What is the experience that anyone has had with shifting that uses dogs versus shifting that uses crashing gerars? If you have been able to get crashing gears to work without tearing up teeth, then have you had trouble with this method at some point, and if so, then what have you done to fix it?
There are pros and cons to both designs. i have seen both dog and mesh shifter work on a first robot. Efficiently team 45 (technokats) has done dog shifting and 180 (SPAM) has done mesh shifting. personally i am supportive to both shifters. but mesh shifter is probably easy to build (i am not an engineer YET). but i am sure that Andy Baker from 45 and Gary Dillard from team 180 can tell you about both shifters… 
http://www.usfirst.org/robotics/2004/Workshops/drivetrainmdj.ppt
this is from the conference at the Championship last year. Hopefully this should give you a lot ideas and many pros and cons of different drive trains/ transmissions.
http://www.chiefdelphi.com/forums/papers.php?s=&action=downloadpaper&paperid=386
This is a link to a whitepaper done by team 33…very interesting 4 speed gear meshing transmission…
good luck
I myself am a big fan of the dog style shifter but from the designs I have looked at, the key to a crashing gears setup is the low D.P. on the gears which make for not only stronger teeth but also much more room between them. From what I have seen something like 15 pitch or even lower. However some of the other designs like to one in another thread the gears look pretty standard such as 32 pitch so I am not sure.
Bill Gold just posted a video of mesh shifting with 24 pitch gears, and I believe that is what team 60 has used for the past several years. The one thing I see with a mesh shifting system, is that while it is simpler to design and build, it requires more gears and more “stages” in the gearbox. First, you have the motor stage, let’s say it is the Chiaphua. Then you have the reduction stage with a big gear that mates with the motor gear and with a long small gear also. Then you have the sliding stage with two gears, the bigger one one mating with the long small gear in the previous stages. Then you have the final/output stage which mates with the sliding stage, depending on which direction it is slid in. So, I’m counting 4 stages and 7 gears. In a constant mesh dog style transmission, Like Matt’s or Andy’s from 2003, you have only 3 stages and five gears (with only the Chiaphua) because the second stage doesn’t have to slide along a previous stage, it stays put and the dog does the gear selection.
That is not neccesarily true. One big reason they have so many gears is because they eliminate any sort of chain reduction entirely and drive the center wheels from the output shaft of the last gear stage. I think that a meshing system could have the potential to have less gears if done in a slightly different manner than they do it. Either way, they obviously have an elegant solution and it seems to be working great for them.
I will still put my vote in for the dog style transmissions. Of course 
Also, Dave, drop me a line sometime. We are creating a service learning class next quarter with the ME department so we can get credit for helping out the high school FIRST teams. You might be interested.