Head Tube Length & Angle
Many riders first look for the head tube angle when judging a new bike design. They want to know how “slack” this angle is because that plays a part in determining what style of riding the bike is designed for.
The head tube angle is the angle of the head tube in relation to the ground. A “slack” angle is anywhere from 63 to 66 degrees. This means the fork of the bike is going to stick out farther, creating a more stable ride at high speeds, but also be slower handling. This angle is also determined by wheel size since a 29 inch wheel requires more leverage to steer, so designers make the head tube angle steeper to compensate. The steeper the head tube angle, the better handling you’ll achieve, but with less stability at speed.
Most trail bikes have settled on a minute range of somewhere between 65 and 67 degrees. Downhill bikes will have angles anywhere from 62 to 64. And XC bikes and road bikes have steeper head tube angles from 67 to 73 degrees.
The head tube length is also related to riding style. It comes down to comfort. A taller head tube will put you in a more upright position which is easier on your back and neck. Touring and endurance road bikes often have super tall head tubes in order to achieve the most comfortable body position possible for longer rides, while mountain bikes will run shorter to achieve a more aggressive riding position for technical terrain.
Fork Rake & Trail
Fork rake isn’t talked about much, but it has as much to do with steering and stability as the head tube angle. Every fork has an offset that pushes the axle forward from the straight line of the steering axis. With more rake, the bike is more stable at speed, but just like the head tube angle, it requires added leverage for precise handling.
Now here is the real head-scratcher. When you combine fork rake and the head tube angle, the result is the fork trail. This is the tire contact point following the steering axis. A longer trail results in a slower handling bike. And less trail amounts to quicker handling.
Manufacturers will tweak the head tube angle, fork rake, and fork trail to get the handling just right depending on the bike’s intended use. Road bikes need to be light and agile but stable on big descents, requiring a compromise between steep and slack angles. And on the other end of the spectrum, a purpose-built DH bike needs to be able to hold a line in technical terrain and be steady at high speeds, requiring a slack head angle and more rake for maximum stability.
Stack & Reach
Stack and reach are two indicators of how a bike will fit.
Stack is measured vertically from the bottom bracket to the top of the seat tube – this is an indication of how tall the frame is. A low-slung frame is going to have more clearance for your legs but will require a taller seat post.
Reach is measured horizontally from the bottom bracket to the center of the head tube – this is an indication of length. You’ll find relatively stretched front ends on modern mountain bikes to provide extra room. This allows for better handling and stability at speed, as well as comfort.
Top Tube Length (Effective vs. Actual)
Top tube length is another indication of length and fit of the front end of the bike. The actual top tube length is measured from the top and head tube junction to the top and seat tube junction.
The effective top tube is measured horizontally from the junction of the head and top tube to the seat tube. Due to most bikes having a sloping top tube, usually for added clearance, the effective top tube is a better measure of length when comparing different bikes.
Seat Tube Angle
A bike’s seat tube is angled relative to the ground. An easy way to picture this would be to draw a horizontal line between the bike’s axles and draw an imaginary line straight through the seat tube to this horizontal line. This angle doesn’t fluctuate much like the head tube angle does. It’s usually between 71 and 74 degrees.
The idea is to position the rider for optimized pedaling, or to target specific muscle groups in order to use them more. A rider can artificially change the seat tube angle by moving the saddle forward or back on the rails. This is up to the rider though and can also be determined by riding style or when the bike is fit after a purchase.
Chain Stay Length
Chain stay length is measured from center of the bottom bracket to the center of the rear axle. This length is determined by wheel size and bike style. Longer chain stays provide more stability but can hamper handling on technical trails. Most bikes these days feature quite short chainstays around a larger wheel (27.5 / 29 inch) in order to maximize the rolling speed of the wheel but keep handling quick. For easy reference, the Santa Cruz Bronson pictured has a relatively short chain stay at 17.01 inches. Depending on the bike, a manufacturer might lengthen the chain stay for added stability.
A bike’s wheel base is measured from front to rear axle. This measurement also coincides with the contact points of the tires to the ground. Every angle and measurement listed above contributes to a bicycle’s wheel base. Where a longer wheel base provides stability and comfort, a shorter one will offer better handling. It all comes down to a balance and the intended use of the bike, and designers are constantly tweaking their designs to achieve the optimal measurement. The Bronson above has a 46.81 inch wheel base, a moderate length that is intended to balance quick handling and high speed stability.
Bottom Bracket Height & Drop
Your bottom bracket (BB) is the point that your cranks and pedals revolve around. The BB drop is a measurement between the center of the BB shell and an imaginary line between your front and rear axles. BB drop will never be a dramatic measurement, usually well under an inch, but its implications in terms of stability can be huge. More BB drop coincides with better handling and stability because it means that your center of gravity is lower. Too low and you could have pedal clearance issues. Less BB drop can aid your pedaling platform for additional efficiency while climbing.
BB height is a bit different. It is a measurement from the ground to the center of the BB, usually around 13 inches. This is more of an overall frame clearance measurement, but can sometimes be used to determine handling and stability. It depends on how the numbers are interpreted.
Now that you know – it’s time to test
Now that you have an idea of what bike geometry is, what does it mean for you as a rider? If you’re out to select a new mountain, road, cruiser, cyclocross, or other bike, what does a long wheel base or a steep head tube angle do for you? It comes down to what style of rider you are, and what bike matches that style.
Do you like to go fast and hit big jumps? Then a downhill bike with low BB and 64 degree head tube angle will offer you maximum stability at speed. But what if you aren’t scared to pedal to the top of the hill? Then those angles and measurements will be altered to accomplish that style of riding.
It’s important to test a bike if you’re aren’t sure. For me personally, I know enough about the type of bike I like to ride to feel comfortable buying without riding it first. But if you don’t know, stop by your local bike shop and ask if they have a demo program so you can be sure you’re getting the right bike for you.
If you’re in Park City, both our Jans and White Pine Touring bike shops have high performance demos from Specialized, Rocky Mountain, Santa Cruz, and Scott – legacy brands that have put a ton of R&D into their products, and that make a variety of bikes to suit any rider.