Weird wheel balancing / vibration issue

[xxaarraa]

Hey guys,

Here is the background: I had the steering wobble at 65-70 mph and THREE different shops tried to road force balance the wheel/tire with no luck. They all confirmed the front tires were "marginal" (tire shop speak for defective) and could not be balanced 100%. The tires are brand new as far as tread wear goes, but they seem to be defective. Well anyway, the third shop was able to isolate it to the front left tire as the particularly bad one. He took me back and showed me the tire spinning on the machine and there was quite a bit of lateral movement on the rubber.

But he was able to add weights to 3 different places on the inside and outside of the wheel and get it balanced. The car was running smooth with no vibrations for a few days but now the vibes are back. But they seem to come and go randomly, which is what puzzles me. If its an out of balance tire, shouldnt it always be vibrating as opposed to every now and then?

Alignment was done recently and front end was checked, so pretty sure its just the tire issue. I ordered a replacement rosso corsa from Tire Rack, but was curious if anyone had any ideas on what this gremlin could be.

[Uwon]

I had the same issue with a Volvo I once had and it turned out that two of the tires had a slight sideways wobble so when they harmonized in a particular way the car would rock back and forth. If I made a 90 degree turn and then went at speed it would go away. Volvo, bless their..., changed the worst tire on warranty and it still persisted but not to the same extent. I became a PITA and they changed the remaining tires, again on warranty, which solved the problem.

By the way, I had those Rossos and gave up on them. I now have Michelin Super Sports and they are great, quiet and long lasting, relatively speaking.

[DBJoe996]

I have experienced this type of problem with other vehicles and tires. Tires are manufactured using various methods and sometimes quality control is lacking. Also, they age and wear so they can develop some weird problems. Think about the heat and stress a tire endures and sometimes it is a wonder they survive the road for very long. The tread design also makes a huge difference as it wears and ages. Sounds like you are headed in the right direction with replacements. I have had the Michelin PS2 Super Sports AS on mine for quite awhile, and like Uwon they are great. My experience is some tires work really well on some cars, and are horrible on others.

Here is the Porsche Approved Tire List http://www.porsche.com/usa/accessoriesandservices/porscheservice/documentsanddownloads/

[xxaarraa]

Thanks for the thoughts guys. I bought the rosso corsa because I didn't want to run a mismatched tire (all 4 tires are almost brand new). When winter comes, I plan to get a full set of forged rims and all season tires anyway, and will go to the Michelin AS or Conti DWS routes.

[Y2K911]

I had this same issue with a front left tire on my Volvo XC. It turned out to be a tire ply separation undetectable to the eye until the tire was worn half-way. They were Pirelli Scorpios. Although I ride on Pirello PZero Rosso on my C2 and quite pleased with them.

Your move to replace it, is the right one, in my view.

[fpb111]

We fought a similar balance problem on a set of 1/2 worn front tires.

They would balance well run smoothly and then then go wildly out of balance and then mostly smooth out again. I finally gave up and bought two new tires. When the shop pulled the tires about 1 cup of water ran out of one and maybe 1/2 cup in the other one?? We figured that I must have added it with air from a source that never drained the air tank. We checked the rears and they had no water.

[logray]

Another thing that can cause this is improper mounting of the rim to the hub.

I've had the same wobble and it has been fixed by correctly torquing the wheels.

At a minimum torque the bolts in a cross pattern starting with 50% of the final value. Then go around again and apply the final value.

[Mijostyn]

Logray is right about torquing. A lot of guys try to do this with just a breaker bar. Bad idea. You have to get a realiable torque wrench.

This is also why Porsche uses two piece lug bolts. They torque much more consistently. The wheel is not indexed to the hub by the lug bolts. The wheel indexes the hub by it's center to the hub spindle. It is good practice to lightly clean the wheel center and hub with a Scotch bright pad to remove any debries prior to mounting and balancing.

Now comes the real shocker. Most of what you are complaining about here are purely tire balance issues. Any time your steering wheel dances between 75-85 MPH you most certainly have a tire/wheel issue. I would estimate that 90% of the time it is wheel balance, 8% a bent rim and 2% a bad tire.

XX, the shake comes and goes because of harmonic dampening by the opposite wheel. As you go around corners the relationship of one wheel to the other changes because they are traveling along different radii. Sometimes they cancel each other and sometimes they amplifiy each other. To you it feels as if the shake is coming and going.

Guys, I hate to say this, Most of the time you are getting your wheels balanced you are actually getting them UNBALANCED. Our Porsches require getting their front wheels balanced to less than 0.1 oz per side. First of all you can't possibly do this with a machine set to read in 0.25 oz increments. If you look at your wheels and see weights in full 0.25 oz increments only, your wheels were not balanced well enough. Occasionally, You will land on a full weight but most of the time you will have to cut the last weight to a fragment. This is one of the benefits of tape weights and one of the reasons steel weights stink. (fortunately I have a huge stash of lead weights) When a machine is in round off mode what it means when it says ".00" is not .00. It is less than 0.25!

The tech has to take the machine off round off mode so that it reports weight down to 0.01 oz.

The weakest link in the chain is the tech followed by the machine. It does not mater how well the tech does his job if the wheel can not index the machine reliably. If that wheel is just a tiny fragment of a mm off you are screwed. Next time you have your wheels balanced watch the tech. When he finishes balancing a wheel, after he takes it off, ask him politely to put it back on and have him spin it up. I have seen wheels up to a full oz off on respin just after the wheel was supposedly balanced! Coats specs its machines at 0.1 oz.! So how can we get our wheels under 0.1? Savy tech using a well tuned machine. Most of the machines in use have had the sh-t kicked out of them. A good tech can tell when his machine is in trouble because it does not give him reliable repeat readings and when he rechecks the mount 4 times he gets readings off by more than 0.1 oz. This is also how you make sure the wheel is mounted correctly. Occasionally, the centering cone does not seat just right and you will get a reading that is way off. Thing is, you have to check. Most do not.

The method of placing the weights is to complicated to explain here. You have to see it done. Your tech probably knows what he is doing if he initially places the weights with packing tape and respins the wheel removing weight and changing the weight's position until he gets it just right. Then he removes the backing and sticks the weights in place.

It is amazing how well a mildly bent rim and a dancing tire will run if the wheel is balanced correctly. If you can not easily tell that a rim is bent just by looking at it then it will probably run OK and I usually wait until I change tires to have the rim trued. You can easily see the slightest hop when you spin the tire up. In 30 years of balancing tires I have never seen a tire that would not balance out fine. I'm sure they are out there and I have had my fair share of defective tires (most of them Pirellis).

Above are three pictures of the same 997S rear wheel on the same Coats 700 computerized balancer. The first shows straight zeros. Think the wheel is balanced? The only thing I did in the next picture was to take the Coats out of round off mode. Now you see 24 and 26, the real weight and not good enough for us. In the last picture I have finished balancing the wheel and we have both sides under 0.1 Good to go. All balancers operate like this, to speed the techs up. It takes a patient 15 minutes to get a wheel under 0.1. NTB would never tolerate that.

Edited June 14, 2013 by Mijostyn

[logray]

The above post speaks volumes.

Having a clean mating surface between hub/wheel and not one done carelessly makes a big difference.

If you track the car, periodically retorque the wheels.

The correct torque value is in your owners manual. Contributing members can also download their owners manual. For example the 996 2003 model year is here: (same specs for all 996 cars, page 208 in the manual below).

http://www.renntech.org/forums/files/file/601-my03-carrera-996-owners-manual/

I always do the following as well when having my wheels balanced.

Take them to the same shop and build a reputation.

I use a place that has a road force balancer.

Always watch them do the work, pointing out much of the things in the above post.

I remove the old weights and clean the wheels before having them balanced. This prevents careless techs from damaging your wheels.

Wheels need to get balanced periodically as they wear.

Carry the wheels to the techs and treat them well, and the techs will treat them the same. If they don't, find another shop.

Worst case, take them to your Porsche dealer and demand the same as above, even if you pay a little more it might be worth it. I've had very competative pricing from Porsche dealers both on new tiers and the mounting/balancing.