Opinions on frame stiffening mods

[stephen wilson]

I was looking for some expert opinions on some additional frame tubes on my RF80. I am making some safety-ralated frame modifications, but would also like to increase chassis stiffness while I'm at it. I plan on running more "modern" spring rates and ride height.

Tube #1, with Blue tape. Does this tube add any stiffness? It competes the load path of the upper diagonal nicely, but the tube it's supporting is 1-3/8"x.080, with a 12" span between supports. It will add a small ammount of intrusion protection, but adds ~ .4 Lbs.

Tube #2, with Red tape. Stiffness benefit? There would be a second diagonal tube, forming an X. It has to be fairly low to clear the bodywork. 1"x.035 would add 1.7 Lbs. , and 1"x.049 would add 2.3 Lbs.

Thanks in advance.

[Brian]

In my opinion both tubes will add stiffness to a measurable degree. If it results in a performance improvement is another question, but I think both are worthwhile. Make sure you braze them in with a compatible nickle-bronze rod like Sifbronze No. 2. Make sure the X in the front doesn't interfere with your right foot. Later VD's have a tube there, and it can be in the way sometimes for those with manly man feet...

Brian

[S Lathrop]

I question the red tube as being desirable. It is adding stiffness but only to one side of the car. It may cause twisting in straight bump loads. My guess only.

The best improvement you could make is to add 1" wide .049 strips the inside of the entire bottom of the car. This will give you a 2" bonding surface for a belly pan. I would use .080 6061T6 aluminum, rivet and bond. You might add a few cross members between the lower rails as well. This will increase the bonding area and add stiffness as well.

You might look at the roll bar bulk head. It does not look like there is much structure there above the original frame. I cannot see in the picture if there is a cross member between the 2 upper cockpit rails, above the driver's shoulders. That area needs to be a fully triangulated bulkhead.

[stephen wilson]

Thanks for your thoughts.

Steve, it's not clear, but the Red tube is just a quick mock-up, there would be another duplicate tube to the Left front corner. My current plan is to carefully weld in a 4130 pan. The roll bar bulkhead is not completed yet, I still need the harness bar, headrest bar, diagonal(s), and rear bracing.

[S Lathrop]

Thanks for your thoughts.

Steve, it's not clear, but the Red tube is just a quick mock-up, there would be another duplicate tube to the Left front corner. My current plan is to carefully weld in a 4130 pan. The roll bar bulkhead is not completed yet, I still need the harness bar, headrest bar, diagonal(s), and rear bracing.

I think that the thick aluminum will give better results than thin steel. It for sure is more work. Aluminum is stiffer because it is 2x as thick as the steel. Stiffness is what you want.

[stephen wilson]

I'll have to consider that, though I was going to use .063" steel if that matters.

[S Lathrop]

I'll have to consider that, though I was going to use .063" steel if that matters.

Use .120 Aluminum then. It will weigh less.

[northarrowracing]

FWIW, Steve Lathrop understands as much as anyone about building safety and strength into a car. I have an older model he built and even with its age it feels like a very safe FF. If he said build a roll bar out of cardboard and tinfoil it would probably be great.

Ric

[RobLav]

No on both tubes. Add 1" steel sheet around inside perimeter of all floor tubes and use at least 090" bonded aluminum floor. I'd also pay more attention to the engine mount methods than adding tubes to the main chassis.

[stephen wilson]

FWIW, Steve Lathrop understands as much as anyone about building safety and strength into a car.
Ric

I would agree. I actually copied the upper cockpit bar from the Citation, though it's substantially different to fit inside the VD bodywork.

Steve, If I did the AL floor, would you omit the additional bars as Rob suggests?

Rob, I agree, the rigidity of the rear suspension mountings on an early VD leave much to be desired.

[Neil_Roberts]

I was looking for some expert opinions ...

Well, here's another one: Euclid was right.

Both of the new tubes will add stiffness and strength. The red tubes will add more stiffness if you add another tube to the aft end of each one that connects it to the tubing joint that is just aft of it. The location that I'm talking about is just forward and outboard of where the steering wheel goes.

If you want to continue this process, add a diagonal tube to the main roll hoop and an X brace from near the top of the main hoop to the rear top corners of the frame. Just make sure that the X will clear the engine.

Here is some sound reasoning about whether or not more torsional stiffness will improve performance. To that, I'll add that a stiffer frame by itself probably won't show up on the stop watch, but a small handling adjustment will produce a bigger change in the behavior of the car, and you are less likely to end up with a handling result that is opposite to your expectation.

On floor pans: steel is 3 times as stiff and 3 times as dense as aluminum. For the same floor weight, either will produce the same frame stiffness, assuming that the thinner steel floor does not buckle when it is loaded, and assuming that the floor is very effectively joined to the frame.

[chrisw52]

when bonding the floor to the frame, what do you recommend as a bonding agent (glue)

any favorite product you like to use?

[stephen wilson]

Thanks Neil, I also used your Swift frame analysis for reference. I was planning on the small tubes from the dash hoop you mention, and running the main hoop braces/diagonal to the rear corners. I don't know if an X will clear my Swift-style headers.

Any thoughts on tubing size for the front braces? I'd like to use 1"x.035 for weight savings.


P.S. Sadly, I had to Google your Euclid reference!

[S Lathrop]

I would agree. I actually copied the upper cockpit bar from the Citation, though it's substantially different to fit inside the VD bodywork.

Steve, If I did the AL floor, would you omit the additional bars as Rob suggests?

Rob, I agree, the rigidity of the rear suspension mountings on an early VD leave much to be desired.

I think the return for the added weight is not good. Looking at the engine bay might be way more useful.

I use a 3/16 aluminum plate that bolts to the water outlet on the head. I also have additional mountings on the head in addition to the 2 water outlet bolts. The plate then attaches to the roll bar. This may be all you need at the rear. This mount will be very close to the upper frame rails you have added.

[RobLav]

The blue tube connects two joints that are already triangulated. It might add a bit of stiffness, but the marginal benefit is probably not worth the marginal cost. In other words, the chassis "system" is nonlinear in gains and costs (weight and time) with (generally) decreasing marginal utility with each additional tube.

I'd also look at the rollhoop where it is NOT triangulated. If you take an upside down hit while going forward, the top of the rollhoop is going to bend backwards and the rollhoop braces are going to bend towards the floor. It's useful to look at each joint as if it were a ball and socket joint with infinite 3D movement.

The red tubes in X-brace will probably add stiffness, but again I don't believe the cost is worth the gain. Here, in side view, you have a very small acute angle between the front of those tubes and the top of the "truss" on each side of the chassis. The effective increase in stiffness is very small because of the trigonometry.

I guess my point to all this rambling is that you can continually add tubes and gain marginally smaller increases in stiffness. In the extreme end (which is always useful to examine when analyzing), you'll have a stiffer chassis with an infinite amount of tubes but weigh too much! You are not optimizing for one variable (stiffness). This type of problem is what we call in my field a multi-variable optimization problem.

[stephen wilson]

Rob, Do you mean triangulating the roll hoop above or at the upper cockpit bar? Turning the Blue tube vertical would prevent the bending towards the floor.

I was assuming the structure above that point was plenty strong, since I have two 1-3/8" hoops to resist bending, where there is normally just one with 1" forward braces.

[RobLav]

See attached Stephen.

What I'm talking about is the parallelogram you have in the red circle. That needs triangulated. You could add tubes along the orange line. Or, IMHO better, remove your existing rollhoop bracing and add tubes along the yellow line. If you do that, then adding along the green would help support the point load at the base of the rollhoop braces. I think this would be more effective and efficient use of structure nevermind safer.

[KodaBear]

Well, here's another one: Euclid was right.



On floor pans: steel is 3 times as stiff and 3 times as dense as aluminum. For the same floor weight, either will produce the same frame stiffness, assuming that the thinner steel floor does not buckle when it is loaded, and assuming that the floor is very effectively joined to the frame.

Unless I'm really misunderstanding what you are trying to say, your statement is wrong. If you increase the thickness of aluminum up to the point the sheet weighs the same as the steel sheet, the aluminum sheet would be many times stiffer than the steel sheet. If you double the thickness of the sheet the stiffness does not just double, it increases the stiffness by a factor of 8.

[stephen wilson]

That would be a more efficient use of structure, but my objective in moving the brace forward was to fully protect the drivers head. The load should feed through the "spreader" bars to the main hoop, which is supported by the back-stays. I will look at fitting in some triangulation. Possibly similar to your Orange line, but tying into the back-stay node?

[Neil_Roberts]

Unless I'm really misunderstanding what you are trying to say, ...

Your description is correct for bending stiffness of a beam, but the relevant loading condition for floor pan stiffness is in-plane shear.

[stephen wilson]

Do you think the added bending resistance would aid overall stiffness by resisting the warp mode ?

[stephen wilson]

I use a 3/16 aluminum plate that bolts to the water outlet on the head. I also have additional mountings on the head in addition to the 2 water outlet bolts.

Similar to the attached picture, which also has a lower AL plate mount? So you can get sufficient rear stiffness by stressing the engine?

[S Lathrop]

Similar to the attached picture, which also has a lower AL plate mount? So you can get sufficient rear stiffness by stressing the engine?

That car uses a Swift pan. It also has front motor mounts on both sides of the engine that give the effect of a motor plate across the front of the block. This triangulates teh roll bar bulkhead.


In short you can unbolt every thing at the rear of the engine and the engine hangs off the roll bar. You might be able to drive the car that way with the mount on the top of the head. The top mount was an update for both the 87 and 94 Citations.

[Neil_Roberts]

Do you think the added bending resistance would aid overall stiffness by resisting the warp mode ?

Loads mostly travel along the stiffest path in the structure. In this case, the stiffest load paths are through the tubes in tension/compression and the floor pan in shear. Other than providing panel buckling resistance, the bending stiffness of the floor pan is a tiny contributor to frame stiffness.

[rg2]

when bonding the floor to the frame, what do you recommend as a bonding agent (glue)

any favorite product you like to use?

I've always favored a bonding compound for belly pans along with rivets. The bonding compound I've always used was called PRC from the aircraft industry. I believe it's better that the epxoy everyone wants to use. The problem with epoxy is it eventually losses bonding because it sets up hard and due to chassis flex it cracks so the bonding is then lost. The PRC I refer to is the stuff that they use when building or repairing aluminum based large aircraft. It's put between the aluminum pieces or steel before riveting. The stuff is also used when they build the pressurized areas of the fuselage which also expand and ccontract. The beauty of PRC is it will set up with the consistency of hard rubber and will give a little but not break bond. Ask yourself how much flexing aircraft wings and the fuselage go through and how much expansion and contraction takes place with the entire aircraft when it goes from the ramp in the summer in Arizona with a skin temp of God knows what to 40,000 ft at 50 below zero. Those same wings are carrying fuel and they don't leak. I've used it on all my cars and years later when it's time to rebuild, I have to use a puddy knife and the malletr to get the stuff to release.

[S Lathrop]

Earlier today I recommended that the belly pan should be aluminum that was twice as thick as a steel alternative.

Neal Roberts then posted that the steel pan would be much stronger.

This evening I did a simulation. The model was a 15" in by 20" in rectangular of 1" square, .049 tubing with a sheet bonded to the tubes.

I did 4 tests. Tests 1 & 2 were single load applied to one corner with the one side anchored. The load was in the plane of the belly pan. Test 3 & 4 had a torque load as well as the lateral load.

In the lateral load tests the aluminum belly pan had 40% more displacement for the same load than the steel belly pan. Thus in this situation the steel belly pan was very much stronger.

Test 3 & 4 had the lateral force and the torque force applied. In this case the aluminum was slightly stronger but not significantly.

My recommendation now would do what you like best. As a belly pan, there is little difference between steel or aluminum when the aluminum is twice as thick as the steel.

[Swift17]

Solely for safety concerns, weld assorted steel tabs to the chassis at no less that 6" from each, affix carbon fibre anti intrusion panels with 10-32 and washers and jet nuts ......

... oops -- that is another thread --SORRY

Back to our regularly scheduled comments !!!

[stephen wilson]

Thanks for running that simulation Steve. I tried searching for more pictures of you current engine mounting, but i couldn't find anything.

EJ, I will be adding intrusion protection. The lower panel will be the common AL panel on 6" centers, probably .080" or .090". the upper cockpit either AL, or Kevlar on the bodywork, possibly with a foam core for rigidity. The uppermost "window" would have to stay mostly open for periferal vision.

[racerdad2]

Very informative thread ! Thank you !

[Alan Oppel]

An area often overlooked on older cars is the main roll hoop bulkhead area, also described as the area where ones harness is attached. With the advent of Hans devices, and full body formed seats, harness mounting locations should be moved up, and closer together accordingly, allowing you to make an additional bulkhead for the main hoop. Other note, full torso molded seats do not collapse as fiberglass shells can, and in my opinion are much preferred. plan to line the inside of the chassis cockpit area with light allum so you can pour or mold a seat. Alan

[KodaBear]

Your description is correct for bending stiffness of a beam, but the relevant loading condition for floor pan stiffness is in-plane shear.

Ah, gotcha thanks for the clarification.

[stephen wilson]

Alan, I will adjust the shoulder mountings for the HAN's, and make a bead seat with an outer shell of Kevlar/glass.

[stephen wilson]

One more question, is there any benefit to the proposed tube in Blue, in either safety or rigidity? It just bothers me to leave that dead node at the upper cockpit bar. It seems it would at least add some bracing for the main hoop, with a cost of 2-2.5# . Would 1"x.035" be considered too thin-wall for this piece?