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Photos above and right are courtesy from Mike Curiak, of Big Wheels, Grand Junction, CO www.lacemine29.com
Going to wider rims, the tire's circumference doesn't grow taller. I've closely measured rollout curcumference which calculates to tire height, and measured knob width too. Using the same 2.3 Pacenti neo-moto tire on a 650b x 40mm Derby Rims compared to a 28mm Blunt, the tire doesn't grow taller, the tire is no higher at the center knobs, and the edge knobs are actually a very small measurement narrower, about 0.0225 inch or 0.6 mm narrower. But the edge knobs do "stand up" a little higher making a more "square" tread profile, and these edge knobs do come closer to the arch of a fork or yoke of a chain-stay by about 1.5mm, so it is slightly closer clearance above the edge knobs.
Tire tread measured on a 40mm wide rim, edge knobs stand up and flair slightly less to the sides
Same size tire tread measured on a 28mm narrow rim
When the new wave era of trail bikes were "pioneered" in the late '70's until the mid '80's, rims were commonly 35mm to 44mm wide. The widest tires were 2.1 inches.
In the mid '80's road racers invaded some less challenging mountain bike races to grab pro category purses. But they were not familiar at handling bikes off road. Road racers brought big brand name sponsorship attention, and the race promoters wanted more big budget sponsors involved, so race courses became easier, mostly grass and fire-road race courses so the sponsored road riders could survive mountain bike races without crashing.
Pro road racers knew that climbing was the easier time to gain ground when racing compared to descending. They knew that lighter wheels, less rolling weight, was most important for the climbing advantage. The trend to drop wheel and bike weight was started by these road riders racing on dirt. Aluminum replaced steel frames, handlebars where chopped narrow for weight with much longer reach stems for aerodynamic advantage on faster easier handling race courses.
Keith Bontrager in 1984 cut some 700C (ISO 622) 40-hole Mavic MA-2 tandem rims 27" x 1" rims, re-rolling them to create a 32-hole 26" rims, the first light weight 25mm outer width trail bike rim. Dropping trail bike rim weight from over 600 grams to about 450 grams each, more than 300 grams lighter rolling weight, the most important weight to drop for climbing.
The dominate racers quickly adapted these narrow light rims. Mavic and other rim makers jumped in to make light weight trail bike rims. And the narrow road bike width rims have remained common for trail bikes since this time.
Tires became smaller too, down to 1.8 x 26" and tread was nearly eliminated for the easy mountain bike race courses dominated by road riders from the mid '80's until recent years.
In the mid '90's Downhill was a rapidly growing core mountain rider race format. DH racing grew in spectator popularity due to the much more difficult real tail bike courses, using heavy duty bike frames, now with suspension. A long travel DH bike in the mid to late '90's had 3 inch travel forks and swingarm travel. The lightweight narrow XC rims could not endure the demands of difficult trails. Rims were reinforced and widened, the added wheel weight was less in performance loss compared to the increase grip and stability for DH speeds, with the Sun Rhyno-lite 29mm wide rim leading the wider rim rebirth.
Most trail riders continue to prefer lighter narrow rims for lower rolling weight enabling quicker climbing, rather than the better handling and greater traction with wider but heavy aluminum rims. In the recent years, old school trail bike riding re-emerged into popularity, climbing for the rewards of downhill challenges, high speeds, jumps, big rocky drops, and the industry marked this pioneering roots style of riding as something "new" labeling it "AM" or "All Mountain". And bigger volume, more durable "Trail" and "AM" tires became in demand again, but high air pressures are needed to use light narrow rims with larger volume tires.
To better optimize Trail and AM handling requirements in recent years a couple of companies produced 35mm wide rims. Salsa made a 35mm wide 29'r rim called the Gordo. A couple years later Velocity, an Australian rim and wheel factory, was commission by Kirk Pacenti to produce a 35mm wide rim to compete with the Salsa Gordo, called the P35. These rims were heavy, nearly 600 grams for the 650b P35, and almost 700 grams for each Gordo 29'r. The Gordo was discontinued. The P35 is still selling well to riders preferring handling performance over climbing speed.
In summer 2013, Derby Rims introduced 35mm '30i' and 40mm '35i' wide carbon fiber rims, the world's first carbon rims wider than 30mm. Carbon fiber is well proven in aerospace technology and elite motor racing to improve strength, durability, stiffness, but with much lighter weight than other materials. Derby's first rims were a 35mm 30i wide 29'r and 40mm '35i' wide 650b rim for advanced trail and mountain riders, having exceptional durability and stiffness, and easy flat tire flat tube repair without needing levers, and a tubeless compatible design that locks the bead next to the rim wall with a floor pump or short compressed air shot of 35psi. In 2014 40mm '35i'wide 26" rim were introduced, also DH layup versions of the three wheel rim sizes. In 2015 a 45mm '40i' wide 29'r rim was introduced, the widest carbon 29er rim available.
Now without adding any weight you can take the huge leap forward in wheel strength, durability, stiffness, stability, and tire optimization by riding wide Derby Rims. And compared to aluminum rims just as wide, large chunks of rolling weight pounds are dropped going to Derby Rims.
The wider sidewall raises the edge knobs without raising the center knobs, which makes a less round or more "square" tread cross-section profile.
Also the tire casing tension under the edge knobs is increased to be firmer, increasing the tire's stability, and allows lower tire pressure when desired for less rolling resistance over rough terrain from increased bump compliance, and a larger tire patch when weighting the tires while braking and cornering.
Cornering traction is much improved on dusty hardpack to very loose gravelly dirt or loamy covered with leaves, as a result of this combination of flatter tread cross section, stiffer edge knobs, more knobs in contact, and lower air pressure, all enabled with wide rims.
Changing direction has quicker response with the more stable and firmer structural tire edge with wide rims, without feeling harsh and choppy, and no more sudden washing-out of the front wheel, or as easily skidding the rear tire while cornering.
The whole bike becomes more stable on wide rims, feeling like a longer wheelbase bike in directional control but with quicker more positive turn response for more nimble handling and corning grip. Holding a line over loose round rocks though "rock gardens" becomes much easier, without getting kicked side to side when not centered over each loose rock.
The result of going to wide rims with the same tires is immediate increase in confidence while riding loose dusty, gravely, rocky, and wet conditions; enabling higher speeds, harder braking, and more precise and predictable cornering.
Carbon rims are expensive to produce. They require many hours of skilled labor cost compared to aluminum rims, and many times the materials cost. Each carbon fiber steel mold is CNC machined for many days to finish, about 1 week of machining per rim. In China the carbon fiber costs about $40 per rim. Then to produce rims requires big machines to pump and vacuum the inner bladder and heat the very heavy steel molds, then after cooling the mold, pop the rough rim out and hand file and sand the resin creep while in the mold, holes are drilled, and then finish painted, before QC inspection and finally packing for shipment.
In contrast, aluminum rims are machine made, from raw metal bars fed into a machine and pinched into shape in metal rollers. Then the rolled aluminum is machine pinned and welded, holes drilled, and finish painted, rarely touched by hands except to stack the raw metal bar onto the machine, transferring to the hole machine, then moving to the paint booth, before QC inspection and packing for shipment.
Each carbon fiber rim costs 6 to 8 times more in labor and materials than an aluminum rim.
Ray AKA 'derby', the designer and producer of Derby Rims, has been asked, "Why Derby Rims vs. other carbon fiber rims?" Here are many great reasons to ride Derby rims vs. any other carbon fiber rim option.
- Mountain biking guru Keith Bontrager once came up with the moto: 'Cheap, light, durable - pick two.' ...However, with Derby Rims, pick all three!
- Derby Rims are the lowest priced US designed and supported carbon rims. Yes, there are lower priced aluminum rims or direct mail order carbon rims from China web-store merchants, but Derby Rims leading durability and quick US based customer support when rare damage occurs also matters in the long run.
- Derby Rims are light in weight. Far lighter in weight than aluminum rims at similar widths. Also lighter than some of the other more expensive carbon rims following Derby Rims leading design with near the same rim widths. Narrower carbon rims can be more than a few grams lower weight, but narrow rims are a huge performance sacrifice, slower climbing, slower rolling, poor braking and cornering, and less stability. And a few grams of weight difference to other wide carbon rims is a very minor factor when considering the next major factor advantage of Derby Rims, durability.
- Derby Rims are the most durable, at any price, as reported by Pro and Elite category championship winning racers riding Derby rims, who are well experienced with many other carbon rim options. Less than 1.5 % damage was reported of all Derby rims in 2016, of more than 7000 rolling. Elite/Pro category Enduro and DH carbon rim riders report that other carbon rims failed in the same race and training conditions well over 20% rate, some over 50% fail rate per year. Only the Derby 27.5 rims used on the most aggressive long travel bikes were reported at greater than 1% damage rate. All Derby 27.5 AM/Enduro was 1.65 % rate in 2016, and the DH/FR was 3.5% rate in 2016. Improvements in durability continued. The 27.5 DH was reinforced with 35g added carbon fiber layer and began shipping January 2016, and the reinforced DH reported only 0.7% rate in 2016, a phenomenal durability rate for any type of DH rim. 27.5 AM DH was reinforced with 25g added carbon fiber layer and began shipping December 2016, and the expected rate will become less than 1% rate going forward. Some models have never been reported damaged, including the CX, RC and RD road rims. I wish there was 0% damage, but mistakes and bad luck does happen. Compared to Derby carbon rims, aluminum rims have a much higher rate of damage where replacement is required. The durability advantage is due to the industry leading Derby rim design, not by added weight.
- Derby Rims design introduced thicker hookless rim walls to carbon rims, increasing durability by an exponential magnitude. Nearly all hookless carbon rims designed by other brands afterwards have closely copied the Derby Rims designed thicker rim wall.
- Derby Rims design includes the most durable parabolic rim profile. A parabolic structure supports load and impact-force inputs at all angles far greater than any other rim profile design. A parabolic rim profile, for given materials and layup, is more durable for load inputs compared to rim profiles that are ‘U’ shaped, ‘triangular’, ‘I-Beam’, round, square or rectangular or other multi-faced profiles.
- Derby Rims design introduced the first bead seat retention ridges to hookless rims. Previously, only the 1999 Mavic UST (Universal Standard Tubeless) design had a similar bead seat ridge in the hooked rim specific UST system. Nearly all hookless carbon and aluminum rims designed by other brands after Derby Rims design have closely copied with bead seat retention ridges.
- Derby Rims introduced the first ever documented water drain holes to production bicycle rims. The water drain holes quickly eliminate water trapped inside hollow rims when water is easily able to enter through the rim spoke holes past nipples while washing wheels, riding through deep streams, or riding in the rain. Otherwise, nearly an ounce of water remains trapped inside any hollow rim with spoke holes for many months. In carbon rims, trapped water inside the rims enables galvanic corrosion of aluminum alloy nipples, producing rapid alloy nipple failure. Aluminum rims share in corrosion with alloy nipples, so the problem of nipple corrosion failure is not so rapid in aluminum rims as the problem is for carbon rims when water is retained inside hollow rims.
- Improvements in both traction and rolling resistance and bike handling feel are noticeable when going to wider and stiffer Derby rims. Riders of Derby rims report they immediately break all their Strava speed and time interval records, while climbing and descending and all timed interval conditions, only by upgrading to Derby rims on the same bike from narrower rims. And they continue to break their new Strava records on Derby rims, as they gain confidence in the much improved climbing traction, to be able to pedal harder without spinning out, as well as cornering and breaking harder with the added traction. And negotiating rocky or rooty and irregular trail with much improved stability and improved handling feel of traction limits and directional control. Rolling resistance is noticeably reduced, conserving rider energy and enabling more speed, with the lower tire pressures enabled with wider rims using the same favorite tires.