There are only two mechanisms I can think of: fasteners becoming loose, and body/frame damage happening and being repaired enough times to fatigue the metal and cause cracks. Or, weak parts that were not designed for the repeated high forces of track driving with DOT-R tires failing and causing cracks (like the upper control arm bracket on the AP1). In either case, braces aren't really a solution. In the former case, you would need to replace or at least re-tighten all the fasteners. The collars in the link above are not helpful here: once the bolt is tight, the size of the hole around it does not matter -- the friction between the panels is so great that they will not slide. In the latter case, you would need to replace the cracked parts.

 

A good weld in cage is the best choice for chasis rigidity, but a less expensive alternative would be stitch welding or adding welds between factory seam and spot welds.

 

why doesnt steel weaken with cycling? if cycling leads to a lower endurance limit it seems to make sense that it does weaken????

 

Just to make sure stiffness and material strength are not getting blurred here...

Stiffness of a component is defined by the material's modulus of elasticity, poisson's ratio, and physical dimensions. As has been stated, the stiffness of a component or assembly will not change unless the physical dimensions change (damage) or fastened joints loosen up, etc...

Strength is a material property that does decrease as the number of stress/strain cycles increases. S-N curves are used to determine a material's endurance limit (or fatigue stress) for a given number of load cycles and specific loading conditions. There are many variables that go into a fatigue analysis, so it's not nearly as cut and dry as a stress analysis.

So, steel does weaken in the sense that it will take a decreasing amount of stress to yield or fail it, as the number of load cycles increases. Hopefully Honda designed the chassis with the goal of "infinite life" and a large enough margin of safety to allow for increases loads from stiffer suspension, sticky track tires and other racing conditions.

 

Here's a question -- According to J.E. Gordon in his book on structures, steel and iron have a "fatigue limit" which is an amount of strain below which the metal will never fail, no matter how many cycles. I'd like to know if a strain cycle below the fatigue limit still weakens the material. My intuition says that it doesn't because the metal can comply with the strain purely elastically without having defects migrate to relieve the stress.

 

Steel will weaken over time, and this chassis design is optimized for rigidity for a given steel type and material cost. The chassis is a big spring.

Crushing the frame rail doesn't help, but you are unlikely to cause it to fail this century. If you track a lot get a roll structure.

 

As long as you keep stresses below the fatigue limit of steel you will have infinite life with no changes to material properties. Think of a steel high-rise building swaying in the wind. The loads are well below the fatigue limit and it's expected to stand for 100+ years barring earthquakes and crazy people in planes.

Of course that's assuming you have no defects in the material, which is pretty hard to achieve. It's a probability game; you assume that x% of the base material is as expected, and that y% will fail sooner than expected. QC tries to raise x and lower y, but they'll never get it to 100:0. That's what warranties are for.

In my career we guarantee our structures for 13 years, that's a really long time considering the abuse a transit vehicle sees! 3G loads in the structure are expected and designed for, but I've seen 11G in testing on a 'normal' route on public roads at ~25 mph. You didn't need an accelerometer to cringe over that one!

 

By defects, I assume you mean gross manufacturing defects, not lattice defects!

 

Yes. Both, really. We've missed some welds and discovered it 4+ years later. It's quite impressive that everything held together that long! We've also had a few bad batches of tube. All were replaced with no costs to the customer obviously, plus the structure warranty got extended.

Re: Material defects - someone I know in another company has explored getting some complex steel castings made overseas. X-raying them is cost-prohibitive, so they're checking the density of them. The first batch failed pretty miserably with various non-metal pieces deep within the castings, obviously put there to reduce material costs. Scary.

 

Corey, you pulled 11 lateral g's in a bus?? That must be one heck of a bus.