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View Full Version : Busting the "shortshifters cause excess synchro wear" myth



thetunersgroup
05-01-09, 12:00 AM
There is a trend we are seeing more and more on a number of forums of people claiming that "short shifters cause excess synchro wear".

It's complete BS, and here's why ...

All a properly engineered short shifter does is reduce the throw at the top end of the gear lever.

With a properly engineered short shifter, the bottom end of the gearlever is moving exactly the same distance as with a standard shifter. In other words, your gearbox doesn't "know" if you are using a standard shifter or a short shifter.

All that changes is the distance moved at the TOP of the gearlever.

Here is a photo of a Porsche factory short shifter ...

http://www.pelicanparts.com/catalog/images/new_images1/911_short_shift_kit.JPG

Given that Porsche invented the synchromesh system, if short shifters caused excess synchro wear, then Porsche wouldn't make factory short shifters.

Here is a photo comparing a Porsche Factory standard throw shifter (A) to a Porsche M241 Factory Shortshifter (B) and an aftermarket shortshifter (C) ...

http://www.seinesystems.com/Images/3Levers.jpg

The most important change in lever design between the Porsche factory standard shifter and the Porsche factory short shifter in the photo above is the distance from the pivot hole to the bottom of the shift lever. Look at the differences in the distance "x" in the photos above.

Note that the distance from the pivot hole to the top of the shiftlever on shifter A and the distance from the pivot hole to the top of the shiftlever on shifter B are the same.

A shiftlever is exactly that - a lever. By moving the pivot point relative to the bottom of the lever, you reduce the required throw at the top of the shift lever.

Many short shifters also use a pivot that adds a second pivot for the side to side movement of the lever, so that you don't "crowd" the gates. ie such systems give you a short back and forth throw (like 1st to 2nd in a standard h-pattern gearbox), but maintain wider spacing between the 1st to 2nd plane and the 3rd to 4th plane etc. This helps ensure you select the correct gear.

Synchro wear simply comes down to how the car is driven. If you crunch through gears, don't match revs, and don't give the synchros the time they need to do their job, then it doesn't matter if you use a short shifter or not - crunching gears and not matching revs will always cause gearbox synchro wear.

There is a simple to understand article about how synchros work here ...

http://en.wikipedia.org/wiki/Manual_transmission#Synchronized_transmission

It reads ...


In a synchromesh gearbox, to correctly match the speed of the gear to that of the shaft as the gear is engaged, the collar initially applies a force to a cone-shaped brass clutch attached to the gear, which brings the speeds to match prior to the collar locking into place.

The collar is prevented from bridging the locking rings when the speeds are mismatched by synchro rings (also called blocker rings or balk rings, the latter being spelled "baulk" in the UK). The gearshift lever manipulates the collars using a set of linkages, so arranged so that one collar may be permitted to lock only one gear at any one time; when "shifting gears," the locking collar from one gear is disengaged and that of another engaged. In a modern gearbox, the action of all of these components is so smooth and fast it is hardly noticed.

The modern cone system was developed by Porsche and introduced in the 1952 Porsche 356; cone synchronizers were called "Porsche-type" for many years after this. In the early 1950s only the second-third shift was synchromesh in most cars, requiring only a single synchro and a simple linkage; drivers' manuals in cars suggested that if the driver needed to shift from second to first, it was best to come to a complete stop then shift into first and start up again.

Anyone who says that a short shifter has caused excess wear on their gearbox needs to learn how to shift gears properly.

As it says above "The collar is prevented from bridging the locking rings when the speeds are mismatched by synchro rings". So clearly the simple solution is to avoid mismatches in engine revs vs the speed the wheels are turning.

You do that by learning to blip the throttle properly on downchanges so that your engine revs match the gearbox shaft speed, and not flat shifting on upchanges. Simple.

Excess synchro wear is caused by one thing only - shifting with mismatched engine speeds vs wheel / gearbox shaft speeds with no mechanical sympathy for the job the synchros do. Period.

When you understand that undisputable fact, it becomes obvious how the "shortshifters cause excess synchro wear" myth got started. Someone clearly wore out a gearbox's synchros by not matching revs, not having any mechanical sympathy for the job synchros do, and then instead of looking at how they drive, decided to blame the short shifter instead.

Synchros are supposed to wear, just like clutches and brakes. The point is that if you don't match revs, then your synchros will wear out faster than someone else's synchros who understands how they work and drives accordingly.

It's exactly the same kind of wear issue you find in clutches if you constantly do hard standing start launches. If you drive your car like that, your clutch will wear out faster than someone who doesn't launch like that. The cause is not the clutch itself - it's how the clutch is used.

Conversely, the more you match revs and have mechanical sympathy for the job that synchros do, the longer your synchros will last.

- Adam

cazSW20
13-01-09, 06:57 PM
I have a B&N short shift kit in my car, and i also have very high levels of mechanical empathy for my tired old S54 gearbox. Heel-toeing is a habit (albeit a good one) i cannot force myself to break.

Where did such a nonsense myth originate Adam?

thetunersgroup
13-01-09, 07:12 PM
I have a B&N short shift kit in my car, and i also have very high levels of mechanical empathy for my tired old S54 gearbox. Heel-toeing is a habit (albeit a good one) i cannot force myself to break.

Where did such a nonsense myth originate Adam?

It's been circulating on a number of forums for a while now.

As I said above ...


When you understand that undisputable fact, it becomes obvious how the "shortshifters cause excess synchro wear" myth got started. Someone clearly wore out a gearbox's synchros by not matching revs, not having any mechanical sympathy for the job synchros do, and then instead of looking at how they drive, decided to blame the short shifter instead.

Fortunately a simple explanation of the engineering of how synchros work and how short shifters work exposes the "short shifters cause excess synchro wear myth" as the complete BS that it is.

Here's a post from another forum that is typical of some posts that perpetuate this myth ...


"I'm running a dieselgeek ss on my Mk4, and I've already had my tranny rebuilt. Mechanic said 2nd gear was chewed up and third was getting there too. He said the ss kit was definitely part of the problem, as the tranny took serious issue to the reduction."

All I can say about that is:

1. A "mechanic" should know better
2. The comment that this guys mechanic "said the ss kit was definitely part of the problem, as the tranny took serious issue to the reduction" is ludicrous. As detailed above, the transmission has no idea whether a short shifter is fitted or not, and there is no "reduction" at the gearbox. With a properly engineered short shifter, the shaft at the gearbox end moves EXACTLY the same distance as with a standard shifter. There is no "reduction" at the gearbox.

That post continues ...


"He also mentioned that the true heart of the problem wasn't so much the ss kit, but the fact that the transmission just won't take the kind of abuse that the ss kit combined with hard driving puts on the already really bad syncros."

Ah - do I see a light coming on over someone's head ? Almost ... but no ... A short shifter does not put "abuse" on synchros. A short shifter is a simple lever with a different pivot point. What puts abuse on synchros is how the driver shifts.

Fortunately we don't see that kind of stuff on JDMST very often.

- Adam

FRpilot
13-01-09, 07:18 PM
good work mate

i know a handful of ppl who believe this myth. good to see theyve been proved downright wrong@!

thetunersgroup
13-01-09, 07:30 PM
good work mate

i know a handful of ppl who believe this myth. good to see theyve been proved downright wrong@!

I'll put the article at the top of this thread up on our website so that you can point people to it.

Maybe not the best idea to point people to this thread on JDMST - we probably don't want to encourage people who "know" that short shifters cause excess synchro wear to discover JDMST ;)

- Adam