How to understand bike geometry

Bike websites and magazines often feature diagrams or tables listing frame dimensions and angles. This is what those numbers mean.

Bicycle geometry affects how a bike fits and rides. It doesn't tell you everything; components and materials are at least as important. And it's less prescriptive than some would have you believe. Bike geometry grew out of practical considerations rather than pure maths. To fit comfortably, the saddle went here, the bars went there, and the front wheel had to be there so it wouldn't hit your feet. Nevertheless, the numbers do tell you useful things about what a bike will be better or worse at, and whether it will suit you.

Will it fit me?

In the days when bikes had horizontal top tubes and short seat posts, the seat tube length told you whether you'd be able to reach the pedals and stand over the frame when you stopped – in short, whether it would fit. So that's how bikes were sized. This idea hasn't died, despite the fact that most bikes now have a downward sloping top tube and 200mm or more of saddle height adjustment.

Assuming that you can comfortably stand over the frame, the most important frame measurement for fit is the effective top tube length (ETT). This is the length the top tube would be if it ran horizontally from the top of the head tube to the seat post. It tells you how stretched out you will be on the bike, subject to a given handlebar type and stem length.

Once you know the ETT of a bike you find comfortable, you can use this to size up other bikes with the same kind of handlebar. If your 600mm ETT hybrid feels just right and you're in the market for a new one, you want one with something close to a 600mm ETT, irrespective of whether the bike is described as Medium or Large or has 530 or 550mm seat tube. To fine tune the fit, see the Bike Fit Basics article in issue 11 of Cycle Commuter. And don't forget you can try for size before buying at your local bike shop.

Frame angles

Seat tube angles range from about 70-75 degrees. A steeper angle moves the saddle forward while a shallower angle moves it back. Steep is fine for racers. It allows them to ride hard in an aero crouch, as the more open hip angle doesn't constrain breathing or power transfer. It's not so good for more relaxed riding as it transfers more of your weight from your backside to your hands, which can cause aches and pains.

You can change the effective seat tube angle by sliding the saddle forward or back in the seat post clamp, or by fitting a different seat post with more or less 'layback' (the distance the clamp is behind the seat post centreline).

Head tube angles range from about 66 degrees (gravity-oriented mountain bikes) to 74 degrees (razor-sharp road bikes). Other things being equal, a bike with a steeper head angle with steer more sharply and a bike with a slacker head angle will be more inclined to travel in a straight line. Small-sized bikes often have a slacker head angle too, not to change the steering (although it does) but too keep the front wheel from hitting the rider's feet.

The head and seat tube angles on bikes with suspension change as the suspension compresses. Riders cope easily with this, which shows that small differences in geometry aren't critical.

How a bike steers

The biggest influence on how a bike steers is not head angle alone but a measurement that head angle helps determine: trail. This can be calculated if it's not listed. Trail is the distance that the contact patch of the front tyre on the ground trails behind a line drawn through the steering axis (the head tube) to the ground.

Trail figures for bikes range from the low thirties to the high nineties in millimetres, with sixty-something being typical. Lower is good for a more immediate steering feel but can feel 'nervous'; higher is for good for holding a line on bumpy trails but can feel 'lazy'.

These things produce more trail: a larger diameter wheel; a shallower head angle; a smaller amount of fork offset (which is the distance between the fork dropouts and a line through the steering axis). A smaller wheel, steeper head angle, and increased fork offset all reduce trail. Have a look at the diagrams of trail on Wikipedia to see how this works.

This isn't the whole story. Steering feel is also affected by the wheelbase of the bike, your weight distribution relative to the front wheel, the weight of the wheels themselves, the size of the front tyre, the length of the stem, and the width of the handlebar. These things change enormously between types of bike but not so much within types.

Other useful numbers

Wheelbase is the distance between the wheel axles. It's determined by the front centres (the distance between the centre of the front axle and the centre of the bottom bracket) and the chain stay length (the distance between bottom bracket and rear axle). The modern trend is for shorter wheelbases and particularly shorter chain stays. Shorter makes the bike feel more lively. Longer improves stability and, since you're not 'on top of' the rear wheel as much, comfort; it also provides heel clearance for panniers.

The bottom bracket needs to be high enough that the pedals don't hit the ground and low enough that you can still get a toe down. Bikes ridden off-road benefit from extra clearance – a bottom bracket height of more than 300mm – whereas commuter bikes ridden in stop-start traffic are more manageable with a low bottom bracket (270mm or so).

There's more to bike geometry than this, not least because of how the different measurements interact. Just don't be blinded by pseudo-science or someone else's opinion: if your bike feels right, it is right.

Resources:  http://www.cyclescheme.co.uk/community/how-to/how-to-understand-bike-geometry

The Geometry of Bike Handling

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Over the past year I've gotten to know Darren Baum of Baum Cycles. He's always been very generous with his time and knowledge when I come to pick his brain. He's refreshingly honest and he says it like it is. He loves to talk shop and is a wealth of information. Last time we spoke I asked him the seemingly simple question about his views on frame geometry and how it affects the handling of a bike. He talked my ear off for nearly 2 hrs! I'll try to give you the quick version of it.

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First of all, when we talk about bike “handling”, we are basically referring to how much input is required to make the bike turn. When you hear about how”compliant” a bike is, it has nothing to do with handling. A misconception is that a soft riding bike is a slow turning bike. Seat say angles or how compliant the bike is has very little to do with handling.

One of the biggest myths out there is that small fork rake dimension is a faster steering bike (fork rake typically varies from 40-55mm). This is completely false. People tend to believe this because most touring bikes have a lot of fork rake. Touring bikes are actually very fast steering bikes. At the other end of the spectrum, track bikes are not fast handling bikes. Contrary to popular belief, they’re very slow steering bikes.

Do you ever look at the geometry specs of a bike and wonder what you’re actually looking at to determine if that bike is for you? Does anything make sense to you except for the top tube length? Me neither…

Does anyone else know how to translate this into how a bike handles?

I want to be able to look at a geometry chart and know a couple things. First, what size I need. I usually look at the top tube to determine this (‘B’ in the above chart). Second, it would be nice to understand what handling characteristics I can expect from the geometry the bike is built with.

Before we begin, let me start with a disclaimer. As with most things that have as many variables as a bike there are many opinions. If you ask 10 engineers how a bike is to be designed you’ll get 20 different answers. Darren offers one perspective based on his experience and opinion (which is highly regarded in the industry).

The handling of a bike is related to four factors:

1. Trail

2. Bottom bracket drop

3. Chain stay length

4. Stem length and position

Trail

The product of head angle and fork rake gives you what is referred to as “trail“. Trail is a figure that will reflect how fast a bike actually steers. More trail equates to slower steering, less trail will make faster steering. Increasing fork rake for a given head tube angle will decrease trail, therefore giving faster steering at the front end. More trail is good at high speeds, but at slower speeds it can make the bike feel sluggish. Trail can be thought of as the tire contact point trailing behind the steering axis.

This is often confused with the curvature or angle of the fork blades. As more rake will equate to faster steering, more trail will actually result in the opposite. See the chart below to make sense of this.

Bottom Bracket Drop

In Baum’s opinion, the lower the bottom bracket (BB), the better the bike handles. What is meant by “better” is more responsive ‘rear end‘ steering. This provides a lower center of gravity and makes the bike more stable. Unfortunately the disadvantage of a low BB is that you can’t pedal through a corner if it’s too low. A bike that might be referred to a “criterium bike” is considered to be good at criteriums because of the high BB drop. As the BB is brought higher, the steering needs to be slowed down. A characteristic of criterium riding are it’s fast corners. In order to have good clearance so you can pedal around those corners, the bottom bracket needs to be high. As you bring the BB up, it’s harder to steer the bike with your backside (you steer a bike using two forces: one with your backside, and one with your hands). In order to lighten the steering up to slow it down, the bike needs to be built with more trail.

For example, you don’t want a BB drop of 65mm and a trail approaching 70mm as this will give a slow rear end that you’re going to need to put in a fair effort with your rear to steer whilst at the same time having a front end that is moving with the smallest input.

For example, you know when you descend and reach speeds of 65-70km/hr and the bike all of a sudden starts to stiffen up? Momentum seems to get under it and the faster you go the harder it is to steer (more countersteering required). If you have a lower bottom bracket and more rake, this sensation of the bike stiffening up comes along at faster speeds. In other words, a lower BB is better for descending. Touring bikes (without racks and panniers) are actually missiles on the descents!

Another example of this the way a track bike is typically built. A track bike has a high bottom bracket so you don’t hit the pedal on the banking of the velodrome, but it also has very slow steering.

When talking about a bike with 700c wheels, a BB drop that would be good for criteriums would range from 65-68mm. A low BB drop that would be on the opposite end of the scale that’s good for touring would be about 80mm. Most bike manufacturers have settled on 65-70mm of BB drop in their frame geometries for race bikes.

So, a higher bottom bracket makes it harder to steer the bike from the rear, and the less rake that’s required to balance the two steering forces. A very awkward feeling bike is when you have very light steering on the handlebars and heavy steering with your bum (i.e. a low bottom bracket). The trick is getting the geometry of the bike balanced between these two steering sensations.

Chain Stay Length

In Darren’s opinion, longer chain stays help deliver a better quality ride. When you climb, the bike has more traction. When you go through a corner, the bike trails further and you can hop on the pedals earlier. The intended use of the bike is a big consideration however.

When chainstays started getting shorter throughout history (late 60′s, early 70′s), it was when riders started getting more powerful and the technology didn’t exist to make the materials strong enough for the desired stiffness. Therefore the chainstays were designed shorter in order to make the bike stiffer. A good bike was considered one that you could barely fit a Tally-Ho cigarette paper in between the rear wheel and the seat tube. This design had nothing to do with handling. It was all about making the bike stiffer. People started identifying this small rear triangle as a “race bike”, and therefore a race bike must handle better. This never changed as materials progressed.

These days the materials exist to make a long chainstay that is still very stiff. However if you have a longer rear end, the bike naturally needs to be manufactured with more material and therefore will be heavier. These days in the industry there is a race for the lightest spec’d bike. What does Baum do with their bikes? They recommend making the rear end as long as acceptable by the customer.

If you’re flexible and can bend forward, 412mm is what Baum will recommend. If the rider sits more upright, the chainstay might go as long as 420mm. If the rider is really tall (i.e. over 6’3″), and the femer is very long, 430mm might be required. The reason for this starts to relate back to seat tube angle and pedalling technique, however I think we might leave that one for another discussion.

Stem Length And Height

The stem isn’t part of the frame per se, but it’s position does affect handling. The longer and lower the stem, the bike will handle with more stability. This doesn’t mean it’s necessarily a good idea to put your stem as low and long as possible however. You actually have to be able to reach it comfortably with your elbows bent and have the flexibility to be in that position.

If you draw a line down to the front axel from where your hands are placed on the handlebars drops, the closer those are together with falling in line with each other, the more stable the bike becomes. For example, track bikes usually have long and low stems.

Conclusion

From all of this you still probably won’t be able to look at a geometry chart to get an indication of the handling characteristics of a bike. The information that’s included in the geometry specs is up to the manufacturer and how informed they want to keep the customer. Cannondale has been including trail and BB drop in their charts for years. Many others don’t. If you want to work out these measurements for yourself based on a geometry drawing of a bike you’re interested in, you can plug the figures into BikeCAD.

The Finished Product

One of the most beautiful works of art that I’ve every laid eyes on. How does it handle? Hopefully one day I’ll find out…

Resources:  http://cyclingtips.com.au/2011/02/the-geometry-of-bike-handling/

Trek Road Bikes

Trek began building bikes in 1976 with a clear mission: Build the best bikes possible. Trek still make the world’s best bikes however they do a lot more than that. Trek actively look for better ways of doing things in every aspect of their business. From creating the worlds most advanced carbon technology, to recycling programs that takes scrap carbon to be re-used somewhere else on a bike. They have even gone as far to develop an Eco range of bikes that aim to create the smallest carbon footprint possible. Trek continue to set the bar while the rest of the bike industry scramble to catch up and nowhere is this more evident than in their carbon Trek road bikes.

Trek road bikes have come from years of development with the world’s best riders. The geometry, that has come from the development of the Madone has meant the road bikes Trek produce not only handle well, with great performance, they are comfortable to ride. In fact, the higher the series of carbon Madone you go, the more compliant and comfortable the ride will be even though the stiffness and responsiveness increases as well. In addition to Trek's world leading carbon technology each Trek road bike is packed with innovative features that set them apart from the rest of the pack. BB90, E2 steerer tubes, Duo-Trap and seamless Di2 integration are some of the stand outs here, and provide Trek riders with exceptional value & performance. Trek road bikes come in three different fit options. H1, H2, & H3 offer three very different riding positions giving just about anyone the chance to be fitted to the world’s most successful bike.

Check out the range of Trek road bikes

H1 fit

Developed for athletes with extraordinary range of pelvic rotation, superior core stability, and the desire to get low and aero, H1 offers the lowest hand position options available for Madone. The choice of many of our Pro Team riders

H2 fit

H2 retains all the handling geometry and ride characteristics of our pro-fit H1 platform, with a slightly higher head tube height to put less strain on a rider’s back and neck. It will also accommodate riders with slightly less pelvic rotation and core stability. The H2 higher head tube is the right way to get most riders in the right place. No goofy high-rise stems, no huge stack of spacers—the look is nothing but pro.

H3 fit

H3 increases head tube height a bit beyond our performance H2 fit, with a slight geometry adjustment to increase the bike’s stability. Perfect for those who need a back-saving, neck-saving, more upright position, but still want pro-level performance and a pro look. No high-rise stems or huge stack of spacers. Fast, strong, and the perfect choice when comfort trumps aero.

Resources: http://mymountain.com.au/g/1059850/trek-road-bikes.html

Generation X, Y, Z

Generations X,Y, Z and the Others - Cont'd
William J. Schroer

Generation X

Born: 1966-1976
Coming of Age: 1988-1994
Age in 2004: 28 to 38
Current Population: 41 million
Sometimes referred to as the “lost” generation, this was the first generation of “latchkey” kids, exposed to lots of daycare and divorce. Known as the generation with the lowest voting participation rate of any generation, Gen Xers were quoted by Newsweek as “the generation that dropped out without ever turning on the news or tuning in to the social issues around them.”

Gen X is often characterized by high levels of skepticism, “what’s in it for me” attitudes and a reputation for some of the worst music to ever gain popularity. Now, moving into adulthood William Morrow (Generations) cited the childhood divorce of many Gen Xers as “one of the most decisive experiences influencing how Gen Xers will shape their own families”.

Gen Xers are arguably the best educated generation with 29% obtaining a bachelor’s degree or higher (6% higher than the previous cohort). And, with that education and a growing maturity they are starting to form families with a higher level of caution and pragmatism than their parents demonstrated. Concerns run high over avoiding broken homes, kids growing up without a parent around and financial planning.

Generation Y, Echo Boomers or Millenniums

Born: 1977-1994
Coming of Age: 1998-2006
Age in 2004: 10 to 22
Current Population: 71 million
The largest cohort since the Baby Boomers, their high numbers reflect their births as that of their parent generation..the last of the Boomer Is and most of the Boomer II s. Gen Y kids are known as incredibly sophisticated, technology wise, immune to most traditional marketing and sales pitches...as they not only grew up with it all, they’ve seen it all and been exposed to it all since early childhood.

Gen Y members are much more racially and ethnically diverse and they are much more segmented as an audience aided by the rapid expansion in Cable TV channels, satellite radio, the Internet, e-zines, etc.

Gen Y are less brand loyal and the speed of the Internet has led the cohort to be similarly flexible and changing in its fashion, style consciousness and where and how it is communicated with.

Gen Y kids often raised in dual income or single parent families have been more involved in family purchases...everything from groceries to new cars. One in nine Gen Yers has a credit card co-signed by a parent.

Generation Z

Born: 1995-2012
Coming of Age: 2013-2020
Age in 2004: 0-9
Current Population: 23 million and growing rapidly
While we don’t know much about Gen Z yet...we know a lot about the environment they are growing up in. This highly diverse environment will make the grade schools of the next generation the most diverse ever. Higher levels of technology will make significant inroads in academics allowing for customized instruction, data mining of student histories to enable pinpoint diagnostics and remediation or accelerated achievement opportunities.

Gen Z kids will grow up with a highly sophisticated media and computer environment and will be more Internet savvy and expert than their Gen Y forerunners. More to come on Gen Z...stay tuned.

Next time we will start to take a more in-depth look at the most significant and impactful of the generational cohorts and what implications there might be for libraries and librarians.

Resources:  http://www.socialmarketing.org/newsletter/features/generation3.htm