Noise when getting into 1st gear

[Boule]

Hi,

I recently got a 2007 s2k, with 20k on it.

When I stop at a red light or a stop sign, and put the car in neutral, it does a weird "clunk" noise when I put it back in 1st gear, before the usual gear "clac". It's like it gets stuck and I have to force it through (although it doesn't require additional force on my part).

It doesn't happen when I first start the car, or if I shift to 1st after second. Only whe the car idles in neutral and I have my foot off the clutch for several seconds.

Any idea what it can be ?

Thanks for the help !

[repiv]

This question has been asked for as long as the car has existed. Relax! It's perfectly normal. You just have to understand how a manual transmission works.
When you put it in neutral and let out the clutch, you have re-connected the engine to the transmission. When the engine idles, it turns the transmission at whatever your idle speed is (except for the output end of the tranny, which is connected to the rear drive wheels, which isn't turning at all when you're just sitting there). When you go to put it into first, the back end of the transmission is then connected to the rest of the transmission. Since the front end is spinning at about 1000 rpm and the back end is not spinning at all, the spinning parts have to come to an abrupt halt. When that happens, all the backlash (little spaces between gears and cogs) in the tranny all the way back to the drive wheels, slam together. That's the clunk you hear.
If you don't want to hear the clunk you can do one of 2 things. Push the clutch in and wait 3 to 4 seconds before moving the shifter into first. This will allow the viscous nature of the tranny fluid to slow and stop the rotating bits before you hit first - NO CLUNK.
Another thing you can do is to throw it into 4th first, then into 1st. Since the gear ratio of 4th is what it is, it can absorb the backlash coming together in such a way so that you don't detect it - NO CLUNK.
It's just the physics of manual transmissions. You may not hear it as clearly in other cars because they tend to have far more sound deadening than an S2000 and also because the gear ratios of those other cars are not so conducive to slamming the backlash together in quite the same way.

Oh, you will likely have noticed something similar when selecting reverse. If the tranny is spinning due to idling in neutral, you will have a tough time getting into reverse or you may even hear the grinding of gears. Throw it into 4th to stop the spinning bits or just hold the clutch for a few seconds so the fluid can stop the spinning.

[Boule]

OK, thanks for the reply !

What threw me off is that:
1. It doesn't happen consistently.
2. I didn't hear it in any of the 5 other manual cars I drove regularly, even with the windows down. I only noticed it when I would would force 1st gear when the car was still running at 2nd.

I will try the 3-4 seconds wait and the going in 4th first.

[repiv]

It likely isn't consistent because none of us are always consistent in the way we go into gear. If you idle in neutral with clutch pedal up, then immediately clutch in and throw it into 1st, I'll bet you'll do it every time.
As I said, other cars are not exactly the same as the S2000. How many other close gated, close ratio, 6 speed transmissions are you comparing the S to? Ratios in a 4 speed or a 5 speed are not the same and they aren't spaced apart the same way. Even most other 6 speeds will have different gear ratios and different gear spacing. I've owned 6 speed Corvette and Viper and you have to really pay attention to detect the clunk, but it's there. Getting rid of 1000 rpm in a fraction of a second involves dissipating the inertia of a lot of moving parts. That inertia has to go somewhere. Some cars just absorb it better.
We also run about 1.7 liters of tranny fluid. This is quite a small amount compared to other trannies. Fluid can take up shock quite well. Same goes for the diff. Most other rear ends take double what we use and that's where most of the clunk comes from.

[Boule]

Thanks for the explanation. I tried the 2 things you suggested, and couldn't hear the clunk. I'm not worried about it anymore :)

I have a couple of other "issues", I'll open new threads for them.

Again, thanks for the help !

[marks_lude]

It likely isn't consistent because none of us are always consistent in the way we go into gear. If you idle in neutral with clutch pedal up, then immediately clutch in and throw it into 1st, I'll bet you'll do it every time.
Every time, the first time :p. If you get it to clunk, then put it back into neutral and let the clutch up, it should not clunk again. There a few things required for the clunk. Spinning of the trans input shaft from the idle (if you press the clutch for a few seconds this stops, so no clunk), and slack in the drive line.

When you stop a light and put it in neutral, the last thing your drive line saw was deceleration, so it is mated on the "deceleration side" of the gear teeth. When you get the clunk into 1st, you are hearing the momentum of the spinning input shaft shifting the driveline to the acceleration side of the gear teeth.



Whats funny is you can get more clunks if you get the 1st gear clunk, put it back into neutral and let the clutch up, then you can get a clunk going into R... after that, back into 1st haha.

[repiv]

Every time, the first time :p. If you get it to clunk, then put it back into neutral and let the clutch up, it should not clunk again. There a few things required for the clunk. Spinning of the trans input shaft from the idle (if you press the clutch for a few seconds this stops, so no clunk), and slack in the drive line.

When you stop a light and put it in neutral, the last thing your drive line saw was deceleration, so it is mated on the "deceleration side" of the gear teeth. When you get the clunk into 1st, you are hearing the momentum of the spinning input shaft shifting the driveline to the acceleration side of the gear teeth.



Whats funny is you can get more clunks if you get the 1st gear clunk, put it back into neutral and let the clutch up, then you can get a clunk going into R... after that, back into 1st haha.
This is directing your attention to the diff, whereby the spaces between the teeth of the ring and pinion gear close and come slamming together to make the clunk.
When you put the tranny into neutral, you are also disconnecting the input side of the tranny with the output side and the input side begins to spin up to the engine's rpm at the time. When you suddenly engage 1st gear, you are also stopping the stuff that was spinning and all those little spaces suddenly close. That will also contribute to the clunk. You just have to listen harder for it. And this happens every time you throw it into neutral and let up the clutch. This "clunk" is a combined effect. The diff is likely the loudest. And let's not forget about the CV joints.