Hi, I'm having a bit of trouble practicing this section of the piece. I've tried playing it slowly and gradually increasing the tempo, but I just cannot get it and it's quite frustrating. The problem is mainly in my bow; my left hand is fine. Does anyone have any tips?
This is a standard "excerpt" used in orchestral auditions. William Preucil has an album with him talking through his advice about various excerpts and demonstrating them. I highly recommend buying the whole album, but here's what he has to say about this excerpt:
... and his demonstration:
If you're playing in an actual orchestra, you don't have your own choice about bowing (unless you're the concertmaster). And if you're taking an audition, I would play it exactly as printed in the Breitkopf and Hartel edition and not try to invent any improvements; the challenges presented by the bowing are an expected part of the audition.
As Preucil notes, those slurs are the problem. In general, in fast passagework, our left hand can manage much more dexterity and speed than the bow can, and our job is to manage the bow to keep up with the left hand. First of all, the slurs cross strings. Yes, you could play the first couple of bars (mm. 16-17) in second position; this would have the double benefit of putting the slur all on one string, and putting the entire measure on only two, with no trip to the A string for the B flat. BUT we have no such option in mm. 18-19 (the ones that start on F); short of some crazy scheme of shifting back and forth between positions (inadvisable) we have no comfortable way to keep all the pitches on only 2 strings, or to avoid a string-crossing during the slur. That being the case, I don't know that I'd even bother with 2nd position in mm. 16-17; better to get the bowing pattern set up from the start. (Plus, if you have any interest in historicity, I doubt Mozart would have shifted to avoid string crossings.)
Recognize that each subsequent measure will reverse bow directions (I would not advise doing anything to change that). Recognize that the actual physical movement you make during the slur is very different on a downbow than an upbow (think about the shape your right hand traces in the air—an arch, or an inverted arch). Get used to putting beats on upbows in the in-between measures.
The best bit of general advice is to use as little bow as you can and still sound good. The slurs will move you (slightly) out of the ideal spiccato spot of the bow and you must work to get back; the less bow you use the lesser this problem. The "scoopy" arch-shaped motion of the downward string crossing on a downbow is less unwieldy on a shorter bow. If you're practicing by starting at a slow tempo and gradually increasing the metronome (a good idea), consider: if you use 2 cm of bow per note at 80 bpm, you should use 1 cm at 160 bpm. Can you get away with 0.5 cm? Produce a good, clean, sound, but see how little bow you need to do so.
In the slurs, work on the timing of the string crossing. Don't just sort of "blur" from D string to G, moving in a continuous arc; "pop" from one string to the other at the exact timing of the 16th note, just as you would lift and drop your left hand fingers with precise timing. (This will help keep m. 20, a few measures later, ...from coming out simply as a measure of double-stops!)
Have you worked on collé? That is, a bow stroke that is powered by "squeezing" and extending the fingers? Although this passage doesn't use true collé, the finger motion that powers it is a magic ingredient in many (maybe all?) bowstrokes, from a beautiful lyrical legato to off-string strokes to string crossings. Consider minutely the way every part of your right arm, from shoulder to fingertip, contribute to the motion of this string crossing. You can accomplish a string crossing by moving the whole arm, from the shoulder; you can do it by moving only from the elbow; you could keep your forearm stationary and merely bend at the wrist; and you could even keep your whole arm and the palm of your hand in one plane and compress or extend your fingers to reach different strings. Start with this finger motion, and then add in as much of the larger motions as needed to stay natural and comfortable. In rapid passagework, do the most you can with the smallest muscles; nothing is worse than trying to whip your whole arm through these string crossings as if you're beating egg whites.
It's great that you're practicing slowly and then at increasing tempos; that's the best start. Since so much of what makes these measures challenging is the bowing, try practicing the pattern of open strings that you'll be playing, without using left hand at all, so you can concentrate on the bow mechanics (e.g. "Slur D to G; G; G; D; D; A; D"). Do the same for the left hand, moving your fingers without bowing.
Consider what you can do to keep your fingering efficient. E.g. in the measure starting with F, it doesn't have to be "Play 2, then 3, then 1, 3, 2, 4, 2, 4"; we think that way because of the dots on the page. Instead, look at your fingers moving in physical space. It could be: "Place 2 and 3 on the F and C. Raise 3 to play the Ab. Lower 3. 2 is still down; rock slightly so that it contacts A string as well. Lower 4. Keeping 2 and 4 down, go to A string to play the C, and come back to D string for another Ab."
One challenge in passages that are all the same note value is to keep them even and equal. One way to practice these is to superimpose rhythmic patterns on them; try a "long-short" patter, dotting the first 16th and turning the second into a 32nd. Much harder, try the opposite, short-long! One long, plus a triplet-y 3 short? "Short-long-long-short" for syncopation? One benefit of these schemes is that it forces you to think about timing in the first place, and about how the mechanics of these motions conspire to rush or obstruct the timing. Another is that the "short notes" force you to make very quick motions, while the "long notes" give you a moment to think, so by combining multiple patterns you get the benefit of practicing very quickly without having to be so hyper-alert.
How are you bowing it? In your picture it looks as if even-numbered bars are bowed differently from odd-numbered ones:
Dow-hown up down Up down up down | U-hup down up Down up down up
I doubt if that's what was intended.
In the edition I'm looking at, the third and fourth semiquavers in each bar have staccato marks. They could both be played with a bouncy up-bow. Then the bowing of odd/even bars would be consistent:
Dow-hown up up Down up down up | Dow-hown up up Down up down up