Why would blipping the revs before gear change help?

pault123

My understanding on changing gear with the clutch, is ...

foot off revs - clutch in - gear change - clutch out while matching revs to current speed.

Now when cold in the morning 2nd gear and sometimes 3rd doesn't want to go in, I end up having to double declutch and then it seems to be ok. Other times instead of having resistance to go into gear it drops in too easy with a crunchy feel to it. Once warm I get no problems. (i've changed fluid and had gearbox checked all ok)

I found out this week if I add some revs/blip the revs at the exact moment i'd normally take my foot off the revs - as i'm starting to press the clutch in - 1st to 2nd it seems to slide in perfect.

Mechanically what is occuring here?

I understand when the clutch is pressed in the revs have no *connection* to the gearbox, but what is happening by adding revs a second before and as the clutch pedal goes in to ease my gear changes? Could the flywheel be adding some spin to the clutchplate and easing the change?

Hintza

http://en.wikipedia.org/wiki/Double_declutch

gilbert_and_sullivan

As explained in Hintz'a link, you are matching the speed of the engine to the speed of the required gear, as the revs rise or fall at some point the speeds coincide and the gear just slips in.

When the gearbox is cold and the oil is at its thickest the synchromesh does not work at its best, the oil is making the rings drag, once the oils warmed up, normal service is resumed and the vast majority of drivers can get back to abusing their gearboxes at the normal rate..;)

On old gearboxes that had no syncromesh, you would have to learn to match those gear speeds with engine revs all the time, took considerable driving skill, but a certain pride was gained by being able to drive properly.

Those old fashioned ''crash'' gearboxes are still in use in many parts of the world mainly in trucks, if you go to Gaydon to the classic truck show in May/June you'll see hundreds of trucks with those old type boxes,they are very robust and last for years once mastered.

It might just be worth changing your car gearbox oil, most people never change them, and like all oils they do deteriorate over time.

pault123

But since this is when going up in gear, from 1st to 2nd and i'm doing the rev "blip" just before I press the clutch in and leave 1st gear this is different to matching the revs when unpressing the clutch pedal and in the next gear?

gilbert_and_sullivan

Ah, it would help if i read the post correctly, you have already had the gear oil changed...doh...

You are blipping the throttle just as you put your foot on the clutch, therefore as the clutch releases the revs are rising and as they fall (you will be ready to engage 2nd gear) the gear just slips through, if you didn't blip the throttle, you'd probably find that the revs would drop quicker than 2nd gear lines up, hence you're effectively double declutching without pressing the clutch pedal twice.

I've noticed if you practice clutchless changes with a sunchro gearbox that the gears baulk slightly, this must be due to the synchro or possibly the way the gears are cut, but if you perform clutchless changes on a non synchro box, they slide in perfectly with barely a feel let alone sound of touching.

pault123

gilbert_and_sullivan wrote: »

You are blipping the throttle just as you put your foot on the clutch, therefore as the clutch releases the revs are rising and as they fall (you will be ready to engage 2nd gear) the gear just slips through, if you didn't blip the throttle, you'd probably find that the revs would drop quicker than 2nd gear lines up, hence you're effectively double declutching without pressing the clutch pedal twice.

That sort of makes sense :rotfl:

Been looking at this diagram, mechanically how does a faster spinning clutch plate once the fly wheel has disconnected affect the gear box cogs?

http://www.dorwey.co.uk/images/Lessons%201&2/engine%20clutch%20pedal%20up4.gif

moonrakerz

pault123 wrote: »

gear box cogs?

COGS !!!!!

Conor_3

pault123 wrote: »

But since this is when going up in gear, from 1st to 2nd and i'm doing the rev "blip" just before I press the clutch in and leave 1st gear this is different to matching the revs when unpressing the clutch pedal and in the next gear?

Completely pointless blipping when changing up as well as doing it with your foot on the clutch.

If you're doing it before you put in the clutch to change gear, you don't achieve any speed matching but take the load off the layshafts. The problem with this is that if you're having to do it rather than choosing to, it can indicate that the bearings on the ends of the main and/or layshaft are worn and the shaft isn't running true.

pault123

Conor wrote: »

Completely pointless blipping when changing up as well as doing it with your foot on the clutch.

If you're doing it before you put in the clutch to change gear, you don't achieve any speed matching but take the load off the layshafts. The problem with this is that if you're having to do it rather than choosing to, it can indicate that the bearings on the ends of the main and/or layshaft are worn and the shaft isn't running true.

Conor thats exactly what i'm trying to explain, i'm adding a little excess revs before i'm pressing the clutch in to change gear. If I don't and the car is cold 2nd gear is either hard to engage or slips in very fast with the crunchy sound/feel.

So by the sounds off it i'm easing my layshaft bearing load and helping the gear mesh Car does have 170,000 miles on it could explain it :beer:

oldagetraveller

moonrakerz wrote: »

COGS !!!!!

Yes, cogs (or gearwheels)! http://www.askoxford.com/results/?view=dev_dict&field-12668446=cog&branch=13842570&textsearchtype=exact&sortorder=score%2Cname
When changing gear it used to be described as changing up/down a cog.:D

Conor_3

pault123 wrote: »

Conor thats exactly what i'm trying to explain, i'm adding a little excess revs before i'm pressing the clutch in to change gear. If I don't and the car is cold 2nd gear is either hard to engage or slips in very fast with the crunchy sound/feel.

So by the sounds off it i'm easing my layshaft bearing load and helping the gear mesh Car does have 170,000 miles on it could explain it :beer:

Quite possibly. It could also be that it just needs the oil changing. I take it for it to achieve that mileage that you have been looking after it anyway and topping up the oil as required. Only problem is that, like my capri rear axle, you just end up adding a bit of clean oil to the brown useless slop that's in there. It might be worth getting the lot drained out and some fresh put in. At worst you've wasted a few quid.