Does double-clutching extend the life of synchros?

 

It would if synchronizers wore, but synchronizer rings don't wear. The inside of a SR is a cone which mates to the cone on the gear it mates with. When you shift you are moving a collar over the SR which pushes the SR against the gear cone and the oil hydraulically couples the rotating constant mesh gear to the collar (which goes to the xmsn output shaft) and forces the gear to go the same speed as the collar. The inside cone on the SR never touches the gear cone while it is rotating because of the oil between them. When the gear and SR are going the same rpm, the collar is also and a small amount of additional movement moves the collar (which has an internal spline) over the teeth on the gear (these teeth match the spline on the collar). At that point the shift has completed without clashing between the collar spline and the matching teeth on the gear. This hydraulic coupling will also greatly resist a shift when you haven't pushed the clutch in fully. Anytime you do hear and feel clashing, it is the collar clashing with the teeth on the gear except for reverse which does not have a synchronizer. The synchronizer is there so you don't have to double clutch and showed up in manual transmissions about 1925. Double clutching is totally unnecessary in modern manual transmissions.
To answer your question though, no, double clutching doesn't affect the life of your synchronizer rings if you fully disengage the clutch upon changing gears because SR's don't rub metal on metal.

 

I appreciate all the great info. So what is it that causes a manual transmission to grind going into certain gears (usaually second) as the transmission ages. I thought the sychros were often the reason you would get a transmission rebuilt. I remember reading some of the tech info on zfdoc.com and they were trying to determine the useful life of the ZF transmission using the BMW Castrol gear oil. Their measure on transmission life was based on the amount of synchro wear. See the following excerpt from http://www.zfdoc.com/faq.htm:

The phosphor-bronze lined synchronizers had no glazing and experienced an average mass loss of approximately 4% based on reserve-wear-range mass equivalency between 0.062"(new) and 0.048"(spent) gap wear/mass measurements.

If not the synchros, what is it that constitutes a manual transmission rebuild and why do gears grind at the end of the transmissions useful life?

THANKS!

 

I disagree. It takes many miles but they do wear out eventually. When they do wear out you get gear clash because the hub spline contacts the gear without the synchro ring matching the gear speeds. The ring gets too thin and by the time it contacts the gear the hub has already contacted the gear and you get the grind. Think of syncho rings as little clutches.

Also, if we're talking ZF6 here, the ZF6 is synchronized in reverse as well.

 

Modern manual transmissions have all forward gears in constant mesh, and "grinding" occurs when the clutch is not fully disengaged and you force the shift collar into the gears' teeth that engage the spline on the collar. In this situation, the synchro is unable stop the relative rotation between the gear and the shift collar.
I have owned only manual transmission cars and have driven several for more than 200k miles. The only transmission I ever had worked on had a bad 2nd speed synchro when I bought it, all others worked perfectly for the entire time I owned the car. Wear on synchros are microscopic because the oil provides a load bearing film between the synchro and the cone on the gear while they have relative rotation between them and prevents metal to metal contact.
If you use good shifting technique, synchros will outlast the car and probably the driver.
PS. I didn't know the ZF was synchronized in reverse!

 

and that's the key "good shifting techniques", however, if you're like me, and have done a little drag racing- you're banging gears and that will take out the syncros-one of the first signs other than grinding while shifting is also popping out of gear under compression braking.In my 64-muncie 4 speed I became quite proficient at rebuilding them lol.

 

Check my post in C4 tech, has some good pics of the borg warner guts.

The $40 syncrhos are great, but the $600 gears are chewed to hell.

 

Yes, but all I'm saying is the grinding is not directly caused by the clutch, it's directly caused by the synchro ring which is worn out because of poor clutching. The rings are made of bronze, they are designed to generate friction when pressed against the gear. They do wear out over time but transmissions are designed so that if you shift properly, by declutching and using the proper shifter force, the rings should last the life of the transmission. If you don't completely declutch the engine from the transmission it will wear the rings very fast, on any manual shift transmission, and in time you have gear clash (grind).

When you rebuild a transmission there are specs on how much syncho wear is acceptable. I've done many transmissions and seen many worn rings. I've also seen a few broken rings caused by hard shifts or no clutching at all! Bill Boudreau references this spec for the ZF6 on his web page discussing oil testing:

 

Yeah it maybe true that the ring rides on a film of oil,but not all the time. If that were true the little threads you see in the ring wouldn't be necessary and you wouldn't see worn gear cones from thread indentations. Plus a ring flattens over a period of time. You hit it hard enough and it looses it's clearance on the taper of the cone of the gear. New rings sit at about .060 to .050 on a Muncie, a worn ring is almost flat. Ring concenticity also affects how it will lock the gear. There is more to Synchros than meets the eye. Few people understand "ring indexing", block out conditions, or grinding. When you grind in gear , you grind from a defective ring. Yes poor clutch release can cause a grind,or ruin the ring but if you have a good release, the slider is now past the ring, with the gear not stopped, thus the grind. This wouldn't happen if the ring slowed the gear down to meet the speed of the slider.

What you are missing here is the following fact. Gears are free floating in constant mesh. When you shift you LOCK the speed gear to the output shaft via the synchro assembly. The Slider and hub are splined to the output shaft. The RING is the only component that slows them down to mate together. The ring is INDEXED to the slider via the strut keys. The ring does lock to the gear when operating correctly, and that is why it has threads and in a sense is a tapered cone clutch. The idea of "slowing the gear down" maybe relative to the mainshaft, but relative to the synchro assembly the gear is stopped, or synchronized at the same speed... a hard concept to grasp.

Sincerely,

Paul Cangialosi
5speeds.com

 

This is news to me. What does that .050/060 dimension refer to? Gap from the gear caused by the taper?

I'm familarizing myself with all this after gutting my ST10, I'm crashing into the gears before they're slowed down, and the locking teeth on the gears are chewed up now. On top of that, the replacement syncrhos I bought just don't look as good as the ones in the tranny, which have pointy teeth on them. The replacement ones teeth are more like nubs, and would require the slider to move further before reaching the synchro.

The other issue, is the slider slams into the side of the gear and chips the teeth in 2nd and 3rd. I see nothing preventing this. The side cover doesn't prevent from 'overshifting'. I'm not too impressed with this 30 year old design.

 

Yes. If you have the gear in your hand, and set the synchro ring on it's cone snugly, measure the gap between the ring and gear face. I'm not sure what the spec is for your transmission but generally 0.050 or less is a worn out ring.

I'm not sure I know what you mean about the teeth chipping on 2nd and 3rd? Do you have a picture?