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Theory of operation of the early DB transmission in 3rd gear


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When I first bought my '22 touring car in 1985, I met Jim Mallars of Stockton, CA (RIP), who was an ardent DB fan and parts re-seller.  His father had owned DB cars since the '20s.  Jim told me that his dad had told him that, before Horace Dodge designed the early DB "reverse shift pattern" transmission, car manufacturers had been having a hard time keeping their transmissions' countershaft bearings quiet because apparently a non-synchro, non-constant mesh transmission puts a lot of stress on the countershaft.  And when the countershaft bearings got noisy, it sounded really terrible, especially in second gear, and made the car sound like a real piece of junk.  Since cars like the DB were driven mostly in top gear, Horace thought it would be a really good idea to disengage the countershaft in third and transmit the power through the main shaft only, thereby avoiding wear on the countershaft.  I have never been very clear about how Horace accomplished this, though.  I've read all the descriptions I can find about how the early DB transmission worked, but I'm afraid I'm not grasping the concept.  Now I'm going to overhaul my transmission, and obviously it would be a good idea to understand its theory of operation first. Could anybody please explain to me how power is transmitted exclusively by the mainshaft in very simple terms?  Thanks very much.  

Edited by 22touring
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I think you mean "constant mesh", rather than "non constant mesh".

There is a large sliding gear at the front of the cluster which, in the indirect gears and neutral, engages with the countershaft.  You will have noticed a howl in neutral from the meshing of these two gears and the occasional crunch when engaging on the move.  When top gear is selected the sliding gears on the mainshaft are disengaged from the countershaft and the large gear at the front is moved back into an internal gear.  This takes the drive off the countershaft and straight through the mainshaft.  This is clearly illustrated in the Book of Informationimg004.jpg.69dd3e786e051b1617edb0554c45f459.jpg

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Posted (edited)

Without a 3D model or seeing it live, that's the best description I've seen on how it works.  These annotated photos may also help a bit.  The key thing that the diagrams don't really explain is that the speedometer drive spiral gear is really 2 gears in one (it drives the speedometer gear/cable and see following).  The first photos shows the main shaft of the trans with cover removed.  The red arrow points to the input shaft, which will be turning at engine speed unless clutch pedal is pushed down (disengaging engine from transmission).  The blue arrow points to the speedo spiral gear.  The first sliding gear on the main shaft (yellow arrow) can slide forward and back on the main shaft as can the other straight-cut gears on main shaft.  When you move the shift lever, the shift forks slide the gears forward or aft depending on gear selected.  This lets the main shaft gears engage (or not) the gears on the counter shaft (the gears do not slide forward/aft on counter shaft).  Getting back to the speedo gear, the second photo shows what this gear looks like in its hollow interior (blue arrow).  It's really a 'negative' of the first sliding gear on the main shaft.  When you select top gear, the shift fork moves this sliding gear into its 'negative' pocket inside the speedo gear, thus locking them together.  At the same time, the other shift forks move the other two gears on main shaft so they do not mesh with counter shaft gears.  The result is that drive torque is transmitted through the main shaft alone and and the counter shaft just sits there unengaged, not turning.  I added a few other photos that may help too.  

 

While the idea of disengaging the counter shaft in top gear may make the transmission quieter, etc., it also makes it much more difficult to achieve a top gear to 2nd gear downshift.  In order to down shift in a non-synchronized gear box, the driver must try and bring the main shaft gear and counter shaft gears up to the same rpm so the teeth can mesh (otherwise, they will grind loudly).  This trick is called 'double clutching' and it goes something like this: push in clutch, blip the throttle, shift to neutral, let out clutch, push in clutch, blip throttle again, shift to 2nd (this is how it's explained for a conventional gearbox where the counter shaft is never disengaged), But in the DB transmission, when driving along in top gear the counter shaft is sitting still.  Only the viscosity of the transmission oil will (hopefully) drag the counter shaft gears along to help a downshift.  This is one reason why really thick trans oil will (sometimes) help with downshifts although I have never been able to get a clean downshift myself (which may be a lack of proper technique on my part).  

1919 Trans w-top off arrows.jpg

Gear Clusters 3.jpg

IMG_6133.jpg

IMG_6188.jpg

Edited by MikeC5 (see edit history)
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Mike, when changing down to second using the double clutch method the countershaft will start moving when neutral is engaged.  The front gear moves out of the internal part of the spiral gear and engages with the front gear of the countershaft (see scan above).  The trick is to get the countershaft moving fast enough to mesh with the gear on the main shaft.  In theory, this is achieved by reengaging the clutch and blipping the throttle.  In practice this requires a sustained pedal-to-the-metal rev during which time your speed up the hill which has necessitated the change down substantially diminishes.  In fact, one might be in danger of rolling backwards!  A silent change down to second is possible but it takes time - considerably more than when changing up.

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The secret is to depress the clutch pedal 

Select neutral

release the clutch pedal

Rev the motor quite hard

depress the clutch pedal and select 2nd gear

release the clutch pedal

 

All this is done much faster than you can say it

perhaps over two to three seconds

 

A clean change is also possible when changing down to 1st using this method

 

You need to be familiar with what road speed you need to change down at in your car (Practice)

 

If done quickly at the correct road speed then very little momentum is lost making hill climbing easier for you and the car following

 

If you slug up the hill until the engine is really labouring then you will always crunch the gears

 

It is more feel than watching the speedo

 

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22, I think you're correct.  But I think any 'crash box' that requires sliding of gears would be classified as such.  This photo shows a constant mesh 5 speed (Subaru) gear box.  The gears are always in mesh.  A 'modern' standard trans uses synchronizers to get the gears to equal speed for shifting and things called 'dogs' do the sliding the engage or disengage the gears.  

 

I see what you mean Tony, when you shift from top gear to neutral, the counter shaft is reengaged.  I think I'm just not revving the engine enough to get 2nd gear to mesh.  From your description Minibago, it sounds like you need to shift quickly after getting the revs up.  I'll keep trying...

Trans.jpg

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Mike, the secret is to change down into second before the road speed drops too low. 
Perhaps if you drive your car in second gear and see what speed you are doing just before you need to change up.

This is the road speed at which you need to change down when going uphill.

The pedals should be worked together but quickly so……..

 

Clutch depressed neutral selected clutch released

As your left foot comes up your right foot is revving the engine (A big rev)

(Left foot Up Right foot down together)

 

Left foot down to select 2nd. Your right foot is lifted off the throttle momentarily 

(Left foot down Right foot up together)


Left foot up right foot down

Clutch released.

Accelerator fully depressed.

From start to finish this should all be done inside one or two seconds.

Road speed will drop a fraction but the quicker the change the less forward momentum is lost.

The only way to get it right is to practice on a small hill to start with preferably on a straight road with no traffic.

 

I hope this is of help, these cars are nice to drive when you can go up and down the box with no noise.

😊

 

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Thanks, I'm going to print this out and have it with me in the car.  Maybe even just sitting in the car in the garage (not running) and going through the motions will help.

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I took a drive this afternoon to a nearby deserted industrial park (Sunday) and practiced shifting.  I did have some success but still need much more practice to be consistent.  Thank you all for your tips on this.  One thing I noticed while doing this is no matter what I tried, I could not prevent a short duration 'crunch' when shifting out of top gear to neutral.  When the mainshaft gear is sliding out of its pocket inside the speedo gear, it's rotating at mainshaft speed (which is dictated by car speed through the rear end and driveshaft).  At essentially the same time, this gear is engaging the more/less stationary counter shaft gear.  Not sure how you can avoid some grinding here.  Or does it indicate my clutch is not adjusted properly (not fully disengaging?)

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No, that crunch is part of life as the moving gear meets its mate on the stationary countershaft.  Your only hope is to fill the gearbox with something which will transfer motion to the countershaft. Banana skins, mercury, wet concrete, small river gravel - the possibilities are endless! 

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