Talk to me about “migrating fenders” i.e. front strut towers moving upward!

[Minivansomeren]

Hi everybody, I’m new and in the market to buy an E9. I’ve read most of the excellent resources on here about inspecting and buying. One thing I’ve seen mentioned several times is the “migrating fenders” where the front strut tower bulges up and rubs the hood.

I haven’t seen any discussion of the significance. Is this caused by rusting of the inner fender spaces? I.e. does it signify hidden decay? Additionally, have there been problems caused by this, such as affecting hood alignment? I know rear shock towers occasionally blow out (metal fatigue) but has anybody seen this front tower “migration” lead to a blowout, or affect drivability?

The reason I’m asking urgently (my first post) is that there is a car on BAT that looks very nice but does have this phenomenon. The auction ends in a couple days and I’m wondering how important an issue this is.

 

[eriknetherlands]

It's a good questions, but straight up: i do not know the answer. I've heard people say it, and i think it can happen if there is a lot rust.

But i try to be precise, and hear say isn't enough for me....

So I've tried researching it by measuring an all original non-welded survivor. I can dig that up for you on thursday.

Curious though what others know about it.

 

[ScottAndrews]

I suppose that could happen on a car with VERY severe rust that basically allowed the inner fender to move relative to the body, but I would imagine that would be pretty obvious, since there would be movement between the two in other areas, not just one spot

The weight of the front end is carried by the strut, and that attaches to the shock tower, so the primary force is at the ring/hole where the strut attaches, and in your photos, that looks normal. If the body was dis-attached from the inner fender (allowing the entire inner fender to rise relative tot he body) I would imagine you would see lots of cracks or wrinkles or gaps in the surrounding area.

My 635 has a bowed shock tower top on the left side. This happened when my wife hit a big pothole in the rain. Destroyed the wheel and left that shock tower domed up about 1/2 inch. But this is right around the strut ring, not elsewhere in/on the inner fender...(One of my upcoming tasks is to build a hydraulic jig to flatten it back down again...)

Other possibilities:
Could be rust under the bitumen pad, causing the pad to rise up (I'd consider this to be the prime suspect. Check the same spot on the inner fender. Could be rotted through from the other side);

Might be some sort of swelling of the pad itself.

A good ice pick would tell you a lot about the two above possibilities in about 5 seconds..

Is the hood perhaps sitting low relative to the fenders? That might lead to the wear pattern you are seeing -although that would be fairly obvious with the hood closed.

Here is a discussion on this topic on another very detailed thread. https://e9coupe.com/forum/threads/fixing-that-little-rust-spot.18521/page-10
Look around post #197

 

[Minivansomeren]

I suppose that could happen on a car with VERY severe rust that basically allowed the inner fender to move relative to the body, but I would imagine that would be pretty obvious, since there would be movement between the two in other areas, not just one spot

The weight of the front end is carried by the strut, and that attaches to the shock tower, so the primary force is at the ring/hole where the strut attaches, and in your photos, that looks normal. If the body was dis-attached from the inner fender (allowing the entire inner fender to rise relative tot he body) I would imagine you would see lots of cracks or wrinkles or gaps in the surrounding area.

My 635 has a bowed shock tower top on the left side. This happened when my wife hit a big pothole in the rain. Destroyed the wheel and left that shock tower domed up about 1/2 inch. But this is right around the strut ring, not elsewhere in/on the inner fender...(One of my upcoming tasks is to build a hydraulic jig to flatten it back down again...)

Other possibilities:
Could be rust under the bitumen pad, causing the pad to rise up (I'd consider this to be the prime suspect. Check the same spot on the inner fender. Could be rotted through from the other side);

Might be some sort of swelling of the pad itself.

A good ice pick would tell you a lot about the two above possibilities in about 5 seconds..

Is the hood perhaps sitting low relative to the fenders? That might lead to the wear pattern you are seeing -although that would be fairly obvious with the hood closed.

Here is a discussion on this topic on another very detailed thread. https://e9coupe.com/forum/threads/fixing-that-little-rust-spot.18521/page-10
Look around post #197

thanks Scott, I had seen that big rusty thread but not seen the discussion about fenders buried in the middle. I’ll read more about it there.

Looking back, it seems like it is @Markos who is mostly talking about this. First in the rust inspection thread

Thread 'Notice for all Newbies considering buying an E9 coupe / rust warning / steps to inspection / BEFORE YOU PURCHASE'

Jan 13, 2021

Recently I have been involved in four Coupe inspections, some before the sale, some after.

Three of the four cars were cosmetically presentable but suffered from really substandard metal work and paint work. In two of these cases the owners have substantial investments in their projects and are not pleased to hear that their foundation is cracking and slipping into quicksand.

I am appalled at the low quality metal work on these recent examples. I would like to offer an early warning alternative to any newbie considering buying their first Coupe. See attached.

Being pillarless...

• paul cain
• Replies: 65
• Forum: E9 Discussion

• 

And later in this thread, where he posts a bunch of reference pics.

Post in thread 'Driver quality e9 on BAT'

Jun 1, 2021

If anyone is a nonbeliever of the migrating inner fender phenomenon, ask yourself where this dent in the sound deadening comes from. Obligatory hood alignment photo included.

• Markos

• 

Up til now, I hadn’t seen anybody link it to rust specifically. For instance the white one in my examples is supposedly a New Mexico car relocated to California. It has original fenders (you can see the dimples along the side) and the rockers are original with only a little bubbling. So hopefully not bad rust in the hidden spots, but that’s why I was asking here.

M30B30-Powered 1971 BMW 2800CS 5-Speed

Bid for the chance to own a M30B30-Powered 1971 BMW 2800CS 5-Speed at auction with Bring a Trailer, the home of the best vintage and classic cars online. Lot #200,741.

bringatrailer.com

I guess that big rust thread has more info, although it seems to focus on separation of the fenders rather than upward bowing of the strut support structure.

 

[ScottAndrews]

Can you access the car and poke at it??

 

[ScottAndrews]

Also, since there is not a lot of info on this issue, I wonder if this is really related to metal issues or is it thickening and buckling of the bitumen? I have seen pitch and other similar compounds drift bubble and crack without any participation from the underlying substrate

 

[HB Chris]

I don’t see anything unusual there. There may be other areas to worry about.

 

[Wes]

Have seen some previous talk, and examples of this.

Can think of only three ways that this can occur.

1. Structural rust causing the inner fender, and more importantly, the shock tower bracing to fail as it cannot withstand the upward pressure from the springs. If you put your head in the wheel well and look up you will see a rectangular section at the top of the inner shock tower. In this scenario you would also see bonnet mis alignment. Unless you are planning a full restoration run away..

2. Significant rust on the top of the inner fender underneath the sound deadening, which then lifts as the rust grows. This is probably the pre-cursor to (1). This may impact bonnet alignment but think unlikely as there are rubber seals to seat the bonnet.

3. Crash damage or poor past repairs. If a car had a punch in the nose it could push the inner fenders back, which would distort them and possibly see them want to lift around the shock tower. Think of what happens if you try to bend a ruler how the middle rises up. I'd be looking for signs of past repair or crash damage.

Another possibility is just poor bonnet alignment, or a combination of more than one of the above.

Hope this helps.

 

[Fritzie]

Some pictures for example. The first pictures is before the restoration (only a tiny little rusty spot ) and the same car after the coating is removed and the fender is removed. You can see the impingement mark of the hood in the sound isolation. Is the front strut higher?!

 

[Wes]

Some pictures for example. The first pictures are before the restoration (only a tiny little rusty spot ) and the same car after the coating is removed and the fender is removed. You can see the impingement mark of the hood in the sound isolation. Is the front strut higher?!

Scenario 1..

 

[eriknetherlands]

Great info here!

Maybe a long shot, but can we give this some certainty if we measure a few cars that are entirely original vs ones that show kiss marks of the hood on the fender?

It seems that the fender-hood gap is already small to begin with, as the hood already has a depression in exactly that location, which is quite a bad place to "thin" the cross section from an engineer's perspective. BMW must have done that as a compromise.
That would be one measurement value I would be interested in: in an original car, how big is the gap there? But it's diffucult to measure as it need the hood closed (think like the plastigauge test) + we'll also measure any bowing of the hood which mixes two aspects.

Second way to measure, more direct, is the height from top ridge of the inner fender to the top of the frame rails.
To do this effectively I think it needs a line from L to R across the engine bay, from and to the tops of the inner fender. From that line, measuring vertical to the frame rails. If the towers bend up, I think it must show in that direction & measurent.
For this to be clear, I think it needs a drawing.... let me spend a lunch break on that.

 

[Fritzie]

Hello Erik, in my car there was an metal sheet welded over the original (rusted one). So that takes approximately 2 mm of the free space.