hello all
When i pedal the cassette is spinning but not driving the bike. I have been told that the freehub is most likely screwed up but not how to fix it. May someone please enlighten me?
thanks

Very much dependent on the hub make. Some are an easy fix, some mean a new hub.

What brand/model of hub?

Hard to tell until you pull the hub apart as it varies depending on hub design – what make / model is hub? It could simply be the pawls, the whole freehub (e.g. Shimano) or even the hub body (broken ratchet ring)

best replace the whole bike just to be sure. and keep the old one too.

the model of the hub is a dt swiss am1900

If you know what you’re doing and you have the tools take it apart and have a look if all the pawls are in place or just if anything seems amiss. Generally hubs are not that complicated although there might be little pieces that can get lost easily. Otherwise take the cassette and tyre off and take it down your lbs.

That’s the hubs I had on my Canyon. They are the renamed from something else in DT Swiss’ range but I’ve just checked through my emails and can’t find it right now.

It was pawls on mine but I got a full rebuild kit (but got the LBS to do it), I think from Just Riding Along.

That DT Swiss may be a ratchet hub, if so, the spring that holds the two drive ratchets (wee gears – instead of pawls) in contact with each other may be broken

They are very easy to work on.

It’s the same as a DT Swiss Onyx, I think.

DT Swiss hubs on cheaper wheelsets are very often rebranded Onyx hubs. Two pawls, single circular spring, made of cheese.

If so, manual:

https://pictures.dtswiss.com/uploads/WXD10000000897S/MAN_WXD10000000896S_WEB_EN_001.pdf

and spares:

http://www.chainreactioncycles.com/mobile/dt-swiss-hub-service-kit-two-pawl-hub/rp-prod81209

Two pawls, single circular spring, made of cheese.

Not so. They’re good hubs and every single part is available as a spare. Easy to strip and rebuild with a minimum of tools too. I’ve literally just finished doing one.
OP – if you’re unsure, go to a decent LBS. They can get the parts from Madison and do it for you.