Correct Offset for 15" x 7" Wheels For Rear of '61 C-1

[Rob V.]

Hi-- DISCLAIMER : I am not an Engineer; Mechanic; or even mechanically inclined.
QUESTION: What is the correct offset for 15"x 7" Torque Thrust D wheel rims to be used on a 1961 Chevrolet Corvette?? The wheels are to be mounted on the rear only; and the car is equipped with a stock differential/drum brake assemblies, and has a stock (unmodified) body. The offset for the two (2) 15"x 7" wheels (VN1055761), I just purchased is -5mm. The American Racing website lists the following offsets for the 15"x7" Torque Thrust D's as: -18mm, -11mm, -6mm, -5mm, 0mm, 1mm, 8mm.
What is causing me consternation is that currently, the four (4) 15"x 6" Torque Thrust D's, (VN1055661), that I have on the car have a +3 mm offset. (If my math is correct, that is a difference of 8mm between the offset of the 15"x 6" and 15"x 7" rims). Note: I have Goodyear Eagle 205x65xR15 tires mounted on the 15" x 6" rims with no interference problems.

Thank You,
Rob Vanderhoeven

[Terry D.]

Re: Correct Offset for 15" x 7" Wheels For Rear of '61 C-1

How much room do you have from the tire to the leaf spring? As far as I can tell the original 15x7 rally wheel has a -.28 offset, which is 7.11mm. I have 15x7 rally rims with 245x15x60 on my 62 with no clearance problem. I have about3/8 of an inch between tire and spring and maybe a little less between tire and fender. There is about 5/16 difference in your rims. If my calculations are correct you should be fine.

[Duke W.]

Re: Correct Offset for 15" x 7" Wheels For Rear of '61 C-1

If you widen the wheel one inch with the same offset the inside of the wheel will move one-half inch inboard and the inside sidewall will move inboard about 0.2". (Maximum tire section width increases/decreases about 40 percent of the increase/decrease in wheel width.) But since the offset change is .31" in the negative direction (outboard), you will only lose .19" clearance on the inboard side wheel and should actually gain .11" clearance at the maximum tire section width, but outboard clearance will be .81" less at the wheel and 0.51" at the maximum tire section width.

If you are using different size tires you can get the dimension data from the manufacturer's Web site for the specific model and size tire. The data usually includes the measuring wheel width, so you can use the above 40 percent relationship to estimate the maximum section width on a different wheel width.

With the above data and a tape measure you should be able to measure current clearance and compute an accurate estimate for the new clearance.

If the above is confusing it will vastly help by drawing a sketch of the situation.

Duke

[Rob V.]

Re: Correct Offset for 15" x 7" Wheels For Rear of '61 C-1

Hi Terry--

Thank you for helping me with this issue! I really appreciate you taking the time to provide me with some meaningful conversions/calculations. As you requested, I went to the garage and took some measurements (albeit rather imprecise). Anyway, the distance between the tire and the inside rear fender lip is approximately 1"; and the distance between the inside of the tire and the leaf spring is approximately 7/8". Measurements were made at the point where the sidewall protruded the most/was closest to both the fender lip, as well as the leaf spring. Measurements were take with the 15" X 6" rims and 205/65R15 Goodyear Eagle GT tires in place.

My understanding is that with the -5mm offset on the 15" x 7" rims, the hub that bolts to the brake drum has moved 5mm closer to the centerline of the car. If this is true, does the mounted tire in essence move the opposite direction, and therefore closer to the inside lip of the rear fender? Anyway, I believe that there is now a 8mm total difference between where the 15" x 6" vice the 15" x 7" mounts to the brake drum; and if that is correct, then the tire moves 8mm away from the centerline of the car (or 8mm closer to the inside fender lip of the rear fender?) If this is all correct, exactly how much in inches is 8mm?

Thank you again for you assistance!!! Assuming the wheels are in fact correct, I will proceed to look for tires.

Happy Friday,
Rob Vanderhoeven

[Rob V.]

Re: Correct Offset for 15" x 7" Wheels For Rear of '61 C-1

Duke--

Thank you for your response as well!! I guess while I was responding to Terry's comments, you posted your helpful response as well. I will also do as you recommend and sketch the dimensions. It is 0820(L) MT, and I already have a headache thinking about this; and I still have to go to work.

Thanks Again,
Rob

[Rob V.]

Re: Correct Offset for 15" x 7" Wheels For Rear of '61 C-1

My simple project is complete; however, I did not get the "bang for the buck" that I had hoped to achieve. The American Racing 15" x 7" Torque Thrust Ds, with -5mm offset worked on the car. However, in retrospect, I would have liked to have seen if the 0mm or +1mm offset would have worked better. I had hoped to get a larger tire under the rear of the car, (with a larger contact patch), than I had with the Goodyear Eagle GT 205/65R15s mounted on the American Racing 15" x 6" Torque Thrust Ds with +3mm offset. As it worked out, I only felt comfortable putting Kelly 215/65R15 tires on the car; and if the contact patch is greater with the new wheels and tires than what I had previously, I would be surprised. The only discernable difference, (and then perhaps only to me), was that the wheel tire assembly moved closer to the inner rear wheel well fender lip (away frpm the centerline of the car); and the rear tires were perhap no more than 1/2" taller than the 205/65R15s. Rough measurements taken from the inside of the wheel were approximately 1 1/2" (1.5") from the lip of the inside wheel rim to the leaf spring; and approximately 1 1/4" (1.25") from the side wall of the tire to the leaf spring. If I had used a wheel with positive offset, perhaps I could have used a bigger tire (aasuming I could have found one in the correct size).

I wish to thank Bruce Bursten and Terry Deusterman for their initial discussions with me on the size of tires that they run on the rear of their C-1s; as well as Terry Deusterman's and Duke Williams' wheel fit verification calculations!

Best Wishes,
Rob Vanderhoeven

[Terry D.]

Re: Correct Offset for 15" x 7" Wheels For Rear of '61 C-1

You do realize that by running the tires you have (215x65x15) that they are about an inch shorter than the stock tire and therefore your speedometer will be running faster than you are actually going. Also it will increase your rear end ratio. There are several web sites out there that give you the dimensions of different tire sizes. I especially like www.rimsntires.com/specs.jsp it gives you a chance to compare two different size tires and offsets.

[Rob V.]

Re: Correct Offset for 15" x 7" Wheels For Rear of '61 C-1

Hi Terry--

Thank you for the reminder concerning the effects of changing tire size with respect to speedometer readings. It has been quite awhile since I purchased the car; but if memory serves me correctly, it had some Cragar rims with 65 or 70 series tires installed when I got it. (The Cragars lasted one day, only because I was more interested in removing and jettisoning the installed radar detector and Sanyo AM/FM cassette radio, both of which found their way to the garbage). Suffice it to say, you are once again correct with regard to my speedometer being inaccuarate--in fact, it is grotesquely incorrect. In a very unscientific test, when driving on the freeway, and indicating 80 mph on my speedometer, my wife reported that the GPS on her telephone was reading 65-68 mph. I was going to change the nylon speedometer gear in the Muncie transmission to attempt to recalibrate the speedometer; however, I had not taken into account the effects of the lower profile tires on the actual speedometer readings. Thanks for the tip on the "rimsntires" website that you recommended; I will take a look at it. Once again, I want to thank you very much for finding the time, and having the interest to assist me with this effort!

Rob