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...




Back to blog


Hi Tom -Words straight from A2 yesterday – Cda column is perpendicular to the axle, not in the direction of travel, and disregard all values at 0 AoA. We're actually working on a "volatility index" for each wheel to address that behavior, but in general each wheel has a very smooth progressive curve through the sweep. All of the wheels except in two instances have a reversal in the 2.5 to 7 range, with the two exceptions being tire dependent – they do it with one tire but not the other width. Interestingly, the two wheels that do this do it with opposite tires – in one case the 23 doesn't show the reversal, in the other, the 25 doesn't do it. But we've been thinking about the volatility, and it's clear that it's not that big a deal and it's relatively consistent among all the wheels in the group. About two years ago (blog I posted about the 2012 TdF TT), I came to the conclusion that if I were fast enough to bring the AoA to near zero and keep it there, on a calm day, and all I cared about was straight line speed, I'd use an H3 or H3D with a skinny ass tire. Apart from that circumstance, which doesn't exist in my life, I've no interest in using that setup for any purpose. Dave

Dave K

All well and good, but if the data output you received from A2 is consistent with what I've seen, then the column you're calling "CdA" is actually labeled "Body Axis CdA" and it's not representative of the transverse force value, but the force in the direction of travel.Secondly, one thing you may want to be careful with about using a wind average weighted value for this measurement is you may end up "missing" cases where the slope of the steering torque curve reverses, which is something I've seen on data for certain wheels in the past (Cough…trispoke…cough) which leads to highly unpredictable handling in gusty conditions.

Tom A.

Son of a… Done. We changed the image to put the long bars on top because it looked better, but the labels didn't follow. We're still getting used to this graphics maker. Thanks

Dave K

I think that the labels for CoP and CdA are reversed.


Rick – As you can see, we've separated the two components to simplify the discussion. The center of pressure should be relatively easy to envision at this point, hopefully. As to the lift being the main source of instability, I wouldn't agree there. I'd instead say that the capacity to go from a high lift to a high drag situation is what goes on there – big swings in the l/d ratio. Walk down the beach with a 12.5m2 windsurfer sail, which is a HUGE sail – max size racing sail. You carry it overhead, with the "leeward" side (the low pressure side) up, one hand holding the mast above (as in further towards the top of the sail, but in this case it will be a lateral not a vertical dimension) the boom, one hand holding the boom. A skilled person can make relatively micro adjustments to respond to small changes in wind velocity and direction to keep the sail in flow, and it's no problem. The lift produced by the sail, kept in harmony with small adjustments, makes this big sail easier to hold than a tiny sail, because you would have to hold the tiny sail up, it doesn't "fly" itself. But make a bad adjustment where you "open" the leading edge of the big sail too much, and all of a sudden you are going to get lifted up HARD. Worse, "close" the leading edge too much and you will immediately be pummeled into the ground. If you walk down the beach with the small sail, you might have to hold it up, but if you make bad adjustments, a 98 pound weakling could overcome them. If you make bad adjustments with the huge sail, Charles Atlas himself could not overcome them.On a bike, obviously you are trying to go the way you are trying to go. You are not primarily concerned with "trimming" your direction to respond to changes in wind speed and direction. And if you are traveling at a constant speed and you are going other than directly into the wind or directly away from it, a change in windspeed always acts like a change in direction – always. Let's say you are traveling along at 20mph and there is a 10mph wind coming from 20* to your right. In your face, you feel that as a 29mph wind coming from about 6.5* to your right. All of a sudden, a 20mph gust comes along, with no change in direction. Instantly, instead of the 29mph wind at 6.5*, what you feel is a 39mph wind at 10* to your right. The net change in wind direction you feel is as significant as the net change in wind speed that you feel. Having thought about the dynamic of why the alloy wheels keep the center of pressure behind the hub so readily, I'm coming to the conclusion that it's because the aft half of the wheel is always producing more drag than any other part is producing lift. But in any case, with those wheels, the forces never get so big that they are not easily overcome with a rider correction.

Dave K

Leave a comment

Please note, comments need to be approved before they are published.