How synchro works - a question....

[FTO-UK]

OK, I know the basics of how baulk-ring synchromesh works - but here's a question:

On my Mitsubishi I get some synchro 'crunching' on a 2-3 upshift - presumably due to some wear or deficiency on the 3rd gear synchro ring. However, I can do a fast downshift 4-3 with no crunching. Why is this? I would have expected any problem to manifest itself on any shift into third gear. Can someone explain this?

 

[Palut]

I can't explain why your problem happens, but I can relate my experience.

I get really bad crunching on the 4-5 upshift when the car is cold. If I double clutch that upshift (let off gas shift, to neutral, let clutch out, let engine speed drop, put clutch back in, shift to fifth) I get a shift that's as smooth as can be! You could try that on your 2-3 shift.

 

[drive it forever]

Some cars use double cone synchronizers for the first 2 gears foe smoother operation. Some, like the 6 speed in the 05' dodge trucks even use triple cone sync's in the first 2 gears and double cones in the remaining gears. The reason I'am saying this is that some vehicals dont have the best shifting tranny, new or worn out. A manual tranny that shifts like glass would cost more and possibly be not as reliable in the long run, its why semi trucks dont even have sycns. The 3 to 4 and the 2 to 3 synchronizer hubs dont correlate each other, that alone explains the shift quailty difference.

 

[FTO-UK]

quote:

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Originally posted by drive it forever:
Some cars use double cone synchronizers for the first 2 gears foe smoother operation. Some, like the 6 speed in the 05' dodge trucks even use triple cone sync's in the first 2 gears and double cones in the remaining gears. The reason I'am saying this is that some vehicals dont have the best shifting tranny, new or worn out. A manual tranny that shifts like glass would cost more and possibly be not as reliable in the long run, its why semi trucks dont even have sycns. The 3 to 4 and the 2 to 3 synchronizer hubs dont correlate each other, that alone explains the shift quailty difference.

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Surely the same third-gear synchroniser components are used whether I shift 2-3 or 4-3; in both cases I am moving the shift lever through the neutral position and into 3rd gear - so I would think I am engaging the same synchro unit?

 

[t-squared]

Your 3rd and 4th gears share a syncro assembly. On your 4 to 3 downshift the assembly is already spinning so the syncro and blocking rings don't have to bring the gear up to speed as in a 2nd to 3rd upshift.

 

[noob]

quote:

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Originally posted by t-squared:
Your 3rd and 4th gears share a syncro assembly. On your 4 to 3 downshift the assembly is already spinning so the syncro and blocking rings don't have to bring the gear up to speed as in a 2nd to 3rd upshift.

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Exactly. If you were rev-matching on your upshifts, they'd be smooth too.

 

[FTO-UK]

Thanks guys - that makes sense. Appreciate your input.