Rim - Round and Round We Go

Rim - Round and Round We Go

What good would a bike be without wheels?  It would be a bi... the cycle-type things define the beast.  The frame would get nowhere without the wheels.  The hubs are necessary, as they support the axle.  In terms of bike wheels, the hub contains the bearings, any extra gearing or brakes, freewheel, and house the skewer.  It also anchors the spokes that connect it to the rim.

The rim is the metal hoop that also anchors the spokes. The rim also supports the tyre, the inflated rubber tube that ideally is the bike's only contact with the rest of the world.  Crashing changes that, but that can wait for another entry.

A simple arrangement, but with a few issues and a history of its own.  If you step sideways to older forms of transport, we can see wagon wheels have the same arrangement of hub, spokes (wooden!) and rim, with an iron band to stop the rim wearing away.  This was the same design as the first bikes.  Even as late as a few decades ago (the realm of living memory!) the top-line racing wheels used wooden rims.  
As a material wood can be as strong as steel in tensile strength, and is much lighter.  The classic examples of wood used in high-tech structures is in aircraft, notably the De Havilland Mosquito fighter-bomber, and then the Vampire jet fighter of the 1950s.  
The main issue with bike wheels is that rim technology advanced beyond the scope and strength-weight limits of wood, which has to always be solid.


The first steel rim was the Westwood, essentially a single flat strip of steel press-formed into a ridge with rolled edges to support the tyre.  These rims are still being made in India...
This progressed to the Endrick rim (also available new from India!), the first design that gave a hollow, box-section rim. The square tube was pressed into the collapsed box shape, and was still found on bikes through to the eighties.

Modern rims are now engineered around a V-design, providing greater torsional and compressive strength, and allowing for aerodynamics.  As the bike moves forward air strikes the inner edge of the rear rim surface.  Older square profiles were lousy in terms of aerodynamics, as the air would be striking a flat surface.  The development of the V-rim allowed a smoother, shaped airflow around the wheel.  Also, as the tyre strikes the oncoming air, the deep V gives the wheel a leading-edge profile similar to an aircraft wing, whereas the old square rims simply created a zone of turbulent air amongst the spokes.  If you like to have a stable, smooth ride this is not the way to go.  


Today, modern companies, for example Zipp, continue to develop aerodynamic rim profiles to give not just a smooth, controlled ride, but to achieve the time-energy boosts from having low-turbulence wheel-air interactions.  The drive to develop the perfect aero wheel gave us the disc wheel, which eliminates the turbulence generated by spokes and also eliminating the slight, but distinct flattening of the wheel shape at the road contact point.  Trouble is, these wheels can only be practically used in indoor velodromes, as they become wind-traps as soon as you venture outside.

My hubs are 28h, so the rims for the build have to match. Of course.  Also, I want regular clinchers, as I like to be able to repair flats alongside whatever road I may puncture on.  I cannot afford a team support car to follow me everywhere with spare tubeless set-ups.  Surprisingly, there is not a terrific range of rims available internationally online (posting to New Zealand). Sun is common, but also available are Velocity rims and, my preferred rim, DT Swiss.

Founded in 1994, the Swiss company expanded its business from spokes to everything attached to them, and they do it well.  My last build used a DT Swiss RR415 rim, and I am staying with this rim.  Of note, the DT model number is the rim weight - the RR415 weighs 415 grams. Rim weight does have some effect upon the performance of the wheel, mainly through rotational momentum.  

To those students of physics, this is angular momentum. A 700c rim has a diameter 622 mm, and radius of 311 mm. It's circumference is (622 x π =) 1953 mm. A rim mass of 415 gm at 0.311 m from the centre has a momentum of (415 x 1.953 x 0.311 =) 252 gm², for one revolution.  Boost the rim weight to, for example, 450 gm, and the angular momentum goes up to 273 gm². More energy is needed to get the wheel turning, and any time that you want to accelerate you use proportionally more energy. Being a middle-aged kind of bloke, that is energy that I am not too keen to use needlessly.  Fortunately I am light enough (78 kg and holding...) to get away without a heavier rim.  So long as I can see my toes when I look down I should be fine.

I bought the rims from Chain Reaction Cycles, far away in Northern Ireland. Two rims and postage to New Zealand, from Ireland, cost me NZ$187.33 (US$151.50).  Really, for what you get (Swiss rims!), and delivery time of 7 days and 10 hours from the other side of the world, is an unbeatable deal.

And now, I have to find not just spokes, but a truing stand to build the wheels on...