The Problem with Numbers

Seat Tube Angles
Zoom

After growing through their first couple of bikes, most people start looking at specs pretty carefully before making a purchase. This is particularly true of transportational bicyclists who may not have access to a good selection of potential models at their local bike shop. Because of this lack of support at the local level, online comparison shopping takes on an important role in the process of identifying just the right bike for our intended uses. And because bike design in the commuting/transpo realm is still all over the map, and not anywhere near as codified as it is with road and mountain bikes, understanding spec sheets and geometry charts is an important part of the selection process. And yet, while I acknowledge that geometry charts can tell us a fair amount about how a particular bike may ride, I’m rather dubious about putting too much emphasis on one particular number. Let’s look at the seat tube as an example.

After growing through their first couple of bikes, most people start looking at specs pretty carefully before making a purchase. This is particularly true of transportational bicyclists who may not have access to a good selection of potential models at their local bike shop.

In the above diagram, the seat tube length is the same for all three geometries. When looking at numbers in a chart, it would be easy to assume that different bikes with the same seat tube length also have the same standover height. But as is illustrated above, standover height, as well as saddle height, saddle set-back, and top tube length are all affected by seat tube angle. In other words, assuming the other dimensions remain the same, as the seat tube angle slackens, a bike becomes “shorter”, and as the seat tube angle steepens, a bike becomes “taller”¹. Other specifications such as bottom bracket drop and wheel diameter also affect standover.

The same type of thing happens at the front of the bike. Again, assuming the other dimensions remain the same, as the head angle slackens, the top tube becomes shorter and the handlebars move closer to the rider, whereas a steeper head angle results in a longer top tube and more reach to the bars.

Unfortunately, none of this provides a definitive answer to the most important question, “What is best for me?” But, it’s important to remember that isolating a particular number in a geometry chart, and using that number as a benchmark in the selection of a bike or bike size, is not a good idea. It’s better to take a holistic look at frame geometry and try to comprehend how all the numbers work together, which, unfortunately, is not a simple thing to do.

Of course, the best bet is always a real test ride on a real road. Alas, if only we had more widespread support for commuter and transpo bikes at the local level, this problem with numbers would be moot!

1. This is the design element behind Electra’s patented “Flat Foot Technology“.

23 Responses to “The Problem with Numbers”

  • kanishka new england says:

    fittings change over time even for one bicyclist. seems like emphasis should be micro + on-the-fly adjustability of seat setback, handlebar height, angle (and even sweep?). with some basic frame sizes. i think the dutch bikes handle the handlebar adjustability

  • Tim in SB says:

    You’re right Alan, it almost takes a Phd in cycling to be able to check specs on paper and know how they will affect a ride. I feel like I have a decent grasp of it finally, but I’m sure there is a lot more to learn.

    It took me years to realize this, but seat tube angle happens to be one of the most important aspects of geometry for me. I can’t get comfortable on a bike with more than a 71 degree seat tube angle. Unfortunately, STAs that slack are rare in today’s newer bikes, even the ones designed for upright riding. Electra’s got it right on all of their bikes and the Breezer Uptowns come at 71 degrees. Hopefully more bike manufacturers will let go of the steeper performance oriented angles on their comfort upright bikes and embrace the slack. I even have the equivalent of a 69 degree STA on my touring bike.

  • grrlyrida says:

    You’re absolutely right Alan. My mass-produced carbon road bike was bought over the internet and I’ll never do that again without riding a bike first. Plus just as you stated, many of the LBS’s around LA have racing bikes like I have now and I don’t even race. Sure I see a few townies and mass produced hybrids, but not the type of transportation bikes I would like to see. I had to make a side trip while attending a wedding up north just to try out a quality transpo bike.

  • Helton says:

    Wow, where did you get that drawing from? Did you make it yourself? Very beautiful!

  • bryantrj@gmail.com says:

    I own a modern MTB style transpo bike with steeper angles and I’ve previously owned an Electra Townie and a Breezer Uptown 8. I’ve never been able to get comfortable with the steep angles. The Townie geometry was too laid back for me — and lacked the bike-like handling feel. The Breezer geometry was okay, but it wasn’t until I got into fixing up old bikes that I found my sweet spot at 68-70 degrees — and maybe 71 if all else is good.

  • Alan says:

    @Helton

    I created the drawing in Illustrator. I’m glad you like it… :-)

    Thanks,
    Alan

  • Spencer says:

    Another thing to remember is there may not be just one bike that’s right for you! I’ve enjoyed riding many different bikes with different geometries that have been comfortable. (this may have something to do with my constant search for yet another bike to ride)
    great post!

  • Lovely Bicycle! says:

    The more I learn about bicycle geometry, the more I realise that I need another 5 years of sustained interest to truly understand everything… And I find that both frustrating and exciting, in more or less equal measure.

  • Pete says:

    Nice illustration.
    Good post about the pitfalls of going “by the numbers.” In fact, you can have two bikes with nearly identical numbers and they still may not ride the same. Not only do all the numbers affect each other in complex ways, but the frame material and construction, and the spec of the bars, saddle, wheels, tires etc all play into the ride characteristics.
    It’s very curious to see how the industry (until recently) has pushed “racy” geometry and steep seat tube angles with one hand, while selling a ton of “super set-back” seat posts to effectively slacken the angle with the other hand!
    I wish I could figure out how to make money owning a bike shop ;-) – there need to be more that cater to the utility/transpo cyclists.

  • Alan says:

    @Kanishka

    I like the idea of on-the-fly adjustability, but I’m afraid the obsessive fiddler in me would go crazy on a bike with those capabilities… :-)

  • Alan says:

    @Tim in SB

    The predominance of overly steep seat tube angles is a result of racing’s influence on mainstream bike design over the past 20 years or more. Fortunately, it seems a growing number of manufacturers are looking at Euro/Dutch bikes for inspiration when designing their modern city/commuter bikes.

    Alan

  • Alan says:

    @Spencer

    “Another thing to remember is there may not be just one bike that’s right for you!”

    So true!

  • Alan says:

    @Lovely Bicycle!

    “The more I learn about bicycle geometry, the more I realise that I need another 5 years of sustained interest to truly understand everything… And I find that both frustrating and exciting, in more or less equal measure.”

    I hear you! I’ve been staring at geometry charts for more or less 30 years and I still don’t feel like I can discern precisely how a bike will ride based purely upon the numbers. I suppose it’s prudent to think of geometry charts as rough guides (at best).

    Alan

  • Don says:

    Thank you Alan! I’ve been waiting for someone to address this more directly. I’d love to see a series that talks about the main parts of the frame and how differences affect ride. This will allow people to factor in their own anatomy and riding styles better to narrow down choices. (But like the pharma disclaimers, it would not be a subsitution for test rides.) It’s also another way to celebrate the bicycle in all its deceptive simplicity.

    I agree heartily that it is easy to get into a mindset of thinking that there is one ideal all-around geometry, at least for the idealist in us. It is only after messing with my two bikes over an extended period that I have embraced the unique rides of each. Part of the fun and the propioceptive pleasure is mixing it up. One favors pure efficiency and the other is more practical about carrying loads and sitting upright, but both are very comfortable–after years of experimentation, of course.

  • Roland Smith says:

    Don’t hang yourself up on those numbers.

    Here in the Netherlands it is very unusual (for general purpose / commuter bicycles) to see any number other than the frame size listed on the manufacturer’s webpage! I just double checked on the Gazelle and Batavus website. Almost no numbers except frame size.

    Millions of people ride these bikes, often every day. So I don’t think those numbers are that important. Although I’ll admit that most local bike shops here probably have a much wider selection of this kind of bike available, which makes picking one that’s right for you easier.

  • Monday bike roundup » Cyclelicious says:

    […] EcoVelo on frame geometry and bike fit. […]

  • Lovely Bicycle! says:

    Roland Smith – I think the reason you don’t see those specs on Dutch bikes, is that they have more or less the same geometry and feel similar to ride. A Gazelle may have more extras than a Batavus, but they are still the same general type of bicycle. American commuter bicycles are much more varied, and the ride quality can differ drastically.

  • kfg says:

    Looking at the numbers is a bit of looking through the wrong end of the telescope and trying to reverse engineer from that. When designing a bike you don’t (or shouldn’t) start by thinking something like “I want a 74 degree seat tube angle.”

    What you do is lay out where you want the bits relative to each other, then draw the lines between them and the angles are whatever the angles are. Seat posts with large set backs or bottom brackets on forward extensions are, by and large, the result of, for whatever reason, someone simply being unwilling to draw a simple, straight line between the bits.

    However, one who wishes to “design” an ideal transportation bike frame could do a lot worse than simply obtaining a Raleigh sportster and knocking out copies of it. The design did not reign supreme for decades without reason.

  • edde says:

    after riding recumbents for 20 plus years, riding an upright just feels weird. RANS crank forwards are the versatile upright variants that works for me. Check out RANS CF slack seat tubes. I’d love to hear others comment on this design;-) edde

  • Roland Smith says:

    @Lovely Bicycle:

    Thanks for pointing that out. I hadn’t realized the US market was so fragmented. I do wonder if it is a good thing though, seeing the discussion here.

    The standard “Dutch bike” seems to be a good enough general geometry if you don’t mind stepping off when you stop. OTOH, I’d say that most people here ride with their saddle too low, so they can get their feet on the ground easily while staying in the saddle. In that respect the RANS crank forward bikes seem a lot better.

    It is one of the reasons why I personally prefer recumbents. But the main reasons are comfort (no saddle-sore) and less wind resistance.

  • Joseph E says:

    Alan, can I use your image, with attribution and a link back to this post, on Bikes For The Rest Of Us? It’s the best one I’ve seen to show the importance of seat tube angle.

  • Alan says:

    @Joseph E

    “Alan, can I use your image, with attribution and a link back to this post, on Bikes For The Rest Of Us? It’s the best one I’ve seen to show the importance of seat tube angle.”

    Sure, go right ahead.

    Thanks,
    Alan

  • Yan says:

    Good subject! In contrast to the majority of repondents I have started looking at a STA that fits my wierd body shape – If I am too set back from the vertical I experience knee problems, add to that large feet and we get toe overlap issues (more numbers!)

 
© 2011 EcoVelo™