Should I Ride A Disc?

August 28, 2014

 

By: Dave Ripley

 

When you show up to a race and you see half the bikes in transition with disc wheels on the back, and the other half without, what goes through your head? Nothing? Maybe it's too windy for a disc? Must be a small person? The course is too hilly?

 

In reality, if you have the means to do so, you should ride a disc pretty much regardless of whether or not it is windy, or there are hills or how big you are.  Yes, there are certainly exceptions to the rule.  But, the rule should be, if it is a triathlon, ride a disc-Period!

 

As with any wheel you put on your bike, it should be the right one for you.  Meaning, it should be within a weight range to your liking and it should be laterally stiff or compliant enough for you to be comfortable on it. 

 

Let’s talk more about some of the common myths that keep people from riding disc wheels…

 

Rider Size

 

A common misconception is that small riders can’t ride a disc in windy conditions because you will be blown off the bike.  In actuality, the disc is the last thing that will be adversely affected by wind speed and gusts.  The disc is behind the rider’s body and is underneath your bodyweight effectively pinning it to the ground.  Also, the disc does not steer the bike.  A gust of wind will not shift the bikes direction by blowing the disc sideways.  The things that are affected by wind are your body and the front wheel.

 

Body positioning is very influential on how much the wind will affect your forward trajectory.  If you ride with a more upright position, you are exposing yourself to more wind force.  And, since your body is the highest point over your center of gravity you will feel these effects.   Get a good fit on your bike then adapt and get used to it.  Don’t assume that a by riding a more aggressively aero position that you will be uncomfortable.  If your fitter gets your angles right, it is actually more comfortable.  Also, if you spend the majority of your time training on your road bike or out of the aero position then don't expect to race comfortably in your aerobars when it gets gusty.

 

Keep in mind that most discs are stiff, both laterally and vertically, which aids in direct power transfer.  However, there is a bit of a myth that the ride quality of a disc can feel harsh.  If you find the disc to be too harsh, try lowering your tire pressure to the optimal range for your recommended weight.  Or you can run a slightly larger tire for additional air volume and some added comfort.  You can get away with a larger tire in the rear on most of today’s tri bikes since the seat tube hides the tire and leading edge of the rear wheel.  But, don't fall victim to hearsay and wives tales about a disc being too vertically non compliant.  When put on an Enstron machine and measured for vertical displacement, there is almost no measurable difference between a carbon clincher disc, an 808, a 404 and even a 303.  Bottom line, all carbon clincher rims are super stiff, and they don't have any inherent give in the vertical axis.  The tire does the heavy lifting for comfort and control, so read up on good tires and optimal tire pressure for Crr.

 

Wind

In windy conditions, the single biggest variable to pay attention to is your front wheel.  Your front wheel is attached to your steering lever and depending on the shape and depth of your wheel can get blown off axis.  Pick the front wheel that you are most comfortable with, and ride it…A LOT.  Get used to it.  Do not expect to leave the race wheels in the bag until race day and expect to be comfortable if it gets windy. 

 

The fastest wheel for you is the one that allows you to stay planted in your aero position for the majority of the race.  Every time you get up off of your aerobars, you are giving back time and effectively negating all of that money you spent on aero goodies.  Pick a wheel that has a shape that is designed for stability and speed.  Pick a depth that you are comfortable with and then ride it! 

 

And, to finally debunk the “too windy” myth, the faster the wind gets, the faster the disc gets.  Because the disc has a constant surface area there is nothing to disrupt the airflow across it.  This accelerates the flow across the windward side and releases the low pressure pocket on the leeward side- stabilizing the entire bike.  This balancing act also shifts the center of pressure of the whole bike and rider system towards the rear, relieving some pressure off of the front  end.  

 

If you want to test this for yourself, throw a disc on your bike, but keep your shallow section training wheel on the front.  You will realize pretty quickly that the disc is not affecting your handling in a negative fashion, except for making the bike feel more stable.  Now, throw on your deep section race wheel and go ride.  You will see immediately that this is the culprit.  The key is practice…ride this combo and get used to it. The speed is worth it.

 

Hills

Is this course too hilly to ride a disc?  Maybe the Alpe de Huez triathlon, but almost every other triathlon that you will ever compete in that allows you to use a disc…DO IT!  Even on a “hilly” course, until the gradient gets up to 6%-7% for an extended period of time, wind resistance still trumps gravity in terms of your major resistive force to going fast.  As long as your rolling speeds stay around or above 16.5+mph, then your biggest hurdle to overcome is wind resistance.  Because of this, you want aerodynamics on your side and a disc has no adverse side effects if your speeds are a bit slower. 

 

 

Since we don’t often do short, high threshold sprints or uphill attacks, the rotational inertia benefit of a lightweight hoop doesn’t gain you anything.  You are better suited to maximize your aero profile and save watts over the other 90%+ of the bike course that is rolling, flat or downhill.

 

 

In closing, if you can ride a disc, do.  But, as with anything new, don’t save it for race day.  Take it out and practice so that you know what to expect.  This has been made easier as clincher technology becomes better, you’ll have the freedom to practice on your race wheels more often without concern for wearing out an expensive tubular tire.  I’ll talk more about the clincher vs tubular subject in a later blog. 

 

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