RIMS - An Overview


Rims are made from aluminum alloy in a near molten, but not liquid, known as a "plastic" state. The aluminum in plastic state is squeezed through a piece of steel that has a hole in it, in the shape of the rim cross section. This steel piece is referred to as a "die". The single die generally has several of the rim shaped holes side by side placed horizontally permitting several pieces of aluminum to be squeezed out the die at the same time. These pieces of aluminum after they are squeezed out and cooled are known as "extrusions". The squeezing out of aluminum is known as "extruding". The extrusions are made up to about 22 feet long. The exact needed length of the extrusion varies depending on what diameter rim you intend to make from it. An effort is made to try to maximize the yield in the number of rims it will create, by making the extrusion long enough to cut from it a fixed number of rims with no extra length left over. The extrusions are cut to the length required for a specific rim, then rolled into a hoop shape. The neds are joined and the rims are drilled or "pierced" for the spokes and inner tube valve. The same extrusion can be to different lengths to make rims of different diameters, and this is frequently done.

Types of Rims: When you look at the cross sections of the rims we sell, you will see that fall into three types. Those that have a single wall with channels where the sidewall and spoke bed walls join, those that have double walls, these have a lower wall that the spoke nipples rest on and just above it a second wall that holds the two sidewalls together above the first. There are also instances of mountain bike rims that have a double wall construction and channels. The three types we refer to as, "single wall, channeled" - "double wall" - or "double wall, channeled". Generally, better rims are of at least the double wall variety.

You will notice in looking at the cross sectional images of the rims that they appear to have different shapes. We have noticed these fall into four types, Box, Aero, "U" shape, or V-Box. The Box shape is the more traditional type with a flat hub facing wall and sidewalls, the Aero shape is fairly recent with a rounded hub facing wall and sloping sidewalls. The "U" shape is also rather recent having a flat hub facing wall with an expanding sidewall at the base, followed by vertically rising, flat sidewalls. The V-Box shaped extrusion has a "V" shaped hub facing wall, that generally adds aluminum thickness at the point of spoke nipple contact, and has vertical, flat sidewalls.

The component parts of a rim: The sides of the rim are called "sidewalls". The sidewall includes any part of the rim that has a vertically sloping, of just vertical exterior surface. The part of the sidewall where a brake would usefully contact is known as the brake pad contact area or zone. The lower wall of a double wall extruded rim is known as the "spoke bed" wall. This is the wall that the spoke nipple will directly (or indirectly with an eyelet) rest against. We sometimes refer to the spoke bed wall as the "hub facing wall" when we want to speak about the exterior, exposed and finished surface of the spoke bed wall. In a double wall extrusion, the wall above the spoke bed wall is referred to as the "cross-tie" wall. This wall is built into the extrusion to cross-link and tie together for added strength and stability, the sidewalls and the spoke bed wall. Occasionally, we will refer to the rim "interior". This is the area between the sidewalls above the cross-tie wall. The rim "exterior" is another reference to the hub facing side of the spoke bed wall. At the top of the sidewall on the inner side is the "tire bead hook". Since we are writing about clinchers rims only, all the rims we've written about have them, with the hook the appropriate size to retain any steel or Kevlar tire bead.

Once the rim extrusion has been cut to length and rolled into a hoop shape there is the problem of how to join the ends so that the brake pad contact area has a smooth or near smooth and continuous surface. There are presently three common solutions in use for joining the ends. The most common is to insert a single separate piece of metal into a cavity or hole in both ends of the rim. In the case of channeled rims, an aluminum or steel rod is pressed into the channel on both rim ends with a glue or bonding agent, then the rim is evenly compressed to close the gap between the rim ends. In the case of a double wall constructed rim, a piece of aluminum with the approximate shape of the cavity is inserted with a glue or bonding agent, into the cavity of the extrusion between the cross-tie and spoke bed walls. The rim is again evenly compressed to close the gap between the rim ends. The aluminum insert for the double wall rims is usually long enough, that when the drilling or piercing for the spoke is performed, at least two spoke holes are drilled through the insert making the joint even more stable. This insertion type of joint relies on some fairly exacting standards in the square cutting of the rim ends and compression stage to make sure the ends butt correctly and there isn't a rise in the brake pad contact surface at the joint.





A severe rise between the common surface will have the brake "grabbing" or hanging onto the raised edge, causing a "thumping" sound, un-even deceleration, and accelerated brake pad wear as the pad rubber is scraped away trying to fill the lower pad contact area. In some instances the brake pad contact area of the rim is re-polished, but not often because the anodized surface maybe polished off. Generally all the rims we've written about, have joint brake pad contact surfaces within a small and quite reasonable tolerance. The last solution to joining the rim ends is to weld them together using a wire feed Tungsten Inert Gas welder. Three manufacturers are doing this now. The process electrically fuses the rim ends together, joining them with a metal filler wire of a similar aluminum alloy pushed into the gap between the ends while the fusing process is taking place. This type of welding can also be done with no filler metal, in which case it's called an "autogenous" weld. The welded seam leaves an uneven surface along the entire weld, and requires further machining and polishing to smooth the surface, but this type of rim joint is the only type that guarantees the brake pad contact surface is continuous.

Rims constructed with double walls generally have steel "eyelets" or "ferrules". These eyelets have three purposes, foremost is to increase the area of the rim that is supporting the spoke nipple. The other reasons are to provide a concave seat for the nipple to rest in, rather than the angular edge of the drilled aluminum extrusion, and lastly to possibly provide a metal surface that is smooth and soft enough for the spoke nipple to rotate against easily while under load during the wheel truing process. Aluminum as a metal isn't known for this "self-lubricating" property and brass, nickel or steel is an better metal surface for this type of contact.

There are three styles of eyelets in the rims we have written about, the least expensive and least useful is what we call the "one-piece, single wall" type. In this instance a single piece, small steel tube is inserted in the spoke nipple hole through the spoke bed wall, with pressure applied from the rim interior the eyelet protruding on the hub facing wall is forced to roll back against the hub facing wall holding it in place. This one-piece, single wall eyelet may be found on both single wall channeled, or double wall constructed rims. The reference to "single wall" in this case means the eyelet loads the spoke weight onto just one wall of the rim.

Another type of eyelet in use, we call the "one-piece, double wall". In this case a single piece of steel, stamped with a cup shaped upper and a tube at the bottom that's the rough diameter of the spoke hole. The top surface of the eyelet cup has a flange that rests on the cross-tie wall, with the bottom of the cup resting against the spoke bed wall and a steel tube that protrudes through the spoke bed wall. With pressure from the top or interior of the rim, the eyelet is pressed against a mandrel that forces the eyelet to roll back on itself, gripping the hub facing side of the spoke bed wall.

The last of the eyelets in use is the most complicated, and a better example of what an eyelet should be. We refer to it as a "two-piece, double wall" eyelet. The "two-piece, double wall" performs the same task as the one-piece, single wall but more exquisitely. It has the upper cup portion, stamped as a separate piece, generally of steel. The second piece is in the form of a tubular rivet made of steel, brass or nickel plated brass. Again pressure applied on the top of the rivet from the spoke bed interior, against a mandrel, forces the bottom of the rivet to expand into a round shape on the hub facing wall. This is the preferred method. The pressure of insertion is displaced across several surfaces, and the rivet can be made, if desired of brass to assist in truing while the strength of a steel can be used for the cup piece. If the machine pressure for "peening" the rivet is set too high, it's possible the shape bed wall could suffer damage as the press in essence tries to force the eyelet through the rim. This damage is seen as a distortion on the hub facing surface of the spoke bed wall and usually shows as a bulge, or crazing (fine cracks) around the eyelet. This type of over pressured insertion befalls all rim makers using eyelets occasionally. The opposite of the over pressure condition is one where not enough was used to properly to hold the eyelet firmly to the rim. In this case, it is seen as a loose or vibrating eyelet.

Rim anodization: Aluminum and aluminum alloys form a thin coating of aluminum oxide on their surface when they come in contact with air. This aluminum oxide is a very fine white coating and serves as protection against corrosion in many conditions and environments. Contact with some acidic solutions or moist corrosives prevent access of oxygen to the aluminum oxide film, which results in a breakdown of the film, and then potentially severe corrosion. To enhance the corrosion resistance of aluminum the anodizing process is used which accelerates and thickens the oxide coating. It is an electrolytic oxidation process which artificially thickens the oxide layer. The anodized surface has improved resistance to chemicals, abrasion, and further atmospheric oxidation. Hard anodizing is the same electrical process performed at higher voltage in different acid bath. It delivers a thickening of the hardened surface layer to a greater depth in the metal's surface.

Hard anodizing is said to increase a rims rigidity between 10 and 20 per cent. Somehow that seems a little high, but is certainly adds something to a rims weight, as the Rim table demonstrates. Anodizing has its drawbacks however. One of aluminum's greater characteristics is its thermal conductivity. It conducts heat half as well as copper and four times as well as low-carbon steel. In recent years truly effective braking has been linked to a brake pad and rims ability to dissipate heat. Anodizing an aluminum rim, reduces the effectiveness of the outer surface to dissipate heat, which could reduce the maximum potential when braking, this is why some truly serious rim makers leave their rims highly polished, but un-anodized. Remember an un-anodized aluminum rim naturally develops this oxide film, but it is very thin.

Where the alloy composition of the aluminum is known we've listed it in the table. Several makers are either secretive or ambiguous about the exact composition of the aluminum alloy, though nearly every variation with the specific alloying elements is metallurgically known and given a numeric designation.

Most of the rims we sell come drilled expecting the rider to be using a Presta valved inner tube. While the Presta valve for the ardent cyclist is the valve of choice, it may not always be the valve of use. Modifying a rim made with a Presta to accommodate a Schrader is easy. A tapered hand reamer will easily enlarge the dimension of the hole to the Schrader size. There are also Schrader to Presta valve adapters made for the reverse. In fact they are a rubber grommet The grommet presses into the hole and has an upper and lower lip that holds itself into the rim from both sides.

Now we need to explain some of the features of the Rim table. The "section height" is the height of the rim from the bottom most surface to the top most surface. The "inner bead width" is the dimension in millimeters between the tire bead hooks on both sides of the rim extrusion. The "outside rim width" is the measurement of the extreme most edges at the top of the bead hook. The "rim height at brake pad contact" is the height of the flat area of the sidewall that the brake pad can usefully contact. The column of the table labeled "weight per drilling & finished weighed", is a little complex. What we want to do is tell you the weight of the rim for each hole drilling, and the exact finish of the rim we weighed because hard anodized rims seem to weigh slightly more for the same drilling. When we have been able to weigh more than one finish of the same hole drilling, we abbreviated the finish and listed both. The "price of each surface finish" column lists the price for each surface finish available of that rim regardless of what hole drilling it is.

Extrusion Wall Thickness Deviation - We've used a Mitutoyo digital micrometer accurate to the 1000th of a millimeter for the measurements of the rim. We noticed that our measurements to the thousandth of a millimeter are at variance from the manufacturers specifications. In our weights, we've used only production models not prototypes, and have also noticed that the weights are sometimes grossly out of what manufacturers specify they will be. So the question becomes why are the physical characteristics so far out of design spec? The answer has to do with the basic process of extrusion. Remember at the top of the overview we described how aluminum was squeezed through a steel die in "plastic" state, to make long pieces aluminum with the same cross section as the rim? The steel die has a limited useful life before it becomes worn out. At the beginning of the die's use, it has sharp edges that capably shape the aluminum to tolerance in the designer's shape. With each bit of aluminum that is pushed through the die a small amount of the steel shaping surface is worn or abraded permanently away, with the effect of changing the die's shape slightly, as well as its specification tolerances.

After a certain amount of aluminum has been extruded through the die, the figure we are told is between 20, 000 and 40, 000 lineal feet, the nice crisp edges of the die which form the extrusion's shape become worn, to the point where it becomes "washed out" and has to be replaced. Therefore the early aluminum extrusions represent fully the designer's intention, and the later ones, from the same die, make the extrusion with thicker or wider dimensions. The later extrusions when cut into rim pieces are necessarily heavier. We call this phenomena "extrusion wall thickness deviation" and have found that some rims with the same drilling can weigh up to 8 or 9 per cent more, though they appear to have the same shape.

The rims selected for writing, represent those sold through our retail store that have a reasonable rate of sale, or as new product introductions are likely to. We've avoided writing about tubular rims deliberately, because like tubular or sew-up tires, the interest in them even by ardent cyclists is waning.

Lastly, for photograph or imaging purposes Silver is a very difficult color to take an image of. Generally, something in a bright Silver color shows up as with an overabundance of White. To take an image of many of the Silver rim sections it is necessary to reduce the light to the camera which makes the object appear to be or have shades of Grey. When a rim is described as "Silver" and the images show a great deal of Grey, it is really Silver but is underlit so we can show some of the detail and relief of the aluminum.