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Old July 5th, 2002, 05:04 PM   #1
Larry Nakamura (In SJ)
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Porsche Tire Balancing........

I have had the "shimmy" in my front end since I got the car. From reading other threads where other Babblers were experiencing the same thing, it was apparent that there is a tire balance issue. I took my car to a shop that has the Hunter balancer and even stood next to the guy while he did the work. This was to make sure he knew where to jack tha car from, take the tire off without chipping the paint on my brake calipers, and didn't scratch my rims. They did a good job at those points (with alittle help from me).

Anyway, My front left tire was off initially by 44lbs and my right was off by 35lbs. This was found by doing the dynamic balancing. He said that if they didn't do this, they would probably think they were fine. We turned the tires to try and get them better as there was no way they could be properly balanced. The left was down to 25lbs and the right was down to 24lbs needed to balance the tires correctly. This is out of specification.

Since I have seen (and keep seeing) people complaining of the "shimmy" in the front end, and since I found my tires were off so much, I'm now thinking Porsche does not do dynamic balancing on their tires. This is the only way to explain getting new cars from the dealer that do not have correctly balanced tires. Maybe we should start a poll to gather data to see if this is another issue that needs to be addressed.

I'm pretty pissed that this could be so. I have never lost any tire weights, (you could tell by the mark on the rims if one was missing), never tracked the car yet, still have most of the little nobies on the tire.

So I talked to the rep at AB and let him know what I found. I will make an appointment with them next week so they can take a look and see. If they find the same thing, then I will need new tires for the front. I will expect to have them replaced for free (installed). Also, did not check the rears, but am very suspecious since both my fronts were off so much, the rears are probably off too. I let the rep know about others having this problem too and that maybe they need to add a tire balance check to the dealer prep list.

I'll let everyone know what happens at the dealer.
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Old July 7th, 2002, 07:49 AM   #2
Magic Mtn Dan
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About Hunter GSP9700 Road Force Measurement™ (don't leave home without it)

44lbs. is way too high as is 25 & 24lbs. (we're talking about the reading from the Hunter Vibration Control System machine - see picture below).

From the Web site:
The GSP9700 Vibration Control System includes Hunter's exclusive Road Force Measurement System. This system utilizes a "road roller" which measures the uniformity of the wheel and tire, simulating a road force test and verifying if the assembly is "round when rolling" under load. Now nearly all wheel/tire-related vibration problems can be diagnosed and/or eliminated in seconds.

The number (in lbs.) should be as close to zero as possible. The high numbers that Larry has mentioned are too high but the problem is you can't tell by the numbers alone what the problem is (probably a combination of wheel and tire but it's possible that one of them could be the cause of most of the problem).

It's great that you went and had them checked. As many of you know, I have long been a proponent of the Hunter GSP9700 machine. I will not have tire/wheel work done on my cars if the shop doesn't have and use this machine. Note: even if they have the machine you'll need to ask them to use it - it takes more time and costs a bit more $ for them to use it - don't assume because they have it that they'll use it on your car - they typically won't unless it's a high-performance shop.

For more information:
http://www.hunter.com/pub/product/sensors/4240t/index.htm

Click here to find a Hunter GSP9700 near you:
http://www.hunter.com/pub/product/sensors/4240t/index.htm

From their Web site:
Why the GSP9700?

Eliminate Tire and Wheel Vibration with the GSP9700 Road Force Measurement™

Hunter's GSP9700 is both a balancer and a vibration control system. It is also a Road Force Measurement analyzer for passenger car and light truck tire/wheel assemblies.

Each tire/wheel assembly is tested for the amount of dynamic/static imbalance and Road Force.

The GSP9700 simulates a "road test", with a unique "load roller" which applies up to 1400 lbs of force against the rotating assembly. The roller measures the loaded runout of the assembly (deflection while under load) and automatically recommends corrections when needed. The GSP9700's technology eliminates many of the time consuming, subjective and often non-productive manual measurements previously used to diagnose and repair ride disturbance concerns.

The following image shows an example of a GSP9700 diagnostic screen.





While driving a vehicle, consider all the wheel-related items that can cause vibration

Wheel Imbalance
Runout of the:
Rim
Tire
Rim and tire as an assembly
Force Variation of the Tire
Mounting Error While Balancing
Issues Related To The Vehicle:
Bent hubs
Mounting of the wheel to the vehicle hub off center
Excessive radial runout of the studs
Out of balance rotors and drums
Up until now the wheel service industry could just balance wheels. The GSP9700 fills the gap doing 75% more than wheel balancing.

Let's review some of these issues:

Wheel Balance
Have you ever heard a customer say he is still receiving wheel vibration complaints after he has balanced wheels? Many people do not realize that balancing is only a small portion of wheel vibration. If the wheel never came in contact with the road, then balancing would be sufficient. A square wheel can be balanced, but a square wheel will not give a smooth ride.
Runout
Not usually checked until there is a problem. Bent rims or defective tires might be found "by an experienced eye," but we all know that most operators of this type of equipment have little experience or training. Is it the runout in the rim, tire or a combination? How much is too much? Is it affecting the ride quality? These questions are tough to answer. Runout is usually considered after the customer comes back complaining. This diagnostic time is usually done at the expense of shop labor profits and can be time consuming. What about the customers who have a problem and don't return going to another shop looking for an answer?
Many rim manufacturers measure runout during the assembly of the rim . The valve stem location is sometimes drilled to the request of the OEM at the low or high side of runout or a paint dot or stamp is placed at the identified location.
Many rims on the market today are also purposely designed so that the pilot hole is .002-.004" off-center to allow for adjustment capabilities by the assembly lines when encountering force variation on the tire. It gives the chance to adjust for tire force variation if runout is induced into the rim. But up to this point ONLY BY THE OEM!
Up until now, anyone could measure rim runout at the bead seats with a dial indicator. The drawbacks? It's too time consuming. The GSP9700 will automatically measure the inner and outer bead seat and determine the effects on ride quality.
Force Variation (Tire Uniformity)
Radial force variation is the amount of change in stiffness of the sidewall and footprint when a load is placed against the tire. There are specifications and acceptable limits from tire manufacturers. Up until now, force variation has been measured only by OEM's in an industrial setting using equipment in the range of half a million dollars. For example, a tire and rim could be perfectly round when measured with a dial indicator (free runout) and the wheel will still vibrate when driven due to excessive force variation. Many tires on the market today have a "paint dot" or mark on the tire that indicates either the high side or low side of force variation. Many have attempted to solve vibration problems by blindly '"Lining up the dot" with the valve stem. The operator assumes the "dot" is the high point of radial force of the tire and the wheel valve stem hole is the low point of runout of the rim. Unfortunately, this is not always true. Many tires are not marked and many wheel manufacturers do not use the valve stem as the low point. Other OEM's use the marks to indicate high points instead of low points! The GSP9700 eliminates any guesswork. The locations of runout and tire force are quickly located and measured. The GSP9700 also automatically determines if they can be used to solve an existing non-balance related vibration, before any extra work is performed.
Mounting Error
The wheel, when balanced, must be mounted on the vehicle the same way it came off the balancer. All too often, the technician improperly mounts the wheel on the vehicle off center and the balance is compromised.
Today's vehicle designs are lighter and more sensitive to road feel. It has become more critical to be aware of wheel mounting on vehicles to eliminate vibration during balancing.
The GSP9700 will automatically alert the technician when a mounting error is detected on the balancer, thus preventing improper centering on the balancer from taking place.

In Conclusion:

With the GSP9700, virtually all possible complaints due to Wheel Balance, Tire Runout, Rim Runout, Tire Force Variation, and Mounting Error are all eliminated before the tire and wheel assembly is placed on the vehicle.
100% Seamless Quality Control of all wheels going out the door. It takes no more effort by the operator, no more steps than doing an everyday type of wheel balance.
If there is a vibration coming from the vehicle when it is driven after the wheels are measured on the GSP9700...then the problem IS NOT WHEEL RELATED! No more wasted diagnostic time. No more swapping of tires. No more falsely blamed tires sent back to the manufacturer. No more loss of profits tracking down blind vibration problems that cannot be traced. This is good for the tire dealer, this is good for the tire manufacturer, this is good for the consumer. There is no other piece of equipment available doing such a task.
Now if there is a problem. What can we do to fix it?

1) Grinding the tire? Ugh...Who wants to have their brand new tire ground away? There are numerous serious problems associated with this type of technology.

2) Match the heavy spot of the tire with the light spot of the rim? No. For years, traditional wheel balancers have offered this form of weight optimization. The primary benefit here is reducing weight; however, this does not take into consideration force variation. In some cases, weight optimization can even increase force variation complaints. The vibration becomes worse.
Hunter Engineering chose a better direction.

How to eliminate tire and wheel vibration with the GSP9700
Match the Tire to the Rim

This can be helpful if the first harmonic of the rim can be indexed to cancel the force variation of the tire. As stated previously, this is not the same as the relatively ineffective common practice of most balancers to match imbalance of the tire to rim to minimize weight.

R & R Tire or Rim
Replace the defective rim or defective tire and also be able to locate and quantify the problem.

Record the Value and Reuse the Tire on Another Rim!
If a rim and tire cannot be matched and the tire is not out of spec, the actual value of the force variation can be written on the sidewall of the tire and placed back on the shelf. At a given point in time the entire inventory in that tire size can be "measured." Eventually a "measured" tire can be hand picked to match a given rim. This would be analogous to picking an alignment shim to fix the alignment condition and bring the wheel as a "tire and rim" back into specification. The GSP9700 moves vibration solving in wheels to a new level of professionalism. "Road Force Measurement" against the wheel before the balance has been needed for years!

Here's a picture of the machine:


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