Vibration, struts, alignment ...

You will recall that recently I took my 2005 Boxster to the dealer for an alignment, after experiencing a vibration between 70 and 80 mph.

The technician noted that the right front wheel could be wiggled slightly when grabbed at the three and nine o'clock positions and shaken. He attributed this to the strut shaft being loose in the strut tube. He recommended replacement of the right front strut. A new strut was ordered.

In the meantime, the government-mandated motor vehicle inspection became due, so I took the car into the local Goodyear garage for an inspection and asked them to pay close attention to the front suspension. The Goodyear garage has performed alignments on my BMWs for 15 years. I have a good working relationship with them. They understand I'm fussy, they allow me into the bay to examine my vehicle when it is on the hoist, and are willing to show me any components which they feel they need to be replaced. In short, I find them honest and trustworthy. The tech who does my work has a very good working knowledge of cars and suspensions, but is not a BMW or Porsche specialist.

The technician did not see any suspension problems. While he noted there was a very slight looseness in the right front wheel, he felt it was well within spec. He pointed out that the looseness could be generated better by grabbing the wheel at 12 and 6. He felt, if anything, it might be a wheel bearing starting to go.

We discussed an alignment. He explained he does not have software to do the steering angle calibration. However, I discovered this can be done with my Durametic software. Here is a link:

[www.renntrack.com]

The actual menu is slightly different for my Boxster and the current version of the Durametric software. It's pretty slick. (I continue to be impressed with Durametric. They also have good online and phone support.)

Because I like the tech and find him to be very conscientious and forthright, I went ahead with an alignment, prepared to toss away $80 if it didn't produce a positive result.

Here are the before and after measurements from the alignment:



You'll note that there is too much rear negative camber. The tech said that this is the best he could get using the adjusting cam to its fullest extent. He wondered if there might be other cams available which would provide a greater degree of adjustment.

Interestingly, although it appears there is too much negative camber, my rear tires actually wear a little more on the outside than the inside.

The following questions arise:

1. There are 15 mm spacers on the rear of the car. (And 7 mm on the front). Might they account for the excessive negative camber?

2. Are there other rear adjustment cams available which would allow a greater degree of camber adjustment?

3. Should I be concerned about this amount of negative camber?

4. Any other thoughts on the ‘after’ measurements?

An initial roadtest suggests there might be a lessening of the vibration. However, our roads are less than mirror-smooth, so it's hard to tell. A longer road test on smoother roads is in order

Incidentally, next week when I'm in Halifax I will have my independent mechanic (he works on BMWs, Porsches and Audis) take a good look at my front suspension. He is very good. I do not mind replacing the front struts, IF that's the problem. I am yet to be convinced.

Thanks, all, for your input.

R

Here's what i don't like:

1. rear cross camber. I would prefer to have them even, even if they are off
2. differential or cross-caster - seems something is moved in the RF strut - note that's the one you spoke of an issue with, correct?
3. rear cross-toe, nto a biggie, but why?

In general most people shoot for as much negative camber as possible for performance driving (track, autox) and less for street driving. I think on the street i had -1.5 / 0x rear and -0.2 / 0x front. I now have something like -1.8 / 0x rear and -0.5 / 0x front. I have darn near zero toe everywhere and like it that way.

if you are at the limit of adjustment i suspect either:

1. something(s) is worn or
2. something(s) is bent

Note that in the rear there is a trade-off between toe and negative camber.

I am of the opinion that things in good condition do not move.

I doubt that you have a bad wheel bearing - they make noise well before they move.

proper measurement is to rock 10-2 and 12-6. At 9-3 you are introducing steering rack and that must be zero'd out. Hold wheel or "feel" for rack rather than struts.

Car should be OK with numbers as presented.

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

and tire life. However, I do not recall just how much excess negative camber the rear tires had and do not have the time -- at this time -- to dig through my files (it is up to two folders now) of paperwork for my Boxster to look this up. The car did end up with what I was told was the max. amount of negative camber (at the rear tires). My impression at the time was that the adjustment was at the max. allowed or what was permissible.

The tech assured me the camber would have no effect on the car's handling/feel or tire life. I decided to trust the tech (this was at Custom Alignment in Mountain View, CA an indy tire/alignment shop with a very very good rep) and the tech proved to be right.

But I do not know that what rear negative camber your car has is comparable to what my car had.

Looking at your car's before and after numbers, I note the right front caster is out. So is the one on my 996, but not as much as that of your car. My car has just a few tenths of a degree difference (call it 7.5 vs. 7.3 while your car's difference is 7.5 vs. 6.9 which for caster is I've been told a lot.

At one time both were the same but at the time of the mule deer repair the front suspension was removed to apply some paint behind the areas the front suspension sits in front of and the tower was not bolted back in quite the same location. To move this now would possibly compromise the paint at the left front strut tower mounting.

At some point I will have fitted after market suspension/steering arms that permit caster adjustment and then have the tech dial in the caster to bring them both to the same reading again.

15mm spacers seems like a lot of spacer. What does the factory offer? If less then the 15mm spacer thickness can account for the excessive camber. The stock setup may not provide enough adjustment range to deal with this. There is not much adjustment to begin with. These cars are assembled to quite tight tolerances at the factory and lots of adjustment costs money. Aftermarket parts offer more adjustment for those that want more or less of the various alignment settings.

I'm not aware of any other cams or such from the factory. I do not some companies make aftermarket replacement parts which have a greater range of adjustment to deal with cars with modified suspension.

As for the negative camber. I can't say for sure it is not an issue. I know that some extra is harmless, but I do not if what your car has is 'extra' or way too much. You will have to ask a Porsche tech.

But they also do quite a few negative things. In the front they add quite a bit of scrub radius, ruining the steering geometry. This results in kickback and odd forces on the wheel (e.g.: a lever arm pushing back on your hub).

In all cases, F & R, they increase the lever arm for all forces operating on the wheel bearing, hub and suspension. The torque on the wheel bearing in particular is increased quite a lot. If its not obvious, exaggerate it and think about it. what if the spacer was 3 feet long. Roughly 1/4 weight of the car (750 lb give-or-take) would be on a 3 foot lever, placing a torque on the wheel bearing. That's 2,250 lb-ft. The spring and damper rates would also be similarly decreased, since they would now need to resist the same weight of car, but moving a smaller amount with 3' rather than, say 1" of effective leverage. In that case a factor of 36x.

I admit many run spacers, btu that does not make it a good idea on balance, and certainly not without downsides.

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

From your comments, I've gathered:

1. My post-alignment numbers are acceptable.

2. It's possible there's something awry in the right front;

3.15mm wheel spacers might account for the high negative camber and might cause an acceleration in wear.

I've had the spacers for almost 5 years now. While they result in a wider track, to be candid my goal was to fill out the wheel well better, aesthetically. They do that well.

Porsche offers 5mm spacers. Clearly the 15mm on the rear are well beyond that. The 7mm on the front are closer.

Needless to say, with the vibration occurring only recently, the spacers didn't cause it per se, but may have accelerated some wear, which could now have a role in the vibration.

Over the weekend, I'll run my car on my old wheels and tires and see if there's any improvement - if not, then it's likely not an out-of-round wheel/tire, and with the alignment done (most specifically, the toe within spec), I'll know to focus on the suspension and related parts.

On Monday, I'll see what my independent guy has to say about the suspension, and take it from there.



Edited 1 time(s). Last edit at 06/21/2013 02:40PM by Roger987. (view changes)

1, wearing the bearing and thsu causing camber
2. bending something (un likely)

I do think they coudl accelerate all kinds of wear.

In the rear they at least dont mess with steering geometry.

5mm is autie a small difference. I sometimes change front offset by up to 10mm (same as a 10mm spacer, but less to go wrong) on my Audi to clear brakes.

Never more hwoever

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

Sorry, I am late to the thread. I had my wheels balanced twice to get rid of it, and it would work for a bit.

These worn control arms are evil and, apparently, hard to diagnose as a sorce of problems.

Peace
Bruce in Philly

Thanks very much, Bruce.

Interestingly, my independent (who will look at the car tomorrow) mentioned on the phone that a control arm could be a possibility.

Certainly, in my BMW, control arms are almost consumables and are often the vibration culprit. I've replaced so many, I actually bought the BMW factory tool for separating the ball joint.