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Chassis reinforcement


CrazyZ

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The closest I have to any empirical data is some FEI analysis done by Bill Savage at T-Mag back when he did some work for me on the rusty old Datsun.

Credit to you and Bill for doing that and it's better than nothing, but I think an actual test on a chassis would eliminate the garbage in garbage out problem. Cary was talking about it before, maybe we could goad him into it... :D

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The closest I have to any empirical data is some FEI analysis done by Bill Savage at T-Mag back when he did some work for me on the rusty old Datsun. The biggest weak areas on the model he and I made of the S30 was the connection of the firewall to the rockers and the floors. That area acted like and hinge. Within the limited rule set I was building to we built a triangulated strut tower brace and I (illegally) added some reinforcement to the upper frame horns and filled in the open box (my car was a 1970) on each side of the firewall connecting the rocker, fire wall, and cowl. Those changes, along with a welded in roll bar, let me run spring rates 350% higher then OEM. It stopped the chassis cracking that had regularly been occurring at the firewall/floorpan junction when I was running 225/50-15 Hoosiers - even after upgrading to 275/45-16 Hoosiers.

 

If I had a full roll cage with braces to the top of the firewall and through the firewall to the strut towers the little analysis we did says we could have gone 600% higher on spring rates. None of the analysis we did showed any strength issues with the rockers.

 

Now, its was a pretty rudimentary analysis done 11 years ago and FEI has been superseded by much more sophisticated tools and techniques. Maybe we screwed up the input parameters and got garbage results. There are a whole host of things that could (and maybe did) have gone wrong with that analysis. Unfortunately that's the best I got and on track results sorta confirmed what the software said.

 

One way to help figure this out is to cut up a 280Z. I've cut a few and the reinforcements are mostly in the roof (for rollover protection), front frame rail/firewall/cowl junction, rear bulkhead/floor pan junction, seat mounting, suspension mounting points, front crossm ember, front frame rails. I honestly don't remember if the rockers were reinforced so hopefully someone who has cut a 280Z up and remembers can chime in here.

 

Did you mean FEA?

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I was reading one of the project builds on MotoIQ today, and one of products they installed on their project s2000 caught my eye. It is an upper front subframe brace.

 

1125903013_srX39-L.jpg

 

Bolting to thick metal of the door mounts and reaching forward to the strut towers, I'd imagine something like this could add some substantial stiffness to our cars, especially if tied into the rockers and used in conjunction with something like the rocker to TC brace pictured below. If done well, I think this could resist some of the 'hinge' action of the rocker to firewall connection, allowing the rockers to do their job a bit better

 

jeffery-R1-E028.jpg

 

Anyone have thoughts on this?

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What size tubing do you think from the TC rod to rocker? And what size for the rocker to strut area.

I took my car apart and cut the sheet metal splash shield, but I am unable to make that bar straight and long.

If memory serves it is about 24" long and we had to put about 15 degree bend in it towards the center of the car. Other pictures look like it's a straight shot.

My tubing is 1.5" Maybe something smaller dia. that would tuck tighter?

thanks

Lewis

Edited by luigi
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I was reading one of the project builds on MotoIQ today, and one of products they installed on their project s2000 caught my eye. It is an upper front subframe brace.

 

1125903013_srX39-L.jpg

 

Bolting to thick metal of the door mounts and reaching forward to the strut towers, I'd imagine something like this could add some substantial stiffness to our cars, especially if tied into the rockers and used in conjunction with something like the rocker to TC brace pictured below. If done well, I think this could resist some of the 'hinge' action of the rocker to firewall connection, allowing the rockers to do their job a bit better

 

jeffery-R1-E028.jpg

 

Anyone have thoughts on this?

 

Like this?

IMG00107-20110925-1559.jpg

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Bj, yours is the inspiration for what I'm doing! What size tube did you use from rocker up and from rocker over to tc? Did the rocker to frame horns have a bend in it? Did you use any plate on the rocker to weld the tube to?

Sorry for all the questions.

thanks

Lewis

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Found this old article in a file amongst my z 'stuff'. Thoughts on this approach? Can't locate the magazine it came from only a semi-decent copy I had made.

 

That's interesting. The tube in the rockers seems to be overkill to me since the original rocker is pretty stout. I'm in the process of doing something like that but with more, lighter weight tubing.

 

I would really like to figure out a way to measure the torsional rigidity of a chassis. I have seen some measurements of spaceframes expressed in lbs/degree. I suppose if the car were heavy enough you might get a part of a degree of twist before it lifted the wheels off the ground. I might experiment with this when my Z is rolling again.

 

Here are some other ideas.

 

Miata test

 

Apparently they test this in Australia as part of certifying some cars(?) 4000nm/degree is the standard.

post-11686-014961300 1321197784_thumb.jpg

Edited by Snailed
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There is a good bit in the book "Think Fast" about testing torsional rigidity, which is done in the same way as the race car in the AU picture. By testing rigidity at the hubs, you include suspension component and bushing compliance, which is not accounted for in the Miata test. It is a pain in the ass to do it right, and while that Miata test shows an obvious improvement, it's tough to quantify how much of an improvement without more accurate measurements, and it skips the testing of control arms, etc. So you can say "it got better" but not "it's as good as it needs to be to run the spring rates that I want to run".

 

The thing that is missing in 1 tuff Z's links is any attachment to the suspension attachment points. I like the rocker mods, but they don't do anything particularly useful in and of themselves. They need to be tied in to where the loads enter the chassis or where the chassis is weak. This is the reason why cages that don't attach to the strut towers aren't nearly as useful as ones that do. Ultimately all loads go into the suspension pickups. While a cage that doesn't take this into account might make you safe in an accident, a large part of the benefit to a cage is that it allows the car to respond accurately to things like heavier spring rates. If you have a cage that attaches to the wheel well as in the articles, or with backstays that attach to the rear of the car and miss the strut towers, then you've added the weight of the cage, but haven't used it to stiffen the first thing that is going to flex, which is the strut towers. You might make a large change in spring rate and feel no difference in the balance of the car, because the chassis is flexing instead of allowing the spring to do its job.

 

That said, if you're going to build a roll cage, it's going to tie in near the rockers. I think it makes a lot more sense to hit the rockers with the cage than to just attach to the floor, because the rockers are much stronger than the floor. The cage is likely to be stronger and stiffer than the rockers that it attaches to. That being the case, then I think modifying the rockers is worthwhile. If you're just looking to make the chassis stiffer and not looking at a full cage, then I would not start with modifying the rockers. But if you're going to tie the rockers into the suspension, then it makes sense to do the rockers. I'd do STB's first, then a 4 pt roll cage that mounts to the rockers (this is really where you need to know how far you're going to go). If you are going to then add bars from the rockers to the upper frame rails or the TC buckets, then reinforcing the rockers makes a lot of sense.

Edited by JMortensen
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Its not steel.... My goal is under 2500lbs. Which i should be. If i get around 2200-2300 ill be extremely happy. No ac/heat, all glass minus front windshield is lexan, no interior, etc you get the idea. My friend is in the process of making my door shells, hatch and hood in carbon fiber. He builds lemans prototype chassis and he assured me he can get all four items under 15lbs so there is a possibility of getting close to 2000lbs...

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