). This figure
indicates which alloys will be suitable for correct operation and lowest bearing wear. It
does not specify the exact measurement where the alloy will be central to the
suspension and outer wing. Although the standard specification
for the E31 is an offset of 15mm - 25mm it is interesting to note
that the Sport models were fitted with Style 21 Throwing Stars
which have a front offset of 10mm. However, other standard
alloys for the E31 (Style 8 and Style 9) have offsets of 15mm.
The correct offset becomes more apparent when wide alloys are
fitted with oversize tyres, in this situation the use of spacers are
often used so that the tyre clears the suspension. Spacers and
hub-centric rings should be used as a last resort as they can
effect the performance of the road wheels.
Timm's BMW Wheels and Tyres Page - Updated 2018
J-numbers - Offsets - Profiles - PCD It's all a bit confusing
Using a tyre calculator
List of tyre sizes for the E38
List of tyre sizes for the E31 and E32
TYRES
The measurements on tyres are just as confusing as those that specify alloy dimensions. Let's take a tyre that will fit the alloy shown to the
right, a 255/45/18 which is the standard size of an E38 staggered 18" rear. The last number is obvious, the '18' specifies that the tyre fits on
an 18" diameter alloy....at least that bit is simple!
The first number '255' is also reasonably simple, apart from the fact that we have now changed to Metric measurements (mm), the '255'
refers to the width of the tread. 255mm = 10", so this tyre has a tread which is 10" wide. It fits nicely on a 9.5J alloy.
The second number '45' is not so simple, you might have hoped that it was a measurement of the sidewall of the tyre, which it is, well sort
of. It isn't a Metric or Imperial measurement, it is a ratiometric measurement! It is the height of the sidewall as a percentage of the width.
Now that's just plain confusing. To calculate the height of the sidewall we divide the width by 100 (= 2.55mm) and multiply it by 45 (=
114.75mm). So the sidewall height is of this particular tyre is 114.75mm.
This strange measurement is sometimes called 'the aspect ratio' because it is the ratio between width and
height. We have all heard of 'low aspect' tyres, this is where the aspect ratio is less than 50%. This means that
the tyres are wider than the sidewall height. Our 255/45/18 tyres can (just about) be classed as a 'low aspect'
tyre......they are fatter than they are high!
The important bit about tyres for your BMW
It is important to keep the correct diameter (that is the measurement from the top of the tyre to the bottom of
the tyre) correct for your model of BMW. Although the E31, E32 and E38 share the same offset, PCD and
centre-bore they do not share the same tyre diameters. This means that if you pull a wheel and tyre from an
E38 it will not be suitable for the E31 or E32. This is because the E38 tyres have a greater diameter.
However, the E31 and E32 DO share the same diameter, you can successfully pull a wheel and tyre from an
E31 and use it on an E32. Because tyres are only supplied in discrete diameters (5% increments of the width),
the diameter cannot always be matched exactly. However, the diameter should be matched to within 2%.
The correct diameter for the E31 and E32 is 25.6" +/- 0.5" (650mm +/- 12mm)
The correct diameter for the E38 is 27.0" +/- 0.5" (685mm +/- 13mm)
The correct diameter for the E63 is 26.5" +/- 0.5" (673mm +/- 13mm)
Fitting the incorrect diameter tyres on all four wheels will alter the accuracy of the speedometer and will affect
the overall gearing. Fitting the incorrect diameter on one axle will cause problems with the ABS and DSC
systems.
The pictures to the right show the same alloy (M-Parallel 9.5J x 18" - 20mm offset) fitted to the E31 and E38.
Although the tyres are the same width, the tyre on the E31 must have a lower aspect ratio (35%) compared to
that on the E38 (45%).
The same M-Parallel alloy can be fitted with a 285mm-wide tyre as long as the aspect ratio is reduced to 40%
(285/40/18).
The other important bit about tyres for your BMW
Never use tyres that are not as wide as the alloy. Although fitting wider tyres than the alloy is standard
practice, the reverse cannot be done as the tyre may come off the rim. Below is a table that shows the
minimum width of tyre (measured in mm) for a given alloy width (measured in inches):
There are a number of tyre calculators on the internet, these make it easier to visualise how the tyre will look on the alloy and ensure the size
is correct. For a quick comparison I use the Miata tyre calculator, this has been around for ages and is extremely useful.
For more detail I use this calculator
BMW Tyre, Alloy Wheel, Affects on ASC and ABS and other Articles
BMW E31 Recommended Winter Tyres
Gen BMW Tyre Wear
Gen BMW Function Of Tyres
Gen BMW The Wheel Assembly
Gen BMW Shimmy Caused By Tyre Problems
Gen BMW Tyre Size effects on ASC
Gen BMW Tyre Recommendations
Gen BMW Nitrogen Inflation
SEARCH MEEKNET
SEARCH MEEKNET