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Old 08-27-2009, 08:56 AM   #1
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Road Force Balancing

FYI, if you have had issues with getting a good balance on your wheels with your Boxster and it's still not right, find a tire shop offers road force balancing. I just had this done on my Boxster and the difference is amazing, my car drives absolutely vibration-free to 100+ mph.

This is similar to the machine my shop used:

http://www.hunter.com/pub/product/balancer/4159T/index.htm

This balancer is able to apply as much as 1250 lb of force to the tire to fully seat the bead after mounting, and to simulate road conditions. The system is able to identify the exact position the tire needs to be rotated to on the wheel to minimize imbalance, so that a minimum amount of weight is needed to bring it to perfect balance. It also measures tire and wheel runout and helps you ID problems with flat-spotting or other damage.

My shop charged $60 to do all four wheels, it took about an hour. As I expected, my left rear tire had the worst imbalance. Now, it's perfect. I did this on my M3, too, with similar results.

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Old 08-27-2009, 10:53 AM   #2
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Rfv

Road force variation (radial force variation) is NOT about tire/wheel imbalance. Radial force variation occurs when there is a variation in the effective "spring rate" of the tire, as mounted on the wheel. Put more simply: you can balance a square wheel/tire, but it still won't roll smoothly.

Imagine that the tire construction has a "stiff spot" at one point in the rotation of the tire. This stiff spot may not necessarily correspond to a visible or measurable "high spot" in the tire itself, and would not necessarily correspond to a point of imbalance of the tire/wheel. What will happen when this "stiff spot" contacts the ground? The tire/wheel will be forced up, causing a thump that you can (potentially) feel as the tire rolls. The tire may look round, but it doesn't act round as it rolls with vehicle weight applied.



The Hunter GSP-series equipment (during force variation checks) will turn the tire/wheel slowly with a load applied to the tread via the roller, as it checks the center-to-center distance between the wheel centerline and the roller centerline. The result of the test is force variation in 1st, 2nd and 3rd order. The next step is to follow the machine prompts to reposition the tire on the wheel in a manner that reduces the force variation. Put simply, the machine helps you find the low spot on the wheel (via a wheel radial runout check) and directs you to place the "stiff spot" of the tire in the low spot of the wheel. The net result is reduced radial force variation and a smoother ride.

The rule of thumb is that for every 0.001" radial runout at the wheel's bead seat, you will have approximately 1 pound of force variation. Different vehicles have a different level of sensitivity to radial force variation, but high performance tires (and high performance vehicles) are generally more sensitive to radial force variation than a more typical passenger car. That is, you might feel a 15 pound radial force variation on a Boxster, but not on a minivan. The trick is to get the radial force variation down to a level that cannot be felt or noticed on that particular vehicle.
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Last edited by Blue-S; 08-27-2009 at 02:52 PM.
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Old 08-27-2009, 12:00 PM   #3
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Thanks for the detailed explanation and correction to my post.
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Old 08-27-2009, 12:44 PM   #4
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anyone knows a tire shop in the Peninsula or Santa Clara/Sunnyvale area who has this? I think my tires need it

Thanks
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Old 08-27-2009, 12:58 PM   #5
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Should have known, it's in wikipedia:

http://en.wikipedia.org/wiki/Radial_Force_Variation

And here's another article from Hunter, very interesting:

http://www.gsp9700.com/pub/technical/4202T/5THEO004.cfm
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Last edited by pbanders; 08-27-2009 at 02:19 PM.
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Old 08-27-2009, 02:43 PM   #6
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I will not balance tires any other way anymore.
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Old 08-27-2009, 02:48 PM   #7
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Quote:
Originally Posted by ChrisZang
anyone knows a tire shop in the Peninsula or Santa Clara/Sunnyvale area who has this? I think my tires need it

Thanks
Hunter has a website that you can use to find a shop near you that is equipped with a GSP9700 road force balancer:

http://www.gsp9700.com/

I didn't have time to put the Hunter force variation illustration in my post originally, but I have added it now for clarity. Although the tire and vehicle manufacturers have had equipment to measure radial force variation for decades, no such equipment existed for vehicle service until Hunter introduced the GSP9700 about 10-12 years ago.
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* M030 coming soon! *

Last edited by Blue-S; 08-27-2009 at 02:57 PM.
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Old 08-27-2009, 08:08 PM   #8
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Just make sure your shop uses the Hunter 97xx equipment and has competent personnel and you'll be fine.

To under-complicate; No Wheel or Tire is perfectly round (you'd need a gravity-free environment to achieve this), nor is the mass evenly distributed around their circumferences.

The Hunter machine identifies the anomalies which then allows the operator to rotate the tire to best cancel these out.

Just because a shop has a Hunter 97xx is not enough. It also needs to have skilled operators who know how to properly seat the tire with the wheel to achieve the best possible balance.

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Old 08-27-2009, 08:24 PM   #9
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I was wondering what good the space station would be after Obama cut funding for NASA.

Now I know what we can use it for. Porsche tire and wheel manufacturing!!!!!!!!

LB, you are a genius. A genius I tell you!
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Old 08-27-2009, 11:36 PM   #10
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Quote:
Originally Posted by RandallNeighbour
...LB, you are a genius. A genius I tell you!

Yo... my Mother, wife, and friends have been telling me that for years...

Not that I believe any of them at all ...

I'm just a goofy f*ck trying to keep my Porsche going like all the rest of you...


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