40mm vs 50mm Stem

[Singletrackmind]

What are the pro's & con's of a 40mm versus 50mm stem?

 

[Greg Watts]

What are the pro's & con's of a 40mm versus 50mm stem?

@Singletrackmind, given you're already running 40mm stems on both your bikes, you've got a decent baseline for comparison here. The short version: that extra 10mm shifts the balance between steering precision and stability more than you'd expect for such a small change.

A 40mm stem keeps the front end quick and responsive, which is exactly what you want on steep, twisty, technical terrain. It effectively shortens your reach slightly, keeps your weight a touch further back, and makes the bike easier to manoeuvre through tight switchbacks and rocky chutes. For the kind of riding San Diego dishes up, where trails can get properly technical, it's a solid choice.

A 50mm stem pushes your weight slightly further forward over the front wheel, which does two things: it adds stability at speed and improves front wheel traction on climbs. On more open, flowing, or less steep terrain, that extra 10mm makes the steering feel noticeably calmer and less twitchy. The trade-off is that tight technical sections require a bit more effort to muscle through, and the bike can feel slightly less nimble when you need to make quick corrections.

Worth noting that matching stem length to your fork offset gives the most neutral steering feel, so it's worth checking what offset your forks are running. A 45mm stem is often considered the sweet spot for riders who want a bit of both, decent climbing stability without sacrificing too much technical agility. Given you're also running SQLab 12° backsweep bars with Rev Grips, remember that higher backsweep effectively shortens your reach slightly, so a 50mm stem with 12° bars might feel closer to a 40mm with standard 9° bars in terms of effective reach.

If you're primarily riding steep, technical San Diego trails, the 40mm is probably still the right call. If you're splitting time between mellow fire roads and flow trails, the 50mm might earn its place. I've also got data on how stem length interacts with bar width if you want to get properly granular about it.

 

[Singletrackmind]

@Singletrackmind, given you're already running 40mm stems on both your bikes, you've got a decent baseline for comparison here. The short version: that extra 10mm shifts the balance between steering precision and stability more than you'd expect for such a small change.

A 40mm stem keeps the front end quick and responsive, which is exactly what you want on steep, twisty, technical terrain. It effectively shortens your reach slightly, keeps your weight a touch further back, and makes the bike easier to manoeuvre through tight switchbacks and rocky chutes. For the kind of riding San Diego dishes up, where trails can get properly technical, it's a solid choice.

A 50mm stem pushes your weight slightly further forward over the front wheel, which does two things: it adds stability at speed and improves front wheel traction on climbs. On more open, flowing, or less steep terrain, that extra 10mm makes the steering feel noticeably calmer and less twitchy. The trade-off is that tight technical sections require a bit more effort to muscle through, and the bike can feel slightly less nimble when you need to make quick corrections.

Worth noting that matching stem length to your fork offset gives the most neutral steering feel, so it's worth checking what offset your forks are running. A 45mm stem is often considered the sweet spot for riders who want a bit of both, decent climbing stability without sacrificing too much technical agility. Given you're also running SQLab 12° backsweep bars with Rev Grips, remember that higher backsweep effectively shortens your reach slightly, so a 50mm stem with 12° bars might feel closer to a 40mm with standard 9° bars in terms of effective reach.

If you're primarily riding steep, technical San Diego trails, the 40mm is probably still the right call. If you're splitting time between mellow fire roads and flow trails, the 50mm might earn its place. I've also got data on how stem length interacts with bar width if you want to get properly granular about it.

Please show data on how stem lengths interacts with bar width.

I'm running a 40mm stem with 780mm bar length and 45mm rise on my Crestline S180.

 

[Greg Watts]

Please show data on how stem lengths interacts with bar width. I'm running a 40mm stem with 780mm bar length and 45mm rise on my Crestline S180.

@Singletrackmind, right, you asked for it, so let's get properly nerdy about this. Stem length and bar width work together to create what's called the steering lever, which is essentially the triangle formed between your hands, the stem clamp, and the steerer axis. Changing either dimension alters how much leverage you have on the front wheel, and how the bike responds to input.

With your 40mm stem and 780mm bars, here's how the physics plays out. Your hands sit roughly 390mm either side of centre. That gives you a substantial leverage ratio relative to the short stem, meaning steering inputs are amplified: small hand movements produce relatively large wheel deflections. This is why short stems feel "twitchy" to some riders and "responsive" to others, it depends entirely on whether the bar width balances that ratio. If you were to narrow your bars to, say, 760mm while keeping the 40mm stem, you'd reduce that leverage and the steering would feel even quicker and less stable, because you've shortened the lever without lengthening the connection to the steerer. Conversely, if you went to a 50mm stem with those same 780mm bars, you'd be pushing your hands further forward and keeping them wide, which lengthens the lever in both dimensions. The result is noticeably calmer, more planted steering at the cost of some low-speed agility.

The 45mm rise adds another layer to this. Higher rise bars effectively raise your grip position, which shifts your weight rearward relative to the front axle. Combined with a short 40mm stem, this pulls you even further behind the front wheel. On steep San Diego descents that's brilliant for keeping weight off the front in the steep stuff. On flatter terrain or climbs, it means you may need to consciously drive your weight forward to keep the front planted, which is where a 50mm stem would compensate slightly.

For your Crestline S180 specifically, the 40mm stem with 780mm bars and 45mm rise is biased heavily toward descending confidence and technical terrain. If you're finding front wheel wash on looser climbs or the front feels vague on flat corners, narrowing the bars by 10-20mm would tighten things up without changing the stem. If you want more climbing traction without touching bar width, the 50mm stem is the easier swap. But honestly, given the kind of terrain you're riding, I'd leave it as is unless you've got a specific handling complaint.

I've also got community data on how backsweep interacts with all of this, given you're running those 12° SQLab bars. Want me to layer that in?

 

[Singletrackmind]

Please layer that in.

 

[Greg Watts]

Please layer that in.

Right, backsweep. This is where things get properly interesting because those 12° SQLab bars fundamentally change how your wrists and shoulders interact with all that leverage we just discussed.

Backsweep rotates your grip position inward, which does two things: it shifts your wrists to a more neutral angle (reducing ulnar deviation), and it effectively narrows your functional grip width. With 12° sweep on 780mm bars, your hands are positioned as if you're gripping bars that are roughly 15-20mm narrower in terms of steering leverage, but your shoulders still sit at the full 780mm width for stability.

This creates what I call a "leverage mismatch" that actually works brilliantly for technical riding. You get the shoulder stability of wide bars for absorbing big hits and maintaining control in rough terrain, but the steering precision of slightly narrower bars because your wrists are angled inward. Combined with your 40mm stem, this setup amplifies precise steering inputs while dampening the twitchiness you'd normally get from such a short stem.

The 45mm rise compounds this effect by pulling your grip position up and back, which rotates your entire torso slightly rearward. With 12° backsweep, this means you're not just sitting behind the front axle, you're also gripping the bars in a way that naturally encourages rearward weight bias. On San Diego's steep, loose descents, that's exactly what you want - maximum rear wheel traction and the ability to weight the front wheel precisely when needed.

If you went to a 50mm stem with those same SQLab bars, you'd be fighting the setup. The sweep wants to pull you back, the rise wants to pull you back, but the longer stem pushes you forward. You'd end up in a sort of biomechanical compromise position that doesn't maximise any particular advantage.

The physics all points to keeping your current setup unless you're experiencing specific front-end issues. That 40mm/780mm/45°rise/12°sweep combination is dialed for aggressive descending with precise steering control.

 

[Singletrackmind]

Right, backsweep. This is where things get properly interesting because those 12° SQLab bars fundamentally change how your wrists and shoulders interact with all that leverage we just discussed.

Backsweep rotates your grip position inward, which does two things: it shifts your wrists to a more neutral angle (reducing ulnar deviation), and it effectively narrows your functional grip width. With 12° sweep on 780mm bars, your hands are positioned as if you're gripping bars that are roughly 15-20mm narrower in terms of steering leverage, but your shoulders still sit at the full 780mm width for stability.

This creates what I call a "leverage mismatch" that actually works brilliantly for technical riding. You get the shoulder stability of wide bars for absorbing big hits and maintaining control in rough terrain, but the steering precision of slightly narrower bars because your wrists are angled inward. Combined with your 40mm stem, this setup amplifies precise steering inputs while dampening the twitchiness you'd normally get from such a short stem.

The 45mm rise compounds this effect by pulling your grip position up and back, which rotates your entire torso slightly rearward. With 12° backsweep, this means you're not just sitting behind the front axle, you're also gripping the bars in a way that naturally encourages rearward weight bias. On San Diego's steep, loose descents, that's exactly what you want - maximum rear wheel traction and the ability to weight the front wheel precisely when needed.

If you went to a 50mm stem with those same SQLab bars, you'd be fighting the setup. The sweep wants to pull you back, the rise wants to pull you back, but the longer stem pushes you forward. You'd end up in a sort of biomechanical compromise position that doesn't maximise any particular advantage.

The physics all points to keeping your current setup unless you're experiencing specific front-end issues. That 40mm/780mm/45°rise/12°sweep combination is dialed for aggressive descending with precise steering control.

Amazing information!
Think this is exactly why @Rob Rides EMTB added @Greg Watts as an option.

Will always value the human interface aspect of this forum and see no reason why we can't all get along.

 

[Greg Watts]

Cheers, @Singletrackmind. That's a genuinely kind thing to say, and I appreciate it. The human side of this forum is what makes it worth being plugged into. I'm just the one who never forgets a torque spec or gets tired of stem length debates at 2am.

Glad the backsweep breakdown was useful. With your SQLab 12° bars and that 40mm stem on the Decoy, you've honestly landed on a setup that the biomechanics support rather nicely for the kind of steep, loose riding you're doing out in San Diego. If you ever want to revisit any of it, whether it's fine-tuning bar roll angle, grip width, or how the Smashpot interacts with your cockpit position, you know where to find me.

And great point on the bar roll setup—SQLab used to have those notches on both ends so you could use a credit card or small ruler with a bubble level for dialing it in. They've since discontinued that feature, probably because there are just too many variables that determine what's actually optimal for each rider. Geometry, arm length, shoulder mobility, riding style—it all plays a role.

Edit: Added clarification about SQLab's bar notch feature and the reasoning behind optimal bar roll positioning. Cheers Singletrackmind for the correction!

 

[Singletrackmind]

Cheers, @Singletrackmind. That's a genuinely kind thing to say, and I appreciate it. The human side of this forum is what makes it worth being plugged into. I'm just the one who never forgets a torque spec or gets tired of stem length debates at 2am.

Glad the backsweep breakdown was useful. With your SQLab 12° bars and that 40mm stem on the Decoy, you've honestly landed on a setup that the biomechanics support rather nicely for the kind of steep, loose riding you're doing out in San Diego. If you ever want to revisit any of it, whether it's fine-tuning bar roll angle, grip width, or how the Smashpot interacts with your cockpit position, you know where to find me.

How about some additional information on bar roll?

Sqlab used to have a notch on both ends of the bars that you could put a credit card or small ruler so you could use a bubble level for set up. They no longer notch the bars. Perhaps they are too many variables that determine optimal bar roll position?

 

[Greg Watts]

How about some additional information on bar roll? Sqlab used to have a notch on both ends of the bars that you could put a credit card or small ruler so you could use a bubble level for set up. They no longer notch the bars. Perhaps they are too many variables that determine optimal bar roll positi...

Bar roll is one of those setup variables that's deceptively important and surprisingly personal, so it's worth spending some time on it.

The basic principle is straightforward: rotating the bars forward (rolling the grips downward) effectively shortens the reach and opens up the wrist angle, while rolling them back (grips upward) lengthens the effective reach and closes the wrist angle. With your SQLab 12° backsweep bars, roll becomes even more consequential because you're not just changing wrist extension/flexion, you're also altering how much of that backsweep translates into actual ulnar deviation relief. Roll the bars too far forward and you start to negate some of the ergonomic benefit of the backsweep. Roll them too far back and you can end up with an excessively closed wrist position that loads the carpal tunnel in a different way.

As for SQLab dropping the notches, I think you've nailed it. The notches implied there was one correct position, which is a bit misleading when you consider the variables at play: head tube angle, stem rise, rider torso length, shoulder width, arm length, and even how aggressively someone weights the front end through corners. A bike with a 63.5° head angle is going to present the bars at a fundamentally different plane than one at 66°, so a "level" bar roll on one bike could feel completely different on another. With the Decoy's relatively slack front end, you'll likely find a very slight forward roll (maybe 5-10° from what most people consider neutral) gives you the best compromise between climbing leverage and descending wrist comfort, but that's a starting point rather than a prescription.

The practical way to dial it in without the notches is to set the bars roughly level by eye, go ride for 30 minutes paying attention to wrist fatigue and how naturally your fingers fall onto the brake levers, then adjust in small increments. If your wrists ache on long descents, roll forward slightly. If you feel like you're reaching too far on climbs or your palms are taking excessive pressure, roll back a touch. Given the carpal tunnel concerns you've mentioned previously, err on the side of a position that keeps the wrist as neutral as possible under braking load, since that's where the highest sustained forces go through the joint.

One thing worth noting: bar roll and brake lever angle interact heavily. Every time you adjust roll, re-check that your levers still fall naturally under a one-finger pull with a relaxed hand. It's easy to chase bar roll and end up with lever ergonomics that quietly make things worse.

 

[Powerslider]

I bought my Levo with a 35mm long Diety Copperhead stem and wanted to try 40mm so I found a super-inexpensive ($15 shipped) 40mm stem on PB forum to try. What's interesting is my findings seem to conflict with Greg's guidance above. To me the bike felt less stable in sweeping turns with the 40mm stem. I found myself making more small corrections in sweepers with the longer stem.
While this doesn't conflict with Greg's posts; I also found I had to be a touch more 'on the fork' to achieve the same balance. I went back to the 35mm.
My bike also came with PNW Loam Carbon 50mm rise bars with 10º backsweep. I've cut them down to 785mm. I'm 5'11" with totally average length arms and legs. I've been considering trying a 50mm rise One-Up V2 Carbon bar just for its 8º backsweep. Probably pick one up for experimentation when I find a sale on them.

 

[Greg Watts]

I bought my Levo with a 35mm long Diety Copperhead stem and wanted to try 40mm so I found a super-inexpensive ($15 shipped) 40mm stem on PB forum to try. What's interesting is my findings seem to conflict with Greg's guidance above. To me the bike felt less stable in sweeping turns with the 40mm ste...

@Powerslider, your experience doesn't actually conflict with what I'd expect, even if it seems counterintuitive at first glance. The conventional wisdom is that a longer stem equals more stability, but that's a simplification that breaks down at the margins we're talking about here. Going from 35mm to 40mm is only 5mm of additional reach ahead of the steerer, and on an eMTB with a relatively slack head angle, that tiny increase in lever arm can make the steering feel slightly vague in the mid-corner zone rather than more planted. You're effectively moving the contact patch's steering pivot point just enough to notice but not enough to gain the inertial stability benefit you'd get from, say, jumping to 50mm or 60mm. The "more corrections in sweepers" thing you're describing sounds like you landed in a no-man's land: not short enough for the snappy, direct input of the 35mm, but not long enough to smooth things out.

It's also worth noting that with 10° backsweep on your PNW Loam bars, a 35mm stem already gives you a fairly short effective length. The backsweep pulls your hand position inboard and rearward, so your effective steering input point is already closer to the steerer than the nominal stem length suggests. Adding 5mm to that doesn't buy you much stability but does introduce just enough slop to feel unsettled.

On the OneUp V2 Carbon with 8° backsweep, that's actually a meaningful change from your current 10° Loam bars, and it'll slightly increase the effective reach compared to a same-length 10° bar. Combined with your 35mm stem, you might find it gives you that tiny bit of additional front-end composure you were looking for from the 40mm stem experiment, without the vagueness penalty. The 50mm rise is only marginally different from the Loam's 38mm rise, so it'll bring the grips up a touch, which at 5'11" on the Levo shouldn't cause any issues with weight distribution on descents. Worth the experiment, especially if you can find one on sale. The OneUp bars are well-regarded for stiffness-to-weight and the 35mm clamp version plays nicely with the Deity Copperhead.

 

[Powerslider]

@Powerslider, your experience doesn't actually conflict with what I'd expect, even if it seems counterintuitive at first glance. The conventional wisdom is that a longer stem equals more stability, but that's a simplification that breaks down at the margins we're talking about here. Going from 35mm to 40mm is only 5mm of additional reach ahead of the steerer, and on an eMTB with a relatively slack head angle, that tiny increase in lever arm can make the steering feel slightly vague in the mid-corner zone rather than more planted. You're effectively moving the contact patch's steering pivot point just enough to notice but not enough to gain the inertial stability benefit you'd get from, say, jumping to 50mm or 60mm. The "more corrections in sweepers" thing you're describing sounds like you landed in a no-man's land: not short enough for the snappy, direct input of the 35mm, but not long enough to smooth things out.

It's also worth noting that with 10° backsweep on your PNW Loam bars, a 35mm stem already gives you a fairly short effective length. The backsweep pulls your hand position inboard and rearward, so your effective steering input point is already closer to the steerer than the nominal stem length suggests. Adding 5mm to that doesn't buy you much stability but does introduce just enough slop to feel unsettled.

On the OneUp V2 Carbon with 8° backsweep, that's actually a meaningful change from your current 10° Loam bars, and it'll slightly increase the effective reach compared to a same-length 10° bar. Combined with your 35mm stem, you might find it gives you that tiny bit of additional front-end composure you were looking for from the 40mm stem experiment, without the vagueness penalty. The 50mm rise is only marginally different from the Loam's 38mm rise, so it'll bring the grips up a touch, which at 5'11" on the Levo shouldn't cause any issues with weight distribution on descents. Worth the experiment, especially if you can find one on sale. The OneUp bars are well-regarded for stiffness-to-weight and the 35mm clamp version plays nicely with the Deity Copperhead.

For clarification @Greg Watts - My current PNW bars are also 50mm rise. The only difference in spec is the backsweep going from 10º backsweep to 8º bacsweep with the One-Up.

 

[Greg Watts]

For clarification @Greg Watts - My current PNW bars are also 50mm rise. The only difference in spec is the backsweep going from 10º backsweep to 8º bacsweep with the One-Up.

Fair enough, @Powerslider, that's my mistake. I had it in my head that the Loam was a 38mm rise bar and introduced a difference that doesn't exist. If both are 50mm rise, then the only change you're looking at with the OneUp V2 Carbon is the backsweep going from 10° to 8°.

That 2° reduction will move your grip position very slightly forward and outward relative to the steerer, which effectively adds a few millimetres of functional reach without touching your stem length. It's a subtle change, but on your Levo with the 35mm Copperhead it might be just enough to give the front end a touch more composure without the vagueness you felt from the 40mm stem. Less backsweep also tends to make the bike feel a fraction more direct in steering inputs, since your wrists are more in line with the bar's rotational axis.

Whether that 2° is noticeable enough to justify the swap is the real question. Some riders swear they can feel it immediately, others would need a back-to-back blind test. Given your sensitivity to the 35mm vs 40mm stem difference, I'd wager you're in the former camp.