when the wheel still has the tire on it. I laid a straight edge across the rubber and measured down to the mounting surface. 4.2" Then I measured down to the outer edge of the wheel lip, but of course, this is the outside of the sealing edge, not the inside where the measurement should be taken. .7" So the difference is 3.5" but that isn't to the surface of the bead. So on an aluminum mag wheel, how thick is that bead? 1/4"?? So these wheels would have a 3-1/4" backspacing? No wonder the fender skirts won't fit. Isn't the back spacing supposed to be 4or 4-1/2" on a 67 skylark?
Jon, backspacing is a measurement between the wheel's mating surface to the hub and the edge of the inner lip on the outside of the rim; it has nothing to do with the thickness of the bead where the tire mounts. Devon
I always thought the measurement was from the back side of the mounting surface to the outside edge of the wheel lip. o No:
That is how the wheel companies measure them. Mine are 15"x8" with an advertised 5 1/5" backspace, which is what it measures from the mounting surface to the far outside lip of the wheel. If it were to be measured from the tire bead surface it would most likely be closer to 5" BS.
Confusing isn't it!! The 15 x 7" Buick wheels measure 7" between the tire mounting surfaces and 8" between the outside edges, BACK SPACING to that very outside edge is 4-3/8".....which is the number you always see posted here. The FRONT SPACING is 3-5/8 A much better term to use is OFFSET. Zero offset would be equal front and back spacing. The 15x7 Buick wheels have 3/8" OFFSET to the rear. A 66-67 likes a little more offset than stock. An extra half-inch is ideal, so 7/8" offset whould be the ticket. I wish we could just abolish the term 'backspacing' from the industry and stick to 'offset'.:grin:
You may be right. Devon's statement doesn't match his picture either. The picture show's the measurement to the outside of the lip. o No:
Maybe I didn't verbalize this correctly. My tires are still mounted, so there is NO WAY I can measure anything to the inside of the rim where the tire bead seals. That is my problem. So I am measuring from the locating hub of the wheel, the surface that bolts to the axle face, out to the back face of the TIRE, not the outer part of the tire, but the inside face against the inner fender. Then subtracting the distance from the back face of the tire back down to the outer edge of the wheel. 4.2-0.7=3.50" My measurments according to this picture, come out at 3.50" for back spacing. I was assuming the back spacing was to the surface that the tire bead sits against, however by this picture, that isn't true, therefore my back spacing is 3.50", which seems small, and would therefore force my tire out further from the center of the car, resulting in the tire rubbing the inside of the fender skirt and/or the fender lip. What is "correct" back spacing on a 67 skylark. EDIT: Since my tires are still on the wheel, I really don't know what my wheel width is, can I measure the outside edges of the tire and just "assume" approx 1.0" less is the wheel width? Because you cannot know your offset unless you know your width, correct? EDIT 2: Also, looking at the picture, is that considered positive, or negative offset as drawn. ALSO.... The statement of Walt above says that this car likes a little more "rear offset". What is rear, positive or negative?
Jon, I don't like the term 'backspacing' since it depends too much on the wheel width.... it only considers the measurement on one side of the wheel..... let's determine width and offset first.... I would suggest you cut a piece of wood about 14 or 15" long, so it sits across the outside edges of the wheel rim, not on the tire. Measure from the mounting surface of the wheel to the wood (straightedge)....that will be your backspacing. (on the back of the tire of course) Now repeat the measurement on the front side, putting the the end of the tape measure thru the bolt hole so you're measuring from the same surface as the b.s. measurement. Let's call that frontspacing. Add the two measurements together.....On my Buick mag 15x7 example, the total edge-to-edge width was 8". But between the tire mounting surfaces, it was 7".....it is a 7 inch wide wheel. I don't know if your alum wheels will have that same 1" difference. To determine the OFFSET: Divide the total width by 2.... The Buick rim would be 4" Subtract the b.s. or the f.s. from that number.... that will be your offset. For the Buick rim, (4") - (4-3/8")= 3/8" offset. Same result for the front: (4") - (3-5/8")= 3/8" offset. ZERO offset would be 4" b.s and 4" f.s. The Summit Racing site states Negative offset is when f.s. is greater than b.s. .....that will push the two tires further apart. Positive offset will put the two rear tires closer together. (EDIT: fixed now, had it backwards.) Post back with your measurements. Is your goal to buy new rims that fit better? What size tires are you using? We'll get this figured out....
It is at work and I'm at home. I will get those measurements this weekend. As to why, since my rear axle is laying on the floor right now, I saw the tire sitting there on the ground and started wondering. It is a 225/60/14 tire,fyi. As they are now, they stick too far out to even allow the fender skirts to be held in up in place. Somebody cut the fender skirt "L" brackets off the top of the opening so I got some to replace them. Since I don't want to spend the big bucks to get the rear chrome moldings and I like the look on the skirts, I have been thinking about new wheel/tire combo. For a Saturday night cruiser, nice stance with skirts on, and just laying a huge burnout rolling out from under, COME ON, how cool is that? So I follow local craigslistings, and find various wheels on there sometimes, I want to know what will, and won't work. Since the car isn't running now, I need to be able to measure and know if this deal or that deal is going to work or not. Anyway, that's the long version of just saying, yeah, I want new wheel.
I can just barely fit 255/60-15's on my 66's with the 15x7 Buick Mags. It's close against the quarter panel, but there's room on the inside. 275's will fit with the right offset....actually my blue GS has a 12 bolt chev rear that is one inch narrower, so that's equivalent to an extra half inch offset per side....7/8" offset. My best guess to center tires in the wheelwell of a 66-67 would be 1" offset: Buick mag 15x7 (8"wide edge-to edge)= 5" backspace Buick mag, custom 15x8 (9" wide e-e)= 5.5" backspace. With 225 tires you'll have some extra room to play with.
Since the rearend is out of the car now, measure it from outside to outside with the wheels and tires mounted on it, then take a couple pieces of string, tie a nut or washer on the end of each one and with the car sitting level on jackstands, tape the strings to each quarter panel and let them hang. With that done you can measure the distance between the two strings and should be able to figure out how much more clearance you need.
Ok, so I went out and measured what I have now. Again, the tires are still on there, so these dimensions are from the OUTER EDGE of the wheel. Distance from wheel mounting surface to front exposed edge of wheel measures 4.5" Distance from wheel mounting surface to back edge of wheel measures 3.5". Allow for the outer edge to bead mounting surface of 1/2" per side, I think I probably have 7" wide wheels. Since the difference between them is 1.0" and the front side is deeper than the back side, my current wheel is a 1.0" NEGATIVE offset. THAT must be the problem, forcing my fairly skinny wheels too far out. Going from the picture I put up above, the more positive offset will suck the wheels closer to car center line and away from the fenderwells. Looking at earlier posts, what I really want is 7/8 or 1" POSITIVE offset? Unless I am completely confused, which is likely.
Yes, most likely they're 7 inch wide wheels. Your offset is 1/2"......it's half of the difference between fs and bs: -Zero offset would be 4 and 4. -Move it 1/2" and it becomes 3.5" and 4.5" -Move it 1" and it becomes 3" and 5" And yes, you do want positive offset (I had it backwards in my earlier post). 1/2" might be enough with your 225 tires, 1" would be better.:TU:
DOH! Yeah, simple math gets me sometimes, and I'm a machinist dealing with decimal's every day and still miss by 1/2" Thanks for the input, now I know just how to measure and what to look for if I find something on Craigslist around here. If not, I will have a lot more faith in ordering new wheels in the future. Amazing that you can fit 255's and maybe even 275's inside there. At least now I have faith I can get a fair sized tire behind the skirts. If I keep this car for a long time, I'm sure I will have a couple sets of wheels depending on what I'm looking to do. Thanks again everyone for all you help and clarity. Jon
