A Study in Geometry

A Study in Geometry

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.

 

Originally posted October 21, 2009.

 


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13 comments

  1. Tominalbany

    All said, I think you really need to know how a particular geometry feels and responds before you can compare to another just by the numbers.


    1. Author
      Padraig

      It took me a long time before I could look at a geometry chart and get a feel for how it correlated to previous experiences I’d had. And all of this can be thrown off if the bar is too high or too low. There does seem to be a sweet spot where there’s enough weight on the front wheel to impart a strong sense of how the bike is behaving that makes a big difference in how confident a rider will be on a descent. But all other things being equal….

  2. Greg B

    FWIW. I’ve had my E1 since 2007 (if I recall correctly) and compared with other bikes I’ve owned in the intervening years (Giant, Specialized, Cervelo, Ritchey, more Specialized), in my opinion the E1 is the most stable descending bike I’ve owned. I’m not going to speculate why – I’ll leave that in your capable hands. While it’s pretty stiff and a bit heavy, it is the bike that has fit me the best and is wonderful to ride. It’s not a crit machine; instead the handling is the most predictable of any of my bikes. Thank you Ernesto!

  3. Chris Hobbs

    I am a big fan of stack and reach as opposed to tube lengths. Just moving the saddle around can generally fix variations in seat tube angles, and head tube length is no longer a good comparator in an age where forks are getting taller to clear bigger tires. My Open has a very slack seat tube, but that is because it is designed for a non-offset post. The top tube length in this case is much longer than I would normally ride, but the fit works.

    I have noticed that Specialized, at least, and likely others, are increasing BB drop to adjust for larger tires. I discovered it accidentally/unpleasantly on my favorite windy road that I used to be able to pedal through. And different tire sizes will impact trail, though not by enough that I can notice it.


    1. Author
      Padraig

      I’ve taken some down time so sorry for the slow responses to comments that will now come flooding.

      Stack and reach are critical to good fit, but a completely different animal than actual frame geometry. Geometry is about how the bike handles and that’s mostly independent of the frame’s size.

      The Open is a really interesting bike and that long top tube you reference is a sign that the bike is in some significant ways more of a mountain bike than a gravel bike; it benefits from a short stem to make it handle right, but it doesn’t feel like other gravel bikes I’ve ridden. And, yes, BB drop has increased some, which shows that product managers are paying attention; it’s a great example of why Specialized is renown for making bikes that handle well. You touch on a really interesting point in that many times a small adjustment to one dimension or another won’t register above the threshold of perception, but as part of an overall geometry package can help give a bike better manners. It all adds up.

  4. BigWheel

    I have a 2015 Ritchey Break-Away steel, cyclocross frame and recently upgraded the tires from 28mm to 38mm. I don’t use it for cyclocross but rather for road riding. (My initial plan was to use it just for travel, but I enjoyed the ride so much, it became my everyday ride.) I was pleasantly surprised at the noticeably more stable handling, but also noticed i needed to “initiate” (countersteer?) a turn more aggressively. Obviously the trail increased from the larger tires, but I hadn’t considered that the change of the distance from the bottom bracket to the ground may have also had an effect as obviously the BB drop did not change. Great article, thanks.


    1. Author
      Padraig

      Glad you liked that piece; thanks!

      Part of what you’re experiencing also owes to the fact that the bigger tires weigh more and increase rotational momentum, making the bike more resistant to steering input. Wild stuff, huh?

  5. Michael

    I’m curious about the effect a long Chain Stay length (longer wheel base) on stability or otherwise. I have a steel Rivendell Rambouillet that is the most stable descending bike I’ve ever ridden and my favorite handling bike overall (I’ve owned several high end race bikes). 62CM frame, Seat angle: 72, Head Angle 73, Fork Rake 4.25, Top Tube 59, Chain Stay 44.5, BB Drop 7.7. Maybe it’s the combination of long wheel base and BB drop. This bike has a very long head tube (21.2 cm), 12 cm stem, and is designed to be ridden with the bars high (level with seat or higher) although I have them lowered as much as possible. It’s a heavy bike and seems slow on climbs so not one I generally take to fast group rides but it’s great for supported touring. Maybe in more capable hands it would be slow handling but it sure seems perfect to me for general riding. Wondering if there is a downside I don’t understand as it seems there is a big emphasis on short chain stay length for race bikes. Any thoughts appreciated.


    1. Author
      Padraig

      Grant Petersen has a long history of making bikes that have terrific handling. That his bikes handle so well is no accident. His mantra might be summed up by one of the bikes he designed, called the “long and low.” Drop the BB and increase the wheeeeeeeelbase and you’ll get a bike that is calm on the drop. The thing to keep in mind is that chainstay length is kept in proportion to wheelbase, so as chainstay length increases, so does front-center in order to keep the rider’s weight distribution optimal. Short chainstays are a thing on race bikes because if you want to quicken a bike’s handling you want to both decrease trail and wheelbase. Think Mini Cooper.

  6. Fuzz

    Wheel flop has become a driving issue for me. As road and mountain bikes have gone to slacker front ends, low speed handling has suffered. What works at 35 mph isn’t necessarily good for climbing at 7 mph. The steering feels loose and less connected. As I look to buying a new MB, I find myself drifting towards races bikes to get that more intuitive, connected feel.


    1. Author
      Padraig

      That’s become a real issue. And this proves why it is really important for bike companies not just to give the head tube angle but also the fork rake. You can slacken the head tube angle to get weight off of the front wheel, but if you want to make the bike climb well, you need to decrease the fork rake some to avoid awful wheel flop. Scot Nicol over at Ibis writes well about this; the Ripmo has been praised for its handling, precisely because they cut fork rake when the head tube angle got slack.

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