Felony Felony Gearing Math Not Adding Up F/R

[Mr.Duke]

I’ve been toying with the idea of using the front and rear tires and diffs from the Felony on a speed runner but the math just doesn’t add up for wheel speed front to rear.
Front tires are 100mm diameter
Rear tires are 107mm diameter

Front diff uses 2.8 ratio (15t pinion / 42t crown)
Rear diff uses 3.214 ratio (14t pinion / 45t crown)

No matter how many times I run the numbers the front tires are always spinning faster than the rear tires.

Example:
Stock Felony with stock 16t pinion installed 46t spur
2050kv (6s)

front tire speed = 67.11mph
rear tire speed = 62.56mph
* 4.55mph difference


26t pinion 46t spur

front tire speed = 109.05mph
rear tire speed = 101.66mph
* 7.39mph difference

27t pinion 34t spool

front tire speed = 127.68mph
rear tire speed = 119.02mph
* 8.66mph difference

Let’s say you want to go crazy and run a TP4070 2700kv with 30/34 gearing (6s)
No load - no drag- no voltage sag

front tire speed

Rear tire speed

 

[Mr.Duke]

My thinking is that you can get away with a slight variation in tire speed front to rear because of the available slip within the stock center diff (100k) but would see undesirable side effects when running much thicker fluid or a spool for speed running. The difference in tire speed front to rear increases as speed goes up making the problem worse the faster you go

 

[bicketybam]

If you look at the detailed specs on the Felony "Giga site" they show a front final gear ratio of 8.58 and a rear final gear ratio of 9.43. In reality the actual front final gear ratio is 8.05 and the rear is 9.24. What's up with that?

I fudged the numbers in my calculator to get the final gear ratios they have listed. Even doing that, I calculate a 1.63 mph difference between the front and rear wheel with a 16t pinion on 6S. That's closer for sure. At 154 mph the difference grows to 4 mph. Either way, the tires will not be spinning at the same speed and that could be a problem if you run a spool in the center.

I'd love to know how they came up with their numbers.

 

[LibertyMKiii]

I am pretty certain they figured the center diff can eat up the 7% difference.

I really wouldn't recommend investing in this idea if you plan on going beyond 110mph. The rear tire's extra mass is a very bad thing for the glue's job holding to the wheel.

As you can see on the calculator having 2.8 ratio diffs is not ideal.

 

[jondilly1974]

What about the rear tires ballooning more than the front? Wouldn’t that compound the issue as well? I certain they would balloon more than the 100mm Hoons.

 

[Outcastblxnut]

I don’t believe it. Seeeeend it and let’s find out ??????

 

[Mr.Duke]

All my white hoons measure 102mm new vs the advertised 100mm. What are you guys seeing on your regular hoons new?
Also has anyone actually measured a new rear 107 Felony Hoons tire to see if it’s also larger than the advertised size?

 

[suchtragedy]

If you bump the rear felony input gear to a 15T the ratio then becomes 3.0 making the difference now 7% over the front instead of 14%, and with the rear tire being 7% more circumference this makes the ratio front to rear now 1:1. Again this is just what the math says on paper.

 

[LibertyMKiii]

All my white hoons measure 102mm new vs the advertised 100mm. What are you guys seeing on your regular hoons new?
Also has anyone actually measured a new rear 107 Felony Hoons tire to see if it’s also larger than the advertised size?

I can measure the tires that came on my felony tonight if someone else does not chime in before then...

 

[3d-rc.myshopify.com]

My mind is absolutely blown on this lol... in real world 1:1 that is common in 4x4 vehicles. But a high speed rc rig that could spell disaster with a spool

 

[Mr.Duke]

If you bump the rear felony input gear to a 15T the ratio then becomes 3.0 making the difference now 7% over the front instead of 14%, and with the rear tire being 7% more circumference this makes the ratio front to rear now 1:1. Again this is just what the math says on paper.

Your math is correct
With stock gearing I get a difference of 0.09mph front to rear if arrma had gone with a 15t diff pinion mated to a 45t crown gear instead of the 14t diff pinion. I wonder how much plastic would have to be machined off the housing and cover on the bearing side behind the crown gear to get a 15/45 to fit correctly ?

 

[3d-rc.myshopify.com]

Your math is correct
With stock gearing I get a difference of 0.09mph front to rear if arrma had gone with a 15t diff pinion mated to a 45t crown gear instead of the 14t diff pinion. I wonder how much plastic would have to be machined off the housing and cover on the bearing side behind the crown gear to get a 15/45 to fit correctly ?

I feel like we are about to find out

 

[Mr.Vylan]

okay, you made me bust the open the new meat. I hope it doesn't spoil.

 

[bicketybam]

Stock out of the box, the rear tires need to be 1.15 times taller than the front tires in order for this to work. So if the front hoons are 100mm, the rears need to be 115. If you used 80mm tires on the front the rears would need to be 92mm. That's the deal. If the fronts really are 102mm then those rear hoons should be 117.3.


Edit: You need 93.3 fronts to work with the rears and a spool.

 

[LibertyMKiii]

My digital caliper battery died, but with a metric ruler it appears to be darn close to 100mm front and 107mm rear.

 

[jondilly1974]

All my white hoons measure 102mm new vs the advertised 100mm. What are you guys seeing on your regular hoons new?
Also has anyone actually measured a new rear 107 Felony Hoons tire to see if it’s also larger than the advertised size?

 

[Mr.Duke]

Alright, I’ve crunched the numbers to infinium and here’s as close as it gets for the front and rear wheel speeds to match up (within 0.02mph)


- New 42/100 Hoons measure out of the package @ 102mm in diameter (feel free to measure your own but all of my new ones measured out at 102)

- New 53/107 Hoons measure 107mm diameter

This requires you to shave off 1mm of rubber off the circumference of a new set of rear 53/107 Hoons making them 53/105mm


-Matched diff gearing and tire size front and rear-

Front = 102mm with 14t/45t diff (3.214 ratio)

Rear = 105mm (shaved down) with 13/43 diff (3.308 ratio) Typhon diffs or Limitless Infraction v1 diffs

Front 102mm 14t/45t________Rear 105mm 13t/43t
____108.08mph_____27/39 @20v____108.10mph
____123.97mph_____27/34 @20v____123.99mph
____137.75mph_____30/34 @20v____137.77mph
____129.70mph_____27/39 @24v____129.72mph
____148.77mph_____27/34 @24v____148.79mph
____165.30mph_____30/34 @24v____165.33mph

 

[jondilly1974]

Alright, I’ve crunched the numbers to infinium and here’s as close as it gets for the front and rear wheel speeds to match up (within 0.02mph)


- New 42/100 Hoons measure out of the package @ 102mm in diameter (feel free to measure your own but all of my new ones measured out at 102)

- New 53/107 Hoons measure 107mm diameter

This requires you to shave off 1mm of rubber off the circumference of a new set of rear 53/107 Hoons making them 53/105mm


-Matched diff gearing and tire size front and rear-

Front = 102mm with 14t/45t diff (3.214 ratio) v2 Infraction diffs or Front Felony diff

Rear = 105mm (shaved down) with 13/43 diff (3.08 ratio) Typhon diffs or Limitless Infraction v1 diffs

Front 102mm 14t/45t________Rear 105mm 13t/43t
____108.08mph_____27/39 @20v____108.10mph
____123.97mph_____27/34 @20v____123.99mph
____137.75mph_____30/34 @20v____137.77mph
____129.70mph_____27/39 @24v____129.72mph
____148.77mph_____27/34 @24v____148.79mph
____165.30mph_____30/34 @24v____165.33mph

Isn’t the front 15/45? Or are you suggesting it should be 14/45?

 

[bicketybam]

Alright, I’ve crunched the numbers to infinium and here’s as close as it gets for the front and rear wheel speeds to match up (within 0.02mph)


- New 42/100 Hoons measure out of the package @ 102mm in diameter (feel free to measure your own but all of my new ones measured out at 102)

- New 53/107 Hoons measure 107mm diameter

This requires you to shave off 1mm of rubber off the circumference of a new set of rear 53/107 Hoons making them 53/105mm


-Matched diff gearing and tire size front and rear-

Front = 102mm with 14t/45t diff (3.214 ratio) v2 Infraction diffs or Front Felony diff

Rear = 105mm (shaved down) with 13/43 diff (3.08 ratio) Typhon diffs or Limitless Infraction v1 diffs

Front 102mm 14t/45t________Rear 105mm 13t/43t
____108.08mph_____27/39 @20v____108.10mph
____123.97mph_____27/34 @20v____123.99mph
____137.75mph_____30/34 @20v____137.77mph
____129.70mph_____27/39 @24v____129.72mph
____148.77mph_____27/34 @24v____148.79mph
____165.30mph_____30/34 @24v____165.33mph

While my calculator comes up with different speeds, the front and rear speeds are within .04 mph of each other, so your math definitely works ???

Isn’t the front 15/45? Or are you suggesting it should be 14/45?

The front is 15/42 stock.

 

[LibertyMKiii]

……... with 13/43 diff (3.08 ratio) Typhon diffs or Limitless Infraction v1 diffs...…...

Was this a type-o? Just want to make sure you have the number you need.