Gearbox Engagement Explained - The Albins Edge

[MSI]

MSI has written this brief technical article to help explain the differences between two different styles of gear engagment systems. We are also trying to relay information that is unique to one of our partners at the same time demonstrating differences between manufacturing techniques.

 

Gear Box Engagement Explained

Over the years, we have been asked on various occasions “what is the difference between dog engagement and synchromesh (synchro) engagement gear sets” and “which one is best for me”. Within these two types of gear sets, you may also have straight cut as well as helical cut options. Hopefully we will be able to relay some useful information to you about myths and truths as well as some features that go into making a quality gear set. In a future article, we will describe how to use dog engagement gear sets as well as what signs to look for when rebuilding and inspecting these kits for rebuilding.

MSI (Moore-Sport) would like to thank our long time partners ALBINS and Rocket Rally Racing for contributing their expertise to this article.

To begin with, we should differentiate between the two different types of gear engagement.

 

 

Synchromesh Engagement.

Most modern cars are fitted with a synchronized gear box from factory. This modern style cone system was developed by Porsche and introduced in the 1952 Porsche 356. In a synchromesh gearbox, to correctly match the speed of the gear to that of the input shaft (running at the same speed as the crankshaft = engine RPM), a collar initially applies force to a cone-shaped brass clutch attached to the gear. This allows the two different shaft speeds to be matched prior to the collar locking into place. The synchronizer acts as a brake.

The collar is prevented from bridging the locking rings when the speeds are mismatched by synchro rings (also called blocker rings or baulk rings). The synchro rings have a sloping engagement so as long as they drag rotationally; they hold the dog clutch out of engagement. The brass clutch ring gradually causes parts to spin at the same speed. When they do spin the same speed, there is no more force on the sloping surfaces of the synchro rings, and the dog clutch is allowed to fall in to engagement. The action of all of these components is so smooth and fast it is hardly noticed. As a note, the Reverse gear, however, is USUALLY not synchromesh, as there is only one reverse gear in the normal automotive transmission and changing gears into reverse while moving is not required.

 

It may also be noted that synchromesh gear engagement is best done at lower engine speeds. Gear engagement with synchromesh also requires a short amount of time to take place, it is not instantaneous. This differs completely from dog engagement where gear selection is instantaneous at any engine speed.

Main limitations of synchromesh gearboxes are slow shifting at very high RPM (i.e. 9000rpm) and slow gear selection when rapidly decelerating (i.e. selecting the 1st gear for a hairpin) as well as the need to use the clutch. For this reason, double and triple cone synchros have been developed to aid 1st to 2nd and 2nd to 3rd gear changes on many OEM applications.

 

 

 

Dog Engagement

Dog Gear engagement is facilitated by numerous large teeth (dogs) that mate into matching openings machined into the opposite surface of the driven gear. Unlike the synchro engagement, the two rotating gears are operating at different speeds (unless the revs have been matched) and there is no synchronising mechanism to assist in bringing them up to a synchronised (equal) speed.

 

The progression of a dog gear being engaged is demonstrated below:

Dis-Engaged

http://www.mooresport.com/upload/Dog%20Dis-Engaged.jpg

Engaging

http://www.mooresport.com/upload/Dog%20Engaging.jpg

Engaged

http://www.mooresport.com/upload/Dog%20Engaged.jpg

 

The number of dogs (teeth) and the size of the openings determine the window of opportunity that the dogs have to engage on the shift event. It is for this reason that often we find a smaller number of dog teeth which offers a better (easier) shift quality. The downside to this easier engagement is an increased noise and abruptness on the shift. It is the profile and design of these dogs that are unique to each manufacturer. Finding a balance between performance and longevity is the key.

If the dogs do not line up to facilitate a gear engagement, the faces of each opposing surface (dogs) will clash and over time can wear. Wear will depend on the speed of the dogs and the force applied.

A unique feature to all Albins dogs are a CAD designed “pent-roof” pentagon shaped surface used to deflect the dogs apart on a miss-shift. This drastically enhances dog gear life. All Albins dog gears are verified using a robotic CMM (co-ordinate measuring machine) to assure perfection with tolerances better than .015mm on each and every part. Any misalignment and poorly machined parts will drastically effect performance and wear characteristics on dog engagement systems. Tolerances on synchro engagement systems are much more forgiving due to the slipping of the brass cones.

 

http://www.mooresport.com/upload/gear%204.jpg

 

Ideal gear selection to minimize clashing and wear of the dog rings is achieved by a momentary break in the engine's driving load removed until the shift is completed. This is achieved by a quick throttle blip or clutch depression. (The opposite is true of a synchromesh gearbox as used in passenger cars, where slow movement helps to allow the synchros to match shaft speeds). When timed properly with practice, the movements are very quick, measured in milliseconds. A sample video may be viewed here:http://www.rocketrally.com/website/index.php?option=com_content&task=view&id=37&Itemid=69 ( Download the "Face and Crazy Foot Cam (7.5 MB .MOV) " video )

 

Remember there will be no dog wear when the dogs are fully engaged (car is in-gear). The damage can only take place when initiating contact during a shift, (miss-shifting) therefore this event must be made as short as possible. If a driver moves the gear lever slowly, or if the linkage is poorly secured, dog wear will occur in various degrees. It is probably worth mentioning here that dog wear is inevitable to some degree, but shift “style” amongst other things will have a bearing on the amount of wear experienced.

 

The Albins Edge

Some features that we have found work well and give Albins products a dedicated edge (for both synchromesh and dog engagement kits) is their attention to detail and design. Their pent-roof shape dogs differ from their competitors by minimizing wear on mis-shifts which is ideal for first time dog box drivers. Albins also uses a very specific (and I will also mention secretive) computer controlled heat treatment process. Heat treatment and anti-fatigue shot peening process are unique to each manufacture and in our opinion what separates quality parts from parts that just do not last. These processes increase strength in the alloys by aligning particles (at a microscopic level) minimizing chances for hairline fractures and cracks. Each manufacture will design a certain amount of give into their products. Too much give will create the parts to twist and they will go out of alignment. Not enough give and the parts become brittle and do not offer enough impact resistance.

 

http://www.mooresport.com/upload/gear%202.jpg

 

 

Most entry level manufactures may also use commercially available materials, which are normally readily available to the general public. One advantage of companies like Albins and MSI being in business for many years is that we have developed strong relationships with our suppliers. This also allows us to often create our own proprietary materials and designs that are not commercially available to the public. The advantage to the end user is that they are able to purchase these quality controlled products using highly developed materials and designs without supporting the development costs that the manufacturers incur.

 

http://www.mooresport.com/upload/gear%201.jpg

 

Albins for example works very close with their foundry that they have been able to produce their own specific blend of ingredients to offer a unique and proprietary alloy that is used in all of their gear kits. This alloy is so specific, that the consistency of the finished product is far superior to mass produced, low cost items. End users can be assured that from batch to batch, and year to year that replacement items will remain available with the most current alloys in production.

 

MSI again would like to thank Rocket Rally and Albins for their contribution to this article as well as assisting us in offering quality products to the Subaru community.

 

Mark Moore

MSI

http://www.moorsport.com

[legacyCT]

Nice writeup! That video is intense..

 

How much would a typical Albins gear set go for?

[MSI]

We will post some LGT specials in the vendor classfied adds shortly. This article was to offer information and explanations

[BAC5.2]

This article was to offer information and explanations

 

And be an advertisement for Albins?

[MSI]

We do support ALBINS obviously, and have had great success with their products. That being said, naturally any technical references we refer too would INCLUDE, but not be limited too that relationship ....

[schwinn]

FYI, for a simplistic view of MTX transmissions, HowStuffWorks has some nice images and animations: http://auto.howstuffworks.com/transmission.htm

 

Regarding the video above... I noticed he was clutching (third pedal) during upshifts, but not during downshifts... does that vehicle have a de-clutching mechanism in the brake pedal for downshifting?

 

Lastly, for the slow people here (ie, me) can you point out the "dogs" versus the "gears" in the images you showed above? Just curious about each segment of these parts...

[BAC5.2]

FYI, for a simplistic view of MTX transmissions, HowStuffWorks has some nice images and animations: http://auto.howstuffworks.com/transmission.htm

 

Regarding the video above... I noticed he was clutching (third pedal) during upshifts, but not during downshifts... does that vehicle have a de-clutching mechanism in the brake pedal for downshifting?

 

Lastly, for the slow people here (ie, me) can you point out the "dogs" versus the "gears" in the images you showed above? Just curious about each segment of these parts...

 

Ignition made a really informative video with PPG describing the differences between synchro and dog engagement arrangements.

 

[ame=http://www.youtube.com/watch?v=-fA94Sxiu4c]YouTube- PPG Gear box - Ignition DVD[/ame]

 

The dog teeth are what the shift collar actually grab onto.

[schwinn]

^ Thanks learned a lot more about dog-boxes with that video... that explains why the video referenced by the OP showed downshifting without clutching... still, I wonder why he clutched during upshifts?

[BAC5.2]

The dogs have to unload in order to disengage. You can either lift off the throttle, or simply STAB the clutch really quickly.

 

You can even put pressure on the shifter, and when you hit the rev-limiter, it'll pop into gear.

 

When I had a PPG dogbox in my old car, I would rarely use the clutch. I had that box in the car for like 8 months. Tore it down because I was selling the car, and everything looked brand spankin new!

 

Here is another video about how to actually drive one, and how they can be driven every day.

 

[ame=http://www.youtube.com/watch?v=n_1jO64tuQA&feature=player_embedded]YouTube- Pfitzner Performance Gearbox - How to drive a dog box[/ame]

[MSI]

As you said BAC5.2 ... ON A DOG BOX you need to remove the load off the input shaft for the gear to engage. The advantage of a dog box vs. a synchro is purely the amount of time it takes to do this. You can either stab the clutch quickly, or blip the throttle. When down shifting, you can manipulate the brake / throttle controls to do this, hence why you do not see clutches being used on down shifts.

Depending on the gear manufacturer, SLOW engagements on a dog box may or may not be preferred. At the end of the day, if there is no wear on the dog rings, you are OK. If there are, change your driving style !

[BAC5.2]

A synchro doesn't really need to have the shaft unloaded to shift out of gear. You can, if you try, pull the shifter out of gear under load. You cannot do this in a dogbox. It's all in the shape and orientation of the engagement teeth and load carrying surfaces.

 

Engagement is all about knowledge. You have to understand what is happening inside the transmission to be able to really have confidence in your ability to drive a dogbox. After driving a dogbox for 8 months daily, traffic and all, you get pretty good at understanding what's going on. I kind of miss it now. It's so satisfying to drive a dogbox. Every gear shift is effortless, crisp, and smooth.

[MSI]

edited above post .... ON A DOG BOX.

Sorry for any confusion.

-mark

[MSI]

Inserted some new photos of how a dog gear engages.

-mark

[jorgemurano]

Hello, could anybody please tell me who builts dog engagement kits for 2.2 Legacy front wheel drive gearboxes? Thank you very much in advance!