Understanding Bicycle Frame Geometry

Alee October 4, 2013 4

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.

Trail

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.

Steering Summary

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.

Chainstay Length

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.

Wheelbase

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.

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 assess the position of the headtube in relation to the bottom bracket, essentially standardizing 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.

Get a professional bicycle fitter to determine your appropriate stack and reach and it'll be much easier to find your next perfect fitting bike!

Effective Top Tube Length

The effective top tube (ETT) length is the simplest way to determine a bikes size, although just because the ETT is the same between the two bikes, doesn't mean the bikes will have the same reach - see above. The ETT determines roughly how far you will have to reach from the saddle to the handlebar.

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 their stack and reach if you can.

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.

Many MTB touring bikes use more spacers than they have head tube!

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.

Summing Up

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!

 

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  • jedrek

    Alex, have you considered running a long, angled stem like the Amoeba riser stem instead of all the head tubes? They’re specially designed to run drops on MTBs. I currently tour on an LHT (pic included), but I’ve built up a 29er for off-road touring and am going to give it a go with drops and a riser stem.

    • http://www.cyclingabout.com/ Alex Denham

      There are many ways to get the front end higher: long steerer tubes with lots of spacers, ‘heads up’ steerer extenders and angled stems to name a few. They all work, but in our mind a long headtube with a few spacers will always be the stiffest and most attractive option.

  • Raymond A. Poythress

    Interesting article. Nice to see how the biking concepts have evolved
    and grown into their own. The packs shown seem like a must have. Great
    piece.

    https://www.spinlister.com/

  • Paul Stead

    Fork rake and trail sounds a bit like caster and camber.