Weekend is here, so time to get started again. The goal is to have the brace mocked up this weekend. Then I can sketch out dimensions and get the prototype built.

Side note, how many people want camber braces? I have the RD engineering design on my car and it would not be difficult to make a chromoly version. It's a simple product, so price would be very reasonable.

Thanks,
Brandon
www.riceperformanceproducts.com

 

Seems to me there are two approaches to chassis braces. One is just to have some kind of rod in tension, which stresses the unibody. The other is to have an actual brace that reinforces the unibody.

So why not reinforce the unibody and make it last longer, in addition to getting more stiffness? Here is an example of reinforcement welded to the unibody rail of a Jeep XJ. But I think you could get just as good results by epoxying something like this to the framerail.




You can also tie parts of the frame together with a rigid frame, instead of just tying them together with tension rods. Check out the Miata Butterfly Brace:
http://www.flyinmiata.com/index.php?...umber=13-69920


It seems like something like the butterfly would also act as a skid plate and protect the exhaust piping etc. Also the stresses are distributed better than with just 4 mounting points like an X brace. They also sell the framerail portion of the butterfly separately. Same idea as the Jeep one http://www.flyinmiata.com/index.php?...umber=13-69010

 

I like the butterfly brace system in the picture above, but there are not a lot of good tie in locations on the C4. The front and rear frame sections seem to be the optimum location. I am considering rivet nuts as an option for attachment method. That would keep modification simple(drill a couple holes and install rivet nuts). I am looking into low priced rivet nut installation tools so I can make it part of the kit.

I finished the basic prototype this last weekend and I started making some drawings. I am looking into availability of Chromoly tubing to determine the final design for the prototype. I intend to make 1 prototype and install on my car for initial fitment and measurements. After that I will email those of you that are interested in testing for the first 10 sets. I should have a good price estimate in the next 3-4 weeks. Since I am already past 10 on my initial testing list, I will let everyone know now that I plan on offering a group purchase for the next 10-20 people that are interested. I may make the list slightly longer depending on the discount I can get on production.

Here is a picture of the basic design in 2d. I am currently working on the CAD model for production. It will probably change slightly after the prototype.

I decided to make the connection for the x-brace near the front because it had the least affect on ground clearance due to the exhaust.

 

If the flexing with the top off experienced in my vett was due mostly to worn out suspension why would it feel so stiff with the top on?? Just saying. The members of the suspension brace are in tension not compression right? The top off causes the top of the windshield to move closer to the top of the halo when crossing bumps, that is why putting the top back on solves the problem. The whole weight of the car is suspended between the front and rear tires and is constantly trying to flex downward in the middle of the car. That puts the frame rails in tension especially since they are at the bottom of the car. A brace running under them is mostly in tension. It is really interesting that your vett doesn't seem to exhibit the flexing problem with the top off, you are in a minority though. I know that I seldom drive with the top off, it's really horrible to experience, I have to REALLY want to drive with the top off to put up with all the flexing.
Since the x brace members are in tension most of the time they could be made much lighter than if they were in compression how much lighter I don't know, I'm not an engineer. I am also interested in the design you are working on, good luck.

 

You don't mention that titanium is a bear to work with.

Since the design would be the same (except for wall thickness),
and the jigs would be the same, have you considered doing both
mild and chromoly, with, say, a $100 price difference?

I never drive my '87 with the top off, I'm not personally interested,
just tossing out an idea that might be helpful.

Chuck

 

You might be well served designing a solution by doing some reading about chassis design. Carroll Smith wrote a series of books a long while ago about race car prep and design. They are still available as reprints and good engineering does not go out of style like they are making better triangles.. Personally I think GM dropped the ball with the transmission tunnel they could have made it structural and had a rigid center spine to eliminate flex easily. It is an old idea used on dozens of very well designed sports cars.

 

What is the point of links D1 and P1? Doesn't the car provide that support on its own?

Also if the braces get too thin they'll just buckle in compression. (During twist one is in compression). So you are either relying completely on the other one in tension, or you have a significant vertical dimension in the braces at the expense of ground clearance.

 

So how about building braces for the transmission tunnel to make it structural?

 

I have looked in detail at drawings and under the car to create the best design that is reasonable for installation. The issue with connecting under the car is that nearly everything is fiberglass or really thin sheet metal. To tie in the transmission tunnel, I would first need to build a structural tunnel. I can completely redesign the frame for the C4, but it won't be cost or time effective.

As far as the side braces, I plan on doing a few simple tests on my car with the prototype to determine effectiveness of each brace section.
Thanks,
Brandon
www.riceperformanceproducts.com

 

If the floor pan had been 1 piece, it is 2 pice structurally last I knew anyway it would have been a lot better. The whole idea is to run tubing from the front bulkhead to rear one and from there to A and B pillars then weld sheet over it and obviously add proper triangulation between the ube. Doing that along with a so called butterfly brace on the bottom would make a very stiff box structure that would stiffen the weakest part of the chassis as is evidenced by taking the roof off and driving on a bumpy road.

The problem I have with all forms of X-brace is the basically only work in 1 plane as generally an inch tall or less and lots of bolted joints to flex. They are certainly better than doing nothing and doing the tunnel is not a brace it is a major fabrication project no doubt well beyond the scope of most for a street car and might make it no legal for some race cars.

My take on it all anyway..

 

Maybe these pics will give you some more ideas

 

I have considered making a brace that incorporates the seat brackets. That would simplify the other brackets to keep cost down and I could also design optional attachments to mount racing harnesses. I believe I can keep the system low profile to minimize effect on ground clearance issues.
Thanks,
Brandon
www.riceperformanceproducts.com

 

Exactly why I said major fabrication involving I would bet involve taking everything out of the interior and possibly pulling engine and trans. I blame GM of dropping the ball in the begining by not taking advantage of it's use structurally.
You have more time under the car than me as mine was only to install new exhaust on jackstands and very happy to be done and did not linger there on my back wonder what if. I have seen frame pics here I am sure and remember it showing front floor pan and part of the firewall as steel. If there was a way to tie that steel to across the B pillars I would think it would add a ton of stiffness. I wonder if you put a bar across the rear monts of the X and then tied that into the B pillars what you might get.

http://www.waxner.com/?tag=lotus-elan-1962

Here is a link to a book that might cut the R&D time way down and provide easy ways to test if you are making an improvement and how much. Something I would bet most have done by seat of pants rather than real engineering and testing otherwise they would advertize the old and new deflection rates.

http://books.sae.org/book-pt-90

 

What you guys don't know about the C4's development and later the C5's development won't hurt you.

I won't argue with you here. Go find out for yourself from the man himself, Chief Engineer Dave McLellan. Corvette from the Inside, I own a copy.

Your chassis ideas are not NEW at all. Dave wanted to do a lot of stuff on the C4 that was later on the C5. Some of your comments are things Dave wanted to do. GM Management, as usual, F-ed things up.

 

Never said the ideas were new or even mine what made you think I said that? It really does not matter in the end as to what was wanted, we got what GM allowed. What was wanted is interesting for informational purposes but if changes made on a bare chassis the odds of implementing them now are probably impractical.

If you have read the book why not enlighten everyone that has not might be the real question.....

 

I would love a brace that tucks up to the floor not under the exhaust. Everyone will have different headers and exhausts and different welders who have done work on their exhausts and they all hang at different levels.

 

The car originally was designed as a t-top car with low door sills towards the front and higher towards the back.

The center spine is basically the C-beam as it eliminates the space eating cross member of the C3. Dave liked the driveline of the Porsche 928 with its torque tube and rear mounted transmission. However, they could not afford the rear mounted transmission. That came later a la C5.

Originally had a torque tube a la C5. However, they had a drive line vibration with that the best solution was to open up the tube (C-beam) and vibration out of driving range of engine RPM. Also this allowed for a conventional prop shaft. Unlike the C5 and up, drive line service does NOT require a complete tear down from the rear.

A lot of time and testing went into the chassis even using a Grumman Aircraft state of the art CRAY computer for simulations and finite element analysis. Dave says that even with all that they did not predict the hinge like nature of the transitions between 2 rail and 3 rail structures. Thus they did not predict the stiffness of the chassis well at all.

GM Lloyd Reuss "requested" (McLellan's type) that the car be a targa. All that time/effort on the structure was for nil without the t-top.
They added the high door sills, still too flexible, added reinforcements where it was moving the most. They got the chassis to be just above wheel hop frequency with the top on. Wheel hop frequency in a C4 is 14.5. Structure stiffness is about 17hz. Compared to the 21hz of the C5 with the TOP out. With the Top out in a C4, the chassis flexes below that of wheel hop so it shakes a lot. The reason wheel hop is important is that it is the "highest frequency of road input."

In the late 80's Dave asked Lotus how they might do a Corvette structure. Backbone, center tunnel is the answer to front engine rear drive. The C5 structure was built from the ground up to be stiff, the hydro forming has a lot to do with this, as does the center tunnel design.

Long story short, the C4 is a flawed chassis design due to GM management interference in something they didn't understand.
NO amount of bracing will significantly increase the stiffness of the structure, there is NO way to easily create a center tunnel for the drive line to run thru and to hold the car together.

If there had been a practical cost effective way to do this, don't you think they may have pursued it? Rumor is Vette to Vette's tension rods are Gordon Kilebrew designed as he? discovered the metal was flexing about the spot welds. However, it was so late in the C4's development cycle it was not cost effective to make any changes.

The only reason the C4 is a good handling car is that the suspension is very well designed for the time. Its obsolete compared to the C5 and up, but still a good design.

You really need to read the book to get a complete picture.

 

Interesting... How about welding a roof into place, will that make it as good as the original T top design?

 

I am more willing to bet they were not allowed to fix it as it would not have been that difficult. All the tools were available a decade earlier to have a solution in less than a week without building a bunch of test mules. Do you honestly think they built airplanes in the 70's with a bunch of guys sitting around saying let's try this. If the car had a full flor pan attached to the frame it most likely would not flex.

There are loads of pics on the net that shows the floor pan and tunnel being two piece rather than a rigid tunnel. It makes me wonder if the fix is on the inside. If you bonded aluminum honey comb to the existing floor pan and then a layer of either fiber glass or carbon fiber to that to make it all one piece and structural from firewall to rear bulkhead and cover those as well. It would not be thick and not add very much weight either.

In the end anything done needs to be tested and there are simple ways to do it given a little bit of work.

 

In the end anything done needs to be tested and there are simple ways to do it given a little bit of work.[/QUOTE]