[sqwert]

Originally Posted by calvarez

Spent a little more time just running around the local area, and yeah, the 13t front is really the best choice for this bike. It simply improves everything from mild dirt roads to 70-some MPG highway time. The biggest improvement is in 40-50 MPH street time, which is common where I live.

Yes, your choice of gear ratio is a major improvement.

Stock sprockets are 44/14, 3.143:1 ratio. Too tall out of the hole, especially when loaded or in a power demanding situation, such as uphill or in a material such as mud or sand. Also, uncomfortably fast pace in 1st gear engine can't pull at idle, especially in terrain. Engine dies, rear wheel locks, bike stops and falls over, but not much one can do to prevent the fall when standing on the pegs using counterbalance steering.

44/13 is 3.385:1 ratio, which is much better overall. If you need a change and want to run out the OEM chain and wheel sprocket, a 13t countershaft is an inexpensive solution in the short term. The 44/13 set is a fantastic improvement in hole shot and slow speed control. The fewer miles on the chain and wheel sprocket, the longer the set will last after being changed, so final drive ratio needs to be one of the first mods considered.

47/14 is 3.357 ratio, almost as much an improvement as 44/13. 48/14 is 3.429:1. Yes, you'll need a longer chain. However, if you are waiting until your OEM chain and wheel sprocket wear out to change your final drive, not much more money involved than 44/13, and will last much longer, so a good choice.

If you are going to run a taller rear tire, such as 130/80 or 130/90, especially if switching to a significantly more aggressive, taller tread, you are going to need lower gear than that.

Starting at 44/14 (stock), effect of sprocket changes on engine speed at a given RPM:

44/13 = 7.1%
47/14 = 6.4%
48/14 = 8.3%
49/14 = 10.2%
50/14 = 12%

You'll need to calculate your engine rpm at specific speeds to predict the effects of changing final drive geometries--sprocket tooth counts and tire outside diameters are mathematically related to engine speeds at various rpms. Simply changing from a 130/70 tire to a 130/80 tire reduces engine RPM by over 4%. As narrow as the stock RX3 power band is, that's significant. A 130/90 tire reduces engine RPM by 7.8%, and a 130/100 tire provides an 11.3% reduction compared to a 130/70. Since the RX4 is already over geared, that isn't what you want unless building a road racer.

That is assuming the tires are of the same type and build. Tires of the same size but different purposes can vary 1 1/2 inches, (38mm) in diameter, easily a 6.2% reduction in engine speed at a given RPM. Changing tire size or type can have a far more radical change on final drive ratio than minor sprocket changes.

For instance, going to a 130/100-17 knobby can reduce engine RPM 17.5%, which would require a 54/14 sprocket set simply to retain the stock final drive ratio when tire size is considered. To end up with the same engine RPM at a given speed with 44/13 sprockets, you will need 58/14 sprockets if you switch to taller aggressive knobbies.

So, you can see that tire diameter, or better, rolling radius times 2, is also a critical dimension to include in calculations.

Thoroughly confused? Don't worry, so is just about everyone else. They just think they will appear stupid if they ask a question, so they choose to remain ignorant instead.

Rolling radius (RR) is the radius of a tire, measured from the ground vertically to the axle center, when the rider, fluids, and accouterments are loaded on the bike, and the tire is properly inflated. It is a 2 person job since simply loading weight on the seat does not result in a representation of the real rider's weight distribution. Even with the same tire, most everyone's rolling radius will vary a bit.

Tire rolling diameter (RD) for closest calculations is rolling radius times 2. First you have to actually measure the rolling radius, then multiply by 2 for the rolling diameter. RR x 2 = RD.

Then, your rolling circumference (RC) is determined by RD x 22/7. 3.14159265359 . Not enough decimals? Try this. Yes, that is the constant referred to as Pi. Sorry, don't have a button for that on my calculator. I use the fraction 22/7 or the decimal 3.1416. Close enough.

Once you have RC, convert to your measure of distance that goes over time on your speedo to calculate how fast your rear tire has to turn to provide the appropriate engine rpm for you. If you have and like 44/13 and the stock tire, you'll need 54/13 or 58/14 to maintain about the same engine speed with a gnarly knobby with a 90 or 100 cross section.

Oh, crap, I just gave away the answer! What a Super Highly Intelligent Teacher I am.