BMW Wheel Upgrades - Street and Track
Wheels are near the top of the list as an upgrade on any BMW. Few modifications have more visual and performance impact than a new set of wheels. Wheel choice depends largely on your intended use - street, show, track days, racing, winter, etc. And there are many many options and choices to make which can leave even knowledgeable people scratching their heads. This page is to educate and aid you in making the right wheel choice. Upgrading for looks or moderate performance improvements is easy. It's only when trying to maximize performance does upgrading become tricky.
BMW, and other manufacturers, set the default handling to understeer by using a narrow front wheel/tire. With less grip from a narrow tire the front will lose traction first, which is arguably safer and easier to detect and control than oversteer (the rear end losing grip). There is nothing wrong with having a narrower front wheel for street use - chances are slim that you will regularly exceed the handling capabilities of your front tires. Having a wider rear wheel also gives the impression of power so BMW designers insist on it. With the front wheels narrower than the rear this is known as a staggered wheel setup.
For the track, we desire the same size wheels and tires for the front and rear. The front of the car needs more grip to reduce understeer so we try and cram as much wheel/tire into the front fenders as possible (or as the rules allow) and make big changes to the alignment. Big front tires and aggressive alignments are not good for the street so these wheel packages are usually best left for track cars. The same sizing also allows the tires to be rotated. This is known as a square wheel setup.
On this page we will define the various terms and technical features of wheels, give you background information and notes on BMW wheel fitments, and provide you with some examples of easy and aggressive wheel fitments. We hope this information proves valuable to you. If you can't find what you're looking for leave a comment or email us.
|the diameter of the wheel from one side to the other|
|the width of the Barrel, measured from the inside of one lip to the inside of the other|
|the outer ring of the wheel|
|Lip||the inner or outer-most edge of the Rim|
|the inner ring behind the spokes that spans the width of the wheel|
|the front of the wheel|
|the opening in the bodywork and chassis for the wheel/tire to sit inside. It's made up of the frame and body skeleton on the inside and the fender on the outside. Within the wheel well sits the brakes and suspension. The wheel and tire combo must fit within the wheel well and around the various components.|
|the area on the back of the wheel that sits on the suspension hub. The Pad will generally consist of the center bore, bolt holes, and machining/alignment holes. In the late 2000s BMW began changing mounting pad sizes but has not made changes across all models. The pad size differences only come into play when mis-matching wheel spacers and wheels. It's OK to use a wheel with a smaller pad even if car originally used a larger pad.|
Center Bore (CB)
|the hole in the center of the wheel usually covered by the center cap. The wheel mounts to the vehicle's hub through this hole. A lip on the suspension hub will fit snugly into the center bore and provide strength and alignment. The hub lip - not the wheel bolts - centers the wheel on the hub. This is known as a 'hub-centric' fitment. Most BMW wheels use a 72.6mm center bore. It is very important that your new wheels have a hubcentric fit to the car.|
|Beveled Edge||the back of the center bore will either have a straight or beveled edge. Nearly all factory and aftermarket BMW wheels have a beveled edge. The bevel or chamfer makes it easier to fit the wheel onto hubcentric lip on the car. But because the bevel is machined inward, it takes depth away from the center bore. This is important when using small, flat wheel spacers that eat into the space on the hubcentric lip. The wheel rests on the larger beveled section instead of the full center bore which may lead to a vibration.|
|the number of bolt holes and the diameter of the bolt ring. Most BMWs have a 5/120 bolt pattern - 5 holes and a 120mm diameter for an imaginary ring formed by the circle of bolts.|
|Seat||the area of the bolt that is tightened against the wheel. There are several different kinds of bolt seats but all BMWs use a cone seat with a tapered 60* angle. However, there are some aftermarket wheels that use a rounded ball seat in their wheels. It is important not to mix seat types because the wheel will not be securely fastened to the hub.|
|the distance between an imaginary centerline of the barrel and the back surface of the mounting pad. Expressed in millimeters. Offset and wheel width determine where the wheel sits - if it is tucked under the bodywork or poking out. All BMWs have a positive offset - the mounting pad is offset towards the outside of the wheel as opposed to the inside of the wheel (negative offset). A high offset means there is a large gap between the wheel mounting surface and the centerline - the wheel will tuck under the bodywork more. A lower offset will push more of the wheel to the outside of the car. In general, 1 and 3-series cars have a high offset and 5, 6, and 7-series cars have a low offset. It's common with aftermarket wheels to use a lower offset to push the wheels to be flush with the bodywork. High end wheels like Forgeline or HRE can supply wheels with a custom offset.|
Wheel Design and Construction
There are three methods of wheel manufacturing: cast, flow-formed, and forged in either single or multi-piece assembly.
Wheel Construction and Manufacturing
Cast. A cast wheel is one where liquid aluminum is poured into a mold and cooled. When removed from the mold and painted you have the final wheel. Cast wheels are always single-piece and nearly all factory BMW wheels are cast alloy. The casting method is quick and inexpensive compared to the other methods. Depending on the density and materials used to make up the alloy, cast wheels can be very light weight. Unfortunately, the casting process leaves wheels with a porous cross-section of material. There are air pockets and voids that makes the material soft and prone to flexing and bending. To make the wheels stronger manufacturers will force more material into the mold which makes the wheel heavier. Cast wheels are generally fine for ordinary street use as long as they don't come in contact with curbs, potholes, etc. But because the alloy is soft it can be repaired easily by heating and re-shaping.
Forged. Forged wheels are made from a single block of high-grade aluminum. A precise CNC milling process machines and shapes the block into the wheel design. Forged wheels can be single-piece (often re-named 'monoblock') or multi-piece designs. The alloy mix that is used is typically better and a higher grade than cast or flow-formed wheels. Because the process starts with one solid mass of material, there are far fewer air pockets and voids so the wheel is always stronger. Most forged wheels are far stronger than any force or condition they might see on the street as well as many racing environments. Since there is no mold involved, forged wheels can be almost any design and style although most manufacturers will test their designs using finite element analysis and motorsports. Using FEI additional machining can be done to reduce weight without affecting strength. Because of the time and costs involved, forged wheels are almost never used on production cars. The materials and craftsmanship demand a higher price than cast or flow-formed. A monoblock forged wheel can be repaired if bent or damaged but requires different techniques than on a traditional cast wheel. A multi-piece forged wheel can usually have the damaged piece replaced, usually the outer rim, without paying full price for a complete wheel.
Flow-formed. The flow-forming process is relatively new and combines traditional cast methods with a semi-forging process. The wheel is technically cast but with a much denser concentration of alloy material. The wheel center (face) is cast with a much narrower inside barrel. After casting and when the material is still pliable the barrel is pressed and formed using hydraulic rollers with tremendous force. The rollers press out the rest of the barrel to its final size and shape. Flow-formed wheels are light like cast wheels but have excellent strength similar to forged wheels. OE wheel manufacturers like BBS and Ronal use flow-forming on some factory wheels, including select high performance factory BMW models.
Check out this very video on the flow-forming process:
Wheel Design and Type
Single-Piece (cast). The wheel is all one piece, cast from one mold.
Monoblock (forged). The wheel is all one piece, but machined from a solid billet.
Multi-Piece (2 or 3-Piece) Forged.
The wheel is made up of 2 or 3 pieces joined together. Two-piece wheels have an inner barrel and the outer barrel and center together. The center/outer is bolted to the inner. A 3-piece wheel consists of an inner barrel and the outer barrel and center are separate pieces. The pieces are stacked together with the center being the final piece. The hardware can be installed from the front or the back through all three pieces. The exposed hardware can also factor into the aesthetic of the wheel too. Forgeline wheels also have a hidden hardware option for a cleaner look. The bolts are inserted from the back into Heli-Coil inserts in the back of the wheel center. Multi-piece wheels are designed to be taken apart and rebuilt when one piece is damaged. This makes them ideal for racing and track use and doesn't require a wheel specialist or unique tools. The cost for one single piece is less than an entire wheel and sometimes less than other single-piece wheels. In some cases the wheel center can be changed for an entirely different design.
Flat Lip vs. Stepped Lip
In many custom wheel listings there may be options for the outer wheel lip on 3-piece custom wheels. There is the standard flat lip and a stepped version. The flat lip is your typical smooth outer lip. For the last several years, wheel styling favored a very wide flat lip with a polished or contrasting color to the wheel center. The flat lip also allows the wheel spoke to extend as close to the edge of the wheel as possible (sometimes making the wheel appear larger than it is). The one downside to wide flat lips is that water can actually pool in the lip itself. The water stays pooled on the lip even when the wheel is rotating and that can lead to a vibration. That won't be an issue in a dry climate but we notice it in the Northeast.
Forgeline's solution is a stepped lip design where the outermost area is flat but the inner section where the wheel center is attached is a smaller diameter. It actually 'steps down' in size. This gives the water a way to drain off and not pool inside the lip. The downside here is that the smaller wheel center makes the whole wheel appear smaller overall - a 19" wheel looks like an 18". In our New England climate we favor the stepped lips but for less rainy regions or for maximum visual affect the flat lip is the best.
Offset In Greater Depth...
Wheel width and offset are the most critical dimensions you need to be aware of. Especially when looking at wheels that offer multiple width and offset combinations. Wheel offsets change based on the desired final placement of the wheel within the wheel well. You can't just put a wider wheel on without adjusting its offset so that it A) sits in a similar position as the original, or B) it clears other components on the car (brakes, strut/shock, suspension arms, inner and outer bodywork including fenders). Read on for more in-depth explanation of offset.
When talking about offset, visualize the Mounting Pad in a fixed position. As the offset changes the barrel and rim of the wheel slides in and out in relation to the Mounting Pad. A higher offset puts the inner lip closer to the suspension or inner bodywork. A lower offset will shift the rim to the outside of the car. In most situations, you want as wide a wheel as possible that sits flush to the fender on the outside but still clears your suspension and brakes.
How Width and Offset Interrelate:
In general, BMW will vary the offset based on width. If the stock wheel is 8.0" wide with a 40mm offset and they offer a factory wheel option in 9.5" they will increase the offset by 6mm so the outside of the wheel sits in the same position as the base wheel. There is quite a bit of math going on to get that result. The width is in English Standard; the offset is in Metric. So the formula looks like this:
First, convert the wheel width to metric mm. The original wheel = 8.0" x 25.4 = 203.2mm (203mm). The new wheel = 8.5" x 25.4 = 215.9mm (216mm).
Second, Calculate the difference between them - 13mm. The difference in width is split evenly between the inner and outer rim halves. So add 6.5mm to both and the wheel sits that much closer to the front fender (and to the strut on the inside). But remember, we want the wheel to sit in the original position.
Third, Add the outer rim extension to the inner rim so now the wheel is not poking out any more. Add the 6.5mm to the original 40mm offset = 46.5mm (round down to 46mm).
That is the basics of how offset and width affect each other. But let's look at more examples -
If you have a factory BMW wheel with an 8.5" width and 40mm offset (ET40) and you replace it with another 8.5" ET40 wheel the position of the wheel in the wheel well does not change. If the new 8.5" wheel has a ET50 offset, the new wheel is going to sit 10mm further inboard than the original ET40 wheel. This may put the wheel too close to the inner wheel well, suspension, and brakes. Conversely, an ET30 offset will place the wheel 10mm further outboard than originally. This puts the wheel closer to the inside lip of the fender. Replacing one wheel with another of the same width is easy. Now factor in a wider width.
8.5/ET40 replaced with 9.0/ET30. The wheel is a half-inch wider but now the offset is lower. You have added 12mm of extra width then shifted the barrel 10mm further outward. The new wheel sits 4mm further away from the strut and 16mm closer to the fender.
8.5/ET40 replaced with 9.5/ET43. The new wheel is 25mm wider but the offset has increased by 3mm. The end result is this wheel will sit 16mm further inward but also extend 10mm further outward. We're worried about this fitment because the offset is high and will leave the wheel tucked too far in. Since a stock wheel/tire may only have 10mm of clearance to the strut this wheel/tire setup may rub on the strut. Plus, a tucked under appearance just looks strange. A wheel spacer can be used to push the wheel/tire away from the strut. Or use a lower offset wheel - a 9.0/ET35 wheel will be 8mm closer on the inside and 18mm closer on the outside. Just make sure that your stock wheel/tire has adequate room on the outside or fender modifications will be required.
We test fitted three 18x9.5 wheels with different offsets to the rear of an E90 330i to demonstrate the changes that offsets can make.
Won't Wheel Spacers Change My Offset?
No. Wheel spacers do not change the offset of the wheel. When offset changes are made to the wheel itself, the profile or concave or "dish" of the wheel also changes and the spokes are designed differently. Usually the wheel is finite element tested as well to ensure the spoke design can handle the loads based on the new position. Wheel spacers change the mounting point of the wheel. They are really 'hub spacers' or 'hub extensions'. Spacers have a similar effect as a lower offset in the overall stance and track of the car.
Wheel Clearance and Fitment Notes
These are some of our own notes on what works and doesn't work when fitting larger wheels to BMWs. It's not a complete how-to guide but just some things that we have witnessed or experimented with over the years.
Use BMW for inspiration.
BMW has their own upgraded wheel sets. These wheel/tire upgrades tend to be conservatively sized and with a higher offset than desired. We consider these to be a good basis for selecting new wheels and tires because BMW's size and offset are almost guaranteed to fit without issues. But improvements can still be made for a better fit.
We assume you have already decided on a staggered or square setup. Next you should decide how large you want to go. If you want to stay conservative choose a wheel that closely matches BMW's own upgraded wheel sets. For a more athletic appearance and stance look for wheels that are half-inch wider in the front and rear than the factory upgrade. In either of these cases you can almost guarantee a slam-dunk fitment with minimal issues. But going far beyond these sizes will take much more consideration.
Changing Wheels and Tires on AWD/4WD Models, Cars with Limited Slip Differentials, or DSC
Because AWD cars proportion torque based on loss of traction it's very important not to upset the ratio of transfer. BMW has built in a ratio where the system delivers a pre-determined amount of constant torque to the front and rear axles. Using tires that alter the front:rear torque bias can trigger the AWD system into directing more torque to the tires it thinks are slipping. The same logic applies to the left:right proportioning on RWD cars with limited-slip differentials. If the system is over-working because of the incorrect tires it will wear out considerably faster. It will also result in inconsistent or dangerous handling.
It can have a similar effect on newer cars with ASC or DSC if the wrong size is used. If a lower sidewall height is used the DSC computers will think the rears are slipping compared to the fronts. And that will trigger the DSC into activating and applying the brakes and/or cutting engine power. It's more prevalent on some cars than others but customers and owners have reported it. This is why a lot of track tires are built with a taller/fatter sidewall rather than the slim low profile.
When shopping for new tires, or going with a larger size, it's very important to note current tire sizes on your car and their revolutions per mile (available from all tire manufacturers). As long as the ratio in front to rear sizes remains the same as factory you should not experience any driveline issues. Also on AWD cars, strongly consider buying a fifth wheel/tire to keep as a spare and include it in your regular tire rotations. This ensures the spare will not contribute to driveline issues when put to use.
Stock Wheel and Tire Sizes for Reference
Use the table below when shopping for new tires to fit to your wheels. Remember, large deviations in the front:rear sizes can have serious effects on AWD systems.
|Model||F/R||Stock Wheel Size||Stock Tire Size*||Rolling Diameter**|
|E85 Z4 M||F||18x8.0"||225/45-18"||25.97"||R||18x9.0"||255/40-18"||26.03"|
|* = stock tire size is a sample from the available sizes offered for this model.
** = rolling diameter is not the same as revolutions per mile. The rolling diameter is simply sidewall height (x2 and converted to inches) added to wheel diameter. It's a useful reference when jumping to a larger wheel/tire. If your overall diameter stays close to the original spec you can avoid AWD, LSD, DSC, and speedometer issues. Revolutions per mile is a better metric to use for comparisons but will vary by tire brand, tread style, inflation pressure, etc.
Major factors to consider when fitting aggressive wheel sizes and offsets:
Sample Wheel Sizes and Offsets by Model
Use the chart below to see standard sizes and offsets, a mild/safe size upgrade, and aggressive sizes and offsets. The below information is for reference only and should not be considered or used for a guaranteed fitment on any particular car.
|E36 M3||F||17x7.5 ET41||17x8.5 ET40||17x9.0 ET41||R||17x8.5 ET41||17x9.0 ET41||17x9.0 ET41|
|E39 M5||F||18x8.0 ET20||18x9.0 ET31||18x9.5 ET22||R||18x9.5 ET22||18x10 ET33||18x9.5 ET22|
|E46 non-M||F||17x7.5 ET41||18x8.5 ET38||18x9.0 ET31||R||17x8.5 ET50||18x9.0 ET42||18x10 ET43|
|E46 M3||F||18x8.0 ET47||18x9.0 ET42||18x9.5 ET35||R||18x9.5 ET27||18x10 ET25||18x10.5 ET27|
|E82 135i||F||18x7.5 ET49||18x8.5 ET45||18x9.0 ET42||R||18x8.5 ET52||18x9.5 ET62||18x9.5 ET62|
|E82 1M||F||19x9.0 ET31||18x9.5 ET35||18x10 ET25||R||19x10 ET25||18x10.5 ET27||18x11 ET25|
|E85 Z4 M||F||18x8.0 ET42||18x8.5 ET38||18x9.5 ET35||R||18x9.0 ET30||18x9.5 ET22||18x10.5 ET27|
|E9X non-M||F||19x8.0 ET37||18x8.5 ET38||18x9.5 ET35||R||19x9.0 ET39||18x9.5 ET35||18x9.5 ET35|
|E9X M3||F||19x9.0 ET31||18x9.5 ET22||18x10 ET25||R||19x10 ET25||18x10.5 ET27||18x10 ET25|
|F30 335||F||20x8.0 ET36||18x8.5 ET38||18x9.0 ET42||R||20x8.5 ET47||18x9.0 ET42||18x9.5 ET43|
|F8X M3/M4||F||19x9.0 ET29||19x9.5 ET20||18x10 ET25||R||19x10 ET40||19x10.5 ET34||18x11 ET44|
The Effect of Wheel Diameters On Performance
One of our race team tire engineers, and a veteran club racer, came up with some great insight in wheel/tire diameters. If you have ever asked yourself why there aren't many 19" or larger racing tires this bit of insight should be very interesting to you.
"In the grand scheme of things wheel diameter doesn’t make much of a difference as long as the revolutions per mile are the same, and in many cases running a smaller diameter wheel with a taller sidewall is desirable on track cars as long as they clear the brakes due to lower unsprung rotating mass and cost and availability of tires. The further away from the center of the wheel the weight is, the more difficult it is to speed it up or slow it down, and the heavier the wheel becomes. In most cases, a 18” diameter wheel is the theoretical ideal for the balance of weight and tire performance. There is an increase in the cornering performance of the tire by going to a shorter sidewall, but there comes a point where the extra weight and further outboard location of the weight offsets the benefit of the shorter sidewall."
Wheel Galleries by BMW Model
Click here to view photo galleries of wheels by BMW model.
Lost or unsure? Did I throw too much info at you? Leave a comment or contact me through the site and I'll help if I can.