Yum. (photo courtesy: www.crownvic.net)
Long-time TTAC Commentator 86er writes:
Hi Sajeev,
Could Piston Slap furnish me with a be-all/end-all explanation about wheel offsets? The more I try to read up on it on the web, the more confused I get. I'm pretty clear that RWD (at least traditionally) went with the low-offset while the FWD revolution made high positive offsets the industry standard, at least in passenger cars.
A few years back, I had purchased a set of winter tires on rims for my trusty ol' 92 Vic and later after research found out that the rims were medium-offset that went on a 4×4 Ranger of similar years. I've heard that putting on a different-offset wheel can hurt steering/suspension parts like ball joints, but I've never seen it in black-and-white, so to speak.
Sajeev answers:
Let's cover the basics of both wheel offset and backspacing: offset is the location of the mounting hub in relation to the center of the wheel's barrel. This mounting hub goes to flat surface where car's suspension holds the wheel (i.e. the hub on the spindle).
Image Courtesy: www.fastcar.co.uk
A positive offset pushes the hub away from center, closer to the outside of the car. Negative offset is the opposite: sucking the hub closer to the inside of the car. Zero offset is smack dab in the center.
I question if the traditional FWD/RWD offset difference still holds water. While FWD wheels often have a more positive offset than their RWD counterparts, all (most?) modern vehicles have flat faced wheels (for aerodynamics and countless suspension needs?) stemming from a more positive offset wheel. Need proof? Look at your own platform: peep the redesigned front clip and the mandated wheel redesign of the 2003+ Crown Vic.
(photo courtesy: http://www.ridetech.com)
Oh wait, the Crown Vic barely changed at all from 1979 to 2011. It was such an antiquated pile: must remember to toe the autojourno line, never speak of Panther Love! But I digress…
In theory you should keep a close-to-factory offset to optimize steering geometry and wheel bearing health. In practice, it might not matter: especially for a set of winter tires. You probably can't drive aggressive/fast enough to care. Probably…
There's also the matter of torque steer on FWD machines, mostly for those with unequal length half-shafts. But most modern vehicles use equal length shafts? (Have at that, B&B!)
You also need to consider backspacing. This ensures the width and offset of wheel you chose will clear your body or suspension, especially on cars with strut suspensions. Instead of my usual ramble, I think this video really nails it.
Click here to view the embedded video.
Send your queries to sajeev@thetruthaboutcars.
The post Piston Slap: A Primer on Wheel Offset and Backspacing appeared first on The Truth About Cars.
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