How is Overdrive % calculated?

[rockspider]

I know that VP declared 6,5% and 33% OD for the Phoenix, but how do they get those numbers?
I tried getting my calculations with teeth counting, ratio and all, and got 6,3 for low (acceptable tolerance) but 60% on high! How does?

Ok, maybe I could have messed up numbers?
Letr's try the old fashioned way, a piece of adesive tape on top of each tire, and a full rear revolution.
On low OD, the front made one full turn and a little bit more, let's say on the 5 minute mark if comparing to a swatch. Ok.
But on High OD the front made more than one and a half turn, about on the 35 minute swatch mark. That's compatible with my calculations of 60%.
If it really was 33% like VP declared, shouldn't the front tire have turned one and one-third of a turn (20 minute mark) for a single rear revolution?
What am I missing?

[themountain]

https://www.youtube.com/watch?v=RZS3i59okq4&t=804s

[rockspider]

Thanks for the link to that very informative video, I had missed it

[TKW75]

Originally Posted by rockspider

I know that VP declared 6,5% and 33% OD for the Phoenix, but how do they get those numbers?
I tried getting my calculations with teeth counting, ratio and all, and got 6,3 for low (acceptable tolerance) but 60% on high! How does?

Ok, maybe I could have messed up numbers?
Letr's try the old fashioned way, a piece of adesive tape on top of each tire, and a full rear revolution.
On low OD, the front made one full turn and a little bit more, let's say on the 5 minute mark if comparing to a swatch. Ok.
But on High OD the front made more than one and a half turn, about on the 35 minute swatch mark. That's compatible with my calculations of 60%.
If it really was 33% like VP declared, shouldn't the front tire have turned one and one-third of a turn (20 minute mark) for a single rear revolution?
What am I missing?

So, I believe there was an error when VP calculated the high OD gear ratio. Like you I cannot determine how the 33% OD is true as the tire rotations do not match up with anything close to that number.

I calculated the various ratios of the transfer case output and found that the mistake is most likely in how the high OD is calculated. The design of the transfer case uses 3 shafts with a 23T input gear and a 32T output gear to the rear.

This results in a ratio of 1.39:1 (32/23 = 1.39)

The standard front OD output uses the same 23T input gear, then power flows through the 32T rear output gear, to a 20T idler gear, and finally to the 30T output gear.

This results in a ratio of 1.30:1 (32/23 * 20/32 * 30/20 = 32/20 = 1.30)

The high front OD output swaps the final 30T output gear for a 25T output gear, but this is where the error occurs. The transfer case is laid out with the following power flow.



The above image shows the power flow from the 23T output gear, to the 32T rear output gear, then to the 20T idler gear, then to the 25T idler gear (locked to the same shaft as the 20T idler), then to the final 25T output gear that is connect to the front driveshaft. I believe it is this 20T to 25T idler shaft that is source of the error. As the 20T and 25T gears are on the same shaft, their ratio is 1:1 meaning for every turn of the shaft they both rotate once. I believe VP has added the 20T idler to 25T output gear as a ratio of 25/20 while in actuality it is the output shaft spins at the same speed as the idler shaft due to the 25T to 25T gear set between the idler and output shaft.

This results in a front high OD ratio of 0.87:1 (32/23 * 20/32 * 25/25 = 20/23 = 0.87)

Taking all of this, and using the ratios shown below for the rest of the drivetrain provides the following overall gear ratios:

Rear output gear ratio = 57.25:1
Front standard OD gear ratio = 53.67:1
Front high OD gear ratio = 35.78

Using the OD formula provided by Harley (OD ratio = Difference in Ratios / Average of Ratios) the OD ratios are as follows:

Standard Front OD: (57.25 – 53.67) / ((57.25 + 53.67)/2) = 3.58 / 55.46 = 6.5%
High Ratio Front OD: (57.25 – 35.78 ) / ((57.25 + 35.78 )/2) = 21.47 / 46.51 = 46.2%

So, in the end the high OD is 46.2% overdriven as compared to the rear, not 33% as indicated in the manuals.

Below are the ratios for every gear to gear combination in a standard Phoenix so everyone can check my math.

I hope this helps and if I have made any mistakes, please let me know!

TKW75

All ratios for Phoenix in stock configuration:

Spur / Pinion Gear Ratio:
Spur: 52T (driven)
Pinion: 15T (drive)
Gear Ratio = Spur / Pinion = 52 / 15 = 3.47


Transmission Gear Ratio
Input Gear: 23T
Output Gear: 52T
Gear Ratio = Output / Input = 52 / 23 = 2.26


Axle Gear Ratio

Ring and Pinion Ratio
Ring Gear: 30T
Pinion Gear: 9T
Gear Ratio = Ring / Pinion = 30 / 9 = 3.75

Portal Gear Ratio
Input Gear: 20T
Output Gear: 28T
Gear Ratio = Output / Input = 28 / 20 = 1.40


Transfer Case Gear Ratio

Rear Output Gear Ratio
Input Gear: 23T
Output Gear: 32T
Rear Output Ratio = Output / Input = 32 / 23 = 1.39

Front Output Gear Ratio - Standard OD Gearset
Input Gear: 23T
Output Gear: 30T
Front Output Gear Ratio - Std OD = 30 / 23 = 1.30

Front Output Gear Ratio - High OD Gearset
Input Gear: 23T
Output Gear: 20T
Front Output Gear Ratio - High OD = 20 / 23 = 0.87

[rockspider]

Thanks for the detailed post, with all ratios and calculations.
Apart from the formula provided from Harley (which I didn't know) our considerations were quite similar.

Looking at the most simple evaluation of ratios, in High OD the relation between rear output and front output is 32t to 20t: 32/20=1,6
for one turn of rear output shaft, the front output shaft make 1,6 revolutions.
Which leads to having front tire making 1,6 revs for 1 rear tire rev.
Does this computes to a 46.2% OD using Harley formula? Yes.
Has that formula any practical sense, considering that everybody can check and the front tire is making 60% more revolutions, or 60% more distance if not subject to the rear tire drag? ... mah...
Personally, I think that calling it a 60% OD would describe it more correctly, but will accept the result of Harley formula if that's what's being used from everybody...

[boldfin]

I'm no math whiz, but I do like to look at FDR and OD when I'm building a new rig, so I have set up a simple spreadsheet that does the calcs according to the same approach you guys have taken. And as far as I can see, your calcs are correct. I'm getting 46.25% OD on the High OD setting:



My spreadsheet allows for inputting all sorts of different portal and axle gearing combos - as well as Spur and Pinion combos.

I don't know if anyone noticed, but Vanquish actually lists two different portal gear sets in the Parts List in the instruction manual. Interestingly, though there is a "Standard" portal gear set, the one included with the kit is supposedly VPS08353 - which is the so-called "Overdrive" portal gear set.





But I was still unable to come up with a combination of Portal or Axle R&P gear combos that would get to a 33% OD on High OD setting. I may be missing something obvious though.

So yeah, I do think it's very interesting that Vanquish is calling it a 33% OD. I'd really love to hear their explanation.

[TKW75]

Originally Posted by boldfin

I'm no math whiz, but I do like to look at FDR and OD when I'm building a new rig, so I have set up a simple spreadsheet that does the calcs according to the same approach you guys have taken. And as far as I can see, your calcs are correct. I'm getting 46.25% OD on the High OD setting:

My spreadsheet allows for inputting all sorts of different portal and axle gearing combos - as well as Spur and Pinion combos.

I don't know if anyone noticed, but Vanquish actually lists two different portal gear sets in the Parts List in the instruction manual. Interestingly, though there is a "Standard" portal gear set, the one included with the kit is supposedly VPS08353 - which is the so-called "Overdrive" portal gear set.

But I was still unable to come up with a combination of Portal or Axle R&P gear combos that would get to a 33% OD on High OD setting. I may be missing something obvious though.

So yeah, I do think it's very interesting that Vanquish is calling it a 33% OD. I'd really love to hear their explanation.

Boldfin,
Thanks for the second opinion; glad to see we are getting the same result. I think there was simply an error in a spreadsheet at VP that had a few numbers crossed. Very easy to do for sure. The interesting thing is that if you look at the drive ratio charts that VP included, the 33% OD ratio chart is incorrect, and the ratios listed match with my previous post where the front OD is calculated with a 23T input and 25T output. However, if you calculate the OD percentage using the values in the rear ratio and 33% front ratio, they result in a 25% overdrive, not the 33% overdrive as listed. Very strange for sure.

Originally Posted by rockspider

Thanks for the detailed post, with all ratios and calculations.
Apart from the formula provided from Harley (which I didn't know) our considerations were quite similar.

Looking at the most simple evaluation of ratios, in High OD the relation between rear output and front output is 32t to 20t: 32/20=1,6
for one turn of rear output shaft, the front output shaft make 1,6 revolutions.
Which leads to having front tire making 1,6 revs for 1 rear tire rev.
Does this computes to a 46.2% OD using Harley formula? Yes.
Has that formula any practical sense, considering that everybody can check and the front tire is making 60% more revolutions, or 60% more distance if not subject to the rear tire drag? ... mah...
Personally, I think that calling it a 60% OD would describe it more correctly, but will accept the result of Harley formula if that's what's being used from everybody...

Rockspider,

The choice of formula to use for this calculation can be thought of two ways, which is why the resulting overdrive is either 42.6% or 60%, as you have stated. The reason for this is viewpoint.

If you consider the rear drive the ‘standard’ drive, then calculating a simple overdrive percentage results in the following:

Overdrive = (ratio difference) / rear ratio = (57.25 – 35.78 ) / 57.25 = 37.5%
This indicates that the front is 37.5% faster than the rear, when considering the rear as the basis.

If we consider the front drive as the ‘standard’ drive, then calculating a simple overdrive percentage results in the following:

Overdrive = (ratio difference) / front ratio = (35.78 – 57.25) / 35.78 = -60.0%
This indicates that the rear is 60% slower than the front, when considering the front as the basis.

The reason for the use of the average of the two is to remove this perspective (the front is 37.5% overdriven compared to the rear or the rear is 60% underdriven as compared to the front) and create a single value that relates them in the same manor.

Overdrive = (ratio difference) / (average ratio) = 46.2% as shown in previous post

This can either be positive (front is overdriven compared to the rear) or negative (rear is underdriven compared to the front). It is really a ratio difference rather than a pure ratio.

I hope this helps and have a great day!
TKW75

[rockspider]

Ok, I see your point. It would be messy to have different numbers if considered front overdrive or rear underdrive, so that average formula makes more sense now

[DRED805]

Did we reach a conclusion on this? Has anyone marked their tires and measured the OD on the vfd twin the no-brainer way?

[TKW75]

Originally Posted by DRED805

Did we reach a conclusion on this? Has anyone marked their tires and measured the OD on the vfd twin the no-brainer way?

Only anecdotally with observations that it appears the OD % is much higher than 33%. If I can, I will try to put together a quick check sometime this evening and report back the 'scientific' findings.

[twade984]

Originally Posted by DRED805

Did we reach a conclusion on this? Has anyone marked their tires and measured the OD on the vfd twin the no-brainer way?

I have reached a conclusion…

Math is hard, and I will leave it to people smarter than me.


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[TKW75]

Originally Posted by TKW75

Only anecdotally with observations that it appears the OD % is much higher than 33%. If I can, I will try to put together a quick check sometime this evening and report back the 'scientific' findings.

I completed the 'tape the tires and count rotations' measurement and here are my results.

First, I placed the truck on some jack stands to get the tires off the ground and put a small piece of tape at 12 0'clock on a front and rear tire. I used my cell phone to record a video of 10 rear tire rotations both in low and high OD conditions.

For the low OD, I recorded 10.7 front tire rotations. This gives an average of 10.35 tire rotations and a delta of 0.7.
OD % = rotation delta / average rotations = 0.7 / 10.35 = 0.0676 = 6.76% OD
This aligns well with the 6.5% given the accuracy of my counting partial rotations and amount of backlash in the drivetrain.

For the high OD, I recorded 16.7 front tire rotations. This gives an average of 13.35 tire rotations and a delta of 6.7.
OD % = rotations delta / average rotations = 6.7 / 13.35 = 0.5018 = 50.18% OD
Again, this aligns well with the 46.2% OD given the accuracy of my counting partial rotations and amount of backlash in the drivetrain.

I would consider this proper supporting evidence of the actual OD %, which is significantly larger than reported in the documentation provided by VP. Funny how a simple error can propagate.

I hope this helps.
TKW75

[DRED805]

Originally Posted by TKW75

I completed the 'tape the tires and count rotations' measurement and here are my results.

First, I placed the truck on some jack stands to get the tires off the ground and put a small piece of tape at 12 0'clock on a front and rear tire. I used my cell phone to record a video of 10 rear tire rotations both in low and high OD conditions.

For the low OD, I recorded 10.7 front tire rotations. This gives an average of 10.35 tire rotations and a delta of 0.7.
OD % = rotation delta / average rotations = 0.7 / 10.35 = 0.0676 = 6.76% OD
This aligns well with the 6.5% given the accuracy of my counting partial rotations and amount of backlash in the drivetrain.

For the high OD, I recorded 16.7 front tire rotations. This gives an average of 13.35 tire rotations and a delta of 6.7.
OD % = rotations delta / average rotations = 6.7 / 13.35 = 0.5018 = 50.18% OD
Again, this aligns well with the 46.2% OD given the accuracy of my counting partial rotations and amount of backlash in the drivetrain.

I would consider this proper supporting evidence of the actual OD %, which is significantly larger than reported in the documentation provided by VP. Funny how a simple error can propagate.

I hope this helps.
TKW75

Wow! How TF did Vanquish miss that?!

Thanks for doing the science experiment for us.

[Fricker08]

Well guys… Rockspider and TKW75 you guys were on to something. DRED805 stirred the pot on Facebook.. and after testing the od on my twin in the rc speedy along with receiving video from guys with stock Phoenix builds, they all matched. And to Harley’s video it equaled ~46% overdrive.
Man were people not happy!
How could you question VP!? It says so on the box! You guys are trolls and liars and set your rigs up like that on purpose, and that wasn’t even all of it. They said it was because we had straight axles and the trans was meant for portals. They also didn’t seem to care that our low overdrive matched the 6.5%
Well Brandon Coonce from vp said he was going to look into it.
Low and behold… they messed up. The vfd twin transmission does in fact have ~46% overdrive!

From Brandon: “I will look into it more this morning but quickly reading it last night I think they are right. I used an incorrect gear in my high OD calculation, it was based on the original gear layout. Thanks for your persistence. I will get the numbers sorted and update the product pages if necessary.”


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[CamoColton]

Holy shit. This is my first time reading through this thread.

I thought the "2nd gear" seemed a bit aggressive while I was bench testing and setting my servo endpoints. This confirms it.
Honestly I would have been just as happy with this kit if it came with the standard VFD.

[twade984]

Originally Posted by CamoColton

Honestly I would have been just as happy with this kit if it came with the standard VFD.

Same here, I like the idea of the vfd twin, but personally I don’t have any interest in selectable o/d.

I would be more excited about a more compact version with dig only.


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[CamoColton]

Originally Posted by twade984

Same here, I like the idea of the vfd twin, but personally I don’t have any interest in selectable o/d.

I would be more excited about a more compact version with dig only.

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Yup. I know I can lock out the OD selection entirely, or change how my servo to a two position, but that doesn't change the overall dimensions.

And I might be in the minority, but I'd also take an actual 2-speed over the OD selection.

[Doomp]

Not to beat a dead horse...

After running about 8 packs through the phoenix the last couple of weeks, i was just thinking about the "33%" overdrive and how extreme it seemed.
I just read through this thread and it makes much more sense now. I generally only run R&P overdrive up front and have in the past used underdrive gears in the rear axle, So the extreme 30%+ OD is foreign to me as far as noticing something was off with VP's math. Things get overlooked in R&D all the time in industries and occasionally make it into production but i absolutely do not understand why people are so pissed off about it. I do wonder if the 46% OD is contributing to people breaking the stock sintered gears?

I've been beating on the phoenix hard with the slipper locked down and getting it into binds and generally abusing the hell out of the truck with a HH torquemaster expert 30T on 3S and its still going strong with no clicking...
Yet.

I personally like the selectable OD for it being innovative and how well it works/designed but at 46% it is abit much IMHO. No way im doing the math but i wonder if its possible to change the OD gear ratio to something abit more moderate without having to completely redesign the transmission internal layout Or maybe release a version of the phoenix with the regular vfd although the loss of dig would suck. A VFD dig only trans would be pretty sweet though.

[Fricker08]

Originally Posted by Doomp

Things get overlooked in R&D all the time in industries and occasionally make it into production but i absolutely do not understand why people are so pissed off about it.

I’m not pissed off about them making a mistake, and I may be speaking out of place for DRED805 but he was told multiple times, his math was wrong and he rigged his truck to spin that fast overdrive. He was told it was because his truck had straight axles and not portals. By VP fanboys and VP themself. I don’t care if there’s a fix, but I do think the product description should be changed instead of “oh, we may change the product description if we feel it’s necessary.”
End Rant


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[Panther6834]

Not to "beat a dead horse"...but, for a quick second, is like to take this back to the 'core' of what this thread is (or, was supposed to be) about - how to calculate OD %.

With 2 gears, calculating an overdrive (or underdrive) ratio is simple enough. With 3 gears (the same 2, plus an idler), it's just as simple, as the idler gear plays no part in the calculation...its ONLY purpose is to have the final gear turning the same direction as the first gear, and wouldn't make any difference on the calculation either the idler had 12 teeth, or 30 teeth. In other words, as with the 2-gear calculation, ONLY the "driving" gear, and "driven" gear matters.

Back to the percentage matter. As stated, anyone can easily calculate an overdrive/underdrive ratio - "driven gear" divided by "driving gear" - and, if the result is above '1', it's "underdrive", and if less than '1', it's overdrive. Going back to what was originally asked, how do you calculate the OD percentage?

Using a typical 3-gear T-case, let's say the "driving" gear (which is also the output for the rear) is 20T, and the "driven" gear (with front output) is 14T, what is the OD %? Better yet, what is the formula for calculating the OD %?

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