How Do They Synchronize Gears?

A guy at work once got to drive a Model A antique car. A co-worker remarked how that was before they knew how to synchronize gears (so they mesh smoothly the instant they engage). So, how do they do this?

I believe prussian blue is applied to both surfaces to see how they mesh. Once determined, they are ground accordingly to maximize surface to surface contact.

You should pm Tuckerfan. I’m pretty sure he’s a machinist by trade.

I’m confused here. Doesn’t “synchronized” mean somehow the gears are always spinning to facilitate on-the-fly shifting? [Of course, I’ve never known a multi-speed fly! :wink: ]. Even when in neutral, the selector (I think its called) remains spinning for smooth engagement? How is this done?

Oops! I’m not awake yet. I was thinking of a rear end. We have a older technician here and actually have a model T in the shop right now that he’s been working on for the owner of the dealership. As soon as my help gets here I’ll go down to his stall and ask him.

I’m pretty sure there’s not a lot of difference between a model A and T.


If you don’t have a synchronised transmission, you use a technique called “double declutch”. You first shift from the gear you are in to neutral, then allow the engine speed to drop to the correct speed before changing from neutral to the next desired gear.

This is pretty much what the tech said. He said the gears aren’t synchronized at all. He also said that once you become familiar with the vehicle, ie what rpm to shift at, the clutch isn’t needed to shift gears, only to start off.

Please forgive the slight hijack… but I’ve never understood why you can’t just keep the clutch in while the rpms drop in neutral, rather than clutching in and out of neutral?

With the clutch pedal in you are spinning the throw-out bearing continuously and it’s not designed for that.

As to a synchronized transmission, once a vehicle is moving you don’t need the clutch if you carefully pull it out of gear and bring the RPM up to match the next gear. The syncrhonizers will allow you to slide into the next gear.

Letting the clutch out will bring the input shaft to the lower engine speed. It is faster.

For an up shift that is the proper way to do it.

For a down shift you need to speed the input shaft up to match the lower gear. There are only two ways to do that:

1: Clutch in, push the shifter to the lower gear. You are making the synchros speed up the shaft. This wears the synchros. No problem if you plan to keep the car until it’s 3 years old and trade it in on a new one. Definitely a problem if you plan to put 200K miles on it.

2: Double clutch. Clutch-in. Shifter to neutral. Clutch-out. Rev engine to a bit higher than it will be in the lower gear. Cltuch-in. Shifter to lower gear. Now the synchros do almost nothing as the input shaft is going almost eactly the correct speed.

I’ve had a couple cars without synchromesh, and the last two VW beetles didn’t have a synchro on first, so they needed double clutching to downshift to first (which you don’t often need to do anyway).

If you are good enough at judging the speeds, you don’t need a clutch to change gears at all. This lets you drive a manual transmission car with a clutch that won’t disengage. Though, to start out, you have to kill the engine, shift, then start as the car lurches forward. I imagine it’s hard on things.

You have to match the speed of the input shaft to the output shaft for the gear you are shifting into. You do this by either raising engine RPMs for a down shift, or letting RPMs fall a controlled amount for a upshift. If the clutch is in the input shaft will not slow down or speed up with the engine.

As far as I know all modern truck (18 wheeler) manual transmissions do not have synchromesh transmissions.

There are basically three techniques for driving a truck without a synchomesh transmission.

  1. Double clutch - The by the book way, clutch in, slide gearshift to neutral, clutch out, raise or lower engine rpm with the throttle, hold the rpm at the correct point, clutch in, slide shifter into gear (while still maintaining rpms at the correct speed) release clutch.

  2. No clutch - Use just enough power so the transmission is under no pressure, (you are not accelerating or engine braking) slide gearshift to neautral, raise or lower rpms for next gear and hold them at the correct speed, slide shifter into gear.

  3. Single clutch - Holding power just right to keep the load off the gears to get into neutral can be the trickiest part, so sometimes it is easiest to use the clutch to get the transmission into neutral than continue with the no clutch method.

Note - I have been a truck driver for 15 years and almost always use the ‘no clutch’ method.

Edited to add this link – How Manual Transmissions Work | HowStuffWorks

There was one that showed a simplified diagram of syncros but I couldn’t find it.

I have had to do this several times when I had a broken clutch cable. The most memerable was when I broke a clutch cable in a parking garage DT San Francisco. I had to get home 100 miles just outside Salinas Calif. This was at 4:00 PM, rush hour. This was when the 101 was mostly 2 lanes each way and freeway only to San Jose. I had to use the no clutch method all the way lhome is stop and start traffic.

First thing to understand is that in nearly all transmissions, all the forward gears are meshed with the teeth of their partner all the time, whether that gear is engaged or not.

One of each pair of gears that is not engaged is spinning free on it’s shaft. In an automobile transmission this is nearly always the gear on the main shaft. (FWD transaxles may be different, I’m not familiar with them)

In order to engage a gear, drive dogs connect the gear to it’s shaft. When you miss a shift and “grind the gears” it is these dogs that are grinding, not the gear teeth. In many transmissions reverse is an exception to this, and you might actually be engaging the gear teeth.

In a synchronized transmission, there are friction clutches that engage before the drive dogs engage. This brings the shaft up/down to the same speed as the gear about to be locked to it. These synchronizer clutches only slip as the gears are changing, and thus are small. Using them for lots of down-shifts will eventually cause them to wear enough that they stop functioning. Double clutching makes life easier on the synchronizers.