Double Clutching / De-Clutching / Shuffling – or how I learnt to love the box.

Alfa’s 105 series gearbsynchro insituoxes are a great design,  borrowing their synchromesh system from Dr Porsche –  the boxes are rugged,  can deal with a lot more power transfer than they were called to when the cars were delivered from the factory  – and are pretty simple to repair.   This last is good – as the synchro system unfortunately wears pretty rapidly – leading to the infamous 2nd gear crunch that the boxes are known for – much earlier in a cars lifetime than is reasonable I think.

 

There are some great modifications you can make to the gearbox and running gear to reduce the load on the synchromesh wearing parts – but one of the best ways is just to adopt a few new routines when changing gears – primarily double de-clutching.

 

Many of us learnt how to Double Clutch / De-Clutch or Shuffle the gear change from 3rd back to 2nd to get around a worn out sychroniser  – but it’s a good habit to get into to both prolong the life of the box,  and gain a little performance !     I’ll leave it to you to decide whether the performance is physical or theatrical…   (we all love a well shuffled downchange into a corner)

 

Some pointers (from a post on the alfabb.)

in this example let’s assume we’re slowing for a corner, changing from 3rd back to 2nd….

* A standard shift –  using no rev-matching and no double de-clutching,  you depress the clutch, change 3rd to 2nd which requires the synchros to work (wear) to bring the weight of the clutch disc, input shaft, layshaft and 2nd-5th gear (they’re constant mesh – always spinning meshed to the layshaft – only the gear you select is locked to the output shaft) to a higher speed and then you let out the clutch, requiring the clutch to work (wear) to bring the engine revs up to speed to match the input shaft, then off you go. 

* A rev matched shift –  You depress the clutch,  blip the throttle and then let the clutch out.  This removes the work/wear of the clutch as you let it out, as you’ve blipped the throttle to bring the engine up to speed for 2nd already. The synchros are still working (wearing) to bring the input shaft, clutch disc layshaft and gears up to the right speed while the clutch is in and you change from 3rd to 2nd.   No help for the synchros there.

* A double de-clutched shift –  you depress the clutch, change to neutral, then let out the clutch and blip – this rev-matches the engine, but also brings the clutch disc, input shaft layshaft and gears up to the right speed in one action as the clutch is engaged, then you depress the clutch, select 2nd (little work/wear on synchros as your clutch, input shaft, lay shaft and gears are already at the right speed) and then let out the clutch – little work/ wear for the clutch as the engine is already up to the right speed too. And you have to do all that quite quickly so the revs of the engine and the clutch/input/layshaft/gears are still moving close to where they need to be. While you’re learning and changing slower – you can blip a little higher than you need to keep things spinning faster longer until you get the hang of it.

synchro rings* On an upshift, eg 2nd to 3rd – you depress the clutch and let off the gas, the engine starts to slow down, as does the input shaft – which is good because the synchros need to slow down the clutch disc, input shaft, layshaft and gears anyway. Then when you let the clutch out it has to work/wear very little to match the engine with the input shaft – as the engine has been dropping speed anyway. Obviously if you take a long time to change gears, the engine / input shaft will be going *too* slow and the synchro / clutch will need to speed them up a little just like a downshift. If you ram the change through, you’re making the synchros work/wear to slow down the input and layshafts, clutch and gears. 

* if you blip between upshifts, you are increasing the engine speed, making the clutch have to work/wear to bring the engine speed back down again. However that also translates as a bit more oomph heading to the back wheels as the clutch transfers some of that energy through. That’s why popping the clutch helps smoke the tyres. That’s the only reason I can think of for blipping on upchanges – if your box is a bit slow on upchanges, blip in the middle and dump the clutch to make up for the delay in changing (Bullitt’s a good example perhaps . Besides – it sounds cool.

* if you blip and double clutch on an upshift – you’re making the synchro work/wear to slow the clutch disc/ input shaft / layshaft / 2nd-5th gears down, and then you’re making the clutch work/wear to slow the engine down, but it sounds cool.

 

A few notes.

Whatever the condition of your box, I think it’s usually more power to the road sooner if you just change up as quickly and smoothly as you can without mashing the synchros too hard by ramming it from one gear to the next unduly. If you are into a little performance art – then blip on the upshift by all means 

Gear and layshaft lightening makes a difference because it reduces the rotating mass of the gears and the input / layshaft the synchros have to speed up / slow down – as do lighter competition clutches. 

It’s also partly why twin plate clutch discs are used – they are a smaller diameter – lower rotating mass – easier to spin up / down by the synchros

Flywheel lightening reduces the rotating mass of the engine – so you can blip up quicker between changes, and have the clutch do a little less work matching the engine speed to the input shaft. lighter flywheels will also make the engine lumpier / pulsier / more likely to stall on takeoff as the flywheel mass usually averages the engine pulses out / allows you to apply clutch at lower revs.

Once the wear on the synchro rings / and the sleeves that bear on them reduces their efficiency, ramming changes will start to grind the dog teeth on the side of the gears / sleeves together. These teeth actually lock the gear in question to the output shaft. More wear and even light changes will start to grind a little. The dog teeth are a fairly cheap replaceable part – but if you snap one them off and it manages to go through the teeth of two drive gears / or into one of the unsealed bearings – bad things will happen. (luckily the gears / bearings usually spit/squeeze any large junk back out until it ends up in the bottom of the box.)

broken dog teeth ring

 

Parting Tip.

When you’ve been sitting at the lights with the gearbox in neutral  – the input, lay and gears  are spinning with the engine.   The lights change and you depress the clutch and shift into first  – this wears the 1st gear synchro as it has to quickly brake all those spinning gears from engine rpm to zero (to match the rear wheel speed)..   this is a pretty hefty load on that synchro – particularly if you are in a hurry.   A great tip here is to actually use the 4th gear synchro to do the work  – both it and 5th gear see very little hard use (5th in particular is still basically brand new in most boxes I rebuild)

So – you’re idling in neutral – the lights change,  depress the clutch,  pull the lever down towards 4th until you feel the resistance of the synchro as it works to match speeds.  As soon as it feels as though the gear will pop in – slide back up into 1st instead – snick !      You could of course use any of the gears to do this – but you tend to want to preserve 2nd and 3rd,  5ths is a bit far away – 4th is the obvious choice.

 

 

One thought on “Double Clutching / De-Clutching / Shuffling – or how I learnt to love the box.

  1. Great article.
    You need more credit for such good and well informed writing.
    Would love some labels of the photo’s though?

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