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Where exactly does your bum go on steeper descents? You lower the saddle (by all of the 120-150 mm it will go); and you pull as much of your weight as far back and as low as you can go to decrease the chance you'd go over the bars.

But does your bum still remain above the saddle, or do you manage to pull it behind the saddle?

Getting one's bum behind and also below the saddle seems ideal for safety. A big issue is then that baggier MTB clothing can snag on the saddle, and if a climb immediately follows the descent, getting stuck behind the saddle becomes again risky.

Is this (one of) the basic MTB bike fit tests? If you can't go cleanly behind the lowered saddle, does that make you wonder whether your bike is one size too large?

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    Yup, weight back and low. Just don't sit on the back wheel. As for getting clothing snagged on the seat. Been there, done that myself but it's the type of thing you probably only do once or twice and learn the lesson. Also suggest that if even baggies are often getting snagged in the crotch probably more an indicator the size of your pants is too big than the bike
    – Hursey
    Commented Feb 10, 2023 at 1:11
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    @Hursey Thanks for confirming. I think MTB clothing is rather silly anyway (bicycles.stackexchange.com/q/85402/48599). Now we have an additional functional deficiency.
    – Sam7919
    Commented Feb 10, 2023 at 1:25
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    I got my camelbak chest strap hooked under the nose of the saddle once on a very steep section. Do not recommend!
    – Andy P
    Commented Feb 10, 2023 at 1:29
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    There is a difference between baggy clothing and baggy clothing designed for MTB of course. My purpose MTB baggy shorts seem to take this into account, where as street clothing probably doesn't
    – Hursey
    Commented Feb 10, 2023 at 3:21
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    Look up the YouTube video by Ben Cathro/Pinkbike on the “boss stance”. The focus should be getting low and remaining balanced between your two wheels to maintain traction and maneuverability
    – Paul H
    Commented Feb 10, 2023 at 14:52

3 Answers 3

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As I elaborated upon in considerable detail in this answer, it's mandatory that you treat body position while MTBing as a continuously variable thing, not a binary switch. You position your hips where needed for that specific instant. Unless you're trying to do a manual the entire way down the trail, putting your weight behind the saddle all the time is a bad idea.

I usually have my hips above the saddle. They only go any farther back for dynamic moves. You can't control the front wheel as well when your arms are fully extended like they would be in that position. Also, I don't think having difficulty with this move is indicative of a too-big bike. In fact, one benefit of newer MTBs with their long top tubes is having more maneuver room in front of the saddle.

A specific problem with having your butt too low and too far back is that it may intersect with the rear wheel. This is especially likely on full suspension bikes. It's neither comfortable (even with padded shorts) nor does it help you with your riding.

Also, 120-150mm is a fairly small dropper these days. There's 240mm ones on the market now!

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Depends entirely how steep the hill is.

The aim is to keep the bike balanced maintaining reasonable (50%) of weight on the rear wheel, and when that can no longer be achieved, keep the center of gravity behind the front axle. To do this, as the hill angle increases, the center of your gravity (which is higher than the front axle) rotates forward if you remain in the same position. To counter this, you need to move you weight backwards.

Thinking only of the static situation for a moment, at moderate angles, you weight shift can maintain the weight distribution between the front and rear wheels. At more extreme angles, you need to sit on the rear wheel and can no longer move you weight far enough back. At this point, the weight on the rear wheel is reducing. At some point of steepness, you can no longer keep the center of gravity behind the front axle, all the weight is now on the front wheel. In a static situation, this is when you are about to go over the bar.

In the over simplified dynamic situation, you are also dealing with forces from bumps - front wheel bouncing will keep you from going over momentarily, rear wheel bouncing with flick you over in an instant. Braking affects things as well and changes weight distribution quickly (and reliably for an experienced rider). Ultimately it is an incredibly complex set of dynamics that gets a mountain biker down a hill upright, but weight too far back has far fewer disadvantages than weight too far forward. Reality is above certain angles, you no longer ride down the slope, you jump it, free fall and no longer keep weight all the way back, but level the bike and balance the weight distribution for the landing.

Quickest way to learn this (speaking from experience) is descent a particularly long, steep hill, hit a bump that kicks the unloaded rear wheel high in the air. Unable to get the rear wheel down, but not (yet) over the bars gives more than enough time to both contemplating the errors you just made while simultaneous wondering how this is going to end....

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I agree with centered over bike, I would refine as my center of gravity over the bottom bracket. If you put bike pedals resting on 2 buckets/benches, then climb aboard, you can rotate bike around the bottom bracket axis as body stays stable. It's easiest to start on a pump track and keep body steady as bike pivots below. You might look at this, especially after about 5 min.

Cheers!

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