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Change in scrub radius - a concern?

eljay1

Passed Driver's Ed
Location
Ottawa, Canada
For winter tires, I'm eyeing a set of after-market rims that are almost OEM spec (rim width, bore size, bolt pattern). The only difference is the offset: OEM is 49; these rims are 42. The calculator at wheel-size.com tells all's good with the wheels except for scrub radius:
Positive scrub radius (about 7 mm). Scrub radius will be changed by 7 mm, this may affect the car's handling. Please check manufacturer recommendations. ...
Does anyone know what VW specs / recommendations are, and is a 7mm change in scrub radius something I should be concerned about?

Any input / advice is appreciated. :)
 

RacingManiac

Drag Race Newbie
Location
MI
You might notice it slightly, in that the car maybe more sensitive to road bumps and camber change. But probably not nearly as noticeable as by running the spongy and vague snow tires....
 

eljay1

Passed Driver's Ed
Location
Ottawa, Canada
Thanks for the input, guys. Sounds like it's nothing to get too concerned about.

I googled the tech specs on the 2018 GTI but can't find any mention of the value for scrub radius. Is it out there and would anyone have a link to it?

Thanks again. :)
 

RacingManiac

Drag Race Newbie
Location
MI
You don't need to know the number, the fact that you are changing the offset by 7mm you are changing it by 7mm, because it is determined by the axis formed by the top of the strut, and the location of the lower ball joint. Where that axis intersects the road surface, the distance between that point and the center of the tire tread is the scrub radius.

https://c1.rt-static.com/how-suspension-affects-wheel-choice/640x480_illustration.jpg

In most modern FWD car you actually run a negative scrub radius to help the self aligning action to keep the car going straight. Usually when you reduce the offset it reduces the negative scrub and brings it closer to zero scrub, this reduces the self aligning torque and makes it likely to feel busier in the steering because you might be correcting for the car wandering due to road surface changes....
 

eljay1

Passed Driver's Ed
Location
Ottawa, Canada
Thanks. :) I was just wondering if the 7mm increase would push the scrub radius from negative to positive which, presumably, would be worse than zero scrub. I guess it doesn't really matter if the effect is simply more of things like busier steering.
 
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