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Coupe owners: Is a Backbone worthwhile?

7K views 30 replies 18 participants last post by  LatinVenom 
#1 ·
Hello Coupe owners!

I've been thinking about the cost / benefit ratio of installing a DDM Race Backbone in my GXP Coupe. It was the first mod I made to my GXP Roadster, but I (wrongly?) assumed the differences in coupe body design stiffness vs. a topless cockpit made it unnecessary.

How many Coupe owners have added a Backbone and is there a noticeable difference in stiffness, handling, suspension control, etc.? Thanks for your opinions on this question. :thumbs:
 
#2 ·
I added the 3/8" thick Backbone as one of my first mods. I definitely notice less cowl shake. And that was after having the stiffer stabilizer bars added at the dealer before picking the car up originally. With the added stiffness the Backbone added, it felt even better in the corners!
I'd say go for it!
________________________
'09 Pontiac Solstice GXP Coupe - (man.) Mysterious / Black - (NIICE) Vin: 000116
'09 Saturn Sky Redline - (auto.) Ltd. Ed. Hydro Blue - (COOOL) 1 of 49

Host for the 2012 PNWR 4th Annual Regionals
July 19-22, 2012 Clarkston, WA
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#4 ·
To build on TS's input, the coupe starts life as a roadster. They have different trunk liner, tails, interior plastic, side window glass and then the plastic top is bolted onto the now transformed roadster. The top does not add a whole lot of stifness:agree:
 
#6 ·
Having owned both a GXP non-coupe and now a coupe, I can tell you that the coupe is much much stiffer than the non-coupe. This comes from experience driving in Oklahoma with our renowned terrible roads.
 
#7 ·
That is probably true, but if you want to get the car to handle better, you need to upgrade the suspension.
 
#10 ·
The coupe isn't that "inherently stiff" , really. My friend Stan ( we both retired from Wilmington and Stan worked for a vendor rep for the poeple who made the fascia's and rear couple assembly and targa) was just showing me pictures of the coupe "fastback" installation. Essentially, the fastback just rides on the body and is attached so that it doesn't blow off. A bit of a silly way to put it but that's about how it is, really. The main difference from the convertible was the use of a couple of solid aluminum brackets that were installed to the inner quarter panel structure exactly where the tulip panel would have attached. The fastback would just drop onto those brackets and bolted in place with two bolts each side. The bolts that are used to attach the fastback to the bracket are quite large. From the pictures they look like they are about 16mm. The reason they bracket and hardware is so substantial is to prevent the targa from creaking while the suspension is loading and unloading. In fact, mine creaked a bit before I installed the Backbone and is silent now. The only other attachment point is at the rear under the duck tail spoiler and their are only two small anchors there. If you remove those two stainless screws on each side you can see them. From what I hear they had to make a special tool to secure them during assembly. In fact there is only about 20 pounds difference in weight between the two cars. ( I think the coupe is lighter) And if you think about it the same backbone pan that is under the convertible is under the coupe so there must be similar structural dynamics present in both cars, wouldn't you say?
 
#11 · (Edited)
The only way to know is if a coupe rigidity test was performed (pre & post backbone). I admit the coupe feels more solid but feeling vs fact are two different subjects.

For instance; a 3 ton vehicle may "feel" more rigid/solid than a 2 ton vehicle but once you factor in how weight affects feeling that doesn't mean a 3 ton vehicle will perform better than a 2 ton vehicle (all things being equal).

Maybe a better question would be; would a backbone instill more confidence. Thereby giving you more confidence to push it closer to its limit before physics takes over.

Or another way of saying it is; if you believe a backbone will instill more confidence to push the platform to its limit/or enjoy testing it...than chances are you may be rewarded.
 
#12 ·
The Backbone and Probeam in combination make a significant improvement in handling - that's enough for me!

I wish the coupe HAD been done as a true fixed head, and therefor more rigid.
 
#15 ·
Sorry I'm 10 months late with this replay. Somehow this thread slipped through my subscription filter. With that said, I've thrown countless thousands of dollars into Porsche 951's (a.k.a. 944 Turbo) since 1993. I was a tier 1, powertrain electronics Supplier to GM and knew of the Pontiac demise prior to the general public. As the collectors were scrambling for GTO's, I committed to buying one of the 1st 100 (non-sequential vins) Captured fleet evaluation Coupes. It became available in May 2010 and had accumulated 15,000 miles. I'd never driven a Solstice Coupe or vert of any type but my closest friends in GM Powertrain told me the of the GXP's vast, untapped performance potential.

At this time, I was still in possession of my "LAST" Porsche 944T which had over $10k in suspension mods over and above the highly sought M030 option and $15k in the engine to achieve 350bhp. This is arguably the best handling production car ever built. The GXP Coupe was located at a Wisconsin dealership some 700 miles away. Prior to picking up the Coupe, my GM Contacts recommended the GMPP Turbo/SW upgrade so I had the dealer install it prior to picking up the car. So I've got a double disadvantage, I've never driven a GXP Convertible or Coupe with factory HP/TQ and the Coupe came equipped with FE3 Suspension.

Now that you've got a clear understanding of my Solstice background, I'll try and answer your question regarding the backbone and rigidity of my Coupe. When I left the Dealership in Wisconsin, I felt a little claustrophobic and vision confined. The claustrophobia was gone after the 1st 100 miles and the blind spots remedied with a stop at an Autozone and some 1" concave stick on mirrors. Finally feeling comfortable driving the car, I looked for some non interstate options to see how the car felt cornering, braking, etc.. Breaking was good, handling was, at best, marginal.

Once home, I was immediately on the phone with my friends in GM powertrain and performance division. That conversation led to an immediate order for a ZOK Suspension package, a Trifecta Tune and Eibach Springs to lower and stiffen the suspension. I gave Dave a call at DDMWorks and scheduled installation of the suspension upgrades as well as his mandrel bent IC Tubes. By pure luck, I was looking for REAL Tires on the Tirerack site and saw a set of 20"x8.5", one piece aluminium wheels on closeout at $75 each. They had the correct bolt pattern and offset so I made some quick measurements and ordered the wheels and Conti Extreme Contact DW 275/30/20's on all 4 corners. These were installed on receipt with no clearance issues and marked improvements in cornering and noise. There's a lot of negativity on the forum regarding 20" wheels so all I will say is "bring it on, stopping distance, auto-cross, 1/4 mile or full fledged track", I'll let the car speak for itself. I then flashed the Trifecta tune atop the GMPP upgrade. Wow was my only thought.

At DMWorks, Dave and crew made short work of the suspension mods which lowered all 4 corners by 1 1/4" still with no rubbing or Clearance issues. Dave then asked me if I was interested in a backbone. Not knowing what improvement I should expect from the backbone, I drove it before and after. There was a noticeable difference in the cars perceived rigidity and the Coupe was now equal to or better than my "very expensive" 944 Turbo S.

Dave then proceeded to take me for a ride in the 1st GT2871 Turbo upgrade car he'd just finished and once again all I could say was Wow! I placed the Porsche on Auto Trader the next day as I had already accomplished superior performance gains in the Solstice with a ten cent on the dollar investment as compared to the Porsche. With a small percentage of the money from the sale of the Porsche, I called DDMWorks and gave them a deposit on a 2871 upgrade as well as ordered a Solo Free Flow cat and exhaust.

The 2871 with an AEM water/meth kit, Solo Cat/exhaust and DDM's new Latin Venom front cross brace were installed in June of 2012. Depending on the Air Density on any given day, 400 dyno'd whp are achievable. My GXP Coupe now sits in the company of Z06's and pulls over 1.1g on the holy 200ft diameter Motor Trend test setup using a G-Force Monitor. I'm really at a stopping point with HP and TQ until I strengthen the internals (Rods, Pistons and Valve Springs). Getting current power to the road, even with the sticky 275's, is an issue. The rear will support 285's so I may end up with a staggered tire set when I replace the current 275's. A little over 2 years ago I'd have laughed at anyone that told me I'd have to nurse the throttle on a 2 liter turbo through 1st, 2nd and sometimes 3rd gear in order to keep traction.

The remainder of the 944 Turbo proceeds went into the purchase of a 1996 BMW 840 Ci. This wasn't something I was looking for but a close friend found himself in a situation where he needed to part with the BMW. It is an awesome car and I had full intention of making it my daily driver. Now if I can just find a way to walk past my GXP Coupe and actually get in the Bimmer for daily driver purposes, I'll have the best of two worlds. Thus far, I've only been successful about 1 in 4 times of making it to my daily driver.

To get back on topic, buy the DDM Race version of their backbone and the Latin Venom front cross brace. You will not be disappointed.
 
#16 ·
Our coupe isn't really a coupe, as they wanted to avoid a complete new certification process to save money. If it had been, it would have been dramatically stiffer which would have been a significant benefit to handling.

An example is the BMW Z4. The convertible has a torsional stiffness (notionally grab each end in a giant hand and try to twist it) of 16,000 Nm/deg. The coupe, because it has a truly fixed roof, unlike the Solstice, has a value of double that, or 32,000 Nm/deg.

Had the Solstice been properly designed as a coupe, it would no doubt have been an even better handling platform - not that it is shabby as it, of course.
 
#17 ·
I love this Forum. The answers are always here if you just "search and dig". Will be going with the DDM probeam, backbone and Latin Venom Brace for my Coupe. Plus will get the polished aluminium DDM LNF pipes. Next year GMPP tune and CAI. I want my Coupe to drive and handle as well as my roadster. I have done all that I have listed here to the roadster plus Solo Hiflo Cat and GXP Mach exhaust. She is way more fun and sounds "mean". Really notice a difference between the two of them. >:)
 
#18 ·
As soon as I purchased my coupe I installed the DDM brace kit. I also have a GXP convertible and knew it made a great difference in handling. I'm not sure how bad my coupe was, since I quickly installed the brace kit. I think the backbone and probeam should be on every kappa. Have told this information to many and have assisted in installing many kits. Brace kit does not cost that much and if an easy install go for it!!
 
#19 · (Edited)
Not being familiar with the coupe, when the hard top is connected, makes it hard for an opinion. The back bone brace really is not designed as a suspension assist. It is a frame strengthening idea. Convertible flex. The back bones are to stop that. If the coupe becomes integral, ie it bolts or clamps to both sides it would act as the back bone. If it does not connect rigidly then the back bone in my opinion would be needed. The backbone adds strength to make up for the missing loop over the passenger compartment on a convertible.

But alas, I would guess most are driven with the top off a large percentage of the time. So in that context, the back bone brace is required for the protection of your investment.

As to backbone, I have the sport 3/8 inch ones on both of my Sky's. You can feel the difference when traversing low drive ways no questions. ON the Mallett I have the 1/2 inch, and it is much more rigid than the 3/8th ones. Tires come off the ground normally.

I would look at the cross bay engine brace over the venom to. I have the venom on my green one, and the cross bay on the yellow. To me it seems much more stable in the corners with the cross bay. It actually forces the inner wheel down. I did not notice much difference if any with the venom brace.
 
#21 ·
Thanks for the feedback. Appreciate it. I just might look at cross bay brace. I have the LV one in yellow on my roadster and am happy with it. The coupe is becoming it's own distinct Sol. Having the cross bay in Chrome could be very cool.:thumbs:
 
#22 ·
"The roof’s structure was strengthened with additional support elements, including an aluminum roof bow attached with aluminum brackets. The bow and brackets are stiff, yet low in mass, a design feature that pervades the coupe’s overall construction. The roof includes a lightweight sheet molding compound cover affixed to a rigid aluminum frame for additional lightweight support. A lightweight magnesium roof panel frame weighs just 31 pounds (14.2 kg) and can be removed by one person."
The cover on the coupe top is SMC.
 
#23 ·
Sorry but there should not be much difference between the 2, base on the CAD simulations done by Dave on his and what I did with mine when it was design. If anything the Venom brace is actually more in line, since our cars do not have a tower type design.
If you do not have the complete suspension set, meaning all 3 pieces plus the ZOK sway bars then is really hard to evaluate the outcome.
By the way the coupe is really a convertible with the added roof, but no structural difference other than is a metal piece.
 
#25 ·
I have all three, on both cars. The one with the cross engine brace to me feels more stable, it seems to force the inner wheel down. I am not a race car driver, just happen to live on a mountain road, and drive the cars back to back. I do not get the same feeling with the Venom brace. Could be good or bad, not sure. Just like the tower brace better.
 
#26 ·
With the tower brace as you know you have to move the power steering resovior and the engine cover. With the Coupe I am not sure I want to do that.
 
#27 ·
I took off the engine cover, but I have my cam cover hydro dipped carbon fiber, with a DDM valley cover on it. The moving the power steering deal is a pain in the butt though. Neither of us most likely drive the cars hard enough to make a difference is my guess. I like the look of the tower brace. I did mine in red like the rest of the accessories. It seems to me to make the engine look bigger. Once you have a V8 in there, everything else looks tiny. lol
 
#28 ·
I didn't think the PS move was very difficult when I did mine. And I prefer to see the engine without that ugly cheap plastic cover on it, anyway. I bought a second cam cover and polished it, but haven't gotten around to replacing the original yet. Sigh.

YMMV :)
 
#29 · (Edited)
OK, but do you have the sway bars as well?. Both braces are within an 1" of each other where they are bolted to the chassis. The cross member brace is just higher in the plane (tower piece welded to the chassis) vs the Venom is bolted directly to the chassis.
Also do you have the same tires on both cars and are the tires filled to the same psi?.
Is one coupe and the other a convertible etc...
All I am saying theoretically base on our CAD simulation, there should not be any noticeable difference between the 2 braces if everything else is equal.
 
#30 ·
Different tires, but noticed the difference with any of the tires I tried. Actually the venom braced car had the better (supposed) tires all through the comparison. Running Michelin SS with 28 pounds, compared to the stock Goodyear that were old, but kept in a controlled area. No cracsk. Still felt the yellow car performed better for me. I then went to the Bridgestones RE71's and there was no comparison in any way. My guess getting rid of the Michelins may make the green car close, but doubt it will be the same
 
#31 ·
Look all I am trying to point out is, if all the parameters are equal there should not be any discerning difference between the 2 braces.
By the way the tire pressure should be 29 not 28 when cold.
 
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