Synchros on the output or input shaft ?

[Ryan L]

Is there a theoretical advantage to having the majority of the synchros on the input shaft or the output shaft? I’ve looked at the exploded diagrams posted for the ZF as well as the Porsche manuals, and noticed the differences.

Will a box with one or the other produce quicker and more reliable shifts over a longer period of time than the other. I’m not trying to be brand specific, it was more from a theoretical point that I was interested.

Are you putting more strain on the synchros trying to get the cogs to match the speed of one shaft or the other?

Question came about in my head the other day after reading for the n’th time the benefits of a lightened flywheel, and was wondering if there is a benefit in the shift speed possible between various designs. Just never seen it come up before?

 

[Russ Noble]

You want quick shifting? Forget the syncros. Go for a dog box.

 

[Ryan L]

the same pluses or negatives would apply for wear rates on the dogs ?

 

[Jac Mac]

Ryan,
The Synchro unit only has to match the difference in speed between the gears each side of it in any normal gearshift. If you were to do an 'abnormal shift' say from 4th to 2nd etc then the synchro unit would have a greater speed differential to overcome. Remember that these trans are constant mesh and all gears are rotating at all times the car is in motion at these relative speeds regardless of which shaft the synchro unit is mounted on.

Jac Mac

 

[Ryan L]

Ryan,
The Synchro unit only has to match the difference in speed between the gears each side of it in any normal gearshift. If you were to do an 'abnormal shift' say from 4th to 2nd etc then the synchro unit would have a greater speed differential to overcome. Remember that these trans are constant mesh and all gears are rotating at all times the car is in motion at these relative speeds regardless of which shaft the synchro unit is mounted on.

Jac Mac

That was my understanding of it as well, the synchro is matching the speed of the gear being selected with the shaft that it is located on.

If all synchros are on the input shaft, then when you engage a synchro you are causing the input shaft to match the speed of the output shaft with reference to the gear ratio. The weight of the components being accelerated to this matched speed will be the input shaft, un-engaged synchro rings, and clutch components. The gears speeds would remain relative to the output shaft.

If all the synchros are on the output shaft then you would have to accelerate or match the speeds of constant mesh gears as well, putting more force through synchro mechanism and causing it to do more work?

That is how I’m thinking of it at present, but am about to go and look at the speed/moment changes under acceleration of the gear sets so that I have some idea as to weather it would be beneficial to have the major momentum components on the input or output shafts. If you have to change the moment less for a given acceleration it’s like having a smaller flywheel?

Regards Ryan

 

[BruceA]

I dont think it matters which shaft the syncros are on, because which ever gear you select, you will be increasing or slowing the input shaft speed to match the selected gear speed accordingly. I get where you're coming from with the inertia thing though. There would be merit in having all the 'inertia capable' components on the one (secondary) shaft. I think you may also find that it is 'packaging' that is also helping dictate where syncro units are located.