A Study in Geometry

Colnago Geo Chart

To the degree that this site might struggle to find a broad readership within cycling, I take full responsibility. I can geek out on aspects of equipment that some cyclists couldn’t care less about. I read geometry charts the way some folks read biographies. If they could examine geometry charts on Mythbusters, I’d record it and watch it daily.

I’d apologize, but the reality is, I enjoy it and from time to time I write something that actually turns out to help other cyclists.

One of RKP’s readers, Sophrosune, commented on the “Road Feel” post and asked me for my take on the Colnago E1’s geometry and to respond to Bicycling’s assertion that the bike was squirrelly on descents, something our reader claimed to experience as well.

I’ve talked a lot about geometry in theory, but this is a great chance to look specifically at one bike and just what information you generally get and what information you ought to get. So let’s start with the basics.

The Basics: If you really want to know a bike’s personality on paper, there are a handful of dimensions you absolutely need. They are:

Top tube length: this is the single greatest determiner of a bicycle’s size. Simply put, bigger people need longer top tubes.

Seat tube angle: this will have a huge effect on saddle setback and can change the effective top tube length by more than a centimeter on small size bikes and more than two centimeters on larger frames. The longer your femur, the slacker the seat tube angle you need to achieve knee-over-pedal spindle, but that’s only meaningful if you believe in that standard.

Trail: steering geometry is defined by the interplay of head tube angle and fork rake; if a chart lacks one or the other, there’s no telling how the bike will handle. More trail means the bike is more resistant to steering input; less trail means the bike is more reactive to steering input.

Bottom bracket height (or drop): these two numbers are virtually interchangeable, though one, BB drop, is absolute because it defines the distance the BB is below a line that bisects both axles. BB height is influenced by the tires used and any given BB height is dependent on a specific tire. A lower BB (a drop of 7.5cm or more) makes the bike easier to lean into a turn; while it makes a bike more responsive, many riders report that a bike with a lower BB feels unusually sure-footed. A bike with a higher BB (a drop of less than 7cm) requires a bit more countersteering to execute a sharp turn but feels more stable when out of the saddle.

Wheelbase: changes in front-center distance (center of the BB to center of the front axle) and chainstay length can have a big effect on wheelbase length, even though the difference between many road bikes at a given size may only be 1cm, which translates to roughly a one percent difference. Ultimately, most bike designers will tell you wheelbase isn’t as important as BB drop, but not everyone agrees.

Colnago Geo Numbers

The Colnago E1: Colnago is one of a teaspoons-full of European manufacturers that didn’t completely abdicate its manufacturing in favor of a factory in Asia. This is significant.

Let me try to make a tedious story short.

The first Asian production of carbon fiber frames was largely set up in conjunction with American companies producing bikes for the United States market. The frame designs were based on a CPSC requirement for pedal clearance; most manufacturers comply with this requirement by designing their road bikes with a 7cm of bottom bracket drop.

Italian companies don’t have this same restriction. Italy also contains the Dolomite mountains. That last detail may or may not have a lot to do with why most Italian bikes had a BB drop of 7.5cm or more. I can’t say for certain because most Italian companies treat their geometry charts like state secrets. They will, on occasion, say something like, ‘We want our bicycles to descend with proper confidence.’

Any bike designs available from Asian factories through what are termed “open molds”—our engineering (reverse engineered from one of our clients with a great engineering team), your decals—were all built around 7cm of BB drop (not to mention a shorter wheelbase and less trail).

Suddenly, a great many Italian bike companies had bikes with a BB a half centimeter (or more) higher than they traditionally were.

If you were someone who had just purchased your first Taiwanese- or Chinese-made Italian bike, you might not notice the change in handling. However, if you’d been with that brand since the 1970s, you’d notice the difference.

However, that’s not the case with Colnago. In the case of the E1, the BB is a millimeter or two higher than it was in the steel bikes I’ve ridden; same for the chainstays and front center, a millimeter or two shorter.

The one shortfall in the geometry is in the fork. The fork rake for each frame size is 43mm. The head tube angle, though not given here, gets steeper as the sizes go from smaller to larger. As a result, the trail decreases (handling gets faster) as the frames increase in size. Each size will handle a bit differently due to the variance in trail.

An aside: I’ve wondered from time to time if more trail in a small frame would be useful in overcoming the decrease in wheelbase length and lower center of gravity that comes with a smaller frame. I’ve talked to women who have ridden a variety of bikes in smaller frame sizes and those bikes with the slackest head tube angles (72.5 degrees or less) and relatively little fork rake (some had 40mm of rake) did what was expected—they didn’t want to turn. Scratch that idea.

So many builders will tell you they build a given model around a given trail. While a custom builder can build a fork to any rake, at best, bike companies will offer two different fork rakes for their size range.

Okay, so what about the question? Is this a squirrelly bike as Bicycling suggested? There are two problems with our data set: We have no wheelbase length and no head tube angle. Despite that, a quick comparison of this bike with a few other frames I’m familiar with shows the BB is 3-4mm lower (as it should be) and the chainstays on average a half centimeter lower than typical American bikes. The front center is pretty typical for a given size.

But is it squirrelly? Based on what I see on paper, my gut says it’s great on fast descents. For anyone not already accustomed to Italian bikes, out of the saddle, this bike is a bit more maneuverable than might be comfortable. I could see how someone might drift off their line on their first few out-of-the-saddle sprints.

The are other unknowns that will make a huge difference in how this bike handles, and two of the most significant are the combination of stem length and handlebar height. Suppose you set up one 56cm frame with a short (say 10cm) stem with 4cm of headset spacers and another with a longer (12cm) stem and no headset spacers will handle so differently as to seem like a completely different bike.

I’d expect this bike to seem rather maneuverable out-of-the-saddle, but great on descents, unless, of course, it had a short stem and a high handlebar, and then it would seem squirrelly all day, every day.

Moving beyond the specifics of this bike, this geometry chart shows how leaving out one or two key details can render the rest of the geometry chart almost useless. For me, the question is why so many European bike manufacturers treat head tube angles as trade secrets. Changing stem length and height will make a much bigger difference in handling than even a full degree of head tube angle. In a way, we’ve brought it on ourselves. So few riders look at geometry charts the manufacturers aren’t motivated to offer more; if they aren’t going to give a complete set of information, then they ought to just give top tube and head tube length and stop there.

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35 comments

  1. SuperDave

    When I worked for Colnago, all the forks had 43mm offset for all sizes (48-65cm) and the head angles, while not published, were offered in the custom building guide and varied from ~70 to as steep as 74 degrees depending on size.

    Head tube torsional stiffness is another key dimension not on geo charts that has a great deal to do with precision and stability on a frameset.

    Good read, thanks.
    -SD

  2. Sophrosune

    Fantastic, Padraig! Just brilliant. Thank you so much for taking the time to do this. I haven’t even read it yet. I’m just so impressed that you did this. Thank you very much.

  3. Sophrosune

    After actually reading this, Padraig, I am even more impressed. To give you a little more insight, I have 3cm of headset spacers and I use a 130mm stem. The size frame I am using is 56.3 cm top tube (sloping geometry)–it’s given as a size 54 in the chart above.

    In anticipation of your question about using such a long stem, I am not certain I made the right choice either. Lacking any professional bike fitting here in Spain (I think most people who ride seriously here usually had some coach fit them) I took on the job myself and used Competitive Cyclists’ measurement tool to get some figures to go by. Here are my measurements and the resulting figures for the “competitive fit”.

    Measurements
    ——————————————-
    Inseam: 84
    Trunk: 73
    Forearm: 35
    Arm: 65
    Thigh: 62.5
    Lower Leg: 56
    Sternal Notch: 153
    Total Body Height: 182.88

    The Competitive Fit (cm)
    ——————————————-
    Seat tube range c-c: 54.4 – 54.9
    Seat tube range c-t: 56.1 – 56.6
    Top tube length: 56.2 – 56.6
    Stem Length: 11.8 – 12.4
    BB-Saddle Position: 71.9 – 73.9
    Saddle-Handlebar: 55.8 – 56.4
    Saddle Setback: 5.6 – 6.0

    Since the top tube of the E1 was 56.3, I thought I could go with a 130mm stem rather than a 120mm stem since it would bring me closer the combination of 56.6 and 12.4. I just know that I have long torso and shorter legs so I thought this would the way to play it safe.

    I hope this is not boring you or your other readers. I think there are probably quite a few others who have some question about how stem length, amount of headset spacers and other factors can change the handling of our bikes.

    Thanks again for this. BTW: You should have some “Share” button so I can link this on Twitter or Facebook. Cheers.

  4. MCH

    Great stuff Padraig. I too, geek out over this stuff. As an example, it drives me crazy when I see a famous ex-pro write in a bike review that the fork rake should be increased in order to slow down the handling! A Bike Geometry 101 course should be required for all journos conducting bike reviews. Geek rant aside, the bane of many tall riders has been frames with poor tortional rigidity and short trail. The result often is high speed wobbles. For those that have never experienced this, imagine your front wheel, fork and head tube vibrating in ever increasing ocillations while decending at high speed – definitely not fun. The advent of carbon frames, at least in my limited experience, has cured the tortional rigidity problem. But, when I read about some manufactures using a single fork offset for all frames and larger frames having steeper head angles, I get concerned. Short trail on larger frames doesn’t make a lot of sense to me.

    I’d be very interested to see some research on the effects of CG (higher for tall riders, lower for shorter) and weight distribution on bike handling. I’ve also heard some fitters talk about the concept of “stem trail” – the idea that stem length has an effect on bike handling.

    Thanks, your insight is always appreciated.

  5. Larry T.

    Grazie Padraig for pointing out the stem length/handlebar height details! It’s difficult to convince folks that shorter stems and higher bar positions can make a bike less stable. My experience in high-end shops and since 2005 setting folks up on our rental bikes in Italy bears this out. We ask for three key dimensions from a rental client A) seat height from center of BB to top of saddle measured in line with seat tube B) distance from saddle tip to center of handlebar near the stem C) distance from front axle up to center of handlebar. From these I select the best frame size and dial in the position with adjustable stems for fine-tuning. This does not mean the bike is the perfect size, just puts them in the same position as they have on their own bike at home, right or wrong. Your Spanish friend reminds me of a client we had recently, his specs seemed to indicate short legs and long torso/arms as well. Sadly, when set up on a bike to match the specs he sent me, his position was awful, he was way too stretched out. If I’d had a smaller bike available I would have tried to convince him to switch and tried to dial-in a better position for him. I can’t help but wonder if this is the case with the Spanish fellow? As you know, proper weight distribution plays a part as well and any bike without enough rider weight on the front end (since one likely ends up stting too far back on a too-large frame and too high on one with a short stem and high bar position) will quite likely feel out-of-control on fast descents. Based on what he’s shown as BB-Saddle Position 71.9-73.9 cm and Saddle Handlebar 55.8-56.4 (assuming these are roughly equivalent to A and B as outlined above) I’d set this fellow up on a frame with a shorter top tube, probably one shown in your chart with a PCCS of 475 rather (unless I’m reading the chart incorrectly) than his with PCCS of 515. To sum it up, I think he bought a frame of the wrong size and nothing much in the way of stem swaps, handlebar heights or other adjustments is going to fix it. It’s hard to believe there’s not someone knowledgeable near him that can sort this out in-person rather than over the ‘net. You and I both have bikes custom-built for us by Antonio Mondonico and I’m sure you believe like me, that besides having Antonio take the measurements, letting him look us over in person made a difference in the way the bikes he built for us.

  6. Sophrosune

    Larry, I am confident that I have the right size frame. I have some doubts about my stem length and Padraig made me realize that stem length not only affects fit, but also can impact handling.

    This is where the question originally came from since I had read that some reviewers of my frame had noted that it seemed “squirelly” on descents and I noticed the same. But I remained unsure if this was due to my poor descending abilities or something inherent in the geometry of the bike.

    According to Padraig’s reading of the geometry, It seems that the problem could be just the poor descending skills of myself and the Bicycling magazine reviewer…as well as possibly the particular combination of headset risers and stem length. However, the wobbly descending would be more the result of a short stem and high headset risers, which is not exactly my set up.

  7. mark

    MCH: the speed wobble problem is not the exclusive bain of tall riders. My size 51 Lemond wobbled horribly at speeds over 40mph. Since I live at the top of a 1500 foot climb that’s 8% on one side and 10% on the other, getting down the hill to ride anywhere would induce speed wobbles unless I was riding the brakes. My solution was to buy a new frame that was much stiffer.

    As for guarding geometry figures like state secrets, I simply wouldn’t buy a bike if the manufacturer didn’t publish its geometry. I’m short and hard to fit, so 90% of my decision is made by looking at geometry tables without ever throwing a leg over the frame.

  8. Larry T.

    Perhaps I did not understand the exact description of the handling problem? Diagnosing a classic “speed wobble” is a whole different thing and can be caused by too many things to list here. Rarely is it some inherent fault in the bicycle frame itself and there are forums galore on what to do to eliminate or reduce it. The fellow who “solved” his LeMond’s wobble by replacing it quite likely got rid of a fundamentally sound bicycle for no good reason as lack of frame stiffness is rarely the cause of speed wobbles. I was interested in the specs you showed since the topic was (at least I thought it was) geometry and fit specs, a subject I have interest in, especially when it comes to setting up clients on our rental fleet. If you don’t mind would you give me the exact A, B, C specs from this bike? I’d like to see them so I could update my sizing chart. You can send ‘em to larry@cycleitalia.com if you’d rather not publish them here


    1. Author
      Padraig

      Everyone: Thanks for the terrific comments. There’s a lot to respond to and I promise to do so once I’m out of court later today.

  9. Sophrosune

    Larry,

    My numbers for your A, B, and C measurements are as follows: A: 74.2cm; B: 58.5cm (note that the “saddle/handlebar” measurement for Competitive Cyclist (CC) is from the nose of the saddle to the nearest edge of the handlebar, not the middle of the handlebar as yours is); C: 60cm (tricky measurement that could be off slightly). My seat is 3mm higher than CC’s suggested ranged for a “Competitive Fit” (I go by feel and comfort rather than maintaining strict static measurements) and when using the “saddle/handlebar” measurement that CC uses I am about dead on at 56.5cm.

    Unfortunately I don’t have a good plumb line (moving around the world you lose some of these items) so I am not certain what my saddle setback from the center of the bottom bracket is. But again, it’s a feel thing for me and my saddle is only slight back from center.

    Again, I am quite comfortable on the bike. It is probably the most comfortable bike I have ridden: no aches or pains and four or five hours on the bike causes no discomfort. It’s just that I have gotten some high-speed wobble on particularly steep descents and high speeds.


    1. Author
      Padraig

      Okay, lots to respond to and with a nap under my belt, I’m recharged and ready.

      SuperDave: My memory is that steel Colnagos came with one of three fork rakes: 40, 43 or 45mm of rake. What I’ve never understood is why all that went out the window when they began offering carbon bikes. It doesn’t seem like it could have been a SKU issue as at this point they could easily have justified the other two molds, especially if they were spec’ing the same fork on multiple bikes.

      Sophrosune: Reading everything you sent, I do believe you have the right size frame, something the CC info echoes. Here’s where being in the same room with a really good fitter becomes so important: I am deeply suspicious of the number of spacers and the stem length you have, but I (and the other smart guys reading this) can’t say anything about your choice because we can’t see just how flexible you are. Without seeing how flexible you are, we can’t tell if you’d do better with an 11 or 12cm stem and fewer spacers. However, I do know that change (three fewer spacers and a 12cm stem, would definitely make the bike handle more slowly. It would, in essence, kill the squirrel.

      Bear in mind that while many companies refer to “sloping” geometry, a top tube’s slope, despite what they seem to be suggesting, has nothing to do with a bike’s actual geometry. Without going too far afield, it would be easy in a frame design to replace a top tube horizontal to the ground with a sloping one after decreasing the length of the seat tube and the length of the seatstays (as well as their angle). All that really changes is that the CG drops a millimeter or two.

      Back to the squirrellyness: This quality, despite its ungainly name (no better a term, huh?) has nothing to do with the rider (either you or the Bicycling editor) and everything to do with how weight is distributed on the bike. If you are on the wrong bike, it won’t handle well. If you’re on the right bike with the right fit, you’ll go downhill like Sean Yates. Okay, maybe not that fast, but the bike won’t feel unstable.

      MCH: I think stem trail is an interesting concept and certainly deserves greater consideration when fitting a bike than it usually gets. The bigger issue is that we need some sort of metric the way we have for the trail that results from the combination of fork rake and head tube angle, and I don’t know of any way to achieve that.

      Mark: You’re right that speed wobble issues aren’t strictly the bane of tall riders but because of how CG height increases much more quickly than wheelbase length does as frame size increases, the proportion of CG height to wheelbase length is very different for a 50cm frame than it is for a 60cm frame. Consequently, solving a speed wobble on a big frame can be a much greater challenge. Ben Serotta once told me that they had a number of frames from tall riders from the Rocky Mountains who were having a speed wobble issue on one of their steel frames and they couldn’t solve the issue until they put a stiffer top tube (.8-.5-.8 wall instead of .7-.4-.7) in every bike; no bigger/stiffer down tubes were available due to the Colorado Concept down tube. The stiffer top tube solved the problem, which points back to MCH’s point about frame twist.

      If ever anyone wants to feel the effects of frame twist (or “lash”), that is the head tube twisting out of plane with the seat tube, just get a ride on an older steel tandem. No matter who is on the bike, if you both stand up and pedal up a hill, you’ll feel the frame twist and once it does, even trying to go in a straight line is a challenge. As an aside: This is why out-of-phase crank setup was created for tandems, so that when the pair stood up, they would be forced to keep the bike as upright as possible, never rocking the bike and decreasing the likelihood that the frame would twist. It was a silly solution to a tubing problem.

      Re: speed wobble. In my experience, the difference between a bike being squirrelly and getting a speed wobble is generally just a matter of degrees. A speed wobble is the expression of maximum turning responsiveness; it is a bike that handles so quickly, it can’t be handled safely. At the other end of the spectrum if the bike that is so stable it won’t turn at all. Ideally, you want a bike somewhere in the middle.

      Sophrosune: Saddle setback can be a big contributor to bike instability. Too much setback can shift weight off the front wheel, and increase how quickly a bike turns. Get a piece of string and tie it to a freewheel tool and check your setback. While Keith Bontrager has done a lot to shoot holes in the biomechanics of knee-over-pedal spindle, bikes are still designed around a particular weight distribution between front and rear wheels. If you move too far forward or too far back, you can disrupt the bike’s handling as much as you can by using a short stem mounted high. If you’re more than 3cm back from KOPS, you need to reconsider your fit, top-to-bottom.

  10. Jan

    Thank you, Padraig for a most interesting (and lucid) exposition. I’ve come across it while searching for some dis/confirmation of my own interpretation of the colnago size chart (I’m looking for a frame for 88cm inseam, 62cm arm, sternal notch 153, and total height 183: Larry, your suggestion here would be much appreciated).

    Sophrosune,

    What is your bar drop? What if you brought it down to about 7.5cm?

  11. Larry T.

    Thanks! Reviewing these specs (rather than the more vague specs I took from your earlier post) compared to my chart data suggests (if you were renting a bike from us)I would set up one of our “G” bikes for you. All are named for past winners of the Giro and the G’s correspond to 54 c-c X 54.5 in the conventially measured steel frame and around 50 X 55 in the sloping aluminum/carbon frames. Your front axle to handlebar spec looks reasonable as well. Based on this I certainly could not blame any high-speed wobbles or squirrely handling on an incorrect frame size. I have a hard time believing any frame geometry issues are to blame either as you’d think if this was a widespread problem there’d be more discussion about it and Colnago would have addressed it by now. I’d suggest visiting some of the various forums where these wobbling issues are discussed (Padraig probably knows about plenty of ‘em)and trying some A-B tests if this is a serious issue for you. Swapping out wheels would be the first place I’d start as it’s the simplest, then proceed with handlebar height by flipping the stem or moving the spacers around. This idea assumes the basic mechanical checks have been done to make sure nothing is loose or misaligned. PS– the client I referred to as too stretched out was set up on a G bike. Once I saw his position on it I would have preferred to put him on a C (52 X 53.3 conventional/47 X 53.5 sloping)but I didn’t have one available. He was content with the comfort and position and had no handling problems but I wasn’t the only one on the tour who commented on his less-than-optimum position. Thanks again for sharing your specs!

  12. Sophrosune

    Jan,

    I use an ITM Wing Shape Lite Luxe handlebar that has 154mm bar drop. It’s a shallow drop handlebar and I love it. I now ride in the drops much more regularly and for longer periods of time. The top of the saddle measures 99cm to the floor and the top of the handlebar to the floor is 95cm, meaning that there is just 4cm or a little over 1.5 inches difference between the handlebar height and the seat height.

    I rode a 55cm DeBernardi for many years and before that a Bianchi of the same size. I always felt a little crunched in the cockpit of these bikes. Now with a 56.3cm top tube I feel much more comfortable.

  13. Sophrosune

    Padraig, I have two spacers that came with the Cane Creek S-8 headset. Their precise measurement together is 2.88cm, not the 3cm I quoted your previously.

    I did attempt to measure the saddle setback (dropping a line from the nose of the saddle and measuring from the center of the bottom bracket). I was a little far back at around 7.2-7.5 (but still in the ball park for CC’s Eddy Fit: 6.8-7.2). I have moved the seat forward slightly so it seems to measure at 6.5cm and now the saddle handlebar measurement (to the near edge of the handlebar is at 55.5. I take from this that in terms of fit the 130mm stem is right for me. If I went shorter the cockpit would really get a little too small for me.

    I haven’t ridden the bike with the new alterations yet and I’ll let you know how they feel once I do.


    1. Author
      Padraig

      Sophrosune: Two points I should clarify—
      1) The bar drop another reader was referring to was, I believe, not the drop of the bar itself, but rather the difference in height as measured from the ground between the top of the handlebar and the top of the saddle. That tends to be the more important number overall.

      2) Bear in mind measuring saddle setback on the bike isn’t at all the same as measuring KOPS. KOPS will tell you something about your weight distribution but saddle setback may or may not tell you much.

      While I love that readers are willing to continue to make suggestions, I really think that the only way someone can further diagnose any issue you are having will involve a thorough fitting that checks your flexibility. Nothing about your bike sounds wildly off, though I’m still a little suspicious of your bar position; there are enough unknowns that the best answer will come from someone who can see you on the bike.

  14. MCH

    Padraig, you opened a very interesting can of worms with the Keith Bontrager knee-over-pedal spindle biomechanics comment. Perhaps an article on the current state of fit science would be a nice follow-up to this topic.

  15. Larry T.

    Padraig sums it up well with his final sentence. In-person help from a qualified person is worth whatever it takes to get. Went through this myself back-in-the-day when an elder statesman of the South Bay Wheelmen corrected my position (on a way-too-big frame!) enough to eliminate a persistent knee pain no docs, magazines or phone calls could fix. Much later, in-person measuring and consultation with Antonio and Mauro Mondonico (finally!) put me on the correct sized frame. At the same time, Mondonico built a bike for my wife based only on specs I gave them, but when they met, measured and saw her on it later, they built another one slightly different and to this day it’s her favorite! PS — none of ‘em wobble or are squirrely on fast descents though I’d bet they’re far less “stiff” than most carbon-fiber frame/forks.

  16. Tom B.

    This bike is NOT squirrelly. Precisely the opposite. I’ve cycled and raced for 20 years, and this bike is the stablest and least apt to choose my line for me of all bikes I’ve ridden extensively.

    You have to tell the E1 where to go, but it goes there and nowhere else. No twitching at all.

    My Trek 5900 in particular had a mind of its own. My Lemond Maillot Jaune was closest, dynamically, to the E1 among bikes I’ve ridden. I rode an aluminium Orbea w/ carbon rear triangle for a week once, and it scared the pants off me on long descents. So wobbly and dodgy I had to slow down probably 10 mph below what I’d otherwise do.


    1. Author
      Padraig

      Tom: One of the biggest points behind my post was that as sizes change for a given bike, handling often does as well. To know anything about a bike, you’ve got to have the full geo chart, but even once you have that, you really only have half the story. The E1 is one of those bikes where as size changes so does handling, from what I could tell.

      If every size of a bike has a different head tube angle, but they all use the same fork rake (in this case 43mm), you end up with a new trail for each size. Ultimately each size handles differently and your experience on that model could have nothing in common with anyone else’s on a different size. Even if you were on the same size, a bar height substantially higher or lower and a stem length much longer or shorter could render that bike unrecognizable.

      This issue is intertwined with the post I wrote on quill stems:
      http://redkiteprayer.com/?p=500

      From what you said about your experience on the E1, I’d be willing to guess it was sized quite well for you. The Orbea, I’d also guess, was a demo bike and not sized to you to any great degree; I’d bet you didn’t have enough weight on the front wheel and that scary experience you had on that bike, I could create on any bike with a bar high enough and stem short enough.

      To the degree that I’m willing to generalize about bike handling, I’ve realized I can’t generalize, except to say, with the right fit, I think you can make any bike handle well … or with the wrong fit, it will handle like crap.

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  18. rich_mutt

    indeed- there is a lot of emphasis these days on “fitting systems”. i’d also love to hear what your take on them is.


    1. Author
      Padraig

      Rich Mutt: I’ll be taking a look at some of the various fitting systems in the future. I think most of the systems are much better than they used to be.

  19. rich_mutt

    padraid – on stem lengths: i was led to believe that stem choice was something formulaic. if you rode a 51-54 (TT) you chose a 90 or 100mm stem length, if you rode a 55-58 you should be riding a 110-120mm, and 59-60+, you were in the 130mm range. this does seem logical.

    when i chose a stem for a new build, i used this formula. i had some idea of what TT length suited me, and what my cockpit reach should feel like. it felt pretty good, and when i was fit by a professional fitter and coach, he seemed to think i chose the correct stem length. i ride a 54cm TT and chose a 90mm stem.


    1. Author
      Padraig

      Rich: Yes, I also believe that stem length, in general, should be proportional to top tube length. There are exceptions to this, of course, and I’m not talking about Italian PROs riding 49cm top tubes and running 14cm stems. I’m willing to allow a little more latitude on stem length, but that’s the basic idea.

  20. Henry

    Trail might also have an effect on ideal stem length and handlebar width. Higher trail bikes requiring more leverage could do better with wider bars and longer stems then a bike with lower trail. The one caveat being that as trail gets lower more weight should be shifted to the front – so stem height and the bikes geometry all have to be in sync to have everything working in harmony.

    Then there are brifters effect on geometry as they have caused stem heights to get lower as the hoods have become the default cockpit position.

  21. Forrest

    Padraig,

    I appreciate your comments about fork rake and head tube angle and geometry, particularly as they relate to trail. When bikes of various frame sizes are equipped with the same fork, the resulting geometry and bike handling is almost always compromised for riders that require either the smallest or the largest frame sizes.

    For smaller frame sizes in particular, the CPSC standard for toe clearance, often result in bikes that have slack head tube angles and fork with a rake of 43 mm or less. These are bikes have a longer, non-neutral, trail – bikes that don’t want to turn when at speed. This kind of trail would be unacceptable to the majority riders that require ‘standard’ frame sizes, but it is passed on to consumers that require smaller frames.

    In my experience, having a smaller frame and fork that is designed to comply with the CPSC standard for at least 3 1/2″ of toe clearance from the pedal spindle to the front wheel when turned, creates a more hazardous risk to consumer safety, than having the front wheel overlap with a rider’s toe. Think of handling a slow handling bike with a longer trail, while descending on a less than perfectly smooth road, at high speeds, with an unexpected cross-wind.

    I recently decided to purchase a bike with a 52 cm frame with a more neutral 5.7 trail and use a 10 cm stem rather than a 50 cm frame with a longer 6.1 trail and an 11 cm stem. This decision contradicted the conventional wisdom to choose the smaller frame to minimize total weight.

    Finally, there are two additional factors that are easily overlooked. The first is handlebar reach. Reach varies significantly between different handlebars and has the same kind of impact as when a stem is too short or too long. The second is the stiffness of the handlebar and stem. On my older steel Mondonico, I recently switched from a 26.0 mm bar and stem to a 31.7 bar and stem ,and found that a considerable amount of flex in the front end was been eliminated.

  22. Damon

    From playing around with stem length on my MTB in the last year I can tell you that stem length makes a MASSIVE difference in stability of steering. I have ridden a 580mm wide flat bar with:

    a 60mm 35 degree rise stem (35mm)
    a 80mm 10 degree rise stem (70mm)
    a 120mm 17 degree rise stem (97)

    The bike has a 71degree head angle with 25mm sag on a 115mm fork and 38mm offset. This results in the horizontal distances from the steering axis to the centre of the handlebars shown in brackets above. Think of the numbers in brackets as “equivalent” stem length; a number that allows stems of varying rise and length to be compared easily.

    The shortest stem was a nightmare in terms of stability, I crashed several times purely because I could not exert sufficient force on the handlebars to correct the wheel when bumped off line. The in between sixe stem was not too bad, but had moments where you did not feel in control of the front wheel movement. The longest stem however was VERY much more stable than the shorter stems, the wheel tracked true and straight through the roughest sections. It also placed more weight over my front wheel which is not exactly what I want, but there is no disputing that longer stem length gives a much more control.

    I believe that this is because the greatest force on the handlebar is the weight of your two hands pushing forward (call this force F). When the handlebar moves from the straight ahead position the forward force of your hands (F) creates a force acting perpendicular to the axis of the stem that resists the turning of the wheel; call this force Z (Z=F*cos(theta) where the angle theta is the angle through which the wheel turns away from straight ahead) Regardless of stem length the force Z is the same for a given angle theta; HOWEVER on a longer stem the force Z acts on a longer lever.

    So if you look at the stems I tried out the 120mm stem with an effective horizontal length of 97mm can create +/-30% more torque to resist turning of the wheel than the 80mm stem that had an effective length of 70mm! This I believe is why the stem length is so important. To runs shorter stems you need more trail to have similar stability, ie you need a slacker head angle and/or less fork offset(rake).

    Any coments?

  23. Steve

    My Colnago C40 53cm has a head angle of 71.1 degrees. Colnago used to publish the head angle in their catalogs. What makes a Colnago a Colnago is the shallow front end and steeper seat angles relative to other Italian brands.

  24. walter

    fyi…your colnago geometry chart is from pre-2007 production. colnago did a wholesale change on the geometry of all their road-bikes in 2006-2007. i put together a handy comparison PDF showing them side-by-side.

    http://dl.dropbox.com/u/29495821/ColnagoGeometryReference.pdf

    i’d be very interested to get your views on how that change affected the handling of colnagos across-the board, as i was a huge fan of the old-school geometry and would be seriously bummed if the new stuff didnt handle or feel quite the same.

    thx,
    walter from motown

  25. Salsa_Lover

    walter,

    I think you are comparing the geometries for external headsets ( like the C40,C50, EP,EC ) that is the one on your pre-2007 chart with the ones for semi integrated headsets like the EPS, C59 etc ( on your 2007+ chart )where obviously the head tube is longer to accomodate the headset bearings.

    However the EPS was introduced on 2009 if I am not wrong

  26. walter

    perhaps. but i have my doubts.

    why would head-tube length affect *everything* else about a bikes geometry? it doesnt and shouldnt necessarily. i cannot attribute such as meaningful change in a “philosophy” for geometry simply to a move from std to integrated headsets and longer head-tubes.

    the odd way colnago measures frame size (BB center to lower-edge of seatpin clamp) is the same pre/post 2007. yet if you compare those 2 sets of geometry tables and they are different in every way for every size of frame.


    1. Author
      Padraig

      Walter: Head tube length has a huge effect on how a bike handles because it determines, to within a dozen centimeters or so—at absolute max—just how high or low a bicycle’s handlebar will be positioned. Bar height goes a long way (though it’s not the whole story) in establishing just how much weight is on the front wheel, and that is a big piece of the story in how a bike handles.

      It’s important, when comparing head tube lengths, to compare integrated headset bikes to other integrated headset bikes. A bike with a traditional pressed-in headset will have a head tube roughly 2cm shorter, depending on a few design parameters.

      How you measure seat tube length is virtually meaningless. It has no bearing on a bike’s effective top tube length (that is, the distance from the center of the seat tube to the center of the top tube, measured parallel to the ground), which is the most important determiner in a bike’s size. When examining which size bike I will review from a given manufacturer, I look at the top tube length first, then head tube length, so that I know where the bar will be positioned. All the rest is negotiable.

  27. walter

    ok, so the headtube matters in a very meaningful way. that supports my observation that colnago changed the geometry of their entire lineup. their geometry “philosoph”y if you will.

    back to my original question: any thoughts on how this change has affected how new colnagos handle in comparison to the old? if all their headtubes are taller, then surely they must handle differently as you say.

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