Torsional Rigidity Testing, 280Z

[Chris Duncan]

It is possible I'm using the wrong welding technique. I did 3/4" stitch welds but left no gaps between them. Welding 3/4" beads then leaving a 3/4" gap, then another 3/4" bead, then a 3/4" gap. Then when a full section with gaps was done and cooled off going back and welding the 3/4" gaps so the weld ended up continuous on each section/area. Thinking back the original plan was small stitches spaced with bigger gaps between. Then the plan changed to the full weld to effectively "seal" the seams as it would be a PITA to reseal the chassis after burning all the existing sealer out. But more is not always better.

 

I think it's problematic adding tubing for chassis rigidity. If it's done it has to be well thought out and the best way would be complete from front strut towers to rear strut towers.

Something like this... http://s160.photobucket.com/user/psanders240/library/BSR-260Z-IMSA?sort=3&page=1

This car is probably getting up in the 6000 lbs/deg range.

 

There may be some benefit to a single tube across as a STB, and a triangulated one to the firewall is even better. (Where the bottom of the cowl structure perpendiculars into the firewall is pretty strong.) The key is STRAIGHT tubing. Bends or kinks defeat the whole purpose. It's too much force and too small of a movement you are trying to hold rigid. This is difficult with a Z because the valve cover sticks up too high. There have to be rigid "raisers" on top of the strut towers so the tubing can go straight across.

 

Out to the garage now to unbolt and rebolt to check repeatability and also to get some readings at each end of the rockers as requested. The cage isn't going in until the seat gets here next week, then there will be another test.

Edited January 7, 2014 by Chris Duncan

[johnc]

Then when a full section with gaps was done and cooled off going back and welding the 3/4" gaps so the weld ended up continuous on each section/area.

 

 

Oh!  You you welded all the seams with a (essentially) continuous bead.  That's not the normal practice when stich welding a chassis.  Normlayy the gaps are left between the short welds.  I don't know how that would affect your results but it does mean that the entire seam has gone back to the original yield strength of base steel if there was any hardness added via alloying or after the initial rolling.

[rsicard]

I did something kind of like this. I don't think you want a bar across the floor on the main hoop and then go forward from there. You would be putting the loads from the front struts into the middle of this bar across the main hoop, and the bar is most flexible in the middle of the span. What I did was a V to the diagonal in the main hoop, a bar from there straight down the middle of the car, and then a V from the front struts that hits the dash bar in the same spot. Dr. Sideways did the same thing but better. Rather than have a single bar straight down the center, he has an X between the dash bar, the diagonal, the A pillar bar, and the main hoop. The problem with his solution is that it completely eliminates the possibility of having passengers, and I love giving rides.

 

In the book Chassis Engineering by Herb Adams he does some scale model testing with balsa wood frames and something close to what I have is used and found to be effective.

 

This relates to what others have been saying both in that the strut tower loads are tied together front to rear, and also the notion that the front clip is the weak part of the chassis. On my 70, when I put jackstands under the TC rod buckets and let the jack down, you could SEE the front end of the car droop about 1/2". You have to figure that the same thing is happening when you step on the brakes; essentially the car is trying to fold at the firewall. 

 

With regards to the STBs, I don't think their main objective is to increase torsional rigidity per se, it's to tie the strut towers together. When the outside tire takes a lateral load it tries to pull the strut tower outward. When both tires hit a bump simultaneously they try to push together. Tying both strut towers together even without an attachment to the firewall should help both of these situations. I have doubts about the benefits of tying to the firewall unless the firewall is reinforced.

Jon:

 

Thanks VERY MUCH for your comments.  It's really nice to have a discussion on this level.  Agree with you totally on the STBs with the same thoughts.  In reading your comment, the longitudinal tube you installed from the diagonal in the main hoop must have been elevated such that it intersected the firewall near the top of same and onto the strut towers via a "V".  This is a much more DIRECT approach to increasing the "Y" axis torsional rigidity especially the front struts towers and structure.  I used an indirect method by having the tube just above the floor hump then extending upward the inside of the firewall with steel and then downward and outward on the front of the firewall to the framerails.

 

I have also purchased the Chassis Engineering book by Herb Adams and read it thoroughly.  I want to keep the original Datsun 240Z dash.  That is why I may place the longitudinal tube just above the hump.  Also want to beef up the forward frame rails by adding rectangular tube to the bottom of the existing frame rails forward of the front towers and far enough to the rear to gain rigidity in the "X" axis of the vehicle.

[tube80z]

If you're looking for easy things to do to increase torsional strength try gluing in the windshield.  We did that to a chump car to save a gasket and I was amazed at the change in handling.  It was similar to what you'd see with a strut tower bar.  For fun windshield related trivia, aluminium and glass have almost identical Young's modulus and density. Glass has a yield strength about 10 times higher than plain aluminium and obviously less allowable plastic strain.  A 5 mm windshield is equivalent to an almost 2 mm thick steel sheet.

 

Cary

[LLave]

If you're looking for easy things to do to increase torsional strength try gluing in the windshield.  We did that to a chump car to save a gasket and I was amazed at the change in handling.  It was similar to what you'd see with a strut tower bar.  For fun windshield related trivia, aluminium and glass have almost identical Young's modulus and density. Glass has a yield strength about 10 times higher than plain aluminium and obviously less allowable plastic strain.  A 5 mm windshield is equivalent to an almost 2 mm thick steel sheet.

 

Cary

 

 

I wonder if those figures apply to safety glass, I would imagine it would be quite a bit weaker. Windshield adhesive is no joke.  I have experimented with filling factor motor mounts with 3M windshield adhesive, it is amazing. 

[JMortensen]

Tempered glass is stronger than untempered. Safety glass in a windshield is two tempered sheets with rubber in between. I would think it would be stronger yet.

Edited January 9, 2014 by JMortensen

[Chris Duncan]

If you're looking for easy things to do to increase torsional strength try gluing in the windshield.  We did that to a chump car to save a gasket and I was amazed at the change in handling.  It was similar to what you'd see with a strut tower bar.  For fun windshield related trivia, aluminium and glass have almost identical Young's modulus and density. Glass has a yield strength about 10 times higher than plain aluminium and obviously less allowable plastic strain.  A 5 mm windshield is equivalent to an almost 2 mm thick steel sheet.

 

Cary

 

Yes, the GT40 has a triple laminate windshield that is a definite part of the rigidity of the chassis.

 

Wonder if lexan comes close?

 

More on the testing on Sunday.

[Phantom]

Interesting discussion.  bottom line is that, compared to new cars, the S30's basically have little torsional rigidity. The 240Z has a nickname: "The Flexi Flyer". It got that because of all the body flex that occurred when raced hard.  The 280Z is a different animal but it still is no where near as rigid as the newer vehicles.  I have a front strut tower brace that triangulates across the strut towers and back to the firewall, a rear strut tower brace that triangulates across the two towers and a 4-point roll bar. All have improved the rigidity of my car but, if I really work it hard, my LS1 can still flex the front windscreen frame which will result in a leaky windshield.

I've often thought about welding tabs to the roof and then bolting them to the roll bar to see if that will help.  Probably would a little but I think it will take a pretty extensive cage to really firm one of these cars up.

Edited January 14, 2014 by Phantom

[JMortensen]

I've often thought about welding tabs to the roof and then bolting them to the roll bar to see if that will help.  Probably would a little but I think it will take a pretty extensive cage to really firm one of these cars up.

A friend of mine had a very simple 4 point bar built and had one 5" wide plate welded from the bar to the roof. He said it completely eliminated the rear interior panels squeaking when he went in and out of driveways. A better way to do it is with a long shear plate or with a couple shorter ones at each end. It does work, and it's easy enough to do. Doesn't do anything for the front of the car though...

[LLave]

Tempered glass is stronger than untempered. Safety glass in a windshield is two tempered sheets with rubber in between. I would think it would be stronger yet.

 

Didnt think about it like that. I bet it is pretty rigid. 

[Phantom]

A friend of mine had a very simple 4 point bar built and had one 5" wide plate welded from the bar to the roof. He said it completely eliminated the rear interior panels squeaking when he went in and out of driveways. A better way to do it is with a long shear plate or with a couple shorter ones at each end. It does work, and it's easy enough to do. Doesn't do anything for the front of the car though...

Jon,

Thanks for the info.  My son and I have discussed this frequently and it's nice to know it's been tried with some success.

[Snailed]

Thanks for sharing your tests with us! Very cool.

[Sam280Z]

I don't think that windshields are tempered. If they were, wouldn't the outer layer of glass crumble when hit by a stone?

http://en.wikipedia.org/wiki/Windshield

 

That said, even normal glass is extreemly ridgid. Once it deflects any appreciable amount, it breaks.

[Sideways]

Take this with a grain of salt, of course, but:

http://en.wikipedia.org/wiki/Toughened_glass

"It (tempered glass) is used for its characteristic of shattering into small cubes rather than large shards and is sometimes referred to as safety glass in this context. (The windscreen or windshield is instead made of laminated glass, which will not shatter when broken.)"

[Chris Duncan]

Here are the measurements at different points along the side both in front and back of the rocker. This was with 1 deg twist at about 1900 lbs. It looks like the movement is pretty evenly spread from front to rear.

 

On closer inspection I am getting some movement in the rear. Not on the rear jig itself but on the body really close to the jig. And also about .077" @ 12" behind the rear jig. So it maybe would have been better to also anchor to the upper rear strut mounts and not just the mustache and inner arm mount points.

 

Edited January 27, 2014 by Chris Duncan

[Chris Duncan]

Front setup showing jackstand central pivot, dial gauge and front jig

 

 

Front setup with 1.5" x 1.5" x .125" tubing jig.

 

 

The 2 x 4 x 1/8 steel beam loaded, you can see the jack is extended and the 4x4 wood beam coming down from the rafters.

 

 

System unloaded, you can see the straighter angle of the beam and the retracted jack. Also note the two 1/2" bolts holding the rear jig to the floor on the left side of the jig. Securely holding the rear jig down is critical.

 

 

The chassis scale at the end of the beam (only using one pad), and the 4x4 wood beam coming down from the ceiling. It is MUCH easier and quicker to just extend and retract the jack to conduct a test than it is to manually load and unload weights from the end of the beam. I've performed about 15 separate tests so far in a small fraction of the time it would take to load and unload 200 lbs of weight from the end of the beam. And with more rigidity it can get up over 500 lbs.

[tube80z]

Thanks Chris for sharing the setup and data.  Very few have ever done this.  

 

best,

Cary

[m1ghtymaxXx]

This is cool! It would be awesome if someone could provide a stock 240Z for the sake of comparison with hard numbers. 

[NewZed]

Somebody linked to the video below from another forum.  Seems related to the topic here, he's fixing a tweaked chassis.  Shows how malleable the bodies are.  As noted earlier, don't over-stress (strain) the body to get test results.

 

http://www.youtube.com/watch?v=3vOk3WPb_3k

[Snailed]

I wonder what the results would be like with the rear strut towers being used as your rear pickup points. Seems like that would give you a better idea of how the body deflects while driving.