kkell3232

Hoping to get some opinions on next steps here:
I replaced my old C4 engine mounts with RS-style mounts over the winter.
I replaced my 6-7 yr old 17 inch Sumitomos (205/255) this spring with Firestone Firehawk Indy 500s (225/255) which were well reviewed on some posts and by Tire Rack. I also had my cup 2 wheels refinished (painted with powder coat clear finish).
I now have an annoying vibration ~80mph and above.
I had the wheels road force balanced and was told one of the tires should be replaced (51 lbs?), I got a replacement and had that road force balanced and told everything looks great. Vibration is the same.
I just swapped out with a friend's 18 inch heavy solid twists/Contis and the vibration seemed much better though not totally gone.

What would you do next? Get the wheels checked elsewhere? Replace the tires? The car drove smoothly last year on the old tires at speeds >100mph. Shocks/springs and front control arm bushings were replaced over a year ago.

TIA

fatmike

You've figured out that most of the problem is wheels. Find another shop and get the balance better. Are the wheels true?

I've used the Firestone's on a different car. I thought they were fine. One of the things about the Michelins is they have a reputation of balancing really well.

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pp000830

Hi TIA
RS Engine mounts are a good bit stiffer and may be transmitting vibration into the body.
Also, see the section on this page "Engine vibration and body boom after replacing the transmission mount:"
https://993servicerepair.blogspot.co...nsmission.html
Andy

PFDGSB

If tires are balanced, next usual suspect is control arm bushings.

https://fdmotorsports.net/shop/ols/p...od-bushing-kit

kkell3232

Front control arm bushings have already been replaced thanks to @AOW162435
If road force balance is redone elsewhere and still found to be ok I'm not clear how there could be an issue with the tires, but guess I'll cross that bridge when I get there (Monday).

TIA=thanks in advance

orangecurry

Are you still on the original tie-rod ends?

Heuerman

Have tried having the wheels dynamically tested (i.e. on the car) which will eliminate tyres, wheels, warped brake discs and bearings?

kkell3232

According to the service records, the tie rods were replaced when the leaking steering rack was repaired 13 yrs/26K miles ago...

Have not done wheels-on dynamic balancing; my understanding is road force is preferable if done by someone who know what they are doing. Because the car was driving fine before replacing the tires and engine mounts I don't think the brakes or hubs/bearings are the issue, but will certainly keep those in mind if the repeat road force balance is unrevealing.

Thanks for the suggestions/comments.

orangecurry

26k miles shouldn't be a problem.

Personally I think your 993 deserves better than Firestones.

...and this doesn't fill me with confidence on your tire shop
...but you also say that the vibration was still there with the other wheels set..... so it's not the wheels or tires.

fatmike

It mostly is the wheels and tires. So fix that.

I’d be concerned that something in the suspension has come loose.

/

ClassicAndy

I’ve experienced the same issue as OP, and in my case it was the axle on the balacing machine that was slightly bent (giving false balancing results = not driveable at all).
This was with brand new wheels and Michelin tires and the shop were not aware of it by themselves. I almost ended up claiming new wheels from a company, because of this.

After two failed balancing jobs I changed shop and it all was fixed.
Later I got verified that the first shops machine needed replacement and it all fell together as what was really the issue.

As I see it, its worth handing the job over to someone new, someone preferably with a new or true balancing maching. (Lower cost rather than starting with new tires or wheels).
I hope you get this sorted.

cheers,

ClassicAndy

Ps,
rotors with lots of brake pad resudue in them will also contribute on the vibration part.
The rotor will not bend, but they would collect lots of pad dust over time if not cleaned. Especially with pads like EBC.

how long have you had your front rotors on? Taking them off and beadblast them internally, should do the trick. Spray on some galvanizing paint in the end and they look brand new again.

rlme36

If you are taking off the rotors, just replace them, not worth the hassle from a down time perspective. I like the swap the installer idea to narrow down if equipment / poor technique is at issue.

95_993

Start with having the wheels checked for "straightness". Costs nothing. You'd be surprised, but I have seen wheels balance that were not "true" which will cause vibration.

kkell3232

@orangecurry I won't disagree that the Firestones (Bridgestone Potenza RE003 rebranded for the NA market) are considered more of a budget option but hard to find a negative review on them, and for a street-only weekend car I thought I would give them a try. One of the only criticisms I've read is that they can flat spot easily and I wonder if that's what the original shop encountered and misdiagnosed as a manufacturing defect. The place is a very busy and widely recommended local wheel refinishing shop, but I suppose anyone can make a mistake- maybe a consequence of being too busy.

The plan for tomorrow is to get the wheels inspected for straightness and road force balanced elsewhere by someone who I'm told knows what he's doing. The rear rotors do need replacing, however I was going to wait until end of the year. Thanks for all of the suggestions and will report back with the results.