Why does 2nd gear go bad?

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Why does 2nd gear go bad?

Postby midlife_crisis » Fri Dec 19, 2008 11:56 am

TAPnTX wrote:Are all year models of the YZF plagued by 2nd gear problems?

The answer to your specific question is "all years are subject to this problem".

However, a related answer is that all motorcyles, particularly sportbikes, with sequential transmissions are subject to this problem. There are two reasons: 1) you seldom stay in first after starting up, so almost every start involves a shift into second, and 2) there is a big jump in ratios between 1st and 2nd, making speed mismatches more significant in the 1-2 shift.

Understanding why the transmission fails is easier if you know what happens when you shift, and what has worn out when you get that dreaded "popping out" of 2nd. I knew nothing about this, even after 3 years of riding, until I actually rebuilt a transmission. You may already know this, but I've never seen this explanation here so I'll attempt it.

The gears actually are mounted on two shafts side-by-side, the input shaft (bottom) and the output shaft (top). This is in 1st gear:

and this is in 2nd:

Notice the two gears in the top right corner. The large one on the far right is the 2nd wheel, and the smaller one just to the left of it is the 6th wheel. Notice also that in the top picture (1st gear) these gears are separated, while in the bottom picture (2nd gear) they are pressed together. All shifts involve a combination of the three shift forks pushing the gears from side-to-side; all the gear teeth are always engaged (that's why these transmissions are sometimes called "constant mesh" transmissions). There are splines on the shafts and, as the gears move from side to side, they engage and disengage from each other, sometimes "freewheeling" on the shafts, and sometimes engaged with the splined gears so that they turn with the shafts.

The final piece to the puzzle - and the piece that causes the problem - is the engagement between various pairs of side-by-side gears. Each one of these pairs has "dogs" - basically, ~10mm diameter pins cast into the gears - that engage into holes in the adjoining gear of the pair. You can just see one of these dogs and one of these holes between the 2nd and 6th gears in the top picture, where the gears are separated.

The gears, dogs, and holes look like this. Here's 2nd, with a worn out one on the left (notice how rounded-off the edges of the holes are):

and here's a 6th, with a worn out one on the right. Notice how sharp the dogs are on the new gear, and how rounded-off they are on the worn out gear:

In fact, you can see that 2nd is a "freewheeling" gear (all you can see in its center hole is a bearing surface, no splines), and 6th is a "splined" gear. So, 6th is always turning with the shaft, while 2nd only turns when it is pressed against 6th and the dogs are engaged.

Anyway, when the dogs and holes are that rounded, the dogs pretty much "ramp up" out of their holes under load, forcing the gears apart, forcing the shift fork sideways, and pushing on the shift drum (the internal part that moves the shift forks) which makes the transmission pop out of gear. At that point, you're in neutral - without wanting to be.

There are variations of the wear that lead to slightly different symptoms. If the shift fork gets worn or bent enough (it wears out, too), sometimes the transmission will disengage unexpectedly, but won't actually end up in a clean neutral. In some early-stage 2nd gear failures, the rounding of the dogs isn't fully developed and the gears may grind and "ratchet" instead of there being a single popping-out effect.

You can imagine how these parts get rounded off: each time you shift into 2nd, you are pushing the 2nd and 6th gears together, hoping that the dogs on 2nd will mesh smoothly into the adjacent holes in 6th, making the pair of gears turn "as one". If you are hesitant or otherwise not decisive with your shift, you might just push the gears closer, but not fully together, and the dogs and holes will grind together without fully meshing. This wears metal off the parts pretty quickly!

In fact, I believe that every shift from 1st to 2nd wears some infinitesimal bit of metal off the gear dogs, meaning that this is a progressive wear event that occurs no matter how you shift. Weak shifting makes the problem progress much more rapidly. Also - and you can't do much to avoid this if you do a lot of stop-and-go riding - lots of shifts, even clean, decisive, correct shifts, will wear out the transmission sooner than fewer shifts (such as would occur if you do lots of long trips with few stops).

The thing to remember is to shift as firmly as you can and even, as Loki mentioned, actually hold the shifter up a fraction of a second after shifting into 2nd just to make sure it's firmly engaged.

The bad news is that if the previous owner of your 'Cat was a lazy shifter, there could be significant problems with 2nd gear already. I bought my 1997 "EuroCat" - wrecked - with 16,700 miles on it, and 2nd gear was already TOAST (those worn gears in the pics were from that tranny). On the other hand, my 2000 "Millennium 'Cat" rolled for 52,000 miles - including a lot of stop-and-go riding - before 2nd gear went bad. And Morecowbell's '97 'Cat just passed 100,000 miles with no transmission problems whatsoever, although Morecowbell says he does a lot of longer rides.

Hopefully this long post helps explain what goes wrong in the transmission, and provides some perspective on the frequency and causes of the failures.
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