Bob's Studebaker Resource Website

Studebaker Avanti

Magnum 500 Wheels

If you plan on changing to an alloy wheel, you may be able to use some of this data...
The Avanti Magnum 500 styled steel wheel is a 15 X 6 (7 inches wide), 5 bolt X 4.5 inch pattern or 5 X 114.3 mm. In measuring the characteristics of this wheel, the '6' inch is determined by measuring bead-seat to bead-seat, where the tire bead rests. If you lay the rim on the ground, the edge to edge measurement is 7 inches due to the extra lip which curves out. Offset is measured from the dead center of the rim and in this case, '0' offset is at 3.5 inches. See
OFFSET

The backspacing of the rim is 4.125 inches (4 1/8). The center of the rim is 3.5 inches. Difference is 5/8. The offset of this Magnum 500 is '+15.875MM. (1 inch = 25.4 MM). 5 X 3.175 = +15.875, The '+' due that the mounting surface is on the outer side of the centerplane. This makes the wheel more like a Front Wheel Drive wheel and believe it or don't, the only other car in this situation is the Jaguar. ( but then don't Avantis use Jaguar brakes in front?)

There is one other design quality of the Magnum and a lot of 60's and 70's wheels that are a detriment. The 'drop-center' is the concave portion between the bead seats. This area can be deep or shallow depending on who designed the rim and for what purpose. Deeper is stronger, but it will prevent you from using this wheel with any kind of disk brake caliper other than the the Studebaker application. As disk brakes became more commonplace, wheel designers revised that area of the rim to accomodate different type of calipers.

From my perspective, in order to find a wheel that will fit, one must alter the original fit to a small degree. I believe that any corner of the car can stand up to a 3/4 inch pushout. You get the pushout by installing a more negative offset version of the same wheel or a wider wheel with some positive offset.

At "0" offset the stock wheel will be pushed out 5/8". Most rear wheel drive cars have a very low positive to a high negative offset. They range from +16 to -20mm from what I've seen, with the positive being rather scarce. To compound matters, 15 X 6" wheels are getting a bit rare also, at least decent looking designs are.

In applying the math to different wheels, I get these results. 15 X 7 @ +15mmm offset will add around 1/4" to the outside and 1/2" to the inside of the existing setup. Using a 16 X 7 @ +20mm will add 7/8" to the inside and 3/8" to the outside. You have to consider the tie rod position. On my stock Avanti wheel, it's positioned about 7/8" from the rear circumference of the rim and closer yet to the tire. Adding a larger (read wider) rim may interfere, but you can avoid this problem by installing quick-ratio steering arms and move the tie rod closer to the spindle.

There are wheels available in 15, 16, 17, and 18 inch diameters. If you plan on using a larger than 15 X 6, do the math, it isn't hard. Don't forget to include the extra inch on the rim when calculating how close those front tires get to the frame in a turn. A 15 X 6 is 7 " wide, 15 X 8 is 9" wide. The rear wheels can stand almost anything, I've mounted a 96 Jeep Grand Cherokee alloy on the rear and it fit fine at +30mm, the front are the problem.

I've researched using spacers between the mounting surfaces of the drums/rotors and the allow wheels. This is a no-no. My innovative thinking brought me to the idea that installing longer studs in the axles and rotors would allow mounting of sharp hi-offset wheels. The result in doing this would be disastrous. The longer lugs would not stand up to the increased shear stress spanning the spacers and would certainly fail in a turn. The numbers were worked out for me by a very capable engineer. Not a nice thought.

The alternative to this is installing an adapter. Basically the same principle, but different mounting technology. An adapter is a machined thickness of billet aircraft aluminum. The adapter is custom made to your requested specifications of thickness and bolt pattern. They bolt to your drum/rotor surfaces and in turn have lugs that extend to mount the wheels. So all you need, is to find ANY wheel you like and have a set of these made to make up the spacing and / or bolt pattern difference between what you have now and what you need to mount the late model alloys. I have been advised that if you go this route, you should always plan on at least a 1.25: thickness to avoid having to cut your lugs. A place I found them available is Coy's Wheels & Tires in Denver, CO.

This is not a promotion to use adapters either tho' In the many calls to Coys, They will never offer any downside of using them and I don't know if they're approved for use in all states. I'd actually leave them as a last resort.


Torqueing a wheel

 

 

 

 

 

 

These pages and programming contained therin
are ©
Bob Johnstone, 1996 - 2009, All Rights Reserved. Many technical opinions are my own from experience, some informational data is from other sources with credits when available and while care has been taken to be as a accurate as possible, it is offered only as a guide and caution should be exercised in the application of it.