td_enterprise

I don't want any kiddies posting crap in here about guessing speed. I need some Gear or Math Wizards to look at this formula and figure out what's misplaced.

Here's how it works out. All of the ratios are the drive gear divided into the driven gear. Normally, these are all multiplied together, then figured into the normal rollout speed chart. The problem is that with these specs the top speed only comes to 27.72 mph, which is about 6 mph under what the truck is actually radar'd at. I also need to add the diff ratio which is 3.31:1 just like an 1/8 scale buggy.

Spur Gear Clutch Bell Ratio
52 15 3.4667

Input Shaft Slipper Shaft Ratio
19 16 1.1875

1st Gear 1st Input Ratio
43 18 2.3889

2nd Gear 2nd Input Ratio
38 23 1.6522

Forward Drive Forward Output Ratio
30 25 1.2000

Wheels Tire Diameter RPM
Stock 6.625 38000

nitromacguyver

what rpms are you using to calculate this? and did you for get that the faster you go the more the tires berm out of enlarge in diameter due to the centrifugal force?

td_enterprise

Those are the easy parts. They change, and the layout I am using to figure this can be altered to check the speed at whatever tire diameter gear ratio, and rpm I choose. The problem is that with the ratios that the gears come out to, the math doesn't work out for a truck that is supposed to do 34 mph +. Stock tires are 6.625 sitting still, and the engine has a practical peak at 38.000 rpm, so I am using those numbers just to start. Even if I consider the tires to balloon to 7.25", they still don't make enough difference on the numbers to get the truck much over 30 mph.

nitromacguyver

ok so this is it:
38000/3.4667= 10961.4

10.9614/1.1875= 9230

9.230/1.6522= 5586.9

5.5869/1.2= 4655.7 rpm *wheel circumference 6.625*3.14 =

4655.7*20.8inches = 96850.199/12=8070.8499 feet per minute*60 = 484250.99 feet per hour / 5280 =91.71414/ 3.31 diff ratio =27.708

if the tires balloon to 7.25 the speed equals 30.3256mph.

ive seen the tires on my mt2 go diameter + 1.5 inches so lets try that.

that came out to 33.985589mph so thats pretty close to the mgt's advertised speed. maybe you were underestemating how much the tires could ballon.

i just did this twice so it shold be correct.

SavageRCRacer

My Feedback: (2)

[sm=spinnyeyes.gif][sm=confused.gif][X(]

td_enterprise

Here's a link to the formula sheet. It's in excel, and if you aren't familiar with excel, just click on the gear ratio and it will show the formula used to figure the number up top. Basically, all the gear ratios are multiplied together for a final ratio after the diff reduction to 27.0154. The numbers work if I figure in the tires ballooning to 8" or more, but I can't believe they would get that big. Need a still shot from a video I guess. It usually works best to save link as to your desktop if you cannot use the chart by clicking it.

[link=http://www.renegaderc.com/pics/directions/MonsterGTgearratios.xls]Excel Monster GT Speed Chart[/link]

SH ByA

Or you could go buy a Venom Speed Meter and it will do the work for you. DOnt know how accurate they are, but its worth a try.

td_enterprise

This is for figuring out gearing. You can basically plug numbers in, and figure out real world speeds. The problem with people using the speed meter is user input. With numbers, there is no chance for error, it's math. With the speedmeter, you don't know if somebody doesn't know how to do their rollout correctly and properly setup the unit. Personally the only people I will believe with it are people I actually know. There will be too many kids with it that don't know what they are doing, or will fabricate the results.

CaNaDiAnRaCeR

Heres a speed calculator if its any help to you. http://www.jagrc.com/speed.html

fhm101

Why not just work with the recorded speed as a known in the formula and then work out RPM from that. The only other variable you will have is tire size and I've seen some bowties blow up an to easy 10" dia pizza cutter on a berg unlimited truck. Another thing to take into account is, do the front and rear wheels blow up the same, or do the fronts get bigger than the rears? If the truck is fast enough actual ratios and rollout are just more numbers like rated HP. It should be easy enough to go up or down on the clutch bell to dial the truck to your needs.