Root cause of 915 1-2 slow shifting ?

[avendlerdp]

Drive an Alfa Romeo for a few years and you'll never have trouble shifting your 915 again.

[ChrisBennet]

Originally posted by avendlerdp
Drive an Alfa Romeo for a few years and you'll never have trouble shifting your 915 again.

At one point, Alfa's used the Porsche synchro setup. I don't know when that was though.
-Chris

[MichiganMat]

I have a theory:

Is the change in gear ratio greater from 1st to 2nd than 3rd to 4th? Could it be that the 2nd gear syncro is doing more work than other syncros in the box?

[KobaltBlau]

Mat, more likely that the 2nd gear synchro gets used more so it's just more tired.

Chris, yes, pre-transaxle post-war alfas (giulietta, giulia) used the porsche patent balk ring syncros, not sure about the transaxle (alfetta, milano) cars.

[avendlerdp]

The transaxle Alfa's sucked too.

[NY65912]

I was taught to drive a stick in a '59 Mack B61 and a '60 White cab over 3000gal fuel oil trucks, a '58 GMC 5yd dump truck and a '63 Ford service van, (family run fuel oil company). I am no stranger to double clutching and learning to have patience in shifting.

You have NO idea what is was like TRYING to shift the White, being a COE design, the shifter (bent in a z like shape) was basically a "remote" setup, about six feet plus ahead of the trans and two feet above the trans, it was like extending your arm fully and mixing in a bowl.

On the later trucks, '70 Intl, '78 Mack MB, '77 Mack U Model, '90 Mack R Model and a '88 DM (5500gal tanks), I could shift each gear both up and down with NO clutch. All you have to do is match the input shaft speed and output shaft speed by using the throttle (blipping) and they meshed seamlessly! Clutches lasted many miles even hauling huge amounts of weight, the only time the clutch was used was taking off from a stop.

I'm completely satisfied with my trans in the 911.

So, could the synchro's be remanufactured to give smoother shifts.
Again this is just an exercise in what could be.

Cheers......

[Geary]

I think the earlier BMWs also had the Porsche-patented synchro.

The second gear synchro does indeed have a harder job to perform (and gets worn more quickly due to lots of downshifting action) because the idler gear is physically larger/heavier than 3rd and up. More weight = harder to synchronize. This is why it's good from a manufacturing standpoint to remove as much weight as possible from large diameter idler gears. (GT-brand 915 gears are right around 15% lighter than comparable "drilled" gears, because they are side-profiled for weight reduction.)

[rick-l]

I think this animation is so cool
http://auto.howstuffworks.com/transmission5.htm

Never having taken a Posche transmission apart, is this how the 915 syncronizers work? What is different?
http://auto.howstuffworks.com/transmission6.htm

[Bill Verburg]

I would take Geary's reply as the one to pay attention to.

As to Alfas, I always thought my '69 boat-tail was one of the sweetest shifting transmissions I ever used. That was back in my younger and umm .. more exuberant days. I beat the snot out of it getting rubber going thur the gears etc. never a glitch or missed shift.

[MichiganMat]

ok, so lets fire up the lathe and get to work!

Who's first?

[kucharskimb]

the only thing swepco is good for is differentiating between your transmission and engine leaks!!...

[midlife]

The second gear synchro does indeed have a harder job to perform (and gets worn more quickly due to lots of downshifting action) because the idler gear is physically larger/heavier than 3rd and up. More weight = harder to synchronize. This is why it's good from a manufacturing standpoint to remove as much weight as possible from large diameter idler gears. (GT-brand 915 gears are right around 15% lighter than comparable "drilled" gears, because they are side-profiled for weight reduction.)

I'm no transmission expert. (I have just started dismantling a 915 though), but it seems to me that every shift has to syncronize the mass of the input shaft, the clutch, and every slider and gear fixed to the input shaft, regardless of the gear you are selecting. The output shaft is hard linked to the wheels, and at a fixed speed. Third and fourth gear are fixed at the output shaft, and hence are part of it’s rotating mass. The rest of the gears are fixed at the input shaft, and act as one rotating mass.
That being said, when you select fourth gear, the fourth gear syncro has to accelerate all the input shaft mass including both halves of second gear.
Lightening the mass of second gear would help all syncros not just second. Lightening third and fourth should have no effect, (other than vehicle weight and overall inertia).

I think that Porsche maybe should have fixed all the heavy gears to the output shaft, and fixed third and fourth to the input.

[Geary]

What you say about rotational mass is basically correct, and I oversimplified my last statement, so let me try to explain another way. Let’s compare two 2nd gear ratios: a low 2.19 (large diameter idler) and a 1.500 (this is a typical range of 2nd gears used in 915 racing). The weight of all parts on the pinon shaft (except for the idler gear that you are shifting into) can be negated, because this shaft is already spinning at the targeted rotational speed. The 2.19 is about 100 grams heavier than the 1.50, and about 3/4” larger in diameter. Weight removal from the 2.19 therefore, is more important (than from the 1.50) because of where that weight is located in relation to the center point.

Secondly, because shifting into a low ratio idler on the pinion shaft is less influenced by the rotating mass of the input shaft (than with a taller ratio), weight removal from the physically larger 2.19 idler is relatively more important than from the 1.50.

Racers have always found that removing weight from the idler gear translates to a more quickly energized synchronizer -- not only because of the above, but also in part because it take a nano second for all the other rotating pieces to “catch up”, due to backlash clearances. The importance of this initial energization really becomes more pronounced with the later 6-speed transmissions, where the gears are huge in comparison to those of the 915, and where almost an additional half-pound can be removed from a 2.25 2nd gear idler with careful profiling. We get a fair amount of positive feedback in this area.

Beyond this layman’s explanation, we really need input from a guy like Hayden, who not only is a 915 transmission pro, but has an engineering background. (a lot smarter than me, because I am/have neither)