Wheel and Tire FAQ

[Red Lion]

Let's start with the basics:

Understanding Tire Size:

Lots of people wonder what the three sets of numbers mean so I am going to explain them. Let's take a popular one for the first gen:

215/45/17

The first number represents the width of the tire in milimeters; in this case, 215mm.

The second number represents the size of the sidewall, called the aspect ratio. It's a percentage of the width (the first number), and in this case it's 45% of 215mm.

The last number represents the size of the wheel diameter in inches.

So, 215 milimeter width, 45% of that number is the sidewall's size, and it fits a 17" wheel.



Diameter:
When upgrading the wheels and tires to a larger size, it's important for most people to keep their wheels close to the stock diameter. When your combined wheel/tire diameter steps outside the factory wheel and tire size, your speedometer cannot read the exact reading and will show up as faster or slower than your actual traveling speed.

Let me explain how to convert the above number to determine the overall tire diameter:

(width x aspect ratio) /25.4 x2 + wheel diameter

(215 x .45)/25.4 x 2 + 17

First you want to find out the size of the sidewall. In order to do this, you multiply the width of the tire by its aspect ratio. This gives you the size of the sidewall in milimeters. In this case the width of 215 is multiplied by a ratio of 45%, which equals 96.75mm.

Now, you're wondering what that size is in inches. In order to convert milimeters to inches, you divide the number of milimeters by 25.4 (because there are 25.4 milimeters in an inch). In this case, the 96.75 divided by 25.4 equals 3.8".

Of course, since the tire is a circle, there is a sidewall on top the wheel and a sidewall underneath the wheel. So, obviously you multiple this by two, giving you the measure of inches that the sidewall adds to the wheel. In this case, 3.8 x 2 = 7.6"

So, there is 7.6" of sidewall sitting around your wheel. The last step is obviously to add that to the diameter of the wheel, which is 17. 17+7.6 = 24.6. This means the wheel/tire combination is 24.6" in diameter. In case you're wondering, this is a spot-on match to the OEM first gen tire size of 205/60/15, which also equals 24.6". Using this tire size will match perfectly with no discrepencies in the speedometer.


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Offset Explained:
Simply explained, offset is a measure in milimeters from the middle of the wheel's width to the wheel's mounting point.



In this picture stolen from Tire Rack you can see there is a distance from the middle of the wheel's width to the mounting hub. That distance is the offset. When you see this number is a wheel measure, it's typically seen as a + number or - number, such as +35 or -16.

There are two types of offset: positive and negative. Positive offset refers to the mounting point being on the half of the wheel width that is closer to the mounting hub (aka closer to the wheel's face). It also means that the majority of the wheel width sits closer to the inside of the wheel well. Obviously, it is denoted in sizes by a + sign.

Negative offset means the mounting point is on the half of the wheel width that is further away from the face of the wheel. It also means that most of the wheel's width sits further away from the inside of the wheel well. It is denoted by a - sign.

Offset Effects:
So what effect does this have on how a wheel is mounted? Well, the higher your offset is, the further the majority of your wheel's width will sit inside the wheel well. This isn't necessarily a bad thing, but you need to be careful that you don't hit any suspension components such as the struts or other links. Inversely, the lower your offset is, the further it sits outside of the wheel well. Too low and the wheel will stick out past the fenders.

In addition, in front-wheel and all-wheel drive cars there exists a dynamic of physics called torque steer. Torque steer means the torque your engine produces, when driving the front wheels, causes the wheels and steering suspension to pull in different directions, which can be a pain if you are trying to launch a drag car as you are struggling to keep it going in a straight line.

The lower your offset, the more weight sits out away from the steering components and this increases the amount of torque steer your car is subject to. In front-, all- and rear-wheel drive cars, it can increase torque steer under braking, making the wheels squirm and shake during hard braking.


Tire Sizes And Width:
It's generally a good idea to stay within a certain reasonable range when placing a tire on a wheel. For example, you wouldn't want to put a tire that was 10 inches wide on a wheel that's only 6 inches wide, at least if you're looking to handle somewhat decently in a sporty, non-rock crawling, off-roading fashion.

So how do you find a tire's width? Above I explained what the tire sizes mean, and the first number in a series, such as 215/45/17, denotes the size of the tire in milimeters. To find that number in inches, divide it by 25.4 (because there are 25.4 milimeters in an inch). So:

215mm / 25.4 = 8.46"

Normally you want to stay pretty close to that size, being just under it. So, a 7 inch wheel or an 8 inch wheel. Also remember that the closer the tire width gets to the wheel width, the less sidewall you have bulging out past the wheel. This can be a good thing if you're looking for responsiveness from a tire (see more below) but a bad thing if you're hoping for a little rim lip protection for the wheel.

I said "normally" above because you can stretch a tire past the width it would normally be, making it fit over the wheel (such as stretching an 8" tire to fit an 8.5" wheel); I will elaborate below.

[Red Lion]

Effect of Tire Sizes In Handling:
Let's talk about how tire sizing effects handling, starting with width. Obviously, the wider your tire, the greater contact patch (amount of tire face that actually touches the ground) you have. The more contact patch, the more grip you have. Simply put.

Let's talk about sidwall and its effect on drag racing. In drag racing, when the car launches its weight squats down onto its tires. When it does this, the sidewall flexes and bulges out, and as this happens the contact patch of the tire spreads (thereby creating more grip). This occurs a little bit in FWD and AWD but it's more prevalent in rear wheel drive because the weight of a car shifts towards the back, causing the weight to squat down on to the main driving tires.


In terms of sidewall's effect on handling around corners, the less sidewall the better. Why? Think about rubber bushings and how easily they squish and move around (sometimes moving a lot) when you try to turn sharply, then think about how polyurethane bushings resist movement better than rubber and make the car's handling feel sharper and quicker reacting. The same can be said about sidewalls.

When you have less sidewalls, you have less area that will squish and move around. This is in both terms of sidewall height and sidewall width, because if you place too wide of a tire on a wheel you will have too much sidewall hanging off the wheel. No matter if the sidewall is only an inch in height there is more area that will move around.

Keeping a nice firm sidewall allows the wheels and tires to move and react faster, allowing you to change directions faster. They also allow you to get more feeling and feedback from the steering, allowing you to know when a wheel and tire is about to oversteer or understeer. They can also allow you to oversteer or understeer more or less depending on which side has the stiffer sidewalls. In addition, it keeps the contact patch square and in contact with the ground during hard cornering, instead of allowing the tire sidewalls to scrub the ground and move around, taking away from the over all contact patch.

While you can't have too stiff a sidewall for cornering, there are downsides to stiff sidewalls. On obvious effect is there is less tire to absorb road imperfections. Another is if your tires don't fully cover your rims (as can be the case often in tires that are stretched to fit a rim) you run the chance of exposing your rim's lip to curb rash. And of course the larger the wheel you get in order to achieve a smaller sidewall the more weight you typically add (such as getting 18's or 19's instead of a 17" wheel).

Increasing tire pressure inside a tire has the same effects on their handling as mentioned above: less pressure means the tire will squat easier, but also slop around more under handling. More pressure means the tire will react better under cornering circumstances but won't allow as much contact patch to spread during a drag launch.


Wheel Effects On Handling:
Any time you add weight to a car it will handle more sluggishly in corners and use more gasoline in order to drive. Wheels (and tires), however, have a much larger effect on handling and gas consumption than adding weight to other parts of a car.

Though it's hard to measure, some people speculate that adding one pound to a wheel has the effect of adding four pounds on the chassis. Of course, heavier wheels are harder to get moving and require more effort from the engine (and therefore more gas) to get the car rolling.

In addition, the rolling mass also requires more braking power to get the car to a halt. This of course can overload the tires to their breaking point. And lastly, a heaver wheel will cause increased momentum during cornering, which can lead to either over or understeer depending.

Not only a heavier wheel will do this, but a larger wheel too, because more weight is placed on further from the wheel's center. This means that between a 17" wheel that weighs 17lbs and an 18" wheel that weighs 17lbs, the 18" wheel (in and of itself) will be worse on handling because the weight is placed further out and adds the weight to momentum.

The only possible plus side I can think of to having heavier wheels is that their heavy weight might make them a little less apt to spinning tires during high RPM launches, however you're better off keeping your tires from spinning by other means (LSD, grippy tires, etc).


Tires and Stickiness:
Some tires grip more than others, simply put, and one of the biggest factors in this is the contact patch and heat.

Contact patch is the amount of actual rubber on a tire's face that comes in contact with the ground. In summer and racing tires they tend to have fewer water channels in order to make sure more rubber hits the ground, and the depth of the tread is usually more shallow to prevent squirm and movement, where as in all-seasons and winter they have taller, more narrow treads to allow water through and to dig into snow, water and ice.

All season and winter tires work best in colder temperatures and their rubber starts to break apart easier as temperatures rise, losing their grip. Summer-only tires work best in hotter temperatures and they can resist the increased temperatures better, however they don't grip well in snow and ice.

When it comes to comparing street rubber to race compound tires, however, the difference is night and day. The best example I can think of in terms of the effects of heat on street tires was once made by Sport Compact Car, comparing it to gum. When you put gum in your mouth and chew slowly it stays sticky, but when you start chewing rapidly it starts to get harder. The same can be said about street tires. As you put more heat through the tires via friction, the tires begin to lose their gripping abilitiy.

Race compounds on the other hand start to stick better the more heat is put through them (if you want to stick with a food comparison, compare them to cookies that just came out of the oven vs. cookies that have sat and cooled for an hour). Of course that being said, they also do not grip very well until they have had a certain amount of heat put through them (a heat cycle) to get to the part of the tread that grips very well. This is why you see drag racers burning out when they line up (and also why you shouldn't be a ricer and do this on your street tires).

They also tend to have little to no tread channels, or if they do the channels are very narrow, this is to keep the tread from moving and squirming and thereby reducing contact patch. In addition, race compounds tend to have very low life. Generally, a tire's tread life is an indication of how sticky it is on hot asphalt, and low tread life tires stick better than high tread life tires.

The down side is obviously that the tire will wear out quicker. Some tires will only last one event before showing cords, depending on how much you use them and what event you are participating in.


Braking:
One last thing to mention is that tires are the most important factor in braking. A car with 15" rotors and 6-piston calipers on cheap all-seasons would never brake in less distance than a car with 1-piston calipers and 12" rotors on, say, Michelin Pilot Sport PS2's. The reason for this is the increased grip works both ways; it digs into the asphalt better to keep you planted during cornering, and also creates more friction to and more grip to bring you to a hault quicker.

Basically, your tires are more powerful than your brakes; it's when your brakes become too powerful for your tires that you run the risk of locking up your wheels, as the tires cannot grip enough for the braking force generated.

[Red Lion]

If I can remember any more, I will post it HERE.