How low to go?
Answers:
daveowenville has given you a good answer. Your new wheel and tyre package will come with lower profile tyres probably 40 or 45 profile tyres. This will be to reduce tyre rub and also try and keep the rolling radius of your new wheels close to the old wheels.
These lower profile tyres will make you feel the bumps more and will help improve your handling a little too. But you cant get improved handling without reduced ride comfort.
I would do as daveowenville suggested. Fit the wheels and drive around for a while but for the purpose of seeing if the wheels are catching the bodywork anywhere. Because you might end up having to fit stronger suspension springs to stop the bodyroll.
Most aftermarket lowering springs are about 30% uprated, not a lot but enough to remove enough bodyroll to prevent rubbing.
A final note about lower cars, if you keep the original shock absorbers then never lower more than 25-30 mm. Otherwise you will be removing all of the original shock absorber travel and they will bottom out over bumps, causing interenal damage and leading to them leaking and failing the MOT.
Dont lower it more than 4 inches in my opinion
i think the question is, why? are you trying to look like michael caroll?
i wouldn't drop it more than 5 inches from ground should be able to clear bumps but i would also recamend a med tire rather than a low profile
Here's the thing about lowering a car that already has a +1, +2 or +3 wheel size conversion: your car's suspension consists of a combination of (a) the amount of spring travel and firmness of the spring, (b) the damping of the shock absorber, and (c) the amount of travel, firmness and flexibility of your tire sidewalls. Think of the sidewall of your tire as a separate, miniature spring/shock combination.
So when you convert from, say, a 15-inch wheel to a 19-inch wheel (gz, that's a +4 conversion.the kids these days!) you're pulling 4 inches of sidewall out of each tire. That's well over 65% of it, and the new tire will have a significantly less-flexible sidewall to compensate for it.
Why am I telling you this? To answer your question, of course. The Ford Focus (like the Nissan Sentra, and other small economy cars) don't have a lot of "spare" suspension travel as it is, especially in the rear. When you put on the new wheels and tires, you're pulling out a lot of suspension travel, and what's left from the tires is now substantially firmer.
In short: do the tire/wheel swap first, then drive the car around for a while. It is very likely that this increase in firmness and reduction in travel will by itself make the ride too uncomfortable for you -- and if it doesn't, you'll be in a better position to know how much you can lower it. After all, we can't answer that question, because we don't know what you consider "a really bad ride".
Side note: 19 inch spinners may look nice (to some folks), but you're doing a lot of negative things, too -- increasing unsprung weight, the impact to the ride as mentioned above, and likely altering the wheel offset. If you really care about things like ride, you're not going to like these wheels. You might want to find someone else with a set on their focus, and convince them to let you take a test drive before you drop the cash. Ebay is full of people who made the investment before they realized it would be a mistake.
Oh, and if you're trying to impress the ladies: it isn't the wheels themselves that the girls find impressive -- it's the outlay of cash required. Some girls really, really like boys who have extra money and like to spend it on frivolous things, because they want you to spend the money friviously *on them*. So keep that in mind.
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