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To start, I've got an 89 bird with T-tops. Chassis stiffening is on the books right now.
The one thing I am not doing is converting it to hard-top. I think T-tops are the $hit and I almost never drive with them installed.
I've done some searching, and I can't seem to find out if there is any potential improvement for the T-top roof. Will I see any improvement if I build additional bracing in the structure of the roof? Would a set of bars running across the gap at the edge of the roof-line add any useful rigidity?
My idea is to add some bracing up the Pillars with 1/2" square tubing and additional spot welds on the seams, build additional bracing from there along the width of the roof and then as far out as possible under the middle bar. This is the blue lines. The yellow lines would be a removable set of brace bars as close to the edge as possible without getting in the way of the T-tops. My goal is roof stiffness, or at the very least a cool set of grab handles. Has anyone done something similar?
I already have a set of Heidts inner, and an outer set I made. I'm just wondering if this will give more support on an extra plane, since the sfc's are all down under the floor and a stiffer roof would make more of an A-frame structure of the car.
IMO this will not get you what you are looking for. Unless you can triangulate your additions, you will just be adding weight, up at the top of the car where you don't want it. When you apply force to the parts of the car that are in contact with the road, the majority of the flexing happens along the longitudinal axis of the car, your brace will just be flexing with the rest of the car. If you have a good (one that triangulates) strut tower brace, you will be way further ahead. If you feel you must add support to the rear of the car, consider a main hoop with supports to the rear upper shock mounts, and has a good cross brace.
ps: 1/2 square tube is not the best choice either, IMO....
To start, I've got an 89 bird with T-tops. Chassis stiffening is on the books right now.
The one thing I am not doing is converting it to hard-top. I think T-tops are the $hit and I almost never drive with them installed.
I've done some searching, and I can't seem to find out if there is any potential improvement for the T-top roof. Will I see any improvement if I build additional bracing in the structure of the roof? Would a set of bars running across the gap at the edge of the roof-line add any useful rigidity?
My idea is to add some bracing up the Pillars with 1/2" square tubing and additional spot welds on the seams, build additional bracing from there along the width of the roof and then as far out as possible under the middle bar. This is the blue lines. The yellow lines would be a removable set of brace bars as close to the edge as possible without getting in the way of the T-tops. My goal is roof stiffness, or at the very least a cool set of grab handles. Has anyone done something similar?
Huh, interesting idea... it makes me wonder if you pulled the headliner if you could find room to weld in maybe some 1" tube the whole way around the perimeter of the t-top opening without losing much headroom. I've also had the thought of doing something like this with a rollcage, running 2 bars down the middle instead of around the outside.
How tall are you? My biggest concern is that you're going to end up with this stuff invading your head room making for a dangerous situation in a side impact. I'm 6'4" and I typically lower my seats (I've had a hardtop car and 3 t-top cars, the hard top I couldn't sit in with a helmet without lowering the seat, and the t-top cars typically I would smack the roof pannels in a bump), and I was once in an accident where I was t-boned and put my head into the bar between the t-tops and even that ended up giving me a concussion.
Originally Posted by 427seven
IMO this will not get you what you are looking for. Unless you can triangulate your additions, you will just be adding weight, up at the top of the car where you don't want it. When you apply force to the parts of the car that are in contact with the road, the majority of the flexing happens along the longitudinal axis of the car, your brace will just be flexing with the rest of the car. If you have a good (one that triangulates) strut tower brace, you will be way further ahead. If you feel you must add support to the rear of the car, consider a main hoop with supports to the rear upper shock mounts, and has a good cross brace.
ps: 1/2 square tube is not the best choice either, IMO....
Triangulating will make it stronger and you don't want to add weight anywhere, but the roof is exactly where you want to add rigidity. It's easy to add a lot of structure to the bottom/floorplan of a car, where just a little bit of structure added to the roofline will make a great big difference in rigidity (the farther the structure is apart the more rigidity it will add. The only reason that subframe connectors are the popular way of stiffening a chassis is that they are a way you can do it easily without messing up the look of the car.
Thanks for the input,
I've got the interior out, with an abs headliner and some clever routing, I think it could happen. I'm not too concerned with headroom if I do this, or a roll cage since I'm 5'8.
My goal is of course a roof that is at least as stiff as a hardtop. With ~1" round tubing, The roof would essentially be three rectangles, and if properly supported by the pillars, I'm curious as to what types of flex can I fight.
My thoughts: (none of these are my car)
It would certainly help with flex like this, where the roof is under compression. This is what I was picturing initially.
This is what I'm picturing as far as torsional forces of the body. Cornering for example. If I've got this right, then an X shape in the roof would be ideal to channel the force to the opposite side of the car and keep the roof in square. I can't keep T-tops with an X shape, so I would need to use the rectangle approach, but that should still be better that an open H shape(factory T-tops).
IMO I think you might be overthinking this. Rather than stiffen up the T-Top area, you would want to just redirect that stress away from the T-Top area during hard cornering, or severe sixty foot. Normally a three point strut tower brace, along with welded in sub frame connectors will take care of the issue. You can also ad a roll bar (no need for the full cage), just a two point roll bar tucked behind the T-Top opening for additional strength to be used with the STB and SFC's, you won't need to worry. Hell, even without the roll bar I wouldn't worry with just the STB and SFC's. I'd be more concerned with securing the T-Tops to their latch lol...
Too much is made of the "weakness" created by the addition of t tops, but the only reason a hole in the roof is significant is bc the chassis is inadequate to begin with. You have to address the underlying issue. Arguably the roof isn't needed at all for a rigid chassis, many race cars prove that (even top fuel cars don't have a roof). With our cars, if you want to have a t top or not, you have to work with the chassis. Rob's comment re-enforces my original suggestion, and I have attached a diagram from Herb Adam's "Chassis Engineering". Look closely at fig 12-13 and 12-14, in both diagrams the "car" has a sheer plate (like the roof inn your car) but the strength difference is from triangulating support off the front and rear suspension points. It would, in fact, be the same if there were no roof at all.
My car is a t-top but it also has a full cage in it and isn't street legal. I've never lost a t-top going down the track.
Good weld in SFC stiffen up the chassis on even a daily driver street car. Adding an anti-roll bar to the rear suspension will keep the chassis from twisting under acceleration even more.
I saved these pictures from here on the forum a few years back. These are the braces Cars and Concepts added to the roof of an F-body when they cut it to add t-tops.
Being thin, they should take up close to zero headroom, and they should not weight too much. But they would indeed add weight, and at the worst possible place height-wise.
While I would not expect these to do a measurable amount of good for whole chassis torsional rigidity, I had thought if I stumbled across some in a bone yard, that I might think about adding them to make the roof more stable, and perhaps reduce a t-top squeak or two. And God forbid a roll-over, they might add a modicum of extra strength.
But I suspect sub-frame connectors would provide a far greater bang for the buck at stopping these noises, as well as adding a lot of help everywhere else.
What about door bars something like (Miata in this case) this in addition to your current subframe connectors? Basically tying close to the same points together(as SFC) but on a different plane??
That looks to function on the same principle as the door bars on a six point cage.. I didn't know anyone put standalone door bars in, I wonder if there is enough structure in the jambs of these cars to see an improvement. Here is the 4 pt bar I put in
I may just have to make a set of door bars if my current setup doesn't satisfy me.
two pieces isnt enough to stop the twisting motion, you'd have to build pretty much an exo-skeleton brace. it would have to be your two pieces, then have additional pieces to come up and over the t-bar. probably would be adding a decent amount of weight (unless you know someone who can weld and bend aluminum tubing) but i like the concept idea of that you could take it off when youre not racing tho. if it were me though, i'd prob just do STB and a similar brace in the rear inside the hatch.
The guys here in Tallahassee, building new Trans Ams are making T-Top cars from the new 6th gen fbodys. They insert a thick chunk of billet aluminum, wrapped in carbon-fiber, in the altered roof.....of course I suppose these new bodys are pretty ridgid to begin with (compared to a third gen).
The end result is nearly $100k or more, but they are built to handle the 1000hp LS.
My car is a t-top but it also has a full cage in it and isn't street legal. I've never lost a t-top going down the track.
Good weld in SFC stiffen up the chassis on even a daily driver street car. Adding an anti-roll bar to the rear suspension will keep the chassis from twisting under acceleration even more.
Do cages make cars not street legal? If you just mean engine mods, he's got MN plates. Unless he's running nitromethane or even worse, has window tint, he's fine.
@427sevenComment covered alot of this. The best way to test "will this make my car stronger.".. get a old large match box sleeve or similar cardboard box. Buy a box of Popsicle sticks. Hold the box on the corners and see how it twists (channeling the wright brothers here, this is how they invented the twisting wing control system except it was a bike tube box ) Sit it on the table and put something under the corner of it and press down note how it twists. Then glue a Popsicle stick where you would want to reinforce your car, repeat the twist test and see if it helped. One reason the inner frame connectors seem to work better than the outer is it hangs lower thus triangulating the twist force into the opposing corner. You can cut the box up and make it basically the same shape as the car if you want to get real detailed. There is also a 3d printed shell on thingverse that you could use.
You'll find long run sub frame connectors, tied into a rear hoop with a x or cross support with rear support bars would take most if not all of the flex out of the car. Door bars tied into the front mount of the subframe connector would make it even more ridge. But personally I wouldn't run a cage on a road car just because I'm already too tall for these cars as it is and I would hit my head on the cage In fact I can only drive t-top thirdgens as my head touches the head liner on the hard top.
The one thing the T-top did cause it it moves the force from the a-pillar which would normally conduct through the upper window frame over to the T-bar. which when it t's back to the C-pillars you have a lot less metal carrying the load, which does translate into higher fatigue and stress thus more twisting of the frame. So if you consider lifting the front driver corner of the car, the force is conducted up the A-pillar, to the T-bar, which tries to twist and applies a lift force on the driver c-pillar and a downward force on the passenger c-pillar. With a solid roof this force for the most part would stay in the window frame and only apply a upward force on the driver c-pillar. (obviously forces are conducted across the windshield and tries to lift that side of the car also but I'm simplifying) In a race car the lower chassis is strong enough you don't need a roof (and typically doesn't have door gaps in the structure) so its structural box is below the roof and only nominal stresses are placed in the roof structure.
You can run a cage on the street if you want. Cops don't really like it because with a cage, you should be also wearing a helmet to keep your head from hitting the tubes.
Mine just happens to be a race car and is not street legal but I still have functioning t-tops and without any body flex, I have never lost a t-top..
The key to making a car stiffer with little or no flex is triangulation. My cage has triangulation all over the place. It also makes it so still that you probably wouldn't want to drive it on the street. I don't need to worry about handling in cornering as I have zero body roll.
There's always a fine line for what works best on the street and what works best in a race car. A street car does not need all sorts of race car parts. As I've already mentioned, SFC and a rear anti sway bar are all that's really required to make a street car better. A simple 6 point roll bar helps but it also makes the back seat unusable.
You have the right idea and a great start. You will notice less longitudinal flexing now. With a given amount of torque and the rear half braced, the energy will want to move to the weakest point. You mentioned the door brace, and that is exactly what it's for. As the rear is now braced, the weak spot is the front half. If you can tie the new strength you have in the back to the front the energy stays in the suspension/tires instead of getting used up twisting the car. The closer you can get to tying the front suspension mounts to the rear, the less flex you'll have.
As the hoop is still open (as is the rear braces), there still is some room to reduce twisting by cross bracing those two areas.
very rigid, rear hoop and bracing tied into the front subframe and firewall
Thanks for all your help, I am actually in the process right now of laying out more triangulation for my roll bar. I really like that door bar design you have in the picture there. I might try to copy it.
Thanks for all your help, I am actually in the process right now of laying out more triangulation for my roll bar. I really like that door bar design you have in the picture there. I might try to copy it.
Just fyi it's very tight under the dash so it's a lot of monkey-ing around to get it to all fit, but it's worth it....
I have made a little progress on my roll bar extension. Started on the door bars. I am fairly certain these will not be track legal, but as this is a 100% street car that shouldn't be too much of a problem. Mainly, this is for chassis stiffness and the pure joy of chopping and welding. Space is pretty tight with my Subaru seats, which are wider than stock, crank windows, and factory arm rests. Fortunately, I have no heat or A/C, so the cavernous space under the dash makes my job a lot easier. I am just tacking it together with MIG for test fitting, but I plan to TIG the majority of the cage itself.
I had to put the door bars a lot lower to clear the arm rests. There will be additional triangulation from the main hoop to the door bar that will follow where the back of my seats sit. I've got a bit that goes from the lower part of the firewall up to the top portion, tied into the door bar. I had to put a drop in to clear the window crank, but I may put a 3/16" gusset plate in the gap that follows the sweep of the crank handle, that, with the triangulation of the door bar should be reasonably strong. There is no plate on that part of the floor yet, but there will be. Just need to decide if I want to run a tube along the firewall to the trans tunnel area on the driver side.
) Sit it on the table and put something under the corner of it and press down note how it twists. Then glue a Popsicle stick where you would want to reinforce your car, repeat the twist test and see if it helped. One reason the inner frame connectors seem to work better than the outer is it hangs lower thus triangulating the twist force into the opposing corner. You can cut the box up and make it basically the same shape as the car if you want to get real detailed. There is also a 3d printed shell on thingverse that you could use.
