Frame Distortion

I hit a small to medium size pothole on the NY State Thruway today. I felt it on the left side of the car. The car continued to drive fine. When I got home 20 miles later, I checked the hood gaps. When I checked the gaps two weeks ago I could get a quarter through the gap, from the windshield to the headlight on both the left and right sides. Now, I can get it through on the right side, but it gets tight just above the headlight on the left side. Not easily noticed visually, but a definite small change. In addition, when I checked the gap above the front bumper from right to left two weeks ago, the quarter got stuck just to the right of the left headlight. Now it can make it through the entire gap from headlight to headlight. I do not want to over react. Could this be just a slight shift in the hood because of the pothole? The slight shift in body panels was mentioned in the BMW Munich letter posted on McFly's website. Or does it sound like minor frame distortion? It seems to me that the hood has shifted slightly since there is a change at the side gap and at the gap above the bumper. I appreciate any opinions/advice.

I would be a bit worried. Check your shock towers for any sign of doming. My car is exactly like yours on the left side, the qtr gets tight just above the headlight, but the right side it slides right thru. I have no sign of shock tower deformation. You could be suffering the "cosmetic" deformation bmw cites---yeah right!

Since I have a Z8 with no miles on it, I thought I'd pass along the following information to help people determine whether their cars have sustained any shock tower deformation damage.

Let me begin by stating that the following measurements should serve as a reference point, not a definitive diagnostic technique.

I do not think measuring the gap between the edge of the hood and the fenders is a reliable way to determine if shock tower deformation has occurred because there is too much variation that is within production tolerances. There may be exceptions to this, particularly on a car with severe deformation and/or frame bending but even my unaffected Z8 has a noticeable narrowing of the gap towards the front of the car so I would be reluctant to trust this method. I would also avoid using the flat top of the shock tower as a reference point since my unaffected Z8 has a slight curvature to this area. Of course, you CAN use this information to alleviate your concerns in case your car has no damage since these small variations are perfectly normal.

Based on the pictures I've seen of cars with obvious shock tower deformation, it appears that the relatively flat plateau area on top of the shock tower is being pushed up into a dome shape. The maximum rise is taking place in the center of the tower (where the shock mount access hole is), as opposed to being a uniform deformation spread across the entire plateau, which results in a dome shaped deformation. This doming also causes the 3 strut mounting studs with nuts visible on top of the shock tower to splay outwards from the center.

On an unaffected Z8, if you lay a straight edge of a metal ruler across the upper surfaces of any pair of strut mounting nuts, you will find they lie in a parallel plane. As a result, there should be no visible air gaps between the edge of the ruler and the surfaces of the nuts. This was remarkably consistent on my car. If doming has occurred, you can expect to see gaps between the straight edge and the surfaces of the nuts due to the splaying. To take this one step further, if you remove the plastic dust cap covering the shock mount access hole, and lay your straight edge across the top surfaces of a pair of strut mounting nuts, you can measure the gap between that straight edge and the top of the raised rim of the access hole. Using a set of feeler gauges, I measured .052 inch. There is paint on that rim so variations are possible but both sides of my Z8 were identical. If doming has occurred, you may find a significant variation either from my measurement or from side to side. Of course, if the studs are splayed, your straight edge will not be lying flat across the surface of the nuts and it is possible that you could get, by sheer coincidence, a measurement similiar to mine despite having some doming so I suggest you also make a visual check by sighting across the straight edge resting on the upper surfaces of the nuts and view the upper edge of the access hole's raised rim. On my car, the two lines are parallel from any angle. If doming has occurred, you should expect to see a curvature to the raised ring or a non-parallel relationship when referenced to the straight edge.

I realize these are just rough guidelines but they come from a guaranteed unaffected Z8 and should provide a reference point from which to begin your examination. Plus, your own measurements should serve as a baseline to keep track of any changes over time. Hope this helps!

P.S. Don't forget to replace the plastic dust covers when you're finsihed, i.e, don't leave them lying under the hood, or you may end up with some new "domes" in your hood when you slam it shut!
Grease Monkey
Grease Monkey

After thinking about my car and the quarter test most of the night, I went first thing this morning to a high grade body shop that I know very well. I showed them my car and all the info and pictures from McFly's website. They said that in their opinion, all cars, with steel or aluminum chassis, have some degree of flex built into the area by the shock towers. They also felt it would be impossible to eye ball the distortion of the shock tower but it could be measured with the correct laser instrument. After carefully re-reading all the posts, particularly on McFly's website, and the BusinessWeek article, could we be creating our own version of 60 Minutes Audi faulty unintended acceleration, and killing the value of our cars over a non issue? I ask this question after I read that Jurgen Wunderlich, a technical board member from the German club that started the entire investigation says, "The Americans seem much more fearful". He also says that you would need a HUGE impact at high speed to compromise the structure of the frame. I am sure my hood seam gap ever so slightly narrowed after hitting a pothole yesterday, but is it frame distortion or normal wear and tear? I ask the same questions about McFly's pictures with the quarter. I am sure that a big percentage of us over time, will fail McFly's quarter test, but is his test really valid for frame distortion? I am really confused.

Thanks GM. I did your suggested check of the shock towers with a straight edge and all is normal. I removed the caps and see no doming at the center of the plateau where the shock is attached. I also made sure I put back the plastic covers. Your explanation is very helpful and reassuring. I just wish some owner's would stop "falsely yelling fire in a crowded movie theatre."

This is no bogus issue, and it is not exaggerated either.

I myself made the miserable experience a couple of weeks ago, driving another owner's Z8.
We measured the towers using a ruler before we started to drive. They were almost flat, the "bulging" was about 1 mm on either side, nothing to worry about.
During our short trip, I was allowed to drive a short time myself. Believe it or not, half a mile down the road there was the famous pothole waiting for me, I saw it too late and hit it with the left front wheel. All I could do was to take my foot off the brake pedal to ease the pain.
After the impact, we immediately went back and measured the strut towers in the same way we had done before. The left one's bulging was now 3 mm. One single pothole, speed was about 50 mph. In my 7 series I wouldn't have thought about this kind of impact for a second.
Here it bend a strut tower such that it was easily measurable with a simple ruler.
Needless to say that this was a most embarrassing experience for me, and I can only thank the owner for the way he handled this.

Consequence:
Measure the top of your strut towers, if they are not flat (follow Juergen Wunderlich's suggestion), you have an issue. It is certainly correct that a lot of cars have some kind of damage in this area, however, most of those are steel cars, which is a completely different story.
Andrew warned several times that driving may cause further damage, and he was laughed at for it. I can only confirm, that a single pothole was enough in this case.

Cheers,
Dieter

Not to belittle the potential for damage, but I would not be surprised to find a variance in the thickness of quarters. I suggest if you are going to use a quarter as your reference, then use the same one.

What you should do is as Dieter (DWZ8) says and measure the tops of the towers, and also have a 4 wheel alignment done on your car.

I will tell you that my car does have some doming, though it is not severe, and my alignment was way out. When they aligned the car they were able to get it well within the spec- and the key thing is to look at the after results for side to side- (eg- front right vs. front left) if the side to side numbers are really close you are looking pretty good.