Why does 2nd gear go bad?

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Why does 2nd gear go bad?

Postby midlife_crisis » Fri Dec 19, 2008 11:48 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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Re: Why does 2nd gear go bad?

Postby henks » Fri Dec 19, 2008 11:52 am

Really great write up Mid!

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Re: Why does 2nd gear go bad?

Postby thesnowgod » Fri Dec 19, 2008 12:04 pm

:bow Mid

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Re: Why does 2nd gear go bad?

Postby heathen » Fri Dec 19, 2008 12:49 pm

Thanks Mid.. being the mechanical nerd that I am, I really enjoyed that..
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Re: Why does 2nd gear go bad?

Postby G35CO » Sun Feb 15, 2009 12:25 am

I think shifting in high RPM from 1st to 2nd will speed up the griding process. I notice a smoother shift if i'm under 6k RPM. I take it easy on the shift from 1st to 2nd and enjoy the high RPM with the rest of the gears.
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Re: Why does 2nd gear go bad?

Postby migz » Sat May 30, 2009 9:54 pm

agreed, im a gas turbine mech tech in the navy and i ride my cat as smoothly posible. i noticed the fact about the last post around the 6k mark when the you have a diffrent feal when shifting. what do you say about this yall?
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Re: Why does 2nd gear go bad?

Postby RatherBRiding » Tue Aug 11, 2009 4:28 pm

Great post Mid :cheers Thanks for the explaination. I just hope I never need to use the fantastic "2nd gear rebuild" thread.

Oh great, now I've jinxed myself :doh

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Re: Why does 2nd gear go bad?

Postby ilovekandypaint » Tue Feb 09, 2010 3:52 pm

Now I'm wondering why our bikes tend to have more 2nd gear failures than many other bikes? I know there are some, but in another old post there was at least 30 percent of those that voted that have had problems. This seems to me to be more than an average 'design' occurance
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Re: Why does 2nd gear go bad?

Postby Insulinboy » Tue Feb 09, 2010 4:35 pm

ilovekandypaint wrote:Now I'm wondering why our bikes tend to have more 2nd gear failures than many other bikes? I know there are some, but in another old post there was at least 30 percent of those that voted that have had problems. This seems to me to be more than an average 'design' occurance

The YZF600R has been around for over 10 years. Bikes from around the same era (like the 97 CBR600) had similar issues, but there are far less of them around (since they stoped production of that particular model in 99 I belive) and the YZF has stuck around until recently. Lots of the CBRs were wadded up and never heard from again, and most of them never made the mileage marker you see on some YZF's. There cheap bikes that lots of people learn on and they switch hands frequently. People learning to drive a 600CC supersport dont typicaly shift properly as it takes a season to really get it figured out so you end up with junked transmissions. There are yzf's above the 100K mark that never needed a transmission overhaul so its not a design flaw as much as it is a rider flaw, when you ride a bike hard and race it things break, most people dont buy there YZF to keep it below 6k rpm :)

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Re: Why does 2nd gear go bad?

Postby butler142000 » Wed Jul 21, 2010 3:46 pm

Thanks for the info guys and great write up. :ok
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Re: Why does 2nd gear go bad?

Postby yamacat99 » Sat Feb 23, 2013 12:36 pm

Well this helped me figure out alot about whats going on inside there. im starting the teardown today of my 99 cat. put 10k on it last summer total of 25000 on her. this is my first time doing anything of this sort to a yamaha. so some stupid mindless questions may be in order here. any feedback is helpful. just wondering a few things. i have a new 2nd gear. should i get a 6th as well jsut b/c those have the most contact wear frmo the "issue"? i was also thinking of just buying a rebuilt trans for $250. but i like to do things myself.(future knowhow).i do know i have to pull the motor. but would it be simple enough to just replace with a manual my only issue is i dont like the insides of transmissions haha. alot can go wrong. i couldnt imagine it being all that bad.
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Re: Why does 2nd gear go bad?

Postby vashtsdaytona » Sat Feb 23, 2013 12:57 pm

"just buying a rebuilt trans"? meaning all the gears and shafts? can you explain?

check the link in my signature for videos on the process. might help
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Re: Why does 2nd gear go bad?

Postby mbrown4851 » Sat Feb 23, 2013 5:26 pm

Yes, you are going to need 6th gear also, otherwise 2nd gear will go out again.

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Re: Why does 2nd gear go bad?

Postby junkie » Fri Jun 28, 2013 2:58 pm

I've read through this thread today and thought I'd try a couple of things. I'm a noob with motorcycles in general and I know I shift like shizzle. I am desperately trying to improve before I kill the tranny on my cat oh-so-fair. :)

I already know that I have to preload the shifter, then flick gas and clutch simultaneously, then feather clutch out. In theory, this is not that hard. In reality I was doing it wrong. I have been driving cars with granny-trannies all my life, and turned out I was way too soft on the shifting on the cat. Despite the preload and as-quick-as-I-can shifts I got that unpleasant KLKLKLUNK sound way too often when shifting into second, many times 2 to 3 also. What I've tried today is putting much more force on the shifter and kicking it even more aggressively into gear. Turns out, it works. I can easily shift to second even at higher rpms like this (I've tried around 8k), it just slips into gear smoothly.

So what I've learned, maybe people go too easy on the shifting, afraid of damaging something, but a sequential tranny needs to be kicked and pushed.

So Why does 2nd gear go bad?
Crappy shifting. :ok
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