MK IV tubular spaceframe drawing

Doc Watson

Lifetime Supporter
Yes that was what I was suggesting... although triangle bracing on the lower sill would be good, and please don't take anything I said as criticism of what you are doing, sounds like a great project.
 
1) can you run FEA on your models? The only potential drawback of the vertical outside members is that they might allow "lozenging" through the cross section, but I can't say that with any confidence, thus FEA.
2) your rear suspension architecture is definitely more modern than original. Drawback is that these cars are small, and space in the rear end gets really tight, especially with starter/coolers/plumbing in place. Can make the car a pain to work on, with lots of interlocking systems. The old rear suspension allows easier access to gearbox etc.
 

Neil

Supporter
You might also look at a 5-link rear suspension. I based mine on the Porsche 996 but I found that I had to modify the Porsche geometry slightly to minimize bump steer. I suspect the original Porsche geometry was designed to reduce bump steer due to compliance in its rubber bushings. My suspension is all spherical rod ends so its deflection is minimal.
 
Tom, what do you mean by "lozenging"?
at the moment I only see the advantage of the big sidebox in relation to the tank.
The solution with the 45° struts is lighter, but offers hardly any space for a tank, especially if it should be removable.
I once sketched a solution where the tank can be removed to the rear when the cross strut (red) is screwed on.
Pink is the tank, grey are crash elements
Stand_29_04_35.JPG
 
@Neil,
I still have no idea what type of axle I should use.
At the moment I tend to use a normal double wishbone arrangement with additional guide strut.
I will try to use the same wheel mounts on all four wheels.
Do you have some information about your suspension for me?
 

Neil

Supporter
@Neil,
I still have no idea what type of axle I should use.
At the moment I tend to use a normal double wishbone arrangement with additional guide strut.
I will try to use the same wheel mounts on all four wheels.
Do you have some information about your suspension for me?
I used Porsche 996 rear hubs and uprights. The brakes are an unusual combination. I used Porsche 928 S4 FRONT rotors & calipers mounted on the 996 hubs. To accomodate the SCTA wheel requiremtns I had the Porsche 130mm bolt circle re-drilled to 5" BC and inserted 5/8" NC threaded studs in the hubs. This way I can use 15" steel racing wheels. Cheap, too. BTW, the SCTA also requires 1"hex steel lug nuts.
 

Attachments

Mike Pass

Supporter
Can I suggest a few things.
The sill outer frame to be angled in towards the bottom and to use a fairly big diameter round tube for the lower rail. This would look more like the original shape.
The two tubes in the centre of the roll cage to be made into a bx section as this would be much stiffer.
A rectangular frame is not stiff. under load it can deform into a parallelogram or lozenge shape which is why this is called "lozenging". A rectangular frame can also twist very easily. Looking from the end twist the near tube clockwise and the far tube anticlockwise. A triangle is the best for rigidity. Make everything a triangle.
The original cars gained much of their torsional stiffness from the sill structures. Panelling with welded panels could have the same benefit for your space frame chassis.
Cheers
Mike
 
I once subjected the sill area to some FEM analyses in a simplified form.

1. sill made of 30x30mm 25CrMo4 steel. Weight 32,5 kg
2nd sill made of 40x40mm ALU T6. weight 14,4 kg
3. 40x40 mm T6 aluminium sills welded with 2,5 mm ALU plates Weight 30kg
4. sills made of 40x40mm Alu T6 welded with 1mm ALU plates. Weight 20,7 kg

The results are torsion and bending.

Force application was 35000 Nm each time
Wall thickness 2mm each

I find the results very interesting, although the force is certainly too high, but this way the differences become clearer.
Edit: Sorry, I set the wrong dates for the covered aluminium frames yesterday

Result:

1. 30x30mm 25CrMo4 steel. Weight 32,5 kg
Bending = 0.827 mm
Torsion = 2.537 mm

2. 40x40mm ALU T6. weight 14,4 kg
Bending = 1.472 mm
Torsion = 5.267 mm

3. 40x40 mm T6 aluminium sills welded with 2,5 mm ALU plates Weight 30kg
Bending = 0.9605 mm
Torsion = 0.7896


4. 40x40mm Alu T6 welded with 1mm ALU plates. Weight 20,7 kg
Bending = 0.9904 mm
Torsion = 1.422 mm

I find the comparison between 1 and 4
With -57% weight saving, + 70% torsional stiffness and only - 20% less bending stiffness


Stahl_Biegung.JPG
Stahl_Torsion.JPG
40er_Alu_Torsion.JPG
40er_Alu_Biegung.JPG
03_01_Biegung_40x40_2_5mm_Beplakkung.JPG
03_02_Torsion_40x40_2_5mm_Beplakkung.JPG
04_01_Biegung_40x40_1mm_Beplakkung.JPG
04_02_Torsion_40x40_1mm_Beplakkung.JPG
 
Last edited:
Tom, what do you mean by "lozenging"?
at the moment I only see the advantage of the big sidebox in relation to the tank.
The solution with the 45° struts is lighter, but offers hardly any space for a tank, especially if it should be removable.
I once sketched a solution where the tank can be removed to the rear when the cross strut (red) is screwed on.
Pink is the tank, grey are crash elementsView attachment 106231
Lozenging basically means the square cross section of the side pod becomes a parallelogram. It's not clear to me, but I thought I'd raise the question. If FEA is not available, building a scale model out of balsa wood might give you indications. Edit didn't see the most recent posts, subject covered.
 
Hello,

I have been dealing with the topic FEM for a very long time now. For this I used Fusion360. Unfortunately it took me longer than I thought.
The insights I gained during this process have led to a new frame design.
My design now consists of an inner structure that absorbs all forces.
On the outside there are now only tubes which are used to reproduce the shape of the original frame.
The total weight is now 120 kg without safety cell
This weighs 23 kg, so together 143 kg
The pipes have the dimensions 40x40 and 25x25, each with a wall thickness of 2mm

Reihe_F_V_10_01.JPGReihe_F_V_10_02.JPGReihe_F_V_10_03.JPGReihe_F_V_10_04.JPGReihe_F_V_10_05.JPG
 
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