[SNIP]
Ahem...anyway -- just picked up an 07 Mazda3 Sport with a manual tranny. I just love the way this car drives...except for one thing. The car has a scary tendancy to hop its rear end sideways on uneven pavement. My wife just about lost it on some broken up pavement on the 401 last week.
I can't figure it out and am going to head to a dealer for a look. Basically it tracks great until you hit a big crack in the pavement then the rear end jumps sideways. Very much like the handling you can get on washboard gravel roads where the car just begins to drift sideways due to the constant hammering. Only with this car all it takes is a single thump. It has to hit both rear wheels at once though. As single pothole under one wheel won't do it.
After the bump and jump, you can feel the vibrations up through the steering column too and they continue for a second or two past the bump as the impact reverberates through the car's body for what seems like an extended period of time. I've driven behind the car to watch the rear wheels and bumps seem to trigger a lot of hammering up and down. It feels and looks for all the world like the rear shocks are shot. But if you push down on the back of the car it's all very firm and the car only has 50K km on it.
So, basically you're saying that, when taking a curve on the freeway at speed (assuming >speed-limit) and you hit a pavement seam with both left and right tires simultaneously (meaning they
both lose contact with the road surface for an instant) the lighter rear end of the car seems to slip out a bit? It seems to me that this is what would happen to
any car in a similar situation. Additionally, it sounds like a pretty big pavement seam if it's having that much impact on the handling through the turn. Again, on the assumption that you're exceeding the posted speed limit for the turn (and why wouldn't you, right? (glare) I mean, that's where the fun is!), it might be worth exploring what happens if taken at the posted speed limit.
There's a curve in the highway near me that I looove pushing through without slowing down (50MPH curve taken at 65-70MPH) just because I can, and there's a seam a bit more than half-way through it. When I hit it, the back end definitely reacts more than the front does, but it doesn't shudder the way you describe -- it slips and then digs right back in, which can be jarring if it's not expected. When I take the same curve at the posted 50MPH, the seam has basically no effect.
So, maybe there is something wrong with your rear shocks... (dunno)
The car has 17 inch rims and 45 series tires. Is this a factory setup? Seems like big wheels for a car this size. I'm not up on the specs for the various 3 models. They do look factory though and the car was advertised by the dealer as a GT if that helps. I wonder if it's those 45 tires. Maybe those low sidewalls are just too stiff for the suspension and so launch the relatively light rear end over rough pavement?
Anyway, that was my first thought. My second was that perhaps the air pressure was too high. The door sill says 32 psi and I discovered they were closer to 40. Letting them back down to 32 has hardly made a difference at all. My third thought was that perhaps the previous owner did a bad job of installing some performance suspension mods. The car's suspension doesn't appear to have been lowered or otherwise altered, however.
If the wheels appear to have a 5-spoke "snowflake" pattern (each spoke splitting into a 'Y' toward the outside), then they're factory (see my sig pic). And yes, the Sport GT comes with 17's, but the factory tire size for them is 205/50R17. I'm betting the current tires are something like 225/45R17, which will fit with minimal trouble, but will also necessarily change the handling aspects, although I would think it would make them
more grippy, not less. (uhm)
And, yeah, 40PSI is way too high to run the tires -- harsh ride and poor handling, especially over bumps in a turn. Had a silver 04' M3H that had an overinflated mismatched set of tires when we bought it used two years ago. Rode like crap until I set them at 32PSI and the center of the tread was almost worn to the bars with plenty of tread left on the outsides. I'm surprised that you didn't notice any difference at all after lowering the tire pressure. (uhm)
I'd love to hear any other ideas. Other than some complaints about front wheel hop on previous pages, no one has mentioned this particular problem.
In my experience, front wheel hop seems to have more to do with the tire, the road conditions and the weather than anything to do with the suspension, at least with this car. The factory BadYear RS-As never gave me a lick of wheel hop if I stomped on it, but that was largely due to them sucking and preferring to spin instead, instantly engaging the TCS. The new tires I have (Dunlop SP Sport Signatures) have given me
some hop, but only on damp pavement with no standing water with an ambient temperature ~50F.
I hope at least some of this helps.