Wheel choice can make a big difference to your vehicle’s ultimate performance potential. When choosing a new set of rims for your track car, you need to consider the following factors; diameter, width, offset, construction, caliper clearance, hub centricity, bolt circle, bolts versus studs, and air flow/spoke pattern. Let’s examine these differentiating factors to better understand what will ultimately make for a better choice when it comes to new wheels.
If you have ever watched one of the discovery channel automotive tv shows where it seems the bigger the wheel you can stuff into a fender, the better. Then you tune into a Formula 1 race, and all the cars are circulating on 13-inch wheels. In the case of the former it is style over function, and with the latter, it is due to the rules package. If you take a closer look at F1 wheels you can see that they just barely clear F1 brakes. In reality, a Formula 1 car would probably perform better with a 15” rim with commensurate larger brakes. This approach has already been suggested by many of the race engineers over the years.
The deciding factor for choosing the right wheel diameter is the relationship with the maximum size brakes, and the overall weight of the vehicle they are fitted to. Simply put, bigger heavier cars require bigger brakes and bigger wheels. So, what’s the downside to a larger diameter wheel? Essentially, it’s weight. Aluminum is heavier than rubber, and combined, the tire and wheel, along with the brake rotor, make up the rotational unsprung mass of the car. It is estimated that for every one pound of rotational unsprung mass saved, it is equal to four pounds of mass removed from the chassis in terms of performance gained. This is the primary reason behind the development of carbon brakes and lighter forged alloy rims.
When it comes to choosing the right diameter, pick the size that will just clear your front brake calipers, and will also provide you with the greatest selection when it comes to tire choices. Quite often it is advantageous to go up or down a diameter if it allows for better quality tire choices. Another consideration is the location of the rotational mass, quite often a larger wheel will move the moment of mass further from the hub center. This will increase the gyroscopic effect of the wheel, which in turn has a negative effect on braking and handling. Currently for production-based street cars, the sweet spot for wheel diameter is between 18 and 20 inches.
When it comes to width, wider is usually better but it is not as simple as just selecting the widest possible wheel. What you are actually selecting first is the widest tire that will properly fit the wheel opening without rubbing at any point of deflection, or steering angle. This will be a function of tire section width, wheel offset, and suspension geometry.
Once you have determined the optimum tire size and type for your vehicle, you can then choose the optimal wheel width to extract the best handling traits out of the tire. From a visual standpoint we are talking about the visible bulge of the tire as it relates to the rim. Most tire manufacturers will list an optimal rim width for a given size tire, however engineers may go up or down a half inch to a full inch in order to enhance handling characteristics. Usually, if you go with a narrower than optimum rim you can increase the edginess of the tire when on the limit. Conversely, a slightly wider wheel may numb some of these characteristics. This type of tuning is usually only seen in a half inch increment from optimal width, anymore than that, and the overall grip of the tire can become compromised.
A prime example of this type of tuning is evident with the 2014/15 Camaro Z28. This car runs on the same 305 section width 19” tires on all four corners. The wheels however are a half inch wider in the rear. The engineers purposely did this to improve the predictability of the back end of the car while simultaneously sharpening up the steering at the front. Another manufacture that has done this to great effect has been Porsche, specifically in order to tame the 911. Always remember that your handling is built around your tires and how effectively you use them. Wheel width is a critical component to extracting the best performance from your chosen tire.
Wheel offset is the distance that the wheel mounting flange is located in relationship to the centerline of the rim. If the mounting flange is further to the outside of the wheel the offset is positive. If the flange is to the inside the offset is negative. Formula 1 cars of the 70s often featured a positive offset front wheel and a negative offset (deep dish) rear wheel.
When choosing a wheel to maximize performance, the wheel offset becomes critical as it relates to handling. Wheel offset affects the following, positioning of the tire within the fender well, loading on wheel bearings, and critically the steering geometry.
Because wheel offset can change the wheel’s centreline position, it can also move that position in relationship to pivot points that the front suspension steers around. Altering this relationship too far one way or the other can create the following undesirable traits, kick through the steering wheel, steering effort, tramlining, premature bearing wear, and the dreaded shopping cart wheel wobble. The effects of a change in offset are felt less when the change is on the rear wheels. If a square set up is desired, try to build it as close to the front wheel off set specifications as possible
When selecting wheels, it is important to select an offset that will position the centerline of the wheel/tire close to the original geometry as possible. The centerline to centerline measurement is what is commonly referred to as wheel track.
Increasing the track (less positive offset) has the following effects, better handling, increased tramlining, and increased steering kick back. Decreasing the track (more positive offset) has the following effects, reduced handling, increased steering effort, increased body roll, and reduced steering feel.
Please note that these effects are amplified once camber and caster settings are set for the track.
Wheels can be made from steel, aluminum, magnesium, and carbon fibre, or a combination of these materials. Generally speaking, the lighter the wheel is for a given size, the more expensive it will be. In order to lose weight, while simultaneously meeting a specified design strength, more exotic materials and time-consuming construction processes are required. Because the wheels are part of the unsprung rotational mass, significant gains in performance can be achieved by selecting lighter options.
Steel is commonly seen as the wheel of choice for NASCAR and dirt track racing series. The close proximity, occasional wheel to wheel contact, wall proximity, and sustained high speeds of these racing series demand a high durability wheel. Steel has proven to be the ideal wheel material for these types of racing series.
Aluminum wheels come in many forms, cast, pressure cast, flow cast, forged, billet, spun, and combinations of these manufacturing techniques. You may also see mono block, 2-piece, and 3-piece wheels. The multi-piece wheels have an advantage for repair ability, durability, and customizable widths and offsets. The disadvantages of multi-piece wheels are higher cost, higher weight due to multiple fasteners, and a higher tendency to lose air over time. Besides cost, the biggest difference between cast and forged aluminum wheels, is that the forged wheel will have a higher strength to weight ratio than the cast wheel. Because of the high strength to weight, and improved manufacturing techniques, the forged aluminum wheel is currently the wheel of choice for manufacturers of high performance cars. When you consider all of the factors that make up an optimum wheel it can either be very difficult, or very expensive to find a better option than OE forged rims.
Magnesium, either die cast or forged, provides the ultimate strength to weight ratio, along with the durability required for motorsports. Magnesium is about 33% lighter than aluminum by comparison. Magnesium is also more costly to produce, and has a higher rate of corrosion and therefore must be properly protected. Forged magnesium wheels are currently popular in the highest forms of motorsports including Formula 1, Indycar, and Le Mans prototypes. Various OEM auto manufacturers have offered magnesium wheels as special options over the years. The 2000 model year C5 Corvette had magnesium wheels as an option. Aftermarket manufacturer Dymag has been making magnesium wheels for years and is currently offering magnesium and carbon fibre two- piece wheels.
Carbon Fibre has the highest strength to weight ratio of any material currently used to manufacture automotive wheels. Manufacturers are now starting to offer carbon fibre rims as original equipment. Notably Ford has led the way by offering this technology on the Shelby GT 350 Mustang. Previously carbon fibre was only found in the domain of the ultra-exotic hypercars, like Koenigsegg. So why don’t we see carbon fibre wheels in motorsports? The main reason beside cost, is safety. Although possessing a high strength to weight ratio, carbon fibre is not as impact resistant as an alloy wheel is.
Carbon does not bend like alloys when subjected to blows, it will either hold its shape or break, sometimes even shattering. As long as racing occasionally has wheel to wheel contact, or wheel to barrier contact, it’s not likely that we will see carbon fibre wheels allowed for use in racing.
Previously it was mentioned that the wheel should often just clear the brake caliper. This was in reference to the diameter of the wheel along the inner portion of the rim. Just as important, it is critical that the spokes of the rim clear the brake caliper. This is usually not a problem with wheels specified for vehicles in OE specification. Problems occur when brakes are upgraded, and different caliper types are selected. Many vehicles come equipped with floating type brake calipers which feature the piston(s) only on the inside of the brake caliper. An upgrade to this design is to change a fixed caliper with either 4 or 6 pistons squeezing from both sides of the brake rotor. This type of caliper will occupy a lot more space towards the spokes of the rim, often negating the figment of the OE wheel. Wheels designed for large brakes typically feature spokes that are designed and positioned very close to the outside of the wheel, this is what is commonly seen in motorsports these days. Often when a wheel is chosen for an older classic style it can be very difficult to fit adequately sized brakes. I’m sure you have noticed this when attending classic car shows. The key point about caliper clearance is to consider the brakes being used, and to also consider future possible brake upgrades before selecting your rims. Often the brake supplier will have cross section downloads available showing the minimum requirements for caliper clearance.
Wheels are either of hub centric or bolt centric design. Bolt centric rims are centered to the axle by the wheel bolts or wheel nuts. This is a design feature on older vehicles and is no longer used on modern vehicles. For this reason, we will focus on hub centric wheels. The hub of the wheel features a machined seating surface of a specified diameter. This surface mates with the flange that protrudes from or through the front or rear brake rotor. The purpose is to centre the mounting of the wheel in relationship to the axle centreline. The bolts or wheel nuts clamp the wheel to the hub. The tapered seat of the wheel’s nuts/bolts will accomplish the final positioning of the wheel.
Therefore, it is important to torque the wheels unloaded, in a star pattern. The added safety benefit of the hub centric design is that the mounting flange will provide additional support for the wheel should the wheel lugs become loose. This is why bolt centric wheels have all but disappeared from the OEM.
When purchasing aftermarket wheels, it is very important to make sure the mount is hub centric to your vehicle. Some wheels are model specific with the proper hub size bore built in, others are universal fit and require adapter inserts specific to the vehicle. It is important not to use these wheels without the correct adapter installed. Without the correct adapter you essentially have a bolt centric wheel that could easily work itself loose on the race track with catastrophic results.
Bolt circle is essentially exactly what it sounds like, and each manufacturer will have their own specific bolt circles. Some will have the same bolt circle as other manufacturers; however, there may be other critical differences such as hub centric size and wheel lug size. The bolt circle is a measurement, either in millimetres or inches, of the wheel studs, as if a circle were placed over the centreline of all the studs. The corresponding diameter of the circle represents the listed measurement.
Bolt circles are often expressed with the number of lugs first, followed by the diameter of the circle. Here are some common sizes;
- 5 on 120mm
- 5 on 4 3/4”
- 5 on 4 1/2”
- 5 on 130mm
- 5 on 108mm
In some cases, there will be an imperial size that is very close to a metric size. For example, 5 on 4 3/4” is the equivalent of 5 on 120.65mm. Some would say these are close enough to fit, those people are the ones that make very dangerous mistakes. A difference of .65 of a millimetre can cause a dangerous preload on the wheel bolts or studs leading to failure. Additionally, there can be differences in the lug size itself 1/2”, 12mm or 14mm. These differences, if not properly matched, can cause problems with proper wheel clamping. Some manufacturers have even confused matters more as they transitioned into the metric world. For example, Corvettes have an imperial bolt circle, 5 on 4 3/4” combined with metric wheel studs, 12mm by 1.5 thread pitch. If you have the correct bolt circle and correct hub centricity, you then need to pay attention to the wheel stud or bolt size so the wheel can be properly clamped to the wheel hub.
Bolts vs Studs
For everyday driving wheel bolts are fine and do the job very well. Several manufacturers have gone the way of wheel bolts for a variety of reasons. Weight savings, fewer parts, and reduced costs are some of the reasons manufacturers are using bolts instead of studs. Another reason is that they do not expect that the consumer will ever be changing a wheel on their own. If you have ever had to change a wheel in the dark without the aid of a flash light you will find yourself expressing a lot of choice words about wheel bolts! Trying to position the rim so the bolt can be threaded in can be a real pain, even in good lighting situations. This is why many enthusiasts who track their cars will install stud conversion kits on their tracked vehicles.Besides being easier to change, wheel studs have other advantages over bolts. Studs are better at applying clamping forces due to the fact that they do not have torsional (twisting) forces being applied in conjunction with tensioning forces as the nut is torqued. This results in a truer torque of the wheel nut and a more even clamping around the bolt circle. This even clamping can also have the effect of reducing brake rotor warping due to the fact that the internal stresses are more evenly balanced.
Another benefit of wheel studs is that a wheel will take much longer to loosen and will have more support should a wheel nut come loose. This is a great safety feature to have when you are hot lapping a race track. With wheel bolts the rocking associated with a loose wheel can cause all the bolts to unwind themselves fairly quickly. A couple of our members discovered this phenomenon the hard way this past season. The bottom line is that studs and nuts are a far better option for vehicles that see frequent track use and frequent wheel changes. If your favourite track car has wheel bolts then it would be a good idea for you to search out a wheel stud conversion kit.
Finally we can talk about spoke pattern, for the most part this is what makes the wheel and car look cool. Most modern performance wheels will feature spokes that have some form of triangulation to increase the dynamic strength of the wheel. The key thing with wheels for road racing applications is adequate airflow so your brakes can be cooled. Try to select a wheel that has good airflow for your brakes. Once you have found wheels that meet all the above listed requirements, the final deciding factor will be which wheels look the best on your car. What colour they are is all a matter of personal taste!
Beware of Replicas
One final note, beware of replica wheels, never use these on a race track. Replicas are often way cheaper than OE wheels, this is because they are made cheaper, often not conforming to the same rigorous standards, and often not possessing all critical design features. Often you will see Porsche or Corvette replica wheels offered for sale sometimes without the word replica present. The problem is the original forged wheels are being replicated with cheap cast wheels. Often these cast replicas will be as much as 10 pounds heavier per wheel than the forged wheel they are replicating. This is because these often poorly cast wheels have to be made substantially thicker to meet the minimum standards for road use. In fact, they will be far weaker and more dangerous than the OE forged wheel they are replicating. Save the replica wheels for boulevard cruising and car shows.
Enjoy the selection process!
Bill Drossos, Crew Chief