Sunday, February 24, 2013

Energy saving while drafting (IM Focus)

So it's now less than 4 weeks to IM Melbourne in Australia, and just in time for a somewhat light hearted look at drafting, or conserving energy legally, whichever you prefer to be the correct term.

It's been widely accepted that there is benefit drafting behind another rider, in fact Kyle(1) published data as early as 1979 showing a 47% reduction in drag at 0 m behind another cyclist, and 27% reduction at 2 m.

Some time ago our local triathlon forum had two schools of thought emerge on "pacelining" in IM, where to be paceline "legal" a cyclist has to be 12m from the cyclist in front; the first being there was no benefit at 12m, and the second that there was benefit but that it was fairly minimal.  Knowing that there would be some benefit, but unsure how much, I designed an experiment that would try to test the reduction.

This involved mounting a laser pointer on the bike (Cervelo P3) pointing at the ground 12m away from the front of my bike.  When following another rider, all I had to do was make the laser dot on the ground match the cyclist in front's back wheel and bingo accurate measure of the correct drafting distance.  Since this test was done on a velodrome, it was pretty easy to concentrate enough (and safely) to ensure that I was pretty accurate with the trailing distance (perhaps +/- 0.2 m error over the interval durations).

But I was concerned that I would possibly be biased in the way that I extrapolated the results; I believed the benefit to be there, so I might unwittingly find what I was looking for without knowing that I had somehow biased the analysis.  The solution was to get friend and previous coach Alex Simmons to do a blind analysis of my power file WITHOUT telling him what I was doing, only that I'd done a number of intervals and that sometimes I was in front, and sometimes not. Each interval had no intermediate or lap markers to denote changes from in front to drafting, and Alex did not know whether I started in front or behind on any of the intervals.

My quick analysis of the results was enough to confirm what I thought, notably an approximate 10% reduction in power required at 12m separation (front wheel to front wheel), which is pretty darn significant, and could mean the difference between running the IM marathon of your life, and lying huddled on the side of the road somewhere imbibing flat coke in a desperate attempt to get to the finish.... but I digress.

Alex, however, went to town on my file; presented with a challenge he left no scientific stone(2) unturned in the quest for knowledge....

Results

He found (to cut to the chase):

So in summary, the gain by drafting the other rider was a reduction in apparent-CdA of:

Interval 1: 0.035m^2
Interval 2: 0.033m^2
Interval 3: 0.026m^2

In terms of energy benefit for for Rob when drafting over leading, when riding at 40km/h this equates to wattage savings of:

Interval 1: 29W
Interval 2: 27W
Interval 3: 21W

The intervals ranged around 240, 260, 280 watts average in respective intervals 1, 2, and 3.  So a very handy saving - roughly 7.5 - 12%.

How significant is that you ask? Well if you're typical triathlete drag (CdA 0.285) at fairly typical triathlete race watts for IM (190) and you got 10% free speed (209 effective watts) your speed would increase from 9.71 m/s to 10.08 m/s, sending you 0.37m (1 ft 3 inches) further down the road for EVERY SECOND you are on the bike!  Your bike time would drop from a hypothetical 5:08:58 to 4:57:37 (3).  A whopping 11 minutes and 20 seconds saved. 

OK, so after all this nerdy work what would be great is if we could get some practical advice on how to use this to our advantage in an IM cycle race. Assuming you have a power meter, and you know what average watts you intend to target for the entire race, I've scribbled up some cheat sheets give you a pictorial and rule based guide to using the ~10% energy saving you'd get from drafting at 12m as you interact with other riders.  It's available in PDF in both right hand drive (Aus, NZ, UK, Japan etc) and left hand drive (Euro, USA & Can etc) versions.

Have fun out there!

Big thank you to Alex Simmons for taking the time to write up the results with proper analysis and detail.
You can read the full analysis on Alex's blog.

References:

(1):

Kyle, C.R. (1979) Reduction of wind resistance and power
output of racing cyclists and runners travelling in
groups. Ergonomics, 22 (4), 387–397.

in


R.A. Lukes*, S.B. Chin† and S.J. Haake*


The understanding and development of cycling aerodynamics,
ISEA Sports Engineering (2005) 8, 59–74


* Sports Engineering Research Group, Department of Mechanical Engineering, University of Sheffield, UK
† Department of Mechanical Engineering, University of Sheffield, UK


Note: Lukes et. al. is a great read (not overly scientific) and a copy is available on this site.

(2) Science: it works b*tches.

(3) Yes it is a hypothetical calculation, and no you can't hold exactly the same aerodynamics for an entire IM bike leg. Despite this, of the relatively few predictions  for IM based bike times where I've known the rider characteristics (3 total), I've been within a minute or so of the riders actual time.

Saturday, February 16, 2013

Confused aero testing

So inside triathlon have published a new aero test on some big name aero bikes... and come to a conclusion that one particular model is 53, 81 or 153 seconds faster than the competitors over a 70.3 bike leg.

What's interesting and slightly confusing is their statement immediately following the results:

"As stated in the printed article, the test is imperfect, as are all bike aerodynamic tests. Several factors—most notably the influence of a rider—could make the real life performance of these bikes different than the results of Inside Triathlon’s test."

Er, well, yes next time I want to send the bike round the course by itself I'll bear these tests in mind... so why are we testing like this again??  Tunnel time isn't cheap, and sweeping through yaw also complicates things, particularly with drive and non drive side aero that varies between the sides tested.

Real world aero tests where n=1 and n=me obviously do have a lot of benefit and should, of course, not be included in "all bike aerodynamic tests".

Perhaps they're still trying to figure out how to use and interpret aero tests that actually have meaning... kudos for doing that, but there are other ways to design an aero study.

Side notes:

The drag estimations they recorded did have variations in drag at zero yaw after swinging through +20 to -20 and back to 0. A better way of presenting results would have been with the error(s) associated with each measurement.

Trek Speed Concept 9 Series: 435 grams +/- 18 grams
Cervélo P5 Six: 495 grams +/- 10 grams
Specialized S-Works Shiv: 525 grams +/- 28 grams
Orbea Ordu GDi2: 606 grams +/- 19.5 grams

Even better, some statistical analysis of the results including for example, standard deviation, standard error or even better a confidence interval (we're 99% confident the value is within a certain range).  It's unclear how many measurements were actually taken.

Secondly, they've weighted the yaw results *again*.  (Sigh).

"In addition to the amount of wind resistance, yaw angle also changes with rider speed (faster rider, shallower yaw; faster wind, wider yaw), so we calculated the fraction of time a rider would spend in various yaw angle ranges when riding at 23 miles per hour and weighted the drag created at each yaw angle accordingly. These were the results for a rider traveling at 23mph in 8.1mph wind."

Er, does this mean you have a course where the rider spends equal time in every compass direction?  I don't know about you, but often race courses are out and back on a single stretch of road that will spend a large amount of time in certain directions of yaw.  They've collected met data from 49 major cities from Seattle to Miami and arrived at an average of 8.1mph of wind... perhaps they should have also analyzed a variety of courses and determined an average yaw value percentages based on typical conditions for those sites.

Weighting results completely obfuscates the value of testing at yaw.  Objects performing at high yaw, may not perform as well at low yaw and vice versa.  When combined the final results may be closer by the nature of weighted averaging, but may not be representative of real world experience.  It would be better to present cases for zero, low and high yaw situations and allow people to have the data that may influence choice in those conditions... certainly you might not swap a bike under low or high yaw situations if you only have one, but other choices like helmets or wheels might easily be changed given different conditions.

"Ideally, an athlete would measure real wind speed on each individual racecourse and run a calculation to find the drag they expect to face during a race then pick accordingly from their stockpile of race wheel. This is, of course, not practical for most athletes so we use this approximation."









Saturday, July 14, 2012

Are Zipp Firecrests faster?

Recently I was able to stack up a few front aero wheels for a velodrome aero testing session...  In particular I was interested to try to get hold of a Firecrest rim to be able to test against my current quiver of race rims.  While I wasn't able to get a tubular FC, a mate was able to provide a FC clincher for the tests.

Method:
Ride repeat double laps of the velodrome at constant position, power and speed (in this case between 20 and 45 kmh) Select sections of each speed, and run through aero calculator to determine CdA (co-efficient of drag = aero) and Crr (co-efficient of rolling resistance)

Total weight: 91.7kg
Air density: 1.221

Here are the bling-bling contenders in the test rig, in no particular order:
 

Campag Scirocco G3: (my wheel)
















Zipp Firecrest 808: (loaner wheel courtesy of BDB)
















HED Stinger 9: (oopsie with the finger, loaner wheel courtesy Andrew)
















HED 3C: (my wheel)
















First, some more homework on rolling resistance: Based on Al Morrison's tire roller data we have these tire rolling resistances:

Vittoria Corsa Crono*      11.5w    +0.0w
Veloflex Record*           12.1w    +0.6w
Vittoria Open Corsa CX*    12.8w    +1.3w
Zipp Dimpled               13.8w av.+2.3w
Continental GP4000s        15.4w    +3.9w
 
* = tubular

Now with the rough resistance rule of 5's (at 48 kmh):

50 g of drag = 0.5 s/km = 5 W = 0.005 CdA = 0.0005 Crr

With a maximum of 3.9w of difference in tire rolling resistance (or 0.0004 Crr), it might be hard to pick up the difference in Crr on only one wheel being changed. So we're probably expecting the Crr to stay relatively the same across all wheels. Fair enough, this is supposed to be an aero test.  

Results:
Here's the graph of total power vs speed for all the tests, including some other data that we'll get to in a sec.
The more normal spoked wheel (Campag Scirocco 3G) is obviously worse than the other aero wheels (and I could feel this on the bike), but the other wheels are pretty similar looking for total resistance. This correlates reasonably well with the on-board experience, none of the aero wheels "felt" dramatically faster or slower than each other, but then it can be hard to subjectively judge that while riding 40+kmh. So with the total resistance looking pretty similar across the aero wheels I started looking at the CdA/Crr pairs to see if there is any difference, and there is:

                     CdA  Crr     R2
Campag Scirocco 3G 0.259  0.0038  0.989
Zipp FC 808        0.226  0.0047  0.996
HED Stinger 9      0.211  0.0057  0.997
HED 3C             0.211  0.0044  0.956
HED 3C - 8 Dec     0.248  0.0043  0.969
 
R2 is the estimate of how accurate the data is with 1.000 being perfectly accurate. Lower R2 = more variable data, = less certain result, closer to 1.0 = better data.
Crr is varying by 0.0019, rather more than the hypothetical 0.0004 we thought we'd see.

There are a number of interesting points here:
- Obviously the spoked wheel (Scirocco) is a lot worse... meaning if you ride 34 kmh or slower, having the aero wheels above won't make much difference to you compared to using the standard wheel. Once you get to 39-40kmh averages the difference is getting much more significant, and the value of the aero wheels is starting to become much more apparent. If you're strong enough to be averaging over 40kmh, then aero wheels become an absolute necessity over a spoked wheel as the resistance difference is massive. I'm not 100% confident on the Crr value on that wheel though ... seems a little low on the rolling resistance for theoretically the slowest tire of the test.
- The Zipp FC 808 has the next best CdA of 0.226, while the 2 HED wheels end up with lower, and identical CdA but different Crr's. Both the Stinger and 808 data have a very high R2 value, meaning we can be reasonably confident that this difference does actually exist. I'm a little surprised that the Stinger wheel ended up with the highest Crr of the tests. On chatting to Andrew afterwards, it appears that there may not be quite enough glue on some of the tire/rim, and the starved joints/missing glue might be contributing to that higher Crr. It might also explain why that wheel was noisier than the others with a lot more tire noise. Now the other thing that was apparent was that there might have been some issues with the Hed3C data. There were a number of data points that had a higher spread away from the trendline than the other wheels (the R2 value was lower at 0.956). So I went back over old rides on the same equipment and position looking for more data on the H3 and ended up with some data from 8 December (a training session not a testing session), which had more wind (up to 22kmh) and slightly lower air density (1.181). That was plotted as a separate series (H3 - 8 Dec) which ended up as a higher CdA, but almost identical Crr and a slightly higher R2 value, but still not as good as the other test data. In this case the higher CdA is probably due to more wind on that 8 Dec day. The fact that the Crr is only 0.0001 different (around a watt difference) between the 2 tests does suggest that Crr of ~0.0044 may be pretty accurate. If we hold the Crr to be accurate, then it means the 0.211 CdA must also be pretty accurate (since we need that to balance the Crr to match the total resistance). Other spot and time trial data suggests that 0.248 is a high value for CdA for that wheel in calm conditions, and that 0.211 is probably a good estimate. So how significant is the difference between these front wheels over 40km? If we ride 40km at an average of 250 watts we get the following times:

                      CdA    Crr    250w        40km time Diff.
HED 3C              0.211  0.0044  11.70 m/s = 0:56:59 + 0:00:00
HED Stinger 9       0.211  0.0057  11.46 m/s = 0:58:10 + 0:01:12
Zipp FC 808         0.226  0.0047  11.40 m/s = 0:58:29 + 0:01:30
Campag Scirocco 3G  0.259  0.0038  11.07 m/s = 1:00:13 + 0:03:15
 
Calculated using:
http://aerocalc.triathlete.com.au/aerotools/
1.211 air density,
91.7 kg weight

Wowsers! That ends up being a pretty big difference, given both HED wheels had the same CdA - and probably underlines the importance of tyre selection, and thorough gluing in looking for quick times. It also surprises me that there is that much difference by swapping a front wheel only... however it is a leading edge cutting into essentially undisturbed air, so it probably has a lot more effect than a rear wheel which is trailing along through air already disturbed by the front wheel and fork, down tube, legs, cranks and drivetrain...

Conclusions:

In these tests the HED wheels did come out faster than the other wheels. The fastest wheel in relatively calm conditions was the HED3, although the Stinger was aerodynamically the same, it lost ground due to the tire/glue job being used against that on the H3. The Zipp FC 808, was slightly slower than the HED wheels, but still significantly quicker than the spoked alternative.  It was a very smooth wheel though, which could be attributed to 23C rim width/wider tire/clincher version, or possibly the engineering Zipp has used to attempt to reduce axial forces from affecting the wheel.
If you're riding around 34 kmh or less, you can largely ignore the benefits of an front aero wheel, it won't assist that much compared to a spoked alternative. If you're riding around 40 kmh you probably should be riding an aero front wheel if you want to be competitive, but if you don't care, cool, ride a spoked wheel. If you're riding above 40 kmh you should definitely have an aero front wheel to be competitive. Thanks to Andrew and Bill DB for the loaner test wheels - without them, we would have had 1/2 the result. :-)

Notes:
As with any data, there exists the potential for errors. The only way to truly measure the difference is to visit a wind tunnel under controlled conditions (particularly if you want to know what happens in cross winds). However this testing was conducted using pretty robust techniques which have seen me get to 4 decimal places of accuracy compared with measurements in the wind tunnel of the same setup. To improve reliability however, repeat testing is always desireable to be more confident that the data variation is not occurring by chance. Going out and slapping money down for an aero front wheel may not give you the same results as those described here, in general terms you will probably receive a similar benefit from the wheel, but specifics such as the rest of your gear (helmet, bike, bottle choices), the fork the front wheel goes in and other factors (body size, position) will influence the final result.

Sunday, March 27, 2011

Last race of the season - race footage.

Last race of the season - went out equipped with a camera on the bike that records in HD (1920 x 1080).

Such a great day out there - one of those rare windless and very beautiful days for racing. Had a blast ... 1st out of the water in my wave, 1st off the bike, then ran up the white flag on the run (as per usual) and finished 6th in A/G. 4th fastest A/G swim and bike - bike time was 30:28, which is probably spot on 40kmh av on the road and about 14 sec at each end mount and dismount to the timing mats.

The camera is a Contour HD which I ordered from the manufacturer in the states, but there are places in Aus you can order online at the same price. About $350. Awesome little camera records in full HD (1920 x 1080) at 30fps (which is almost too much info to process for most computers to play easily) onto a small SD memory card. At 3/4 HD size and 30 fps you can record 1 hr of footage on a 2 Gb card, will take up to 32 Gb card, which could possibly hold a full IM bike at full HD quality.

The 25 min odd recorded on Sunday was about a 1.1 gb file at 3/4 HD res. Started the camera on the bike once I had my feet in and wasn't going to be a danger to anyone. Left it running the whole race and switched off in the rack (would have auto shut off anyway).

Here's a few speed increasing tips and times from the vid you can use to hopefully ride faster as well:

0:25 - 0:50
Get aero as possible on the downhills (even slight ones) - doing so can produce a much higher speed, and by keeping up the power output you can get a good head of steam up that can carry you up the next hill, or further along the course.

1:25 - 1:44
Make sure if you do pass any packs you go over the top with sufficient speed they can't suddenly jump on (may not be that easy!). Remember to hurl appropriate abuse as you steam by.

2:12 - 3:44
Slipstream up behind people, just remember you have 15 seconds to make the pass stick and get out of their draft. Don't get too close or pass too close - a swerving cyclist is hard to miss at full tilt. Also ride in the wheel tracks where the cars have been - they are typically smoother, but may be more prone to potholes (keep an eye out!) Remember to pass only on the right, and move left as soon as you overtake and it is safe to do so. Failure to do so may incur a blocking penalty.

3:44
Take it easy on the turn arounds, particularly if it is wet. No prizes for wiping out. Don't blow a fuse building speed on the way out from the turn around - get up to full race pace again without overcooking yourself... it's usually faster than riding out too hard from the turns.

4:48
Wave to the draft busters, they are your friends.

5:08 - 5:20
Know the course, and ride the corners hard; get in the right gear beforehand and nail it in, through, and out.

6:00
Practice those dismounts and squeeze out your opposition over the timing mat! At least then if they get away on the run you can say you dismounted first.




Time to learn how to run properly over winter...

Oh and if you do end up buying your own Contour HD, you just have to put yt:stretch=16:9 into the Youtube video Tags field to tell it it should be a 16:9 widescreen movie.

Saturday, March 12, 2011

Cervelo's LSWT tests against the Big 4

As published on Slowtwitch.com recently, Cervelo recently tested the big 4 TT rigs against the P4 in the San Diego LSWT wind tunnel, in a long series of pretty significant tests.

These tests are interesting because Cervelo is using their famous DZ (Dave Zabriskie) mannequin - although Dave is super good at sitting in the same position each time, wind tunnel testing is largely a repetitive chore, and I'm sure Dave has better things to do than freeze his goolies off sitting in the same position in a couple of hundred tunnel runs.

< Super Dave - the mannequin.

This means that the test results are actually representative of what we'd expect to see in real world use - ie. someone actually riding a bike round a course, rather than bike only drag figures which are often published from tunnel tests. The second reason this is so significant is because arguably the widely available current best 5 TT bike/frame combos are there being tested, and thirdly, independant observers from Slowtwitch were present during the tests (what level of exposure to tunnel testing they had is unknown however).

Cervelo has stayed quiet on the publication of this material, possibly choosing to release the info via an "independant" third party, rather than publish the test results themselves. Given how much hype surrounds tunnel tests by manufacturers this may be a smart way of releasing the results without incurring the suspicion of "manufacturer bias" prevalent when a manufacturer goes to a tunnel, particularly since their bike comes out either on top or close to the top (depending on the wind angles being tested).

So to results:

Firstly, the graph of Bikes, with Super Dave on, and trying to get the fastest possible ride - ignore-hydration-do-whatever-it-takes-to-win. This run sweeps from -20 yaw to +20 yaw, also important, because the drive train side affects the run numbers (if you look at drive side and non drive side in isolation you can get an idea, but not the full picture of what is going on.)



What does this show? Well by and large the P4 with a bottle on is certainly better than its competitors in the narrow range of -7.5 to +7.5 degrees of yaw. So if it is a low (or no wind) day, it could be hard to hang onto that guy on the P4 that just came steaming past. At 0 yaw the P4 is an average of 100 grams less drag than the average of the competitors - or around 5% better which is a significant reduction in drag. Between -15 to -7.5 and +7.5 to +15 the bikes chop and change in results, with the P4 being worse than all competitors in -10 to -15 yaw. Strangely, the P4 is again better than all competitors at less than -15 or greater +15 yaw. All competitors to the P4 test faster with a bottle OFF, making the P4 the only bike to go faster with the bottle on (whether you can actually get at that fluid while staying relatively aero is another matter - if you have a P4 you should be racing with the bottle in, regardless of what it contains).

The second part is to compare these results against the Trek Speed Concept white paper that came out last year. This is also available from Slowtwitch. In those tests, they did use a mannequin similar to super Dave, but only published limited data for tests with a mannequin. Most of their published data was without rider... which isn't a problem here as we have without rider data from the Cervelo tests too. Now this data was collected by 2 companies that have years of experience in tunnel testing, from the same tunnel (LSWT). The only apparent difference between these tests is the use of a H3 front/Hed disk rear in the Trek study, compared with Zipp 808 front/Zipp disk rear in the Cervelo study.

Cervelo results:

Trek results:


OK, so we have to confine ourselves to 0 to +20 deg yaw, but WTF? How is it possible that these results are so dramatically different between these tests? Admittedly, we only have 2 identical bikes in both tests: Trek Speed Concept and the P4 - but even these figures themselves are widely different for each set of results. If we take the data for P4 and Speed Concept from both tests and overlay it against each other we get:



The Trek data (solid line) is markedly different to the Cervelo data (dotted line), and given what I know about the 2 wheelsets (including independant tunnel tests and field tests), I'd be very surprised that either wheelset is the primary contributor to this margin of difference. Given the large (and consistent) gap between P4 and Speed Concept in the Trek data - I'd say "please explain, Mr. Trek".

Dan Empfield (Slowtwitch) does have the following conclusions:

Other bike companies might argue that the superior straight-on performance of the P4 is: 1) Somewhat due to the superior 0° yaw performance of the Ventus (the minimal drag of the pursuit position seems intuitively to be optimized for straight-ahead winds; and, 2) The most important yaw angles are those between 7.5° and 15°. As to the latter point, that's above my pay grade.

What is nevertheless undeniable, based on the results of this test, is that the P4 is the equal, or near equal, of every other bike in the test at these greater yaws, and better yet when as the bike points directly into the wind.


I'm betting the performance of the P4 in low yaw won't vary that much regardless of what bars go on that machine. As to the idea that yaw angles between 7.5 and 15 degrees are more important, that only works if you're a mere mortal spending the majority of your time racing in wind lower than about 10 kmh (6 mph). Once the wind strength goes above 10kmh (6 mph) you'll be seeing a wide range of yaw angles depending on the direction you're pointing, so you need a bike/wheelset that works well across a wide range of yaw. And if race day dawns calm and still, I hope you've got a P4 hanging on the wall of your garage!