Quote:
Originally Posted by jkelleyrtp
This post was more-or-less inspired by the old 4wd physics analysis posted a while back and I was just verifying if any of the old information is pretty much the same considering how much drivetrains have evolved since then.
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Obviously physics hasn't changed in a while, so lets see if we can use the conclusions of the paper to try to understand the justifications of using a six-wheel drive.
The "rules of thumb" listed at the end of the paper were:
- Make the track width greater than the wheel base (LTW > LWB)
- If possible, reduce the lateral friction coefficient while keeping the longitudinal
friction high (i.e., use holonomic wheels or choose a good wheel tread pattern).
- Try to move the center of mass slightly away from the center of the robot. Use
caution to not move the COM far enough so that the robot becomes unstable.
Lets ignore #3 for now, for many teams the COG is set by all the other mechanisms on the robot, and modifying those to get better drive performance seems like a lot of extra work if we can improve the drive by just changing the drive. If you're not at 119.9lbs, then you do have room to add some extra ballast and might be able to use this to your advantage.
#1 is exactly what a 6w accomplishes, it effectively cuts your wheel base in half while keeping the track width the same. So, why use #1 and not #2? First off, maximizing your longitudinal friction has several benefits with minmal drawbacks, mainly it'll take more effort to make your wheels slip (which you don't want to happen in most scenarios, such as pushing matches or rapid accelerations). However, part of friction is material selection. Naturally, if a material has higher longitudinal (forward) CoF compared to another on carpet, it probably has a higher lateral (sideways) CoF. This can be mitigated with various tread patterns, but trying to figure out therotically how the tread pattern affects these coefficients is diffifcult; you're better off just testing for it.
Quote:
Originally Posted by jkelleyrtp
Based on wheel base sizes, why do so many teams opt for the long chassis? In 2012 I feel that chassis shape would have taken a change for wide, but alas I was not around in FRC back then to see what the trends were.
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I agree, wide was an extremely good choice in 2012. The first step to many of our pick lists in 2012 was simply crossing off all the long-orientation robots at the tournament.
Quote:
Originally Posted by jkelleyrtp
This year (as rookies) we ran with the KOP chassis and had a significant rock. As our weight distribution ended staying with the part of the robot with the lift, we mainly stayed on the four front wheels the entire time. Has anybody experienced how bad the scrubbing is on long 4wd systems? And would have a 4wd been have feasible in 2014? Thanks for the input.
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For a long time, the rule of thumb with a 6w, drop center drive was to have about .125" of drop, sometimes a bit more for some treads that dug deeper into the carpet. However, this was with the bigger robot dimensions, when most people were 38" long. With the shorter robot frames we have now, I also think that a full .125" of drop is a bit excessive. I understand why the kitbot has a similar drop to that though; it is built to work for all teams, regardless of how they use it; erring on the side of a bit of rocking and being able to turn well is much better than not being able to turn well.