When it comes to mountain bikes there are many components that split opinion and stem length is definitely one of them. I’m a firm believer in the merits of a 50mm stems on any bike.
For me it is a simple matter of physics, balance and control. I’m also of the opinion that the 50mm stem is a great leveller when it comes to nailing down the performance characteristics of different bikes. I would much rather ride a bike with geometry and ‘fit’ that complement a 50mm stem than create the correct reach through fitting a larger unit. A longer toptube and the resulting wheelbase usually equates to a more stable handling bike anyway.
The more directly you are over the centre of the front wheel the better. If I want the force of my weight to be transferred anywhere when going forward I would rather that force to be as direct as possible towards the centre of the front wheel and through the suspension.
This may be less of an issue if the terrain is particularly mild or if you are doing most of your miles on the road where aerodynamics and pedal efficiency take precedent over weight transfer and dynamic body position.
Running a 50mm stem has saved many an over-the-bar moment, giving me and my upper body the ability to take a hit and push that force back into the wheels instead of sending that force over the bars away from the forward momentum of the wheels and out of my control. I believe they are safer for this reason – ou are forced to ride a little further back on the bike and as a result you ride ‘in’ the bike more. Lengthen the stem and your bodyweight is constantly being shifted out of line with the bike. You are forced too far forward and forever on the edge of being pitched over the bars.
I want to know that any downward force I put into the bars is being directly translated to the tyre. If I push down I want that force to compress the tyre into the ground. As we already know, more pressure on the tyre is going to equate to more grip. A shorter stem puts you further back on the bike meaning your weight is always pushing the bike forward and carrying momentum while maintaining balance and control.
Picking the front of the bike up and pumping through terrain is also a lot easier to do with a shorter stem. However, climbing with a shorter stem the front end is essentially unweighted. This is something I have no issue with and it is something that you quickly get used to, and with the range of adjustability around these days, dropping the forks will add more weight over the front-end when climbing anyway. The advantages the shorter stem has in all other areas of handling and control far outweigh this minor niggle.
Why any complete mountain bike (aside from cross country race bikes) comes fitted with anything longer than a 50mm stem remains a mystery to me.