Topstone 105 – Wheels, Tires, Fenders

Tires & Wheels

The most common bicycle tires in the early 21st century are pneumatic clincher or bead clincher tires. The wheel has a rim, and tire has a bead. The tire contains an inner tube, which is the part that is inflated. Tubeless tires are sealed and inflated directly – no inner tube. A tubeless rim will have a sidewall with a bead hook to catch and hold the bead. A tubeless tire may have a specially machined bead to fit into tubeless rims. Tubeless systems generally require sealants installed in the tire. The wheels and tires on my Topstone are “tubeless ready” – tubeless rims and wheels inflated with inner tubes like standard tires.

The wheels on my 2019 Topstone are WTB i23, a metal alloy with a 23 mm rim; the 23-622 model. The 23 mm measurement is taken at bead seat. WTB has versions of i23 including a “26 inch” mountain bike rim, and i23 (23-622) for 700c tires from 700 x 32c to 700 x 46c. WTB is one of the manufacturers that have refined the bead hook at the outer edge of the rim into a channel.

The tires were WTB Nano 700c x 40. Tire sizes are normally described by the nominal exterior diameter of the tire – the diameter at the tread on mounted and inflated tire. A tire is always marked with a tire size in a nominal diameter system, and a size in the International Standards Organization ISO or ETRTO system. The most common tires on gravel and all-road bikes are 700c tires – considerably fatter than 700c road bike tires. 700c is a standard for tires with a nominal outside diameter around 700 mm (28 inches; 700 mm. = 27.559 inches.) There are skinny 700c x 25 (ISO 25-622) tires and fatter 700c x 36 (ISO 36-622) tires. The ISO descriptions of 700c tires refer to the 622 mm diameter of the wheel and the tire measured at at the inner bead seat where the tire bead fits into the bead seat of a wheel. 650b is a standard for tires with a nominal outside diameter around around 650 mm (27.5 inches). The ISO descriptions of 650b tires refer to the 584 mm diameter of the wheel and the tire, measured at at the inner bead seat of the where the tire bead fits into the bead seat of a wheel.

The other number in descriptions is the bead seat diameter (“bsd”). A higher bsd means a fatter tire, mounted and inflated, with a larger diameter and circumference than a narrow tire. . Height is diameter across the wheel at the tread on the outside of the tir , measured on the mounted and inflated tire. The height, assuming the tire is a perfectly round torus, should be the diameter of the rim plus the 2 x the diameter of the tire. For a 700c x 28, this works out to 678 mm which is almost perfect in relation to the real world. The tire, inflated, has an irregular oval cross section, sort of egg or pear shaped. Diagram below. Thickness (width from sidewall to sidewall, from the outside)≥ bsd. For higher bsd numbers, the thickness starts to push out. Height (outer diameter) and thickness can vary slightly depending on the construction and inflation of the tire. This illustration shows a bead channel and hook:

1: rim, 2: rim strip, 3: rim braking surface (with rim brakes), 4: bead core (engaged by the bead hook), 5: inner tube, 6: casing, 7: tread. The diagrams and pictures at Bike Gremlin and Bike Touring News also illustrate. The height and thickness of some tires:

Tire Size Circumference ≅Diameter (C/π )ThicknessSource
700c x 18 (ISO 18-622)Generic2070 mm658 mm
700c x 25 (ISO 25-622)Generic2111 mm672 mm
700c x 28 (ISO 28-622)2130 mm678 mm≅ 28 mm
650b x 50 (ISO 50-584)Generic2149 mm684 mm
700c x 38 (ISO 38-622)Panaracer GravelKing SK700 mm41.4 mmMeasured

Clearance

The rear tire has to fit between the chain stays behind the bottom bracket and between the seat stays below a bridge below the seat. The bottom bracket shell accomodates a bb and a crank with a suitable Q factor.

On the Topstone, the chain stays are straight and parallel to each other for a few cm behind the bottom bracket, where the tire normally passes. The distance between the chain stays is about 45 mm at the eyelet for the front of the rear fende; 56 mm in the parallel section. The stays bend out to fit around the rear axle assembly (cassette and disc brake rotor). The seat stays are straight. The distance between the seat stays at the bridge with the eyelet for a fender mounting bracket is 48 mm, opening to 66 mm where the tire runs. The front tire has to fit between the fork blades under the fork crown. The distance between the fork blades where a tire runs is 61 mm or more. (The measurements were made with a caliper with the wheels on the bike).

The WTB Nano 700c x 40 tires were not quite the largest tires that would fit on that bike. They were too big to allow for fenders. For subsequent model years, Cannondale shipped the Topstone with 700c x 37 WTB Riddlers. The WTB Nano tires had a mountain bike type tread with widely spaced knobs. I thought the tread caused vibration; experienced gravel riders reviewing it thought it made for a smooth ride. I did not need that feature for my riding which is on asphalt and packed gravel. I downsized the tires to 700c x 38 Panaracer GravelKing SKs which have a file tread pattern of small knobs.

Some all-road and gravel bikes can be adapted to use 650b wheels and tires. The 650b standard is based on a 584 mm. diameter wheel. A 650b x 48c (ISO 48-584) wheel is as tall as a 700 x 28c. It has more rubber, and is wider and heavier. 650b x 48 is not as tall as a 700c x 38; it will fit into the fork under the fork crown and the seat stay bridge. A few tire manufacturers will state the expected width of a tire. René Herse states that its 650b x 42 is 41-43 mm wide, and its 650b x 48 is 49-50 mm wide. These will fit in the Topstone. Fitting a 60 mm diameter fender for 48 mm tires will be awkward with the clearances of the Topstone.

Tubeless Ready

Some manufacturers (e.g. Mavic) make rims without holes and manufacture rims and tire beads to the Uniform System Tubeless (UST) standard. The bead seat is designed to accept and hold the beads of UST tires without tape and without tire sealants. They are super-tight. WTB’s rims have holes and should be taped for tubeless set up. Howver the rim channel and wheel beads are beadlock – which is WTB’s way of saying manufactured to the UST standard. WTB calls this a feature. It is hard to unmount a tubeless tire from a rim to fix a flat with an inner tube. This is a downside of this feature as noted in this YouTube review of the Topstone (at the end of the 3 minute video).

Fenders

Fenders were normal part of the design of all-purpose bikes used by commuters and city riders, and installed by manufacturers for most of the 20th century. Road bikes, imitating racing bicycles, were built without fenders or even mounting points for fenders. While fenders were useful for some uses of mountain bikes, fenders were hard to design. Fenders for road bikes, hybrids, and mountain bikes became accessories. Fenders were more popular on touring bikes and endurance/randoneusse bikes.

To fit fenders, the rider needs to know the thickness of the tires, and the clearances in the bike frame. The front fender has to attach to the fork at or near the fork crown. The rear fender needs to attach to the frame behind the bottom bracket between the chain stays, and at a bridge between the seat tubes. The attachments at those points support the front parts of the fender in the proper position. Fenders stays hold the back parts of the fenders in position. Fender stays normally attach to the frame and the fork near the axles.

The exterior diameter of the fender, measured across the base of the arc of the fender indicate that the fender needs that horizonal distance to fit between the seat stays or fork blades without cutting or bending the fender A fender should fit around an inflated tire, with clearance. Velo Orange, a manufacturer of aluminum fenders, suggests an 8 mm. difference between the exterior diameter of the fender and the bead seat diameter of the tire. This suggests a 700 x 40c tire needs a 48 mm exterior fender. A fender should clear the tire by about 20 mm where the fender covers the tire. There has to be a vertical gap of more than 20 mm from the inside of the fork crown or the seat stay bridge and the outside of the tire.

My first fenders for the Topstone were SKS P50 Chromoplastic Longboard Fenders, a popular model sold in many shops and online stores in the USA and Canada including Modern Bikes and Universal Cycles. SKS marketed these fenders as wide enough to cover 700c tires in the range from 700c x 38 to 700c x 45. The P50 fenders had an exterior diameter of 50 mm. Plastic is light but it is prone to twisting and vibration. The rear ender mounted easily to the eyelet between the chain stays behind the bottom bracket. SKS, like other fender manufacturers provides a fender bridge that has to be bent around the outside of the fender and crimped around the edges. This bridge has to be centered on the fender to align to the frame mounting fitting on a bridge between the seat stays, slid into position and crimped onto the fender. If this is not done successfully, the fender will be twisted out of true. There is very little margin for error . SKS chromoplastic fenders use V shaped stay ; each stay has 2 arms. These stays are designed to attached to eyelets at the ends of the chain stays. The stays did not quite line up to eyelet . I had to adapt some left over parts from a rack mount kit to fabricate little fins. I had to bend the stays out to balance the tension to keep the fender from rubbing, but it worked.

The front fender fit under the fork crown and appeared to have adequate clearrance over the tire, but did not clear the original 700c x 40 knobby/cleated tire. I downsized the tires. The next problem with those fenders was the lack of eyelets for fender staysat the ends of the fork blades. The only eyelets stays are on the inside of the fork, 15 cm above the end of the fork. This was not a good place for the SKS breakaway tab or for a V shaped stay. I was able to improvise an attachment for the SKS break-away fender stay mount, and bend the V arms of the stays to slide into the attachments on the fender. This worked for my rides in the winter of 2020-21. I began to get a rubbing noise at higher speeds on chipseal. The V stays did not support the plastic fender under those conditions.

Other riders reported installing SKS plastic fenders on Cannondale road bikes in forums, Reddits and YouTube. These riders referred to bikes with eyelets at the end of the fork blades. One English rider reported, in a YouTube video, using the Bontrager NCS plastic fenders on a Topstone with carbon forks. That fender has a single straight (adjustable) stay, which normally attaches to an eyelet at the end of fork blade, but can be attached to the eyelets on the Toptone. I installed aluminum Velo Orange fenders. These are light but rigid, and attach with a single stay on each side of the fender. This is how the stays fit, using the pre-drilled holes for the stay:

Carbon Fiber Fork

Carbon fiber forks are offered on many production road and gravel bikes with steel or aluminum alloy frames. A carbon fiber fork is light and stiff, which is supposed to improve steering, although this can be debated. A carbon fork on bike with disc brakes has to be stiff. A stiff fork does not dampen vibration. A carbon fork lacks eyelets for racks and other devices, which limits some uses of the bike.

Pressure

The pressure marked on a tire is a consumer protection warning – a fraction of pressure that will blow the tire off the rim. It is not a recommendation for performance. The operating pressure is normally much lower than the marked pressure. High pressure was believed to lower rolling resistance, but that theory or belief has been contradicted. The appropriate pressure depends on several factors, and can be estimated with a calculator. Fatter tires run at lower pressures. The maximum presssure for 700c x 38 with an inner tube is 75 psi. I ran it at 60 psi; I got less rolling resistance in the low 50’s.

Crankarms and Cassette

I changed the stock 172.5 mm crankarms to 165 mm – a shorter radius and less stress on knees. I replaced the Shimano 105 11-32 cassette for an SRAM PG-1170 11 cog 11-36 for a few more low, climbing gears

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