Torsional rigidity test: 67 coupe

Discussion in 'Classic Mustang Specific Tech' started by mustbereel, Oct 7, 2007.

  1. Inspired by the Mustang II suspension thread I spent the last couple of days testing the torsional rigidity of my 67 coupe. Since I am installing a MII front end from Rod & Custom this gave me the opportunity to compare stock to MII. I also tested various additions like front torque box and subframe connectors.

    The car: 67 Coupe, rust free and solid. 67s are odd in that they have a single torque box on the driver's side. Earlier cars have none and will be more flexible. Later cars have boths sides and should be stiffer.

    The setup: support the car at three corners and use a ten foot bar with weight (me, 210lbs) to twist the frame and measure deflection with a dial indicator.

    I placed the rear jackstands where the shackles attach to the leaf spring. I placed the single front jackstand under the frame just behind the radiator support. I attached the 10 foot bar across the lower part of the radiator support. Although this setup does not accurately reflect how the frame is loaded by the suspension it gave me a way to test both suspensions as well as any future modifications, front or rear, with the same twisting force.

    Opposite corner must be securely lashed to the ground.

    Torque bar attachment.

    Dial indicator to measure deflection.

    This test does not address beam strength as I couldn’t come up with a good way to load the chassis other than getting about 10 volunteers to get in and out of the car after every change and measure the deflection under the center of the car.

    To begin I removed all factory bracing. The doors are not latched and the trunk is open. All glass removed. This limits the strength to the unibody alone.

    The results:

    Baseline 0.66"
    Factory crossmember 0.64" I'm surprised this added anything
    Export brace 0.48" The biggest single improvement
    Monte Carlo bar 0.48" Not surprised with this.
    Passenger side torque box 0.48" Very surprised. I guess this is more for beam.
    Subframe Connectors 0.48" I thought I would see a little improvement.
    Subframe X brace 0.475" Very dissapointed. Did I make a mistake?
    Remove export brace/MC 0.65 Nearly back to baseline.
    Remove all parts 0.67" More flex than baseline? Torque box still there.

    Cut out shock towers 0.69" Lost some but not much.
    MII X member 0.63" Slightly better than stock crossmember.
    Subframe connectors 0.63" Still nothing here.
    Subframe X brace 0.61" Slight improvement.

    Notes: I didn't have the factory shock tower braces so I couldn't compare to the export brace. Subframe connectors were bolted in. I will retest after welding them. I haven't added the repair panels to replace the shock towers yet. I also haven't removed the strut rod brackets yet.
    I plan addition bracing for the front end and will retest as I go.

    At least in torsional rigidity, the MII setup is slightly stiffer than a stock chassis without an export brace. However, it is no match for the stock chassis with an export brace. We'll see what happens when I add additional bracing.

    When bolted in, subframe connectors (at least the Heidt's model) offer little torsional resistance even with the cross brace. Hopefully this will change when welded. The Heidt's subrame setup is similar to the TCP setup except Heidt's uses smaller diameter tubing.

    Attached Files:

  2. I feel better about installing a Boss 302 chassis manual style crossmember already.

    I'm suprised too. Wonder if they were worth the trouble at all.

    It was my understanding that the subframe connectors only helped with beam. I remember reading so much in a post by SN65.

    Awesome post. It's nice to see somebody actually do testing so we can qualify what does what. I wish I had better understanding of loadpath and the rest of that engneering stuff. I guess that's what my wife the SE is for.
  3. bout time somebody did some real-world engineering testing. Good work
  4. Mustbereel,

    Excellent post!

    The Export Brace does really well: flex is reduced by 25%. That the other parts help next to nothing is a little surprising, though.


    The torque box doesn't seem to help, but I don't think that means that it's not worth the trouble. After all, Ford put them in and I doubt if they would add parts that didn't help any. Heck, they didn't even put on an Export Brace on most cars and that part reallly works.
  5. The torque boxes are definitely worth the trouble. I had my fastback on a rotisserie before and after the torque boxes and the chassis was noticeably stiffer. I just thought it would make a difference in torsional rigidity too.

    I wasn't expecting much from the subframe connectors but the cross brace that connects the two sides is supposed to help with torsion.
  6. Another modification which would be interesting is the welding/bolting a plate behind the rear seat. It would be cheap to make and almost invisible when everything's done. It makes a great firewall too.

    I've often seen it in race cars and I once read a torsional stiffness overview of BMW 3 series. The ones with a fold down rear seat (which obviously didn't have any bracing behind the rear seat) were considerably less rigid than the ones with a fixed seat. I forgot the numbers, but it was at least 25%.

    Of course that wouldn't automatically mean that the same applies to Mustangs, but it would be interesting to see how much it helps.
  7. while i was expecting some improvement with sub frame connectors, i wasnt expecting much in torsion. that is why i always recommend adding stringers from the connectors to the rocker panels, and welding in the connectors.
  8. This is one of the changes I plan to make and one of the reasons I chose to secure the chassis at the extreme rear.

    I'm considering adding stringers to the subframe connectors. I don't want anything to hang down too much so I thought I might run a length of 1"x.5" tubing along the rockers and then tie them to the sfc with the same tubing. I'll have to look at some of the kits for newer Mustangs. Any sugestions on stringer design and attachment?
  9. do it like kenny brown did with the fox body cars. he used 1x1 tubing that ran along the rocker panel, and 3/4" tubing straight across, and also in a pair of diagonals, welded in place. he also used tabs welded at the ends to create a bit more connection area. i think BBK still has the extreme matrix brace in their catalog.
  10. You should also seam weld your panels; makes it a lot stronger. The factory is just spot welds. Also try adding the support bar from the firewall to the front of the frame rails. It's supposed to help.
  11. can you find another location behind the firewall to isolate where the flex is occurring?
  12. I plan to add a bar from the firewall to the frame rails. I plan to run it under the fender from the corner where the cowl vent and cowl side panel meet down to the frame just in front of the suspension. It just needs to be out of the way for suspension and tire movement. I then want to tie it into the inner aprons at a place where I can add an export brace like diagonal to the center of the cowl panel.

    I will probably do some seam welding but I'm not having much fun lying on my back welding under the car so I may just do the easy parts.
  13. I'm thinking about placing the rear jackstands under the forward leaf spring mount. This will place it right where the subframe connectors end. Maybe I'll try that with and without the subframe x-brace.
  14. Best post ever! :hail2: :nice:
    Im a little surprised the monte carlo bar didnt do anything. I put one on my 65 comet, and kept wondering why my car seemed to ride a little smoother. It wasnt until a couple weeks later I figured out that the ride quality difference and MC bar happened at the same time. Just looking at it, I can see why it didnt do much for your torsional readings, but it does help tie the left and right towers together, so where does the benefit come from? Or what is the benefit, if thats what Im trying to say? What exactly is the beam strength you refer to, especially regarding the subframe connectors? Chassis flex front to back, rather than twisting?
    In any case, it looks like I know the next chassis mod for my Comet!
  15. Thanks, best response ever to one of my posts!

    The primary benefit to the Monte Carlo bar is to keep the shock towers from bending towards each other under load. By keeping the shock towers stable the alignment is maintained and the suspension can work as designed.

    You've got the beam strength correct. Just imagine a board placed across two saw horses. If you stand on the board it will flex. Now attach a 1x4 to each side perpendicular to the board and it will flex less.
  16. Doesnt the export brace do exactly the same (but more)? In your now expert opinion, is the MC bar even worth the $50 if you already have an export brace?
  17. I would say yes. The export brace isn't as good at resisting the inward load imposed by the suspension. This is a different force. Imagine you are driving along and both front tires hit a bump at the same time. The impact is transmitted through the springs to the top of the shock towers which then deflect towards each other. The MC bar is in direct line with the load that caused the shock towers to lean in. Plus I've always found it make a great support bar to lean on while working on the engine.
  18. Mustbereel, what a great post. I have welded Global West SFCs but dont really know what the welded stringers mean? I put the export brace on my vert, along with the "bent" Monte Carlo at the same time. Now I know which to thank for the decrease in front end and cowl shake.
  19. This is the basic idea with the stringers or jacking rails:

    They connect the subframe connetors to the rocker panels to further reduce twist. I found these by searching for "Mustang subframe connectors." I also found examples of the Kenny Bell brand but they look less substantial.

    Attached Files:

  20. I'm curious what you would get if you used all the convertible reinforcements on a hardtop (extra rockers, boxing under seats, torque boxes, one piece seat riser, etc.). SN65 is doing something like that at the moment, but I don't think he has posted the end results yet.

    One nice thing about using factory sheetmetal for the reinforcements (Export Brace, torque boxes etc.) is that they they look like they belong there. If you like that look is of course a matter of taste.