Recently Mark Moore from MSI showed me a good article he wrote in conjunction with Albins Australia and Rocket Rally on the differences between types of gear sets and the advantages of using a Albins gear set. I copy and pasted the article below in hopes this might answer some questions people might have. I found it very informative and hope you will as well.
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 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.
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/patrickri…2004_incar.mov
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.
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.
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.
