Bumper Height WCD and other Drop Center chassis

Some other discussions on Bumper Height have been locked so I’m starting a new one with an extended question.

We have run mecanum for 3 years now so this has never come up for us. It was simple. Put Robot on ground and measure up.

We plan to switch to a 6 wheel WCD with drop center. Do the bumpers get measured with the outer most wheels equally off the ground or do they get measured at the robots at rest angle with 4 wheels in contact with the ground?

I believe it has to be within spec for both. Since the latter one will have the higher height, go for that.

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In the past, it has to pass both ways. Sitting on the ground, your bumpers are not allowed to leave the zone (unless in HAB Zone as defined by the rules this year). The rock is minimal typically (1/8") so unless you build exactly to 7.5", then you shouldn’t have an issue.

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Building on this, your BUMPERS have to be in compliance within the BUMPER ZONE requirements at all times. However, there is no requirement for your BUMPERS to be parallel to the ground. I suspect that this was put in to allow drop center drivetrains, but you can have them as angled as you can fit within the 7.5" BUMPER ZONE, while being 5" wide (ie. one end could have the bottom edge touching the group and the other end of a BUMPER segment could be up 7.5").

At the events we attend, there usually is an inspector with a gauge. The robot is set on a hard floor and the gauge passed around the robot to be sure the bumpers are within the zone. If the the gauge doesn’t pass all around the robot, the bumpers ruled “not legal”.

The key point is that it is a worst case measurement so that entire BUMPER has to be compliant with the ROBOT sitting in the worst case orientation (which is an issue if you have an asymetric drop center drive or angled BUMPERS). The highest point has to be compliant, not the at rest orientation or the optimal orientation.

The other point to consider is that measurement techniques vary from year to year. Past year’s techniques or past event techniques in no way mean that the same technique will be used. For example FRAME PERIMETER may be assessed with a heavy duty tape measure, a fabric (read flexible) tape, or a piece of string. ROBOT size has been done with a go/no go rigid box, a tape outline or a tape measure and eyeball. Some inspections and inspectors allow for a bit of noodle compression to make it fit, some do not. Passing inspection at one event, does not mean you will pass at the next. If you are compliant worst case, to the most rigid interpretation of the rule, you are complaint. Anything less could result in unhappy time fixing your design at an event instead or being out in your practice matches.

For whatever reason, BUMPERS seem to be a source of ongoing trouble. I think I have seen at least one team rebuilding BUMPERS or BUMPER mounting systems at every event I have attended. Some of those have been due to woefully flimsy or poorly mounted BUMPERS, but many were due to dimensional issues.

As with any limit rule DO NOT DESIGN TO THE LIMIT. Give yourself a bit of margin to deal with variation in measurement technique, fabrication, and assembly stackup. This is speaking from designing and building a ROBOT that was less than 1/16" under a height limit for that year.

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