Double clutch and synchro theory question

[JHZR2]

Hello,

I was thinking about double clutching (which I do all the time), and how doing it wrong could potentially be harmful for the engine. I am curious of this as my wife, who only drives MT, never has used a double-clutch technique.

Anyway, effectively my question is, which is worse in double clutching, setting your input shaft speed (i.e. engine RPM) too high, requiring the synchros to slow it down, or too slow (requiring more drivetrain torque to speed the engine up)?

Given that in day to day use, synchros generally are used to help smooth upshifts (which are done far more than hard downshifts), and need to slow down transmission parts that may be spinning faster than necessary, it seems that maybe they are designed to be more robust in the overspeed condition.

Maybe Im just clueless... maybe it does not matter. We always drive gently anyway. That is why its a theory question after all...

Any input would be great. Thanks!!!

JMH

 

[Shannow]

Dunno, but the natural tendency for rotating things is to slow down, so nature is doing part of the work in upshifts, and synchros doing more work in downshifts, bringing the whole lot (and driven plate) up to a higher speed.

Anecdotally this fits, as boxes tend to baulk more on downshifts than upshifts.

 

[mstrjon32]

I think the wear on the synchros going up through the gears is negligible, especially in gears 3-4-5 and beyond. I never double clutch for upshifts into these gears.

Downshifting, however, is a much different story. The 1st and particularly the 2nd gear synchros have to work very hard to speed up the input shaft when shifting down into these gears and they wear significantly without double clutching.

In my Jeep with 160K miles, the 2nd gear synchro is all but shot. The previous owner beat the snot out of it, and when I changed out the transmission fluid it was obvious that it had been ground down into the fluid through many non double-clutched downshifts. Without double-clutching, upshifts feel jittery into second, and downshifts are sometimes impossible without a nice grind. If I double-clutch it going into second, it pops right in, and the same for downshifting. It's definitely saving wear on an already worn synchronizer.

So in daily driving I'll rarely double clutch for upshifting (though at times I'll just skip second altogether, gotta love all that torque from the 4.0 at just 1000 RPM), and I'll almost always double clutch for downshifts for anything besides 5-4.

All that rambling aside, I think that if you're fine with double-clutching and have practiced it enough and have been doing it long enough (I'm no expert myself!) to match the rev's spot on every time, you're saving wear on both the clutch and the synchros. You don't want to rev it too far past the target speed though, you'd be defeating the whole point.

I think the wear on the engine is negligible.

 

[JHZR2]

Thanks for the inputs.

My point being double clutching on downshifts, and being high vs low when I put it in gear... which wears the synchros more for that specific circumstance.

Thanks very much!

JMH

 

[mstrjon32]

I think you're wearing them less being on the high side with respect to the speed of the input shaft. You're wearing the clutch by speeding up the input shaft (albeit a totally imperceptible amount) and the much smaller, weaker synchros don't have to work nearly as hard to match the speed.

You should be matching the revs within 200 RPM at most when you double-clutch, though, otherwise you're just wasting your time. The point of double-clutching isn't to be 'high' or 'low', it's to be as close as possible every time.

 

[sprintman]

Practise practise practise. I can barely get second on my Mazda unless I double clutch.

 

[TheTanSedan]

I'll match engine and road speed when downshifting the New Venture 5600 6-speed in my heavy pickup when getting off the highway, but it's not really a "double-clutch" as it is in a big truck. That's a whole other world of transmissions.

 

[swalve]

What wears synchros is hard pressure on the gearshift. Give it a little easy pressure and it will drop right into gear.

 

[mstrjon32]

Sure, I always rev match on every shift, you'd be jerking all over the place if you didn't. But, for second gear, even on my synchronized transmission, I'll double clutch it into the downshifts.

 

[ffracer]

If you are matching your revs on the downshift and timing the upshifts right, the synchos are doing almost nothing, and the clutch wear is nearly zero.

All my cars/trucks have been manuals and I have always match revs. I have used Mobil 1 (10W030 for Hondas, ATF for Ford Ranger, and gear oil for BMW) in all of them and the gearboxes are like new. The last Honda had 320K with 10+ years of autocrossing and some track time and the Ranger has 160K with towing and both were perfect. Clutches generally last 150K+.

Plus, the car is a ton more fun to drive!