Automobile Wheel Facts
Automobile Wheel Facts Summarized Part 1
If you’re looking for a set of aftermarket wheels it’s a good idea to get your wheel facts straight before taking the action of buying and installing them. I am by no means an authority on wheels or technology, but I felt it might be useful to some who might be shopping for street or race wheels to assemble into a few shorts articles some basic facts about competition wheels and the associated terminology.
Recently I acquired a few sets of Panasport racing wheels as a part my own race car re-acquisition. These Panasports had all been purchased for use on a BMW 2002 race car that I had actually previously owned, and have now purchased back again. I sold the car to my now late friend, George Barbour about 15 years ago. just before I moved away from Columbus.
George was a close personal friend who was also often my racing companion, and who helped me to prepare the car for 2002 in SCCA events while i owned it. I raced it in ITC and later in E Production from 1993 until 1999. George had most recently been preparing the car for GT-3 after I sold it to him.
The bottom line is that I am now purchasing the car back from George’s family estate. My purchase package included several sets of Panasport UL wheels in various 13 inch and 15 inch sizes, many of which I now have no need. I intend to run 15 inch the wheels to which George had already converted, and to finish the conversion to 4 wheel disc brakes.
Broadening the Potential Buyer List
If I hoped to come into contact with a wide audience of prospective buyers for these extra and unnecessary (but extremely high quality) wheels, I knew I was going to have to become informed about potential applications other than for a BMW 2002.
The 15 x 7s with zero offset were chosen for the 2002 on the basis of a calculated desired track width, taking scrub radius into account, and considering the necessity for proper wheel profile to accommodate larger rotors and calipers behind the mounting surface. On the front of the car this meant a consideration for larger than stock rotors and calipers, while on the rear of the car it means accommodation for rotors, period. The 1600s and 2002s all used rear drum brakes, the 2002 version being a slightly larger diameter than those on used on the 1600. George ast ran the car with the 2002 sized drum brakes, and i have not yet done the rear conversion to discs.
The taller hub height of 15 inch wheels also provides plenty of margin to prevent interference between the rim and the control arms/tie rod ends on the front of the car, but I have no idea how much room the rotors and caliper are going to take up in the rear yet. It looks like a safe bet I will stick with the 15 x 7s, but I cannot yet be entirely certain.
To expand my potential audience beyond the BMW, first I simply Googled Mazda Miata wheel applications because I knew that I had seen 15 x 7 Panasports on Miatas. They have the right 4 x100 bolt pattern, but because I do not know Miatas, I have no idea whether the zero offset is a workable option for a Miata.
So, is anyone looking for some attractive Miata wheels?
Searching online for Miata wheels, I found that many sets of 15 x 7 are sold for Miataswith 25 mm positive offset. The offset requirements for a Miata are going to be a function of where the geometric pivot point for the steering arc is, as well as clearance issues. Perhaps an informed Miata enthusiast might be willing to comment here on whether or not a zero offset 15 x 7 is okay for use on a Miata. pacers are a potential solution, but in my opinion are they are a bad idea due to their tendency to cause lug bolts or lug nuts to work loose.
Wheel Fitment Issues for Brake Upgrades
If you have a stock brake setup and just want to change to a larger than stock wheel and you do not care to make the effort to measure to ensure a correct fit, then I would suggest taking the advice of those who have done so for you already. Most of the major wheel retailers online can provide you with reliable information on up-sizing your wheels with standard brakes in place. They don’t like to handle returns, so they have done their homework accordingly.
But how do you choose a wheel that’s compatible with your brake upgrade? The only answer is either to measure it all up after your brake conversion is complete, or to believe someone else who had already measured for your choice of brake components.. If you are changing brake components from stock at all, there will be a possibility that you cannot retain the stock wheels.
Modify your braking system first, and then find suitable wheels to match your brake modifications, never the other way around! Moving your brake components around to match the set of wheels you happen to have on hand is always a bad idea, and may end up compromising some element of your braking upgrade. It;s a band-aid solution. Finding the right wheels is an easier, safer solution to solving an unintended brake interference problem caused by a change in brake component size and location.
Having the right rim profile behind the hub center (because that’s where the calipers will reside) is critical for preventing caliper interference issues.
Caliper locations sometimes must migrate outwards whenever larger, thicker rotors are installed. Bigger brake setups mean both more powerful calipers and larger and thicker rotors. The profile of the wheel inside of the mounting surface must not only be capable of accommodating the larger diameter rotor, they must also be able to accommodate the extra rotor thickness..
Street Brake Upgrades
Picture a thicker rotor mounted in exactly the same location as the stock rotor with respect to the hub. A thicker rotor means that the rotor hub will be a little bit shallower than it was with the stock rotor. Well designed street oriented brake upgrade systems take this into account and generally will allow the upgrade without creating a need for changing wheels.
Race Brake Upgrades
With more radical braking upgrades such as those applied to race cars, there is generally less concern about retaining stock wheels. I think that most readers should be capable of translating these words into a pictorial concept by examining the drawing below, which I robbed from another website.
With a thicker rotor installed, caliper brackets may put the caliper closer to the wheel center than it previously was, not to mention the possibility that the caliper itself may also be bigger. Both factors might cause wheel to caliper interference. I believe the best approach to solving these problems to be a scientific one backed up by actual measurements. You just have to measure, think it all out, and make yourself a drawing of the size of wheel needed to accommodate the brakes you are dealing with.
I plan to document this with a practical example in a later blog post, which may be more useful to many readers looking for specifics. If you’re a good logical thinker, you can figure this all out based on the outline provided here without a specific example, but I promise to eventually provide my own example.
Of course you could always estimate by eyeballing things and using the trial and error methods. Many people do this and get away with it. Personally I like knowing, at least theoretically, how much clearance there actually is going to be between the wheel and the caliper, and being absolutely sure that the wheel is not going to interfere with any other suspension or steering components.
Wheel Terminology – Offsets
When you begin reaching wheel upgrades, it is vital to have a clear grasp what of exactly what is meant by each of the terms that are so freely thrown around.
The wheel offset and the backspace measurement are really the same thing, seen from two perspectives. Wheel offset refers to the location of the wheel mounting plane with respect to the center if the wheel. The following diagrams which I stole from the Tire Rack’s website make that clear. Potential dyslexic thinkers may still however become confused! I had to think this through several times when I wrote it, to be sure i was not explaining it backwards! In fact, the first time I published this post, I DID explain it backwards. Fortunately, nobody really noticed it before i corrected it! Truthfully, writing this post really helped me to firm up my own perceptions of offset and backspacing.
The picture in the center depicts a zero offset wheel. The mounting plane of the wheel is in the center of the rim. That’s easy enough!
A positive offset moves the wheel mounting plane closer to the outer surface of the wheel. So, when you match the mounting plane of the wheel to the hub, the outer edge plane of the wheel is spaced closer to the center of the car by the amount of the offset. | That means that a positive offset has the effect of decreasing the overall track, and that both wheels set further closer to the center of the car by the value of the positive offset.
Think of it like this: You have to push the wheel further towards the center of the car when you put it on before the mounting surface meets the hub. The positive offset moves the wheel surfaces closer to the center of the car, and thus farther from the inside surface of the fender.
A negative offset moves the wheel mounting plane of the wheel from the center of the wheel to a point closer to the inside surface of the wheel. So, when you match the mounting plane of the wheel to the hub, the outer edge plane of the wheel is spaced farther from the center of the car by the amount of the negative offset.
That means that a negative offset has the effect of increasing the overall track, and that both wheels set farther from the center of the car by the value of the negative offset. The negative offset moves the outer edge plane of the wheel surface closer to the inside surface of the fender, and further from the center line of the car.
Think of it like this: You do not have to push the wheel very far towards the towards the center of the car before mounting surface meets the hub. That means the track is going to be wider than it was without an offset. It’s easy to remember when you picture what are called “deep dish” wheels.
Be aware also that if you add width to a wheel but retain the same offset, then the extra width gets added 50/50 between inside and outside. That will often cause a problem with either inside or outside interference with something. Whether or not your car needs positive or negative offset to compensate for clearance problems caused by increasing the width, depends upon whether the potential interference is inboard, or if you are dealing fender well clearance issues. Not to mention handling problems caused by accidentally increasing the scrub radius.
Not to confuse things too much, but the choice of correct wheel offset with respect to additional wheel width is also tied to controlling an important geometric steering factor called scrub radius. Scrub radius is essentially the distance between the pivot point of a wheel when it is being swept through its steering range, to the center f the tire contact patch. Ideally, the contact patch center and the point about which a wheel rotates through its steering range should be as close as possible. Choice of the wrong offset and/or the addition of wheel spacers on the front of a car can upset the scrub radius. Scrub radius is a subject for yet a whole different discussion.
Choose wheel offsets that do not contribute to increasing the scrub radius. A rule of thumb for scrub radius is:
If you are not moving the steering pivot point around, then do not move the center of the contact patch. If you DO move the pivot point around, be sure the wheel contact patch location follows it. .
I hope not! I know i still was when i started writing this! All of the explanations I found online provided nice pictures, and then leave it up to the reader to figure it out in terms if what I just wrote down. Some of the wording I found was true enough, but really not helpful in terms of explaining the big picture. So I hope my words helps to dispel rather than contribute to any confusion you may have entertained about wheel offsets.
For instance, here is a confusing set of words I found that is really saying the same thing, but in my mind it is stated ass backwards:
“Most car makers set the center line of the wheel in from the hub face, inwards is called POSITIVE OFFSET, and outwards is called NEGATIVE OFFSET.”
These words describe moving an imaginary surface around, namely the center line of the wheel. My mind most easily relates to moving around an actual and definable surface, which in my mind is the actual mounting plane of the wheel on the hub surface. So I would re-write this explanation in terms I believe the average reader can better identify with:
“Most car makers set the mounting plane of the wheel (the hub surface), in front of the center line of the rim, viewing the wheel from the outside of the car. This is positive offset.
Setting the mounting plane of the wheel (the hub surface) behind the center line of the rim, when viewing the wheel from the outside of the car, is called negative offset.”
That says the same thing, and now having expressed it both ways and looking at the pictures, you should have a much clearer picture in your head. If you were still confused, I hope that did it for you!
Backspace – the inverse of offset
The backspace is the distance from the hub mounting surface, the mounting surface of the wheel, to the inside edge of the wheel.
Stated another way: Rim width minus the absolute value of the offset equals backspace. I added the words “absolute value” because its possible you are dealing with a negative offset wheel. Look at the pictures above if that is confusing. A negative offset wheel will have a smaller backspace measurement than will a positive offset wheel. Wheel spacers always have the same effect as would decreasing the offset. Likewise, the removal of wheel spacers increases the effective offset.
I sure do hope I didn’t make any mistakes in my logical analysis of offsets!
In the next post I will discuss other vital but perhaps more general and less confusing wheel facts.