synchros, straight cut, and shifting them...

das908kind · 03-16-2005, 10:12 AM

...This may have been talked about before but...

I'm reading Vickie Elford's book on driving Porsche, and the chapter on transmissions gets into synchro vs. non-synchro transmissions, and shifting without the clutch. He also states that the reason most racing transmissions don't have synchros, is that it's faster for a race-driver to synchronize the in-put/out-put gears manually, with out the synchro cones doing it for him.

My question: is it damaging to a synchro, street-based transmission to be shifting with-out the clutch.

discuss.

Jim Sims · 05:36 PM

No damage if you speed shift perfectly everytime; otherwise you're shock loading the operating sleeve and synchro hub teeth together (in a 915). Due to their smaller cross-section the synchro hub teeth usually fail first. The decision to speed shift depends how perfect, lucky or rich you think you are.

masraum · 03-16-2005, 10:49 AM

I think in practice most people wouldn't get a perfect shift every time. In that case I think you will be wearing the tranny. A tranny that is designed to be shifted without using the clutch is quite different on the inside more than just straight or helical cut gears.

dd74 · 03-16-2005, 11:15 AM

With straight cut gears, you still need the clutch to downshift, right?

masraum · 03-16-2005, 11:58 AM

Yes, dd74, I believe the difference is in the syncro/dog setup, not how the gears are cut. It just so happens that the straight cut gears are stronger and noisier, so they are used most of the time in in racecars which are also often setup to shift with no clutch.

There was a long thread about clutchless shifted trannies a while back.

dd74 · 03-16-2005, 12:44 PM

Quote:

Originally posted by masraum
There was a long thread about clutchless shifted trannies a while back.

I'll check it out. Thanks, Steve.

das908kind · 03-16-2005, 01:17 PM

Here's a thread from September...

Clutchless shifting, left-foot braking?

rcwaldo · 03-16-2005, 01:43 PM

A straight-cut gear (in itself) will shift just like any other production gear set, since both types are in constant mesh, standard in just about any gearbox made after the late 1920's. Straight-cut gear sets, like that found in typical racing applications, like mentioned earlier, are indeed noisy, as well as cheaper to manufacture. Race car gear sets are replaced frequently, so there is a cost aspect.

I think what you are referring to is syncromesh vs. dog-type gearboxes. Speed shifting a syncro- gearbox is indeed risky as they were really not designed to do so, much less without driveline disengagement via clutch, unlike dog type gearboxes where its expected for a driver to rev match and be deliberate with his or her shifts.

I will take some pics of both types to better explain these two drive systems..

Chris

rcwaldo · 03-16-2005, 02:04 PM

Here are some pics of the two basic types of gearbox drive mechanisms.. The first picture if of the syncro system. Note how small or fine toothed the syncro ring and hub are.

The second picture is of a dog engagement system, typically found in race cars. For this application (Champ Car), there are four 'dogs' that couple this particular gear set. Also note the much larger 'window of opportunity' for a successful shift at ~80 degrees of rotation.