Road Fan

But the fork is not just a homogeneous mass of thermoset, it contains fibers in a selected set of orientations, which brings its own set of internal stresses and strains. I don''t have experience as a molding or composite engineer and certainly of nothing related to cycles, but I have been a systems/safety engineer on projects that include such materials. Thie means I need to review and consider all discrepancies to see if the problem challenges any product safety requirements or statutory/regulatory requirements. Very surprising discrepancies occur in initially quaified manufacturing processes. And I appreciate how expensive it can be to capture all the deviant parts before they are released to the next level of system assembly.

Kontact 

Despite those stresses and strains, the dropouts are still exactly 100mm apart.

Road Fan

I mainly mess around with road bikes, so for most of my tires there isn't a lot of center-tread build up, at least not so my clearances are challenged. When I calculate trail I have both measured tire diamter and estimated it with a simple equation. Like all models the equation is not perfect, but it is useful:

diameter = bead seat diameter + 2 * measured tire width.

Sometimes it matches reality and sometimes it doesn't.

For the head of a cycle engineering department, it makes sense to need to understand the loss of trail with tire lean. It also makes sense to study a Vehicle Dynamics text book, most of which analyze tire contact patch issues for single-track vehicles, and how the tire interacts with the road under vehicle and dynamic loading.

Road Fan

I thought this thread was mainly interested in variations in offset, or, what's your point?

Perhaps it's just that final inspection is easily able to detect and weed out any forks that show that particular defect, or that the process controls are able to effectively prevent that defect. As I said, I am not a composites expert.

In your previous post you say that the same process controls should control accurate offset as well as accurate OLD at the fork. It seems intuitively correct, but I do think the tests for the two are not equal in labor or production line time, nor are the product requirements the same. Every fork released to retail sale needs to readily and correctly accept the QR front wheel. We seem here to have agreed that a few mm of variation in offset is not very important to the product, so correct lateral spacing would be more important than correct offset. Hypothetically, if it takes more effort to check offset of each fork than to check lateral dropout spacing (a hand-held go/no-go gauge which could itself be a very cheap injection-molded part, I presume) of each, then perhaps only every hundredth fork will be checked for offset, where all will be checked for lateral. So effectively zero forks will be released with a lateral spacing error, and some forks with an offset error could be deemed acceptable, even if they are not at the nominal value.

FBinNY 

No disagreement there. My point is that I agree with the speaker that obsessing over small differences is pointless (if that's what he said) since there's decent latitude before it becomes material. Logically, if small differences were material, we'd expect to have more analysis of how things like tire section affect handling. The fact that there's not so much of that is evidence that there's some degree of forgiveness.

Fork rake tolerance also has to be considered in the context of head tube angle, and wheel diameter tolerance, since trail depends on all three. If you look at a sine chart, it'll be obvious that tiny changes in head tube angle can be more significant than a 2mm change in fork rake. (very roughly equal to 1mm of trail / 0.1° of head angle difference)

Kontact 

Your previous post was about composite molding processes, and so was mine. Now you've shifted to talking about quality control processes.

But I was talking about the production, and I have an incredibly hard time imagining a process that has both a fairly low rejection rate and a great deal of precision locating the dropouts in one axis, but not two.

So unless you're suggesting that a huge number of forks are rejected for the dropout spacing being 2mm off, I don't see why any part of the fork would be 2mm off.

Road Fan

I never said the numbers or fractions of any errors is "huge" - please don't put words in my mouth.

I can't say any part of the fork WILL or WILL not be off by 2 mm. I'm just hypotheticaly discussing how the testing protocols could introduce a bias in the product, in favor of releasing products that could have longitudinal variation but do not have lateral variation.

Again, I'm not an expert in any kind of molding. I have seen some surprising things go wrong in molding of some some smaller parts with high tolerances and complex shapes.

Kontact 

I didn't. Re-read.

fietsbob

like some fork crowns do , when building my touring bike fork the bi plane fork crown was 2 pieces of plate steel..

the steerer and the fork blades, both round, 1".. so offset there, and then the blades rake bend was Less..