HPI Proceed ratio computer
 

Please note: Although this ratio computer was made with the HPI proceed in mind, it can also be used for the Mugen MRX2, Picco Integra and Serpent models. Practically every car with a belt-drive system much like the Proceed is supported. This page is best viewed with Netscape® Communicator 4.7 and up, or Microsoft Internet Explorer 5.0 and up.

 

The Ratio Computer!
 by NoJoke productions (c) 1999

Status Info:  
Rear wheel diameter   2 speed front pully   2 speed rear pully    Front wheel diameter 
              
 Overdrive Ratio: 
   
 Rear pully 
    		 Mid rear pully 
    		 Mid front pully 
    		 Front pully 
   
Calculate:
Gear Axle
gear		 Engine
gear		 Shift
RPM		 Top
RPM		 Speed [km/h] Speed [mph]
1 n.a.
2

How to use the ratio computer

Calculating the overdrive ratio

When this page first loads, it is set up for calculating the overdrive ratio. You can fill in the tire diameters and the pully sizes (or settle for the default HPI Proceed factory setup) and click on the "Overdrive Ratio" button. You should now see the overdrive ratio filled in (0.9901247401247402 in the case of the default factory setup). The status info field should now read "Ready.".

You will also notice some other fields filled in at the bottom of the table, we will deal with that later on.

Calculating pully or wheel diameters

With the ratio computer, you can fill in an overdrive ratio, and calculate the front wheel diameter you would have to drive to get the desired ratio. To achieve this, click on the radio button next to the "Front wheel diameter" field, fill in the desired overdrive ratio (for instance "1") on the "Overdrive ratio" field. You will notice that the text in the button next to "Calculate" is changed from "Overdrive ratio" to "Front wheel diameter". Click this button and the front wheel diameter will be updated.

With the radio button, you can select any pully or diameter you would like to calculate from the other pullys or diameters. As you soon will see, using this ratio computer is easy and fast, and you can try any setup to get any overdrive ratio.

Using the "Top speed" section

Below the ratio computer you can find a table with RPM and speed fields. The "Top Speed" fields and the shift point RPMs are automatically updated every time you use the ratio cmputer to calculate pully or wheel diameters. You can change the number of theeth on the 1st and 2nd gears, and the top RPM you figure (or measured) your engine runs, to see what this does for your top speed.

The "Top speed" section of the ratio computer also calculates a "shift RPM". This is the theoretical RPM your engine would run if the car shifts from 1st gear top RPM to 2nd speed with full clutch lockup. You can use this figure to optimize clutch and/or gear setup.

Formulae used in the ratio computer.

The formulae used in the ratio computer are nothing sophisticated, and you can probably figure out your own formula for overdrive ratio, top speed or just about any axle speed in your own car, whether it is an HPI Proceed or a Traxxas Stampede or an RS4.

Overdrive Ratio

This is the ratio between the front wheel surface speed and the rear wheel surface speed. Please keep in mind that cars with spraques on the front wheels (sometimes incorrectly called 1-way differentials) that have an overdrive ratio of less than 1 are only powered by their rear wheels on the straight.

Formula:

      FFT   RRP   TFP   MFP
ODR = --- * --- * --- * ---
      RRT   TRP   MRP   FFP
Where: ODR = Overdrive Ratio [ratio]
FFT = Front tire diameter [mm]
RRT = Rear tire diameter [mm]
RRP = Rear axle pully [teeth]
TRP = 2 Speed axle rear pully [teeth]
TFP = 2 Speed axle front pully [teeth]
MRP = Mid axle rear pully [teeth]
MFP = Mid axle front pully [teeth]
FFP = Front axle pully [teeth]

Top Speed

The top speed formula pratically speaks for itself, it calculates the surface speed of the rear (!) wheel. The 60/1000000 devision is to get from millimeters per minute to a more usefull kilometers per hour.

Formula:

                   EngineGear   TRP                60
     Speed = RPM * ---------- * --- * RRT * pi * -------
                   AxleGear     RRP              1000000
Where: Speed = Speed [kmh]
RPM = Engine RPM [rpm]
EngineGear = Engine Gear [teeth]
AxleGear = Axle Gear [teeth]
TRP = 2 Speed axle pully [teeth]
RRP = Rear axle pully [teeth]
RRT = Rear tire diameter [mm]
pi = The number pi 3.1415926535897932384626433832795 (approx.)

Small note: To get from kmh to mph, simply multiply the outcome with 0.621371192237

Shift RPM

To calculate the theoretic rpm of the engine when just shifted to 2nd gear with fully locked up clutch. To achieve this, it is simply enough to calculate the shift axle rpm just before shift, and then recalculating engine rpm from this axle rpm at 2nd gear. This results in this simple formula:

                   gear1engine   gear2axle
 shiftrpm = RPM1 * ----------- * -----------
                   gear1axle     gear2engine
Where: shiftrpm = rpm when just in 2nd gear [rpm]
RPM1 = rpm just before shift to 2nd gear [rpm]
gear1engine = 1st gear engine side [teeth]
gear1axle = 1st gear axle side [teeth]
gear2axle = 2nd gear axle side [teeth]
gear2engine = 2nd gear engine side [teeth]

 

Special thanks: The ratio computer is inspired by the "overdrive computer" at team serpent's network. I would like to thank team serpent for giving great support to 1/8 racers out there. I would also like to thank HPI racing and some guys on the forums for tips, hints and answers to my questions, especially Frank, Kent and Buster.

NoJoke productions (c) 2000
Rolf Heller


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