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Saturday, February 2, 2013

Are Your "Stiff" Wheels Actually Stiff? *Updated: New Video (No Flex)*

How many times have you heard or read a testimonial where someone said "these wheels are very stiff" without showing how stiff the wheels actually are?  It would be an understatement to say a lot, but I'm hoping to change this!

1000 WATTS + FLEXY WHEEL = 850 WATTS
The equation above is the reason why I'm writing this post.  After reading the "Great Wheel Test - Part 3 - Stiffness" article at RouesArtisanales.com, I learned that during sprints at 1000 watts, you can expect to lose 150 watts on a flexible wheel.  So instead of 1000 watts, only 850 watts effectively gets used towards the acceleration.  That's a huge loss!

Since I don't have the equipment to measure lateral stiffness, I created a video to demonstrate a wheel's lateral stiffness under real conditions.  If the wheel isn't stiff, you'll either see the wheel deflect into the brake pads or go out of true... or both!
THE E.S.T.S. LATERAL STIFFNESS TEST VIDEO:  A FAILING EXAMPLE
The Oval Concepts 524 rear wheel in the video below was the OEM wheel that came with my full carbon Kestrel Talon road bike.  Not only did the rim hit the brake pads despite having the calipers open and slightly rotated, it went significantly out of true!  Check it out:

Oval Concepts 524 Rear Wheel Lateral Stiffness Test


As you saw, the wheel initially maintained its shape, but immediately flexed into the brake pads as soon as the sprint started.  After a few more revolutions, the wheel lost its true.  Keep in mind that the spoke tension was within spec and confirmed with a Park Spoke Tensiometer.

The video was taken during a 20 second sprint from 14.5 mph to 27 mph.  Since I am a lightweight rider (137 pounds) sprinting on a relatively flat surface, I started at a cadence of 41 rpm to guarantee a high peak power output.  The max cadence I hit was 72 rpm.  I wish I could also provide you guys with some power data, but I unfortunately don't own a power meter.

Here's a screenshot from my Polar CS600X cycling computer showing the data from the sprint graphically.
The spreadsheet below was taken from my Polar CS600X.  The sprint started at the top of the list and ended on the highlighted row.
Here are some of the specs for this wheel:
  • 21 mm rim depth
  • 24 round spokes
  • 2-cross (drive side), radial (non-drive side)
THE E.S.T.S. LATERAL STIFFNESS TEST:  A PASSING EXAMPLE- VERY STIFF!
After discovering how weak my Oval Concepts 524 rear wheel was, I needed to find a rear wheel that wouldn't flex.  The first wheel that came into mind was my 32 spoke rear wheel from my Fuji Roubaix 3.0!  With eight more spokes, it has to be stiff right?  Check out the video below to find out:


As you didn't see, the wheel barely made any sign of flex despite my hardest effort.  This video confirms two things.

  1. My frame DOESN'T flex or twist a discernable amount.
  2. The wheel is more likely to flex than the frame.  Makes sense especially since the wheels are supported by thin metal wire and the frame is a giant piece of carbon fiber.  Frame wins.

CHARACTERISTICS OF A WHEEL STIFF (IN ORDER OF MOST IMPORTANT TO LEAST IMPORTANT)
While there's a compromise of either weight or aerodynamics, wheel stiffness needs to be a high priority in any type of race or course where accelerations, sprints and climbs are to be expected.  Of course, there are tradeoffs, and if a super stiff wheel means doubling weight, then this would be detrimental to performance.
  1. HIGH BRACING ANGLES
  2. TANGENTIAL SPOKE LACING
  3. LARGE FLANGE DIAMETER
  4. HIGH SPOKE COUNTS
  5. SPOKE SIZE
  6. SPOKE TENSION
  7. RIM MATERIAL:  Not all carbon fiber rims are stiff.  Carbon fiber can vary in tensile strength (the amount of force to break the fibers) and tensile modulus (the stiffness of the fibers).  Velocite bikes wrote up a really good article discussing this topic.
HINDSIGHT IS 20/20:
As crazy as this sounds, throughout the entire 2012 racing season, I used my stock Oval Concepts 524 rear wheel in all of my races.  I remember losing the podium by a bike length and sometimes, by a few feet.  I trained harder, but my sprints barely improved because the extra strength only contributed to harder deflections and brake pad rub.  It's without question that a stiffer wheel could have changed the outcome of my results.

To be honest, when I bought my full carbon Kestrel Talon (Oval Concepts 524), I was very disappointed.  It always felt ridiculously slower to accelerate compared to my aluminum Fuji Roubaix 3.0 (Alex Rims DA13 rear wheel).  Despite having a stiffer frame, it now makes sense why my full carbon Kestrel felt slower than my aluminum Fuji in sprints.

While I don't recommend this to anyone, I started riding with my rear caliper open and it has done wonders to my sprints!  There's such a noticeable difference that I don't need data to tell me that my sprints are faster.

TAKE HOME LESSON
After watching my own footage, I've been really careful about how I shop for wheels.  I seem to pay more attention to things that I normally would not notice.  While subjective reviews are helpful in many aspects, going by feel doesn't seem to be the most reliable method to evaluating stiffness.  I hope this post helps everyone else out there make better decisions when buying a wheelset!