calogero_vizzini wrote:What is the measurement unit for revolutions in that context? Is it one full turn of the freehub?
Pardon my ignorance, but could that not be simply achieved by having more pawls (Like Tune Mag - 5 Pawls) or by having more teeth for the pawls to contact?
Yes, one revolution of the freehub body. Since on a road bike your granny gear tends to be more than 1:1 (ie a 39/25) the engagement issue isn't so great. Mountain bikers gear down (like the 22:34 mentioned) so it is more of an issue.
The number of pawls
doesn't matter....its the number of teeth in the outside ratchet ring that counts.
The more indents in the outer ring, the more engagement points. As you add more indents to the outer ring each one must get smaller and shallower (physical limitations of the hub) and so they become less secure or unable to hold a high load. This is usually overcome by adding more pawls to spread the load, such as in Tunes design.
Mavic hubs use only two pawls. Each one is massive and engages a massive indent in the outer ring, hence the freehub never skips and can handle huge load. Roadies never complain about Mavic freehubs but mountain bikers do. As an interesting side-note the pawls in a Mavic hub are offset by about 135 degrees, not 180 like most freehubs. This means that when the pedal load is applies one pawl engages and twists the inside of the freehub slightly to ensure that the other pawl engages. With multiple-pawl designs it is common that under load, when the freehub body starts to twist, one pawl will not engage. In the worst
case senario only one of the pawls would engage, ala Tyler Hamilton's modified ADA wheel (only had 3 pawls in it, not the recomended 6), but this one pawl cannot hold the entire load so it fails.
King use a clutch-plate idea like DT Hugi, with 72 engagement points. Mountain bikers rarely have problems with King hubs but roadies complain about freehub body and seal drag caused by this design.
In Canada it is not uncommon in the winter to see mountain bikers peeing on their freehub bodies because water has worked it's way into the pawls and has frozen, meaning the pawls cannot engage. Nice warm liquid melts the ice and allows the hub to work again.
divvie wrote:In most cases faster engagement doesn't do anything for performance. Not even in regular MTB. Only in very tight and slow technical sections does a quick CK engagement offer a little advantage. It allows you to position your crank better after freewheeling or repeated "ratcheting back" in order to maintain balance and drive force.
In trials riding the quality of the rear hub is paramount. The hub has to have quick engagement (usually running 22:18 - 22:20 gearing) and has to be very strong to cope with the load from pedal-kicking (*). This is why King hubs are so popular, especially in the past when there were fewer other options. Hope make a trials-specific hub from their Pro II freehub body, and DT Hugi hubs are also used quite a bit.
Pedal-kick, from http://biketrials.com/intro/dictionary.shtml
Quick stroke of the pedal making a quarter revolution or less. Release of the power of the stroke usually controlled by the rear brake. Pressure is applied to the pedal while the rear brake is locked, then the rear brake is released and the quick stroke begins. You can pedal-kick a gap, which usually means you were rear-wheel hopping on one side, then pedal-kicked and landed either rear-wheel, bashguard, or both wheels on the other side. If you landed rear-wheel, you could say you tocked across the gap. If you had both wheels on the start side of the gap, then launched across the gap with a pedal-kick, this would be called a lunge.