Fastback inner rockers

[Mach1 Driver 560]

On 1/9/2019 at 4:23 PM, Ridge Runner said:

A buddy had a 72 plymouth duster in highschool ,if you opened the door and powerbreaked you could not close the door untell you let off the pedal ,there was a lot of twist in that body ,you could see it 

My Dad worked in maintenance for Chrysler for 42 years at the LA assembly plant. When my twin brother and I graduated from High school, Dad got him a job at the plant. He would drive the cars from the end of the assembly line to the parking lot. One day he was in a 440 with a 6 pack driving along the outside road of the lot beside a chain link fence. He jumped on it and the car torqued over so much he side swiped the fence. His supervisor grinned and said "they've got a lot of torque don't they?". He knew Dad and wrote it up as a stuck throttle. I think he experienced a little of that twist you mentioned.

[aslanefe 333]

You need to look at this topic in 2 different angles. First, "sagging" of the middle of the car,; bending at the middle and floor in between front and rear tires getting closer to ground. Second, "twisting" of the body; right rear and left front of the car higher or lower than left rear and right front of the car. A solution that works best for reducing "sagging" may not help with reducing "twisting", and a solution that works best for "twisting" may not help with "sagging". Adding vert rockers (doesn't matter if you used your method or not) will reduce "sagging". I can't give you a number without modeling the whole car, loading the model, doing stress calculations etc but I can tell you that adding vert rockers will add negligible amount of torsional stiffness to combat the "twisting".

Are you trying to reduce "sagging" or "twisting"?

[Rich Ackermann 174]

I have only replaced an inner rocker in my 73 vert. It was a lot of work and a lot of spot welds. They were not from Dynacorn and were three pieces welded together. On a 73 vert you have to use coupe/fastback floors which required modification to the lip on the outer edge. I am now replacing the floor pan in my 70 Mach and was planning to use SFCs, but now you have me thinking. Look forward to following this thread closely.

 

 

[RPM 1,190]

3 hours ago, aslanefe said:

  I can't give you a number without modeling the whole car, loading the model, doing stress calculations etc but I can tell you that adding vert rockers will add negligible amount of torsional stiffness to combat the "twisting".

That is not correct. I'm not any engineer, but I play one in my shop. All I know is the difference in how much my car flexed when jacking it up from the side, before and after adding inner rockers. 

Before adding the inner rockers, it would flex several inches when jacked from the rear torque box before the front suspension stated rising. After adding the rockers (and the one piece seat riser) the chassis has no visible flex when jacking. 

[aslanefe 333]

4 hours ago, RPM said:

That is not correct. I'm not any engineer, but I play one in my shop. All I know is the difference in how much my car flexed when jacking it up from the side, before and after adding inner rockers. 

Before adding the inner rockers, it would flex several inches when jacked from the rear torque box before the front suspension stated rising. After adding the rockers (and the one piece seat riser) the chassis has no visible flex when jacking. 

I am an engineer, have a BS in Aeronautical Engineering and have worked on aircraft structures for many years.

As you said in your post, the twist on your car was reduced because of the one piece seat riser; strategically placed tie between right rocker to left rocker reduces twist. The subframe connectors don't help much with twisting, they will help with sagging. A roll cage tied to rockers will help the most for twisting. That's why verts have additional rockers for sagging, and one piece seat riser and the plate on the bottom for twisting. The roof on coupe and fastback is for twisting and sagging. You used both vert rockers and seat riser, so that is the best bang for the buck (or less additional work and weight) for both sagging and twisting.

[Mach1 Driver 560]

9 hours ago, aslanefe said:

You need to look at this topic in 2 different angles. First, "sagging" of the middle of the car,; bending at the middle and floor in between front and rear tires getting closer to ground. Second, "twisting" of the body; right rear and left front of the car higher or lower than left rear and right front of the car. A solution that works best for reducing "sagging" may not help with reducing "twisting", and a solution that works best for "twisting" may not help with "sagging". Adding vert rockers (doesn't matter if you used your method or not) will reduce "sagging". I can't give you a number without modeling the whole car, loading the model, doing stress calculations etc but I can tell you that adding vert rockers will add negligible amount of torsional stiffness to combat the "twisting".

Are you trying to reduce "sagging" or "twisting"?

Yes I'm an electrical engineer and get the difference. I don't have any particular complaints about the chassis as my car is in good shape- at least for a 50 year old sheet metal box. It lived with me for 46 years in southern California where it seldom rains and it only has 77k on the odometer, so its well preserved. As you say, inner rockers give the most bang for the buck. BTW, Bob (RPM) has a fastback, inner rockers and a roll cage so he has the best possible combination. I would just like to stiffen mine up a bit as I intend to almost double the hp (that's pretty easy on a 351w 2v). I'm like Bob and would rather work on the the car than drive it- although driving it is fun.

I have only seen one test done on a classic Mustang and it was for "twist" on a car with SFCs. As you indicated it did absolutely nothing (zero) for twist. We have an occasional poster here whose car is a self proclaimed rust bucket with 500 hp. He said SFCs turned it from a wet noodle into something more manageable, and said it was the best modification he ever did. Even if SFCs haven't been tested there are lots of people who claim that their car "feels" stiffer and better. I'm guessing that not all of this is placebo. I may do both, although I am also thinking about cutouts with side exits and SFCs would get in the way. We'll see.

Hey I see you're in middle GA, and I'm north of Atlanta- we should get together some time.

[aslanefe 333]

If I had your car and wanted to stiffen it, I would add vert one piece seat riser and subframe connectors (welded to floor and rails).

Adding vert rockers the way you described above with an L at the bottom instead of without an L, will have negligible effect in sagging and negligible (if not at all) in twist.

As you rather work on it, use the L bracket method you sketched because that will take longer to install (and you have to custom make the inner rocker to go over the L).

A well designed SFC can help some with twist; just bolting a 2x3 tube from front rail to rear rail on a rusted car with thinned torque boxes, rockers etc mill make a significant difference on that car; but it will not do anything (may be negligible) to your car (for twisting).

My comments are for a car with good metal (no rust damage).

I'll PM you if/when I come up north, I am about 100 miles south of Atl. Usually busy in the shop during the weekends and after work so I do not make leisure trips much.

[RPM 1,190]

4 hours ago, aslanefe said:

I am an engineer, have a BS in Aeronautical Engineering and have worked on aircraft structures for many years.

As you said in your post, the twist on your car was reduced because of the one piece seat riser; strategically placed tie between right rocker to left rocker reduces twist. The subframe connectors don't help much with twisting, they will help with sagging. A roll cage tied to rockers will help the most for twisting. That's why verts have additional rockers for sagging, and one piece seat riser and the plate on the bottom for twisting. The roof on coupe and fastback is for twisting and sagging. You used both vert rockers and seat riser, so that is the best bang for the buck (or less additional work and weight) for both sagging and twisting.

Hmm, I think you've got me there. I envy you engineers, seriously. Since I did the one piece seat riser and inner rockers at the same time, I kinda forget their value working together, and give credit to the inner rockers. In addition to inner rockers and one piece seat pan, I have SFC and an 8 point roll bar. 

[Mach1 Driver 560]

1 hour ago, aslanefe said:

If I had your car and wanted to stiffen it, I would add vert one piece seat riser and subframe connectors (welded to floor and rails).

As you rather work on it, use the L bracket method you sketched because that will take longer to install (and you have to custom make the inner rocker to go over the L).

I'll PM you if/when I come up north, I am about 100 miles south of Atl. Usually busy in the shop during the weekends and after work so I do not make leisure trips much.

Yeah you're right about the one piece seat riser, but the dang things screw-up the shift console. I have a very good sheet metal shop that would do the inner rockers parts so I just have to drill for plug welds and then weld them in place (along with everything else). 

Every 2-3 years we go down to Florida with our Vette club and may pass by, if so I will PM you as well.

[Ridge Runner 1,112]

20 minutes ago, RPM said:

Hmm, I think you've got me there. I envy you engineers, seriously. Since I did the one piece seat riser and inner rockers at the same time, I kinda forget their value working together, and give credit to the inner rockers. In addition to inner rockers and one piece seat pan, I have SFC and an 8 point roll bar. 

I did a 8 point cage in a 65 fast back road racer i built at the end of the 80s ,that thing couldnd flex under any condition . It tied the front anr rear frame rails and rockers together . 

The last i heard ,it was in france and raced 

[Mach1 Driver 560]

16 minutes ago, RPM said:

Hmm, I think you've got me there. I envy you engineers, seriously. Since I did the one piece seat riser and inner rockers at the same time, I kinda forget their value working together, and give credit to the inner rockers. In addition to inner rockers and one piece seat pan, I have SFC and an 8 point roll bar. 

Wow, inner rockers, SFCs, seat pan, and a roll bar!! That's about as good as it gets Bob. My brother-in-law had a 64 T-bird that got a flat. He jacked up one rear corner with a bumper jack and both sides came off the ground. Then it would teeter on the jack and fall over off the jack. He had a heck of a time changing the tire. I hope yours isn't that rigid ;)

[Mach1 Driver 560]

2 hours ago, aslanefe said:

If I had your car and wanted to stiffen it, I would add vert one piece seat riser and subframe connectors (welded to floor and rails).

Whose SFCs do you like?

[RPM 1,190]

None of them. 

51 minutes ago, Mach1 Driver said:

Whose SFCs do you like?

None of them. They all probably work the same, but I don't care for their looks. I'd think someone by now would make them which resembled the front and rear rails. My first mod was a set of Competition Engineering SFC. If I do another Mustang I'll probably make them.

EDIT: Oops, sorry, thought that was for me.

[aslanefe 333]

37 minutes ago, Mach1 Driver said:

Whose SFCs do you like?

Yes, you have to trim your console for one piece seat pan.

I am not up to speed on who makes what kind of SFCs; and do not have much time to search the net and find all of them (I have to work 8 hrs a day during the week which cuts into my time in the shop or other car related stuff). If you are up to speed on SFCs, post me links to them and I will tell you which one I would use (as is or with a little modification) that will give you the most with vert rockers (or your design additional rockers) and without one piece seat risers on your car.

My house and shop is about 3 miles off of I-75.

 

[RPM 1,190]

45 minutes ago, Mach1 Driver said:

Wow, inner rockers, SFCs, seat pan, and a roll bar!! That's about as good as it gets Bob. My brother-in-law had a 64 T-bird that got a flat. He jacked up one rear corner with a bumper jack and both sides came off the ground. Then it would teeter on the jack and fall over off the jack. He had a heck of a time changing the tire. I hope yours isn't that rigid ;)

Ya, each project of stiffening the chassis was a "while I'm at it" deal. They began after I tore down the car in circa 1998 and were done before I started driving it again in 2016. So I was never able to compare each additional mod.

Ha! I doubt I'd have that problem with the jack, but I've never tried it. 

[aslanefe 333]

1 hour ago, RPM said:

Ya, each project of stiffening the chassis was a "while I'm at it" deal. They began after I tore down the car in circa 1998 and were done before I started driving it again in 2016. So I was never able to compare each additional mod.

Ha! I doubt I'd have that problem with the jack, but I've never tried it. 

With additional inner (vert) rocker, 8 point roll bar (tied to rockers or very close to rockers) and one piece seat riser; I would say SFCs on your car is just added weight with no or negligible amount of gain.

[RPM 1,190]

12 minutes ago, aslanefe said:

With additional inner (vert) rocker, 8 point roll bar (tied to rockers or very close to rockers) and one piece seat riser; I would say SFCs on your car is just added weight with no or negligible amount of gain.

Oh I do agree with that. But they were my first mod and I don't think I'd spend the time to remove them unless I went to side exit exhaust. Hmm...

[latoracing 256]

One interesting thing with 69-70 convertibles, they do not use a one piece seat riser from the factory. I don't quite understand Ford's reasoning on this. When I was upgrading my '70 FB I looked everywhere for a specific one for that year, they didn't make them, so I hacked up an earlier model to fit. 

[aslanefe 333]

6 hours ago, Mach1 Driver said:

Whose SFCs do you like?

Let me see if I can explain with words.

Square tubing or a hat section with cross section of the rails and thickness of the rails, tie the front rail to rear. Weld the connector to floor also.

You know the front floor cross member (link below), add 3-4 more of braces like those about a foot apart in the transmission tunnel. Weld them to floor and weld the ends to your SFCs. Then weld tubing or hat section from the SFC to rockers following the braces you added to transmission tunnel. You can use something like 1x4 tubing for this so they are not sticking too much under the rockers. This is how I would do if I didn't want to cut the center console. If you put inner rockers, just tie the 1x4 tubing to them.

Front floor cross member

[Midlife 814]

Just reading this thread, I think y'all are simply off your rockers.

 

Someone had to say it...

[Mach1 Driver 560]

8 hours ago, RPM said:

Ya, each project of stiffening the chassis was a "while I'm at it" deal. They began after I tore down the car in circa 1998 and were done before I started driving it again in 2016. So I was never able to compare each additional mod. 

I was thinking today that I should ask here for help in a "design of experiment" to test the difference made by adding various structural components as I restore my car. OK engineers here is your chance for input. The first step is to review the only other chassis twisting test: https://www.stangnet.com/mustang-forums/threads/torsional-rigidity-test-67-coupe.723029/. This is a long thread running over 5 years with a summary in post #79. Maybe someone can come up with a "sag" test too.

[Mach1 Driver 560]

8 minutes ago, Midlife said:

Just reading this thread, I think y'all are simply off your rockers.

Someone had to say it...

thank you so much, and it explains so much

[Ridge Runner 1,112]

19 minutes ago, Mach1 Driver said:

I was thinking today that I should ask here for help in a "design of experiment" to test the difference made by adding various structural components as I restore my car. OK engineers here is your chance for input. The first step is to review the only other chassis twisting test: https://www.stangnet.com/mustang-forums/threads/torsional-rigidity-test-67-coupe.723029/. This is a long thread running over 5 years with a summary in post #79. Maybe someone can come up with a "sag" test too.

I had a 65 GT convertible that had been hit in the front ,it bowed the driver side rocker to where the door hit the quarter at the top. I bolted the front of the car and the rear of the car to the floor with chains and with a block of wood and a floor jack tried to push up the bowed rocker ,it would not hardly bend . It finally broke the bolt in the front of the car and sounded like a shotgun when it did ,the bow was still there . I wound up cutting a piece of the inner rocker out so i could get in there and heat up the flat plate inbetween ,it took a lot to do ,even with heat to move that rocker and it only had to move a 1/4 inch so there isnt much chance of saging on a coupe or fast back with the convert inner rockers intstalled . I then welded the inner piece closed again 

[aslanefe 333]

3 hours ago, Mach1 Driver said:

I was thinking today that I should ask here for help in a "design of experiment" to test the difference made by adding various structural components as I restore my car. OK engineers here is your chance for input. The first step is to review the only other chassis twisting test: https://www.stangnet.com/mustang-forums/threads/torsional-rigidity-test-67-coupe.723029/. This is a long thread running over 5 years with a summary in post #79. Maybe someone can come up with a "sag" test too.

I have read that twist test before, he did a good job. To test the sagging in your garage/shop is more difficult and costly, you will need fixtures and hydraulic equipment. The rockers do a real good job for sagging because of that you need to be able to apply a lot of force (or use a very long lever and be very heavy so when you sit at the end of the lever, you can exert a lot of force) to sag the car enough to measure it or very good equipment to measure minute amounts of sagging.

[aslanefe 333]

4 hours ago, Midlife said:

Just reading this thread, I think y'all are simply off your rockers.

 

Someone had to say it...

You know, we can't check shorts everyday.