Saturday, March 30, 2013

SRM claims best power meter accuracy based on Triathlon magazine test

SRM claims to be the most accurate power meter according to a test done in Triathlon Magazine, a German publication. Here's a link to the PDF article (German).

I can't understand German, but the second-hand report I read was that the various power meters were compared to a Cyclus 2 ergonometer. This is a trainer system after the LeMond trainer / Wahoo fitness model: it has a cassette to which the chain directly attaches, the trainer replacing the rear wheel:

Here were the results of "accuracy":

placepower meteraccuracy
1SRM FSA0.50%
2SRM Campagnolo-1.3%

The Powertap wasn't measured since it's built into a rear wheel, and the trainer replaces the rear wheel. They used a treadmill to test the Powertap, comparing it to the SRM, but didn't include the results in the data table since this was a different platform.

These results look strong for SRM, until you consider that each of these power measurement systems is measuring power at the crank, and for the powers used in the test (150, 200, 250, and 300 watts) drivetrain losses would typically be close to 3%. For example, I have data listed here.

Suppose I assume 3% drivetrain losses. Then the "correct" number is no longer the number measured by the ergonometer, but rather 3% higher. If I assume this, the test results become:

placepower meteraccuracy
2SRM FSA-2.5%
4SRM Campagnolo-4.3%

Looking at the plots in the article, which are unfortunately at low resolution, I see there's some errant data points in some of the plots, including Quarq. I'm not going to worry too much about these because it's a well known issue with crank or pedal based systems that the Edge 500 (used in the test) expects power numbers each second, while the crank-based systems measure every pedal stroke, so there's synchronization issues which can be resolved in different ways. But to my eye the SRM/Campagnolo, Power2Max, and Polar/Look appear to have the smoothest curves. The Quarq, Rotor, and SRM/FSA have more noise. But producing smooth curves isn't necessary the goal.

Another issue here is we're assuming the ergonometer is accurate.

I'm not going to declare a winner here, but certainly the message I'd take from these numbers is Quarq is looking more like the result I'd expect. Look/Polar is looking poor, but I'm really not too surprised. I expected better from Power2Max, since it uses "instantaneous" instead of rotation-averaged cadence. If the ergonometer is inaccurate, or if it has a built in assumption about drivetrain losses, all bets are off.

I think a conservative interpretation of these results is you can't believe manufacturer accuracy claims, since each of these power meters claims 1.5% - 2.0% accuracy. They obviously differ from each other a lot more than this, assuming the ergometer is precise and stable.

Back to the Powertap: they had this to say, courtesy of Google translate:

They are set on a wheel, but it can be used on different wheels. Conclusion As in the lab test "Cyclus2" Laufräder can not be tested, this system underwent the examination stage twice on a treadmill, with reference power meters. Since the setting is not possible with the same accuracy as the "Cyclus2" was to let himself make any concrete statements. Is the basis of the measured values however, be assumed that the system is well suited for power measurement.

Visual assessment of the low-resolution plot looks like it did fairly well, with relatively constant power where expected.

addendum: The claimed accuracy of the ergometer is 2%. Suppose there's 1% uncertainty in my claimed 3% drive train loss. Then Quarq, SRM/FSA, and Rotor are all certainly in the ballpark for near-equal consideration on accuracy. It gets work if the ergometer has a built-in assumption about drivetrain loss, for example if it were calibrated against an SRM with a geared drivetrain (as opposed to a fixed drivetrain). But if it was calibrated against an SRM then the whole test is biased. So I'm not sure what to make of this. Let me just rank them from highest to lowest power measured:

placepower meter% mean
3SRM FSA0.0%
4SRM Campagnolo-1.8%

Monday, March 18, 2013

randonneuring tire mass versus size

Last week I rushed to my bike to catch my morning train and the tire was flat. There wasn't nearly enough time to swap the tube, but there was an old Mavic Open Pro wheel laying there with an old Continental Supersonic 20 mm tire. I grabbed the wheel, inflated it to 100 psi (it held!), put in on my bike and caught the train.

I couldn't believe we used to race, let alone train, on these things. Every bump felt like it threatened to cause a pinch flat. Even the speed bumps in the parking lot at work seemed like they'd launch me into ballistic trajectory.

James Huang, CyclingNews
Ellis Randonneuring bike, 2011 North American Handbuilt Bike Show (James Huang, CyclingNews)

The improvement in ride quality going from 20 mm to 23 mm tires is profound, and it's even better making the jump from 23 to 25 mm or 26 mm. I love my Michelin Pro 25's, and my Grand Bois Cerf 26's. They're even good for a little fun on the dirt, riding fire roads and smooth hiking trails. But even 26 mm tires have their limits: once last year I was riding to work in an SF2G paceline when I aced a pot-hole not far from home and dented both of my rims (the tires stayed inflated, however). Something fatter with more air volume and there's a good chance I would have been able to finish the ride rather than walking home in my cycling shoes to catch a late train.

It's the advantage of fat tires which I view as one of the primary attractions of randonneuring bikes. Yet despite the rapid growth in 29'er mountain bikes designed for 700C wheels and "gravel racers", there's a shortage of high-quality wide tires. Traditionally the space above 25 mm has been dedicated to either heavily-treaded cyclocross tires or to super-heavy touring tires. Touring tires are designed for maximum endurance and minimal flat probability, but the rolling resistance associated with their thick casings is appalling. These tires have done a disservice to width: it's not that wide tires are slow by nature, but rather they tend to be slow by design.

Jan Heine and Bicycle Quarterly have gone a long way towards promoting the advantages of supple, light wide tires. He even advocates the use of huge 650B/42 mm tires for his dream-bike, a custom-built Rene Herse (built in Colorado, not Paris). The wider the tire, the bigger the bumps it can absorb. And when riding on gravel roads or on typically poorly maintained American roads useful for bike commuting, that's a good thing.

The obvious issue with fatter tires is mass: fatter is heavier. So I decided to look into what's available. I made a plot of some popular randonneuring tires, plotting claimed mass versus tire width. Here's the result:

mass vs width

For a range of sizes (Paris-Moto available in only 650B/38 mm), you can see that the Grand Bois Extra Léger tires are by far the lightest of the bunch. Next come the standard Grand Bois tires. Jan Heine likes these tires and sells them through Compass Cycles, his on-line bike component shop. Curiously, wheel size plays a relatively small factor. The Grand Bois 650B and 700C tires fall on roughly the same curve. The Extra Léger tires are 80% - 87% the mass of the standard Grand Bois tire of the same size, although fewer sizes are available so far. I extrapolate the results to predict the mass of future Grand Bois Extra-Léger using a multiplication factor of 80% for 700C tires, and 87% for 650B tires, based on comparing the sizes which are already sold. I get Grand Bois tires from Box Dog Bikes in San Francisco.

In addition to the Grand Bois, Compass sells a tire under its own brand in the 26-inch size, which Jan advocates for widths greater than 42 mm. This tire falls on a bit of a heavier trend line than the Grand Bois tires.

The Panaracer Pasela is a popular randonneuring tire, and is available in a broad range of sizes. Yet it's much heavier: the 700C/32 mm Pasela is 70 grams heavier per tire than the corresponding Grand Bois standard, and 128 grams heavier than the corresponding Grand Bois Extra-Léger. Deforming all of that extra mass surely comes with a huge increase in rolling resistance losses. Panaracer sells another tire, the T-SERV PT, which is quite similar. In both cases I plot the tire with the synthetic bead, rather than the wire bead: the wire bead is substantially heavier still.

Then there's the Vittora Randonneur. With a brand with the racing tradition of Vittoria (their tubular road tires are excellent) you'd expect high performance. But these tires are real anchors, coming in even heavier than the Paselas. Maybe the goal is to maximize the effort to build strength for racing.

With all of the tires, the mass increase is relatively modest up to approximately 28 mm, but beyond this and the mass increases more rapidly. 32 mm tires are clearly heavier than the tires up to 28 mm, but going above 32 mm you pay an even stiffer penalty. For me this suggests 32 mm is close to a sweet spot. But maybe if I had experience riding fatties I'd feel differently. Certainly if I want to go fatter than 32 mm I'll want to hope that Grand Bois extends its Extra Léger range.

added: For 650B tires, the Panaracer Paris-Moto tires are notable in that plot for their lightness. There's two: the black (lighter, higher thread count) and the tan (heavier, lower thread count). Of course I would prefer the black. I don't understand the retro sidewall aesthetic.

Wheel moment of interia: introduction

One issue with wheelsize is angular momentum. Jan Heine claims that 622 mm rims (700C) are best for up to 32 mm tire width, then 650B are good up to 42 mm, then "26-inch" are best for larger size. The idea is the wider tires yield larger mass and also yield a larger rolling radius relative to narrower tires at the same rim radius. This results in more angular momentum. Turning requires changing angular momentum, so more angular momentum creates more stability: more reluctance of the bike to change direction.

It's been commonly asserted that trail is what controls bike stability, not angular momentum of the wheels (the "gyroscopic effect"). Trail's important, for sure, and you can make bikes where trail is the only contributor to stability (for example, ski-bikes), but the detailed analysis by Andy Ruina at Cornell has shown that angular momentum and trail both contribute, as well as center of mass. Bikes are a complex dynamic system.

Angular momentum is an abstraction of Newton's simple laws of motion which applies to rotation about a center-of-mass. When an object has a velocity v and inertial mass m, classically it has a momentum mv. The rate of change of this momentum is the applied force F. Force and velocity (and momentum) are vectors, so in three dimensions, there's three components, and the rule applies to each component separately. The magnitude of a momentum vector is the square root of the sum of the squares of the components -- the same formula which is used to calculate distance in 3-dimensional space. An applied force may change the magnitude of momentum or its direction or both. If the force is constantly applied perpendicular to the momentum vector then the direction of that vector changes but the magnitude remains constant. For a spinning wheel at constant speed this corresponds to changing the heading of the bicycle (assuming flat ground). Force needs to be applied to the rim perpendicular to its spin direction.

This situation is more readily analyzed using torque instead of force, angular momentum instead of translational momentum, and angular velocity instead of translational velocity. Instead of distance/time, the units of translational velocty, angular velocity has units of radians/second, where radians are unitless so this is just /second. Torque is proportional to force multipled by distance (using the component of force in the appropriate direction). In the case of this rotation-based analysis, inertial mass is replaced by moment of inertia. Angular momentum equals the moment of inertia multiplied by the angular velocity. It's easy to see that the units of angular momentum are distance-squared multipled by mass. For a point mass, the angular momentum is the mass multipled by the distance from the rotation axis. For distributed mass, calculus is used, treating the distributed mass as a series of differential point masses.

So I'm interested in the relative angular momentum of wheels. Angular momentum of wheels is dominated by the rim, tire, rim strip or plugs (if any), and inner tube (if any). The hub is at such a small radius to be virtually irrelevent. The spokes extend from inner to outer radius and end up contributing about 1/3 as much on a per-gram basis as the rim, tire, etc. (this can be derived using integral calculus).

Angular momentum depends on moment of inertia but also on angular velocity. Jan Heine's initial analysis on the subject looked only at moment of inertia, but he later corrected himself by recognizing that smaller wheels spin faster, offsetting some of their inertial advantage. So the proper approach is to compare the angular momentum of different wheels when the bike is moving at the same speed. This will correlate to how quicky the bikes handle, assuming the bikes are built with the same trail.

Sunday, March 17, 2013

2013 Cervelo RCa revealed

I was floored by what I've seen by the new Cervelo RCa, which was just announced. The thing comes closest to perfection in a diamond-shaped bike for it's geometry that I've seen.

A very brief digression: In the past, bikes are designed for fit and stiffness and mass. Aerodynamics was always a factor, but it was relegated to fit: for 100+ years riders have realized they could go faster if their backs were flatter. But all bikes were made out of the same 1-inch steel tubing, as this worked well.

That changed in the 1980's, when Klein came out with his Al frame. With Al, the tubing can be made thinner with the tube diameter larger to get improved stiffness at lower mass. Other companies, like Cannondale, picked up the same trend. This flaunted aerodynamics, since the fatter tube bikes have more wind drag. But the effect of the increase in wind drag was quantitatively hard to assess, while weight and stiffness are directly detectable, and riders kept winning races on the bikes so it was considered to be an acceptable trade-off.

Carbon fiber frames started in the 1970's, picked up steam in the 1990's, and came to dominate in the 2000's. With carbon, as with aluminum, fatter tubes could also be made stiffer and lighter. Aerodynamics again suffered.

Aerodynamics has recognized to be a factor in frame design since the 1980's, when wind tunnel tests first became popularly used in cycling, first on equipment, and later, on specific riders. Early bikes like the Trimble used unified frame structures, but the UCI (international cycling union) ruled that racing bikes had to be traditional diamond-shaped, more out of aesthetics than any rational basis. Kestrel was the first to come out with a real aero-optimized mass-start frame. Cervelo took the next leap forward with the SLC-SL. Others followed. Each of these bikes used airfoil-shaped tubes to some extent. These added a lot of mass, even though the SLC-SL came in at just under 1000 grams, still much lighter than today's not-so-aero Pinarello Dogma, for example. I always admired these bikes for their engineering but never found them attractive. Pro riders agreed, in part perhaps because they were always compromised on comfort and stiffness. The only such bikes which got widespread professional was the Cervelo SLC and SLC-SL. But even there some of their riders, including most notably Carlos Sastre who won the Tour de France, preferred their R-series "Squoval-tube" bikes, despite the engineering data which suggested he had no business winning that tour with the aerodynamic penalty, perhaps 2% total wind resistance, of those bikes.

Scott really broke the mold on aerodynamics with the Scott Foil (first called F1). They realized you could truncate the airfoil shape to the rear and still get much of the benefit. Before Scott, LiteSpeed had done something similar with their C-series, but in that case to allow integration of the water bottle into the aerodynamic profile on the downtube only. Scott applied it throughout the frame. The result was most of the aerodynamic benefit at much less mass cost, and ride feel could be better tuned since the tube profiles were closer to traditional. The Scott was a game-changer.

Other companies picked up on the model, perhaps the best example being Trek with its Madone-7. Trek's probably wishing now they'd dissociated themselves from the Armstrong reference in the bike name, but the bike was the first Trek in that series I thought was really well designed.

But there's a lot of copying and designing for appearance in the cycling industry, not so much real engineering-based optimization. Perhaps it's no coincidence that Trek did such a nice job on the Madone after hiring Damon Rinard, an engineer who first came into my attention when he published on-line instructions for how he built his own carbon fiber frame at home. Trek hired him, but he later moved to Cervelo.

It had long been the practice on bike companies to sell super-priced bikes to skim off the component of the market that viewed spending many thousands of dollars on a bike to be not just acceptable, but even preferable. The price threshold of these "limited" bikes started around $5k for the total bike, but as marketing people realized that even their preconceived notions of how much people with way too much money were willing to spend were low-ball, this price quickly accelerated. A few years ago Scott and Specialized blew through the $10k limit for a complete bike without pedals. But Cervelo has always focused on frame + fork, not total bike. And now they've cracked the $10k barrier with just that.

First it was the R-Ca, which was a gorgeous bike. But it was still firmly grounded in the Squoval tube shape of the R-series. Now they've broken into the truncated airfoil ground with their newly announced frame, the R-Ca. The white paper is here. It describes some of the excellent engineering which went into optimizing the mass-versus-aerodynamic drag on the bike. Not described there is the ride characteristics. Cervelo has previously described how they integrate accelerometers over their test bikes to tune the stiffness versus comfort characteristics. They're wonderfully fact-driven. If something which looks aero, is considered by customers to be aero, but doesn't measure in the tunnel to be particularly aero adds mass, they leave it out. For example I don't see any "hidden brakes" on this bike, even though Cervelo was the first to patent brakes hidden in the fork (from what I know: Storck also did something similar).

So that was a long digression. The white paper is full of great stuff. For example, here's frame mass measurements for different model bikes. Despite the aerodynamic optimizations, you can see the RCa is super-impressive.

But this sort of plot isn't new. What's newer is companies actually providing the weight as a function of frame size, giving not just the best-case number, but the range of values expected due to manufacturing variation.

The curious thing about this is the trend. For the smaller sizes, the frame mass increases gradually with frame size. But then at 56 cm that changes. Presumably the longer tubes + the larger mass of taller riders (on average) suggested the need for fatter tubes. The weight thus increases rapidly from the strength-limited trend.

Suppose you were on the fence between 56 cm and 58 cm. The 58 cm bike appears to be designed with an increased emphasis on stiffness, while the 56 cm frame is designed more for optimal mass. I'm just conjecturing here, but if I was relatively light I'd then tend to go with the 56 cm frame. This is the advantage of custom carbon: you can design not only for size but also weight, power, and riding style. And for the price of this frame you can get close to two top-end custom carbon frames, for example from Crumpton. Still, these bikes won't be as finely optimized as the Cervelo. The small-time builders simply can't devote that much engineering into their designs.

So will I be on the list to buy one of these? No way. $10k is simply too much to spend on a race frame. But I deeply admire the engineering work. I'm not going to even try to compute a $/gram or $/watt number. People whose first response to this is the $ number are off-base. This bike is designed as the best-bike-which-Cervelo can build. Then they put a price on it. If people want to buy it, that's their business, just like it's their business if they want to buy an expensive sports car or piece of useless jewelry. I'm just glad Cervelo made it, because I am impressed, even if I don't much like the geometry with it's relatively slack seat tubes in the small sizes and it's exceptionally long head tubes.

Saturday, March 16, 2013

Milan-San Remo predictions

I have a soft spot for Milan-San Remo since I rode the final portion of the course in 2010.

My picks this year, in order of probability of winning:

  1. Thomas: Sky's been preparing in Tenerife. Traditionally preparation has demanded that riders do either Paris-Nice or Tirreno Adriatico. The only question is which. But Sky's shown remarkable success in their training on Tenerife, doing specific training in an isolated controlled environment.
  2. Sagan: The guy's amazing. It's very tempting to put him first: do I go with the amazing team or the amazing rider? I went with the team.
  3. Haussler: He was a very close second in 2009 and claims he's feeling good.
  4. Bassen-Hagen: Sky's back-up man.
  5. Gilbert: Has been slow getting out of the blocks this year and last but he's a proven candidate for this race. He just needs to control his aggression.
  6. Cavendish: Cavendish came into last year's race as a favorite and Liquigas sacrificed themselves to drop him. Liquigas is no Cannondale and they have the race favorite so aren't as desperate this time. Cavendish is climbing well and sprinting well and unless there's a focused effort to drop him on Manie, he still has a chance.
  7. Cancellara: He's always been strong for the classics, but he's getting old. Perhaps no longer being the favorite here will help him avoid being marked. Bike racing is weird. Either you're so strong nobody can do anything, but just being the strongest can actually hurt if everyone knows it and can mark you. Perhaps it's better to be only very slightly strongest.
  8. Moreno Moser: He won Strade Bianche. He's only 22, but young riders have won here before.

Race begins soon enough!

Tuesday, March 12, 2013

Marin Avenue (Berkeley)

On Saturday, I got an email from Tim telling letting me know that Murphy's Spring Classics was going to start up Marin Ave. I'd missed that. Murphy announces his courses the day before, and indeed it was already well into the night before when the course announcement went out. I'd skipped over the details, jumping straight to middle and end games.

Marin? I laughed at the irony, because not long before when he'd told me he was choosing to repeat the Nifty Twn Fifty which we'd done together last year versus selecting Murphy Mack's Stage Mullett two-day, I responded I didn't think I had it in me to ride Marin Ave two years in succession. Well, so much for that. I was committed, so I went downstairs to swap my 11-26 cassette for my Recon 12-27.

The name is easily overlooked: "Marin Ave" fails to strike the same impression as, for example, "Redwood Gulch" or "China Grade". But among those in the know, Marin Ave is infamous in the San Franscisco Bay Area. I first learned about it reading Grant Peterson's fantastic Bay area cycling guide, Roads to Ride. The profile there was a series of virtually vertical lines. Clearly it was unrideable: beyond sanity.

But over the years I managed to climb every other climb listed in the book within anywhere close the proximity. Marin stood there as an unmet challenge.

Back in the day, when the lowest gear we had was 38/28, that challenge would have been profound. But with compact cranks, I can dip down into a very nice 34/27, which eases the burden considerably. So what was at one time a source of fear is now more an expectation of assured pain. Instead of dread, that yields even a bit of positive anticipation.

I rode it the first time during the Nifty Ten-Fifty last year. It was steep, but not the steepest I've done (that has been Filbert in San Francisco). And the flattening of the road at cross-streets provides considerable recovery before tackling the next block. The only time I was under real distress was the final block before Wildcat. That was again the case at the Spring Classic this year, except here I still faced the sobering challenge of 114 miles remaining.

The map view is here. It's almost comical: a network of roads meander up the steep slope of the Berkeley Hills. Only Marin Ave takes the hill straight on. It's a remarkable stubbernness you've got to admire.

View Larger Map

Here's the profile:


On the ride this Sunday, from which the profile data were taken, some riders were traversing, but I took the climbs all head-on, straight up the hill, focusing on keeping my wheels turning. I compromised this only in the final meters of the last block. The grade in the profile thus tails off slightly here, an artifact of my diagonal path. But other than these few meters the profile is fairly true.

Monday, March 4, 2013

collision probabilities

I take the train around 450 times per year (one-way) and I've been doing that for around 11 years, so that's around 5000 train trips. Caltrain runs around 90 trains per day, or 450 during the week, about the same number I take in a year. Caltrain hits a pedestrian or vehicle around every 3 weeks, so at least one per month on weekday trains. I ride the train around 2/3 the total distance. That means I'd expect to be on a train which hits a pedestrian or car around once every six years. But I've never been on such a train, until now. I'm on the train now, logged in to a wireless network which happened to reach this position.

There's two bike cars per train. I usually get on the southernmost. That's what I did this evening. The northernmost is the lead car for the evening commute. The engine is to the south, pushing the train set ("consist" in train-speak). Cyclists in the northernmost car, the other bike car, reported the collision was loud and "scary". We heard and felt nothing. The lights went out, the train rolled to a smooth stop.

But I'm stuck here in the train for now. Now that the lights are back and I'm logged in it's not so bad: I have stuff to do. It's way better than being stuck on the highway with a vehicle crash. Bad days on the train are way better than bad days in a car.

Probabilities are strange. Ride the train more than a decade, and it's probable you'll be on a train which hits a car or person. So it's not surprising it happened to me. But it is surprising it happened at the moment it did, on the day it did. But it's perhaps not super-surprising it happened where it did. San Bruno is notorious for car-train collisions: this is the third in just a few months. The first was with a truck, the second with a car, and now this one with another truck. At least nobody has been hurt in any of the three incidents. Suicides have got to be way, way worse, mostly for the driver and conductors, but also for the passengers. So far, fingers crossed, I've not been on a train encountering a suicide.

appendix: We eventually got picked up by another train. I'd been sent an email it had been a truck we'd hit, but it was a car: empty. I got home around 8:30 pm. Not so bad: I spent time on the train catching up on emails once the lights came back on and I was able to guess the password of a nearby wireless network.

revising position & design

During a ride yesterday Cara told me she thought my shoulders looked more relaxed than before. I'd been doing yoga recently and also had had a very nice massage at Doctor's Orders (World Gym, Potrero Hill). I have had a chronic issue with shoulder tightness for as long as I can remember: I tend to sit both on the bike and, more significantly perhaps, at a computer with my shoulders rolled forward. Stay in a position long enough the the body decides there's clearly some natural selection advantage to being there so semi-permanently adapts (this is over-simplistic, I realize). Anyway, I need to constantly focus to rolling my shoulders back, which is to say not roll them forward.

The consequence of this on the bike is if I have my shoulders rolled back that brings my chest forward. This means I bend in the hips, lowering my torso. But if my bars are so low my torso is already as low as it can go then this position becomes unsustainable. The only options are straightening my arms or rolling my shoulders forward again. Neither of these is a winning choice. It's certainly more racey to have a lot of handlebar drop but if you're cheating it by distorting your body to get your hip angle back where it has to be then there's no point.

So I raised my bars, pulling out some spacers and then flipping the stem. This felt better, like I had more room to move yet had no problem getting in a decent position. When I got home I photographed myself leaning against a wall, which admittedly introduces a distortion, and compared with a similar photo I'd taken in Jan 2010. The photo from Jan 2012 had tan lines and looked generally fitter than the one from Mar 2013: it was sort of depressing comparing my pasty present self to my rather fit-looking self from 38 months ago. So I blacked out the present image to focus on the outline.

The positions matched up fairly close. Key was with the higher bars my shoulders were rolled further back, which effectively shortens my arms, but keeps my back in the same place. The Slam That Stem crowd would be appalled, but I'm convinced the new position is better.

As I noted, I was playing with a BikCad design of a randonneuring bike. I increased the seat tube and top tube lengths of the new design and put it up against the silhouette and the new position (using the hoods here instead of the drops, to avoid handlebar shape issues) matches fairly well.

revised design

The key issue about this design is a rather steep (75 deg) seat tube. I like riding with a fairly short set-back so no reason to design a frame to handle large set-back. A steeper seat tube opens up clearance for the rear tire and allows for potentially shorter chain stays. However, with 42 mm tires and fenders this is somewhat limited by tire clearance, anyway.

Sunday, March 3, 2013

Playing with BikeCad

I've been playing around with BikeCad, thinking about randonneuring bike designs. I've been fascinated with randonneuring bikes for a few years now, since I started reading Bicycle Quarterly. They seem an excellent option for a wide range of riding, including commuting to work, since a little carrying capacity goes a long way and having a handlebar bag would be super convenient for all sorts of things, liberating me from carrying a backpack for my work clothes, for example, and allowing me to carry more clothing option on certain rides. Fatter tires would be quite nice on the rough roads often encountered around here, and the rolling resistance of performance-oriented 650b/42mm tires has been proven in tests published in Bicycle Quarterly to be quite acceptable. Additionally, Cara wants to do a loaded tour, and while a randonneuring bicycle isn't optimized for a loaded tour, it's far better for that purpose than a traditional racing bicycle.

BikeCad is a wonderful tool for geometry tinkering since it handles all of the self-consistency issues, balancing microscopic parameters like tube angles and lengths with macroscopic variables like reach, stack, and trail. It's an amazing piece of work, and the pro version is widely used by small-scale custom frame builders.

To help with the geometry determination, I superposed a screen shot from BikeCad onto a photo I took of my position a few years ago. I flipped the photo to match the BikeCad camera position (this can be changed in the Pro version). I set the scales to match the tire diameter: BikeCad reported the tire diameter for the bike (670 mm) and I knew the value from my bike based on rollout tests (671 mm). Curiously the 650B wheels with 42 mm tires end up very closely matched to 700C wheels with 25 mm tires.

Here's the result:

The handlebars in the BikeCad model are a bit of a strange shape (this is adjustable), but at the time I was running my brake hoods on the Fuji fairly high, and curiously my bike design ended up with the brake hoods in approximately the same position. The new bike, however, offers a much higher position on the tops. Despite a much roomier design, the position on the hoods on the two bikes would be fairly similar. The position on the drops would be more relaxed on the randonneuring bike, though. That's fine: it's not designed for riding crouched in a pack of racers.

Still, randonneuring bikes traditionally have very little saddle-to-bar drop, and this one still has a substantial one. Is this just a cultural/retro thing, or is it due to rational basis? Honestly I can see moving my hands up a few cm but going to a drastically relaxed position makes little sense. Randonneuring bikes are still performance-oriented, just for longer distances and carrying small loads. There would be some compromise in loaded touring but I don't immediately foresee that being the primary application: there's no way I'd want to commute to work on a touring bike, so if I had to pick one, it would definitely be to compromise on the touring aspect.

Gravel racers are becoming popular but it seems randonneuring bikes are fairly well optimized for "adventure rides". The only issue there is the fenders, which can become clogged with dirt, but for gravel roads this isn't a big problem.

Randonneuring bikes have a lot of other historical associations, like generator hubs and Brooks saddles. Modern rechargeable LED lights are great: I have no desire for a generator hub (if anything, a generator would be useful for charging my electronics, not powering lights). And racing saddles work fine, even for ultramarathon races.

One aspect I'm willing to yield to retro-riders is downtube shifters. These work fine and are lighter and simpler. A fraction of a second lost in shifting is bad during mass-start races but for what I'd plan to use this bike for it's not an issue.

For brakes: cantilever brakes seem fine. Center-pulls exist which can wrap around fenders, but cantilevers are simple and light and by accounts I've read work well except for the possible issue of setting toe-in.

Anyway, it's quite possible my views on these aspects will evolve.