Originally Posted by
cyclotoine
I really don't recommend vintage MTBs for trail riding unless you are in an area with smooth non-technical trails. If you want to go rigid I suggest a modern + size bike like the surly krampus or even a fat bike with a 4" tire. You'll have a lot more fun on these machines. We have learned a lot about the sport in the last 25 years and steep angles and long stems don't really make sense. Slack angles, longer top tubes and short stems with wide bar make a lot of sense. I love vintage bikes. I am restoring a couple MTBs at this time and have stash of sweet early 1990s mtb parts including Mag21s (one of the best), but my choice for recreational fun is modern equipment, more comfortable, safer and a blast to ride.
If you must go vintage I would check out the pre-1997 forum on retrobike to get some ideas.
I was curious. I have a surly 1 x 1 which is a modern mountain bike. In my size (18) inch it has a top tube of 570 mm (596 effective) and angles are 71/73.
I also have a 20 inch trek 950 I just picked up.That's my size as well. It has a top tube of 56.5 so it's shorter; the angles are also 71/73.
That's just one data point but it's not so clear to me that mtbs have gotten slacker.
If you really want slack angles, I wouldn't be surprised if those 1980 era mtbs have slacker angles than modern bikes.