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Umm, gearboxes...


Jack

Question

I may be stupid but...

 

I am asking this for a friend of mine who is slightly a bit dim :whistle:

 

But could some explain/write a 'how this works' explaintion of the mechanics of a gearbox........in extra laymans terms please.

 

So I can pass it on to my chum - *cough cough*

 

Many thanks.

 

Jack.

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But could some explain/write a 'how this works' explaintion of the mechanics of a gearbox........

 

Now what type of gearbox, is it;

 

epicyclic

crash

pre-selector

synchromesh

constant mesh

pneumo-cyclic

automatic

etc.............................

 

We don't want to waste time doing the wrong one ;-)

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Now what type of gearbox, is it;

 

epicyclic

crash

pre-selector

synchromesh

constant mesh

pneumo-cyclic

automatic

etc.............................

 

We don't want to waste time doing the wrong one ;-)

 

 

Umm.......I will ask him and get back to you Richard as I did point the above out to him :whistle: ( cos who wouldn't know that stuff) and I will report back in the morning :computerterror:

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Wouldn't crash and constant mesh be the same thing? :?

 

 

Richard,

 

Your AEC has a crash box, the actual gear teeth sliding in and out of mesh. A good example of a constant mesh box is a Fuller Roadranger, this is much like a motorcycle gearbox where the teeth are in mesh all the time but are slid along the shafts by selectors and dog clutches engage the drive. The Roadranger is a very sound box, we ran them on Scammells and Fodens with little trouble and very nice to use.

 

Richard

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well actually it to know how my Jimmy box works....a crash, straight cut box.

 

This question was prompted by seeing Chapper pictures of his box over on http://www.cckw.org.

 

I will see if he would post them here. I was taught all this when I was at agricultural college but I think at that age I was too busy fighting ladies off :whistle:

 

 

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I took my GMC out for a quick spin at the weekend, :banghead: or rather a crunch grind clatter. :nono: The noise coming from the box when I tried (without success) to get it into fourth or fifth was awful. :argh: Does anyone have any information on how to work the gearbox properly? :rtfm:

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Fourth and fifth gear ratios are very close together, so try changing gear a bit quicker between those two. All the other gear ratios are spaced further apart, so the change is a it slower. Its been a while since i drove a jimmy, but i seem to remember having the same problem. :police:

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I am double declutching (at least I think I am :dunno:) but sometimes it works and other times it just doesn't engage. :? I've never managed to get it into fourth or fifth at all. :oops: I guess I just have to get a demonstration some time, maybe at Beltring.

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I am double declutching (at least I think I am :dunno:) but sometimes it works and other times it just doesn't engage. :? I've never managed to get it into fourth or fifth at all. :oops: I guess I just have to get a demonstration some time, maybe at Beltring.

 

 

Double declutching is easy enough with practice. For changing up:

 

Attain the right road speed for the gear, hold it a few seconds.

Depress clutch and move gear shift to neutral

Release clutch

Pause a moment

Depress clutch and move gear shift into required gear

Release clutch and accelerate away

 

For changing down:

 

Brake or bleed off speed till a gear change is required

Depress clutch and move gear shift to neutral

Release clutch

"Blip" the accelerator such that the engine noticeably revs up

Depress clutch and move gear shift into required gear

 

The problems usually lie in the pause for the up shift and the blip of the accelerator for the down shift - thats where the practice comes in. I always found it to be different on every vehicle, even those of the same type, so getting baulking and grinding for the first few shifts is nothing to be upset about - I drove a 6x4 Militant tipper for a week on exercise and had the shift really sweet then had to hand it in and take over a 6x6 GS Militant. Jumped in expecting to drive out as I'd driven in - but for the first 20 miles or so I had to stop to get a lower gear :oops: Then I got the "feel" of the vehicle and things progressed back to the point I was in the tipper. :-)

 

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Double declutching is easy enough with practice. For changing up:

 

Attain the right road speed for the gear, hold it a few seconds.

Depress clutch and move gear shift to neutral

Release clutch

Pause a moment

Depress clutch and move gear shift into required gear

Release clutch and accelerate away

 

 

 

 

 

The one thing I would say is not to depress the clutch for too long before trying to select the required gear. Remember, you're trying to match the engine speed and the road speed to the ratio of the gear you are going to change to. The second the clutch is depressed the gearbox is no longer linked to the engine and the two start to rotate at different speeds and then you have trouble trying to engage the gear.

The way I learnt with my Militant Mk1 was to not use the clutch. It sounds a horrible thought but in practice you can "feel" the gear in much easier than when using the clutch and the changes were smooth and silent. Once I got the timming right I started to use the clutch. Saying that, even now I still miss the odd gearchange but I think thats all part of the fun of a crash box.

 

Richard

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It might be a good idea to get another GMC owner to try your gearbox, there must be other owners in your general area, any body on the forum in Epsom area that could help?

 

I'm sure Grasshopper or I can offer help if you can limp over to Croydon (Coulsdon) some time?

 

If you're finding you lose too much road speed when trying to change up, you're probably changing too early. Sorry :-) . I know it can be embarrasing and seems wrong but you're driving a 60-year-old truck. You can console yourself that you're way cooler than anything in the queue behind you 8-) . Often you just have to sit in a lower gear with the engine revving a bit until you reach a flatter bit of road.

 

You can practice double de-clutching on your everyday car and you'll find you get much smoother gear changes. (People will tell you that double de-clutching on a synchromesh 'box damages the synchro's but it's valuable practice, and would you rather practice on a venerable Jimmy or a Eurobox?).

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I think maybe Richard has put his finger on it when saying you might be trying to change up too early. Sounds crazy but I found - again with Mk 1 Militants - that you needed to hold the road speed for a minute (-ish) after reaching it before trying the up-shift. And the clutch movement is a rapid down-up, gear out, down-up, gear in action going up.

Oh - and have the hand on the gear lever before touching the clutch; again as Richard rightly says - too long and the synchronised speed of the components is lost!!!

 

Often wonder these days how long it would take to get back into the hang of going up and down a Militants box after not having sat in one for 30 years....

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Often wonder these days how long it would take to get back into the hang of going up and down a Militants box after not having sat in one for 30 years....

 

 

Its only been about 20 years since I drove a crash box on a regular basis, having just aquired the Militant its still taking a bit to get into the swing of things again, going up the box is easy and a lot easier without using the clutch, going down is just timing. Last time we took the Militant out was to fill it up with diesel because we were moving lots of things around the yard and didnt want to bleed it though again :whistle: on the way back from the petrol station there is a long steep hill back to the yard, anway missed the down shift and decided just to coast down the hill, it was great fun listening to those 1600 x 20's rumbling away at 45 mph, what was scary was how quickly the air pressure dropped and anyone who has driven the 22 ton green lump will testify how poor the brakes are on a MK 3 recovery at the best of times. Twas great fun just glad we didnt have to stop in a hurry :|

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Heh -if you think the Mk 3's brakes are bad - try a Mk 1's!!!! :evil:

 

I tried coasting down hill once - and only once - never again.... Had all the windows open and the cupola cover off for a few miles after to remove the funny smell!!! :schocked: :schocked:

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