[DeTomaso] What Wheel Offset Would I Need? Long

[SOBill at aol.com]

Curt,
 
If you want my opinion on 17x8 BS 5.25 Vs 17x9 BS 5.95, jump to the  last 
paragraph.
 
When wheels are quoted as 15x7 the 15 is the diameter of the tire  mounting 
surface and the 7 is the distance between the INSIDE edges of the  rim. In 
order to determine the offset of a wheel, the distance between  the OUTSIDE edges 
of the rim and the distance from the inboard edge of the  rim to the tire 
mounting surface, the backspacing, must be known. The 7  inch Campy outside rim 
width is 8 3/16 inches and a backspacing of 4.25  inches. The 8 inch Campy 
outside rim width is 9 1/8 inches and a  backspacing of 5.25 inches.
 
Offset is the distance from the wheel mounting surface to the  center of the 
rim with a positive offset being inboard
 
Wheel Offset = Backspace - 1/2 Rim Outside Width
 
The outside width of the rim is used for the measurement so this  can be done 
with tires mounted.
 
Campy 7 inch front wheels have 0.25 inches of inboard  offset. Campy 8 inch 
rear wheels have 0.75 inches of inboard offset. 
 
Steering scrub radius is the lateral distance, at ground level,  from the 
centerline of the wheel to the steering axis. The steering axis is  a line drawn 
from the upper ball joint thru the lower ball joint. Scrub  radius gives the 
driver a feeling for what the steerable tires are doing. A zero  scrub radius 
feels like driving on ice and a large scrub radius gives heavy  steering and 
steering wheel kickback when traversing road irregularities.  The stock Pantera 
front wheels have approximately 3 inches of scrub radius. This  is an 
abnormally large scrub radius. Given that the steering upright cannot  be changed, 
scrub radius can only be reduced by moving the centerline of  the wheel inboard.
 
To achieve a more normal 1 inch scrub radius, a 7 inch front  wheel with 
outside rim width of 8 3/16 inches would need to move inboard 2  inches for an 
offset of 2.25 inches.
 
Backspace = Wheel Offset + 1/2 Rim Outside Width
 
A 7 inch wheel would need a backspace of 6.34 inches.  This means the wheel 
mounting surface would be 0.64 inches inboard of the  outboard edge of the 
wheel. This is clearly not feasible from a strength  standpoint.
 
In addition, moving the wheel inboard two inches relative to a  stock 7 inch 
Campy will look very strange in the wheel well, will reduce the  steering lock 
available due to the tire hitting the chassis, and will likely not  clear the 
brake caliper with 15 inch wheels.
 
Wheel backspacings are normally quoted in 0.5 inch steps and  are not 
available in all increments. If a wider wheel is used, the strength  aspects become 
feasible. The rounded off backspacing required for various wheel  widths 
(assuming a tire flange width equivalent to Campys) required to achieve a  1 inch 
scrub radius is:
7...6.5
8...7.0
9...7.5
10..8.0
11..8.5
12..9.0
13..9.5
14..10.0
 
Not until you get to a 10 inch tire will you have a mounting flange  with a 
width of 2 inches. The message is that the scrub radius cannot  be safely 
reduced to 1 inch with wheels alone.
 
So, that's why I am going to power steering.
 
If you look at the wheels of modern, front wheel drive cars, you  will see 
that the mounting flange is at the outboard extreme of the wheel. These  cars 
use essentially 0 scrub radius so that the driving torque applied to the  front 
wheels does try to also steer the car.
 
A 17x8 with 5.24 backspace will have an offset of 0.75 inches .....  the same 
as an 8 inch Campy. A 17 x9 with 5.95 backspace will have an offset of  0.85 
inches. Since the offsets of both wheels are essentially the same, I would  
take the 17x8. The 17x8 will give you more steering lock before the tires rub on 
 the chassis. You will still likely need to roll the fender edges.
 
Have fun,
 
SOBill  Taylor
sobill at aol.com



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