by Charles Owens on 12/20/2010
What Is Final Drive Ratio: The final drive ratio can be calculated by dividing the number of teeth on the rear motorcycle sprockets by the number of teeth on the front or countershaft sprocket. For an off-road dirt bike the result will typically range from approximately 3.5 to 4.5. This number represents how many times the countershaft sprocket will turn in order to turn the rear motorcycle wheel one revolution. A higher number equates to more turns of the countershaft sprocket and is referred to as “Lower Gearing”. Conversely, a smaller number is referred to as “”Taller Gearing”.
Lower Gearing means the engine speed will be higher (Higher RPM’s), while Taller Gearing lets the engine work at Lower RPM’s at a given vehicle speed.
Why Is The Final Drive Ratio Important? Let me fill you in on a dirty little secret of motocross racing….despite what many hop-up shops and dyno numbers are being advertised “It’s Not All About Maximum Power”!! Yes you heard that right…..motocross or any off-road motorcycle & atv racing is about creating “Usable Power For Given Rider Skill & Track Conditions”. It is possible that the engine with the greatest peak horsepower will not produce the best acceleration because the driving force will be deficient at all speeds except where peak power is produced. If maximum power was all we were concerned about then motocross racing would be nothing more than a drag-race. It is acceleration that usually wins races, and this is important!! Do yourself a favor, before spending thousands of dollars on high performance engine “hop-ups” first get your motorcycle sprockets or gearing right to suit your weight, riding ability, and track conditions (You can thank me later).
How To Choose Proper Gearing? First do an honest assessment of your weight and skill level. Generally speaking, beginning or heavy riders have a much harder time maintaining momentum and thus need good low to midrange powerbands to offer maximum acceleration between extremes. These riders are better suited with “Lower Gearing” motorcycle sprockets to allow the bike to stay in the “meat of the powerband” and offer maximum acceleration. Expert and top-level riders understand the importance of momentum and how to properly carry it thru corners & obstacles. In this case maximum top speed is essential, and these riders would chose a “Taller Gearing” for given track conditions.
Next assess your track conditions and obstacles in which you ride. Review elevation changes, soil content, number of turns, and frequency of jumps. Tight tracks with deep soil or slippery conditions, and many turns or elevation changes are going to require “Lower Gearing”. Motocross tracks with fast sweeping turns, and long fast uphills are going to favor “Taller Gearing” to keep the midrange and top-end of the powerband working for these conditions.
What Does Lower Gearing Do To The Powerband? Lower Gearing will allow the engine to move through the spread of gearbox quicker, but that also means the rider will be shifting faster and more often. The bike will hit or pull harder, and rev out quicker.
When should you gear the bike down? There are several scenarios which demand lower gearing motorcycle sprockets. Engines produce less power at high altitude due to less available oxygen, and generally need to be geared down to rev freely. Tight tracks with short straights are better suited to lower gearing. High elevation changes or deep loam, mud, and sand will need more thrust which comes with lower gearing.
Evaluate the rider and determine if he/she is reaching the upper gears on the longest straights of the track. A motorcycle is faster at 3rd gear half throttle than 2nd gear full throttle. The bike will be more effective with a setup using 3rd thru 5th gears, than staying in 1st thru 3rd gear only.
What Does Taller Gearing Do To The Powerband? Taller Gearing will produce engine power in a more linear fashion. The output is more controllable, and each gear will carry longer. While some hit will be lost, requiring more clutch use, the maximum speed potential of each gear will be increased.
When should you gear the bike up? It is rare that a stock dirt bike needs to be geared up, however consider this option whenever more top speed is needed. This is typical for an expert rider when the track is hard, smooth, and flowing allowing momentum to be carried. This is also an option on more powerful bikes when the rider is looking for less hit in the lower gears.
Small Steps With Gearing Changes: Always keep a log and typically never make more than a one-tooth change in gearing at a time. Carry a “Starter-Pack” of motorcycle sprockets which consists of a one-tooth lower countershaft sprocket and one-tooth, two-teeth, and three-teeth larger rear sprockets. By having this selection of motorcycle sprockets on hand, a rider will easily be able to change gearing at the track to suit the conditions.
Motorcycle Sprockets Construction: Not all motorcycle sprockets are created equal as they come in different patterns, tooth designs, and materials. Hardened Steel is better for longevity and cost, while Aluminum is lighter. Anodized Aluminum is often a good compromise offering a hard outer layer for durability, while keeping weight and cost to a minimum. More expensive alternatives such as Stainless Steel or Titanium offer advantages in wear and performance. Always purchase a high quality Case Hardened, Steel front countershaft sprocket as this is a high wear item. Some rear motorcycle sprockets such as Renthal Twin Ring, and Supersprox Stealth offer a two-piece Aluminum Inner Ring with Hardened Steel Outer Ring design. This provides both weight and durability advantages, and combined with good quality motorcycle chains will last for many races.
New Motorcycle Sprockets Mean New Motorcycle Chains: It makes no sense to change to fresh motorcycle sprockets if the motorcycle chains are wasted. Running a new sprocket with worn motorcycle chains costs power and instantly wastes the sprocket thru the improper interface. Do yourself a favor and always buy good quality motorcycle chains anytime you plan on installing new sprockets. Good quality motorcycle chains will drastically reduce the wear on your entire drive setup.
Motorcycle Chains Construction: There are essentially two types of off-road motorcycle chains construction: Conventional and O-Ring/X-Ring designs. Conventional motorcycle chains use steel pins with a press-fit soft metal bushing to form each link. These chains use a mechanical seal to protect the link, meaning there is minimal clearance between the rollers and the link. O-Ring or X-Ring chains use a ring to seal lubrication in the link and keep dirt and water out. These motorcycle chains are much more durable, however they are more expensive and heavier. These chains are great from a maintenance perspective because they do not require adjustment as often or require as much cleaning as a conventional chain.
Proper Motorcycle Chain Adjustment: The condition and adjustment of the drivetrain has a significant effect on both the handling and engine performance of the dirt bike. The simple chain-and-sprocket is still the most efficient way to transfer power from the engine to rear wheel on a motor vehicle with 12+ inches of suspension travel. The forces transferred through the chain and into the suspension can have a very positive effect on handling. During acceleration, the chain forces the rear wheel into the ground. Motocross racers depend on the chain forces when pre-jumping and landing. Racers land from a jump with the throttle-on to provide more resistance to the rear suspension to prevent it from bottoming.
If the chain adjustment is too tight, too loose, or the wheel is not aligned in the swingarm an experienced rider will notice the difference. An improper setup or poor chain/sprocket condition can absorb as much as 5 horsepower at the rear wheel on the average dirt bike!
To set the chain adjustment first start with a perfectly clean chain to get a truly accurate reading. Next disconnect your rear shock to determine where the tightest point in the swingarm travel occurs. Adjust the chain to have approximately 0.5 inches of free-play at this swingarm location. This location is when the swingarm is parallel to the ground and the rear axle is at it’s farthest point from the swingarm pivot. Next ensure the sprocket alignment is correct. Use the swingarm markings as a guide but in production there are slight tolerance differences in these marking which can make a difference to the racer. Most race mechanics prefer to use an alignment gauge, which fits into the centers of the rear axle and swingarm pivot bolt to get this correct. The cost of the tool will easily be paid for with reduced bearing & drivetrain wear/replacement.
This article was picked up from an unknown user, if anyone knows the author let me know and I will give credit where it is due!