So let me just describe what I mean first, since the term is vague.

I have a '90 with the ZF 6-speed. When downshifting, I will first push the clutch in, shift out of gear, let the clutch back out, give the car some gas usually to about what I guess the rpm will be in the lower gear, push the clutch back in, and then shift to the lower gear as normal.

I don't always do this, but generally do as it's not hard and doesn't take particularly long. Though I'm no speedy gonzalez either. I do usually do this before a downshift that puts the car deep into the revs, i.e. downshifting to really hammer it. Though not like on a roadcourse or while a truck is bearing down on me. There's no hurry behind it, otherwise I'll just shift normally if hurried.

My impression is this is easier on the synchros. So what I wonder is, is this true? I'm not trying to debate whether it's worthwhile or whether the car can handle normal shifting, etc. Just does this actually stand a chance of making the tranny last a little longer, or is it completely wasted effort (or worse, does it wear things out faster)?

 

I do a variant of this in which I push the clutch in, blip the throttle to the rpm I think will match the downshifted rpm, then let the clutch out. It's not a true double clutch downshift, but it works fine. I think either your way, or my way, of doing this is MUCH easier on the transmission, drivetrain etc.

Cheers,
/s/ Chris Kennedy

 

The difference you are feeling is just the fact that you are matching the RPMs and it is easier to slip into gear.

What you describe is what I would do if I had to drive the car with a completely non functional clutch. Just put pressure on the shifter, rev the motor and the shifter will slide in as the Gears sync.

 

No need to double clutch with modern day trannies. RPM matching couldnt hurt.

 

That IS double clutching. The closer you match the RPMs the less work (wear) on the synchros. You can do the samething while up shifting, except instead of blipping the throttle, keep your foot off of the gas and let the RPMs drop the proper amount. Semi tricks with the 9, 10, 13, or 15 speed Fuller Roadranger transmissions are unsynchronized, and double clutching is the norm, up and down. You WILL save some wear and tear on the synchros, but you may add a little to the clutch.

RACE ON!!!

 

You are wasting time, gas, engine wear, clutch disc, flywheel, clutch master and slave cylinders. First of all double clutching isn't necessary because all forward gears are in constant mesh, you aren't matching gear speeds! Synchronizer rings don't wear because they use the oil film between them and the cone part of the gear to transmit torque to the clutch disc. My 87 4+3 has 216k miles, driven every day, no double clutching and no repairs ever needed! Also, you are doubling the wear on the clutch master and slave cylinder and also the clutch disc and flywheel face, and on the engine for revving it. If you want to minimize wear, push the clutch in, shift, release the clutch!

 

Indeed, you ARE matching speeds. All the gears are in constant mesh, but, if the speeds didn't need to be matched, you would be in the gear you wanted. You, by double clutching, or the synchros, aren't matching the mainshaft gear speed with the countershaft (cluster) gear speed, because as you said, they are already meshed. BUT you DO have to match the mainshaft gear speed with that of the mainshaft. In order for the teeth of the sliding clutch (the sleeve the shift fork moves) to slide over the clutching teeth of the mainshaft gear, they have to be moving at the same (synchronized) speed. The sliding clutch is splined to the mainshaft. When the sliding clutch slips over the clutching teeth of the mainshaft gear, it locks the mainshaft gear to the mainshaft. Because they are in constant mesh with the countershaft gears, they have to freewheel on the mainshaft until selected. Clear as mud?

RACE ON!!!

 

I will add that when engaging the OD, I do push in the clutch, turn the OD on, release the clutch.

 

JHC!..... There are some pedantic ****'s on here! lol In other words, it is not necessary with modern transmmision's.

 

I am not making a statement as to whether it is desirable or not. I was just clarifying that there is, indeed, speed marching to be done. That IS the function of a synchronizer. If you don't care how a manual transmission works, don't read about it.

RACE ON!!!

 

I do it cause it sounds cool.

 

Syncro no syncro - mesh no mesh - do this experiment - get going about 50 mph in 4th gear - down shift to 3rd and let the clutch out fast! - or better yet go 50 in 3rd - and downshift to 2nd - RPM's jump up and the car slows down fast - now do this going into a corner - under hard braking - and the rear end will slew out and give you all sorts of problems - spin ya out if you do it just right!
(BTW what hapens if you are slamming along at 80 mph in 3rd - and you miss a shift and pull it back into second? all the syncromeshing in the world just helped you blow up your engine - right?)

It's not so much saving wear and tear on clutches and trannys thats the issue here - it's saving engines and rear ends - in my opinion - thats what you need to be thinking about!)

You want to match RPM speed with gear and ground speed,, if you do it right - you slow the car down underbraking - put the car into a lower gear - then up the rpm so it matches the ground speed for that gear and rpm - then let the clutch out - power comes on smoothly - much easier on the rear end of the car - and does wonders for your control.

Racers and HPDE guys do this by "heel and toeing" actually most of em hit the brake with their big toe (you do this and RPMs drop fast - push in the clutch - and they drop faster still) - roll their foot out to blip the throttle (get rpm's up so the speed of the car will match gear/rpm choices), grab a lower gear and then let out the clutch as they go into a corner that will require a lower gear to get a good launch out of.

What you are doing is a fifty's and hotrodders version of heel and toeing- designed to make them feel like they were really cool. In actuality - they way you are applying it, it probably has no real measureable effect on the car - but if you are ringing this thing out on a track - the heel and toe version will make you much faster around the track - and will save lots of wear and tear on the rear end!

Carl "geez - I hope thats not to verbose" Johansson

 

Partial Quote from CFI-EFI:
Clear as mud?


Its clear to me because I understand how modern manual transmissions work. The driver doesn't have to match any speeds of anything in the transmission because the synchronizers are there to do it for him and they do so on the strength of an oil film which prevents the synchronizer cone (metal) from touching the cone on the gear (also metal) the driver is selecting while underway!!! This makes synchronizers non wearing and the oil film is the clutch that smoothly allows the shift collar to engage the spurs on the selected gear. Synchronizers became standard equipment in manual transmissions somewhere late in the 1920's. I don't understand why anyone driving a manual transmission automobile made since 1930 would feel the need to double clutch, it isn't necessary and causes increased engine wear from blipping the throttle and extra slipping on the clutch disk and flywheel. But, like I have said before, its your car, do whatever you want with it!

 

I got a job as a school bus driver when I was 16, partly because I demonstrated that I could double-clutch while downshifting. They gave me a 14-year-old Chevy powered school bus and the only way possible to downshift on the mountain roads where I lived was to double-clutch.

Perhaps the synchronizers were worn out. But my boss told me I would have to double-clutch for every downshift. During training we also had to start off on a steep hill without using the gas pedal.

 

I would concentrate more on rev matching when you downshift instead of double clutching. It just isn't necessary.

 

CFI and Carl got it. Double clutching hasn't been needed since the 40's unless you were driving a Mack. The same thing happened to "rowing down thru the gears coming into a corner" when the brake engineering caught up.

It's important to match mainshaft speeds and be smooth. I've been with a lot of students that wonder why their car went off thetrack when they downshifted.

Besides that, I like " we don't need no stinking clutch" dog ring transmissions.

 

There seem to be a reasonable number of people who have them wear out then...

I can appreciate the comments about rev-matching and heel-toe'ing. I've no skill and big feet, and can't heel-toe. Also I'm talking about driving on the street.

But when shifting, say from 6th down to 2nd or 3rd in order to hammer it, I dunno, it doesn't seem right to just shift from 1,000 rpm to like 5,000 rpm like that. Maybe I'll reserve it just for the times I'm doing something like that, and not on lower speed downshifts like when slowing down. Thanks for the opinions folks.

I haven't heard of too many engines wearing out due to excessive throttle blipping though...

 

Just work on rev-matching, and then heel/toe (if you into tracking it).

 

Again, I am not commenting on the desirability or value of double clutching, just on how the transmission works.


By that same logic, an engine would never need new main or rod bearings. Sure, the oil film helps, but if the oil were the only braking force in the synchro system, the material of the stop rings would never be a consideration, which it is. I'd like a nickle for every synchro ring I've ever sold.



My 1929 Model A Ford was an all sliding gear, non-synchronized crash box. I think you are about 20 years early in your "Synchronizers became standard equipment in manual transmissions" statement. Although transmissions existed that had it, first gear was never universally synchro in a three speed.

RACE ON!!!

 

Theoritically, it would relieve stress on the synchronizers, but I don't know if it would actually extend our transmission life. It won't hurt however. What ever feels good and if it helps you enjoy your ride, give it a whirl