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post #1 of (permalink) Old 06-04-2004, 08:25 PM Thread Starter
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OT double clutching

hi there

what is double clutching and why is it necessary. all i know is that it is necessary in vehicles without synchronizors...what do synchronizors do (other than synchronize)?

thanks
patrick
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post #2 of (permalink) Old 06-04-2004, 08:56 PM
 
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Re: OT double clutching

Double clutching is done while shifting when your shift forks don't quite line up well. Normal procedure is clutch in-shift-clutch out. Double clutching is just what is sounds like. You're driving along and need to shift, clutch in-shift out of gear into neutral-clutch out-clutch in- shift into next gear-clutch out, continue driving.

Synchro's I'm not to fluent on but the way I understand them is that they are basically a mechanism to help your gears to line up better so you flow from gear to gear. Kind of auto adjusting shift forks?

Anybody else want to chime in and explain them more technically?
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post #3 of (permalink) Old 06-04-2004, 09:04 PM
 
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Re: OT double clutching

Syncros are disks that go infront of gears that have points on them instead of flat fronts. So if the gears are not in line when you shift, it doesn't matter, they push the gearset into alignment.
Search for a picture.. they are fairly self explanitory.

Double clutching is normally used for downshifting to bring the engine side gearset back to speed before mating it with the wheel driven side.
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post #4 of (permalink) Old 06-04-2004, 09:39 PM
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Re: OT double clutching

As has been said above, double clutching is necessary to get the engine speed and the speed of the gear you are shifting into nearly the same so that the gears will engage.

here is a link How Manual Transmissions Work that explains it pretty well. In the jeep transmissions I have seen the inside of, the syncros and the forks are different than the illustrations at howstuffworks, but the idea is the same. On my T18 for instance, there is a conical brass ring that brings the actual steel gear up to speed. The ring gets pushed into the gear when you move the shift lever and the friction of the brass causes the steel gear to come to the same speed as the engine driven gear (synchronize).

HTH
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post #5 of (permalink) Old 06-04-2004, 10:23 PM
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Re: OT double clutching

Gears spinning at different speeds can't engage.
The idea of double clutching is to bring the speed of the new gear (usually the one connected to the wheels) to the same speed as the one that needs to engage with it
(the engine driven one.) That applies either upshifting or backshifting.
Upshifting you normally just let the clutch out in neutral, hear them spin, clutch again and engage.
Backshifting is a little harder - you clutch, neutral, let it out, blip the motor, clutch again, engage.

Synchros do essentially the same thing. As the spinning gear approaches the new one, a cone gets pressed into a taper -- that friction gets the new gear spinning. When they are almost at the same speed, the dogs engage - the taperd bumps on the gear mate with the dogs on the hub.
So essentially the taper acts just like your clutch in neutral to spin e'm up.

With a syncronized transmission when you shift, you are actually moving a hub into then through the brass ring that's tapered, then it moves to the dogs on the new gear.
If you shift too fast the synchro doesn't have time to work.

On unsynchronized transmissions, you are moving a gear trying to mate the teeth.

Note -- since a synchro is a friction device, things that greatly reduce friction - like slick 50 and those types, will cause the synchros not to work right. Lots of transmissions have been rebuilt because of that stuff. If you've put it in, it takes several flushings to get rid of it. Yes the can says to use it - but do they care?

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post #6 of (permalink) Old 06-05-2004, 07:50 PM
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Re: OT double clutching

Dear Patrick, The explanations that you have received are pretty much in line with reality. Here is something to keep in mind: The Bigger and heavier and SLOWER the drivetrain is, the greater the need to double-clutch because it takes TIME for all that rotating mass to change speeds. Double clutching keeps the DRIVE spinning at engine speed, and makes it easier to match the DRIVEN to it. In fact, when we used to shift heavy trucks, we would clutch coming OUT of gear.......let the clutch pedal back out, then watch the tachometer reading drop and WAIT OUTSIDE THE GATE for our next gear and then slide it in without ever touching the clutch. We did that because we knew that a drop of say....250 RPM would line up the next gear perfectly, AND we could do that because we learned to MANAGE THE SLACK in the driveline. What you do is rev all the way up to the top of the tach in say...3rd gear; then back off SLIGHTLY to free everything up.....; then declutch and pull the stick into neutral.....; then release the clutch....; then watch the tach drop down to your next gear....... and then slide the stick into the next gear..... and then ease back onto the throttle. Double clutching is a good way to save those synchros. [img]images/graemlins/smile.gif[/img]
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post #7 of (permalink) Old 06-05-2004, 10:13 PM
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Re: OT double clutching

</font><blockquote><font class="small">In reply to:</font><hr />
Synchros do essentially the same thing. As the spinning gear approaches the new one, a cone gets pressed into a taper -- that friction gets the new gear spinning. When they are almost at the same speed, the dogs engage - the taperd bumps on the gear mate with the dogs on the hub.
So essentially the taper acts just like your clutch in neutral to spin e'm up.

With a syncronized transmission when you shift, you are actually moving a hub into then through the brass ring that's tapered, then it moves to the dogs on the new gear.
If you shift too fast the synchro doesn't have time to work.


[/ QUOTE ]

Maybe you don't get what RRich is saying, maybe you do... Everyone thinks of things in different ways. Sometimes you can say something ten different ways and still not get your point across, then change your wording slightly and everything becomes clear for the person you are talking to...

Well here's my stab at it from another thread ((with pictures of synchros (blocking rings)...

syncros bad, hurt anything to drive it this way?

...oh yeah and a few tangents I added [img]images/graemlins/blush.gif[/img]

Everyone that has replied is trying to say the same thing... in their own way, with different points (that may or may not matter) just to add to your confusion.

[img]images/graemlins/tongue.gif[/img]

Good Luck,
Dale
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