We were looking on andy mark and found these monsters
http://andymark.biz/am-0298.html
Too bad they weigh 14 pounds each and cost $710 a set.
Anyways, from what I've seen around here (vids and such) it looks like 8 in. mechs are not gonna have too much trouble getting over a bump, as long as you build up a decent amount of speed. As for mechanical integrity, we've used 8 inchers in the past without too much trouble, but after 3 years the plates are starting to deform. I think they should be fine for competition, but you might want to buy two sets. I have no experience with 6 in. mechs though, sorry.
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