/ South Downs Way by bike
I've got a lot of experience of the SDW as I practically live on it but I don't have a lot of experience of bikes. If you want to ask specific terrain questions I can help.
FWIW, I've never seen anyone on anything other than a mountain bike. A lot of the route is muddy at the moment where I guess a tourer would be ok. But a lot of it is quite 'rubbly' and flinty and it's hard going.
Its many years since I did it on a solid framed £50 mountain bike but from memory, there are very few 'technical' sections. Especially in the dry I doubt that a solid tourer would be any worse than my old bike.
You could do it on a tourer, but would be much more comfortable on a mountain bike.
Most of the way it's chalky/flinty farm track so not too bad, but there are a couple of sections where its quite rutted and rubbly. There are also a number of sections which are open grassland and very fast (butser, harting down, the beacons) where I would not want to be on anything but a mountain bike... that's not to say it couldn't be done on a tourer or cyclocross bike.
The other thing to consider is gearing - not sure what your tourer is like, or whether you are carrying kit with you, but there are some very steep hills that on an MTB lead to you having trouble keeping the front wheel on the ground and/or getting traction on the back wheel.
If at all possible, don't do it in the wet, the chalk surface can be like ice at some points.
Yes, Mountain Bike is your best bet
This. I did about 60km of it on a rigid hybrid with knobbly tyres (approx Pulborough to Lewes). It was doable - there's nothing too technical, but there are plenty of rutted or rubbly bits, some steep climbs etc., where a touring bike is uncomfortable/not much fun. The bits across fields on nice compact earth could pretty much be done on a road bike, but all in all, I would do it on a hardtail now, with something like schwalbe smart sams on (in that, you don't need the knobbliest of knobbly tyres if it's not wet). I remember my wrists hurting a lot by the end of the day, and for a couple of days after! In good weather it's fantastic though.
It's the Winchester to Midhurst end that is a bit more rutted, but none of it is really bad and I have done the full length about 3 times on a Rigid MTB, but it was so much nicer with some (basic 1995) suspension on the front. You do get a lot of speed up on the grassy down-hills, which while smooth enough to ride on a road bike, it was bad enough stopping on an mtb if required - I came off at about 40mph on the way down Butser on dewey grass, simply because I briefly touched my brakes, and then couldn't recover control - had a massive grass burn up my side for about a week after!
It is a great ride though if the weather's kind
I remember a few, loose, rubbly downhills on tracks which were rideable on the hybrid, but could be ridden faster and much more comfortably on a hardtail.
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