Geometry: from plain BMX to mods
The very first trials bikes were merely derived from plain 20” wheeled BMX rigs. These were modified and reinforced to take the extra abuse from hopping around and negotiating obstacles, hence their generic designation as mods. The bash plate was a direct adaptation from motor-trials, so was the triple-crown forks of the early models, with a straight angle that allowed sharp turns. As more riders gathered, more bike designs made their appearance. Pretty much anything could go at that time, sometimes even mixing 24” and 20”, or 26” and 24” wheels, on the rear and the front of the same rigs, respectively. From the late 70′s to the early 80′s, the first mods became available in serial production, favouring the 20” wheel size which made the bikes smaller and more agile.
Some manufacturers also experimented with mountain bike geometries. All the trials bikes built with 26-inch wheels are called “stock bikes”, because they were originally based on general-purpose mountain bikes.
Stock bikes suite riders who prefer the feel of a bigger and “less eccentric” bike. Both the mod and stock categories are allowed in competition, albeit riding different sections. For a long time stock bikes could hardly compete with the purpose-built 20” trials bikes mostly used in Europe. Thanks to recent changes in bike trials rules and geometry improvements stock bikes are closing the gap rapidly. Contrarily to public misconception, suspensions are rare in bike trials. They only add weight, suck energy when hopping around, and make the bikes more difficult to handle.
A smooth progression
Progressively, cable and cantilever rim brakes shifted to hydraulic systems and disc brakes, the wheels also got stronger, built with wider rims and fitted with bulkier tyres. From the year 2000 onwards, seats became less common on trials bikes, they were just impractical dead weight after all. In general, improving frame geometries meant making more room for body movement while shortening the chain-stays to shift the centre of gravity backwards and ease balance on the rear wheel. The bottom bracket height rose year-on-year for greater obstacle clearance and higher side-hop reach.
Since 2005, competition rules no longer require stock bikes to have five full working gears, which the mod bikes never had. On competition grounds, this immediately translated into lighter 26” bikes with fixed sprockets and horizontal drop-outs, taking out the gear shifters and the rear derailleur altogether. Even sprung chain tensioners are becoming a rarity on 2010 models. Nowadays, so-called stock bikes bear no comparison with general-purpose mountain bikes. More and more they resemble mod bikes on larger wheels. At the time of writing, only 20” and 26” bikes are allowed in competitions, which may explains why there are relatively few 24” trials bikes on the market despite their growing popularity for street riding. Some champions equally at ease on 20” and 26” platforms, argue that 24” bikes could bring the best of both worlds if only they were not ruled out. Maybe some day, competition rules will allow all three wheel sizes for competitors to pick their favourite rig and battle for the world title.