How to increase stiffness against twisting on a convertible-subframe

[hannes_slanec]

Building a 68 Impala Pro Touring and would like add twin-hairdryers on a 540 - I expect around a 1000HP.

In that performance-range of course some consiederation should come to the subframe.

In any other car one could say lets go for a 10-point rollcage or something similar but that would make a convertible totally pointless.

What are the technics to improve/modify a subframe.
Some illustrating pics would be lovely.

I saw an aftermarket frame for a C2 or 3, and the left and right frame rails was done from small tubes in order work like the framework of a crane, that looks quite strong to me and I would like to do so too.

[hannes_slanec]

No answer in a Pro-Touring-Forum???

Thats sad.

[derekf]

I'm not an expert at anything so don't take me as gospel, but I have read a lot of threads here and I did stay at a Holiday Inn Express once.

First off, I was under the impression that Impalas were a full-frame car, not subframed? I don't know if by 68 they'd moved away from that wishbone design to a perimeter frame though.

Searches expire, so I can't do it for you and post a link, but it seems like in other similar discussions the real outcome is that you can't get torsional stiffness like you're looking for without adding some height to the frame either in the form of a cage or at the very least some very tall door bars - the tradeoff between how much inconvenience you're willing to accept versus how much strength you're wanting. "Torsional" is a good search term, and the contents of the Advanced Technical forum have quite a few suspension/torsional rigidity threads.

Hope this helps.

[psy4s]

OK, Just a thought, You know how the new Short Sports Removable Hard Top cars have a Roll Bar (Cosmetic) set up that peaks just above the seat? Why not design a Cage that crosses actually just behind the top of the seat? It will not really be seen, unless you walk up to the car.
It may not be as strong as a tall cage, but if it were made from the correct gauge tube, it should get a lot of the twist out. It could even give you a good place to connect a good Race Shoulder Belt System. A cage does not have to be as tall as a Hard Top. I drew one up, but I do not have a way to scan it in, I am sure you could do it this way.:smoke:

https://static1.pt-content.com/images/pt/2010/07/smokinape1MA121312130002-1.gif

[hannes_slanec]

OK, Just a thought, You know how the new Short Sports Removable Hard Top cars have a Roll Bar (Cosmetic) set up that peaks just above the seat? Why not design a Cage that crosses actually just behind the top of the seat? It will not really be seen, unless you walk up to the car.
It may not be as strong as a tall cage, but if it were made from the correct gauge tube, it should get a lot of the twist out. It could even give you a good place to connect a good Race Shoulder Belt System. A cage does not have to be as tall as a Hard Top. I drew one up, but I do not have a way to scan it in, I am sure you could do it this way.:smoke:

https://static1.pt-content.com/images/pt/2010/07/smokinape1MA121312130002-1.gif

I really dont want to loose the stock look except the lowering or the rims.
It also will not be an race car but a street car with big power.

Maybe a 540 with Turbos is still too much for a convertible....

[wmhjr]

You can see my build on this site as well. I'm not running that much hp - only about 600. However, unless you're going to add a cage you're pretty much out of luck. Putting something like a Schwartz chassis under the car will help somewhat, but the upper perimeter is what you need to truly stiffen a perimeter frame car - and I found no way to do that without significantly changing the look of the car. You can add all the frame reinforcement you like under the car, but you can't fix the torsional flex characteristics, because struts, gussets, etc will all be on the same plane.

I sure with there was a better answer but if there is, I never found it. For bigger cars like ours, convertibles are flexy as all getout. When I lift mine you can see the door seams, etc slightly change. Heck, when I fasten my convertible top you can notice a difference if you look closely. There's a reason the top performance cars aren't verts.

But we sure look good!:fingersx:

[BMR Tech]

I can't remember where I saw this but I remember reading a thread somewhere from a convertible owner looking for the same solution. I honestly don't even remember what kind of car it was but he made a short, gusseted hoop both under the dash and inside the trunk to add some height to the frame and minimize torsional flex. I don't know how successful he was but it looked pretty good and was completely hidden. This might be a possibility.

I also saw someone install a late model roll bar from a convertible PT Cruiser (yuck :eek:) in his 68 Camaro convertible. It looked surprisingly good when it was color-matched to his car. They are a functional bar and built fairly stout (doubt it would support a 68 Imp though) but if done properly may actually provide some torsional resistance. Not sure how it would look in an Impala though....

Just did a quick search to show you what the bar looks like.

http://www.automarket.bg/wallpaper/chrysler_PT-Cruiser_Convertible_1.jpg

[frojoe]

I can't remember where I saw this but I remember reading a thread somewhere from a convertible owner looking for the same solution. I honestly don't even remember what kind of car it was but he made a short, gusseted hoop both under the dash and inside the trunk to add some height to the frame and minimize torsional flex. I don't know how successful he was but it looked pretty good and was completely hidden. This might be a possibility.

I also saw someone install a late model roll bar from a convertible PT Cruiser (yuck :eek:) in his 68 Camaro convertible. It looked surprisingly good when it was color-matched to his car. They are a functional bar and built fairly stout (doubt it would support a 68 Imp though) but if done properly may actually provide some torsional resistance. Not sure how it would look in an Impala though....

Just did a quick search to show you what the bar looks like.

http://www.automarket.bg/wallpaper/chrysler_PT-Cruiser_Convertible_1.jpg

Adding additional height to the more or less flat-horizontal-plane of a full frame is the first step in increasing the torsional rigidity, however only adding this height in one plane (side-to-side) has hardly any effect. A small hoop under the dash, as well as hoop under the rear package tray (or convertible top storage area) only really help the side-to-side bending stiffness of the frame if you were to look at it head-on.

In order to truly improve the torsional stiffness by adding such dash/trunk hoops, they'd need to be triangulated into the perimeter frame front-to-back, and likely would require significantly-long down tubes like the dash hoop to the firewall, then ideally from the firewall to front suspension, and the trunk hoop going forward to the c-pillar area. Of course these tubes would greatly impede the stock look, and a cage might as well be installed.

This is part of the reason a simple rollbar has downbars, 1) to prevent the main hoop from just flopping forward or backward when the weight of the car is on it upsidedown, but 2) to increase the total "volume" the cage occupies. The larger the cage is, assuming adequete welds and gussets, the more rigid the entire structure is.