Questions about gearing for solely dirt riding

[Megadan]

Originally Posted by bogieboy

I like how the troll hasnt touched what i said, which is exactly what you said, but said in a little bit different way...

Also, guess not everyone knows that HP is just TQxRPM/5252 To get the arbitrary number everyone drools about...

Don't go quoting math, you might give them the ability to figure out what we are telling them.

Actually, maybe I can help them with it.

Just for fun, lets say our engine makes 10ft-lb's of torque at 4000rpm in 4th gear. Let's say the primary gear reduction is 4:1 and we are in 4th gear, which is 1:1 for some simplicity.

Total Gear Reduction: 4 x 1 = 4 or 4:1 at the counter shaft - where the front sprocket is located.

Since Gear ratios multiply torque by dividing RPM, the Countershaft now has 40ft-lbs of torque and the counter shaft is spinning 1000RPM

We can plug in the engine figures to get hp with the equation:
10ft-lbs x 4000rpm/5252 =
40,000/5252 = 7.61hp

What about the counter shaft, let's do that math, shall we?
40 x 1000/5252 =
40,000/5252 = 7.61hp

So, having gone through ONE gear reduction the HP is the same at the crank and the front sprocket. Let's go to the rear wheel, shall we?

Then, let's say we have a 17 front 46 rear to start with, or 2.71:1
40ft-lbs x 2.71 = 108.4ft-lbs. 1000rpm/2.71 = 369rpm
So our stock gearing is producting 108ft-lbs at 369rpm at the rear wheel. (go ahead and do the math for MPH if you want).

Next, let's say we have a 15 front and 46 rear, or 3.07:1
40ft-lbs x 3.07 = 122.8ft-lbs 1000rpm/3.07 = 325.73rpm at the rear wheel.

So far, the new sprocket has increased torque at the wheel, but reduced maximum wheel RPM at the same engine RPM point - exactly as Thumper, myself, and bogfie boy said.

Not looking too good for you keyboard warriors/experts. But, let's keep going and see if it does indeed increase horsepower at the rear wheel.

To reiterate, HP = Torque x RPM / 5252

Let's do some math!

Stock gearing, at the rear wheel we had 108ft-lbs at 369rpm

HP = 108.4 x 369/5252
HP = 39999/5252
HP = 7.61hp
Huh.. Look at that HP figure...

Smaller 15t front sprocket gearing we had 122.8hp and 325.73wheel rpm

HP = 122.8 x 325.73/5252
HP = 39999/5252
HP = 7.61hp

Gosh... look at that. The HP is unchanged, just like we said.

So, gents, the math has spoken. If you argue past this point you are simply trying to sooth your hurt ego. Truth is, I don't care. We are right, you are wrong. The end.

P.S. If you want to try and argue driveline loss, it's irrelevant in this example as the loss is a fixed value that would apply to all of the figures. It would also be counter point to your argument since it is... a loss.