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We also hand build custom alloys. Rims by Pacenti, Stans, HED and Kinlin. Hubs by Miche, WI, Chris King, DT, Tune and PowerTap.

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Tire Size Reëxamined

Like that umlaut? The Economist does it, so I figure it's probably correct.  Anyhoo... all of this hoo-ha about tire sizes and inflation and tire size versus aerodynamic speed has had us thinking about this quite a little bit lately.  

Step 1 was to to further examine the effects of tire size on aerodynamics.  Ideally, we would have been able to naturally (i.e. without artificially manipulating bead seat width) set two of the same model of tires up to the same inflated dimensions on wheels with different bead seat widths, to test the slope that we'd earlier established that had the 404 and 52 leap frogging each other at inflated width. The 52 with a 23 is, inflated size-wise, a bit smaller than a 404 with a 25 and moreso bigger than a 404 with a 23.  It is a little closer in speed to a 404 with a 23.  What would happen if you put the same tire inflated to the exact same dimensions on a 404 and a 52?  Unfortunately, that's an unanswerable question, but we were able to test a good approximation.  

Continental makes the Attack 22.  It's a different tire than the 4000s II, but its inflated dimension on a 52 closely replicates the inflated size of the 404 with a 4000s II 23 - the Attack 22 on a Rail is .2mm narrower than the GP4000s II on a 404, and about .6mm shorter than the GP4000s II on the 404.  There are tread differences, which are known to influence aerodynamic speed, but net of everything it's the closest we could get to being able to measure exactly what we wanted to.  This was part of a totally separate trip, primarily to do some totally different testing which we can't yet talk about, and we shoe-horned this bit in.  We ran the 52 first with the GP4000s II, and then with the Attack 22.  With this testing reaching the point of diminishing returns and threatening to turn into a bottomless money pit, we kept it to just the two runs.  Since we don't directly compare different tests from different days, we used the Rail 52 with 23mm GP4000s II as the baseline, rather than the Pacenti SL23 which was the baseline for the other round of tests.  Semantics, we know, but it counts.  



As predicted, the narrower tire gains some speed.  Whether it's the same speed gain we'd get from the theoretical GP4000s II that set up to the same dimension that the Attack 22 set up to, we can't say.  We're getting well into the realm of splitting hairs here.  

So now, the million dollar question: which tires should you be using?  We've now examined the relationship between inflated dimensions and effective pressure, and the relationship between aerodynamic speed and tire dimension, and the fact that people don't want to ride around on the road at 60psi, and a few things show up.  The first is that you should ride the tires that you like, at the pressure that you like.  There's no "right answer."  That said, it seems to make little sense that a 110 pound triathlete should choose a 25, on which she'd have to knock the pressure down pretty low to get a smooth ride (also taking into account the rims on which she's mounting them), when she could instead get more aerodynamic speed, better protection against pinch flats, and a comfortable and secure ride on a 22.  Conversely, a 200 pound guy doing performance recreational rides - group rides, gran fondos, etc - on beat up roads can choose a bigger tire to advance his priorities.  

Varying tire size, type, and inflation pressure gives you an array of tools to tailor what's between you and the road more specifically to your priorities.  Rather than try and drill down into making you feel as though there's a specific answer of what, how much, and how wide, what we've tried to do is to give you the tools and information to help you discover your own perfect ride.  


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...





November in the wind tunnel: Steady as she goes

One of the chief aims of the Rail series was to provide a stable and manageable ride.  This was a major consideration in the 52 being the 52, and not the 58 or the 60 or the 62.  The 34 was in large part born from the market demand for a wheel that would be nigh on invisible to crosswinds, even though early 52 reviews were near unanimously positive in regard to manageability.  

The cross sectional design of Rails attempts to create a near symmetry between rim side and tire side.   Obviously, tire choice affects this.  We had a 23 in mind, aware of the size to which most 23s would inflate on our chosen 18mm bead seat width.  Different tires have different shapes, and many people use different sizes. Nonetheless, the general gist stands.

To date, crosswind stability has been measured subjectively and anecdotally, never directly measured and quantified.  Thankfully, a wheel company from Indiana had been pestering A2 for an actual measured match to their CFD predictions.  Recently, A2 completed the measurement apparatus and algorithms to provide these measures. When you test a wheel now, your data sets include two new columns -  coefficient of drag, and center of pressure.  

Ceofficient of drag, simply stated, is how much pressure is pushing against your wheel - how strong is the push.  Units of measure are non-specific, but linear - meaning that .20 is twice as hard a push as .10, and 2/3 as big a push as .30.  Center of pressure describes the position of the push, relative to the hub, measured in centimeters.  Center of pressure of 2.35 would describe a push centered 2.35cm in front of the hub.  -3 would describe a push 3cm behind the hub.  

To date, we've never seen any graphical presentation of this data.  It's too new a concept, so we've taken a stab at it, which we think provides a clear picture of the relative power and placement of the crosswind's push on each wheel.  We have once again used the Tour Magazine angle of attack weighting in creating this chart, but we have used the 25mph weighting.  Our reasoning for this is that as wind speeds increase relative to bike speed, the likelihood of wider angles of attack increases.  We expect and welcome questions about this information and presentation, simply because we want it to be easily understood.  

Here is a link to a page that allows you to calculate apparent wind speeds and angles for any given combination (be sure and use the second box, the first one calculates to true wind speed).  Be aware as you do this that a windy city will have an average windspeed of somewhere around 10mph (the calculator uses knots - 10mph equals 8.7 knots), as measured at that city's airport.  Airport windspeed is measured high off the ground in an unobstructed place, and will overstate what your wheels are riding in by quite a bit - like 50% or more.


To say the results pleased us would be an understatement.  As the initial test of the 34 was underway, I was busily prepping the next wheel to test in the work room and poked my head into the control room to ask if I was in a good mood.  Dave, A2's engineer and a man not given to subjective statements or value judgments, said the aero drag measurements were going right along, but that the pressure measurements should put me in a very good mood indeed.  As the 52 ran and the data came up, my mood improved even more.  


As you can imagine, we're excited to see that our consideration of crosswind stability in the design of the Rail has been confirmed with such excellent results.  


Full of air: tire inflation

Last week's discussion of how tire size affects aerodynamics set off quite a little bit of discussion.  We've provoked some big responses before, but nothing quite like that.  The one thing that we hope people started to think about as a result of it, other than the direct component of narrower tires doing better than wider ones in the wind tunnel, is the importance of measured width.  It's a big factor.  

Now, measured width is a bit of shorthand, what we are really referring to is the actual volume of a tire, which includes the height as a variable.  Height and width aren't in lockstep, as some rims actually hold the tire lower down within the rim, while some let the tire sit a bit higher.  To investigate this more fully, we measured inflated width and height of 23 and 25mm Continental 4000s II tires on every rim we took to the tunnel, as well as estimated what they would be on a representative rim of the old standby 14mm between the brake tracks.  

There is debate over what "counts" as tire volume - does only the inflated portion outside the rim's circumference count, or does the volume in the cavity count as well?  Fortunately, the variances there weren't so extreme that they threw things out of whack.  Our calculation was fairly rough and simple - average the width and the height, take the surface area of that circle, and call that overall tire volume.  To eliminate the debated "dead zone," we took 5/8 of the overall tire volume and called that "effective tire volume."  5/8 simply because we are measuring the "outside half" of the tube, as it were, and that's bigger.  As I said, a bit quick and dirty, but when you reference it against a bunch of other calcs, the way you peel that carrot doesn't amount to much in the wash.  

Using a law of chemistry called Boyle's Law, which simply states that for a given mass of a gas, if you decrease the volume then the pressure must rise, we normalized how much pressure a given volume of air would yield in each tire/rim setup.  The results are shown in the graphic below.


So what does this have to do with anything?  It shows that as you increase tire volume, in order to keep the same "buoyancy," you need to decrease pressure.  There are a lot of different ways to express buoyancy, probably the best of which is illustrated here - the wheel drop methodAsk 10 people what the ideal pressure is for any given tire and you are likely to get 20 responses.  The point we're making here is that tire volume is probably the biggest determinant of how much pressure you should use in your tires, and it varies by a ton.  Put 30 psi in a road tire and you are going to be riding around on the rim, put 30 psi in a 2.2" mountain bike tire and you are going to be bounced all over creation, put 30 psi in a cx tire and you are going to be pretty close to ideal (I know, I know, tubulars, lower psi, etc - I'm making a point).  How big your tires inflate on any given rim will have a big effect on how that tire feels. The 100 PSI default for road tires was established when rims were much narrower than today's. The chart shows that to achieve the same tire volume as 100 PSI on a traditional skinny rim, you should only run only 79 PSI on a set of Rails with a 23mm tire, and only 66 PSI if you've mounted 25mm tires on your Rails. 


November in the wind tunnel: is wider faster?

What a question!  It might be simpler to ask "how long is a rope?" as there simply is no one answer to this question.  

In the simplest terms we can look at, aerodynamic performance of every wheel we tested suffered when the wider tire went on.  There has been much speculation over this one recently, but the results of the tests we ran conclusively show that, in terms of measured aerodynamics, narrow tires are faster.  

The question we were perhaps more intrigued to have answered was whether one rim or another tolerated wider tires better than others.  Unexcitingly, the answer to that is also no; all rims suffered a similar drop off in speed when outfitted with 25mm versus 23mm tires.

Now, back to my "how long is a rope" question - how wide is a 23mm or 25mm tire?  For that matter, how tall is either tire?  As the chart below shows, that answer varies widely (I slay me) based on the rim to which it's mounted.  The biggest determinant of inflated tire width and height (and thus inflated volume) is the interior width of the rim - the distance between the brake tracks.  The relationship between interior width variance and tire inflated volume is steady in direction (wider interior rim reliably equals more inflated tire volume), but the magnitude of the change is not as perfectly predictable.  For example, despite both rims having 18mm between the brake tracks, the tires we measured inflated bigger on Rails than on Pacenti SL23s.  But a basic rough rule of thumb is that for every 2mm gain in width between the brake tracks, you will gain 1mm in inflated width.  So if a tire of a stated size runs true to size on an Open Pro that is 14mm between the brake tracks, it will measure 2mm wider (which is equal to the most common size increment jump) on a rim with 18mm between the brake tracks.  Which means that if you prefer a 23mm tire on a traditional-width rim, you can use a 21 on a Rail and get the same volume (more explanation of that to follow).  And that, I promise, is the absolute last time I will mention an Open Pro in any discussion of aerodynamics!


The interesting part that follows on from this is that, when you measure two rims with the same tire, you aren't necessarily measuring the same tire on them.  The 23mm Conti 4000s II that we used measured 24.3mm wide on the 404, but was a full 1.5mm wider on the Rail (and .4mm taller on the Rail, but to keep things simpler we'll focus on width).  Similarly, the 25mm Conti 4000s II that measured 26.7mm wide on the 3.4 front measured 27.3mm wide on the Rail.  Tires also set up relatively lower on the Enve rim compared to the width increase - the 23mm tire was .1mm taller on the 404 than it was on the Enve, despite the tire being .6mm wider on the Enve than the 404.

Given the negative relationship between width and speed, and given that tires measure bigger on our rims than on any others tested (which we knew they would - those who've followed the Rail story know that design parameter #1 was an 18mm interior width), we had to peel the onion back a little bit on that one.  Interpolating the difference between 23mm and 25mm tires on the 404 creates a line that predicts where tires of widths between those two would fall.  Create the same line with the Rail 52, and you see that for any given actual inflated tire width, the 52's "seconds saved" line is above the 404's.  Of course we wouldn't be us if we didn't point out with equal emphasis that the 34's "seconds saved" line is below the 3.4's, so by using the same metric, a 3.4 is a little bit faster than a 34 for any given inflated tire width.  

The current trend is absolutely for wider tires.  Note that when we decided to test two tire sizes, we chose a 23 and a 25, not a 21 and a 23.  Wider tires have been shown to have lower rolling resistance at equal pressure (don't worry, we're building a better mousetrap to measure that), and as many people have learned, offer advantages in both comfort and handling.  Inflated volume also has serious ramifications for what tire pressure to use, which we will discuss in much more detail later, but the strange looks I've gotten for the past two years when I tell people what psi I use now make perfect sense.  

There is a terrific amount of interrelated data that comes out of this, all of which will come out over the next several installments, but for now the myth (if there really was one) that wider tires are aerodynamically faster is busted.  

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