Components affecting load distribution

• a taller/wider rim is stiffer, spreads the load more
  • each spoke sees less change of tension
• thinner/more flexible spokes spread the load more
  • each spoke sees less change of tension
• more spokes
  • spokes per length of rim — "spokes in the load-affected zone"
  • 36H for 700C approx. same spoke distance as 32H for 26"
  • No accident — light MTB rims: Keith Bontrager: 36H MA-2 cut down to 26"

Rim section

• box-section aka double-wall — most common for more expensive rims
• single-wall — most common for cheaper rims
• twin-hollow — most common for cheaper rims
• taller
• wider
• ultimately you care about weight, strength, durability, etc.

Rim structure

• welded or pinned?
  • summary: makes no difference (more later)
• machined sidewalls
  • summary: reduces rim durability (brake track service life)
• eyelets, double eyelets
  • spread spoke/nipple load to spoke bed — more durable
  • easier to turn nipple — more spoke tension
  • avoid losing nipple in rim box — easier wheel building
  • some eyelets rust and get noisy

Rims and braking

• brake track height
  • precise pad position
  • pad position changes under braking!
• brake smoothness
  • bump bump bump at rim joint — usually wears away
  • machined brake track — smooth for "first impressions"
• brake power
  • dry: most rims similar
  • wet:
    • chromed steel: bad
    • aluminum: good
  • machined
    • good for test ride wet braking
    • wears away
• brake track wear thickness
  • how thin is too thin?
  • many manufacturers do not say
  • approx 0.5mm (4-5 sheets of paper)
  • Velocity Aerohead approx 1.4mm brake track thickness
  • Mavic MA-2 approx 1.7mm brake track thickness
  • "Useful" thickness
    • Aerohead 1.4mm-0.5mm=0.9mm
    • MA-2: 1.7mm-0.5mm=1.2mm
    • 1.2/0.9=1.33 → MA-2 33% more wear thickness
• hard anodizing
  • popular in the early 90's — rare today, still sometimes
  • longer wearing
  • wet braking poor
  • can cause fast rim failure via cracking
• ceramic coating on brake track
  • helps wear life a lot
  • insulator: hurts hot braking
  • expen$e, weight
  • brittle — can flake off
• brake cooling — failures when things get too hot
  • melt pads
  • blow tire off the rim — heavy + hills, esp. tandems
  • valve stem failures, esp. tandems
• brake cooling
  • polished > anodized > painted >> ceramic
  • deep section >> shallow section

Rim weight

• heavier is usually stronger and more durable
• but some rims have design weak spots
• light rim compromises
  • small section builds a weak wheel
  • thin walls crack
  • thin brake track wears quickly
  • thin brake track more likely to ding on pothole
• rim weight varies LOTS
  • light clincher road < 400g, heavy > 800g
  • variation even within a given make/model
  • e.g., 450-500g
  • section is extruded — hot metal through a shaped die
  • die wears
  • rims get thicker
• some makers "bin" rims, same rim different die wear

Rim aerodynamics

• minor effect for slow riding
• significant for fast riding
• deep: less drag for straight ahead in calm winds
• deep: bad in cross winds
  • handling
  • loss of aerodynamic benefit
• power is cubic in velocity
• aero is tricky — not all "deep" rims are aero
• tradeoffs: crosswinds, durability, weight, ...
• modern: "semi-aero"
• rule of thumb:
  • lots of climbing: fastest lightest wheel possible
  • lots of flats: most aero wheels

Offset spoke bed

• asymmetrical or "dished" wheels
• only a few mm
  • looks small
• but if flanges are 36mm and 12mm offset
• 3:1 spoke tension (approx)
• 3mm change in offset
• 33mm and 15mm
• 2:1 spoke tension (approx)
• big difference in maximum load capacity
• big improvement in wheel retensioning interval
  • nipples unscrew when spokes go slack
• spokecalc shows you the improvement!

Rims and tires

• rim too narrow for wide tire
• rim too wide rim narrow tires
• "proportion"
• rim that is too wide
  • harsh ride
  • tire blow-off
• rim too narrow
  • bad handling
  • tire pull-off (MTB lower tire pressures)

Nipples

• less(!) to say

Nipple shapes

• standard: wrench flats one end, flare at the other
• standard: 2.0mm or 1.8mm threading
• splined wrench
  • rare
  • special tool — field service
• hidden
  • better aerodynamics
  • have to take off tire, etc., to adjust spokes
  • special tool — field service

Nipple materials

• most common:
  • chromed brass — shiny silver or black
  • aluminum — dull silver, black, colors
• standard: brass
• aluminum
  • usual reason: 0.3g/nipple vs. 1.0g/nipple for brass
  • but more likely to fail esp. for 2.0mm (thicker) spokes
  • fail:
    • snap — effect like a broken spoke
    • sieze to rim and spoke, round off
• here: brass

Nipple locking

• thread-locking/self-locking
• prevents nipple from unscrewing in use
• usually held by friction, unscrew when spoke goes slack
• but if it unscrews in use your wheel may be near collapse

Tall nipples

• tall may have same amount of threading as standard!

Spoke kinds

• Material
• Length
• Diameter
• Cross-section shape
• Elbow, straight-pull

Material

• galvanized steel — traditional, rare today for loose spokes
• chromed steel — traditional but uncommon, rare
• painted — and underneath, galvanized?
• stainless steel
  • by far the most common
  • usually silver
  • some are black
• titanium
• aluminum
• plastics, carbon composites, ... "exotics"
• here: stainless

Length

• determined by rim, hub, lacing pattern

Diameter

• units: mm or gauge
  • 2.3mm ↔ 13g
  • 2.0mm ↔ 14g
  • 1.8mm ↔ 15g
  • 1.6mm ↔ 16g
  • 1.5mm ↔ 17g
• straight: 2.0, 1.8, occasionally 2.3
• thin in the center, thick at the ends: swaged aka "butted"
• idea: light in the middle, thick at the ends where failures more common
• irregular microstructure and irregular stresses
• Example sizes
  • 2.3/2.0/2.3
  • 2.3/1.8/2.0 (!)
  • 2.0/1.8/2.0
  • 2.0/1.6/2.0
  • 2.0/1.55/2.0
  • 2.0/1.5/2.0
  • 1.8/1.6/1.8
  • 1.8/1.55/1.8
• why do you care? structure, spoke and spoke bed durability
  • Jobst approx 500,000 km same spokes
  • at 2.2m/rev => 230M load/unload cycles
  • Sapim reports 1+M cycle durability
    • sounds good
    • 1M revolutions is approx 2200km — yawn!

Shape (cross-section) [example]

• round
• bladed — aerodynamics but may need special hub
• oval — blading without special hub
• skinny → torsionally flexy → easy to damage when tensioning
  • field service issues
  • less section drag but more skin drag
  • more total air drag in cross-winds?
  • for this class and for most uses: round

Elbow (common) vs. straight-pull