Hello, I am planning to buy 15x7 wheels with the 5 by 4 1/2 in. Lug hole and a 4.25 back spacing. I have seen a variety of choices, some having negative offset, zero offset or positive offset. My question is, does offset matter and if so do I need to get negative, zero or positive? They are for a 1970.
The rim width subtracted from the backspace equals the offset. So you have already determined it. 4.25" back spacing with a 7" wide rim is about as big a front wheel as can be installed on a '70 Cougar without doing some sort of surgery to the wheel wells. Assuming a P225 - 60 R15 tire size.
It matters a lot, this video explains why.
Thank you for your responses. Yes I plan on running 225 60 15’s per Don’s video. My confusion comes from listings of the various different wheel manufacturers. Some reference their wheels with the 4.25 back space, yet they reference a different offset. One may have a negative offset with the 4.25 bs, while the other will list the 4.25 bs with a positive offset. So from what you tell me here, as long as I get wheels that list a 4.25 bs I should be good. Is this correct? I would not be a concern if I ordered from WCCC because I know all of their wheels will work. But the wheels that my son picked out for his Cougar unfortunately are not offered at WCCC.
It will also load your wheel bearings differently.
FYI: i run 235/60-15 with 15 x 8 Magnum 500,s on the front of a 69 and 255/60-15 with 15 x 10 on the rear.
Cant recall the back spacing, but every thing works with no rubbing.
Offset also changes your scrub radius. This link explains it far better than I can with words.
https://www.carthrottle.com/post/what-is-scrub-radius-and-why-is-it-important/
The impact of changing tire diameter and wheel offset (which are the easiest ways to change scrub radius) then impacts steering (returning forces, bump steer, tracking on dished roads, hydroplaning), braking (tendency to spin out without ABS, “duck walking” with ABS, tie rod, center link, steering rack forces during braking, steering wheel torque generated during split-mu wheel lockups), Required steering wheel force while turning, tire wear, wheel bearing wear/failure, and I’m sure there are others I have forgotten.
In short, unless you are an experienced braking, steering, and suspension engineer, DON’T MESS WITH IT!
Not a bad article but has a lot of possibly scary, “what if” scenarios and notes suspension and braking systems these old cars don’t have (McPherson struts and diagonal braking systems). Not many of us drive these cars daily nor push them that hard. We’re also not Mercury building cars for all season year after year use while under warranty. I’ve run a variety of wheel offsets on cars I’ve used for auto-cross and didn’t die even once! Tires are replaceable and I bet most here have tires that look good but have aged out long before being worn out.
I have 17x8 wheels with 4.5” backspace on all four corners or my 1969 with 245/45-17 tires. I do have adjustable strut rods and have centered the wheels in the well and will tweak position for as much caster as possible when I get her back on the road. I wouldn’t be too worried about how scrub radius may affect the car. Get the backspace and offset close and have at it. Discount Tire has a nice tech section with good graphics and charts for review.
Agree. First things that comes to mind is all the other massive amount of people that change the wheels and tires off their vehicles. I think wheel and tire changes have to be the #1 thing people will do to their vehicle when talking about making changes over stock. You can certainly take it to the extreme and affect things like this article mentions but to say to leave it alone is a bit of a stretch.
Back space certainly matters in more ways then one and you can take it too far and negatively affect the handling of your car but you can also greatly increase it as well. As with anything there are pros and cons. As long as your reasonable in your wheel choices you will be fine
Thanks everyone for the information. From hearing these responses and continuing my research I have determined the answer is yes, offset does matter. but I am still at a loss as to what offset I need for the 1970 Cougar with 5x7 wheels and a 4.25 back spacing - P225 - 60 R15 tires all the way around. Does anyone know what the offset has to be on the rims? Would it be different on the front and back? Or, does anyone know of a calculation to determine the offset?
Like Royce said in post #2:
Wheel width: 7"
BS: 4.25"
Offset: -2.75"
BS & offset measure two parts of the same whole (wheel width). if you have two, you can calculate the other. A stock wheel was probably 6" w/3.75" BS (guessing), so offset would be 2.25" (again, guessing). Stock tire was roughly 26 inches in diameter.
4.25 BS is probably OK on the rear, but too much of the tire’s width on the inside of the front (which is what higher BS does), will push the tire’s sidewall too close to the upper ball joint & might rub there. You run less backspace there, maybe 3.75 to avoid that problem. Trouble is, as you move the width further outboard, you risk catching the inside of the front fender lip.
It’s a balancing act. If you want, order 2 of them at a time. This way you can confirm the fit on your front, and if you run into trouble order a different BS/tire size to make it work.
Ahhhhhh, I got it now. Thank you. Much appreciated.
Dave
The various wheel manufacturers are listing a backspace and an offset that doesn’t add up to the wheel width. This is why it was throwing me off. Here is an example
Condition: New UPC:
886821059904
Placement on Vehicle: Left, Right Manufacturer Part Number:
141H5765GM6
Wheel Size: 15X7 MPN:
141H5765GM6
Style: 141 Legend 5 Brand:
Vision Wheel
Load Rating (lbs.): 2100 lbs Series: American Muscle
Size: 15X7 Bundle Listing: Yes
Hub Bore: 83mm Offset: 6mm
Bolt Pattern: 5x4.5 (5x114.3mm) Quantity: 4
Warranty: 1 Year Back Space: 4.25
Wheel Finish: Gunmetal w/Machined Lip
The 4.25 is referenced in inches and 6 is referenced in millimetres. It doesn’t add up to the 7 in
I would say they are quoting hub offset there (vs wheel offset). 6mm is like 1/4".
There is a pretty good article in the November 2020 issue of Hemmings Muscle Machines on this subject. They show some methods to use to figure out what offset will work, total wheel width and all sorts of good information. It may be of some help to you as you figure out what will work on your car.
Randy Goodling
CCOA #95