100 miles on Folding bike?

Seems reasonable to me - most I've done on a folder is about 25 miles (Warrington to Manchester). Depends on the bike and the terrain and the conditions though. The bike I use is a Dahon - it has bigger wheels than a Brompton which is probably good for the longer rides but the 8 speed hub gearing is poor with far too narrow a range for my tastes. I have got it up some reasonably steep climbs but headwinds kill it and the small wheels don't really carry any momentum; stop pedalling and the bike stops! Might almost be pleasant on a flattish route on a still day!

I've often done 60-80 milers on my Birdy & have seen a few Brommys and the odd Dahon on longish charity rides. budget will be your issue for a decent bike, could you borrow one for the event?

As a Brompton user my max is about 70+...The secret is how well the bike is serviced and prepared... high enough inflated tyres,tyre types and certainly not worn ones ,Wheel trueness.Rubbing brakes asnd all other aspects of sercving etc make a lot of difference to your performance even on distances shorter than this..Folders need more servicing than a larger bike to keep them efficient

I have seen some bloke did a 70+miles sportive using a Barclays cycle hire bicycle. so i cant see why you can not do it on a foldie however, as someone have mentioned, you need a foldie with a bigger wheel to ease of the bumps. its doable so good luck on your challenge.

Other aspects that improve distance and effeciency on my Brompton are seat post height (an extended seat post may help .. aslo the seat post height can sink a bit during a long ride if you are a heavyweight and needs correcting en route.....also adding a toe clip to the non folding pedal.helps .(I have just one folding pedal on mime) .Its hours in seat that matters so a comfy brooks B17 like saddle helps too.. On your own 100 mieage is Ok if you can put the hours in..but trying to keep up with others who are on full size bikes is a struggle espec on hills.Best be an early starter if going on a charity ride with staggered start times

100 miles on little wheels -- it's not hard, but be prepared for a bit of pain. In the posterior, in the legs, the hands and wrists, and the back. Most of it is about mind over matter: if you can do it on a penny farthing/standard (http://www.youtube.com/watch?v=91PBQUWZCp8 -- and those look like Brommies in the picture, too) or on a Boris bike (http://www.guardian.co.uk/environment/bike-blog/2011/jul/19/dunwich-dynamo-boris-bike) then you can easily do it on a Brompton. Um, so long as you can keep your mind off the matter of the pain.

I find that most other riders think you're brave or mad for riding the same ride as them but on 16 inchers and that's a great conversation point -- far more interesting than talking about carbon forks and aero spokes! Chatting to people around you really does make the time pass more quickly. The same with single-speeders, although they tend to have other issues, like whether it's a single-speeder or a fixie, like it makes a difference!

It's also possible to be quick on little wheels: I rode the Dunwich Dynamo this year (120mi at night -- missed a turn, though, oops) in 8 hours 8 mins -- including a stop for a bacon butty at 1am and another stop for cake -- which is an average of just under 15mph (average moving speed was just over 16mph), which I hope to thrash next year. There was a guy on a drop-handlebar Moulton who rode it apparently about 30mins quicker, but I bet he didn't stop for that bacon sarnie. London to Cambridge (60mi) in 3hrs 30m = 17mph. Cambridge to Norwich (77mi) in 5hrs2m = 15.4mph (I spent the last 90mins helping a guy get to the end, just chatting with him and sharing jelly babies, so a bit slower then I had aimed for, but Colin did get there in one piece). None of these are stellar times for a roadie, but they are above average for these types of rides, and you have the warm glow of having done it differently

There are things to watch for, though: after the Dunwich Dynamo I couldn't feel anything much in my nether region for two very worrying days (fortunately I am already a father). I swapped from the then standard-fit sporty saddle to the now-standard fit Brompton saddle with-a-grip, which I immediately hated and replaced with a much better and more expensive non-standard Bontrager Affinity RL -- but saddles are very bottom-specific, so find a bike shop that will let you try one for a few weeks and return it if you don't like it (like Haywards in Horningsea, Cambridge) and then try a few.

Other things to consider are the longer seat post to get you into a better riding position -- longer-legged but more bent over. The handlebars -- I have an S-type (straight bar, lower position) and I have ridden the M-type (classic bars, more upright) and I much prefer the S-type. The gearing -- I have the 6-speed, so an amount of choice, with the +8% chain ring for cranking along at speed; it's a compromise, though, because climbing is like being on a single speed and there are times when the only way is to get up out of the saddle and stretch. The pedals -- I have ridden with SPD clipped pedals since the early 90s and I wouldn't ride any other way now; on my Brompton it's the same, even when commuting into London. Lose the rear rack -- it weighs loads. Good tyres -- and pump them up rock solid -- I have them at 60-80psi. Replace the soft or medium rear suspension block with a hard one -- it's a bit less comfy, but the bike becomes a lot more direct in control.

Hand position is the other thing that can be murder: there's only one position for your hands unless you fit bar ends. Unfortunately I have a little Mavic mirror in the end of my bar, so bar ends won't work -- the mirror is a life-saver when commuting, but the bar ends wouldn't be, so the mirror stays. The trick is to learn to ride no-handed and straighten up every half hour or so to let your hands rest. It's tricky to do, but possible for miles at a time.

I have a final sportive ride of the year coming up this weekend and I am doing it on my four-year-old Brompton -- the Wiggle Rut from Oakham in a loop around Rutland Water. There's a fair amount of climbing to be done and I don't have much experience of it, because East Anglia's not known for its hills. 72 miles; there is a longer loop, but I am going to bury myself on the hills on the short loop this time out to find out where my limits are, and so I don't want to have to worry about keeping some back to complete the longer distance. The gold-certificate time is 4:14, which is average 17mph -- if I can get the climbing right then it's just about achievable. And that's what makes it all worthwhile for me: to do 4:14 on a road bike is probably straightforward, but doing it on a Brompton would be a proper achievement!!! (I haven't ridden a road bike for 25 years, so I can't be sure, but my local mechanic Dave in Histon assures me that riding a road bike after riding a Brompton feels like cheating!)

And then being able to simply fold the Brommie and chuck it on the baggage rack on the train home without having to worry about the rail company's four-bikes-per-train limit is just the icing on the cake

Anyway, enough boll's from me -- that's a splurge and a half of rubbish nobody's going to read (it's either write this or watch X Factor over the Internet). The main thing is to get out and ride whatever you've got and if you do it differently to everyone else then enjoy the (jealous) glances and (usually) positive comments and distracting discussions along the way.

And post photos.