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Home > Tech Article Directory > Specialty Articles > Wheel Fitment and Spacing
Wheel Fitment and Spacing
Weíve all been there. The weekend rolls around, the weather is great and you decide that itís a perfect day for a cruise. Youíre rolling down the coast knowing that your car, the one you spent hours and hours meticulously detailing the night before, is looking good. Stock body, perfectly appointed from the factory, and rolling on those ďsportĒ OEM wheels because you want everyone to know this is a performance machine. Youíre proud of your car. But as you pull up to a stop light, it happens. Everything seems to slow down like that time the cute girl from your humanities class smiled at you in the hall. Your car, the very same car, pulls up right next to you. Only something is different. Same stock body, same options as yours but itís not wearing the sport wheels you optioned at the dealership. No, your doppelganger is fitted with some wider, more aggressive aftermarket wheels and all you can think of is how to get a set for yourself. 

Few automotive items can have such a performance and aesthetic impact as a new set of wheels. Stock wheels are typically designed with low cost and a wide range of fitment as their main target points--this includes many "sport" OEM wheels. This may work for your everyday commuter car, but if you are looking to improve the look and feel of your vehicle, then you need will most likely want to make some changes. Before you go and bolt on the biggest, baddest wheels you can find, letís get a broader understanding of what's involved. We'll start with the basics and move on to some advanced and performance fitment guidelines. Weíll also give you an alternative view on the practicalities of winter driving in the northeast (or wherever there is freezing and thawing weather). Lastly, weíll finish with a discussion on the various aspects of wheel design and their impact on performance and driving dynamics.
The Basics
A wheel can be divided into three areas: front, side and rear. Each area is responsible for a specific task in relation to the wheels, tires and mounting.

Front:
Face: The face is the outer portion of the wheel that faces away from the car.

Diameter: The diameter of a wheel is the measured distance from bead seat to the bead seat directly opposite.

Lug nut seats: These are where the lug nuts seat and hold the wheel to the hub. Lug nut seats come in a variety of styles (acorn, ball, mag). Please ensure that you use the correct lug style to mount your wheel. Never mismatch these; for example, do not use a ball lug nut on an acorn seat or vice versa .

 
Bolt Pattern: The bolt pattern is defined by the number of lug nut seats to the diameter of the imaginary circle formed by the circle of lug nut seats.

Valve stem hole: This is the location that the valve stem (a rubber or steel stem used to inflate your tire) is installed.

Center Cap: Located at the very center of the wheel, usually capped off with the car or wheel manufacturerís brand logo.

Lip: The outer most edges of the wheel.

Side:
Barrel: A metal barrel that fills the expanse between the outboard and inboard flanges.

Outboard and inboard flanges: Comprised of the outer and inner lip and bead seats

Width: the width of a wheel is the measured distance from the inside of the lip on the outboard flange to the inside lip of the inboard flange.

Bead seats: The flat areas of the flanges where the tire bead rests on.

Centerline: The line that is center of the wheel and equal distances from the outboard and inboard flange.

Offset: This is the distance between the center line and the wheelís mounting pad. Where centerline is zero, moving the mounting pad toward the outside of the wheel results in a positive offset and moving the mounting pad inward toward the inside of the wheel results in a negative offset.

Backspace: Distance from the mounting pad to the inboard lip.
 
Back:
Mounting pad: The flat center area of the wheel that mounts flush to the hub. This area contains the center bore and lug nut holes.

Center bore: At the very center of the wheel is a large hole called the center bore. Depending on whether your car is lug centric (meaning that your wheel is centered via the lug nuts) or hub centric (which means itís centered via the lip on your hub) your wheel may have a specific diameter to match the suspension hub lip.

All these parts work together to fulfill a wheelís three main jobs: 1) suspend (keep the car off the ground), 2) roll (forward and backward), and 3) turn (change the direction of the rolling movement left or right). Stock wheels work perfectly fine at normal driving speeds and conditions. However once we approach higher speeds, changing road conditions, or a combination of both, the limitations of the stock wheel and tire configuration can quickly become apparent. These limitations can include:

Contact Patch:
The contact patch is the amount of tire that is in contact with the road at any point in time. At normal speeds and conditions the tire flexes and deforms in order to keep the contact patch in contact with the road and within the tireís threshold of grip. Increases in speed, changes in direction, or road conditions, will begin to act on that contact patch and its threshold of grip. Pushed beyond this threshold, grip with the road can be compromised.

Tire compound type: Due to ever increasing demands for fuel economy, manufacturers have outfitted a majority of vehicles with low rolling resistance or all-season tires. These tires are great when trying to maximize EPA fuel ratings, but their hard rubber compound and low rolling resistance means their threshold for grip is very low, not something you want when pushing your car to the limit!

Sticky tire compound: Overcoming the above limitations can be as simple as mounting stickier tires to your stock factory wheels. Sticky compound, usually found in summer and racing compound tires, increases the grip threshold for the same amount of contact patch. Unfortunately stock wheel sizes often have very few sticky compound tire choices available.
Aspect Ratio
Even though this article is about wheels, tires are an integral part of the discussion. And there is an important feature of tires that I need to explain here: Aspect ratio. This is defined as the ratio of the height of the tire to its width.

Letís understand this in more detail. The height being referred to is the distance from the tire lip to the crown of the inflated tire. So itís essentially the height of the tire sidewall plus the thickness of the tread.

The tire is usually wider than the rim itís on. Again, the width is measured with the tire inflated.

The ratio of height to width in many older passenger tires designed for general road and street use was 80%. Sometimes, for the more sporty crowd, 70 series tires were used, with a height to width ratio of 70%. The more sporty and performance oriented tires dictated the lower the ratio. Soon cars were being fitted with 60 series tires, then 55, then 50. And lower.

The idea behind low-aspect-ratio tires is that the lower the sidewall height, the less flex there is in the tire and the tauter the handling becomes. There are other consequences as well. The effect of low aspect ratio on rolling distance and also on winter driving is discussed later in this article.
Advanced and Performance Fitment
Using the basic information above will give you a general understanding of what your stock wheels and tires can do. But what if you want to take your tire/wheel combo the next level? If thatís what you are looking for, there are additional steps you can take in your wheel selection and fitment to really bring out the best in your car's handling. Follow along as we outline some general guidelines for wheel width/diameter and performance fitment that can result in a truly dynamic driving experience.

Yes, a sticky tire will help you round a corner better but those stock wheels still donít have the visual or performance impact that larger, wider ones can have on your car. We all want that low, tight race car fitment but when installing larger or wider wheels, there are a few items you need to consider before installation. *insert picture of race car or aggressive fitted sports car*
 
Clearance: Maximum wheel width is limited by fender and suspension component clearance. In order to select the proper clearance, you will first need to know the distance from the mounting pad to the inboard lip, this is called backspace. With the backspace figured out, you also can figure out your offset using the chart below.
 

 
In addition to knowing where the inboard and outboard wheel lips sit in relation to the fender line and suspension components, we will also need to know the physical dimensions of the tire to be mounted in order to get a complete idea of how the wheel/tire combo fits. Actual tire dimensions vary from manufacturer to manufacturer, but most manufactures list their dimensions on their websites.

Now that we have an understanding of wheel and tire dimensions, we can begin to look at larger and wider wheel combinations. Generally you want to try and work with your original tire and wheel specifications and increase from there.

Example:  If your stock wheel is 18x7.5 with a 40 offset (ET40), keeping your offset the same but increasing the width of the wheel an inch to a 18x8.5 will bring the inboard lip ~12mm closer to the inside suspension components and ~12mm closer to the fender.
 

 

We can see that with our new increased width, the wheel can accommodate a wider tire. This should improve the car's handling dynamics. However there are a few caveats with running wider wheels.

Wider but slower?: Adding more weight to the outer edge of the wheel is the last place you want it located. It takes more effort to accelerate and change direction on a wheel with more weight on the outer edge than one with more weight toward the center. While a wider wheel offers greater contact patch, it takes more energy to get power to the road and depending on the increase in weight, we can actually see a loss in acceleration.

Increased steering effort: Wider wheels can also increase steering effort due to the increase in friction from the increased tire contact patch and grip. This is an issue mostly associated with older, non-power steering assisted vehicles.

Better tire selection: Moving from a stock width to a wider wheel can open a larger variety of tire selections. Stock wheel sizes are usually designed with all seasons or low rolling resistance tires in mind. Increasing our wheel width moves us into more performance based options. Here you will find sticky tire compounds suited for high performance use.

More grip: Increasing the width also changes the shape of the contact patch which allows for more grip in turns. So while we may lose a little straight line acceleration, time and speed is made up in the corners as the wider contact patch grips the turns over a larger area of the tire. An additional benefit of more grip is an improved road feel.

Just like changing the width of our wheels, we can also change the diameter. This change can provide many performance benefits but there are also some considerations to take into account before doing so.
 
Speedometer accuracy: Your speedometer is calibrated from the factory to a certain rolling diameter wheel and tire combination from the factory. This is usually measured from the wheel, axle or drive shaft and the number of revolutions is translated into miles per hour. When we change our wheel diameter or tire aspect ratio, this size is no longer in sync with the speedometer causing it to read higher or lower speeds. We can remedy this by changing the aspect ratio closer to the factory calibrated rolling diameter using the equation below or one of the many tire size calculators available online.
 

Ride comfort:
A larger wheel can also increase the harshness in the way your car rides. In keeping our aspect ratio correct with our speedometer, we have reduced the space between the wheel and road. This space acts as a buffer that will absorb the shock of movement; so the less buffer we have, the more shock will transfer to the suspension and rest of the car resulting in a harsher ride.

Increased steering effort: Similar to increasing wheel width, a larger wheel will also increase steering effort but this is still mainly a concern for cars not equipped with power steering.

Wheel bearings: You should also be aware that the your carís designers calculated wheel size, tire size and wheel offset in order to keep roughly equal loads on the inner and outer halves of the wheel bearings. If you fit the vehicle with wider wheels and tires and accommodate them within the geometry of the vehicle by spacing the wheels out further on the hub, you run the risk of throwing the load on the wheel bearings off so that they wear out quicker.

One should keep in mind these caveats as well as the benefits gained through wider wheel width. If you have dismissed the down side considerations, the benefits of increased wheel diameter can be explored.

Example:  Letís say that after fitting our 18x8.5 ET40 we donít like how much tire sidewall we have and decide that a 19 inch wheel will look aesthetically better. Remembering that our speedometer is calibrated to our stock wheel of 18x7.5 ET40 and 225/40R18 tire we know we have to try and keep our rolling diameter as close to that as possible. After comparing the rolling diameter of the stock wheel to our new 19x8.5 ET40, we find that 225/35R19 has the closest dimensions with a .7% variation (at 60 mph the speedometer will read 60.4 mph). Larger diameter wheel can also provide more suspension clearance due to the increased distance provided by the now larger diameter inboard lip.
 

Big Brake kits:
Increasing our threshold of grip through larger, wider wheels and tires allows for more confidence to push our cars harder. However, this increase in grip also puts more strain on the stock braking system. This can result in spongier pedal feel, higher wear rates, and longer stopping distances. Luckily with larger wheels comes the option to install bigger brake components. Big brake kits, with their larger diameter rotor and larger caliper, increased mass and improved heat management and displacement properties are better suited to handle the increased strain. Performance benefits also include pedal feel, consistent rotor and pad wear and shorter stopping distances.

Image Courtesy of Brembo.com
 
Smaller = Better? : We can also look at a change in the opposite direction by reducing our wheel diameter for a performance increase. By downsizing our wheel diameter, we gain a higher effective gear ratio which increases our acceleration at the expense of some top end speed. Smaller wheels usually have a lower weight and require less effort to accelerate and change direction; giving us a more responsive handling car. But remember, this all depends on whether or not you can find good enough tire compound for the smaller sized wheel.

Understanding the following relationships will help you safely pick a wheel/tire combination that will safely clear suspension components and the fender line and increase performance.
 
Spacers: Letís say you found a wheel/tire combo that safely clears everything and looks better aesthetically but you want to either have a little more suspension clearance or bring the wheel a little more square with the fender line. Spacers are a great way to safely accomplish this. Spacers are machined disks that attach to the suspension hub and move the wheel toward the fender line. Using our wheel width example from before, after fitting our new 18x8.5 ET40 wheels, we noticed that even with our increased width we are still not as square to the fender line as we would like to be. Measuring the distance from the fender line to the wheel, we find we have about 16mm of space between them. Using a 4mm spacer, we can move the face of the wheel closer to the fender line, gaining a squarer 12mm clearance between fender and wheel as well as gaining an additional 4mm of suspension clearance.

 
It is important to note that hub spacers do not change the offset of the wheel. They simply extend the suspension hub mounting surface outward.

Bear in mind also the previous caution about excessive wheel bearing wear caused by uneven load when the wheel is spaced out.
 
Custom offset wheels: While spacers can be very useful in dialing in a wheel set up, in very large changes of offset it is often a wiser solution to have your wheels customized with an offset tailored to your use. Bolt-on hub spacers have been known to loosen from the mounting hub, so having a custom wheel with the offset provided by the spacer already there can increase the safety of your wheel setup. In addition to changes in offset, custom wheel manufactures can also change factors like wheel diameter and width, the material used, cast or forging process, monoblock or multi piece construction and many other options to give you a truly one of a kind wheel.
Tires, Wheels and the Realities of Winter Driving
While performance driving may be something we all aspire to, we also have to get to work every day, or to the grocery store, or take the kid to the doctor. In New England and other parts of the country where it snows and the roads undergo destructive freezing and thawing every winter, keep in mind the following cautions and considerations when it comes to wheels and tires. Wide and sticky tires may work well on dry or even wet pavement, but become toboggans on snow. And do not kid yourself about ďall-seasonĒ tires. To get around on snow you need snow tires Ė at all corners of the vehicle. Snow tires give you additional traction so you can start, stop and turn with far greater safety. As mentioned in the discussion about aspect ratio, one of the performance gains from low profile tires is the result of lower sidewall height. The wheel is that much closer to the road and there is less sidewall to flex; therefore you get tauter steering and cornering response. This is desirable and helpful on smooth roads but on bumpy roads or ones with potholes, you are that much more likely to bottom your sidewall and whack the wheel into the pavement. You wonít like what that does to your wallet. Typically, high performance wheels are made of softer and lighter metals (aluminum or magnesium Ė see next section), so that bending them is easy and there are not many good ways to straighten them out. Keep in mind too that bottoming the sidewall in a pothole is likely to tear the tire as well. One incident like that and youíre likely to be out many hundreds of dollars for the damaged wheel-tire combination. So for winter driving I recommend you equip yourself with alternative steel wheels and taller snow tires. For year-round driving in pothole country, try to back off at least one step from the most aggressive low-profile tires. For example, if you are considering going for 35 series tires, maybe 40 series or even 45 series will give you most of the same look and feel of performance and still allow you to be spaced off the pavement slightly more. If budget is not a constraint, consider having one set of tall wheels and tires for street use and another lower-profile set for the track.
Materials and Manufacturing Methods
Not all wheels are created equal. In addition to all the information we covered above, things like material choice, manufacturing processes and construction can have an impact on your carís driving dynamics when choosing your next set of wheels. Wheels are made of a variety of materials, each with their own individual benefits and drawbacks. The following is a brief overview of the materials that are most commonly used in todayís wheels.
 
Steel: Steel wheels are durable and easy to manufactureówhich makes them cheap. It is also heavy and usually limited on design. While widely used in classic cars of yore, you mostly find them relegated to spare tire duty nowadays.
 

Aluminum:
Aluminum on the other hand carries many of the same pros that steel has but with the added benefit of a much lighter metal base and ease of design. Because of this balance of properties, aluminum makes up the majority of OEM and aftermarket wheels.
 

Moving up the material scale we start to find exotic materials like magnesium and carbon fiber.

Magnesium: Magnesium can be seen widely used in racing and many high performance supercars. This is because of magnesiumís extreme light weight. It is lighter than steel or even aluminum. In performance applications, lightness is a very desirable trait which we will go over in more detail below. All that lightness comes at a cost. Magnesium wheels are expensive, require a lot of up keep and have the nasty habit of becoming an unstoppable inferno if you manage to get it hot enough. Granted it takes a lot to get that much heat in them but once they get going, they are nearly inextinguishable.

Carbon Fiber:
One of the latest developments in wheel materials is the use of carbon fiber or carbon composites. Carbon fiber wheels use long strands of carbon threads woven together to make an incredibly strong material that can be formed and milled to nearly any desired shape. Carbon fiber has seen use in all kinds of automotive componentry where its strength and lightweight properties are prized, so naturally carbon fiber technology is quickly making its way into wheel design. The only real downside at the moment seems to be the price, which can make even magnesium wheels affordable by comparison.
 

Material make up of a wheel isnít the only factor in a wheelís performance. How the wheel is formed and constructed has a large impact on how durable and lightweight a wheel can be.
 
Cast: A majority of wheels, OEM and aftermarket, utilize a gravity cast forming process in which molten metal is poured into a mold and using Earthís gravity, fills the mold creating a single piece wheel that can be easily and cheaply made. Low pressure casting, which uses positive pressure to fill the mold, is another method thatís frequently used to create a denser and more structurally uniform wheel. However this has the unwanted effect of increasing the weight of the wheel.

Flow Formed: One method manufactures have developed to gain the denser and uniform metal structure of low pressure casting, but without all the excess weight is called flow forming. Starting with a low pressure casting, large rollers shape the barrel and lip under extreme pressure and heat. Under these conditions, the formed section of the wheel has increased strength, stiffness and impact resistance closer to that of a forged wheel but with the added benefit of reduced weight compared to a traditional or low pressure cast wheel.

Forged: Forged differs greatly from cast. One of the benefits of forging is that the manufacturing process begins with a solid billet block of high quality aluminum. The wheel or wheel components are then CNCíd or pressed from this block of metal. Casting typically allows for air pockets, impurities and imperfections as part of the process. Forging eliminates these issues. Additionally, the aluminum alloy used in this process is typically superior to the metal found in cast construction, allowing forged wheels to use less metal material to build a stronger and lighter wheel. Milling from blocks of aluminum also allows wheel manufactures to be more creative with their designs.

Multi piece wheels: Another approach manufactures take in their wheel design is to segment the wheel into multiple sections. The main benefit to a multi piece design is modularity. These modular multi-piece wheels allow the user to change the various dimensions of a wheel on the fly. Compared to a cast or monoblock forged wheel (which is milled or pressed from a single block of material) where the dimensions are set, a multi piece wheel can increase or decrease barrel width, change offset positive or negative and increase wheel diameter by changing its individual pieces. This also comes in handy when a wheel is damaged. In many cases the affected portion of the wheel can simply be replaced rather than replacing the wheel entirely. A definite cost saving!


Image Courtesy of Fifteen52.com
Unsprung Weight and Handling
Throughout this tutorial, we have seen how manufactures have used different materials and manufacturing processes, all in the search for lighter weight wheels. But why? Why spend all this money and research on featherweight metals and forging technologies for a few measly pounds? The answer is un-sprung weight and rotational mass.
 
Un-sprung weight: There are two types of weights on your car: sprung weight and un-sprung weight. Sprung weight is all the weight supported by the suspension and un-sprung weight is all the remaining weight not supported by the suspension; usually comprised of the wheels, brake components and some of the suspension itself. Your suspension works by controlling the inertia of the un-sprung weight to keep your wheels and tires connected to the road. The heavier the wheel, the more inertia it has and the harder your suspension has to work to keep it on the road. By reducing the weight of our wheels, we lower our un-sprung weight and inertia, thus reducing the load on our suspension and improving a carís feel and handling.

Rotational mass: Rotational mass is stored energy. Using power provided by the engine, your car uses its horsepower to start rotating the wheels. The heavier the wheel is, the more force is needed to spin it which can sap available engine power. Moving to a lighter weight wheel, your car needs less power to move the wheels, leaving more horsepower available for acceleration. As mentioned above, heavier objects are harder to accelerate or change direction but they are also harder to slow down. So the lighter the weight it is, the easier it is to slow down. Also it is important to note that itís not just how much your wheel weighs, but how that weight is distributed around the wheel. Weight located toward the outer periphery of the wheel results in greater rotational mass energy vs a wheel of the same weight with a majority of its weight closer to the center axis of the wheel.

By focusing on reducing both un-sprung weight and rotational inertia, we can extract the maximum amount of performance out of our wheel and tire setups.
Summary
From this point you can get quite imaginative with your wheel and tire combinations. No matter what direction you go, whether its maximizing your road grip with the widest wheels or using a super low negative offset for that deep dish look, using the guidelines and considerations above will help you find your next set of well-balanced performance wheels. A great place to start your search is here at Pelican Parts. We offer a wide selection of wheels from top brands like BBS, VMR, Fifteen52 and more. Also check out our great selection of wheel care and accessories to keep your wheels looking fresh. Be sure to check out our YouTube channel for the latest Pelican Parts tech tutorials and automotive news. Click the links below for more.

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