Now comes the part for which FEA is ideal.
I did a first set of numbers and got a bit of a dissapointment. 69 kg and about 4000 nm/deg (2950 lbft/deg). Now, this is not exact mass, as I did not remove parts of the tube that were inside of the joints.. but I use it as indication.
One more thing I used, and which I would suggest anyone doing this analysis is to calculate speciffic stiffnes.. i.e. torsionall stiffness / weight
So for the start I got 4000 Nm/deg, 69 kg which gives me a factor of 58.
Frame stiffness goes down with it’s length, and the weight goes up, so I was paying the price for comfort.
Looking at the FEA pics, I coud see where most of the twist was present.
There were 3 main areas to be improved.
1. Front section, from the nose to the driver opening.
2. Driver space that is open from above. It gains most of it’s stiffness from the triangles on the outside, however I wanted to know if I could find more.
3. Rear section. Now, this is not modeled right in the FEA as I did not include the engine. It wil lbe stiffer with the engine but I wanted to see if I can improve it.
Last thing first. In the rear I added one more tube between the rear suspension and the bulkhead. (nr. 1 in the pic). I also added one tube across the rear triangles that support the roll hoop.
I also placed the roll hoop support tubes to connect at a single point, which should improve stiffness. The roll hoop will still be round, but the supports will meet in the middle.
This raised the stiffness to 7074 Nm/deg and 70.33 kg, which gives me a factor of 100... not bad for a couple of tubes..
It could also be a result of bad FEA in the first instance, I suspect there is a lot of stiffnes to be gained from the roll hoop and it probably did not count initially due to the problems FEA has with curved tubing.
I then checked what can be gained by replacing the side supports of the driver space with a big X.
In my original design I had a big X on the side. This is probably the best, but in my V2 design I went with a different design (nr. 4 in the pic). The reason for this was the fact that when imagining I was changing the gears I would bump my elbow on that tube. So I designed the side tubes differently which gave me more space for the elbows, but also better support for the seat and safety belts.
It was a surprise to find no significant difference, so I staid with my V2 design.