Monday
Aug112014

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. 

Thursday
Aug072014

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.  

Tuesday
Aug052014

A day in the life of 52s: IM Whistler

While we were off galivanting about North Carolina, our man Laurier was off to BC for Ironman Whistler.  For the record, I don't think I could do a sub-4 hour marathon if I was driving it in a car, so a full bag of propers to anyone who can do it after all of the tomfoolery that precedes it.  Here is his report:

On Sunday morning at 7:00 am, the gun went off. It was the start of a very painful day for me and 2500 other fellas. The swim was crazy. It was a swim start, which, compared to a beach start, makes things even worse. We were spread on at least 150 meters wide, and at the first corner buoy, I was in the biggest funnel ever experienced. I managed to get out of the water in a good place, although my swimming skills are relatively average compared to the guys I try to beat on the line.

The bike was at least as challenging as I expected. 3 big climbs made most of the course, so managing energy really was key all through the split. We knew there as an hour-long climb waiting for us before the transition that could really make the run 42 km of hell.


With such a course, average speed of the best guys could not be very high. Most guys take it easier in the climbs to save their legs for the run, and very few are able/comfortable enough to smash the pedals in the descents and go 75kmh in the aerobars. I took a gamble and went all-in in the first 2 hours. It turned out well, as I started the bike almost 400th and was 58th after 100 km. but all that had big risks. As I'd been wisely advised, every watt spent pushing the bike over 45kmh were watts I would never get back, and the gain at this speed was marginal compared to the cost in energy. So I saved my legs over the next 90 minutes, making sure I was not over 300 watts unless it was worth it, planning for the last hour of climbing. All through the bike leg I felt very aero, both with the apparel and with the bike-wheels setup. I am used to riding a disc wheel for this kind of races, and many guys out there had such a setup. But the Rail 52s turned out to be perfect for this course. The gain in stiffness all through these long climbs was well above the marginal aero loss in the descents. The 52s are also lighter than a disc setup and feel more aero than most wheels of this depth out there. Plus, we got quite a bit of crosswind in the flat section, in which 52 mm deep wheels way outperform a disc, especially after already 3 hours of racing.

I must also admit that those were the most scenic kilometres I have cycled in the country. Breath-taken, both by the eyes and lungs.

It felt very reassuring to ride carbon clinchers on this kind of course. As it was a one-loop split, chances to catch race support were slim, so good to know I could change a flat in just seconds. Also, I would never have felt comfortable to go down these hills at these speeds with a tubular I had just changed on the side of the road. I was thrilled about the overall gains, both technical and mental, of the Rail 52s.

I managed to get off the bike in the top 5 of my age group, 36th overall. Legs felt good, so good that I had to pace my self after 4 km under 4"50/km and 38 km to go. Did a good first split, and knew that if I kept that pace I was going for the podium. And then - the wall. 25km done, 17 more to go, and the engine was turning off. It was a very hilly run course as well, I guess everything is pretty hilly here. I was cooked, and started calculating my finish time if I walked all the way. I kept being passed. I hate being passed. But couldnt help it. I just could not keep up. At km 35, after 10 very bad km and a few sips of cola, I felt the legs were giving me a second chance. I managed to keep a good pace all the way to the line and finish my 2nd and best Ironman to date with a 10hours 30 min chrono and. 6th age-group place and a sub 4 hour marathon.

Now i' m off to Vancouver for some sightseeing before returning to Quebec.

Monday
Aug042014

Business in the front...

 

One of the tests we were most curious about was "the mullet."  Our lovingly pejorative label for the 34 front/52 rear combo aside, we get asked about this all the time - "is the tradeoff worth it?"  Despite having been accused of pulling the answer out of our rear by various internet eyeball aerodynamicists, we've actually put a bunch of study into it.  VeloNews has covered it, and Zipp used to publish offset depth tests, so becoming informed was easy with some Google-fu. In the end you don't know until you test, so test we did.  

First we have to start off with a disclaimer.  I move around too much on the bike to be used as a pedaling dummy.  My mom would say "of course not - I didn't raise no dummy," but my active riding style meant that it was impossible to do the "rider on" portion of this test.  More on this later, because it takes some special skill to be silent enough on a road bike to be a good dummy, and this has some ramifications.

Anyhow, to answer the mullet question, we ran three tests - 34s front and rear, 52s front and rear, and 34 front/52 rear.  What we found was approximately precisely what we'd guessed - that the rear wheel accounts for less of the aerodynamics than the front.  There's some "there" there, but the assumption that a wheel's benefit is the same front or rear is debunked.  If they were equally important, the 34/52 set should have been a 50% closure of the gap to the 52/52 set from the 34/34 set.  As it happened, it only crossed about a third of the gap.  

The major motivation for people to do the offset set is to get the crosswind handling benefit of the shallower front with the aerodynamic benefit of the deeper rear.  As it turns out, A2 is now able to quantify that as well, which will become part of the story soon.  As sticky as it is to editorialize on data, everyone's still going to ask us if it's worth it, so having tested, our answer will be thus.  If you want the shallower front in order to save weight, we've never seen a model where the sacrifice in aerodynamics would be worth the benefit in reduced weight.  If you want all the aerodynamics you can get but are incontrovertibly certain that a 52mm deep wheel is too much to handle, then there is a bit of benefit to be gotten from the setup.  As ever, my wording on the last sentence turns out to be quite meaningful.

Friday
Aug012014

November in the wind tunnel: "semi-aero"

Tunnel testing alloys was actually more exciting for us than it should have been.  While a lot of people choose to buy builds from us with the rims on test, this essentially amounts to a "public good" test, simply because these rims are so widely available.  The excitement mostly came from the fact that the internet was seemingly ready to have an aneurism there was so much demand for us to do this (which is a funny concept when you think about it - no one else spends the money to do this for people, not even the rim makers), and because so many "eyeball wind tunnel" experts had definitively pronounced the results long ago. 

We have to once again stress that directly comparing this test on this day with that test on that day is a fool's errand.  The way that we have designed our testing protocol, we will always be able to make significant back-references, but direct comparisons between different tests are worthless. Relative results of one wheel vesus another from test to test are valuable, which is why we have tied ourselves to the benchmark of the 404 wheel we used in our original test.  We will always keep that wheel as the reference standard. 

We tested these with a 23mm Continental 4000s II tire - one tire used in all tests, as it must be. That is perhaps the most popular tire used with the rims we tested, and is a standard for the wind tunnel.  Significantly, you must take note of the tire width as it is on each wheel when looking at these results.  The same tire will inflate differently on different rims: the wider the interior dimension between the brake tracks, the wider the tire will set up.  Among the rims in this part of the test, the differences are fairly small.  As we get into other rims, this difference will become more significant. 

 The biggest lesson out of this test is how much closer the Kinlin and Pacenti rims come to the performance of carbons than what the A23 did.  The overall delta between worst and first in this test was much smaller than it was the last time.  Tony Martin's not going to read this and think "you know, maybe I ought to use some nice mid-depth alloys in my next time trial," but the gap is much smaller.  Again, we are only able to make this statement because the A23 was tested against our reference wheel using the same tire (although we used a different type of tire last time), and the same reference wheel was used to measure the gap this time.  There is the noise of "maybe that's just a particularly bad tire for the A23" in there, but that's at most a very small part of the story.  Using our back-check method, the Rail 34 is faster than an RFSC38, while the alloys in this test are slightly slower.  

The Kinlin was the faster alloy.  As we will explore later, the tire's increased width on the Pacenti may account for all of the difference between the two.  It is impossible to normalize one tire to the same width on two different rims, and you wouldn't want to anyway.  What you could do is test a narrower tire on the wider rim and interpolate from there, but with these rims we were more interested in seeing if there were any gross differences between the two, and how they stacked up compared to the carbons on test.  We will explore the effect of inflated tire width in more detail in subsequent posts.