Wheel information resource: Sizing, offset vs. backspace, bolt patterns, ect

I am very frequently getting PM's regarding these topics.

Figured I would post this over here.


this is a great resource for the basics:

https://www.rsracing.com/tech-wheel.html

offset calculator:

http://www.1010tires.com/WheelOffsetCalculator.asp

this one does tire size:
http://www.wheelsmaster.com/rt_specs.jsp



Offset is a measure of how far the mounting face (surface that touches the brake rotor) is from the centerline of the wheel. When you want to run a wider wheel with the same backspacing, the inner edge of the wheel will come closer to your suspension, eventually hitting the frame, control arms, coilovers or bodywork. To avoid this, wider wheels are made with lower (more negative) offsets. The side effect of this is the wheels "poke" out more. This "poke" makes the car look better by getting rid of the wheel gap almost all stock vehicles have. We try to take this a step further by stretching tires and running offsets so low that the wheel lip actually extends beyond the bodywork, while the tire tucks into the body.

Sunken:
(stock Miata wheels)


Good fit = flush


I took a tape measure to my stock wheels and measured the gap to the fender arch from the wheel lip then used the calculator above to determine what offset I wanted. Remember that you can
always get more negative spacing with spacers -but you wanna be close -however if they are too negative (sticking out too far) it will be difficult to get them back in -if not impossible. -typically would require machining and cannot be done on most wheels without a huge compromise in strength = unsafe.

Anyway hope this helps some of you... and of course please add anything pertinent.

-A

 

it would be nice to get a collection of limits on our cars. For example, the most offset each model can support up to and even include wheel diameters. Searching never brings up those specifics but if we could get this stickied then it would be easy to find.

 

Stickied at the top:

https://www.rx7club.com/suspension-wheels-tires-brakes-20/fd-wheel-tire-sizing-416251/

The reality is though, it can only be but so specific. The edges of the envelope depend on what you're willing to live with in terms of fitment, performance, etc.

For instance, I wouldn't refer to the white Miata pictured in the OP as a "good fit". It appears practically undriveable from a ride height perspective because the wheels are so short, tires are less than optimal size for the rim, and will no doubt wear the tires horribly and damage the fenders when you turn the wheel and go over a bump, etc.... so TO ME, it would be outside any "limit" I'd publish.

That said, I know the depicted fitment is what a lot of people are after, so I think if that's what you want, you need to approach it with the same ethos—buy the wheels you want and then make everything else fit them... don't need a calculator for that!

 

IMO there are way too many variables to say wheel X and tire Y fit. With some very basic match skills and a tape measure anyone should be able to figure out a proper fitment with the tools I posted. In general people want to be given the answers -this is bad -people need to learn how to do something as basic as this... it's not rocket science.

Exactly - I agree the Miata is a bit aggressive for even me and goes out of function and is based more on looks.

One of the reasons I posted the calculator and basics up is many people do not know the difference btw positive an negative offset or where to begin. Also just buying the wheels you want and going from there will not work -again there are too many variables -yeah I like the Equips is 17x11" -3 and 17x13"-10 -but in all reality getting them to fit is going to be more work and headaches then the average guy wants to deal with (extensive bodywork, cutting ,welding ect) -you need to have some idea where you need to be in terms of width and offset ahead of time. I have seen so many guys just buy wheels and they don't fit.