Offset alone is useless.
The combination of wheel diameter, wheel width, offset and disk/face is needed to know fitment.
Offset is the distance between the centerline of the wheel and the hub mounting face.
This distance dictates the wheel’s placement within the wheel well. Offsets are described in three different ways, Positive, Zero and Negative.
Positive Offset: Hub mounting face is on the outside (towards the wheel face) of the wheel centerline.
Zero Offset: Hub mounting face is the wheel centerline.
Negative Offset: Hub mounting face is on the inside of the wheel centerline.
There are other important measurements that include offset. Forward Spacing is the distance from the front of the wheel to the hub mounting face. Backspacing is the distance from the hub mounting face to the inside of the wheel. Both of these measurements are key to calculating fitment.
The manufacture of a given vehicle designed their wheels (offset included) around the chassis, suspension, braking and steering of said vehicle. Changing the offset can lead to problems if it is changed.
The change in offset can be calculated by finding the difference between the offset of the new wheel minus the offset of the factory wheel. When this number is large side effects start to become apparent.
Simply, when the difference in offset is positive the wheel will sit further in the wheel wells. When the difference in offset is negative the wheel will be pushed further outward.
This is the effects of offset very simply, ignoring wheel width can lead to serious problems. DO NOT LOOK AT ONLY ONE MEASUREMENT, all measurements of a wheel are key and must be considered when purchasing a new wheel.
Knowing the potential side effects of changing the offset drastically is key.
Potential Side Effects of drastically changing offset.
Rubbing:
- On suspension components
- Springs
- Struts
- Control arms
- Trailing arms
- On body panels
- Fenders
- Fender liners
- Bumpers
- Sideskirts
- On brakes
- Calipers
- Rotors
Knowing your clearances to all of these components is key before purchasing or deciding on a given offset.
I will be going into detail on specific models.
- Acceptable Offsets
- Acceptable Diameters
- Acceptable Widths
Math Explained:
Offset is generally measured in millimeters (mm) although converting an offset from inches (in) to millimeters (mm) is very simple. mm = in * 25.4
Change in Offset = New wheel offset* – Factory wheel offset*
Wheel Width in mm’s = Wheel Width in inches * 25.4
Centerline in mm’s = Wheel Width / 2
Backspacing in mm’s = Centerline + Offset* + 12.7
Forward spacing in mm’s = Centerline – Offset* + 12.7
*this formula applies for both Positive and Negative offsets, use the appropriate sign for a given offset to calculate both back and forward spacing.
Nice article- short, concise and easy to understand. I know I have gotten confused on this issue before and I’m pretty sure others have (even vendors!)
Thanks.
Thank you,
if something wheel related needs further clarification don’t hesitate to ask.
I have a question regarding changing offsets. It just doesnt seem to fit in my head 😛 I found really nice rims with an offset of 18, but my car has 35. Is it possible to fit the new rims by adding spacers?
Alex,
it depends on a lot of things.
1.) what car?
2.) what suspension are you on?
3.) what is the width of the wheel? what is the width of the stock wheels?
4.) what size tire is standard, and what size tire are you hoping to run?
there isn’t a Yes or No answer without more info.
its an 1995 BMW E36 M3.
Everything is stock, wheel width is 235.
I dont want to change any tire sizes, just keep the stock 235/65 on 17″ rims
To Alex…
t3h_Clap asked what you vehicle’s wheel width is, NOT the tire width which was the ‘235’…seeing as you have a P235/65R17’s, you probably have a 17×7 or 17×7.5…when He also asked about ‘stock’, that meant the OEM factory spec….
Also, adding a wheel spacer is just gonna make your tire stick out by a good 2 inch, rubbing against the fenders and the wheel well.
…not good for looks, not good on your suspension…knowing BMW’s have tight suspension, you might have to spend for a ‘lift kit’ if you REALLY want THOSE nice-18mm-offset RIMS…which defeats the whole purpose of a BMW…
Also you might want to measure your OEM backspacing to find out how much room you have for aftermarket rims, so they don’t RUB against your suspension parts…measure from the rim’s mounting pad up to just before the lip of the rim, not to the edge of the tire
So overall, you might for a rim that has a medium to high offset…limit it to 25mm if you have to, and don’t go over 40mm….
you OEM rim is probably a
…17(size)x7.5(width), P.C.D.(pitch circle diameter or bolt pattern)is 5(bolt)x120mm(or 5×4.72 inches), wheel stud size is a 12(mm)x1.5b(tread pitch), center hub bore is 72.56…
thats really great info..and its really helpfull.
i made some calculations based on the formulas above..
i just wanted u to give me ur opinion..
my car is 2004 lancer ES..
there is a guy Running 18×7.5
Offset = 38 mm
Width = 7.5″ =190.5 mm
Centerline = 95.25 mm
Backspace = 146 mm = 5.74″
forwardspacing =2.753″ (no fenders mods needed)
everything is fine with this offset in his car which is 2004 lancer ES as well..
i came up with this wider size with different offset..and its my plan…
18×8.5
Offset = 25 mm
Width = 8.5″ = 215.9 mm
Centerline = 107.95 mm
Backspace = 154.65 = 5.73″
Forward spacing = 95.65 = 3.76″ (i can work somthing out for the fenders)
i am doing this because i really need traction..my car has an evo engine(swapped) and its FWD..
so what do you think abt it..is there anything else that i have to worry abt..?
thank you…
note : both cases has the same wheel with tire diameter..
One inch is not a small amount when it comes to outboard clearance.
you will most likely also have to add some additional camber on top of the fender mods.
what size tire you looking at running?
i think the best tire wud be 225/35 r 18
currently running on my car 215/45/17 no rubbing..
so what do suggest how can i work this out..?
thank you
i wanna add somthing..
but 1st i have a question..
when i go this wide the camber will go postive or negitive..?and if i will have to adjust it..should i put it 0 or a bit negitive..? since i do aggressive cornering..?
i googled..i found a front camber kit..for my lancer..the rear is adjustable..also i will be using BC coilovers they have front camber plates as well..
i could get those and roll the fenders..
do u think i may face any other problems..?
i even thought why dont i go 235/35/18 since the diameter will be simillar to 215/45/17.what do u think.?
too many questions 🙂
thanks.
Good read about alignments. http://t3hclap.com/archives/18
you’re going for negative camber.
Positive and 0 camber are not ideal for performance.
always aim for ZERO toe as well.
If you’re looking for more grip, you need to be sure you’re looking at Extreme Performance Tires. Not some $50 a piece tires.
The BC Racing coilovers will help with the fitment. You’ll want to roll the rear fenders, like we said.
the 235/35/18 may be too tall, initially stick to the 225/35/18 (if you can find any).
the +25mm offset will be harder on the bearings, causing increased wear. Your car will most likely tram line more often.
this isn’t going to be a bolt on setup, you’ll need to do adjustments/modifications to make it fit correctly.
Hi Clap;
You sound like a very sharp guy. I’m looking to put the largest set of wheels that will fit on my ’74 Datsun 260Z.
It has mostly stock f/r susp. except for stiffer springs & sway bars.
It also has fiberglass front fenders w/appx 1.5″ flares as well as fiberglass rear arches w/appx 3″ flares.
I now have 70’s style wheels/tires: Western brand 14X7 running fat Gr60/14 McReary radials (yep, they’re old!)
Can I fit a set of 17″ or 18″ wheels with my stock susp/brakes? I’m not looking for super-wide tires, I just want to fill out the wheel wells w/modern wheels/tires.
I’m willing to use spacers & longer studs, but I don’t want the expense of coil-overs!
Thanks for your help,
Bill
sir may i ask what kind of wheel should i use for lancer 97? thank u