<VV> Backspacing

[Padgett]

>What is backspace?  I know its a stupid question, but at least I admit I 
>don't know.

Not at all stupid especially since different aftermarket companies use it 
creatively. Backspacing is essentially the distance from the inner side of 
the rim to the mounting surface (inside of the hub). It generally increases 
as the rim width increases to balance clearances but too little  and the 
steering effort goes up. A six inch RWD wheel will have about 4" of 
backspacing, 7" 4.25-4.50"
See http://www.4wheelpartsperformance.com/html/faq.html#backspace

Today you usually see *offset* which is the distance from the hub to the 
centerline of the wheel. RWD cars are generally from 0-20 mm and  FWD are 
35-50 mm, here as the rim width goes up, the offset generally goes down 
(RWD can go negative) again to balance clearances.
See http://www.off-road.com/chevy/tech/wheel/

*In general* either will change by half the increase or decrease in rim 
width to maintain the king pin angle aka steering axis inclination but only 
works if the tire diameter remains the same. More likely aftermarket wheels 
will have "enough" to avoid any interference on the inside.
http://www.procarcare.com/icarumba/resourcecenter/encyclopedia/icar_resourcecenter_encyclopedia_suspsteer3.asp

Can get very complicated.

Padgett