BIKE GEOMETRIES

Last Updated: 04/02/2007

UNDER CONSTRUCTION


OVERVIEW

The purpose of this web page is to describe what I consider to be the most important types of bike designs. In my opinion, selecting the right kind of bike is one of the most important decisions when buying a bike. My hope is to provide you with enough information to be able to recognize the different kinds of bikes regardless of the marketing-speak you may encounter in catalogs, web-sites and sales floors.

Some caveats are in order. First, these categories are crude and should not lead you to think that all bikes within any category are the same. For example, different Audax bikes are the same just like different sports sedans are the same. That is, there is a significant amount of important variation within each category that I describe.

Second, this list of categories is not meant to be exhaustive. There is a seemingly infite number of variations of bike designs, just as there is among car designs. Instead, the goal of this page is merely to describe what I consider to be the most important basic design types.

Lastly, the definition of and the boundaries between the categories are as I see them. Many experienced cyclists and bike designers (whose opinions you should listen to more than me) may disagree with some or all of what I write.

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A WORD ON FRAME GEOMETRY

The act of describing the different frame designs will require that we discuss the lengths of various frame tubes and the angles at which they are joined together. We refer to this as the geometry of a frame and it's a topic that almost always leads to disagreements among bike enthusiasts. To avoid confusion, I should discuss how I think about geometry.

I use geometries to distinguish between different kinds of bikes. Bikes designed for the same purposes generally end up being more like each other in terms their geometry and are generally more different from other kinds of bikes designed for other purposes. This is useful since marketing literature often seems designed to create confusion. Use the descriptions of the bike types on this page to help you select the kind of bike that will best suit your needs. Once you decide on a kind of bike, note the basic geometry that defines that class of bike. Then you can begin to use the geometry information published by the manufacturers to help you find bikes within that category. Just as importantly, you can use the basic geometries to eliminate designs that won't serve your purpose, despite of the lofty claims of the manufacturer.

In terms of classifying bike designs, the first thing I look at is the length of the chainstay (CS), which is measured from the center of the bottom bracket to the center of the rear axle. The biggest effect of the chainstay length is whether or not the bike is suitable for carrying panniers. For most people, chainstays of 43 cm are needed to prevent the rider's heels from striking the panniers. Riders with large feet (like me) will generally need 44 cm or more. Chainstay length will also effect the overall wheelbase of the bike, which has an effect on handling. And short chainstays can limit the width of the tires that can be used since the chainstays are forced to come close together at the bottom bracket.

The reason I look at chainstay length first is that it is generally a good way to distinguish between bike types. Typically, road bikes have the following chainstay lengths:

The second figures I look at are the headtube angle (HA) and the fork offset (FO; a.k.a. rake). Taken together, these 2 figures have the biggest effect on the handling characteristics of the bike. Roughly speaking, when the HA is steep and FO is short, the bike will be stable at speed and will handle very crisply. We often refer to this as a "steep" front end and one typically sees this on road racing bikes. Conversely, when HA is relatively shallow and FO is longer, the bike will generally handle loose road surfaces better and will handle front end loads better. We often refer to this as a "relaxed" front end and we typically see this on touring bikes.

Some caveats are in order. First, I would advise against focusing on any particular geometric element in an attempt to understand how that element affects the behaviour of the bike. The handling and performance of a bike frame is the result of the interplay of many variables. Discussions of the affect of any single variable almost always leads to disagreement. My suggestion is to think of geometries as a comprehensive whole that, when taken together, define that particular bike.

I would also advise getting to hung up on formulas such as trail or flop-factor that are derived from a set of geometry variables in an attempt to predict handling characteristics. My experience with such functions is they provide little extra insight over what can be better seen by simply looking at the variables themselves. I have 2 problems with these types of functions. First, it is very common to see very different variable values yielding the same function value. For example, bikes with different HAs can end up with the same trail or the same flop factor. And, because of the different HAs, these bikes will behave very differently from each other regardless of the fact that they have the same trail. Second, these functions are generally based on just a few of the variables while ignoring others which may have a subtle yet important affect on the quality the function is attempting to model. For example, it is possible to find 2 bikes with exactly the same HA and FO (and thus the same trail) but with different wheelbases. As a result of the different wheelbases, the bikes will handle very differently despite of the fact that they have the same trail. Again, my recommendation is to look at the geometries wholistically.

The last note to make is that I will generally describe the geometries of the bike types in terms of a 56 cm or 58 cm frame size. Typically, modern frames will vary the HA and seat tube angle (SA) for different sized frames within the same model. Generally, HA will increase and SA will decrease as the frame size increases. Most bike manufacturers will publish geometry figures for all frame sizes but occasionally a manufacturer will only publish the figures for a single size. The most commonly used sizes seem to be 56 cm or 58 cm.

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ROAD RACING BIKES

Modern Road Racers
  • Chainstays approximately 41.0 cm
  • Head Angle approximately 73.5
  • Fork Offset approximately 4.3 cm
  • Classic Road Racers

  • Chainstays less than 41.5 cm
  • Head Angle approximately 73.0
  • Fork Offset approximately 4.5 cm
  • By far the most popular and most commonly seen bike today is the road racing bike. They are fast and fun. Racing bikes sprint like mad and climb great. They will allow you to dive into a fast corner with confidence. But the longer the ride is, the better shape you had better be in. The responsiveness of a racing bike comes at a cost. The very same attributes that make it quick handling make it hard to keep going in a straight line when your shoulders and neck get tired on longer rides.

    If your primary mode of riding is fast fitness riding and club riding in pace lines, you will probably be the happiest on a racing bike.

    And if you hang out with folks who have racing bikes, you might be happiest on a racing bike no matter what. The reason I say this is that peer pressure among cyclists is amazingly strong - overwhelmingly strong at times. It's very common to see even the most experienced cyclists adapt and change just based on what others in their circle of friends are doing. If none of your friends race but they all own racing bikes and you buy something other than a racing bike, you should prepare yourself for non-stop advice on why you should get a racing bike even though you may be able to ride farther, faster and in more comfort on a different style of bike depending on your riding style.

    There are 2 basic variations of the racing bike design. I call them the modern road racer and the classic road racer, but you won't see those terms used in this way anywhere else. They differ only slightly. The modern road racer has a bit shorter wheelbase and a bit steeper front end geometry.

    Nearly all racing style bikes are designed for short reach brakes. This means that you will be limited to tires no wider than 25mm. Racing tires are often in the 18mm to 20mm range and training tires are typically in the 23mm range. None of these are really wide enough for dirt roads, rough road surfaces or day-in/day-out commuting.

    Short reach brakes will also make it hard, if not impossible, to put fenders on the bike. To make things worse, nearly all race bikes have no eyelets at the drop outs making it much harder to attach fenders. If you aren't looking to ever put fenders on your bike and if you will never, ever, ever put a rack on the bike for any reason, you are a candidate for a classic road racing bike. Otherwise, you should probably look at other designs.

    There are a few other variations of the racing bike that bear mentioning. The are increasing number of racing bikes like the Specialized Roubaix and Trek Pilot that have a slightly more relaxed HA in the 72.5 range and more FO in the 5.0 range. This design promises to give more comfort over long distances but due to the short chainstays and tight tire clearance, these bikes remain limited in their versatility. Another type of racing bike sometimes seen is the time trial or triathalon bike. These bikes are specially designed for the time trail event and are not well suited for general riding.

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    EVENT/AUDUX/CENTURY BIKES

  • Chainstays approximately 42.5 cm
  • Head Angle approximately 73.0
  • Fork Offset approximately 4.5 cm
  • Bike companies are finally answering the needs of the non-racing cycling enthusiast who wants to ride but who also wants to tackle distance events like century rides. These so-called event bikes are essentially racing bikes that have been relaxed a bit for better comfort and stability over long distances.

    A crude way to differentiate an event bike is to look at the chainstay length. Most have chainstay at or slightly above 42 cm. You will see other similarities among event bikes that further differentiate them from racing bikes. Most notably you are more likely to see head angles at or below 73 degrees (racing bikes are often around 74 degrees) and fork offsets above 4.5 cm (racing bikes are typically between 4.0 and 4.3 cm). The relaxed angles and longer wheelbase allow the bike to track straighter, especially on rough road surfaces. It is this property that makes an event bike more comfortable over long distances.

    Another design aspect common (but not universal) among event bikes is a slightly shorter top tube, which allows a bike to be fitted for a more upright "French Style" fit with the bars at the same height as the saddle. Many long distance cyclist find this sort of fit more comfortable when the day's milage creeps up into the triple digits.

    The utility of event bikes as all-rounders depends on the brake set up and the rider's foot size. Some event bikes are set up for short reach brakes, which limits tires to 25mm. Great for going fast but not so great for touring or commuting. The rider's foot size comes into play when you consider carrying panniers. Most (but not all) event bikes have braze-ons to accept a rear rack and fenders. But riders with large feet may find that the sub 43 cm stays will cause their heels to strike their panniers. My sense is that event bikes are designed for fenders and, at most, a rear rack that either carries a top-mounted rack trunk or that supports a Cardice style large seat bag. Rack trunks and large seat bags are often used by century riders to carry extra food and spare jacket. These aren't over night loads; just enough make it through a full day ride without having team car handing you stuff like they do with Lance.

    I am pretty strongly biased against racing bikes as everyday riders for most mortals, so it will come to no surprise that I think most non-racers would end up happier on event bikes. Actually, my bias goes further than that since I think the Sport Touring bikes are really the best all-rounder designs. Never-the-less, I'm thrilled to see the event bike gaining in popularity as it may show that the buying public is starting to redefine it's understanding of performance when it comes to bikes. Performance should mean "it allows me to reach my fullest potential", not "it looks like Lance's bike". The alpine ski market went through this shift in the 1990s, thankfully. Prior to the '90s, performance meant you purchased a racing ski. Today, racing skis are seen as unruly and punishing tools for the hyper-spcecialized activity of running gates. Today's recreational skier is sking on high performance skis for non-racers. Hopefully, the road bike market will follow a similar shift

    A critical part of this shift is the setting aside of the false idea that an event bike is somehow a de-tuned or sub-par racing bike. People do want to be like Lance and the desire to make that association with their bike purchase is a strong one. The reality is that event bikes are the highest performance bikes when you consider going long distances and roads might get less than perfect. As witness to this, consider the Specialized Rubaix, which many point to as making this design legitimate again in the American market. The Rubaix is named for the Paris-Rubaix race which is notorious for its long distances and rough cobble-stone roads. Today, if you look closely, you will the Rubaix under elite racers on the Tour de France during long hard stages.

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    SPORT TOURING BIKES

    Modern Sport Touring
  • Chainstays between 43 cm and 45 cm
  • Head Angle approximately 73.0
  • Fork Offset approximately 4.5 cm
  • Classic Sport Touring

  • Chainstays between 43 cm and 45 cm
  • Head Angle approximately 73.0
  • Fork Offset approximately 5.5 cm
  • The ideal of the sport touring bike is to combine the light weight and speed of a racing bike with the pannier carrying capability of a touring bike. A sport tourer should let you compete in a citizens race or the fast pace line of a club ride one week, then do a century ride the following week and then with the addition of racks and bags, a week-end tour the week after that.

    Materially, the differences between a sport touring bike and racing bike are very small. Sport tourers typically have a) longer chainstays, b) brake bosses and fork crowns that allow for 33mm tires to be used, and c) at least one set of fender eyelets on both the front and rear. It is hard to argue that these very slight differences create a meaningful weight penalty compared to racing bikes. In contrast, true touring bikes will often use slightly heavier tube sets compared to racing bikes and sport tourers.

    I think sport touring bikes should be further separated into two sub-groups based on their front-end geometries. Modern, new-school sport tourers use forks with a rake of 4.5 cm or less, which produces a lot of trail. On the other hand, classic, old-school sport tourers have 5.0 cm or more of fork rake which produces less trail.

    I love the way classic sport tourers handle. But, A big caveat here... While I've racked up thousands of miles over many years on modern racing bikes, I've not spent time riding modern sport tourers with short 4.5 rakes. So, I can't compare them. That said, I must report that I notice a big difference in the way classic sport tourers handle compared to racing bikes. The short version is that I can ride farther, faster and in more comfort on a sport touring bike. I think this is a result of 3 handling differences.

    The first is riding no hands. My bud who is into kayaks talks about initial stability versus secondary stability. This very much describes what I feel. On my racing bike I can take my hands off the bike and the bike sits nice and solid.... initially. But as soon as I start to lose control, the bike really wants to veer off and dive into a turn. I NEVER ride that bike no hands with confidence. Adjusting pit zips, unwrapping pop-tarts.. these are all chores on that bike.

    On the classic sport touring bikes (which have all had HA = 73 and FO = 5.5) the bike feels a teeny bit wobbly when I first take my hands. But... there is a secondary stability that is very noticeable. The bike basically wants to go more or less down the road. I can ride this bike around the block no hands if I wanted to.

    The second place I notice a difference is in the cornering. My racing bike corners with a sense of precision. It feels like the front tire is on a rail. The classic sport tourers aren't as responsive (or overly twitchy) as a racing bike. But, having ridden both, this isn't a big part of what makes a bike feel sporty. Give me some reasonably light wheels, some fast tires and a BB that doesn't sway too much so I can sprint and climb and I'm happy. "Cornering" for me is mostly making turns at stop signs, not diving into hair pin turns in the Alps while riding elbow to elbow with the rest of the peloton. I can understand why racers prefer the sharper, more precise handling of a steeper front end. But for my type of riding, its just not that a big improvement and worse, it offers a bad trade off.

    The third and most important place I notice the difference is on rough roads and bumps. With my race bike, rough road or bumps would cause the bike to want to veer to one side or the other. This is very much like riding no-hands in the sense that the bike really wants to go right or left, not straight. With the classic sport tourer, the bike just hunkers down and rides right up and over the bad road and really and noticeably wants to keep going straight. I ride into rough patches with infinitely more confidence on these bikes. I can peddle more smoothly since I'm not fighting the bike for control. My upper body is less tired at the end of longer rides.

    As much as I love the sport touring bike, and the classic sport tourer specifically, they aren't that common and most people won't want to buy them, which is regrettable. At the time of this writing, the Velo Orange Randonneur is the only bike of this type on the market that I am aware of. The Mercian King of Mercia comes close with 4.8 cm of FO.

    I believe the sport tourer was squeezed out of the market in an attempt to reduce overhead costs when the mountain bike first hit the scene at the end of the 1980s and beginning of the 1990s. This might explain why they became hard to find. But why are they still hard to find?

    I believe the reason has to do with perception and why most people buy bikes. Racers buy racing bikes, as they should. But what about non-racers who would be better served over the course of a decade by a more versatile sport touring design? I strongly believe that most non-racers want to project an image (to themselves or to others, it's all the same) of being fit and fast. Racing bikes are supposed to be fast. They look fast and the magazine reviews say they are fast. Sport tourers on the other hand, look a bit different. The bars look higher and there is more room between the tires and the frame. In the ski American ski industry, racing skis used to dominate performance ski sales. Today, performance skis generally mean high end "all-mountain" skis and racing skis are viewed (correctly) as hyper-specialized tools best left on the race course. Perhaps the American bike market will reach the same point at some time in the future.

    In my opinion, non-racing enthusiasts who are looking to ride faster over longer distances including centuries and other events will be able to reach higher levels of performance on a sport touring frame than they could on a racing frame. And as riding becomes a part of the cyclist's lifestyle and as riding begins to span decades, the sport tourer is, in my opinion, the most versatile road frame imaginable. For it can handle commuting, fast riding, rough road riding and touring with equal aplomb.

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    CYCLO-CROSS BIKES
    Chainstays less than 44 cm

    I know very little about cyclo-cross bikes other than what I've seen and read. I am very glad to see any option in the bike stores other than racing bikes and mountain bikes and today, cross bikes appear to be more common than sport touring or touring bikes. Any way you look at it, this is a good thing, in my opinion. Options are a good thing and it's great to see such a versatile design gaining in popularity.

    One of the primary benefits of a cross bike is its dual nature. It is literally designed to be ridden both on the road and in the dirt. How cool is that? And cross bikes are typically built with cantilever brakes and clearance for fatter tires, so they have room for the cushy tires and fenders one would want for commuting and touring. That's fantastic.

    There are a couple of things to watch for however. Generally cross bike have chainstays around 43 cm long. This may cause riders with big feet (like me) to hit their heels on panniers. Secondly, there are cross bikes and then there are cross bikes. That is, some cross bikes are built up as super specialized competition bikes and loose some of their versatility. For example, some competition bikes will lack rack and fender mounts, will have wide range double cranks (as opposed to triple cranks) and will non-standard cable routing. All of these design choices make perfect sense for cyclo-cross competition. But they should be considered with suspicion if your goal is to choose a bike to fill a wide range of uses.

    If you are attracted to cross bikes because of the appeal of being able to ride in the dirt, you should consider that frugal cross riders will use older sport touring bikes (or even road racing bikes) with center-pull brakes installed. Center-pull brakes offer good tire clearance and great stopping power, especially when used with modern brake pads. This is to say, you don't need a cross bike just to ride in the dirt. All you need is tire clearance for fatter tires and decent brakes. Is it better to put 33mm knobbies on a more road friendly sport tourer to run it in the dirt, or is it better to put fast 23mm slicks on a cross bike to go a fast club ride? Different riders will decide differently based on their preference. The important thing to remember is that there is a choice. Sport touring and touring bikes may provide more than enough dirt riding capabilities and may be a more versatile design.

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    TOURING BIKES

  • Chainstays greater than 45 cm
  • Head Angle approximately 72.5
  • Fork Offset approximately 5.2 cm
  • My first experience with a touring bike was the old Fuji America which was sold in the late 1970's. It was called a touring bike because it shipped with Suntour bar-end shifters and a triple crank. It still had a reasonably tight rear triangle with 43 cm stays and came with caliper brakes, not catilever brakes. This was before super-specialization became the norm in touring bikes and today, I would actually classify the old America as a sport touring design. But then, this was before we regularly made that distinction.

    Somewhere along the way, Miyata started selling a dedicated touring bike with cantilever brakes and by the mid 1980's Trek's touring bikes had cantilevers and very, very long chainstays. Some of those bikes had the reputation of being underbuilt and so in response touring bikes started getting slightly thicker (and heavier) tube sets to withstand the loads of heavy touring.

    The modern touring bike is ideally suited for loaded touring. It is very stable under load and it can withstand the strain of a full load of gear. If you are going to ride across a continent, there is no better tool. However, if you also want use that bike for fast training and club rides, you might find a touring bike too heavy and too sluggish. But then again, you might not. Especially with the aluminum touring bikes like the Cannondales.

    My preference is clearly with the lighter sport touring bikes. I would rather ride across the country on a sport touring bike (and have) than ride a touring bike for the rest of my life. In my description of sport touring bikes, I discussed the difference between low trail bikes and high trail bikes and noted that I very much prefer the greater stability and straight tracking of classic low trail sport touring bikes. The modern touring bike is the same, only more so. Perhaps, too much so. It is not uncommon to find modern touring bikes with very relaxed head angles and lots of fork offset. The "feel" of a bike's handling is a very subjective issue where rider preference trumps all other considerations. This said, modern touring bikes represent the extreme in terms of riding characteristics. If a classic sport touring bike is noticeably better at tracking straight, the touring bike is way better, especially when bikes are loaded down with front panniers. Some people (like me) might find this sort of ride too sluggish in its handling. But I will be the first to admit that others would say the same thing about sport touring bikes (which I love) compared to racing bikes. You are the only one to say. Ride a few and decide for yourself.

    One thing to note about touring bikes for commuters is the great value they might represent. They are typically sold with many of bells and whistles that you would want to add to other bikes to make them usable for rugged commuting duty like heavier wheels, wider tires and racks to carry panniers. So,if day-in/day-out riding for you means the rough rigors of commuting or if dirt roads are a regular part of riding life, a touring bike is a very, very good option to look at seriously.

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