The Geometry of Bike Handling
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:
2. Bottom bracket drop
3. Chain stay length
4. Stem length and position
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.
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…