Steady as she goes redux


We knew that yesterday's blog would have some birthing pains, simply thanks to the new-ness of the data we presented, the lack of a vocabulary and nomenclature around it, and the simple lack of history around it.It hadn't gone through the crucible.  Everyone's familiar with seconds saved in 40k TT, but the quantitative measurement of the off-axis forces acting on a wheel is new stuff.  Heck, we've barely just settled on using "bead seat width" for the measurement of the distance between brake tracks inside a rim.  

Anyway, we had one bobble in how we ranked stuff, which knocked the alloys down off their rightful spot in the hierarchy.  If you take the wheel in isolation, having the Center of Pressure (CoP, think of it as the location of the push from a cross wind) ON the hub axis is ideal.  When you put the wheel on a bike, it's not.  The steering axis, which is the actual line on which your front wheel pivots, is behind the hub.  This is where we get into headtube angle and fork offset and trail dimension, but at the end of a long road somewhere around 4.3cm is a good number.  That simple change made the sniff test go from "something's funny" to "yeah, that seems about right."

The alloy-rimmed wheels are the only wheels which put the Center of Pressure (which I will inevitably call Center of Effort somewhere around 1000 times, because that's what it's called in naval architecture) behind the hub, in it's ideal place.  Why exactly that's the case is a subject we will leave alone for now.  So instead of using the hub axis as our zero point, we are using -4.3 as the zero point.  That is to say that a pressure that is centered 4.3cm behind the hub will exert no steering torque on your bike.  Any center of pressure behind that will turn the front of your wheel into the wind, any center of pressure ahead of that will turn your wheel away from the wind.  


We also had represented the Center of Pressure and CdA (coefficient of drag) both as separate and related values.  For the time being at least, it probably gets this conversation further down the track to emphasize their separate values, as we have on our shiny new graph.  

The emphasis falls more squarely on CoP for now.  Combining the two, with the steering axis correction, does nothing but flipflop the relative position of Rail 52 and 34.  

All told, this is a more correct, cleaner, more easily understood and digested presentation of the data, and a better starting point to the conversation than yesterday's.  

A couple of quick notes:

1. Zero yaw points are excluded.  The measurement formula doesn't work at zero because the formula math doesn't work with a zero in it, and at zero yaw there is no crosswind anyway.  The values for zero are all over the place, and we were told straight away to remove them (which we had done yesterday)

2. As with yesterday's graph, this is weighted per Tour Magazine's yaw-oocurence weighting for 25mph bike speed.  That may or may not be ideal.  The differences narrow down a little bit at wider yaw angles which are more represented at lower bike speeds.

3. These are all measured with 23mm tires.  Putting 25mm tires on does change things a little, but that's a jar we aren't opening for now.  

4. At the end of it all, there is precious little context around this.  How much of an actual "on the road" difference in handling is represented by the gap from worst to first?  There is just not the landscape to put that gap into context.  Where this group of wheels fits into the overall picture is unknown.  

5. The CDA figures (which are measured in m^2) have all been multiplied by 100.  This changes nothing in their relative ordering or the magnitude of the differences between them, it is a facility to make the chart easier to present.  

Soon enough, we will all have a common language and ease with this stuff.   Until then...




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  • Rick R. on

    I'm also confused on how body axis drag is being used to determine the steering force? The lift produced by the wheel should be much larger than the drag. Also, I would suspect that this lift is the main culprit for the instability of deep section rims in crosswind, the side force produced by the larger "wing" is what you're overcoming in a gusting situation.

  • Dave Kirkpatrick on

    Tom – I can't comment on what they've done since June. I know that our tests occurred on 7/28, 7/29 and 8/14. The columns that match in the sheet you're referencing don't match in ours. In the grand scheme of things, it's not THAT expensive to go to the wind tunnel. It's $550 an hour. Spend a day, test a bunch of stuff. :-)Dave

  • Tom A. on

    So…does that mean they've changed the output since June and that the column in the datasheet is mislabeled? If it still is labeled "Body Axis CdA" (which you've never confirmed or denied), then I think the prudent thing would be to have them show you the calcs. You don't want the Body Axis CdA (or, CxA) for this calculation; you need the value orthogonal to that. I find it doubly odd that in the one example of that data sheet I've been able to find (since you won't show us your data sheet) that the values in that column match the values in the ACTUAL "Body Axis CdA" column for the wheels, just multiplied by the unit conversion values. Are you saying this isn't the case for your data sheet? One would expect the 2 columns to be approximately inversely related, not directly.I'm sorry if you think I'm unfairly poking and prodding at this. I like and appreciate the fact you're trying to communicate this information. I just want to make sure we're all looking at the right thing ;-)

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