Shifting gears mechanics [Tech]

[DutchBlackNsx]

Some question which has been bugging my for a while now, but i can't seem to understand why it happens. I had my gearbox in pieces multiple times (NSX and other Honda's), but never understood this part of the mechanics of gearselection.
So here it goes.

In short:
When shifting gear up, lets say 2 -> 3 the shifter falls into the gear at the approx. right rpm. Why doesn't it fall into gear at a higher rpm, what keeps the shifter from going past the synchro-spring?


The long whining and boring version:
The gear selection exists out of 5 major parts (see picture).
Now lets study the case when downshifting from, lets say 3th to 2nd.
3rd gear is released, then the shiftfork moves the synchro-sleeve over the syncro-hub towards the gear. On its way, the syncro-sleeve pushes with the longer teeth (inside the synchro-sleeve) against the synchro-spring, which is sitting on the synchro-ring.
At that moment the synchro-ring will slip against the conical side of the gear, causing the gear to spin up to the same rpm the synchro-ring is running.
When the difference in rpm between the synchro-ring and the gear is minimal, the longer teeth go past the synchro-spring, and move onto the gear-selection teeth of the gear, making a full lock between the synchro-hub and the gear.
So, why doesn't the synchro-spring let the synchro-sleeve pass when the difference in rpm's between the synchro-ring and the gear are way off?

I got one explanation for it, but its a bit science fiction, so lets not publish here...

Any thought?

 

[DutchBlackNsx]

O come on..... nobody got a clue?

 

[Larry Bastanza]

Hi Mich,

You know I love to think about this stuff, but I have not had time to hurt my brain enough to come up with a response.

OK, Now you sucked me in. Amazing what a cup of coffee will do. First off is there any chance you have a gearset and syncro handy?

Fundamentally the syncro is called a "Blocking Ring" It is designed to hold the slider from moving toward the gear until they are moving at the same speed. So how? On the syncro there are three large tangs, that coorespond with three gaps in the INSIDE component of the slider, If you look at your picture you see the inside gear of the slider has the slots,

The slots are wider then the tangs. The tang MUST be centered in order for the slider to move towards the gear.


So if the gears are syncronized the relationship is like this:

NOTE: When you look at these drawings omit the periods, the UI will compress spaces in the text and make is look incorrect.

|.........|...Silder gap

...|...|......Syncro tang Now the gear is not selected


If so you get this when engaged:

|.|...|.|.. Now the gear is selected


If the gears are not syncronized the relationship is like this:

|........|..Slider gap

|...|.......Syncro tang

or.....

|.........|..Slider gap

......|...|..Syncro tang

Which side the tang is on is determined by which part (gear or shaft) is spinning faster, meaning upshift or downshift.

So when the shaft and gear are spinning at different speeds the syncro tang will not be centered. So how does it get centered?? When they are not centered and you move the shifter(slider) towards the gear you apply pressure to the syncro and the cone area of the gear. This acts as a brake, which gets the target gear and the shaft spinning at the same speed, or syncronized. The braking action occurs when the syncro tang is on either side of the slider gap. Also remember the teeth are beveled like a ^ . This allows the slider collar to move easily into engagement when syncronized.

When gears grind(from the resultof a worn syncro), what is happening is that the braking action is not good enough and the slider is forced towards the gear syncro teeth prematurely. The relationship between the coeficient of friction for the conical area on the syncro and gear is critical so that it all works out before the driver gets the shifter into the engaged position. This is why many folks like the magic "GM Modified Friction Fluid" It increases the coeficient of friction and makes the syncro work better.

If you look at how to check a syncro (as I know you have done), you notice you are measuring the gap between the syncro and the gear face while you are forcing the two together. This is measuring the wear of the inside cone of the syncro so you can decide if the braking action will be enough.

If you have a gearset with syncro this is very easy to see by holding the gear steady, then rotating the slider back an forth, the tang will move from side to side in the slot but the slider will only move forward when in the center of the slot.

I hope the drawing makes sense.

Regards,
LarryB

 

[DutchBlackNsx]

Wow, thankx Larry,

Great write-up. Was waiting for you

This also explains why there is some rotational play between the synchro ring and the hub itself, i never understood that part either.
Understanding the shifting system, your pictures make fully sence to me.
Quiet a smart system actually, simple and efficient.

Everytime you learn a little more about the mechanics of a car, the more you handle it with respect.


Thankx again.

Mich

 

[Larry Bastanza]

This also explains why there is some rotational play between the synchro ring and the hub itself, i never understood that part either.

Exactly

Glad it helped. Interesting but I learned about syncro's when I purchased factory Porsche service manuals years ago. The german manual writers decided a complete theory of operation should go into the service manual. I read them cover to cover. The Porsched syncro (older models) did the same thing in a VERY different way. They were actually not that good, Porsche was always known for balky shifting. In 1990(if I recall) with the G50/50 transmissions they finally sucked in their pride and went with the cone type(Borg Warner) syncro setup, like the rest of the world.

Regards,
LarryB

 

[DutchBlackNsx]

Did some searching around on the web (which i should have had done before), and it seems that ALL manufacturers use the same principle these days. Biggest difference seems to be in the way they use the 'synchro spring'.
Which suprises me a little, since all i hear from 'the carmechanics' is how different gearboxes are inside from different manufacturers.
Oh well, suppose they never open one up

Thankx again Larry, great help.

Mich

 

[NSXTech]

Did some searching around on the web (which i should have had done before), and it seems that ALL manufacturers use the same principle these days. Biggest difference seems to be in the way they use the 'synchro spring'.
Which suprises me a little, since all i hear from 'the carmechanics' is how different gearboxes are inside from different manufacturers.
Oh well, suppose they never open one up

Thankx again Larry, great help.

Mich

Hey Mitch, You really have to be carefull when 'carmechanics' speak. :wink:
I've rebuilt upwards of 500 import trannies simce my first VeeDub type I in the early 70's. And I have to opine, that they are all differant inside. My experience is limited to Porshe, VeeDub, BMW, Subaru, Saab (worst week of my life), Honda, Acura, and an occasional 'friend' with a really wierd car.
If you just look at your illustration, then your comment would be correct. But what really makes each gearbox differant, is how the installation of the shafts and the gearchange is accomplished. There are as many ways to do this as there are car manufacturers. There are only so many ways to lay two shafts side by side, but how you move the forks, how you support the shafts, how you connect to the outside shift mechanism, etc is very differant. IMO, this is what makes being an 'all car' trans builder very difficult. It is also why you should always to go to a specialist on your make for trans work.

To your original Q about shifting ease at speed, something that contributes in a large way is the amount of force for any given speed delta, that is required for enough oil to squeeze out from under the synchro ring. I don't think GM Modified Friction is ness sliperier, becuse that would make the synchro tend to work harder slowing down the facing gear. But I know more about tranny's than lube science.

When we moved to our new shop last year, I again threw out the garden o ' gears I was saving. Based on when I last cleaned out storage, this pile represents about 2 years worth of NSX trans work, not counting the ones that got repairs with no upgrades.

Cheers,
MB

 

[DutchBlackNsx]

I've rebuilt upwards of 500 import trannies simce my first VeeDub type I in the early 70's. And I have to opine, that they are all differant inside. My experience is limited to Porshe, VeeDub, BMW, Subaru, Saab (worst week of my life), Honda, Acura, and an occasional 'friend' with a really wierd car.
If you just look at your illustration, then your comment would be correct.

Yup, thats what i meant.

But what really makes each gearbox different, is how the installation of the shafts and the gearchange is accomplished. There are as many ways to do this as there are car manufacturers. There are only so many ways to lay two shafts side by side, but how you move the forks, how you support the shafts, how you connect to the outside shift mechanism, etc is very differant. IMO, this is what makes being an 'all car' trans builder very difficult. It is also why you should always to go to a specialist on your make for trans work.

Suppose your correct there, its just that all i hear overhere is how carmechanics do not even think about opening up a gearbox.
Except from the Honda site (tuningshops, dealers) i do hear that they open them up. Maybe The Honda ones are ez to service?

When we moved to our new shop last year, I again threw out the garden o ' gears I was saving. Based on when I last cleaned out storage, this pile represents about 2 years worth of NSX trans work, not counting the ones that got repairs with no upgrades.

Thats one amazing pile! :biggrin:



Thanks Mark for the reply.

Mich