About the design of the Spa cycles touring frame.
Steering
A touring bike needs stable steering. You don't want to end up in the ditch as soon as you take your eyes off the road. You don't want the bike to be upset by rough roads, sidewinds, or heavy loads. Our frame has a shallow head angle for stability. This is combined with a long fork offset to produce nicely balanced steering: stable with varying road conditions and load, but not too unwieldy for fast descents.
Front centres
The 'front centre' is the distance between the front wheel axle and the bottom bracket axle. This distance has been chosen to avoid overlap between the rider's shoe and the front mudguard with the length cranks appropriate for that frame size and the ball of the foot over the pedal spindle.
Seat tube angle
A shallow seat tube angle was chosen so that riders can get Brooks saddles far enough back for comfort without having to seek out a seatpost with extra long layback. Steep seat tube angles are appropriate for race bike designed for maximum effort and an aerodynamic riding position, not for day long comfort on a tourer.
Top tube length
Once front centres, head and seat angles, and fork offset have been decided, top tube length is pre-determined!
The handlebars on a touring bike need to be positioned so that the rider has a comfortable reach, not a long stretch. This can be a problem on small frames, where a balance must be struck between toe clearance and reach to the handlebars. Some manufacturers use a steep seat tube angle on small frames to get a "short top tube". This is entirely spurious, as the rider will then fit a long layback seatpost to get comfortable. It is also common to use a very shallow head angle on small frames to get a short top tube, but this alters the steering unless the fork offset is changed to compensate.
As mentioned above, our frames have a shallow head angle and a long fork offset. This is an occasion where the laws of physics conspire to help us, as these dimensions not only produce desirable steering, they also reduce the reach to the handlebards (for the same front centres).
Chainstays
The chainstays are relativley long for a stable comfortable ride and to get riders heels away from panniers. Short wheelbase frames are for racing.
Clearances
There is fairly generous clearance for 32mm tyres, mudguards and mud. A bigger tyre will fit, and if you want to run 28mm tyres that works too, just drop the mudguards down to the end of the slot in the mudguard bridge if you don't like to see daylight between tyre and mudguard.