We recieve a lot of questions about geometry on this site, so I have written this article to discuss the reasons why touring bicycle frames have the geometry that they do.
This resource may help you understand how a bike may ride without testing it, and should help you determine what type of geometry is best for what you do. It will certainly help if you're building a custom designed frame!
It is important to note that bicycle geometry is different for different sized bikes.
We would also like to suggest that bicycle touring frame geometry is particularly important if you are carrying front pannier bags. Why? A touring bikes steering is optimised around front loads.
I have incorporated a geometry comparison to road and mountain bikes called 'Touring vs Road/CX' which should give you even more perspective and understanding on the matter. As frame size affects geometry, these comparisons are all for bicycles with a 57cm horizontal 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 says the most for how your touring 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 bikes stability.
Touring vs Road/CX (57cm TT): 71-72 degrees is common on touring bikes. 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.
Touring vs Road/CX (57cm TT): Touring forks often have 45-52mm rake. 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 actually give you an indication as to how fast the bike will steer. This measurement is often not 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 and to keep the heavy load stable on fast descents. On the other hand, they experience more 'wheel flop' than other bikes, making it slightly harder to keep a straight line at low to moderate speeds – although front pannier weight tends to dampen this feeling.
Touring vs Road/CX (57cm TT): Many touring bikes have between 55-70mm trail. Road bikes 50-60mm. XC MTB. CX 55-65mm.
A touring bikes geometry is optimised so that it can be stable carrying front and rear loads. This is evident through a slack head angle and a higher level of 'trail' than a road or cyclocross bicycle. These characteristics slow the bikes steering down.
It is interesting to see that a road bikes geometry is always the most aggressive – the steering is designed to be fast. This makes sense in a racing situation where you need to change direction fast. A cyclocross bikes geometry almost always falls somewhere in the middle between a touring bicycles and road bikes.
One of the more important factors 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 460mm chainstays, my size US12 feet clip bags!
Touring vs Road/CX (57cm TT): 445-470mm is common. 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.
Touring vs Road/CX (57cm TT): 1050-1070mm is common. 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 often need clearence over obstacles, so some manufacturers provide a high bottom bracket (53mm drop with 700c wheels). Others provide a low bottom bracket (78mm drop with 700c wheels) to maximise the bikes stability at the higher risk of pedal strike.
Touring vs Road/CX: Not compared due to vastly different tyre sizes affecting bottom bracket height
Seat Tube Angle
Seat tube angles do not differ a lot between touring bikes and road/cx bikes of the same size. This is because the way that you pedal doesn't change too much between different bikes.
A very general bicycle fitting rule (to optimise pedalling efficiency) is to have your knee reach your pedal axle (see diagram). By changing the seat tube to a slacker angle, your knee would not reach as far as the pedal axle and your pedalling would therefore be less efficient.
Touring vs Road/CX (57cm TT): 71-73 degrees is common. Road bikes 73mm. CX 73 degrees.
Horizontal Top Tube Length
The effective top tube length is the most important length on a frame. It determines how far you will have to reach from the saddle to the handlebar. Make sure you organise a professional bicycle fitter to determine which length is most suitable for you.
Seat Tube Length
This length will not matter too much, unless you have particularly short legs for your body size and need the additional stand over clearance. Make sure to compare bikes based on the horizontal top tube length.
Head Tube Length
Long headtube are common on touring bicycles in order to keep the bars up high without the use of excessive spacers. Head tubes are often 40mm or more longer than the equivalent road or cyclocross bicycle headtube.
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 (increased stand over clearance) and a short head tube (more aggressive positioning).
When you turn a mountain bike into a touring bike, you tend to put lots of spacers under your stem to get the bars up high. As a result, there is often MUCH more flex in the front end of the bike than on a touring specific frame (only noticeable when carrying front panniers).
This flex can sometimes lead to speed wobble aka 'shimmy'. I have experienced this on many bikes including Surly's Ogre and Troll set up as touring bikes with a front rack and pannier bags.
Having said this you can tour successfully on a mountain bike; lots of people do! A dedicated touring bike works better for touring however, just as a dedicated mountain bike works better for single track. And a half-tourer, half mountain bike will no doubt do a mediocre job of both.
Not Using a Front Pannier Rack?
If you are planning on cycle touring with only rear bags, frame geometry is arguably not as important; a mountain bike with rear panniers will not ride all that different to a touring bike with rear panniers.
Touring-specific frames become much more important when you have a front rack and heavy gear, requiring a super stiff front triangle and more fork 'trail' to make your bike handle well.
A touring bikes geometry is specific to carrying loads – especially at the front. The steering and wheelbase have been optimised for stability, the chainstays are long to clear your heels and the headtube is long to get the bars high.
If you haven't ridden a touring specific bike with filled panniers, we recommend that you do to feel the difference!