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. image 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 to make 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. This excess manufacturing capacity is because the defense industry of Southern California, which used these machine shops so extensively, have been abandoned as sub-contractors with the defense cut backs. These sub-contractors looked at the relatively low technology used in the bike industry and vowed to improve it while making sure they were now in a "Green" industry, not dependent on the Defense contractors. Another pool of capacity became available in the Aeroplane industry. (Remember when the saying, "There are no White Tails in Seattle" was a truism? Well, it's not as true, in 1994 as it was in 1987, and if the 777 doesn't succeed Frank Shrontz could be operating a CNC station) Boeing has cut 16,000 in the 12 months previous to March '94 its sale for 1993 were off by 16% and net profits for the same period were off 20%. These highly trained machinists, and sub-contractors are the same ones that are now getting involved in bike parts manufacture. (Like Bob and Jeff Samac at Control Tech) There are several ways to forge cranks. The most desirable way is to cold forge them. Cold forging is a process of continually squeezing the aluminum using several different dies. Each of the dies is closer to the desired final and compress the workpiece slowly into it's final shape. Between each squeezing process the aluminum is annealed to soften it enough to be cold worked again. This cold working, left un-annealed on the last step leaves a harder, stiffer, more brittle piece referred to as "work hardened" or "strain hardened." It produces a much denser metal material, that is stronger, and less likely to crack, bend, or snap under the pressure of a blow or in an accident, because it is anisotropic (it has increased hardness in the length of the arm not the width). The alternative to cold forging is "melt" forging. Melt forging of cranks arms can performed as Impression Die Forging or Closed Die Forging. Impression die forging uses two dies, or half molds that separate near the middle. When the dies are together they completely enclose a cavity that will be filled with near molten aluminum. It is also possible to have just one of the dies contain the entire cavity to be filled while the other die merely makes a near finished surface (crank arm back). As the two die halves close around the near molten aluminum, a small amount escapes in the joint between the two dies. Called "flash," this metal fin around the edge, cools the workpiece raising the pressure inside the dies. The increased pressure inside the dies helps the metal to flow into the unfilled portion of the cavity or "impression" as it's known. Separating the two dies releases the part, so they can be closed to forge another of the same part. All the hot forging techniques produce decent arms but there may be flaws in the forging not obvious to the eye from the outside. There are sometimes weaknesses internally that reveal themselves only after a strong, or maybe not so strong, smack or blow. The weakness shows as a stress fracture, crack, or outright break. The common flaws in forged arms are "hot tears," "bursts" (which are internal ruptures) and "thermal cracks." Cold forged cranks are affordable and desirable. We have mentioned those cranks which are cold forged. Mountain bike drive trains have changed in the last few years. In the 1992 season Suntour, released Micro Drive. Micro Drive was the first of the "compact" drive trains. For the 1994 season Shimano finally copied the concept, gave it their own name, Hyper Drive-C (called "Hyper-C" by users) and its own bolt center diameter pattern. The Shimano version is 94mm mid/outer and for 58mm inner. The Micro Drive and Hyper-C components are able to be used in the same fashion as all the other drive train parts but are considerably lighter, because of a few changes in the chainring and rear cog sizing. Suntour, in the original, reduced the size smallest rear cog to 11 teeth. This is the smallest practical size because of the 1/2 inch spacing between each of the chain rivets, (don't be surprised if someone introduces a new smaller chain spacing in the future). The 11 tooth cog replaces the 12 tooth small cog that was the previous smallest. In reducing the high gear cog by 1 tooth you can reduce the size of the front chainrings by several teeth and still have the same gear ratio. As an example the no longer made standard XC Pro drive train came with cranks that had 24t-36t-46t chainrings, with rear cogs are generally running 12-14-16-18-21-24-28t. In the standard XC Pro if you were using the 36 tooth middle and the 12 tooth rear, one crank revolution would turn the rear wheel three complete revolutions, the gear ratio is 3 to 1, (36t divided by 12t = 3). Using XC Pro Micro Drive or Hyper-C with an 11 tooth cog, to achieve a 3 to 1 ratio you would need a 33 tooth middle chainring, (33t divided by 11t = 3). In fact the Micro Drive and Hyper-C middle is 32 teeth giving you a 2.91 to 1 gear ratio. Not only are the chainrings smaller, but Suntour was obligated to introduce two new bolt circle diameters when they innovated the compact drive train. The middle and outer of both the Suntour and the Shimano compacts sets have a 94mm diameter while the inner is 56mm on the Suntour and 58mm for the Shimano. The smaller diameter chainrings are more rigid and needn't have the same support near the edge as conventional chainrings require, therefore the makers have been able to shorten the length of the chainring fastening arms, and reduce the mass of the arm structure, to drop the crankset's weight considerably. The inner fixing bolt pattern permits this crankset to "nest" around the B/B shell giving a reduced and more favorable "Q-factor." On some frames, to accommodate large diameter tires, the chainstays move out from the B/B shell at a wide angle so they will enclose the tire's width. In many cases these splayed chainstays must have the area near the chainrings "dimpled" or pressed in so the chainrings themselves don't grind against the frame. The compact crankset's smaller chainrings reduce in nearly all cases the need to weaken the frame this way.
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