Overview of Cranks

Crank length is determined by measuring from the center of the bottom bracket spindle, (where the crank arm is fastened to the frame), to the center of the pedal spindle, (where the pedal passes through the crank arm). This measurement is always taken and read in millimeters (mm). For our purposes, when you see the term "road racing cranks," it means that the crank will accept up to two chainrings on it. Road Racing cyclists only use double chainwheeled cranksets. Double cranksets, can be used with only one outer chainwheel. This would make it a "track" crank, for use on nearly flat ground with no brakes. When you see the term "mountain bike cranks," it means that the crank will accept up to three chainrings on it. Mountain Bikers use only triple chainwheeled cranksets. Triple cranksets could be used with only one outer chainwheel on it, or an outer and a middle. So if you had to, you could press a triple into service as a double crankset. There are FIVE fixing bolt patterns used as a standard by bike part makers. The fixing bolt pattern, (or bolt circle diameter), is measured as the diameter of a circle that is drawn through the center of each of the five bolt holes on the chainring, or arm. It is expressed in millimeters. Again, draw a circle through the center of five fixing bolt holes in the chainring. The measurement across the circle is the bolt circle diameter. No one still measures it the old way of center to center between neighboring holes. Mountain bike triple cranks use three of the patterns. One pattern, and historically the most common, uses a 110mm outer and middle BCD. The inner is spaced at 74mm, that is the distance across the circle formed by a line through the center of all the bolt holes. The bolt pattern for the middle and the outer on mountain bikes is the same. The outer and middle are bolted on the opposite sides of the same spider arm piece. The bolt pattern for these outer rings is 110mm. In 1991 Suntour brought innovation to the crankset by introducing for the 1992 season their "Micro Drive" cranks using smaller chainrings yet achieving the same gear ratios. As the first maker of the Micro crank they began using 94mm for the bolt center diameter of the outer and middle chainrings. The inner on a Micro Drive crank uses a 56mm bolt center diameter. Shimano, in the 1994 model year, has made a tremendous effort to stanch the flow of mountain riders to the Micro Drive crank by pressing hard in the marketplace with their version of micro cranks known "HyperDrive C" or "HyperDrive Compact," ( the use of the "M" word "Micro" is to be avoided). Shimano used the established Micro bolt pattern of 94mm bolt center diameter for the outer and middle, and instead founded their own third bolt pattern for the inner using 58mm. Anti-Jam Pin For the 1995 model year, Suntour, according to Sandy Coulter, has chosen not to adopt the 58mm inner chainring Shimano standard. This will be interpreted by many as a mistake as it perpetuates a dying standard, that even small crank makers will no longer support. Road Racing cranks, produced almost anywhere but Italy, use only 130mm spacing for both the outer and the inner chainring on doubles. Campagnolo as an Italian maker still uses 135mm spacing. A current trend in mountain bikes now is the use of a stainless steel inner chainring. The reason isn't always obvious. The inner is the fastest wearing chainring because the chain line is so acute and there are so few teeth. The acute chain line angle occurs when you are on the small inner chainring and shift to a high (small cog) gear in the back. If you were to look at the chain from the back of the bike, you would see a dramatic sweep to the left before it becomes straight again at the front. The stress the chain is under, in this condition, is known as chain line binding. It grinds the tops and edges off of aluminum teeth quite easily. Consequently, replacing the alloy chainring with steel would add a great amount of extra wear. Chain line binding is at its worst between the inner and the small cog. Any occasion where the chain is not in a straight line will wear the chainrings out, and is known as chain line binding. The second reason the inner wears out so fast, is that it has fewer teeth. It has only half as many as a 48 tooth chainring and would therefore wear out twice as fast. A stainless steel or Titanium replacement would have a much longer life. Some aluminum crank manufacturers paint their crank arms with an enamel or epoxy paint. They never offer a reason for this, and most people probably assume that it has to do with fashion. The silver color of aluminum, is an acceptable color for expensive cranks. It is not the aluminum color that crank makers disagree with. I suggest that they disagree with the expense of having to polish the arm, to make it look cosmetically acceptable to the buyer. It is cheaper to paint them, than polish them. There is another reason to be concerned about painted arms. The paint will cover over the manufactures material defects. If you should ever get in an accident, you may not be able to notice to notice damage, with the paint covering it. The natural silver or anodized colors of aluminum is the way to go. When aluminum is anodized to the silver color it is said to be "clear" anodized. All aluminum alloy crank arms fit onto a four sided spindle. The spindle sides taper slightly as the arm moves down them to be seated. This is known as a 2 degree tapered spindle. It may not be obvious which cranks can use, or would require the 2 degree bottom bracket, therefore we mention it wherever necessary so you will know about compatibility. "Q factor" is the distance between each arm, at the point where the pedal spindle is seated, expressed in millimeters. So this measurement is taken outer crank pedal face to outer crank pedal face. It has to do with "How far your legs spread out from the frame because of the crank angle." and "What is the clearance between the end of the crank arm and the chainstay?" "Q" factor has become important recently. A low "Q" is considered desirable. One of the most complicated matters related to "Q" factor is finding a uniform way to measure it. Every manufacturer seems to be giving themselves an advantage by measuring the Q distance using a bottom bracket spindle that is unlikely to be used with their arms because it is shorter than would really be used, or they are tightening their aluminum arms down so tightly that they deform the seating on the spindle to achieve a shorter Q length. We have tried to come up with an objective way to measure Q so that the measurement is uniform and fair for each maker. What we came up with is this; about every parts maker is prepared to admit that Campagnolo does high quality work in machining. We decided to use the spindle from a Campy Chorus bottom bracket for our measurement because it has Campy's well cut tapered sides, and it is just 111mm long which is short enough that it should satisfy everyone equally. We decided not to use crank bolts to tighten or over tighten the arms onto the spindle. Instead using hand pressure, we pushed and held the arms onto the spindle using hand and arm pressure only, then made the measurement, outer pedal face to outer pedal face. All cranks we review come from our general inventory, none are submitted for review so every maker has an equal footing from a quality point of view, provided their QC is good on production parts for sale to the public. If a maker mis-manufactured the taper to their detriment on the production crank set we used randomly for measurement, they created their own happened, (we actually didn't notice this occurring). We call this measurement on the Campy spindle the "Bike-Pro Q Measurement," imperfect or not it seemed fair and objective. Almost all aluminum crank arms are forged. The exception is when a single block (known as a billet) of aluminum is used and is machined to the final crank shape. In the 1994 model year, the number of machined from billet cranks increased dramatically. A part of why this has happened is the device for the automated milling of the billet has become less expensive. Known as a Computer Numeric Controlled (CNC) milling station, or lathe station, these machine allow the user to create a program that a rotary cutter follows. This type of program was once stored on punched paper tape, now they are stored on a mini floppy disk allowing the designer of the part or programmer to take the disk from machine shop to machine shop so tomake the same part exactly the same way. The program set X, Y, and Z co-ordinates for the rotating cutter to follow. Although this style of manufacturing is more expensive than the forging used to make cranks historically, it allows the maker to machine as few or as many as necessary to satisfy present demand without overextending themselves in inventory commitments. It also allows a better use of materials because the same block of aluminum that could become a crank arm if there were demand, can instead become 3 brake arms because on that day the maker is short on brakes. Also if the design doesn't sell after making a few prototypes to show, you've made no big commitment. This style of manufacture also preserves the integrity of the metal's grain structure, forging processes to make parts aren't necessarily as strong. CNC machining retains the billet's isotropic (retains uniform properties and hardness in all directions) quality, forged parts are considered anisotropic and will have different strengths depends on the direction in which they are measured, (more strength in the length of a forged crank arm than in the width). There is quite a bit of excess CNC capacity at the moment, which is why in 1994 we see so many CNC bike parts.