It's a bike ride run under the auspices of Audax UK (AUK) (Ok or a foreign/international audax/randonneur organisation)
The idea is that it is non-competitive, so no times are published, no official placings are given. However there are time limits: the idea is not to ride too slow or too fast. There are maximum as well as minimum speeds and to keep you within these you check into controls around the route. On a 100 or 200 km audax you'd probably get 3 or 4 controls on the route.
Some controls are cafe or village hall stops with food and acontroller to stamp your card; sometimes they are petrol stations and you grab a till reciept as proof of passage. Sometimes you use an ATM. There's a range of luxury, from fully-catered with beds (on an overnight event) to totally shoestring where you are pretty-much on your own. They're all different.
You tend to get out of them what you put in: it's usually possible to find a group going at about your pace to go around with, if you choose, but there's a very strong self-sufficiency ethos. if you puncture they'll check you can cope and most likely leave you to it. If it's a more serious mechanical you'll get lots of help.
Unless you know the riders before hand and/or have agreed to ride together they may not slow down if you get dropped - it's not unfriendliness, it is a recognition that people chose to ride at their own pace and often deliberately drop out of a group if they fancy a slight rest. People have variable paces over a long day: sometimes you'll be flying off the front, other times hanging on the back. You'll all meet up down the road.
If you chat to people, audaxs can be pretty sociable. You may meet some of the more reticent members of the cycling community tho' Wink, and sometimes you have to persevere. I remember the first time I met a leading light of Audax UK. I was younger and quite fit then, and I rolled into the penultimate control of the Elenith (a classic early season hilly 300) in chatty mode. To be cut dead as he turned away and blanked me. I had him down as a miserable old gimp until i got to know him better: he's a great bloke, but just then he was completely shattered and in no mood for my inane idiocy.
Then there's Permanents, Darts,
Arrows, SR Series...
The first three are a bit arcane, but it's probably worth describing an SR series. This is, if you like, the goal of AUK: to get people to ride SR Series. The Series consists of a 200, 300, 400 and 600 km audax in the same audax year (currently 1 Nov - 31 October). An SR Series is also the qualification for PBP.
In the early years AUK existed entirely to get UK riders to PBP; these days the vast majority of its 4000 members ride 200 k events or shorter. Only a few hundred each year do an SR Series, and a handful do several.
More info at
http://www.aukweb.net including the online events calendar. There's an AUK mailing list for the truely hardcore; though it is quite low-traffic it has good info and a pretty good signal-to noise ratio.
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