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17 Oct

Aero test BORG50C are in.

Posted by Bold Apps

Full details here. Test published by Hambini Engineering. 

Most aero tests have issues. A wheel or a wheel in a bike is tested with or without a dummy or rider. Sometimes with half a dummy. All these differences affect the significance of the overall result as it is impossible to tell the overall effect of the wheel in the system as a whole. Not to mention the air flow around the wheel is affected by the presence of a fork or the absences of it. 

Sometimes the wheels are static and a spinning wheel behaves differently to a static one. 

The final problem and this is a big one all aero tests you see are done by running the air flow at 50kph for a time at a static angle. Stop recalibrate and reset at the next increment. This does not predict real world conditions

As you ride the bike rocks, the front wheel and changes direction constantly. The changes in direction are small but if the front wheel did not do this you would fall over. Also when you pass gaps in hedge rows or buildings, even when cars pass on busy roads the airflow will osscilate sometimes widely.

Most wind tunnels don't simulate osscilating airflow. All this, means the results from most wheel aero tests are not that informative because the wheel behaves quite differently when the airflow oscillates and therefore behave differently in the real world. For starters you can spot the wheels that are at risk of speed wobble this way.

So when an engineer at airbus started testing wheels in there wind tunnel as an after hours project my ears pricked up. First the protocol has the air flow ramping up from zero degrees and holding at an angle for 15secs but oscillating +/- 2.5 degrees. A wheel is in a bike with a rider on it. The test is consistent for all wheels.

The error is +/- 2.5% 

The protocol was determined by 6 month of rider monitoring. 

 

 The above data is from the test rider riding down a straight road in the west country at constant speed. The yaw angle is anything but constant. 

So from lots of data gathered on yaw angle vs speed gathered from time trailists averaging 50 mph and road cyclist average 30 kph a protocol has been determined. The protocol does not represent one ride or how your wheels perform on a particular ride under certain wind conditions. What it does is sum wind conditions experienced over 6 month of U.K riding and therefore simulates how your wheel performs over a wide range of condition that would be experienced riding the wheels over a whole year. So this is more like giving a car mpg figure but for a bike.

The test is conducted with a rider on a road bike (the same bike and rider) in a road position (on the hoods). Below is an exaomple of one of the test protocols. 

The testing protocol is very different to manufacturer tests. It  mimics real world riding conditions in the sense it models transient (oscilating) air movement. Emphasis is placed on wheels which handle the separation and reattachment of airflow efficiently, very little emphasis is placed on riding a bike straight into a head wind at zero degree yaw - this is not realistic so why bother testing it. The wind tunnel used was temperature and humidity controlled.

The results

The figues for the BORG50C wheel with the reference Continental GP4000sII 23mm tyres fitted is 186W at 30 kph and 597W at 50 kph. I dont know about you I can't hold 50 kph on my road bike for very long. the 600W output for me is quite realistic. Remember the error is 2.5% so at 30 kph, the error is 4.675W so frankly the wheels that test better are within the margin of error. Also what is clear depth is king. Although profile is important too. 

So at 30kph all the wheels in the from about 183W to 186W are as good as each other. Cost spares availability, ease and cost of repair do vary though. 

If the rim is two blunt that increases drag although cross wind stability maybe better. 

What the detailed results show is most wheels show speration of air flow at 12 degrees yaw. The BORG50C is no different. That's physics. Some blunt profiles can extend this to 14 or 15 degrees but at the expense of drag. Those blunt wheels have one big problem once the airflow seperates it does not reattach well. 

What is shown in the detailed results is some wheels deal with the air flow flutter better than others. Mine do quite well, the wheels that don't deal with it poorly. Thats down to the profile. 

So in short for £800 the BORG50C is really a very good wheelset and has all the charastistics you need. It also benefits from a life of the rim warranty against faigue failures and manufacturing defects. The wheels are repairable as every spare is available and I will dio it here for sensible money. 

 


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