I saw mention of racing gearboxes being unsynchronized to make the shifts go quicker. How exactly is that?
I don’t really understand what ‘synchronized’ means, or at least what I thought it meant doesn’t seem to apply.
I have a 1982 Chevy StepVan (like a UPS truck) that pops out of 2nd gear…it’s a 3-speed plus a creeper. I’ve been told by 3 different mechanics in passing that the 2nd gear synchronizer is bad. What exactly does that mean?
What’s the difference (in the way you drive) between a synchonized gearbox and an unsynchronized one?
The very short summary version is that an unsychronized transmission is the original naive design from the old days (1920s), where the pairs of gears aren’t meshed until you shift them. It’s simple and rugged, but it requires double-clutching, or jamming, to switch from one gear to another.
In a synchronized transmission, the gears, all of them, are always meshed, and shifting is accomplished by locking different pairs of gears to the input or output shaft. The gears always mesh with each other, but are either synchronized (ie locked) to the shaft, or are free-spinning on their shaft.
The “synchronizer” is the part which locks any given gear onto its shaft. When that part starts to fail, the transmission pops out of a given gear, or becomes hard to put into a given gear.
Here’s a decent link with pix that’ll make my quicky summary make more sense
I’m getting what you’re saying though. I’d like to see the link before I ask any more stupid questions, such as what causes the grinding sound when I shift without the clutch at too high of an RPM?
Up into the 1960s, there was often no synchromesh between 2nd and 1st. BMC in particular seemed to be a holdout, as the MGB didn’t get full-synchro until 1967. There were a few other cars that were late synching first, but I don’t remember them right now.
Ok, LSLguy never came back to correct that link, so…
If gears are synchronized between and first in newer cars, why does first gear grind if you’re moving too fast when you try to shift into first?
What causes the grinding sound if it’s not actually the teeth of the gears banging into each other?
I had a 1972 Ford F100 pickup that never did shift well. The only way I could do it without a problem was to either clutch into neutral and then release the clutch and then reclutch to go into the next gear (I guess that’s what double clutching is)…or I’d slide it into neutral without the clutch and then press the clutch to go into the next gear which seemed to be the fastest and most reliable way to shift without grinding. Nevermind that the linkage attached to the steering column was all messed up…it was like that even after I put a floor shifter in it.
I’ve been driving stickshifts all my life but never have understood what goes on inside the transmission.
Here is a description of how a synchro works. (Previous pages in the same report deal with other aspects of a manual tranny.)
IANAMechanic, but I think there are a few errors and misconceptions in this thread.
First, I don’t believe synchros are left out of racing trannies because they slow down shifts, but because they add weight. And because race car drivers are real men (and occasionally women) who know how to double clutch, match revs, and heel-toe brake, and don’t need no steenkin’ synchros.
Second, since it hasn’t been explcitly stated, I’ll just point out that without synchros double clutching is generally only necessary when downshifting, not upshifting.
Third, as shown here, synchros or not, the gears of a transmission are always locked together, and what you’re actually shifting (or grinding, if you do it wrong) are the dogs.
Fourth, the reason a 1-2 synchro wasn’t deemed necessary in older cars was that second gear was generally low enough to drive the car as long as it was still rolling, no matter how slowly. I.e., you weren’t expected to downshift from second to first; you only went into first from a dead stop. Even on cars that have 1-2 synchros, this principle is applicable. Don’t downshift to first unless you’re taking a really slow corner on a race track. In normal street driving, second will probably do fine.
The responses so far don’t really answer the OP, which asks what “synchronized” means w.r.t. transmissions. Here’s an attempt to capture the basic ideas. IANA mechanic, so please take what I say here with a grain of salt:
A typical manual transmission has several sets of gears mounted on two parallel shafts. Each gear is paired with a gear on the other shaft. Each pair of gears is in constant mesh - that is, their teeth are always together. One of each pair of gears is permanently fixed to its shaft, so that it spins at the same rate as the shaft. The other of each pair is not permanently fixed to the shaft - a mechanism called a “dog” hooks the gear to the shaft when that gear is engaged. Each dog has teeth that fit into its gear. The dog slides along the splined shaft and, when its gear is engaged, locks onto the gear . Only one pair of gears is ever engaged at a time. What happens when you shift gears is that one dog is disengaged (for the gear you’re shifting out of) and another is engaged (for the gear you’re shifting into). This engagement and disengagement of gears is always done on the input shaft of the transmission (the side connected to the engine), not on the output side (the side connected to the differential). This is because that shaft can freewheel when the clutch is disengaged.
When you shift gears the input shaft has to change speed to match the new gear ratio. When you upshift the shaft has to slow down, and when you downshift the shaft has to speed up. A synchro is a clutch-like mechanism that gets the shaft and the dog to spin at the same speed as the new gear that is being engaged. Without synchros, the transmission will grind as the dog is brought to the right speed abruptly while the gear is being engaged. With synchros, the dog is brought to the right speed before it engages, so no grinding happens.
Double clutching is necessary to avoid grinding the gears in an unsynchronized transmission. The driver first shifts into neutral, engages the clutch, revs the engine to the right speed, then disengages the clutch and shifts into the new gear. You can get away without doing this when upshifting because the input shaft needs to slow down, and will do so of its own accord through friction. When downshifting an unsynced transmission, double clutching is always necessary because the input shaft needs to speed up, and won’t do so on its own.
Yep, Jason, that’s double clutching. They want you to do that when you test for the CDL. Most trucking companies prefer you to “float” from gear to gear without clutching. It’s easier and it doesn’t burn up clutches.
It’s been a while since I’ve done it, but I recall engaging the clutch to shift into neutral, then disengaging it to rev the engine, and then re-engaging it to shift into first.
Revving the engine with the clutch disengaged won’t accomplish anything, because the clutch is between the engine and the transmission.
My first vehicle was a truck with a broken synchro in second gear. For years I double-clutched every time I downshifted to second to go around a turn or to go up a hill (since I live in the San Francisco Bay Area, this happened a lot).
Here’s a more-detailed description of double-clutching:
1 - disengage the clutch
2 - shift into neutral
3 - engage the clutch
4 - rev the engine to match the speed of the new gear to speed of the vehicle
5 - disengage the clutch
6 - shift into the new gear
7 - engage the clutch
It’s possible to shift into neutral without disengaging the clutch (that is, you can skip steps 1 and 3). I suspect this puts extra stress on the transmission and could lead to premature wear.
Oops - I see the problem here. When you step on the clutch pedal, you are disengaging the clutch, not engaging it. The clutch is engaged when it is closed, and disengaged when it is open.
Maybe we’re talking past each other. When I say ‘engage the clutch’, I mean that I’m pushing the clutch pedal in. This disengages the transmission from the engine. When I say ‘disengage the clutch’, I mean I’m letting the pedal out, which engages the transmission.
Well, I shouldn’t have asked about transmissions because today in my '87 Chevy Sprint, at Mile 21 on the New Orleans Causeway (at one time the longest over-water bridge in the world…might still be) I suddenly lost 5th gear. The car was acting like it was in neutral and there was a grinding sound. I shifted down to 4th and it worked and I made it another mile before that completely gave out and I had to coast to a stop. There are no breakdown lanes on the causeway and I was just past the last crossover where you can pull over. Had to get a cop to push me the rest of the way.
The car would start in neutral with the clutch pushed but when I let out the clutch the engine would die. The car didn’t seem to be trying to move, such as if the shift linkage broke and the car was actually in gear.
In any forward gear, if I revved it up and let the clutch out, it seemed to be trying to creep forward but then the engine would die. In reverse, it would just die. Had to get it towed all the way back. I’ll find out tomorrow what’s wrong with it.