I receive a lot of questions about frame geometry, so I’ve thrown together this resource to discuss the reasons why bike frames are built to the lengths and angles that they are.
This information will help you understand how a bike may ride without testing it, which is especially handy if you’d like a custom frame built up. It should also give you an idea of what style of geometry is best suited for you.
It is important to note that bicycle geometry is always different for each size bike, given our body proportions change with our height.
If you’d like to be carrying front pannier bags, frame geometry is especially important as steering is often optimised around front loads.
I have incorporated a geometry comparison to road and cyclocross bikes. As frame size affects geometry, all comparisons are for bicycles with a horizontal 57cm top tube.
Understanding the Steering
The front end of the bike is a bit complicated but I’ll do my best to explain it.
There are three measurements at play:
Head Tube Angle
Fork Trail, and
Fork Rake (or Offset)
Out of the three, fork trail arguably tells us the most about how a bicycle will steer.
Head Tube Angle
The headtube angle is the angle at which the head tube is to the ground.
A steeper head angled bike has faster steering. There is less effort required to steer it.
A slacker head angled bike has slower steering. There is more effort required to steer it.
Touring bikes are slacker in head angle compared to their road/cx relatives because they carry weight, and a slower steering speed helps with the bike’s stability.
Head Tube Angle Comparison for 57cm Bikes: Touring bikes 71-72 degrees. Road bikes 73-74 degrees. CX 72-73 degrees.
Fork Rake (Offset)
Fork rake is the offset of the fork dropout center from the straight line of the steering axis (centerline of the fork’s steerer tube).
Increasing the fork rake makes steering faster.
Decreasing the fork rake makes steering slower.
Touring bikes have more rake than road and cyclocross bikes to increase the their wheelbase length, provide more toe clearance and to increase the forks comfort.
But hang on… touring bikes have more rake compared to road/cx bikes yet they steer slower. Why? Well, fork rake is only one ingredient to the steering equation. Read about ‘trail’ below to understand the rest.
Fork Rake Comparison for 57cm Bikes: Touring forks 45-52mm. Road bikes 40-45mm. CX 45mm.
The product of the head tube angle and the fork rake is the trail. This is the measurement that will give us the best indication of how fast a bike will steer. This measurement is not often provided by manufacturers, although it is arguably the most important number when it comes to the handling of the front end.
Less trail equates to faster steering. It effectively makes a bike feel more nimble, like it’s steered ‘with your hands’.
More trail equates to slower steering. It provides stability at higher speeds and makes the bike feel like it’s steered ‘with your hips’ (leaning).
Touring bikes have lots of trail to slow steering response, keeping heavy loads stable on fast descents. On the other hand, high trail bikes experience more ‘wheel flop’ making it slightly harder to keep a straight line at low to moderate speeds (although front panniers tend to dampen this feeling).
Trail Comparison for 57cm Bikes: Touring bikes 55-70mm trail. Road bikes 50-60mm. CX 55-65mm.
A touring bike’s geometry is optimised so that it’s stable carrying front and rear loads. This is evident through a slacker head angle and a higher fork ‘trail’ than both road and cyclocross bikes.
Road bike steering is tuned to be fast with a low trail front end. This makes sense in a racing situation where you may need to change direction in a split second. A cyclocross bike’s geometry almost always falls somewhere in the middle between touring and road bikes.
One of the more important measurements on a touring bike is the chainstay length. A longer chainstay length is desirable to increase the wheelbase (making the bike more stable) and to provide ample heel clearance from the panniers. Heel clearance is especially important for riders with large feet. Without long chainstays my size US 12 feet will clip my panniers!
Chainstay Length Comparison for 57cm Bikes: Touring bike 445-470mm. Road bikes 405-415mm. CX 420-435mm.
A longer wheelbase provides a more stable and comfortable ride. Touring bikes have a long wheelbase due to a combination of a slack head angle, long fork rake and long chainstay length.
Wheelbase Comparison for 57cm Bikes: Touring bikes 1050-1070mm. Road bikes 996mm. CX 1018mm.
Bottom Bracket Drop
Bottom bracket drop determines how high your cranks sit off the ground. A lower crankset results in a lower saddle height and therefore a lower centre of gravity.
Touring bikes can need pedal clearance over obstacles, so some manufacturers provide a high bottom bracket (53mm drop with 700c wheels). This is generally offered with bikes designed with off-road use in mind. Other manufacturers provide a low bottom bracket (78mm drop with 700c wheels) to maximise the bikes stability, albeit at the risk of pedal strike. This is more common with road-oriented touring bikes.
BB Drop Comparison: Not compared due to vastly different tyre sizes affecting bottom bracket heights
Seat Tube Angle
Seat tube angles don’t differ a lot between touring bikes and road/cx bikes of the same size, as optimal pedalling positions aren’t all that different between bikes. The seat tube angle varies most between frame sizes.
A very general bicycle fitting rule (to optimise pedalling efficiency) is to have your knee reach the pedal axle (see diagram). If you were to slacken a seat tube angle, your knee may not reach as far as the pedal axle (and would therefore be less efficient, cause knee pain etc). See my resource Understanding Bike Fit for more information on optimising your position.
Seat Tube Angle Comparison for 57cm Bikes: Touring bikes 71-73 degrees. Road bikes 73 degrees. CX 73 degrees.
Stack and Reach: Important for Comparing Bikes
Stack and reach measurements are perhaps the best common information we have to know if a bike will fit us, without testing it first. Stack and reach measurements assess the position of the headtube in relation to the bottom bracket, essentially standardising bike geometry/sizing between brands and models. This is important because bikes from two manufacturers that are both called the same size (medium or 54cm, for example) can actually fit up to 2cm (a full size) different from one another! If the companies you’re looking at don’t have stack and reach measurements, here is a calculator.
You can get a professional bicycle fitter to determine your appropriate stack and reach, making it much easier to find your next perfectly fitting bike. If you’re interested, check out my experience of getting a Precision Fit.
Effective Top Tube Length
The effective top tube (ETT) length is the simplest way to determine a bike’s size. That said, just because the ETT is the same between the two bikes, doesn’t mean the bikes will have the same reach. The ETT still tends to be a good guide for how far you’ll reach from your saddle to handlebar.
Seat Tube Length
Seat tube length isn’t too important for most people with the exception for if you need the additional stand over clearance (often smaller riders). Again, it’s best to compare bikes based on their stack and reach if you can.
Head Tube Length
Long head tubes are common on touring bikes in order to prop the bars up high without the use of excessive headset spacers. Head tubes are often 40mm or more longer than the equivalent road or cyclocross bike.
Short Seat Tubes and Head Tubes on Touring Bikes
One of the main reasons why a mountain bike is less suitable for bicycle touring is because it employs a short seat tube (to increase standover clearance) and a short head tube (to achieve a more aggressive positioning).
When you turn a mountain bike into a touring bike, you tend to require a lot of spacers under your stem to get the bars up high. This can result in more front-end flex than found with touring-specific frames (note: only noticeable when carrying front and rear panniers). Flex can sometimes lead to speed wobble aka ‘shimmy’ – I have experienced this on a handful of heavily laden off-road touring bikes including Surly’s Ogre and Troll.
Having said that, you will travel successfully on a mountain bike. Lots of people do. It’s just that a dedicated touring bike works better for touring, just as a dedicated mountain bike works better for mountain biking.
Not Using a Front Pannier Rack?
If you are planning on cycle touring with front or rear bags only, frame geometry is arguably less important. A mountain bike with rear panniers will not ride all that different to a touring bike with rear panniers.
Touring-specific frames work out to be more important when you use heavy gear front and rear, requiring a super stiff front triangle and more fork ‘trail’ to make your bike handle well.
A touring bike’s geometry is specific to carrying loads. If you haven’t ridden a touring bike with filled front and rear panniers, you’re missing out. The steering and wheelbase are optimised for stability, there is ample heel clearance and the front end is appropriately tall. Don’t compromise next time, try one out for your next adventure!