Tuesday, September 2, 2014

Tour magazine frame mass tests: carbon vs titanium vs steel

Here's a comparison of Tour magazine tested mass data for medium-to-large frames taken from three articles:

  1. steel bikes, Aug 2013, on-line summary
  2. "sub-800 gram" carbon bikes, Dec 2013, on-line summary
  3. titanium bikes, Nov 2013, purchased today in print version


This sample is biased in favor of the carbon since the carbon frames selected are the lightest, most expensive available, while in steel, for example, a custom builder like Rob English, emphasizing weight, can get lower than most stock steel frames, which have less emphasis on weight. But I think the Crema represents this quite well: only 1512 g for the frame.

Forks are heavier on the steel and Ti bikes, in general, but I am focusing on frames, since forks can be selected, especially with custom steel, which is still a lot cheaper than those carbon bikes.

Result: best carbon is 704, best Ti is 1350, and best steel is 1512. So the cost of Ti is around 646 grams, and the cost of steel around 808 grams, relative to carbon.

808 grams: that's not bad at all. Still within typical daily body mass fluctuations.

Monday, September 1, 2014

Fit change from Cervelo R to 2015 Cervelo S5 geometry

Cervelo's new geometry for the S5 is a move toward the industry standard for stack and reach, with shortened head tubes reducing the stack and increasing the reach of each size.

I decided to see what this new geometry did for my fit on the bikes. Would I end up with a hideous tower of spacers?


Here's a chart I posted before, with Cervelo S5 2015 replacing the Cannondale Evo. The Cervelo S5 and Evo geometries are fairly close, and so I removed the Evo curve to reduce color. I also show the Trek H1, an example of an aggressive geometry, and the Fuji SL1, which is my bike.

My bike is small, with an 11 cm +6 degree stem establishing the position set for me by 3D Bike Fit in San Francisco. Then from those coordinates (to the right of the plot), I trace back to the frame using a variety of stem options, using the head tube angles of the Cervelo R-series bikes.

With the R-series (also S2-3, and also the 2014 S5), I could ride a 48 cm frame with a 13 cm +6 degree stem, a 51 cm frame with a -6 degree 11.5 cm frame (adding 1 cm of spacers), or a 54 cm frame with a -17 degree, 10 cm stem (with around 5 mm of added spacers). The 51 cm is the most likely candidate here.

To the S5, shown with dotted lines: now the 51 cm would require an 11 cm, +6 degree stem, removing around 5 mm of my present 1 cm stack height. But with the 54 cm frame, I'd be very close to my present fit using a 10 cm -6 degree stem. It would be even closer if I'd adjust for the fact that the 54 cm has a 0.9 deg steeper head tube and the displayed stem trajectory is just slightly too steep (upward).

The result: with a -6 degree stem, I can use the 51 cm R-series with an 11.5 cm stem, or the 54 cm S-5 with a 10 cm stem. Personally I'd prefer the 10 cm stem.

So really no spacer difference is involved: to the contrary, in fact. With the S5 I end up with fewer spacers than I would have for the R-series. And the reason is with the S5 I can upsize to the 54 cm frame with a -6 degree stem.

Sunday, August 31, 2014

new Cervelo S5: reduced stack

Since the retirement of the RS road bike with more "relaxed" geometry, Cervelo has unified its stack-reach schedule, creating a single geometry shared between the R and S series. This geometry is fairly close to the Trek H2 geometry, the more relaxed of the two geometries Trek supports. The logic used by Cervelo is that the vast majority of riders can be fit with a stem ranging from +6 degrees to -17 degrees using this more relaxed geometry.

And it's true, but many riders object to a bike like the S5 being touted as an "aero" frame had the longest head tubes. This was especially problematic when using the bike with full time trial bars. Indeed the S5 is sufficiently aero that with time trial bars, it's a competitive time trial/triathlon frame. So a lower stack provides more adjustability and allows more reach for the same stack and a shorter stem for most male riders (small women often benefit from a small reach). Riders claim to prefer the feel of stems in the 9-10 cm range over 12-14 cm (although I'm skeptical the difference can be perceived at the hoods or drops).

Here's the geometry data:


I plot it here, comparing to Trek:


So the new S5 is basically middle-of-the-road compared to Trek. A lot of bikes are in this range. So nothing extreme about the new S5.

One interpretation of the results is a 2 cm increase in spacers. But there's others. If you're running a -6 deg stem, which is typical, you can now up to the next frame size and shorten your stem by around 2 cm. If you're running a -17 deg stem you can probably swap to a -6 deg stem. So no hideous spacer stacks required here unless you want an fairly upright position or your arms are exceptionally short. But then you may want to stick with the S2-3, which is still an excellent bike.

Friday, August 29, 2014

Tour magazine sub-800 gram frame test: comparing measured frame mass to claimed

Previously I reported on the Tour Magazine test of "frames under 800 grams" published in the December 2013 edition of the magazine, and summarized on-line.

In the article, they weighed the frame, fork, and bearings from a series of light & expensive bikes. They also bent them, measuring the stiffness characteristics. They then calculate a rating based on these and some other minor factors (warranty, quality of finish).

But it's interesting to compare these weights to what is claimed by the manufacturer. Those aren't listed in the article. The focus of Tour isn't to investigate the dishonesty of bike companies. Rather it's to determine the truth.

Claimed weight is a tricky business. Only Cervelo is totally clear in the matter, publishing a lot minimum, average, and maximum for each size of the RCa frame in their White Paper on the bike. Obviously bigger frames tend to weigh more. If I'm buying a 56-cm frame, I don't care about the mass of a 48-cm frame. If one company sells a 52 cm frame as their small bike, and another a 48, do I take the latter because they can claim a lower number for their smallest bike?

Then there's painted versus unpainted. Some people like to sand the paint off their bikes. But for most, the unpainted mass is irrelevant. Sure, if I'm getting a white bike, which might be relatively heavier, I don't care how good Trek's "vapor coat" is. I want to compare, for example, Cannondale's white to Trek's white. But in general it's more honest to publish the lightest painted bike sold. Key question: would stripping the paint void the warranty? If yes, don't tell me the weight of the unpainted frame.

Then there's the issue of hardware. I'm okay with leaving off the bottle cage bolts, since I can easily ride the bike without those. But the derailleur hanger? Err... no. That stays. And it's not as if I can just go and swap a heavy hanger for a light one, something I can do with bottle bolts.

Let's go through the bikes...

  1. Cannondale SuperSix Evo Black: Google had a cached page from Cannondale claiming "under 700 grams", although the 2015 page has no claim. So I used 695.
  2. Cervélo RCA: I took their reported average for 56 cm frames (see table above). The high for this size was 696.
  3. Corratec Mauro Sannino Prima: "starting at 680 grams." This is a custom bike, which adds a lot of potential variability.
  4. Focus Izalco Max 0.0: I got 750 grams from a dealer.
  5. Neil Pryde Bura SL: There's a big fat "710 grams" on their web page.
  6. Pasculli Altissimo: This is another custom bike. But unlike Corratec, these guys just say "680".
  7. Simplon Pavo 3 Ultra: I got 740 grams off their web page.
  8. Trek Madone 7.9 H1: I got 725 grams from a Jan 2013 VeloNews article (down from 750 grams in 2012).

Only Cervelo was a size-specific mass. The others were all just a single quoted number.

Here's the result:


Amazingly Simplon came in well under the claimed weight. All others were over. I was surprised Cervelo was over. Trek, Focus, and Cannondale are each around 50-60 grams over claimed. The two custom frames were 100 grams over claimed, but I'll cut the custom builders some slack, since they tune stiffness. The Bora came in 90 grams over claimed.

The result is you can't put much faith in claimed mass, comparing different manufacturers. Trek, Cannondale, and Focus each seem to be mutually consistent in how low they are. This is likely because they are referencing a small frame, and Tour measures relatively large 56-58 cm frames. For the customs, you need to say what you want: if you want light, and don't care about stiff, you need to say that. Neil Pryde is the one which is most clearly off target.

Thursday, August 28, 2014

Cervelo S5 and the aero road bars

At EuroBike, Cervelo made two huge product announcements, a new S5 aero-road bike, and a revised on the super-premium "would be fun to ride but I can't comprehend paying $10k for a frame and fork" "Project California" RCa.


The S5 is an interesting case. Cervelo has a range of aero road bikes: the S2, S3, and S5. The S2 and S3 are the same frame but with different part specs. These were designed for the more average rider/racer, with an aero front end combined with the rear triangle which is basically the same as the R-series road frames. The S5, on the other hand, was designed with the racer philosophy that, as Specialized tweets, #aeroiseverything. The idea is that racers, looking for all marginal advantages, would ride the S5, and give up a bit of rear-triangle comfort.

Surprise, surprise. On the Garmin pro team, the S3 got much more use. Racing is a complex dynamic, much different than riding solo in the wind. The riders felt better on the S3 with a bit more compliance than the S5.

And so it was inevitable a revision of the S5 was coming, and soon. So this year at EuroBike, they revealed a new version.

Many were expecting an up-rated version of the S3, an aero-road hybrid. But instead, Cervelo led with the aerodynamics, improving the aerodynamics further of the previous S5 while claimin to have improved the stiffness and comfort at the same time yet without resorting to the less aerodynamic rear-triangle features of the S2-3.

Here's the sources of wind resistance on the bike, according to a BikeRadar report. Initially I thought this represented the sources of improvement, but it was pointed out to me on SlowTwitch Triathlon forum it's more likely total resistance. Defining how each component contributes to total resistance is tricky, since parts interact. In a tandem does the front rider contribute more than the rear rider since the rear rider drafts behind the first? Suppose the rear rider is tucked right behind the captain, in virtually perfect draft. Do I conclude the rear rider has no effect on wind resistance? No -- that would be an error. If I were to completely eliminate the front rider, rendering his wind resistance to zero, the total wind resistance of the bike would barely change: the wind which had been hitting the captain would now hit the stoker instead. So the system is nonlinear: the total is not the sum of its parts.

1% – seatpost 
2% – rear brake 
3% – front break 
5% – rear wheel 
9% – drivetrain 
9% – bottle 
9% – fork 
16% – frame 
16% – front wheel 
30% – handlebar

I was initially shocked the handlebar was listed as responsible for more drag than the frame, but the original S5 has a total CdA of around 0.058 m2 according to Bicycling Magazine data, or 580 cm2. A simple calculation of drag on a handlebar (see later), treating it as a cylinder with air at normal incidence, is 100 cm2 just for the top. If I I assume the drops + the brake levers add an extra 75 cm2 then I'm up to 30% (Damon Rinard of Cervelo clarified that this number includes the brake levers). Of course where the rider grabs the bars there will be no contribution to wind resistance from the bars, however: there's a lot of bar-rider interaction.

Starting with the bottle (9%): they didn't introduce any sort of new aero-shaped bottle. They understand road racers need to use whatever bottles they get handed. Instead they followed the model Litespeed used on its carbon road frames and optimized aerodynamics under the assumption there's a bottle on the downtube. This is an interesting question because at the finish of a race, or in most criteriums, there's often no bottles on the bike. But if you're optimizing the bike for long breakaway then you've got to consider bottles. A quantitative assessment would look at "match burning" episodes from real-race power data. But I think optimizing with a bottle is the way to go.

For the frame, then tweaked transitions from the head tube to the down tube and increased head tube taper.

On the front wheel (and rear), they switched to Hed. Of course you can get Hed wheels with the old S5, as well, if you want to buy them.


The biggest contributor is listed as the handlebar, and for that there was a new $400 aero carbon handlebar. Of course you can buy any handlebar you want and put it on the old S5, but this one comes with the new one. It's slick and aerodynamic, with a claimed savings of 4.4 W, which is apparently at 40 kph based on this BikeRadar article (this was confirmed by Cervelo's Damon Rinard), This corresponds to 7.7 W at 30 mph.

4.4 watts at 40 kph/25 mph corresponds, assuming an air density of 1.214 kg/m3, to a CdA reduction of 52.8 cm2. This makes sense: a standard handlebar has a flat top around 36 cm wide, more or less, neglecting the clamped portion and the transition to the drops, with a diameter of near 26 mm beyond the clamp. The Cd of a cylinder is around 0.95 for broadside impact (Mallack and Kumar, 2014), resulting in a net CdA of 89 cm2 for the top part. If the portion of the rider behind the cylinder can draft the cylinder with 30% efficiency, then that reduces the effective CdA of the bar to around 62 cm2 (if the bar wasn't there, the wind that would have hit the cylinder will hit the body instead: doomed either way). So if I can reduce the wind drag of the top portion of the bar by 80%, I get a reduction of 50 cm2. That's really close for a super-back-of-the-envelope estimate.

Super-simplistic kinetic model: consider a stationary air approximation, with no pressure waves. The handlebar displaces a volume vA or air per unit time, speed v multipled by cross-sectional area A. The air develops a kinetic energy density of 1/2 ρ v2, where ρ is the mass-density. That's total rate of kinetic energy transfer 1/2 ρ A v3. This would correspond to a CdA of 1.

But the air slows. Displaced, it falls behind the handlebar. Eventually the body slams into it. By this time, suppose its velocity had dropped 45%. It now has 30% the kinetic energy it did before. The body now increases the kinetic energy of these molecules back to 1/2 ρ A v3. The total kinetic energy transfer rate has been now (1/2) (1.7) ρ A v3. This is a 70% increase versus if the bar wasn't there. So the bar's effective contribution to wind resistance corresponds to 70% of what it would be without a rider behind the bar.

So basically with 20:20 hindsight it's easy to convince yourself this amount of reduction is "obvious".


Then there's the Zipp bar, formerly the Vuma Sprint, now the $350 SL-70. That claims a force reduction of 6.4 watts at 30 mph. This corresponds to a CdA decrease of 44 cm2, in contrast to 30 cm2 for the Cervelo claim for their bar. We know both used a rider on the bike (Cervelo and Zipp both state this), but there's too many sources of variability to directly compare these numbers, for example the position of the rider and how close his body is behind the bar. Both are in the range of my super-simplistic estimate.

So aero road bars appear to be the real deal: essentially free speed for a modest (on order 100 gram) increase in mass. The Cervelo bar claims 270 grams while Zipp claims 240 grams for their bar, Zipp's bar 70 grams more than the claim for their "SL" round bar.

One slight digression:  the Zipp bars sweep forward on the tops.  I like this.  Ritchey Evolutions, for example, sweep back.  Backward sweep pits you'd wrists in a contorted bend.  Forward sweep yields a more neutral position.  So these bars look comfortable too me.

But which one to choose?

The Zipp claims a bit less aero drag reduction, but that doesn't mean much since it wasn't a 1:1 comparison. It claims lighter mass, but mass claims are notoriously unreliable. And it is selling for $50 less than Cervelo has indicated they'll list their bar for. Overall it seems a win for the Zipp bar but that's not a very reliable win.

Note, though, that while 4.4 watts sounds great, that's out of an aerodynamic drag power of around 266 watts assuming CdA of 0.32 to start with, which saves at most 20 seconds per hour.  This is even smaller than the leg shaving advantage Specialized reported from their wind tunnel on YouTube.  So don't expect any dramatic change from these things.

Wednesday, August 27, 2014

Tour magazine test of "under 800 gram" frames

Tour Magazine did a web feature from an article in the Dec 2013 printed magazine "Lieichte Rahmen unter 800 Gramm." This featured 8 bikes, all exceptionally priced, which presumably get under the challenging 800 gram mark for a bike frame.

Now 800 grams is hardly new. For example, Ruegamer showed a custom frame under 700 grams at the National Handbuilt Bike Show 6 years ago, and that frame is still ridden a ton by weight weenie legend Don Becker of Berkeley. Spin, a company in Germany, was also around the same mass at that time.

But production bikes tend to be overbuilt due to the need for mass production margins and since sometimes the fattest riders buy the lightest bikes, irrationally. It's not a good thing if a 100 kg rider snaps his 51 cm frame after hitting a pothole on his 3 mile bike commute. Additionally, 1500 watt sprinters may perceive an extra few mm of flex in a super-light frame, causing irrational fear that precious watts are being squandered. So the game has more often then not been to claim lower weight by changing the protocol (for example, without paint, without derailleur hangers, even in the case of Pinarello without bottom bracket shells.... I'm still waiting for a company to leave out the epoxy and just weigh the carbon fibers). Cannondale, for example with the Evo, used an "equivalent" mass attained by adjusting for various design features on the frame, on the basis they could have made it that light had they designed the frame differently.

Despite this, big US media almost always report the claimed mass of the frame. After all the claims are from advertisers, and it doesn't do well for advertising revenue to indicate any lack of trust in their claims. It's been said "the customers of magazines are the advertisers, the product is the reader; they deliver the product to the customer."

The German magazine Tour, however, ruthlessly weighs every frame they review, and that includes the frames of complete bikes, which obviously requires disassembling it. They weigh not just the frame, but also the fork and bearings, reporting each separately and rating the bike on the combined total. This is important, because usually everything else on a bike is replaceable, and in any case there's almost always various component options for each frame, so a "fair" comparison comparing like-to-like with different companies becomes impossible.

I say "ruthless" because while it's common to report the lower mass associated with smaller-sized frames (the most infamous case of this might have been the Guru Photon first shown at Interbike, a tiny custom), they target a "medium" frame, medium by German standards, which is typically what is called a 56 cm to 58 cm. The other difference is they are willing to use a white version of a bike, and since white paint needs to be applied generously to fully occlude the black background, white frames tend to be heavier, especially relative to a clear coat option. So the numbers are often on the high side. The result is if you come out light in the Tour magazine tests, that says a lot.

So back to the "800 g" article. They measured the following:

  1. Cannondale SuperSix Evo Black, a personal favorite of mine. I test rode a Cannondale Evo back soon after they first appeared in a local SportsBasement and it was very nice.
  2. Cervélo RCA: the famous $10k frame described in Cervélo's white paper. This is a wonderful combination of light, aero, and optimized stiffness but with an anomalously tall head tube which calls for a relatively long, -17 degree stem to get a racing bar position.
  3. Corratec Mauro Sannino Prima: I've heard of Corratec, but don't really know anything about them.
  4. Focus Izalco Max 0.0: Focus has always been a practioner of the "fatty down tube" style of frame design, which I don't like. But they do claim low masses.
  5. Neil Pryde Bura SL: Neil Pryde made big claims about this frame, but in the end, it wasn't particularly light compared to competition. I suspected this wouldn't do as well as some of the others like Cannondale, obviously Cervélo, Simplon, or maybe even Trek (with its expensive vapor coat paint).
  6. Pasculli Altissimo: I don't think I've ever heard of Pasculli.
  7. Simplon Pavo 3 Ultra: Simplon is one of the frames that perennially competes for the top spot in Tour's ratings. The frames are very light, but like the focus, designed with the fatty downtube philosophy which results in poor aerodynamics.
  8. Trek Madone 7.9 H1: Trek historically didn't brag much about weight, at least until the latest Emonda "fatty downtube" design, but the top-tier Madone with vapor coat does very well. Trek always comes through with a really light, super-priced top-end frame, then they beef out frames lower in the range which are sold at a consideraly lower price.

Here's the results, ranking the bikes by total mass:

Cervelo RCa1082704323555763871.48837
Simplon Pavo 3 Ultra1113717324725593941.41878
Cannondale Evo Black1123760303605593961.41162
Focus Izalco Max1164802293695654021.40547
Pasculli Altissimo1203789336785593941.41878
Trek Madone H11204786379395454041.34901
Corratec Mauro Sannino Prima1209783336905474041.35396
Neil Pryde Bora SL1241802374655623901.44103

Tour goes beyond mass, also measuring stiffness (good and bad) to come up with an overall score. I'll leave the stiffness scoring to them: check out the German article.

One thing which pops out here is that despite the title of the article, two of the frames are over 800 grams: the Focus and the Neil Pryde. The Focus makes up for it to a large extent with its exceptionally light fork: the only fork under 300 grams, something which used to be fairly common back in the mid naughts but which is rare now due to increased safety standards. Note the Cervelo fork is 323 grams, well above 300, despite its expensive micro-think nickel coating for strength. The Neil Pryde, on the other hand, complements its 802 grams with the 2nd heaviest fork in the group, only the Trek Madone heavier and just barely. The Trek Madone, however, has the lightest headset bearings by a decent margin.

On average, these bikes have the following masses:

avg bearingsσ bearingsavg forkσ forkavg frameσ frame

Interesting here is that the difference (as reported by the standard deviation σ) in bearing mass is around 40% the difference in frame mass, and the difference in forks is around 80% the difference in frames. So just looking at frame mass, fairly common, doesn't tell nearly the whole story. And there's only a weak correlation among these frames between frame and fork mass. So you really need to consider both, unless you're planning on replacing a heavy fork.

The total mass has an average of 1167 and a σ of 52.4 grams. This σ is larger than you'd expect if the three mass componts were uncorrelated: the uncorrelated result would be 43.4. So there's a positive correlation among components.

On the specific results: the Cervélo wins on total mass (it's also the most aerodynamic, I'm fairly confident). The Simplon and the Cannondale are close behind, trailing by 31 and 41 grams. The Simplon frame is impressively light at 717 grams, only 13 grams more than the Cervelo and 43 grams lighter than the Cannondale. The Cannondale scores strongly with the light bearings and the light fork.

By the way, even though the Cervelo RCa "won" on total weight, the frame weight is higher than Cervelo claimed for even the heaviest of the size 56's. Did Tour include something with the frame that Cervelo does not? The Cervelo number includes "hardware":

Here's rankings by sub-class: frame, fork, and bearings.

Cervelo RCa1082704323555763871.48837
Simplon Pavo 3 Ultra1113717324725593941.41878
Cannondale Evo Black1123760303605593961.41162
Corratec Mauro Sannino Prima1209783336905474041.35396
Trek Madone H11204786379395454041.34901
Pasculli Altissimo1203789336785593941.41878
Focus Izalco Max1164802293695654021.40547
Neil Pryde Bora SL1241802374655623901.44103

Focus Izalco Max1164802293695654021.40547
Cannondale Evo Black1123760303605593961.41162
Cervelo RCa1082704323555763871.48837
Simplon Pavo 3 Ultra1113717324725593941.41878
Pasculli Altissimo1203789336785593941.41878
Corratec Mauro Sannino Prima1209783336905474041.35396
Neil Pryde Bora SL1241802374655623901.44103
Trek Madone H11204786379395454041.34901

Trek Madone H11204786379395454041.34901
Cervelo RCa1082704323555763871.48837
Cannondale Evo Black1123760303605593961.41162
Neil Pryde Bora SL1241802374655623901.44103
Focus Izalco Max1164802293695654021.40547
Simplon Pavo 3 Ultra1113717324725593941.41878
Pasculli Altissimo1203789336785593941.41878
Corratec Mauro Sannino Prima1209783336905474041.35396

Geometry in this review is primarily parameterized by stack-to-reach ratio. A bigger number is a more upright bike, a lower number is a more aggressive, lower or longer geometry. Here's that ranking:

Trek Madone H11204786379395454041.34901
Corratec Mauro Sannino Prima1209783336905474041.35396
Focus Izalco Max1164802293695654021.40547
Cannondale Evo Black1123760303605593961.41162
Pasculli Altissimo1203789336785593941.41878
Simplon Pavo 3 Ultra1113717324725593941.41878
Neil Pryde Bora SL1241802374655623901.44103
Cervelo RCa1082704323555763871.48837

The Trek and the Corratec are the most aggressive, each at 1.35. Then there's a pack at 1.42: Focus, Cannondale, and Pasculli, and Simplon. This seems like a good numberto me. Then there's the Neil Pryde at 1.44, and in a class by itself, the Cervelo is 1.49.

Geometry is plotted here, where each bike's point is at the center of its label:


Adding spacers (assuming a 73 deg HTA) moves you up the near vetical blue dotted lines. Adding a longer stem (0 degrees) moves you along the near horizontal lines to the right. A +6 deg would move you at an upward slope relative to these lines, a -6 deg would move at a less steep slope. The spacing between lines is 1 cm.

The result is that it would take around 3.5 cm of spacers, and a stem 1 cm longer, to match the position of the Madone or Corratec on the Cervelo.

Which of these do I take? I'll eat the SRratio and go with the Cervelo. It wins on mass, it wins on aerodynamics.

Sunday, August 24, 2014

fitting a Cervelo demo R5?

On a recent ride, I found a shop local to the day job, Cupertino Cycles, had a Cervelo Demo R5. Cool! Since reading the RCa White Paper, I've always wanted to try one. Indeed, I've never ridden any of the Cervelo R-series.

Recall, perhaps, the Peloton magazine review of the RCa, the prototype which led to the present R5:

There is a fury to the way the bike reacts to power -- it leaps from under you, but the feeling continues beyond the initial acceleration. Each pedal stroke delivers a new surge forward.

Who wouldn't want to ride this?

Deal is the Cervelo geometry is, as I've described, a bit tall. So it isn't obvious what frame to try. But is low-stack over-rates?

So what I did is I started with the geometry for the Fuji SL/1, which is my race bike (since I've done little racing recently, the poor thing is getting neglected).

I started with the geometry of the size small, which I ride. I use a +6 degree 11 cm stem at present (I used to use a lower position but found that the lower bars didn't actually result in a flatter back, especially since I don't use shallow bars, but instead just caused my shoulders to roll forward, which causes back strain, so I flipped the stem). In conjunction with the 71 degree head tube, this results in an upward 25 degree slope for 11 cm on the stem, which results in the adjusted reach-stack coordinates shown in the plot below. I have 1.0 cm of spacers on top of a low-stack headset cap as well, not shown in the plot:


The original Fuji stack-reach is shown with the "x" on the open black circle toward the lower left, then the result of the stem is shown with a "*" toward the right.

Note this is not a relaxed position: I still have 9 cm of drop, which is plenty. Really the frame is by most standards "too small" for me. I was lured to the smaller frame by the lower mass and by the possibility for lower bar position. But I overestimated how much drop I really wanted. I'd have been better off with the next size frame as it would have bought me a steeper head tube, even if it had been slightly heavier.

From this point, I want to "undo" the effect of various stems to find the frame stack-reach coordinates which would give me the same position with various stem options. I picked stems from 9 cm to 13 cm long, with angles of either +6 deg, -6 deg, or -17 deg. Note the spacing of stem angles is 12 deg and 11 deg, so they are relatively equally spaced. The target for these stem angles were the Cervelo 48 cm, 51 cm, and 54 cm R5. They have head tube angles of 70.5 degrees (this is how they manage to get such short reach on the small frame -- super-slack head tube, then compensate trail with a long-rake fork), 72.2 degrees, and 73.1 degrees. In general, I consider a 73 degree head tube to be superior to a 71 degree head tube, since it results in less wheel flop: the wheel is less prone to flopping over when the bike is tilted, resulting in better cross-wind control. But I've never done controlled experiments to verify this.

Note if the trajectory from the Fuji-with-stem position passes over a frame size, spacers can be added to the frame to bring it up to that trajectory. Additionally if the trajectory falls under a frame size I could remove up to 1 cm of spacers to bring the fit down. There's blue dotted lines on the plot showing what points are accessible by adding spacers, with roughly orthogonal dotted blue lines marking 1 cm intervals. These lines are drawn for a 73 deg head tube, so would apply best to the 54 cm Cervelo, but would also be a good approximation for the head tubes of the other frames, which differ by at most 2.5 degrees.

Fit coordinates I don't consider to be an exact science, and all I need to do is get close. With this in mind, the 48 cm frame could fit with a 13 cm stem at +6 degrees. That's obviously excessive. The 51 cm stem would fit with an 11.5 cm stem at -6 degrees. That's acceptable, not too far from the Fuji except with the stem flipped. And the 54 cm Cervelo would fit with a -17 degree 10 cm stem. That might actually be the best option since it would give the steepest head tube angle and also a shorter stem which would stiffen the front end.

In any case, the shop doesn't have a 51 cm demo, so I'd need to pick the 48 or 54. Given the choices, a 54 would be the obvious pick. However, I doubt they'll have a -17 degree stem available for a test ride. I can check, however.

Also on the plot I show Cannondale and Trek Madone H1, although the stem adjustments aren't drawn for the head tube angles on these frames. I could fit each of these as well, with shorter stems.

Consider, for example, the Evo geometries:

The 52 cm size with a zero degree stem looks like it would work, or a -6 degree stem with some spacers. The 54 cm size could also work fine: I could remove 5 mm of my 10 mm spacer stack, perhaps removing all of them with a headset cap 5 mm taller, and get the same bar position, using a -6 deg stem. There's a small error in these projections due to the head tube angle difference.

This plot shows an alternate route to the same result. Here instead of starting with the target position, I start with each of the three Cervelos. I then extend the various stems away from these positions.


The 13 cm +6 deg stem from the 48 cm Cervelo scores a virtual direct hit on the position I presently have with the Fuji. The 11.5 cm -6 deg stem from the 51 and the 10 cm stem -17 deg stem from the 54 come in within 1 cm of that position, low. This is easily rectified with an extra 1 cm of spacer: no problem.

So again, the big stack Cervelos work. However if I could choose a Cervelo with the Trek H1 geometry I'd probably take that instead. It would shave several cm from these stem lengths. The 54 cm frame (3rd in the series) would provide plenty of options, using either a -6 deg stem with spacers, or a 0-deg stem with fewer spacers.