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Final drive ratio help
- Thread starter twospeed
- Start date
lemansvk
Donation Time
Many people would say that the final drive ratio does not change at all - "Final drive" generally being recognised as the reduction ratio, found in the gearset of a drivetrain, that is furthest removed from the engine (Typically, the differential ratio). 
What it will change is the "overall drive ratio" (the ratio of engine revolutions to road wheel revolutions). In direct drive 4th gear this will be 3.89:1 The overdrive has a ratio of 0.802:1, meaning that in overdrive 4th the overall drive ratio will be 3.12:1
Cheers, Vic
What it will change is the "overall drive ratio" (the ratio of engine revolutions to road wheel revolutions). In direct drive 4th gear this will be 3.89:1 The overdrive has a ratio of 0.802:1, meaning that in overdrive 4th the overall drive ratio will be 3.12:1
Cheers, Vic
alpine_64
Donation Time
Te guys are right if you use the factory alpine OD.. but there were a few ratios of the overdrive unit, for both other cars, and th rootes ground (as found in rapiers, minx, vogue etc.. ) so you really need to know which OD unit you have. Either way keeping the 3.89 will be a good move.
V_Mad
Donation Time
While on the subject, I have read that there was a 3.7 axle that could be used on the Alpines. I know this wasnt factory on the series cars but I wonder if there is a 3.7 (or anything lower down to about 3.5) gearset from another Rootes car that could be fitted into the Alpine axle casing?
RootesRacer
Donation Time
The 3.70 was used on many arrow based cars, likely for you, the most common would be the hunter.
The 3.70 was the highest gearset offered that would fit in the small rootes case.
Anything higher and it would need a swap from another car (like a mustang II or pinto).
RootesRacer
Donation Time
Laycock made 2 ratios for the D type, 25% and 32%. Both are approximations of course, but the 32% uses a single gearface planet and the 25% uses a multiple gearface planet.
Sunbeams used 25% ratio and most hillmans used the 32% ratio.
Not sure about Singers, but the probably used the 32% also.
V_Mad
Donation Time
Slightly puzzled by expressing ratio as %. What is that in normal money?
(I presume 25% is not 4:1 but 1:0.75 and 35% is 1:0.65).
husky drvr
Platinum Level Sponsor
Chris,
I believe that the 25% and 33% refers to the amount of RPM drop caused by engaging the OD.
The Alpine's OD ratio is 0.802:1 approximately.
The early Rapier's OD is 0.756:1 approximately.
1000 - 802 = 198 >..................> 198 / 802 = .247 or 24.7%
................................................> 244 / 756 = .323 or 32.3%
I am not positive but I think Rootes dropped the 33% OD entirely after the Rapier series II ( about the time of the Alpine's introduction ).
HTH,
I believe that the 25% and 33% refers to the amount of RPM drop caused by engaging the OD.
The Alpine's OD ratio is 0.802:1 approximately.
The early Rapier's OD is 0.756:1 approximately.
1000 - 802 = 198 >..................> 198 / 802 = .247 or 24.7%
................................................> 244 / 756 = .323 or 32.3%
I am not positive but I think Rootes dropped the 33% OD entirely after the Rapier series II ( about the time of the Alpine's introduction ).
HTH,
RootesRacer
Donation Time
You must never have had an over drive huh?
The 25 and 32 is part of the laycock model number and is printed on the data tag.
To find the actual final drive ratio, add 1 (1.25) then take the reciprocal (0.80).
Like I said earlier, Laycock rounded the ratios for thier model numbers, so they are approximations.
Don,
Although your math is OK, that is a strange way to describe a 25% "reduction". Any schoolboy knows that when using math to describe a reduction you use the original number, not the reduced number, as the denominator in the calculation. But, of course, it is quite possible that this is how the British engineers described it.
But more commonly, one would say that the 25% refers to the INCREASE in road speed for a given RPM. The road speed increases by 1000/802, or 1.25, or 25% when the OD is engaged.
Now the math as you described gets to the same value, of course, and may well be how a Rootes engineer would describe it- I don't know- but I know my 6th grade math teacher would have flunked me if I said a reduction of 1000 down to 800 was anything but a 20% reduction.
Tom
Although your math is OK, that is a strange way to describe a 25% "reduction". Any schoolboy knows that when using math to describe a reduction you use the original number, not the reduced number, as the denominator in the calculation. But, of course, it is quite possible that this is how the British engineers described it.
But more commonly, one would say that the 25% refers to the INCREASE in road speed for a given RPM. The road speed increases by 1000/802, or 1.25, or 25% when the OD is engaged.
Now the math as you described gets to the same value, of course, and may well be how a Rootes engineer would describe it- I don't know- but I know my 6th grade math teacher would have flunked me if I said a reduction of 1000 down to 800 was anything but a 20% reduction.
Tom
husky drvr
Platinum Level Sponsor
Tom,
Ah, the lovely world of percentages - always able to twist things around. Percentages must be a sibling of statistics. Together they can prove anything.
I tend to work problems in a reverse direction than most people. I knew there was a simpler way to describe the math but was still mostly asleep and missed remembering your method. It is a matter of deciding to use a constant input speed or a constant output speed when describing the OD effect.
Jan,
You asked how many teeth on each gear. I'm not sure if that was a test question for us or if you wanted to know. But I checked a disassembled Laycock OD unit I have. I found the Sun gear has 19 teeth, Each Planet as 20 teeth, and the Annulus has 59 teeth. After drawing lots of diagrams and straining my brain I calculate that this gives an output of (1+19/59) turns for every input turn. That is 1.322 output turns per input turn. Or 0.756 ratio as normally stated. From this I am figuring it is NOT an Alpine OD. Note that in all these ODs the Sun gear is held stationary and the input drives the planetary carrier. And I see comparing this OD with the WSM pictures what V-Mad means about mulitple gearface Planets. The gears on this OD are single face.
I would guess from this that a corrrect Alpine gear set would have 17 teeth on the Sun and 69 on the Annulus. Or possibly 16 Sun and 65 Annulus. Either combo yields a .802 ratio when rounded. From my diagrams and analysis the teeth on the planet gears do not enter the ratio calculation when the Sun is held stationary. So they could still be 20 or maybe 19 or 21, whatever makes the best mesh and strength. I'm guessing this is why the two-faced planet gears. To make a good mesh with both the Sun and the Annulus.
Tom
You asked how many teeth on each gear. I'm not sure if that was a test question for us or if you wanted to know. But I checked a disassembled Laycock OD unit I have. I found the Sun gear has 19 teeth, Each Planet as 20 teeth, and the Annulus has 59 teeth. After drawing lots of diagrams and straining my brain I calculate that this gives an output of (1+19/59) turns for every input turn. That is 1.322 output turns per input turn. Or 0.756 ratio as normally stated. From this I am figuring it is NOT an Alpine OD. Note that in all these ODs the Sun gear is held stationary and the input drives the planetary carrier. And I see comparing this OD with the WSM pictures what V-Mad means about mulitple gearface Planets. The gears on this OD are single face.
I would guess from this that a corrrect Alpine gear set would have 17 teeth on the Sun and 69 on the Annulus. Or possibly 16 Sun and 65 Annulus. Either combo yields a .802 ratio when rounded. From my diagrams and analysis the teeth on the planet gears do not enter the ratio calculation when the Sun is held stationary. So they could still be 20 or maybe 19 or 21, whatever makes the best mesh and strength. I'm guessing this is why the two-faced planet gears. To make a good mesh with both the Sun and the Annulus.
Tom
jumpinjan
Bronze Level Sponsor
Tom,
I was just curious, because I don't mess around with our OD units, I send them off to the UK for repair.
I have been studying planetary gear sets over the past year, and I can't understand how one can manage to hold the sun gear stationary?
I have seen some Brog-Warner boat transmissions, that use a double reduction planet (cluster) gears. Interesting enough, most all planetary transmissions, I have seen in use are: ring gear stationary, sun gear input, planet carrier output (Input rotation = Output rotation). Have you seen this page:
http://auto.howstuffworks.com/automatic-transmission1.htm
If the planets get smaller in diameter, they revolve very fast, so the bearing life is less, but to add more planets (from 3, 4, 6 and more) they can handle more torque.
Jan
I was just curious, because I don't mess around with our OD units, I send them off to the UK for repair.
I have been studying planetary gear sets over the past year, and I can't understand how one can manage to hold the sun gear stationary?
I have seen some Brog-Warner boat transmissions, that use a double reduction planet (cluster) gears. Interesting enough, most all planetary transmissions, I have seen in use are: ring gear stationary, sun gear input, planet carrier output (Input rotation = Output rotation). Have you seen this page:
http://auto.howstuffworks.com/automatic-transmission1.htm
If the planets get smaller in diameter, they revolve very fast, so the bearing life is less, but to add more planets (from 3, 4, 6 and more) they can handle more torque.
Jan
Jan,
Yes, I saw that page AFTER i figured it out. At first I looked at Wikipedia's explanation, but could not get it. Also I had assumed that our ODs hold the ring stationary. Then I read the WSM and looked at the unit I had. It's really quite simple how it holds the Sun gear. The Sun gear is spined to the clutch assembly which can clutch to the main housing. Then the input shaft goes through the Sun and is splined into the planet assembly. The ring is then splined to the output shaft.
Yes, I saw that page AFTER i figured it out. At first I looked at Wikipedia's explanation, but could not get it. Also I had assumed that our ODs hold the ring stationary. Then I read the WSM and looked at the unit I had. It's really quite simple how it holds the Sun gear. The Sun gear is spined to the clutch assembly which can clutch to the main housing. Then the input shaft goes through the Sun and is splined into the planet assembly. The ring is then splined to the output shaft.
Then why did mine break? I did none of the above, switches were fine, quit at about 30,000 miles. I think the major stress on them occurs when going from OD to direct with the throttle closed. Especially when in third, which I did very often. Just makes sense with a 1600 cc engine.
When Tiger Tom was giving the flashlight tech session in Georgia, he stated the major cause of failure was twisting of the shaft. I've tried to find out which way the shaft twists, i.e. from engine torque or engine braking. Does anyone know?
Bill