I'm in the process of exorcising a strange cyclic, speed related vibration on my 2007 997TT. The car has 54k miles, a DSC PASM controller, new tires (MP4S) and otherwise is all factory original parts. The vibration does not appear wheel related, as the wheels have been road force balanced multiple times and swapped with other known good wheels without any difference. Front wheel bearings have been replace, runouts have been checked, and all axles and the center driveshaft have also been ruled out, so the vibration appears to be a worn suspension issue. Shocks are in good condition and verified to be go firm/soft per PASM control.

Car is street driven and factory ride height. Car otherwise seems to handle fine -- nothing is noticeably "shot" or noisy, although I do get a small bit of (non-traction related) wiggle from the back end under hard acceleration which suggests (I think) that I've got something loose/worn in the rear suspension.

I recently replaced the top strut mounts on the front, and that made a nice difference in the firmness of the suspension, as well as partially damped the vibration I'm having. While none of my suspension parts are obviously loose/broken, the improvement I saw by replacing the top mounts has left me looking to refresh other suspension components. As such, I'm looking to "smart upgrade" -- I don't plan on setting track records and I definitely do not want any NVH issues, but more crisp handling is welcome. And having the vibration go away would be bliss.

I'm currently running a very non-sporty alignment: F: ~-0.5 degree camber / .6 degree total toe in; R: ~-0.8 degree camber / .9 degree total toe in. I'd like to have the car realigned to factory GT3 specs, but to do that, I'll need more camber adjustment in the front to get to -1.5.

Here's my current plan (in order of my perceived importance):

- Front strut top mounts -- done (and nice difference already)
- Front GT3 LCAs -- plan is to run a conservative shim stack (no more than 7mm) to keep the front axles happy. Based on other reports, that should get me to -1.5 degrees of front camber. Also, the ball joints on my current LCAs, while not shot, seem pretty "floppy" versus new, stiff joints -- so I'm hopeful this may be the remaining vibration I'm feeling. Note that I will be using spacers/washers with the existing 997 tuning forks with my new 996 GT3 LCAs.
- Rear shock top mounts -- My thought was that these may add a bit of dampening/control to the back as happened with the fronts
- Rear LCA solid bushing -- My sense is that this bushing is most likely to wear and allow toe changes in the rear (and hence wiggle under acceleration) -- would love feedback on this theory

For now, I'm not planning on replacing the tuning forks or tie rods in the front, nor replacing any suspension components in the rear other than the shock mount and the LCA bushing -- All the rubber ends and ball joints feel fine/solid. Let me know if you think I'm making a mistake. I'm doing the labor myself as I've become quite good as disassembling the suspension and replacing parts piecemeal is somewhat interesting as I try to figure out the source of the vibration -- science and all...

Thoughts on the above?

Thoughts on the solid LCA bushing in the rear? Would a solid LCA be better in the front? I could also put the solid LCA bushings in the front and swap new rubber bushings into the rear LCAs, but am not convinced that would get me as much benefit as stiffening up the rear under acceleration/braking.

Appreciate your feedback.

 

I have no rubber left anywhere in my suspension and it's totally fine for street use and handles like a dream.

Regarding the vibration, are you currently running any wheel spacers?

 

I would strongly suggest getting aftermarket rear adjustable toe links. The bushing on them is very compliant and causes big toe changes to the rear tires when the suspension is loaded and unloaded which is not a good feeling. They also help you get the desired amount of static toe when lowering the car. This is generally what causes toe changes, not the rear LCA bushings. If you're going to get them, get the ones that are bump-steer adjustable (shims allow you to change the angle of the link to minimize bump steer based on your static ride height).

https://www.splparts.com/products/bu...er-cayman.html
https://www.tarett.com/items/996-997...mps-detail.htm

The LCA bushings are pretty thin. Just installing new GT3 LCAs will get you the new ball joints, and new rubber bushings. I'd call that good. You don't really need rear adjustable LCAs since you can get the amount of camber you need by lowering and adjusting the stock suspension but you COULD replace the rear LCAs with new OEM ones or have them rebuilt with new bushings and balljoints. The former options would be easier but more money.

I wouldn't do solid LCA bushings front or rear, personally. I just don't think you'll see much gain from those. If I was replacing ALL rubber bushings with solid, as Petza914 did, then sure, but there are a LOT of components you have to replace to do that and it's a lot of money.

Given the vibration issue you are having, I'd consider tie rod ends too, but those are easy to check by a competent shop who can measure their play at the wheel. If you go to a shop for this, ensure they are using a measuring instrument to perform the test instead of just grabbing the wheel and eyeballing the free-play.

 

Pete, thanks for the reply. Good questions, and No, I'm not running any spacers -- and part of the reason why I want GT3 LCAs is so that I can widen the front track just a bit and not use spacers.

Regarding the vibration, I get a cyclic vibration felt through the chassis of the car at speeds above 75mph -- it seems very strong around 85mph. It's not very noticeable/doesn't exist on normal road surfaces, but grooved concrete roads seems to really excite it. It's the cyclic part that's strange -- it's a very quick "shake, Shake, SHAKE, Shake, shake, pause, pause, pause, and repeat" vibration. Each cycle of the vibration intensity is less than 1 second peak to peak.

Lot's of people have said 2 wheels slightly out of balance can cause "beating" as the wheels travel slightly different speeds -- the theory makes some sense, but hasn't matched my situation including multiple wheel rebalancings and even swapped wheels with a known good set. In fact, the wheel swap seemed to make it slightly worse, which somewhat reinforces my current view that it's a suspension issue -- the test wheels were heavier than my regular wheels, had more offset, and thus may have been harder to control by a worn suspension.

I've also looked at bad CVs and wheel bearings and ruled them out by swapping (rear axles), replacing (wheel bearings) and removing (front axles and driveshaft) from the equation. The biggest improvement so far has been replacing the strut top bushings in the front, which was a surprise, as the vibration feels like it's in the rear/chassis, not the front/steering.

My current theory, is that I've got worn front LCAs (balljoints) and that the grooved road is exciting one or both front wheels slap like a grocery cart with a bad wheel.

Most of my forum reading suggests that the all-solid bushing swap, while not terrible from an NVH standpoint, isn't really very impactful unless you're tracking the car. But I seem to recall, that someone also stated that the solid LCA thrust bushings are probably the most impactful in terms of "bang for the buck" on steering responsiveness (after alignment) -- it makes sense to me given that the bushing locates the wheel both fore/aft which would impact toe if the bushing is sloppy. I'm curious whether that rigidity is more important in the front or rear. I'm guessing both, but maybe more important on the rear on high HP/torque cars given how toe changes really make the rear of these cars feel stable or loose.

Appreciate any other thoughts.

 

Hmmm, that is odd. The wheel balance thing with the wheel balance phase issue would only make sense if this never happens on a straightaway when the wheels continue to spin at the same speed on both the left and right sides of the car. If it shows up after a turn then goes away after then next turn, then that could be a possibility, but seems pretty unlikely. Issues in the front end usually shake the steering wheel. Issues in the rear end usually shake the car. It does sound like something is loose or worn in the rear end, but I just can't tell you what specifically based on the symptoms.

What about this yellow circled attachment point on your LCAs? I could see this moving up and down slightly in turns or under load and creating that type of issue. A loose or worn stabilizer bar link could do the same, but you said you've checked those.

 

Thanks for this -- the solid joint rear toe arm makes a ton of sense for the reason you describe: Maintains toe under load. I'll put it on the list of things to do in the rear.

And I may swap the the rear LCAs for a new ones too -- it's probably easier to just swap the parts than rebuild them. As you suggest, everything I've read suggests that GT3 LCAs are not needed in the rear unless you're going for a very aggressive setup (which I'm not).

New tie rod ends are potentially on my list. As you point out, it's hard to test them at home, but they're not obviously loose. Frankly, it seems hard to test all the bushings at home -- they're pretty stiff even when worn. Even my strut top mounts weren't completely separated -- they were cracked, but intact -- but not doing as good a job as new parts.

Also, you didn't mention new tuning forks -- that's also consistent with all the forum posts I've read. Unless they're obviously shot/making noise, it seems unlikely they are a problem or area for meaningful upgrade..

 

Yeah, the vibration thing has been a very time-consuming head-scratcher. All signs pointed to something rotating being out of balance, but I've pretty much ruled all those options out. The cyclic vibration happens on the grooved pavement (at speed) whether it's straight or curved -- so relative wheel speed does not seem to be the issue. I've even tried changing (meaningfully) the pressures of individual tires (eg, run one tire at 45 or 50lbs) to see if that affects the vibration signature -- it did not, again suggesting that it's not a wheel balance or relative speed issue, but rather looseness somewhere in the suspension.

And that somewhere could well be the LCA balljoints. It very likely might be in the rear (as you circled in your picture), particularly since it feels like a rear/chassis vibration. But I think it could also [partially?] be the same joint on the front LCA, which feels "pretty loose" though not quite sloppy -- it's hard to check the actual tightness of this stuff by hand as mentioned above. Regardless, I've got new front LCAs on the way, so one more item will get checked off the list soon. And per my post above, seems like new rear LCAs and solid toe links are warranted.

Thank you for the thoughts.