Sequential vs. Synchromesh, The Difference

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Sean@Weddle

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A common question. What is the difference between sequential and synchromesh (or H-pattern) shifting gearboxes?

I will almost always recommend a synchronized gearbox to the recreational user, or to anyone that plans on having multiple people drive their car. Synchronized H-pattern gearboxes are somewhat fool-proof and operate like any manual shift transmission in a common production car. They are difficult to damage when used properly and will often outlast sequential shifting components.

Sequential gearboxes require some knowledge and coordination between throttle input and shifter operation to shift properly without causing damage to the gears. They are great for performance applications and are often times the strongest option available for high powered applications. They offer the fastest shifting, can be operated with no-lift-shift electronics packages, and can even be operated via paddle shifters.

The basics: Synchromesh gearboxes will typically be an H-pattern type shift arrangement (but there are sequential type shifters that can be used with synchronized gearboxes). Synchronized gears utilize a system of small engagement teeth on the selector sleeve (or slider) that is forced into each gear with the shift fork. The gear itself has a matching set of teeth that engage the slider teeth, which is what connects the gear to the output shaft through the slider/hub assembly and completes the gear ratio. 

To aid in engagement of these small teeth, a small ring called a synchronizer is placed between the slider and gear. This synchro is commonly made of brass, but can be made of other materials. This synchronizer sits on a cone machined onto the gear. The cone shape allows the synchro to float and spin free when not being used, as the synchro is spinning at the same speed as the output shaft with the gear that currently selected. 

During the shift, the synchronizer is forced onto the cone of the gear as the slider moves towards the gear being selected. The synchronizer will either speed up or slow down the speed of the gear as it is forced onto the cone of the gear. Once these speeds have been matched, the slider teeth can then slip over the synchro and engage the teeth of the gear to complete the connection of the gear to the output shaft. 

This style of gearbox is easy to operate as the synchronizers change the speed of the gear before the gear can be engaged in a smooth and easy operation. The downside to this style of mechanism is that the synchros take time to change the speed of the gears, and therefore is a slower shift actuation before the clutch can be released and power can be driven through the gearbox again.

Synchros will wear with use, and will eventually wear to the point where the synchro sits low on the cone of the gear. This is when the gears will grind as you engage the gear. Synchros can also break into pieces if forced too hard against the gear. A synchro gearbox should be shifted slow and smooth to allow the synchro to change the speed of the gear for an easy engagement. 

Another common complaint with synchronized gearboxes is hard shifting in cold weather. This is often due to thick oil being used to help protect the gears and high powered engines. The cold, thick oil is packed in between the synchro and cone surface of the gear. As the synchro is pushed onto the gear, the cold oil does not get out of the way and the synchro slips on the cone of the gear rather than grab and change the speed of the gear. This will often get better as the gear oil warms and thins out. Another similar symptom is caused by oils that are too slippery, and not allowing the synchro to have enough friction with the gear to slow it down or speed it up to complete the shift.

Sycnhro Gears.jpeg

 
The term "sequential" refers to the fact the gears in this type of gearbox are shifted sequentially from neutral to the high gear. You cannot select neutral between gears, nor can you skip a gear, like shifting from 4th to 2nd for instance. They operate much like a motorcycle transmission.

These gearboxes are shifted with a ratchet type shift mechanism, usually attached to the gearbox, or built inside. The ratchet mechanism turns a drum with grooves machined into the outer barrel that guide the shift forks into each gear. There is a lever on the outside of the gearbox that only allows a forward or back rotation. The main shifter assembly for the driver is also only a forward and back throw. A cable or solid rod is used to connect the main shifter lever to the gearbox and operate the lever at the gearbox.

Sequential gearboxes utilize a set of lugs, or dogs as they are called, machined or attached to the side of the gear and operating sleeve (slider). These dogs are what is used to connect the gear to the output shaft. The dogs have wide gaps between them allowing them to slip into each other at high RPM when shifted quickly. These lugs quite literally crash into each other for each shift as there is no synchronizer in place to match the speed of the slider to the gear. 

Since there is no synchro required to change the speed of the gear, the shifting is much faster than a synchronized gearbox. Shifts can also be completed without depressing the clutch pedal, if the driver has the proper coordination to release the throttle and shift at the same time. 

Downsides of a sequential gearbox is that they can be tricky to operate for drivers that don't have as much coordination between throttle foot and shift hand. They are also trickier to downshift when the vehicle is slowing down as a throttle blip is required to help downshift. They are a bit noisier with "clanking" and gear lash noises due to the slack between the dogs.

They are typically not affected by oil in most cases. Due to the design the dogs and gears, they are usually the strongest type of gearbox that can help get big power to the ground.

AGB Seq. Cutout copy.jpg

 
L.R.S. said:
Just for that I'm bringing it to you for the next time I pull it for a refresh. And I'm staying to watch. 
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We have an hourly rate for that service. Bring your checkbook and current bank statement. 

 
So with a sequential what is better on the trans? clutched shifts or just lifting off the gas? I've heard both ways and have always wondered.

 
Hey Sean, thanks for that. Mind touching on the straight cut gear(s) of an Albins? Why in some cases it's only 1 gear, louder, and what it's use case is? I've heard it referenced many times and now that I'm a new Albins owner, it would be interesting to know more about it.

 
funco4lizzle said:
So with a sequential what is better on the trans? clutched shifts or just lifting off the gas? I've heard both ways and have always wondered.
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I have somewhat changed my tune on this over the past few years.

In the past, we have always said that using the clutch for shifts will help to unload the gearbox, and prevent major damage to the engagement dogs from a mis-timed shift. There is still some truth to that.

The internal argument that I have with myself after we have put so much effort into understanding how these things need to work, and redesign the shift mechanisms the eliminate the issues of the original Gen 1 and Gen 2 designs, is that using the clutch to shift can actually lead to a mis-timed shift and possibly lead to damage on the dogs.

The early Gen 1/Gen 2 (Mendi and PBS) were a bit slow to shift. The actuation of pulling/pushing on the shift lever did not equate to a fast enough action of moving the shift fork from one gear to another. The Gen 3 shift mechanism we designed in house speeds this up, and helps pull the dogs out of the gear they are in, and throw them into the next gear faster. This is the key mechanism that we spent a few years wrapping our minds around and designing a mechanism to speed it up.

Back to clutching, since the Gen 3 mechanism actuates much faster, a simple preload of the shift lever prior to shift actuation, lift off the throttle and yank back on shifter should be all that is required to upshift the trans in proper fast action.

I feel that lifting off the throttle, then depressing the clutch, will allow the car to start to slow down and then "reverse thrust" on the gearbox, which then almost requires a downshift style of throttle blip to help release the dogs from the decel side of the dogs a the weight of the car is pushing against the gearbox. 

Since we have had the new Gen 3 mechanism out over the past few years, we see far less wear and tear on engagement dogs compared to the Gen 1/Gen 2 mechanism. 

This is all somewhat splitting hairs on my part. I would still say that however each driver has managed to learn the feel of a good shift, is how they should keep operating. But, there are lots of bad habits out there that can still damage parts with the new Gen 3 mechanisms, so I will still give advise on what I feel is proper shift action. 

 
acefuture said:
Hey Sean, thanks for that. Mind touching on the straight cut gear(s) of an Albins? Why in some cases it's only 1 gear, louder, and what it's use case is? I've heard it referenced many times and now that I'm a new Albins owner, it would be interesting to know more about it.
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All of our common gearboxes (VW, MD, HV, S45, AGB) use helical cut gears as shown in the pictures above. The only gearboxes we sell that use straight cut gears are the Albins ST6 gearboxes. No need to dive down that rabbit hole in this thread.

 
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Kraut_n_Rice said:
Gonna touch on the shape and function of the teeth on a syncro?
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Nope. Deep rabbit hole.

The only thing I will mention is that the shape of the engagement teeth in all gearboxes should be designed to help "pull" the slider into gear under load. This prevents the shifter from popping out of gear. 

 
Sean@Weddle said:
All of our common gearboxes (VW, MD, HV, S45, AGB) use helical cut gears as shown in the pictures above. The only gearboxes we sell that use straight cut gears are the Albins ST6 gearboxes. No need to dive down that rabbit hole in this thread.
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Seems like every sequential I have owned there is one gear that is louder than others (besides my fortin frs5, which all were loud lol). I have even pulled a brand new gearbox and had it serviced because of it.

Similarly, my friend has a 5 speed ABG H pattern and 3rd gear is very loud, even after service. After a season of running it, the gear never got louder, but is very annoying. 

Any reason for this?

 
What about my old 1959 ford w/ a 3 on the tree?? No sycros, and would grind downshifting into 1st.  :classic_laugh:

 
Reece Pettersen said:
Seems like every sequential I have owned there is one gear that is louder than others (besides my fortin frs5, which all were loud lol). I have even pulled a brand new gearbox and had it serviced because of it.

Similarly, my friend has a 5 speed ABG H pattern and 3rd gear is very loud, even after service. After a season of running it, the gear never got louder, but is very annoying. 

Any reason for this?
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This is typically due to the coarse tooth profiles used for strength, rather than a fine/small tooth designed to be quieter and last longer. 

These gears are not made in sets as some others are. Each gear half is made in it's own batch and goes through its own heat treat batch. Occasionally, we get a louder gear than another. Since we have started profile grinding gears after heat treat, we have fewer complaints about gear noise, but they still come up occasionally.

Rule of thumb, if a gear starts out noisy and doesn't get any noisier, it is generally ok to keep running. If a gear develops a noise with use, it is most likely wearing out and will need replacement. 

Some of those H-pattern gears were made years ago, before they were profile ground, so it's not uncommon for those to be a bit louder that new gears in the sequential gearboxes.

 
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