70white402 Posted December 19, 2021 Share Posted December 19, 2021 I can buy an old set of 1967 torque thrusts. Will they work at 15x 8 1/2”? It says 1 1/4” positive offset on wheel. 1 Quote Link to comment Share on other sites More sharing options...
cny first gen 71 Posted December 20, 2021 Share Posted December 20, 2021 What is the backspacing on them rims? They look kinda deep may cause a rubbing issue. Are you thinking front or rear or both? There is a wheel and tire data thread on here somewhere I'm not sure where maybe someone else can find it or let you know where it is. Quote Link to comment Share on other sites More sharing options...
MCfan Posted December 20, 2021 Share Posted December 20, 2021 Jeff, I assume you are interested in a wider rim in order to run wider tires so it may come down to how large of a tire you plan to run. According to the Wheel Offset Calculator at the Tiresize.com website, a 1 1/4" positive offset (+32mm) is equivalent to a 6" Back Space on an 8 1/2" wide rim. Generally, positive offset (greater Back Space) on a rim is good for our first gen Montes because it moves the wider wheel/tire combo inboard away from the quarter panel, especially in the rear. HOWEVER, too much positive offset can get you in trouble in the front with rubbing against the frame on tighter turns and in the rear with rubbing on the inner quarter. I used the wheel offset calculator to compare some tire/wheel combos using those 15 x 8.5" rims with the combos I am running because my wheel/tire specs are known and clearances are all workable. Here's what I found: On the front: If you mount a 255/60-15 on those 8.5" rims with +32mm offset, your inboard sidewall will be .79" closer to the frame than my 255/60-15s on rims with only +12mm offset. I am positive that combo will rub on tight turns because mine barely rubbed until I put a 1" spacer in the front coils. Dropping back to a 245/60-15 tire on those rims will still move the inboard sidewall .59" closer to the frame and they will also rub on sharp turns. On the rear: If you mount a 275/60-15 on those 8.5" rims with +32mm offset your inboard sidewall will also be .79" closer to the inside quarter than my 275/60-15s on rims with +12mm offset. I can't say for sure that they will rub but I would be nervous that they might. If you drop back to a 255/60-15 tire on those rims the inboard sidewall will be .39" closer to the inside quarter panel and that is probably doable. If you have to run a narrow tire get the inboard clearance you need, what is the point of using 8.5" wide rims? It is important to remember that the offset of any rim is relative to the exact center of the rim and is therefore independent of the actual rim width. However, Back Space is relative to the outside edge of the inner rim and is totally dependent on rim width (see the conversion table in the Wheel Offset Calculator at Tiresize.com). For any wider rim, including the 15x8.5", to work well on our first gens, an offset of +12mm (+1/2") is nearly ideal for 60 series tires. Some have tried 50 series tires on the rear and more positive offset is usually required in addition to trimming the wheel well lip. Because of the almost certain clearance problems on the front with wider tires, I'd pass on those particular rims, even as cool looking as they are. JMO Quote Link to comment Share on other sites More sharing options...
cny first gen 71 Posted December 20, 2021 Share Posted December 20, 2021 See I knew someone smarter than me would be better off with this question, thank you Dennis Quote Link to comment Share on other sites More sharing options...
714024SPEED Posted December 20, 2021 Share Posted December 20, 2021 We have put rims with 5 1/2 inch backspace and they do fit but I'm not sure if you have another 1/2 of room. Quote Link to comment Share on other sites More sharing options...
MCfan Posted December 20, 2021 Share Posted December 20, 2021 7 hours ago, 714024SPEED said: We have put rims with 5 1/2 inch backspace and they do fit but I'm not sure if you have another 1/2 of room. What was the width of the rims with 5 1/2 backspace? Were they used on the front, rear or both? Thanks. Quote Link to comment Share on other sites More sharing options...
MC1of80 Posted December 20, 2021 Share Posted December 20, 2021 15x10, rear. Black Monte- 15x10 American racing, 5 1/2" bs 275/60 r15 White 69 GTO- 15x10 weld wheels, 5 1/2" bs 275/60 r15 wheels powder coated black and factory dog dish hubcaps installed. Wheels were from the orange Chevelle pictured which were installed on a 70 Monte Carlo. 71 Heavy Chevy Chevelle- 15x10 weld wheel, 5 1/2" bs, I believe 325 50 r15 drag radials. Extremely tight! Had to massage inner tub and trim outside lip. The Chevelle and GTO have "bigger hips" and taller wheel well openings but inner clearance is the same as the 70-72 Monte Carlo. 1 Quote Link to comment Share on other sites More sharing options...
70white402 Posted December 20, 2021 Author Share Posted December 20, 2021 19 hours ago, MCfan said: Jeff, I assume you are interested in a wider rim in order to run wider tires so it may come down to how large of a tire you plan to run. According to the Wheel Offset Calculator at the Tiresize.com website, a 1 1/4" positive offset (+32mm) is equivalent to a 6" Back Space on an 8 1/2" wide rim. Generally, positive offset (greater Back Space) on a rim is good for our first gen Montes because it moves the wider wheel/tire combo inboard away from the quarter panel, especially in the rear. HOWEVER, too much positive offset can get you in trouble in the front with rubbing against the frame on tighter turns and in the rear with rubbing on the inner quarter. I used the wheel offset calculator to compare some tire/wheel combos using those 15 x 8.5" rims with the combos I am running because my wheel/tire specs are known and clearances are all workable. Here's what I found: On the front: If you mount a 255/60-15 on those 8.5" rims with +32mm offset, your inboard sidewall will be .79" closer to the frame than my 255/60-15s on rims with only +12mm offset. I am positive that combo will rub on tight turns because mine barely rubbed until I put a 1" spacer in the front coils. Dropping back to a 245/60-15 tire on those rims will still move the inboard sidewall .59" closer to the frame and they will also rub on sharp turns. On the rear: If you mount a 275/60-15 on those 8.5" rims with +32mm offset your inboard sidewall will also be .79" closer to the inside quarter than my 275/60-15s on rims with +12mm offset. I can't say for sure that they will rub but I would be nervous that they might. If you drop back to a 255/60-15 tire on those rims the inboard sidewall will be .39" closer to the inside quarter panel and that is probably doable. If you have to run a narrow tire get the inboard clearance you need, what is the point of using 8.5" wide rims? It is important to remember that the offset of any rim is relative to the exact center of the rim and is therefore independent of the actual rim width. However, Back Space is relative to the outside edge of the inner rim and is totally dependent on rim width (see the conversion table in the Wheel Offset Calculator at Tiresize.com). For any wider rim, including the 15x8.5", to work well on our first gens, an offset of +12mm (+1/2") is nearly ideal for 60 series tires. Some have tried 50 series tires on the rear and more positive offset is usually required in addition to trimming the wheel well lip. Because of the almost certain clearance problems on the front with wider tires, I'd pass on those particular rims, even as cool looking as they are. JMO Thanks Dennis and others. I feel very informed. Quote Link to comment Share on other sites More sharing options...
MCfan Posted December 21, 2021 Share Posted December 21, 2021 46 minutes ago, MC1of80 said: 15x10, rear. Black Monte- 15x10 American racing, 5 1/2" bs 275/60 r15 Thanks, Tom, Let's focus on the Monte. That 5 1/2" Back Space on a 10" wide rim is exactly 0 offset (see the table in the Wheel Offset Calculator). If you put your Monte's setup in the Wheel Offset Calculator and then compare it to Jeff's proposed setup with the same 275x60-15 tire on the 15x8.5" rim with +1.25" (32mm) offset (6.0" BS), the entire tire/wheel combo is moved exactly 1 1/4" inboard, not 1/2" as you may have suggested. A Back Space figure without the accompanying rim width is meaningless. Back Space comparisons are meaningful only when comparing rims with exactly the same width. That's why wheel manufacturers use offset because that measurement is relative to the center of the rim (regardless of rim width). Oh, sure, they will quote you a Back Space measurement but only for a specific rim of known width. Next, I compared your setup to mine. Same tire size but my rims are 15x8" with 5" BS (which is +12mm offset). As you would expect, my tire/wheel combo is 1/2" (.47") further inboard for additional outer quarter panel clearance. I also use my telescoping bore gauges to measure the inboard and outboard minimum clearances of my setup. Inner sidewall of 275/60 tire to inner quarter panel was 1.4" driver's side and 1.14" passenger side. Outer tire sidewall to wheel well lip (untrimmed) was 3/4" driver's side and 1" passenger's side. Naturally, one would have to consider the worst case since the drive train is not necessarily perfectly centered with the body. Further, some shift in the drive train relative to the body during hard acceleration, braking or body shift during turns should be a consideration. Now, as I revisit my earlier comparison of Jeff's proposed combo to my current set up (knowing actual minimum clearances), it appears that he could technically run 275/60s on the rear using those 8.5" rims but will only have 1/4" of inboard clearance. That may not be adequate under all of the various considerations listed above. Alternatively, he could run 255/60s or 245/60s or even a 235/70-15 (which is also 28" tall) on the rear only and the inner sidewall would be within .04'" of my setup, giving adequate clearance all around. Likewise, he might get by with 225/70s on the front with some rubbing on the tightest turns. I have no idea what a 70 series tire looks like mounted on an 8.5" wide rim but that is another consideration. Thanks for sharing your Monte's tire/wheel setup, Tom. Every workable wheel/tire combo we know about lends insight for making decisions on other available combos. 2 Quote Link to comment Share on other sites More sharing options...
714024SPEED Posted December 21, 2021 Share Posted December 21, 2021 Sorry but I don't believe the "science" of the offset calculator. I have a 15x15 rim on my Pro street Firebird with 3 1/2 inch back space,and it measures 3 1/2 " . So if I put a 15x8 rim with the same back space your saying the measurements will be different. I understand what the offset table explains but a 3 1/2" back space is a 3 1/2 " . Just my opinion 1 Quote Link to comment Share on other sites More sharing options...
714024SPEED Posted December 21, 2021 Share Posted December 21, 2021 Here is a cheap way to get measurements.Pvc pipe cut to how wide of a rim you want to use,put the tire where you want it and measure from front of rotor/drum to pvc mark it and that will give you a ballpark for your back only problem is you need a tire 1 Quote Link to comment Share on other sites More sharing options...
70white402 Posted December 21, 2021 Author Share Posted December 21, 2021 7 hours ago, 714024SPEED said: Here is a cheap way to get measurements.Pvc pipe cut to how wide of a rim you want to use,put the tire where you want it and measure from front of rotor/drum to pvc mark it and that will give you a ballpark for your back only problem is you need a tire Love that Aztec bronze! Quote Link to comment Share on other sites More sharing options...
MC1of80 Posted December 21, 2021 Share Posted December 21, 2021 It's actually a flip flop. Hence the name, flip flop Firebird. Depends on how you look at it and in what light. It goes lime green, gold, brown, eggplant, burgundy and all colors in-between. 4 Quote Link to comment Share on other sites More sharing options...
70white402 Posted December 23, 2021 Author Share Posted December 23, 2021 That’s beautiful! 2 Quote Link to comment Share on other sites More sharing options...
MCfan Posted December 25, 2021 Share Posted December 25, 2021 On 12/21/2021 at 9:28 AM, 714024SPEED said: Sorry but I don't believe the "science" of the offset calculator. I have a 15x15 rim on my Pro street Firebird with 3 1/2 inch back space, and it measures 3 1/2 " . So if I put a 15x8 rim with the same back space your saying the measurements will be different. I understand what the offset table explains but a 3 1/2" back space is a 3 1/2 " . Just my opinion Whether you believe the "science" or not has no bearing on the validity or usefulness of the Wheel Offset Calculator. Offset and backspace are different measurements of the position of the wheel/rim mounting surface and are taken from different physical references. Offset is measured from the centerline of the wheel/rim while backspace is measured from the outer lip/edge of the wheel/rim on the inboard side. If two rims of different widths have the same backspace (3 1/2" or whatever) they will have different offsets. in your example, the 15" wide rim has an offset of -115mm (4.5") while the 8" wide rim has an offset of -25mm (1"). Conversely, if two different width wheels/rims have the same offset, they will have different backspaces. A 15" wide rim at 0 offset has a 8" backspace while an 8" wide rim at 0 offset has a 4.5" backspace. The backspace of any rim at 0 offset is always (rim width + 1)/2. BTW, the Wheel Offset Calculator with convert any offset to backspace or backspace to offset if you also input the rim width. The issue is the positioning of the wheel/tire combo relative to the body/chassis structure around it. The location of the mounting surface of the wheel/rim is clearly fundamental to its relative positioning within the body/chassis space. This discussion is about the practical use of wider tires and rims for first gen Montes which have stock bodies and rear ends of known dimensions and a practical range of wheel/tire combinations that will work. The beauty and value of the Wheel Offset Calculator at the tiresize.com site is that you can enter any working/fitting wheel/tire combination for a first gen as the current setup and then compare any other prospective/desired wheel/tire combo to it and visually see how the positioning will change relative to the current setup. It also gives you the exact dimensional changes so you don't have to guess by just looking at the comparison image. You should be able to see and measure any/all sidewall-to-fender/frame clearances with your current setup and know if the change in wheel/tire position of any new setup will fit within them. With online tools like the Wheel Offset Calculator readily available, I don't understand why anyone would guess or take a chance on some wider wheel/tire combo working on their first gen Monte. It's not rocket science ... Quote Link to comment Share on other sites More sharing options...
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