[E92Fan]

So many wheel threads recently


Right, here goes...


There are three essential things when it comes to deciding what wheels to get -

1) Size in diameter (eg 17", 18", 19")
2) Size in width (8J, 9J, 10J etc... ps.. 8J = 8" wide)
3) Offset

For an E9x, the bolt pattern or PCD must be 5x120 - this never changes regardless of the alloy wheel you put on.


Point 1

The most important goal, ignoring the style of the wheel, is that the wheel fits underneath the arches without complication. The actual diameter of the wheel is not actually that important - there are other aspects that should be given greater consideration to. In theory, assuming other parameters of the wheel stay constant (ie. width and offset) you should be able to get even a 21" wheel to fit underneath the arches on a 3 series. The reason is that the entire diameter of wheel and tyre together can be maintained at a roughly constant figure by altering the sidewall profile of the tyre. In other words, if you have a 19" wheel with x mm of tyre sidewall, and want to increase to a 20" diameter wheel, you can reduce the sidewall dimensions of the tyre so that the overall size doesn't change. The downside is obviously less compliance in the tyre, amongst other things.

The diameter of the wheel is actually the least important aspect when it comes to choosing a wheel that will fit.



Points 2 and 3 above are connected and should be considered together.

The overall width of the OEM rear 19" wheel on a BMW 3 series is 9J, or 9 inches. The wheel itself sits in the space between the spring/damper and the inner rear wheel arch. There is a finite amount of space between the two items, within which the rear wheel must be able to turn freely under full load and compression. If you start increasing the size of the rear wheel in terms of its width, ie. go from a 9J to a 10J (or 10 inch) wheel, then you will use more of the available room between the strut/damper and the inner wheel arch. If you go to a 12J wheel, there probably isn't enough physical space.

That's the first, basic aspect of wheel size to consider.



Now look at this offset diagram below




You can see that the offset effects the position of the mounting point of the wheel onto the hub. If the offset is ZERO, the mounting point to the hub is in the centre line of the wheel, and therefore the inner and outer wheel faces are equidistant either side of the hub surface.

A wheel with a positive offset (ET > 0) will move the mounting point to the right of the centre line, therefore moving the entire wheel inwards towards the strut/damper.



Let's take this example -

• Assume a standard E92 has an 9J x 19" rear wheel, with an offset of ET39. That means the wheel is 9 inches wide, and is positively offset by 39mm. Let's assume there is 20mm of clearance when the suspension is uncompressed between the outside of the 9J OEM wheel and the inside of the rear wheel arch.

• We now change the wheel to a 10J x 19" wheel, still with an offset of ET39. The overall width of the wheel has increased by 1 inch, or by 25.4mm. This is an overall increase, so effectively it is an increase of 12.7mm either side of the centre line. Because we have assumed the offset is still ET39, the relative position of the centre of the wheel in relation to the hub has not changed. Therefore the wheel has grown equally on both sides and now protrudes 12.7mm further into the arch, and 12.7mm closer to the strut.

• Remember we assumed there was only 20mm of clearance with the original 9J OEM wheel? Now with the 10J wheel there is only 7.3mm of clearance left, when the suspension is uncompressed. When the suspension is compressed and the wheel pushed into the arch, that 7.3mm of clearance remaining becomes a serious issue.

• To compensate for this, you can alter the offset of the wheel. Increase the offset from ET39 to ET44 gives an increase of 5mm. This means the entire wheel is shifted in towards the strut/damper by 5mm. Therefore this same 10J wheel now with an offset of ET44 has 12.3mm (7.3mm + 5mm) clearance between the wheel arch, but equally 5mm less clearance to the strut/damper. Remember also that the spokes of the wheel will also be 5mm closer to the hub/brake caliper/brake disc surface

You can hopefully see that it's a balancing act when choosing the right size of wheel.

An 8J x 20" ET37 wheel will fit within the arch just as well as an 8J x 17" ET37 wheel. All you have to do is alter the tyre sizes so that the same overall diameter, and therefore rolling circumference, remains constant.



Example -

• 8J x 17" ET34 with 225/45/17 tyres - has an overall diameter of 24.97 inches
• 8J x 20" ET34 with 225/30/20 tyres - has an overall diameter of 25.31 inches
  
• The difference between the two is 1.3%, which is perfectly acceptable.
  
• Both sizes of wheel will fit, because overall they are the same size and width, and located or offset relative to the hub in the same dimension.

So, the bottom line is this -

You can choose any size of wheel, whether it be 17" or 20". If you keep the width dimensions and offsets the same as BMW OEM, then you won't have any problems. The ride will be the only thing that suffers as the sidewall profile decreases.

If you start increasing the width of the wheels, then the offsets will likely need to be altered a notch if the change is dramatic. Remember that the wider the wheel, the wider the tyre, and that also starts eating into available room under the arches.



As a rule of thumb -

1) Front wheel

• OEM 8J front wheel is between ET34 and ET37 (depending on model)

• 8.5J front can be installed, with an offset of approximately ET35 - ET40

• 9J front can just be squeezed in, but with a high offset of about ET42

I wouldn't recommend a 9J front unless you plan on making your car a track car. 8.5J is the widest I would suggest, and ET40 will give the most clearance to the inner arch, without compromising turning circle


2) Rear wheel

• 9J OEM rear wheel is between ET37 and ET39, depending on model

• 9.5J rear can be installed, with an offset of ET40

• 10J rear can be installed, with an offset of ET40 (but you have to go skinny on the tyres, or roll the rear arches to create more room)

• 10.5J rear can be installed, but ideally with an offset of around ET44, and you'll have to roll the rear arches and probably dial in more negative camber (tilt the wheels in vertically) to give more room. You will be getting close to the strut/damper in this configuration.

For what it's worth, my suggestion is to use either an 8J front and 9J rear combination, or an 8.5J front and 9.5J rear together. 19" wheels is the largest I would be happy with personally, as the ride hasn't deteriorated too much, but more importantly the weight of the wheel hasn't increased drastically. A 20" wheel is typically much heavier in comparison to a 19" and adds a huge amount of unsprung weight to each corner of the car, making the handling much more heavy, more cumbersome, and the ride quite atrocious.



Tyres

Tyre sizes are chosen to match the wheels used, and depend largely on the width of the wheel. There are combinations of sizes front & rear that are used together to maintain the same relative rolling circumference.


For instance -

• Example 1
  
  Front - 8J x 19" or 8.5J x 19" = 235/35/19 tyre
  with
  Rear - 9J x 19" or 9.5J x 19" = 265/30/19 tyre

• Example 2
  
  Front - 8.5J x 19" = 235/35/19 tyre
  with
  Rear - 10J x 19" = 275/30/19 tyre (just about, might need arch surgery or increased offset to ET42 or so)
  
  or
  
  Front - 8.5J x 19" = 245/35/19 tyre
  with
  Rear - 10J x 19" = 285/30/19 tyre (will need arch surgery!)

• Example 3
  
  Front - 8.5J x 20" = 235/30/20 tyre
  with
  Rear - 10.5J x 20" = 275/25/20 (will need major arch surgery!)
  
  or
  
  Front - 8.5J x 20" = 245/30/20 tyre
  with
  Rear - 10.5J x 20" = 285/25/20 tyre (will need major arch surgery!)

Finally... Spacers

Once you've chosen your wheel and tyre combination and mounted it on the car, you might suddenly realise that there is more room to the inner wheel arch than you expected, and you want to push the wheels out as far as possible to create an aggressive stance. To do this, you can use a hub-centric spacer which bolts between the hub surface and the mounting plate of the wheel. That's all a spacer does - it pushes the wheel further out to the side of the car by a predetermined amount. A 5mm spacer pushes the wheel out by 5mm, which is the same as reducing the offset of the wheel by 5mm (eg. going from ET39 --> ET34)



So... if you have a question along the lines of "Will E46 wheels fit and what size tyres do I need?", it's quite easy to work out for yourself whether they fit. If the bolt pattern is 5x120, that's a start. If the width of the wheel is an 8J and the offset is ET35-ish, then yes it will work. If the offset of the E46 wheel is ET20 for example, then you're 15mm too far out into the wheel arch, and it probably won't fit. If the offset of the E46 wheel is ET50 compared to the E9x standard ET34, then the wheel will sit 16mm too far inwards in comparison. However a 15mm spacer will compensate for that creating an effective offset of ET35, by pushing the wheel 15mm further out.

Clear?! Hopefully...



I think that covers pretty much all the basic aspects when it comes to choosing alloy wheels