Rear Subframe & Shocktower Reinforcement - 100% Noticable Improvement

[CMM3]

I just recieved my car back from Randy Forbes who did a good deal of work to my car fully documented on his site Gallery page #21 DCCoupe.

Because I'm running a true coilover set up in the rear reinforcement of the rear shock towers was mandatory. Randy fabricated vertical braces on each side tied into his standard trunk/diff mount work and then fabricated a bar to tie the rear shock towers together. (some pictures below). He also installed urethan sub and control arm bushings which I thought were going to make the cars ride become more harsh or jarring.

What I didn't expect was the difference in how solid the rear of the car is now and how much improved the ride quality is. Compared to before the rear suspension is no longer jarring, it's firm but considerably more fogiving in the overall ability to handle road irregularities.

I would have thought just the opposite whould have resulted, but the car is noticably quiter, rides smoother, and there is a distinctly solid feel compared to the loose feeling it had before. I went over some of the same roads that I use to avoid and there is simply an amazing difference.

The car needs to be realigned and some hopefully minor things taken care of but judging by what I've experienced so far I can only imagine the differnence it will make on the track. One regret I have is that I didn't have an opportunity to put it on the track before the work was done to get some quantifiable data on lap time improvements. But there is no question that the rear of the car is seeing a 100% improvement in being connected to the road.

A few pictures:

[BimmerBreaker]

Beautiful work. Randy really knows his stuff. Wish I lived closer so I could have him work on my car...

[albrazzi]

I just did the Poly/Poly upgrade and I have to agree My car seems to absorb rough roads better than with old Powerflex subframes and OEM TA bushings. I followed your build and I am intrigued by the CO rear, I would have to lose my convertable top to brace properly for that and I am just getting used to my ASTs just 6 mo old. I think stiffer everything lets the shocks work better, chassis tuning is pretty interesting and Im just getting started. I also set up a string line to set Toe and can measure Camber acurately with some digital levels, a good alignment is a beautifull thing.

[dkindig]

I also set up a string line to set Toe and can measure Camber acurately with some digital levels, a good alignment is a beautifull thing.

+1 on the home garage alignment. I haven't done the math for a 17" wheel but for a 16" wheel using a standard level for plumb and a machinists rule to measure top and bottom of the wheel, if you can see +- 1/32", you can get your angles within +- 0.08 degrees...

[osborni]

Nice work!

I realize that you want to maintain the interior trim. A bit surprised you didn't tie in the diff mount ears... anyway, that would be simpler now to do that.

Coilovers take out a lot of slop, hysteresis and friction from the stock system.

You did the math on the wheel rates to get the same effective spring rate?

[cleanerPA]

Please tell me that you're not actually running harnesses in the setup that you have pictured above. That's very unsafe- the harness angles are all wrong and you're setting yourself up for severe spinal compression in the case of a crash- your harnesses need to be at a 90 degree angle to your back so that you're pulled back into the seat in the event of a crash, not downwards.

You need to add a rollbar and a harness bar or tabs to the rollbar to fix that ASAP.

[Randy Forbes]

Please tell me that you're not actually running harnesses in the setup that you have pictured above. That's very unsafe- the harness angles are all wrong and you're setting yourself up for severe spinal compression in the case of a crash- your harnesses need to be at a 90 degree angle to your back so that you're pulled back into the seat in the event of a crash, not downwards.

You need to add a rollbar and a harness bar or tabs to the rollbar to fix that ASAP.

To clarify, those harness straps (in THAT picture) are merely draped over that shock tower strut bar. They were later wrapped around it. Do your observations still apply?

The owner (CMM3) supplied a (Sparco?) harness bar, but it kind of scared both the owner and myself, and I thought this was a better set up.

Please explain how the "tabs" should be oriented, as that bar can be removed for additional work. I can't believe I don't have a "finished" picture, but the end result was the harness straps being wrapped around that bar.

Thanks!

[JoshS]

To clarify, those harness straps (in THAT picture) are merely draped over that shock tower strut bar. They were later wrapped around it. Do your observations still apply?

FWIW, the SCCA recommends the shoulder straps at 90 degrees to the shoulder point, but the actual rule says the following:

The shoulder harness shall be mounted behind the driver and supported above a line drawn downward from the shoulder point at an angle of 20 degrees with the horizontal. The seat itself, or anything added only to the seat shall not be considered a suitable guide. Guides must be a part of the roll cage or a part of the car structure.

EDIT: If I were to be concerned about anything, it's that the straps appear to be in contact with non-structural items, like the privacy guard. The concern is that under the stress of an accident, the harnesses could be pulled tight enough across those to break them, allowing the torso to move a lot more than it should. But since this is a not-quite-done picture, it's not clear if that's actually the case or not.

[Randy Forbes]

FWIW, the SCCA recommends the shoulder straps at 90 degrees to the shoulder point, but the actual rule says the following:

The shoulder harness shall be mounted behind the driver and supported above a line drawn downward from the shoulder point at an angle of 20 degrees with the horizontal. The seat itself, or anything added only to the seat shall not be considered a suitable guide. Guides must be a part of the roll cage or a part of the car structure.

Do you consider this to be an unsafe harness setup (I respect your experience in the matter)? CMM3 is 6'1", so the angle may be slightly greater than 20*, but I don't think it exceeds it by much.

Again, the end result was that the harness straps were wrapped around that shock tower bar, NOT secured to the floor, as that picture might suggest.

Thanks Josh!

[JoshS]

Do you consider this to be an unsafe harness setup (I respect your experience in the matter)? CMM3 is 6'1", so the angle may be slightly greater than 20*, but I don't think it exceeds it by much.

Again, the end result was that the harness straps were wrapped around that shock tower bar, NOT secured to the floor, as that picture might suggest.

Thanks Josh!

I didn't see this before I made my edit, but, in my experience, the concerns would be:

1) For a tall driver especially, seat choice is important. The harnesses need to fit properly over the shoulder without touching the seat frame. Use of a HANS device or similar can make this even more tricky. The seats with the large holes like that are usually more successful in this regard.

2) The harness really shouldn't be routed around anything non-structural, such that the harness could collapse it and allow the torso to move forward under extreme loads. I can't tell if that's the case or not.

3) I don't know if 20 degrees is an important number or not but the closer to horizontal, the better.

4) Especially if the driver will be using a HANS or similar device, you should use some bolt-on guides on that bar, or weld some guides on, such that the straps can't spread too far apart from each other. When you wrap the strap around the bar, nothing prevents them from sliding side-to-side. In order to make sure that the straps stay on the shoulders, it's important that they are straight back or even a little towards each other.

[Randy Forbes]

I didn't see this before I made my edit, but, in my experience, the concerns would be:

1) For a tall driver especially, seat choice is important. The harnesses need to fit properly over the shoulder without touching the seat frame. Use of a HANS device or similar can make this even more tricky. The seats with the large holes like that are usually more successful in this regard.

2) The harness really shouldn't be routed around anything non-structural, such that the harness could collapse it and allow the torso to move forward under extreme loads. I can't tell if that's the case or not.

3) I don't know if 20 degrees is an important number or not but the closer to horizontal, the better.

4) Especially if the driver will be using a HANS or similar device, you should use some bolt-on guides on that bar, or weld some guides on, such that the straps can't spread too far apart from each other. When you wrap the strap around the bar, nothing prevents them from sliding side-to-side. In order to make sure that the straps stay on the shoulders, it's important that they are straight back or even a little towards each other.

Thanks Josh!

He's already scheduled to take the car into a shop familiar with race-prep, so it'll be good to have them see it firsthand and make a determination.

1. given seat back ange, I think we're in good shape with the rearward angle; if it's below the 20* number, I do not think it can be much.

2. he's since put the metal framework of the privacy/cargo shade piece in place (with said net & shade cut off), so that will be a lot better than the way it looks in the the picture. I agree with you about non-structural parts collapsing!

3. yes

4. I think it would be easy enough to put a clamp to restrict any movement towards the center of the car, the outboard harness can't go anywhere. (machined aluminum billet comes immediately to mind, but my mill is being loaded into a shipping container tomorrow...).

In my opinion, a rollbar (main hoop, harness bar and diagonals) would be preferred and if the owner lets me work on his car again once I get set up in FL, I know exactly what I'd want to do. In the meantime, I know he'll get a professional's 2nd opinion, and I hope he stays away from any immoveable objects!

Edit: I'd like to see a picture with CMM3 in the car, to know if the harnesses still touch the cockpit divider once the belts are up on his shoulders, as it was a very slight diversion from the straight line with me (5'7"?) in the car...

[clintjg]

Very cool, do you have part numbers/specs for the rear coils?

[CMM3]

I'll get some pictures of the finished set up posted and with me in the car.
But a quick comment is the sparco bar is a piece of crap and folds in a crash. I feel very comfortable with the set up as it sits.

Very cool, do you have part numbers/specs for the rear coils?

The coil over set up is offered as a complete kit from GAZ and consist of front and rear (single knob adjustment which propotionally adjust compression and rebound) coil over shocks, springs, adjusters, front camber plates and rear mounting plates. The shipped directly to me from the UK cost for everything was ~$1600 US Randy can attest to the quality of the product.
http://www.gaz-shocks.co.uk/

Thanks Josh!

He's already scheduled to take the car into a shop familiar with race-prep, so it'll be good to have them see it firsthand and make a determination.

1. given seat back ange, I think we're in good shape with the rearward angle; if it's below the 20* number, I do not think it can be much.


2. he's since put the metal framework of the privacy/cargo shade piece in place (with said net & shade cut off), so that will be a lot better than the way it looks in the the picture. I agree with you about non-structural parts collapsing!

3. yes

4. I think it would be easy enough to put a clamp to restrict any movement towards the center of the car, the outboard harness can't go anywhere. (machined aluminum billet comes immediately to mind, but my mill is being loaded into a shipping container tomorrow...).

In my opinion, a rollbar (main hoop, harness bar and diagonals) would be preferred and if the owner lets me work on his car again once I get set up in FL, I know exactly what I'd want to do. In the meantime, I know he'll get a professional's 2nd opinion, and I hope he stays away from any immoveable objects!

Edit: I'd like to see a picture with CMM3 in the car, to know if the harnesses still touch the cockpit divider once the belts are up on his shoulders, as it was a very slight diversion from the straight line with me (5'7"?) in the car...

First off the Sparco Harness Bar is worthless, it folds completely and has zero strutual integrity in a crash.

With the overall configuration of what I now have I'm very comforatble that it meets the downward angle reccomendations with me in the seat.
I'm not concerned about the deflection of the straps by the divider, With me in the seat it is insignificant. I also don't believe the divider would see enough direct downward force to cause it to give. But again even if it did the amount the shoulder belts would loosen is negligible.

The lateral movement of the straps along the bar are very simple to address. There are aluminum kart akle collars ~$18 a pair on Ebay that will bolt onto roll bars and work well for this purpose.



Randy - What is the diameter of the cross bar I'm going to pick up a pair of these

I just saw a piece of figure 8 webbing made of the same materials as the harness that the harness would go though before being wrapped around the bar that serve the same function. And since the harness cannot move laterally in a crash since all the force applied is perpendicular to this directio, as long as it's held in the right position to begin with this is a non issue, most racers just use some roll bar padding to keep them in place

Finally the construction of Randys reinforcments to the shock towers and the cross, now harness bar appears to be significanlty stronger than any of the bolt in aftermarket harness bar that I've seen offered. So if there were some give in the shoulder belts it would be minimal and ultimately they would restrain me and prevent me from becoming part of any immovable objects that I don't wish to encounter

[pwhitt]

randy - that's awesome work!

i have questions... this is way unlikely to be anything i attempt to tackle even in the next two years, but i know others here are going to have similar questions.

1) why make the support so high relative to the plane in which the shock mounts lie? it looks like the connecting member is a few inches above the shock mounts.

2) is it feasible to make one of these for a roady? judging from the thickness of the typical front end strut support bar, you could get sufficient strength from a bar that might lie beneath the roof storage area, leaving it possible to stow the roof.

3) you plan on making a bolt-in kit???? need a beta tester???

EDIT: just to make myself clear - i'm only thinking of providing some support for the rear shock towers - not prepping the rear end for rear coilovers. i suspect there is a bit of a difference, no?

[CMM3]

Here are some pictures of how the harness looks with me in the seat.

1. The harness is not being supported by the seat = good
2. The total downward angle of the shoulder straps is </= 20 deg = Good
3. No sharp edges in contact with the straps = good
4. The straps are attached to a bar that is no moving in a crash = good

I think it's all good
Looks OEM too nicely finished and I can listen to my favorite CD'S while running laps at NHMS






















I just noticed that I have the buckles for the shoulder straps unsidedown.
If I flip them over it will reduce the angle and deflection by the speakerbox and cargo divider housing

[clintjg]

Yes all good except that in a heavy front impact your weight plus velocity may crush the Sub box cover, and collapse the aluminum privacy cover causing the harness to suddenly have 4-6" of extra 'slack'.

I'l
The coil over set up is offered as a complete kit from GAZ and consist of front and rear (single knob adjustment which propotionally adjust compression and rebound) coil over shocks, springs, adjusters, front camber plates and rear mounting plates. The shipped directly to me from the UK cost for everything was ~$1600 US Randy can attest to the quality of the product.
http://www.gaz-shocks.co.uk/

Awesome, thank you!

[Gofast]

That's actually where my belts are mounted too, except I don't have any stereo parts or trim in the way...



The angle on mine is 15°

[Randy Forbes]

Whew! All good comments = relief.

The bar is 1-1/2" outside diameter.

Looks better since you put in the rest of the trim! Thanks for getting the pictures with the seat occupied.

I like how GoFast's straps are crosseed, can't slide apart that way (but bad for the concours circuit ).

[CMM3]

[quote=clintjg;24033645]Yes all good except that in a heavy front impact your weight plus velocity may crush the Sub box cover, and collapse the aluminum privacy cover causing the harness to suddenly have 4-6" of extra 'slack'.

Not sure bout 4-6", it's most likely more like 2-4".

Randy I've been thinking.

Is there any reason I couldn't have a another cross bar made that had two horizontal bars. One in the present location and another 3-4 inches above it. The upper and lower bars would use the same side plate attaching to the same spot in the car. They could be triangled together for additional support.

Then I could run two smaller 1" bars coming from the center of the upper bar and tied to where the side trim pieces are held in place near the base by that square cover.

Maybe looks something like this:





If I recall that cover is held in place by a screw. If you where to remove this screw would you be able to put a bolt with a backing plate in from the fender well to hold these down bars in place.

This would provide an elevated location for the shoulder straps to eliminate any concern about slack being created, while the down bars would prevent a rotational effect in a crash.

Your thoughts????

[clintjg]

Not a bad option in my opinion, unless you're concerned about weight.

Or you could just remove the privacy cover and notch or otherwise customize the speaker cover to accommodate the harness straps in a direct line from the current mounting bar to the rear seat.

[CMM3]

Not a bad option in my opinion, unless you're concerned about weight.

Or you could just remove the privacy cover and notch or otherwise customize the speaker cover to accommodate the harness straps in a direct line from the current mounting bar to the rear seat.

Not going to add much weight and with race seats installed I'm ahead of the game in this regards.
I'd like to keep the car looking close to stock and be able to return it to as close to orignal condition with as minimal impact as possible.

Right now everthing I've done can be unbolted or isn't visable unless your looking for it.

[Randy Forbes]

Yes all good except that in a heavy front impact your weight plus velocity may crush the Sub box cover, and collapse the aluminum privacy cover causing the harness to suddenly have 4-6" of extra 'slack'.

If you could see how far back (and down) his seat is, you wouldn't think too much about getting a few inches closer to anything (if I'm buckled in, I can't get the clutch in far enough to start the car__seat is "fixed" not adjustable) the double snatch may be uncomfortable, unless it absorbs more energy.

I don't make any claims to be an expert, however passive, on the subject of crash analysis, I figure it's best to avoid them altogether...

Not sure bout 4-6", it's most likely more like 2-4".

Randy I've been thinking.

Is there any reason I couldn't have a another cross bar made that had two horizontal bars. One in the present location and another 3-4 inches above it. The upper and lower bars would use the same side plate attaching to the same spot in the car. They could be triangled together for additional support.

Then I could run two smaller 1" bars coming from the center of the upper bar and tied to where the side trim pieces are held in place near the base by that square cover.

Maybe looks something like this:





If I recall that cover is held in place by a screw. If you where to remove this screw would you be able to put a bolt with a backing plate in from the fender well to hold these down bars in place.

This would provide an elevated location for the shoulder straps to eliminate any concern about slack being created, while the down bars would prevent a rotational effect in a crash.

Your thoughts????

I think I would put the rear-angled bars in line with each harness point, leaving the center area more open to put stuff in.

The problem becomes, how do you put the trim back in? To leave the hatch area mostly stock appearing, as it is now, you have to be able to put the carpeted pieces around the shock tower brace before it's bolted in. Alternatively, you would have two (2) bars; one for track and one for street, but the fallacy then is, that you're more likely to get clobbered on the street than at the track (this takes your skill level into assumption... ).

Let me think about it some more, and of course, you can get another opinion on Monday when you see the local guys.

[clintjg]

Not going to add much weight and with race seats installed I'm ahead of the game in this regards.
I'd like to keep the car looking close to stock and be able to return it to as close to orignal condition with as minimal impact as possible.

Right now everthing I've done can be unbolted or isn't visable unless your looking for it.

Fair enough.

For what its worth, if you keep your eyes pealed you might find someone selling that trim cover piece which you could hack up and keep your original untouched. I've seen them go for less than $50. That way it's still stock'ish but you can play with how to make the harness work (not that it doesn't now) with what you have on the cheap.

[CMM3]

If you could see how far back (and down) his seat is, you wouldn't think too much about getting a few inches closer to anything (if I'm buckled in, I can't get the clutch in far enough to start the car__seat is "fixed" not adjustable) the double snatch may be uncomfortable, unless it absorbs more energy.

I don't make any claims to be an expert, however passive, on the subject of crash analysis, I figure it's best to avoid them altogether...


I think I would put the rear-angled bars in line with each harness point, leaving the center area more open to put stuff in.

The problem becomes, how do you put the trim back in? To leave the hatch area mostly stock appearing, as it is now, you have to be able to put the carpeted pieces around the shock tower brace before it's bolted in. Alternatively, you would have two (2) bars; one for track and one for street, but the fallacy then is, that you're more likely to get clobbered on the street than at the track (this takes your skill level into assumption... ).

Let me think about it some more, and of course, you can get another opinion on Monday when you see the local guys.

I was thinking that I would not have to remove the side carpeted pieces at all.
I'm not really worried about the loss of space in the rear, and the distance from the upper bar to that lower mounting point is not far back at all so I wouldn't lose much anyway.

What I was contemplating is:
1. Remove the square/plastic trim retaining piece.
2. Drill a hole through to the fender well.
3. Add a stainless steel spacer of the appropriate lenght to bring the mounting point for the down tubes out.
4. Add a backing plate to support the bolt.
5. Have the downtubes terminate in a plate with a hole in it
6. Bolt the entire new assmbly in without removing anything

Questions:

a. If I where to remove that square metal/plastic retaining piece and drill a hole straight through where would about would it come out?

b. since the ends of the assembly are supported by being bolted to the towers wouldn't it make more sense to support the center of the upper bar thus providing more of a trianglulated structure?

c. In your opinion will it be easier (please read less costly) to modify the existing bar or fabricate a new one?

I'm not sure but the track may be a safer place for me, there are run offs and guard rails and all sorts of saftey related things. I drive stupidly on the street, had the coupe up to 150mph one afternoon on the highway Skill level being the same I think I'd rather screw up on the track.

Taking into account Randy's comment about putting some support close to the point where the shoulder straps attach I came up with this idea which would appear to offer a lot more inherent strength.

I think it might be able to tie side support bars into the side plates where the upper tubes are attached and have this be the same piece that gets bolted in at the bottom and where the down tubes terminate.

The picture below is what i have in mind.

Randy - I appreciate you comments, and look forward to your response to my questions above. Thanks



An alternative approach could be simpler to Fab.

[cleanerPA]

Do you consider this to be an unsafe harness setup (I respect your experience in the matter)? CMM3 is 6'1", so the angle may be slightly greater than 20*, but I don't think it exceeds it by much.

Again, the end result was that the harness straps were wrapped around that shock tower bar, NOT secured to the floor, as that picture might suggest.

Thanks Josh!

Randy-
Sorry I didn't see your reply until now.

Check this out, read p. 12 about harness mounting angles and the consequences:
http://www.schrothracing.com/sdocs/2...structions.pdf

That is not a safe setup at all- you need a rollbar in there if you're mounting racing harnesses, at a minimum. Also, you do not have a harness bar that is mounted properly- either as close to 90 deg. to the seatback angle as possible, so the belts basically come straight back out of the seatback.

For simplicity's sake, having a rollbar that incorporates a harness bar at the proper height is the easiest solution. Mount the harnesses to the rollbar- if the rollbar is constructed properly and anchored properly, it will be as safe as mounting the belts to the stock seatbelt anchor points and if the bar is mounted properly, you have little worry of the rollbar coming loose or going through the floorpan.

You also mention the height of your client- if your seatback harness holes are below shoulder level, that is also not safe- you need to spec a seat that has the harness holes above shoulder level.

That's actually where my belts are mounted too, except I don't have any stereo parts or trim in the way...

The angle on mine is 15°

At least this car has a rollbar, so that's a good start. I guess the rollbar fabricator measured the proper angles for you so you don't have issues with your harnesses?

I can't measure the angle from a photo, but that is a pretty shallow angle. CMM3's harnesses look closer to 45 degrees if not more.

Those harnesses in Gofast's picture are routed properly, according to Schroth's instructions.

I was thinking that I would not have to remove the side carpeted pieces at all.
I'm not really worried about the loss of space in the rear, and the distance from the upper bar to that lower mounting point is not far back at all so I wouldn't lose much anyway.

What I was contemplating is:
1. Remove the square/plastic trim retaining piece.
2. Drill a hole through to the fender well.
3. Add a stainless steel spacer of the appropriate lenght to bring the mounting point for the down tubes out.
4. Add a backing plate to support the bolt.
5. Have the downtubes terminate in a plate with a hole in it
6. Bolt the entire new assmbly in without removing anything

Please don't take this the wrong way, but if you're going to do all of that, please do it the right way- install a real rollbar!

Racing seats and harnesses are really a fine and dandy way to kill yourself if you don't install them properly.

Read this:
http://www.schrothracing.com/sdocs/2...structions.pdf

Read it thoroughly and understand what they're writing about. This is not something to do half-a**ed or incorrectly, as that can literally get you killed.

Not only that, you will die in a racing seat and harness without proper rollover crush protection- if the roof collapses, you're rigidly buckled down in a fixed-back seat.