How to calculate wheel offset

[stevesei]

Use this procedure to calculate your offset requirements. These calculations can be used to find wheels and tires, verify if tires and rims will clear existing struts, and to calculate spacer requirements:

ABOUT POSITIVE OFFSET: Nearly all front wheel drive cars with McPherson strut suspension use POSITIVE OFFSET wheels. Looking at the inside of a wheel, POSITIVE OFFSET means that the mounting surface of the wheel is spaced OUTSIDE of the wheel centerline. MORE positive off set means the mounting surface is farther from the INSIDE of the wheel. Less positive offset means the mounting surface is closer to the inside of the wheel. NEGATIVE OFFSET means that the mounting surface is spaced closer to the back of the wheel than the centerline. Very wide wheels on a front drive car COULD require "0" or negative offset. THIS IS RARE!

FINDING THE NECESSARY OFFSET FOR YOUR CAR:

1. FIND THE INDEX: Use a straightedge held vertically on the front (and rear, since they are separate measurements) rotor hat WHEEL mounting surface (NOT THE HUB ITSELF!) as a baseline. Measure the distance from the straightedge to the strut (at THE WIDEST PART OF YOUR TIRE, approximately 9" to 10-1/2" from the hub center). This is your INDEX. If you have camber bolts, you will want to crank in all the available negative camber when you make this measurement, so that you aren't surprised later.

2. FIND THE WHEEL CENTERLINE: Measure the exact installed width of the tires you intend to use, or use the manufacturer's published section width specs (not the tread width). Divide by 2 so that (example only) 8.3" section width / 2 = 4.15" wheel centerline.

3. FIND THE INSTALLED CENTERLINE: Add the tire clearance you want ( IMHO, absolutely no less than 10mm. 15-20mm is better, and more is better yet) to the wheel centerline to arrive at the (installed) CENTERLINE. LEAVE CLEARANCE FOR CAMBER CHANGES! if you didn't in step 1.

4. DO THE MATH: Baseline to strut = INDEX, and (tire width / 2) + clearance = (installed) CENTERLINE, therefore INDEX - installed CENTERLINE= OFFSET

5. FIND THE OFFSET: The remainder from #4, in mm, is your OFFSET. A positive number is a positive offset, a negative number is a negative offset. MX-3s normally have a 45mm POSITIVE OFFSET with stock wheels, tires, and suspension. IF your calculation comes up negative, see #6. If it doesn't, read #6 anyway!

EXAMPLE #1: In my case: 205/40ZR17 on 7" rims = 8.3" section width divided by 2 = 4.15". 105.4mm plus 3/4" clearance (20mm) = 125.4mm installed CENTERLINE. Measured INDEX= 155mm (LOTS of negative camber). 155mm - 125mm = ~30mm OFFSET, which is the actual POSITIVE OFFSET of my rims.

NOTE: if the tire does not extend beyond the rim, use the total rim lip width for your centerline measurement and make your index measurement at 8-10", from the hub center, depending on your wheel diameter.

6. SPACERS: On strut type cars, if the OFFSET calculated in #4 above comes up negative, and you already have wheels, spacers are required to regain clearance. So, if you have 45mm offset wheels and your calculated OFFSET is 30mm, adding a 15mm spacer REDUCES the 45mm offset to 30mm and regains the necessary clearance.

EXAMPLE #2: For 225 section tires,
a. 9.1" / 2 = 4.55" centerline plus 1/2" clearance = 5.05"/128mm installed centerline. Installed centerline + 50mm wheel offset = 178.27mm.
b. Measure your index and subtract 178.27mm from it.
c. If you have a 171mm INDEX, you will have an OFFSET of -7.27mm, a negative number. 7.27mm will be your spacer requirement.
c. Spacers usually come in 5mm, 10mm, and so on. go larger rather than smaller! ALLOW for manufacturing tolerances. More clearance is better!

BTW, measured INDEX of a bone-stock RS is 171.5mm (no camber bolts). Calculations verified on an actual car.

KEEP IN MIND, use the specs for YOUR tires, they vary considerably for the same tire size from manufacturer to manufacturer.

I hope this is useful!

Steve

[kulluminati777]

great info. theres sooo much stuff lost on these forums because they are not stickied or added to the FAQ for later mx3 owners or people searching for similar info. cant moderators do these functions?

[Josh]

um its called Google. it takes not but a couple minutes to do a search and come up with info. That is why my browser has book marks.

[MrMazda92]

Josh is right about searching, but kulluminati has a point too... There are an awful lot of things that we should have stickies for, that would prevent so many of the "Search, noob!!" threads we have popping up!

I'll start PMing MoonEggs about stickying a few threads, others should do the same!

[kulluminati777]

um its called Google. it takes not but a couple minutes to do a search and come up with info. That is why my browser has book marks.

I dont know i would just like to see the forum more than what it is. It already grinds my gears 400000 people post on the facebook page and the forums have like 10 of the same people comment or create posts.

[Josh]

um its called Google. it takes not but a couple minutes to do a search and come up with info. That is why my browser has book marks.

I dont know i would just like to see the forum more than what it is. It already grinds my gears 400000 people post on the facebook page and the forums have like 10 of the same people comment or create posts.

I know, I am just playin..

[stevesei]

Guys, I am just trying to lend a hand where I can. There have been a number of posts recently that need to be in the FAQ. I (more or less) humbly suggest that this post and my earlier one on the installation of Energy Suspension LCA bushings, might be candidates, as well as the current one on using Probe/MX-3 hub carriers, the one on Mazda6 brakes (which go perfectly with the Probe suspension post.) and several others. These cars are quickly becoming collector's items and deserve, if not restoration, thoughtful updating to current state of the automotive art. The forum has been really short on good suspension and tire articles; not to mention that these areas add more true performance, to most cars, than any other mods.

Enlightened participation and tolerance for new ideas can keep this forum a true leader. given the wealth of knowledge already here. It might be a good idea to sift through the various forums and pick the best and most accurate information. A new set of forums might be needed for DIY instructions, technical articles, specifications, etc. This may be redundant; maybe they could just be added to FAQs, but any progress is worthwhile. I'd help if there is an interest in such a project

[MrMazda92]

I support that 100%.

Misinformation is inherent in any gathering of individuals, especially an internet forum... Why not have an honest "zero BS" section, full of 100% confirmed how-to guides?

I have over 80 MX-3 forum bookmarks, several of which are simply pages out of random worklogs, where I saw something cool and completely under-rated.

To qualify for the new section, every guide should be concise and thorough, with supporting information(and pictures, where applicable) to verify the steps of the modification.

[benyandthejets]

Also using the forum bookmarks are nice as well, as they are saved to your profile. Then you can access wherever you have internet