DIY - Chassis reinforcement ideas

One thing I notice when driving the car hard on tight twisted roads is that the chassis flexes. As I’m about to get my car repainted I was wondering what people have found to be the most useful for chassis strengthening. I know Coupeking is doing that almost as standard procedure. A lot of the work seems to be around adding welds to things like the rear wheel well to the bulkhead behind the rear seats. I’ve also seen a triangle shape being added in the engine bay between the frame rail and the firewall.

Does anyone have any experience to share in that area?
 
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eriknetherlands

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Well I can confirm that they flex.

The only thing (un do able?) is to tie in the middle of the frame rails (chassis legs) to the front of the sills and/or tunnel. Currently the frame rails are only connected to the sills by just a 0.8 mm sheet flat floor under your pedals, and with a sheet of just 0.8mm of firewall to the tunnel.

The ends of the frame rails are connected to the sills via the floor and the seat structures. But halfway up the length of the frame rails, say at firewall height, it seems to be floating to me... This is where the small triangles may help, stiffening up the firewall.

I've seen quite a few cars with stress cracks and creases from continued flexing around the gear shift opening. On the left and right, running down in the direction of the footwell.

I myself put in a 3d contoured, 2mm u- shaped piece to reinforce the round opening, going 20 cm up front, and 15 cm to the rear, and plug welded it in. There are some pics in my build thread.

Strangely, i fail to understand for sure where the load is coming from: that area is not loaded directly, although the gearbox mount is not far off. I can only see it from torsional forces; pushing one wheel in the air will twist the car; this point (gear shift opening) is the cross point of the 2-axis over which the twisting occurs.

In modern cars, often there is also a vertical connection from the firewall/dash to the floor following (roughly ) the sides of the console. This adds a lot of rigidity. What type of vertical connection can we hide behind those center console panels....?
 
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tferrer

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If you are building a hotrod, and unafraid of non-standard a and c pillars there are definitely things you can do (expensive). A great example is of the approach this builder took w a 911
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Bmachine

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Well I can confirm that they flex.

The only thing (un do able?) is to tie in the middle of the legs to the front of the sills and/or tunnel. Currently the frame rails are connected to the sills by just a 0.8 mm sheet flat floor, and just 0.8mm of firewall to the tunnel.

The ends of the rails are connected to the sills via the seat structures, but the middle, at firewall height seems to be floating to me...
This is where the small triangles may help, stiffening up the firewall.

I've seen quite a few cars with stress cracks and creases from continued flexing around the gear shift opening. On the left and right, running down in the direction of the footwell.

I put in a 3d contoured, 2mm u- shaped piece to reinforce the round opening, going 20 cm up front, and 15 cm to the rear, and plug welded it in. There are some pics in my build thread.

Strangely, i fail to understand for sure where the load is coming from: that area is not loaded directly, although the gearbox mount is not far off. I can only see it from torsional forces; pushing one wheel in the air will twist the car; this point is the axis over which the twisting occurs.

In modern cars, often there is also a vertical connection from the firewall/dash to the floor following (roughly ) the sides of the console. This adds a lot of rigidity.

What can we hide behind those center console panels
All good points, Erik. I also tend to think that the triangle between the front frame rails, the firewall, and the inner fender makes the most sense. I’ve seen pics of that somewhere but I can’t find them right now.
I would love to see what the race teams did in that regards, apart from the obvious roll cage of course.
I wonder if anyone has real data to prove that the front strut braces actually do anything or if it’s mainly for looks. Here is one article which gives mixed results:
 
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autokunst

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It is just a bit hard for me to believe that stitch welding the trunk bulkhead has that much of a benefit.
The 911 guys stitch weld a handful of areas, and there has been testing done that shows an increase in chassis stiffness. I don't know how that translates to our cars, which are quite different in many ways.
 

Baikal Bimmer

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You could hear my car flex without the strut brace, now with one in I don’t hear or feel it as much. Bottom line is our cars will always flex to some degree.
Also added one in the back that combos as a battery box, again, I felt that it made a noticeable difference
2A543E06-EDAE-42E6-AC08-AA2951ED89A4.jpeg
 

Bmachine

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You could hear my car flex without the strut brace, now with one in I don’t hear or feel it as much. Bottom line is our cars will always flex to some degree.
Also added one in the back that combos as a battery box, again, I felt that it made a noticeable difference
View attachment 113076
Some serious upper shock tower reinforcements there! Impressive.
 

Rek

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It's no surprise that the chassis twists, When looking at mine once the rust was removed, it is apparent that the frame rails end prematurely with only thin sheet metal between them and the rear sub-frame mounts. We concluded that this section would likely be the culprit of chassis flex.

As my frame rails needed work, as part of the repair I extended them to the rear and connected them to the section nearly above the subframe mounts. I also replaced the rear seat sheet steel cross piece with a box section cross-brace which meant a more rigid structure, especially as this was also connected to the extended frame rails. As I have not driven another E9 and mine was a non-runner when purchased, I cannot comment on any before and after effect but the car does not seem to flex a lot.

It adds weight but not that much, and I had a more powerful engine to put in so I felt that this might compensate in overall performance.
 

Bmachine

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It's no surprise that the chassis twists, When looking at mine once the rust was removed, it is apparent that the frame rails end prematurely with only thin sheet metal between them and the rear sub-frame mounts. We concluded that this section would likely be the culprit of chassis flex.

As my frame rails needed work, as part of the repair I extended them to the rear and connected them to the section nearly above the subframe mounts. I also replaced the rear seat sheet steel cross piece with a box section cross-brace which meant a more rigid structure, especially as this was also connected to the extended frame rails. As I have not driven another E9 and mine was a non-runner when purchased, I cannot comment on any before and after effect but the car does not seem to flex a lot.

It adds weight but not that much, and I had a more powerful engine to put in so I felt that this might compensate in overall performance.
Brilliant. Do you have any pics by any chance?
 

Ohmess

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Anyone see any differences w/a strut bar?

Like that sweet Daddywad one?

strut-bar-jpg.26184

Two comments:

That picture is my car (with the Konig seats on the table in the background at the start of the color change project). I noticed a slightly quicker turn in when I added the strut bar to my e39. That, and structural reinforcement, is the reason I added it to my coupe. I can't feel the difference in my coupe.

As to chassis reinforcement, Tom Samuelson (VBigDog) had Mario Langston do some type of reinforcement in the door sills. I've not seen it, as it was underneath the trim, but the reinforcement ran from the rear subframe mount area (not sure if it went across the rear as Rek describes), inside the door sills and connected to the front frame rails.
 

Gazz

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It's not just one thing it's a whole bunch of things. Stitch weld the seams for sure. If you've ever had an E9 stripped to the bare metal you'll see that the factory welding is very inconsistent.
Strut braces front and rear. Chassis rail to engine bulkhead triangles. Floor cross brace on the rear seats front bulkhead. Lots of welding on the rear seats rear bulkhead and particularly around where the splined threaded pin goes through the floor to the subframe. Trans tunnel around the engine bulkhead.
I have my own ( over built ) welded in diff reinforcement case with extended sections running cross wise on the chassis. This is through bolt tied to a triangular plate in the boot / trunk which is under the rear mounted battery.
When the car is finished ( Ha! ) I will see if I can add any under car cross bracing, though I'm hoping it's unnecessary.

Coupe King used to have a bunch of pictures in their project section and Larsen architect had a great pictorial as well. Someone may have saved them.

Bear in mind that flex stresses have a way of manifesting themselves somewhere so if you do all of the above don't be surprised to see flex cracks appear in new places, especially if you have lowered and stiffened suspension with HD anti roll bars.
 

oz73csi

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Probably get crucified for this , but I don't believe the rear strut top brace does anything suspension wise as the rear suspension is controlled by the stiffness of the subframe mounting points , they are nowhere near the strut tops which basically mount one end of the shock , not like the front where it controls the top of the strut which is a suspension member. This is different to stopping the tops punching out , and I suspect the rear brace as above actually works better to stiffen the floor , below which is the diff mount.
My 2c
I would be keen if anyone has access to those photos mentioned which do not seem to be available anymore , and yours too Gazz.

waiting!
 

Gazz

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Yes alas, CoupeKing and Sven Larsen have discontinued those pics.

For the welding - that's not difficult, just go over everything. For the diff cage see La Jolla's CS parts section as they sell them. It would take a while for me to get a picture of my set up on here though if it would help the members I'll do it. I feed off likes so.......
You should be able to find something about the chassis to bulkhead triangles on the site.
Here is a picture of Paul Cain's 2002. Note the cross brace bolted in between the longitudinal chassis rails just back of the engine. I would do it differently in that I would weld tabs to the chassis rails and bolt the brace to the tabs. I don't like the idea of drilling holes in the chassis, notwithstanding Paul's considerable expertise.
And while you are looking take a moment to see what he has done to this car - incredible.
 

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nosmonkey

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Yes alas, CoupeKing and Sven Larsen have discontinued those pics.

For the welding - that's not difficult, just go over everything. For the diff cage see La Jolla's CS parts section as they sell them. It would take a while for me to get a picture of my set up on here though if it would help the members I'll do it. I feed off likes so.......
You should be able to find something about the chassis to bulkhead triangles on the site.
Here is a picture of Paul Cain's 2002. Note the cross brace bolted in between the longitudinal chassis rails just back of the engine. I would do it differently in that I would weld tabs to the chassis rails and bolt the brace to the tabs. I don't like the idea of drilling holes in the chassis, notwithstanding Paul's considerable expertise.
And while you are looking take a moment to see what he has done to this car - incredible.

Gazz I'm at the point where I'm looking to reinforce my car around the rear end so some photos could be very helpful to me right about now!
 
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