The xx-4 has a front bulkhead that's U-shaped in the top view. It goes across the front of the chassis and has longitudinal holes running along each side for the hinge pins. Screws in to the chassis at the front and back on each side right next to the hinge pin and is keyed to the chassis. This helps the part withstand twisting action. The original ones were junk, but they began making the part of Delrin (white) and now the major problem is hinge pin hole wear. Point is, the chassis never breaks there. Most twisting stress generated by the control arms stays in the bulkhead. that which reaches the chassis is spread out.
I submit the B4 front end is not the strongest possible design, and there is not much we can do about it. All the twisting stress from the control arms is transmitted directly into the smallest part of the chassis with screws that are relatively far from the pins. There is no keying, the bulkhead is located by the screws alone. Further, that part of the chassis is furthest away from the spot where the plastic flows into the mold, so when the plastic gets there it will be (relatively) cold, causing all sorts of problems, all of which effect strength...
Parts with mold flow characteristics like this made in the beginning of a run will not be quite as good as parts in the middle of the run, because the mold is not up to temperature. Early in the run when the mold is cool, the place in the part furthest from the inlet receives plastic cooler than later in the run when the mold is hot. At the very end of any run, as the supply of plastic runs out, the last two or three parts may have problems, especially in the areas furthest from the inlet. All of the problems described in this paragraph are difficult, if not impossible, to see. The parts look perfect.
So should we throw away the first 10% of the run and the last 5-6 parts? The vast majority of them are fine, and that's a terribly expensive thing to do. Would equal significantly higher prices.
Is any of the above the problem here? Can't say without actually having the part in my hands. Most likely it's simply a freak of the racing game. If we think it's a defective part, we'll work out something with Canon.
Eight years ago Paul and I just wanted to build a better 4WD. Whodathunk we'd have to learn about injection molding???
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