# Tag Archives: torsional stiffness

## Never stop thinking

I can’t, it’s part of my condition. I have the frame almost complete in design phase, so what do I do? That’s right, I change everything forward of the firewall.

I’m trying to get unsprung weight down so the tires will be easier to keep on the road over bumps. This means making everything between the frame and the road lighter, and one way to do that is indirect springing AKA push/pullrod suspension. The thinking is that the connecting bits between where the push or pull rod attach to the moving part of the suspension are : a. lighter than the spring/shock unit, and : b. more sprung than unsprung weight. The coilovers on a rod-activated suspension are believed to be 75% sprung weight with the same said for the rocker. The rod is 100% unsprung, but the total of the rod, unsprung weight of the rocker and unsprung weight of the coilover is (hopefully) less than the unsprung weight of the direct-acting coilover. Another thing is the motion ratio, or the ratio between how much the wheel moves to how much the end of the shock that moves the spring moves. With the rocker arms the effective wheel rate can be anything just by drilling the hole where the shock bolts to the rocker a different distance from the pivot, and a long travel softly sprung shock can be used on a very stiffly suspended car.

Now what this does is change everything that connects the front axle to the frame. There are no spring mounts hanging out in the breeze and everything is tucked up tight. Google a shot of a modern F1 front end as an example. This means the parts of the frame that would have interfered with the tires when they were steered aren’t there any more, but it also means that I don’t have the same radius of gyration to work with as I would with direct springing. And since torsional stiffness is proportional to the 4th power of the radius of gyration that’s kinda important. Like this is going to make the front a whole lot less stiff unless I add a lot of bracing to reduce the average length between pivots or whatever the technical term is for bracing the heck out of the structure. It has been more than 40 years since my last structures class, the one where we built a scale model bridge with 1/16” square balsa and a piece of Bristol board for the road deck, so my terminology is a little fuzzy. Anyway, major loads would be carried by the .060″ wall tubes, and also the braces since I could use the cut-offs from making the major pieces of the frame.

Another thing on my mind has been replacing the heavy steel tube axle and steel spindles with a carbon fiber axle and aluminum spindles. I have even been heavy thinking about how to construct said CF axle. What brought this on is I have had to move the steel axle I bought back when I thought I was going to be using the minivan engine and drivetrain, every time I needed to go in or out of the office, because Mrs the Poet didn’t like where she put something and insisted it belonged in my office. So because leaving the axle on the floor was a tripping hazard and because it blocks access to the door when leaning against the wall, the axle has to be moved every time I go in or out of the office. And I have come to realize that it is freaking heavy and detrimental to handling because it is 100% unsprung weight, and will work to get the car built but needs to be replaced at the earliest opportunity. Between the axle and the forged steel spindles we are talking about 80 pounds (+/-) of unsprung weight on a 1450 pound car. And replacing it with a 16 pound axle carrying 2 7 pound spindles. So replace 80 pounds of unsprung with 30 and the hubs that fit those spindles are lighter than the ones that fit the spindles I have now, by about a pound.

And final note, I screwed up again in my last correction. I used an old value for the rollover hoops of 16 feet back from when I didn’t have a good idea as to how far the rollover structure had to be over my head. The actual value is about 11 feet which takes a metric buttload of weight off the frame. Well a little bit because 5 feet times 1.75 pounds per foot isn’t much, about 9 pounds per hoop, or 17.5 pounds total. But every pound, every gram, adds up, and the fewer there are to add the lower the total will be at the end of the project. Also the less metal I have to buy, the less money I need to spend. Also important.