Is double de-clutching helpful to my gearbox?

[djefk]

One for people who actually have the knowledge here, as there's a lot of conflicting "expert" opinions!

If I'm driving along in 5th in the girlfriends Pug 106 1.1, is it beneficial to the gearbox at all to double de-clutch if I want to change say, straight into 3rd for acceleration? I've always done it out of habit from driving 60s and 70s classics when I was younger, with the result that I now do it automatically on every down change whether I'm in my BMW CS, the 106 or in something brand new!

I ask as one of my friends noticed me doing it and we had a debate as he feels it has absolutely no benefits for the gearbox on anything made with synchromesh on the gears your selecting (so that means all gears on all cars made from around the mid 70s onwards)

Does anyone have the actual knowledge needed on this to prove me right??? ;D

[Adam]

Rev matching, yes, makes for a smoother change.

Double declutching, no. Infact double declutching can cause increased wear on the syncros.

[popuptoaster]

Plus it doubles the wear rate on the clutch mechanism, thats mainly applicable to cables and cylinder seals, so not a major issue, but none the less, twice as much use will cause twice as much wear.

[djefk]

Adam, how do you know this is the case? Are you going on (like me) what you've been told or can you back that up with a deeper explanation for the benefit of us not knowledgable in all things cogbox?

[scruff]

Rev match, it'll give everything in the box an easier time as the differential speed of gearbox/clutch is lower and give a nice smooth transition.

The way to practice it is while cruising in 5th on quiet roads (So you can practice safely) is to practice down shifting with throttle blips. You can quite quickly lean to get it near perfect most of the time. After a while it becomes second nature and cars with a slower rev pickup will annoy you as you can't do it as easily!

Eventually you can move on to heel and toe if your car can cope, you need proeprly spaced and aligned pedals and a quick rev pickup. Takes a lot of practice and doesn't really achieve anything but it's satisfying to get it right. Flying up to a round about, braking at the last minute while dropping 2 cogs smoothly and safely scares the *poo* out of passengers but impresses though that know... a bit... unless you get it wrong and end up coasting round the roundabout in neutral!

[e21meister]

Or to state the obvious, learn how to change gear in the 106. Why knacker a gearbox to prove a point?

EDIT:

May 28, 2011 8:51:02 GMT djefk said:

on anything made with synchromesh on the gears your selecting (so that means all gears on all cars made from around the mid 70s onwards)

Erm, most cars made in the 50's and 60's had syncromesh on all gears (except some povvo-spec cars that had a crash - ie. no syncro - 1st gear)

[dave21478]

I was always of the opinion that the potential for damage to the box was outweighed by the benefits of matching the revs, shifting smoother and causing less strain on engine mounts and the drive train.
I have only had a couple of boxes apart for rebuild, so my knowledge is far from tip-top but I fail to see how it could cause more wear than normal use, apart from as PopupToaster says, extra wear to the clutch actuation mechanism, which I had never really considered.

[rupertw]

As has already been said, rev matching is different to double de-clutching. I always rev match, but only properly double de-clutch on cars with no syncro whatsoever. I would try and explain double de-clutching but I can barely get my head round it myself, but I think it all involves getting shaft speeds to match.
Cars have generally had syncromesh on most gears since the mid thirties and certainly by the sixties it would be extremely rare to find a car without syncromesh on all gears apart perhaps from first.

[Brian Damaged]

May 28, 2011 9:59:19 GMT rupertw said:

Cars have generally had syncromesh on most gears since the mid thirties and certainly by the sixties it would be extremely rare to find a car without syncromesh on all gears apart perhaps from first.

Trucks, however....different story.



Or for the ultimate in rev-matching...'67 Kenworth with 2-stroke Detroit Diesel and 13-speed box....and he only uses the clutch to pull away. Now THAT is skill.

[BenzBoy]

If anyone is reading this and was unsure what exactly rev matching is (like me), this site has quite a clear explanation.

[jrevillug]

May 28, 2011 8:54:06 GMT Adam said:

Double declutching, no. Infact double declutching can cause increased wear on the syncros.

Interesting. I would have thought that double-declutching, done well, would reduce wear on the synchros - you rev-match in neutral (no wear) and then select the gear at the right revs (no wear). I have no proof either way - have you a reference?

The synchromesh on 1st on my car is worn out - it was when I got the car - so if I want to get first while rolling I do have to double declutch.

[automatic]

I only double de clutch if I need to, but I do however rev match on all cars well try to anyway ;D

The driver of the coach that takes us to college told us that it goes into gear far easier withrev matching but has synchro so doesn't need double clutching.

[RobinJI]

I have found that among my friends a lot of people don't seem to realise that rev-matching isn't the same as double de-clutching.

I rev-match all the time and heal-toe when on the brakes, I just find it's a much smoother way of driving.

I've been wondering recently what the consequences of clutch-less shifting would be on a normal syncro-equipped box, as I find that when I do occasionally not rev match a change it'll sometimes turn out that I've not really been pressing the clutch enough.

[spunkymonkey]

Double declutching, if done right, will remove prevent any wear at all to the synchros because you're doing their job for them. The synchros act as individual clutches to equalise the speed between the gear you're selecting and the speed of the gearbox mainshaft.

Remember that, in a synchro box, the gears themselves don't slide in & out of mesh when you change (except for reverse). They sit loose on the mainshaft and the appropriate one is "locked" to the shaft when you select it. Because all the gears are permanently connected, through the layshaft, to the drive wheels, their speed will depend on road speed and will be different for each "cog".

Say you're driving happily in 4th and you want to change down to 3rd. You depress the clutch and everything in the gear box will keep spinning at the same speed because the road wheels are still making it all spin.

Then you move into neutral.

At that point, 4th gear disconnects and the mainshaft, the input shaft and the clutch are no longer connected to the output so they start to slow down. But you're selecting a lower gear so you don't want them to slow down, you want them to speed up so that when you select 3rd they're at the (higher) correct speed for that gear.

Rev matching will bring the engine up to the right speed for the lower gear but if you have your foot on the clutch it won't speed up the clutch, input shaft or the mainshaft. So, as you start to select the gear, the synchro cones literally rub against the side of the gear you're selecting (remember, that gear is being "driven" by the road wheels) and force the mainshaft, input shaft, and clutch driven plate to speed up to match. That is the only time that synchro suffers any wear - as you're selecting the gear it relates to.

If, instead of rev-matching, you bring your foot off the clutch and then rev match, then as the engine speeds up it will also speed up the clutch, input shaft and mainshaft. So when you then dip the clutch again and select the gear, the mainshaft and layshaft are at the correct speeds compared to each other and the synchro doesn't have to do anything.

If you get it absolutely right then you'll prevent any wear at all to the synchro. If you get it slightly wrong then you'll reduce wear significantly. If you get it very wrong then you can increase wear but you'll also end up with very harsh changes because your basic rev-matching will be so far out that you'll know about it!

Whether it's beneficial in real life with modern boxes is questionable because the expected life of synchro (assuming reasonably sympathetic changes) is huge and, as mentioned before, you will be increasing wear on the clutch actuating mechanism (although not the clutch itself).

[popuptoaster]

Its worth learning how to rev match, when the clutch cable snapped on my 24v granny cossie in tintagel in cornwall on a sunday morning I discovered I couldnt get one to replace it so I just drove it the 400 miles back to essex without using the clutch AND I had to get through all the motorway traffic jams as the Farnbourgh airshow was kicking out as we came past.

[Brian Damaged]

May 28, 2011 11:41:07 GMT @automatic said:

The driver of the coach that takes us to college told us that it goes into gear far easier withrev matching but has synchro so doesn't need double clutching.

That's true.......I do it in the wagons at work most of the time, get it right and the changes are seamless. Get it wrong, and you come to a grinding halt in front of a 600 metre tailback. ;D

[stealthstylz]

People use the clutch? I use it for pulling away and coming to a standstill and thats about it lol.

Matt

[datsundrew]

my land rover is smoother without the clutch........unless I have an off day! clutch is really heavy which is why I often not use it, i just take my time with the box and match the engine and gears

[cobblers]

A clutch for a 1.1 106 costs more than a gearbox. Do your worst!

[Deleted]

Any of you double declutchers want to tell me specifically how to get an old no-synchro box into 1st when rolling? Been using them for 15 years and never managed it. Maybe that's why the dogleg first was used - so you can really wrench it in before it breaks all the teeth off first gear ;D