Wheel and Tire FAQ's

[wret]

Please feel free to comment, criticize, or correct these. I will make appropriate changes before these get added to the FAQ’s.

What questions should NOT be asked?

“What is offset?”
You can find detailed descriptions and illustrations with a simple web search.

“Can someone explain tire size?”
There is plenty of information about tire size available if you do not want to learn it, you would be better off with standard recommendations from a reputable vendor.

“Will ‘X’ wheels fit?”
To start with, this is the wrong question. What one should ask is “Will XXX/XX/XX tires fit?” This is a better question for two reasons. 1) Almost any wheel of the right lug spacing will fit. 2) The wheel is really not much more than a support structure for the tires. And tires is what wins races.


How does wheel offset affect tire clearance? (2g)

The primary element to 2g tire fitment is the clearance between the rear tires and the steering knuckles. This clearance is nearly static once you mount your tires making it relatively simple to measure. The secondary clearance of concern, the rear fender flange, is variable depending on ride height and suspension jounce. (See other posts for fender flange issues).

The size variables that affect the knuckle/tire clearance are tire radius, tire width (influenced somewhat by brand), and wheel offset. The baseline for any tire and wheel size changes can be compared to the stock tire and wheel size: 205/55/16 tires on 16x6 inch +46mm offset wheels.

The stock setup has about 20mm of clearance and the shape of the knuckle in that area is rather like an arc so that any tire size changes on the horizontal or vertical axes have about the same effect on the clearance. For example: a change from a 205 width tire to a 245 width tire (keeping the radius the same for now) would close this gap 20mm (the other 20mm of added tire width being on the outside of the tire). Zero clearance is not acceptable, so typically wider tires are used on wheels with less offset. If the same 245 width tire is mounted on a wheel with a 40mm offset, there would be an acceptable clearance of 6mm. (For more detailed information, see post with "2D graphical model" of steering knuckle)

A change from a 205/55/16 tire to a 205/60/16 tire would add about 10mm to the tire radius. The new tire is taller, narrowing the clearance above the tire. In this example, clearance is acceptable but you can see how lessening wheel offset would increase the clearance in this case as well.

Often tire size changes affect both axes. A change from a 205/55/16 to a 225/55/16 tire makes the tire 10mm closer on the horizontal axis and 11mm closer on the vertical. For practical purposes, simply add these changes together for a total change of 21mm. Negative clearance is not acceptable. If the same 225/55/16 tire is on a wheel with 40mm offset, there is an acceptable clearance of 5mm.

What tire size is best?

The size tires you should select should be based upon your performance goals, daily driver considerations and of appearance. 245 width wheels are considered by many to be the best width for a daily driver that also will be used for some Auto crossing. These should be mounted on wheels that are sufficiently wide. You may also choose 215, 225, or 235 width tires based on your goals, preferences, and current wheel size.

What is the widest tire I can fit on my 2g?

A 245 width tire is the widest practical tire for a daily driver. On 38mm or 40mm offset wheels, they will be nearly flush with the fenders. Anything wider will protrude from the fenders by about the same amount. A 255 width tire for example will protrude about 10mm. If the wheels and tires are dedicated to racing, there is no finite limit. 285’s are possible.


Reference:
http://www.miata.net/garage/tirecalc.html

 

[jtmcinder]

Wret -

Good start to a challenging project.

I agree with your first two questions not to ask, but think that you should modify the third. If a person is going to ask a *new* "does it fit" question, then they need to specify the tire size, tire model, and wheel width. When you start to cut things close, you need all three pieces of information to be sure. More on this below.

With regard to 2G clearance, I agree that it's the rear that matters. But I like to talk about this in terms of both the knuckle and the fender lip, which I sometimes call Scylla and Charybdis (not that many people read Greek mythology anymore, but at least Defiant will get it). The advantage of this approach is that it allows you to take it all in steps. First you must decide between three possibilities: (1) I want the tires to tuck in [soft springs and/or lowered car] but the fender lips will be left alone; (2) I want them to tuck in and the fender lips will be rolled; (3) I don't care about the tires tucking in because I'm not lowering [much] and will be running stiff rear springs (such as 400+).

Then address the widths and offsets for each of these options. This is where you force people to be clear about tire size, model, and wheel width, to get the right offset. For example, I agree that 245s on a +37 through +40 is the max for option 2, but only for some tires. 245/40/17 Kumho MXs work great, but 245/45/17 Azenis rub. The reason for this is partly because Azenis are wider at the tread than MXs (for the same stated sidewall width) and partly because the 245/45/17 is taller than the 245/40/17 and the knuckle curves around the top inside corner of the tire. The curve means that as the tire gets taller, you must run a lower offset; running a lower offset brings the outside top corner closer to the fender lip (rolled or not). Scylla and Charydbis don't like 245/45/17 Azenis.

Yeah, this is real nightmare, so with all that said, I wish you best of luck. If you can create a simple FAQ for this, you will have done everyone a great favor.

- Jtoby

 

[wret]

Agreed on all points. I intend to get into fender lip, wheel width vs. tire width, and tire brand width variation.

And, whew! This is a little harder than I anticipated. I have a condiderable experience writing elaborate technical instructions that no one ever reads. If I expect them to be read, the points must be very short, yet descriptive, without much room for personal opinion.