backbone

[Critterman]

Originally Posted by carlintexas I get my race version this next Thursday at our DDM Mod Meet! The price difference between the two was so minimal...and not really knowing how they feel...I figured - better to err or the stiff side. I think this threads great cuz it lets me see all those who have bought the backbone - and the positive comments. Have you seen the front shock brace DDM puts on the stage III S/C? I may need one in starlight red to complement my turbo pipes.

Carl, just for the record the tower brace is not part of the Stage III upgrade, but part of the DDM suspension upgrades, and it looks great in the Starlight red, so good I was almost tempted to add a little red under the hood.

[carlintexas]

critter -

I can't have a tower brace...my pipes are in the way. 1KUL tried to put a brace on his but the elbow to his throttle body is too tall & he is going to get a shorter one to make it fit. On the S/C side the brace fits very close to the S/C & may need a little cutting to give it clearance.

I like your boost post @ 13.5 psi. That's a good number and safe enough with the stock internals.

[Critterman]

Carl,

They had to do some cutting and trimming on my car to get everything to fit. For some reason, there isn't much room under the hood. 13.5 is a good number but I still haven't had a chance to really punch it and see what happens. 1st and 2nd gear are just crazy when you punch it and hold your foot down. You don't have time to look at anything but the road and wonder why it keeps moving around under you.

My local track is open this Friday night for the season and it is a test and tune night. Might have to check it out.

[Flash's Owner]

Is the DDM Backbone made from steel or Al?

Some things to consider:

-When does a vehicle encounter significant torsion loading?
-How does the tunnel section generate "stiffness"?
-Can you pick the change out of several blind swaps?

I am getting a lot of questions in Private Message system about the benefits of the DDM Backbone.

We did work with increasing the tunnel belly pan thickness during the Z0K development, and tried double thick steel plate, approx 3.5mm. The predicted change in torsional stiffness was predicted to be minimal, and was not detectable blind.

Tell someone what part is in the car, though, and they definitely "FEEL" it. It would be interesting to spend a day in a garage, have several people evaluate a controlled blind study (the study would be randomized, the conductor of the study would not be an evaluator, and would consist of 6 evaluations where the belly pan was removed and reinstalled). No cross discussion allowed, as consensus can truly ruin a blind ride. It would take the better part of a day, but would be very worthwhile.

During the development of a vehicle, we do not always have the luxury to do blind evaluations, but I love to do them when possible. Nothing resets your ability to evaluate a vehicle than a blind evaluation. Nothing can be so humbling, either.

We also did a series of blind evaluations when increasing the rear crossmember stiffness, during the development of the GXP Z0K. The results of stiffening the rear crossmember were so dramatic that we went through the trouble of producing a specific beam for use on the Z0K club racing models. Expert evaluators could consistently, 100% of the time, determine whether a stock or a "stiffened" rear crossmember was in the vehicle. Normal drivers, on one evaluation run, had 11 of 12 able to pick the differences between identical vehicles, WITHOUT cross discussion.

My conclusions from this development work are that it is critical to increase stiffness of the rear crossmember.

DO THIS FIRST before moving on to other structural improvements. There are beams, braces, etc., but the critical part is that the rear crossmember must increase the stiffness of the beam and must attach the structure to the CENTER OF THE BEAM itself. If you do not do so, as in attaching a gusset at the corner of the beam and the drop-down bushing bracket, you will have wasted your time.

I know this from experience, my first attempt at strengthening the rear suspension structure was exactly this scenario. It did lead on to understanding how the crossmember functions in the system.

From people using the example of a thin Coke can, remember that Al has a fatigue limit no matter the strain. Steel has a strain limit, under which the fatigue limit is basically infinity. Consider too that this small can, the thickness of two sheets of standard paper, holds at times in excess of 100 PSI. It is all about the proper constraint and optimization of how the material is stressed.


The tunnel belly pan, and the trans belly pan both operate primarily in SHEAR, not bending. It closes out the tunnel, and provides the fourth side of a mostly square "torque tube" that is connected to the rails. That is why the number of fasteners (a dozen by memory) are so important. Constraining the edges is critical. The technicians used to mess with me (in jest, of course), removing a bolt or two in the tunnel belly pan during evaluations and changes. I would come back after and evaluations, puzzled by the change in steering or ride shake, and they would admit they were messing with me and put the bolt or two back in. The center bolts seem to be the most critical. So is cutting into the trans belly pan, so do not mess with this pan as it seems to be critical in providing the proper structural tie in to the front parts of the side rails.

The car is very stiff in bending, there is a large diameter torque tube, two large rails, and good structural tie-in to the loading points. Also, the tower offset at front is so close to the very large rail that we determined (again, by analysis and later evaluation rides) that a front tower brace is basically a waste of mass.

[Flash's Owner]

In fact, regarding a blind ride, I will volunteer my personal time some weekend in August to run a blind ride for the DDM backbone, if someone wants to sponsor it.

We could run a couple of them, depending on the results of the first one, I would suggest a comparison of the thickest one to the stock panel.

If that is successful, we could run a second one on the thinnest backbone vs. the thickest one, or even vs. the middle one.

If it passes both rides, I would consider it very conclusive that there is a detectable effect. It would be fun for me, I always love interesting puzzles. LOL

[Speedhawg]

I am sure that Dave would be happy for you to conduct a test, just PM him directly. I can tell you I am pretty sure I could tell. I know after the install I drove to work, I have a very nice on ramp that I love to take at excess speeds when the opportunity presents itself. I did this with my NA and now with the GXP. The first thing I realized after the install of the backbone was that I had obtained a habit of heading into the turn at speed and waiting for the frame to flex, then accelerating. I did not know I was doing this, it was a subconscious learned response. I headed into the turn the first time after the backbone and found myself waiting for the flex, it didn't happen. That is when I first noticed there had even been a flex, I was used to it. I also notice a great deal less shaking when crossing RR tracks that is just a no brainer. I have the Race version and it is aluminum.

[kennysabarese]

I have the Extreme version 1/2 inch aluminum and i definitely noticed a difference, but i also go the probeam which is the rear brace at the same time

[snaponbob]

Originally Posted by Flash's Owner We also did a series of blind evaluations when increasing the rear crossmember stiffness, during the development of the GXP Z0K. The results of stiffening the rear crossmember were so dramatic that we went through the trouble of producing a specific beam for use on the Z0K club racing models. Expert evaluators could consistently, 100% of the time, determine whether a stock or a "stiffened" rear crossmember was in the vehicle. Normal drivers, on one evaluation run, had 11 of 12 able to pick the differences between identical vehicles, WITHOUT cross discussion. My conclusions from this development work are that it is critical to increase stiffness of the rear crossmember. I know this from experience, my first attempt at strengthening the rear suspension structure was exactly this scenario. It did lead on to understanding how the crossmember functions in the system. (snip) The car is very stiff in bending, there is a large diameter torque tube, two large rails, and good structural tie-in to the loading points. Also, the tower offset at front is so close to the very large rail that we determined (again, by analysis and later evaluation rides) that a front tower brace is basically a waste of mass.

You have been VERY helpful in answering my PM's and I really do appreciated the information. Two observations;
1) With spring rates now in the 400's, I really DO notice the increase in cowl shake. Not at all surprised and I am not complaining at all. It is not class legal for me to install the ProBeam or I would. Reading what you said about all the bolts makes sense which answers a question I had about all of them.
2) Dave sent me a ProBeam to evaluate KNOWING that I would/could not install the three big bolts in the cross member. The three bolts are the "make it or break it" for legality in SCCA Solo Street Prepared and SSM classes. I am here to say that I left the driveway, started hammering the car around, it DID help, and not by a small amount. (If it was legal to install the bolts I would do so as that would undoubtedly help even more.) The results were confirmed by removing it, driving it again, and then reinstalling it. It was a TOO significant change to be psychological!!! After installing poly bushing in the suspension the difference on vs off was even more noticeable. In "normal" driving there is no difference, but when driven in a "spirited" manner (ya' like that one?) HOW the rear end steps, HOW it acts after it steps out, and HOW it catches the slide it MUCH different - more predictable, more consistent, and more controllable.

BTW, PM sent on a different matter.

[Speedhawg]

Originally Posted by snaponbob You have been VERY helpful in answering my PM's and I really do appreciated the information. Two observations; 1) With spring rates now in the 400's, I really DO notice the increase in cowl shake. Not at all surprised and I am not complaining at all. It is not class legal for me to install the BackBone or I would. Reading what you said about all the bolts makes sense which answers a question I had about all of them. 2) Dave sent me a BackBone to evaluate KNOWING that I would/could not install the three big bolts in the cross member. The three bolts are the "make it or break it" for legality in SCCA Solo Street Prepared and SSM classes. I am here to say that I left the driveway, started hammering the car around, it DID help, and not by a small amount. (If it was legal to install the bolts I would do so as that would undoubtedly help even more.) The results were confirmed by removing it, driving it again, and then reinstalling it. It was a TOO significant change to be psychological!!! After installing poly bushing in the suspension the difference on vs off was even more noticeable. In "normal" driving there is no difference, but when driven in a "spirited" manner (ya' like that one?) HOW the rear end steps, HOW it acts after it steps out, and HOW it catches the slide it MUCH different - more predictable, more consistent, and more controllable. BTW, PM sent on a different matter.

I am a bit confused Bob, I assume you mean you are evaluating the Pro-Beam without the bolts? The backbone is illegal and only uses existing bolts?

[snaponbob]

Originally Posted by Speedhawg I am a bit confused Bob, I assume you mean you are evaluating the Pro-Beam without the bolts? The backbone is illegal and only uses existing bolts?

For SCCA SP and SSM classes, on the GXP and RL, the GM GXP/Z0K rear chassis beam IS legal, as is the ReBar. The ProBeam is NOT legal in those classes IF INSTALLED PER THE INSTRUCTIONS, but per the Tech people at Topeka, the ProBeam IS legal if mounted WITHOUT the three bolts that are supposed used to secure the lateral piece to the rear frame channel. If the ProBeam is installed with the existing bolts at the rear pivots of the lower control arms the letter and intent of the rule is satisfied. IF (and this is a BIG if) I trophy at the September Nationals (oh, I WISH) I may be protest meat, but am not worried about it.

[smartin]

All I can say to support the difference is that they installed mine at MM5 and I took the Washington DC Beltway back home that night and with all the construction and changing of lanes and uneven pavement and dips and rises, I had to drive the car differently. It was considerably more stable and stayed truer to the center of gravity of the car. I also took it out on the twisties and I was hanging on the back of a GXP like no tomorrow. I was able to drive the car harder and did not feel the flex and sway that I had before installing it. So, to me, it definitely makes a difference!

[TecSolOnt]

Couple of questions to Flash's Owner:

1) is there any safety issue with changing from the original steel unit to a aluminum unit given the aluminum being used by DDM? This with regards to your discussion of various stresses on the backbone. IMHO In the end if a user thinks it adds value then it does,it's not a huge outlay, my concern is longer term.

2) Does your experience than suggest the Pro Beam is a value add and would improve handling?

I realize these are judgement calls on your behalf without real world testing but your have far more experience and background than 99% of us.

[GXPinKC]

Originally Posted by Flash's Owner Is the DDM Backbone made from steel or Al? Some things to consider: -When does a vehicle encounter significant torsion loading? -How does the tunnel section generate "stiffness"? -Can you pick the change out of several blind swaps? I am getting a lot of questions in Private Message system about the benefits of the DDM Backbone. We did work with increasing the tunnel belly pan thickness during the Z0K development, and tried double thick steel plate, approx 3.5mm. The predicted change in torsional stiffness was predicted to be minimal, and was not detectable blind. Tell someone what part is in the car, though, and they definitely "FEEL" it. It would be interesting to spend a day in a garage, have several people evaluate a controlled blind study (the study would be randomized, the conductor of the study would not be an evaluator, and would consist of 6 evaluations where the belly pan was removed and reinstalled). No cross discussion allowed, as consensus can truly ruin a blind ride. It would take the better part of a day, but would be very worthwhile. During the development of a vehicle, we do not always have the luxury to do blind evaluations, but I love to do them when possible. Nothing resets your ability to evaluate a vehicle than a blind evaluation. Nothing can be so humbling, either. We also did a series of blind evaluations when increasing the rear crossmember stiffness, during the development of the GXP Z0K. The results of stiffening the rear crossmember were so dramatic that we went through the trouble of producing a specific beam for use on the Z0K club racing models. Expert evaluators could consistently, 100% of the time, determine whether a stock or a "stiffened" rear crossmember was in the vehicle. Normal drivers, on one evaluation run, had 11 of 12 able to pick the differences between identical vehicles, WITHOUT cross discussion. My conclusions from this development work are that it is critical to increase stiffness of the rear crossmember. DO THIS FIRST before moving on to other structural improvements. There are beams, braces, etc., but the critical part is that the rear crossmember must increase the stiffness of the beam and must attach the structure to the CENTER OF THE BEAM itself. If you do not do so, as in attaching a gusset at the corner of the beam and the drop-down bushing bracket, you will have wasted your time. I know this from experience, my first attempt at strengthening the rear suspension structure was exactly this scenario. It did lead on to understanding how the crossmember functions in the system. From people using the example of a thin Coke can, remember that Al has a fatigue limit no matter the strain. Steel has a strain limit, under which the fatigue limit is basically infinity. Consider too that this small can, the thickness of two sheets of standard paper, holds at times in excess of 100 PSI. It is all about the proper constraint and optimization of how the material is stressed. The tunnel belly pan, and the trans belly pan both operate primarily in SHEAR, not bending. It closes out the tunnel, and provides the fourth side of a mostly square "torque tube" that is connected to the rails. That is why the number of fasteners (a dozen by memory) are so important. Constraining the edges is critical. The technicians used to mess with me (in jest, of course), removing a bolt or two in the tunnel belly pan during evaluations and changes. I would come back after and evaluations, puzzled by the change in steering or ride shake, and they would admit they were messing with me and put the bolt or two back in. The center bolts seem to be the most critical. So is cutting into the trans belly pan, so do not mess with this pan as it seems to be critical in providing the proper structural tie in to the front parts of the side rails. The car is very stiff in bending, there is a large diameter torque tube, two large rails, and good structural tie-in to the loading points. Also, the tower offset at front is so close to the very large rail that we determined (again, by analysis and later evaluation rides) that a front tower brace is basically a waste of mass.

I met you and talked to you at the First National Solstice Meet in Kansas City, Missouri organized by Kathy G. and I enjoyed your speech later in the Meet from the Podium. I appreciate your comments with reference to this topic on this and other forums.

I do look forward to your blind ride sometime in August, but somehow wish it could be done prior to the Denver Meet for those of us wishing to have it installed at that time. Thanks Sir, for your comments on this topic.

It was an extreme pleisure meeting you at the Meet. Thanks for your information and expertise.

GXPinKC
Ron

[Flash's Owner]

Originally Posted by snaponbob You have been VERY helpful in answering my PM's and I really do appreciated the information. Two observations; 1) With spring rates now in the 400's, I really DO notice the increase in cowl shake. Not at all surprised and I am not complaining at all. It is not class legal for me to install the BackBone or I would. Reading what you said about all the bolts makes sense which answers a question I had about all of them. 2) Dave sent me a BackBone to evaluate KNOWING that I would/could not install the three big bolts in the cross member. The three bolts are the "make it or break it" for legality in SCCA Solo Street Prepared and SSM classes. I am here to say that I left the driveway, started hammering the car around, it DID help, and not by a small amount. (If it was legal to install the bolts I would do so as that would undoubtedly help even more.) The results were confirmed by removing it, driving it again, and then reinstalling it. It was a TOO significant change to be psychological!!! After installing poly bushing in the suspension the difference on vs off was even more noticeable. In "normal" driving there is no difference, but when driven in a "spirited" manner (ya' like that one?) HOW the rear end steps, HOW it acts after it steps out, and HOW it catches the slide it MUCH different - more predictable, more consistent, and more controllable. BTW, PM sent on a different matter.

Bob, I think you are talking about the pro beam, which is a change that I believe would create a significant change in steering feel. Exactly whether or not it is better than a Z0K welded channel crossmember, or Kappasphere's bolt-on is a completely different discussion. ANY improvement to the rear crossmember, if properly applied, seems to be directly felt.

Interestingly, it does increase lateral single-input and lateral parallel input structural stiffness of the rear bushing pocket. It is easy to see why. Analysis predicts this conclusion.

However, on a full vehicle, you would think parameters like lateral force camber stiffness (how much the camber changes when a lateral force is applied at the tire patch), or lateral wheel stiffness (how much the wheel moves with lateral force applied at the tire patch) would also be directly stiffer. In reality, the changes are nearly immeasurable.

I did not believe the results, so I had them re-run the car on our kinematic-compliance machine, but it is true. The overall stiffness does not change measurably.

What does change, and what I believe people are feeling with a stiffer crossmember, is WHERE the displacement happens.

Say, for example, the bushing is 10,000 N/mm. Say the structure is 15,000 N/mm. Total stiffness of the two in series is about 6,000 N/mm (or 6 kN/mm).

Increase the structure to 20 kN/mm, the overall stiffness increase is barely detectable - only 6600 N/mm, or 10% stiffer. The structure change was a whopping 35%.

However, during a 1.5 kiloNewton lateral load, like cornering at 0.5g, the first case has a bushing deflection of 0.15mm and the structure has a deflection of 0.10 mm.

The second case has the same bushing deflection, but the structure only deflects 0.075mm.

Less deflection in structure relative to bushing means overall more damping. Structure has little damping, being primarily steel parts, so deflections in structure tend to be uncontrolled. Bushings have damping. Same rough deflection, more in the bushing means that for a given amount of deflection, there is more damping and therefore more control. Less overshoot, etc.

It is a little more complicated than the above example, because bushings have non-linear deflection curves, but I suspect this is the bulk of the explanation of why there is little overall change in the static stiffness, but a profound difference in how the car feels.

Incidentally, the Z0K rear beam is not cheap. We were working on a way to get it on all cars, but the cost of the part did not present a good business case, and we had significant trouble justifying adding a part like this.

Originally Posted by smartin All I can say to support the difference is that they installed mine at MM5 and I took the Washington DC Beltway back home that night and with all the construction and changing of lanes and uneven pavement and dips and rises, I had to drive the car differently. It was considerably more stable and stayed truer to the center of gravity of the car. I also took it out on the twisties and I was hanging on the back of a GXP like no tomorrow. I was able to drive the car harder and did not feel the flex and sway that I had before installing it. So, to me, it definitely makes a difference!

Was yours a 'backbone' or a 'pro beam', or both?

Originally Posted by TecSolOnt Couple of questions to Flash's Owner: 1) is there any safety issue with changing from the original steel unit to a aluminum unit given the aluminum being used by DDM? This with regards to your discussion of various stresses on the backbone. IMHO In the end if a user thinks it adds value then it does,it's not a huge outlay, my concern is longer term. 2) Does your experience than suggest the Pro Beam is a value add and would improve handling? I realize these are judgement calls on your behalf without real world testing but your have far more experience and background than 99% of us.

2) first - yes, I think the Pro Beam or other crossmember stiffening devices, if executed properly in stiffening the beam first and second bending modes, do add value.

1) I am completely unsure of the changes that would happen to crashworthiness, that area is certainly way outside of my expertise. I know small changes in structure can result in unintended changes to how a particular structure handles crashworthiness, and it is not as easy as saying "stiffer is better". A structure must both protect AND absorb damage energy.

My opinion (and it is ONLY an OPINION) is that the rest of the structure can probably handle a local increase in stiffness, provided it does that. I would not think that it would cause a chain failure elsewhere, but I also freely admit that this is conjecture on my part.

[smartin]

Backbone only.