Team Update 5

[rwood359]

Originally Posted by writchie

Hitch Model showing Rev A and Rev B
http://www.chiefdelphi.com/forums/at...5&d=1232678195

Comes up as invalid attachment.

[writchie]

Originally Posted by Don Wright

I'm sorry but I would have to respectively disagree here. I believe that the intent of the trailer all along was to be towed behind the robots as a typical trailer (boat, rv, etc) would be behind a car or truck... This is a pivoting, freely rotating connection.

A typical trailer hitch would have 3 degrees of freedom (like a standard ball hitch). The original hitch swivel (prior to RevA) had 2 (yaw and roll). The 0.27 hole added on Rev A removed the roll leaving only yaw. If you examine the REV A drawing, you will see that .27 dia through hole was added in Rev A (eliminating roll) and the 0.26 dia hole was unchanged. If you closely examine rev b you will see the change from 0.27 dia to .375 dia. Note that it previously was 0.26 dia not 0.27 implying that the change from 0.26 to 0.27 was some unreleased change between Rev A and Rev B. This by the way would be a proper change given that the plus or minus 0.01 tolerance left a 0.25 nominal pin an a 0.25 worst case hole, a bit tight for a clearance fit.

The Rev B change is not a tolerance issue anymore than changing a shaft hole from 0.26 to 0.375 would be a tolerance issue. This change provides a nominal 0.0625 annular ring around the pin allowing it to slosh around 1/8th of an inch or rotate plus or minus 10+ degrees in pitch and roll and bang against the sides with every change in robot direction. Like using too small of a ball on a typical trailer hitch, it can be expected to de-stabilize the attachment. Note also that the hole is now so big that it encroaches well into the ball radius.

Originally Posted by Don Wright

I believe that designing strategies around small oversights in things like tolerances in drawings in order to gain an advantage is at the risk of the team and should they risk doing such, face the facts that their strategy is null and void when the inconsistency in the drawing is fixed.

That is the lesson that should be learned here for your team, IMHO.

The idea of transferring weight from the trailer, i.e. changing the CG of the robot/trailer system occurred during our conceptual design review. It was a natural side effect of our two wheel drive concept. The ability to transfer pitch load through the hitch was being reviewed and if it was good enough for stability then it was also good enough for shifting CG. The design intent of the trailer is clear from the the drawing. Changing from .26 to .27 or even .270 would be correcting an inconsistency. Changing from .26 to .375 is a change in design intent. This change adds two degrees of freedom (both yaw and pitch).

I fully agree with you regarding designing strategies around small oversights or technicalities. We don't believe in that either. But we are trying to demonstrate engineering to our students which in part means looking closely at the rules (requirements) and specifications and understanding them enough to exploit any competitive advantages that are available. In fact, we were fully embracing the idea that the robot and trailer are a system and were paying particular attention to the specifications for the interface of the two systems. This interface is now different in very substantive ways and this fact may have other unforeseen side affects not sufficient considered or tested.

Thanks for your input.

[Rick Wagner]

I believe the GDC intends to deliver a stable and well-defined game at kickoff, but the GDC members being human, each year the game will generally have a few areas that need clarification and refinement. I don't believe they will ever intentionally make major changes to the play of the game during the build season, so I would not look for new game pieces or major rule changes.

[Daniel_LaFleur]

Originally Posted by writchie

This pretty much invalidates the strategy of transferring the bulk of the trailer weight to the wheels of the robot (through CG), increasing the normal force on driven wheels by as much as 30 lbf.

Any change in nominal force designed to give a traction advantage is illegal, and is inferred here in the Q&A.

No change in the rules, just a clairification.

[writchie]

Originally Posted by Daniel_LaFleur

Any change in nominal force designed to give a traction advantage is illegal, and is inferred here in the Q&A.

No change in the rules, just a clairification.

The Q&A answer was:

"Any vacuum/suction/fan system that alters the traction characteristics of the ROBOT would be considered a violation of Rule <R06>".

Other Q&A answer regarding dynamic wheel height expressly confirms that wheels that can be raised or lowered to change which wheel(s) bear the weight of the Robot do not violate <R06>.

So IMHO transferring mass from the trailer to Robot wheels via dynamic wheels would not be precluded by Rule 06. This, however, is academic as the design change you are calling a "clarification" would seem to precludes this approach.