How Short can—or should—we go?

A Design Manifesto by Kirk Pacenti

1. Riding the Unridable: The Evolution of Mountain Bike Geometry.
Mountain bikes are all about evolution. In addition to obvious items like frame materials and equipment, bike geometry itself is constantly evolving. In fact, MTB geometry has evolved rapidly during the past 5-6 years as riders learn to ride the (previously) unridable. More about that in just a minute. Right now, let’s just stick to what drives MTB evolution.

Three interdependent forces are continuously pushing mountain bikes forward. Advancements in rider skill levels, increasingly steep and technical terrain, and the development of new technology: bikes and equipment equal to the demands of rider and terrain.

  • Advances in skill, fitness, and training allow riders to engage with more challenging terrain, and drive the evolution of technology, including bike design, materials and equipment. In turn, tougher terrain puts new demands on bike and rider. And finally, better design and technology makes new terrain conquerable and, as riders attempt the previously impossible, their skills grow to meet the challenge.
  • As performance demands increase in all three areas, the trend now is more and more toward achieving optimal performance—much the way it is in MX and GP motorcycles—and less about conventional notions of rider/bike “fit.”
  • So here’s the Bottom Line: Once we get past conventional notions of what a bike “ought to be,” that evolution in thinking makes complete sense.

Optimal fit for XC Racing is not optimal for flowing a trail, railing a berm, or nailing a 30-foot step down. And vice-versa.

What’s Changed, What’s Changing, and what’s stuck.

decsend-with-conviction2.1. Riders. We’re physically stronger now; we’re gymnasts as well as cyclists. Compare the upper bodies of a Freerider with an XC racer. But XC riders have evolved to ride Trail and Enduro disciplines. All-Mountain, DH and Freeriders are building strength on pump tracks. And just about everyone hangs out at the bike park on weekends.

Influences from BMX and motorcycles have transformed not just our bikes, but ourselves as riders too. We have body armor for extreme riding. Even foam pits. So mistakes become learning opportunities—part of the process—and not disasters. Well, not as often, anyway.

2.2. New technology is introduced from outside the cycling paradigm, and most frequently from motorcycles. Suspension and disc brakes are two obvious examples. Two not-so-obvious ones are tire technology and wheel size. Plus we have dropper posts now (one example of bike-specific innovation developed expressly for mountain bikes way back in the ‘80s and re-invented thirty years later).

2.3 Bike Fit. For years we’ve been told to “ride the smallest bike you can”. Smaller bikes, they tell us, are more maneuverable under the rider and tend to weight (a little) less.

The Conventional Wisdom of Downsizing.

They’ve been telling us wrong.

Here’s why: although downsizing may sound like a good idea at first, if you think about it you’ll see that the default toward smaller bikes has some serious drawbacks. Most importantly, a downsized bike has a shorter top tube, moving the rider’s center of gravity forward and putting too much weight over the front wheel (endo-prone).

Even worse, the shorter top tube requires a longer stem to achieve proper fit, (which slows handling response) and has a negative impact on weight distribution, moving the rider’s center of gravity farther forward, making rearward weight shifts and steering inputs more difficult and less responsive.

Longer stems also require more steering input from the rider and sacrifice some of the control benefits of wider bars. We’ll discuss that at length (heh) in Part 5.

All too Often, “Conventional Wisdom” isn’t


Endo much? Once you understand the geometry issues, the disadvantages of conventional downsizing wisdom become obvious. Note stem length on rider at right.

Sometimes, it IS the Size That Counts

For nearly 20 years I’ve argued against the conventional wisdom of downsizing for the reasons discussed above. Instead of “smaller is better” I’ve been advocating that riders should go for longer frames instead.

I call it upsizing.

if you’re between sizes,
almost always go for the bigger (longer) bike.

The industry is (slowly) starting to come around to this view. Current trends in geometry are for steeper seat tube angles, longer top tubes and shorter stems; slacker head tube angles and a longer wheelbase.

This is a healthy trend, but I propose bike designers start making bikes even longer. In fact, we should be designing bikes not just around longer top tubes and shorter stems, but longest-possible top tubes and shortest-possible stems. Here’s what one industry pioneer told us on the topic back during an online chat in March, 2015.

“The other thing that stem length affects is the tendency to go over the bars. Short stems force the rider’s CG further back and make it easier to avoid going over the bars in technical situations (steep rocky descents, big rocks or roots, etc.).

Practically speaking, trends dictate stem length to a large extent. The recent trend towards shorter stems is consistent with the way bikes are getting used (in the extreme case). Handling in severe technical terrain is a priority for many riders.”

Who said this?

—Keith Bontrager

We are On The Cusp of a Geometry Revolution.

I agree with Keith, but I also believe the next step in MTB frame evolution requires an MTB geometry revolution.The trend toward steeper seat angles allows big- travel trail bikes to pedal a great deal better… The steeper seat angle also happens to increase the reach to the bars (for any given TT length) and requires a shorter stem. This, coupled with increasing TT lengths, requires substantially shorter stems.

This combination results in easier uphill pedaling, which benefits everyone, especially riders on long-travel bikes. It also creates a longer front center, which provides better stability. And it’s easier for riders stay centered, which means they don’t have to hang off the bike (as much) for descents.

In addition to the longer top tube/steeper seat tube/shorter stem, the new geometry features wider handlebars, improving control in a variety of situations.

  • Another popular trend is lower bottom brackets for increased stability on the corners.
  • But here’s the “stuck” part I mentioned way back at the top of this section. The limiting factor to continued MTB geometry development is stem design: we can’t make top tubes longer without making stems shorter to compensate. And stems are already as short as we can make them.

If MTB geometry is going to continue to evolve, stems have to be made shorter than the current limitation, which is Ø30.2–Ø35.0, the point at which the bar contacts the steerer.

“Long stems are a throwback to the road roots of our industry. They don’t reflect the reality of riding off road in the modern era.

“The kind of trails that the average guy rides on a 6 inch trail bike these days could have been a World Cup DH course in the 1990s and the general standard of bikes and riding means average speed has gone up too.

“Stems have got shorter for more control too and everyone agrees short stems are better! It’s just that the industry seems scared to try even shorter!”

Who said this?

—Chris Porter, Mojo Suspension UK

We can’t get shorter without also getting higher.

Every less-than-30mm stem to date has carried with it a greater-than 30mm penalty in height.

The problem with existing 0mm–10mm stem solutions going back to the 1990s is not so much about stem length as it is that the advantage of these shorter stem is more than offset by the disadvantage of increased height. Which brings us back to the two operative questions presented in the title of this manifesto:

What Is optimal stem length,
and how do we achieve it?

Steering System Mechanics and Bar/Stem Design.

It turns out there’s an awful lot we don’t know about how stem length affects steering, especially with very short stems. This is partly because there hasn’t been a practical sub-30mm solution, so there hasn’t been a lot of point to theorizing about them.

But here are some things we do know about shorter stems, both from rider input and some simple calculations.

  • We know riders (who have tried them) overwhelmingly prefer shorter stems.
  • Shorter stems allow for wider bars/wider bars require shorter stems.
  • Wider bars effectively increase reach, further requiring shorter stems.
  • Shorter stems = increased handling precision: as much as 40% reduction in rider input to affect the same steering output. (25mm vs. 80mm stem).
  • Shorter stems combined with a longer top tube improve fore-aft position* (better centered between the wheels*), for better weight distribution. in extreme climbs, steeper seat angles can offset the shorter stem, allowing the rider to keep the front wheel on the ground.
  • Beyond pure weight distribution, a shorter stem combined with a longer top tube puts more of the rider’s weight behind the steering axis, riders can stay (more) centered between wheels, even on steep descents. Instead of moving the rider’s body back to get further behind the front wheel, why not just put the front wheel farther out in front of the rider?

So, if we suspect conventional stem length of 30-35mm is too long for optimal geometry, is the ASAP (As Short As Possible) hypothesis correct? Do we really want 0mm stems?

Maybe. But I suspect not, for reasons we’ll discuss in Part 4.

The Pacenti Hypothesis:

Optimal stem length = >12 but <30mm.

We believe already know that 30mm stems are too long to be optimal. But 0mm stems are probably not going to be an optimal, either. At 0mm, the sweep of any realistic bar configuration will likely place the rider’s hands too far behind the steering axis.creating a “tiller” effect As a purely practical matter, I believe this will tend to be the case for anything less than 10mm.

Shorter-than-30mm stems must become available for bike geometries to progress. This will become even more critical if industry adopts 35mm diameter handlebars (as Easton, Race Face, and others are doing), which will push conventional stems out to 32mm.

So we know where we want to go, but how do we get there?

Sometimes One Ride Changes Everything

About eighteen months ago I was riding some of the very steep and very twisty trails we have in the Southeast and thinking about how my bike’s geometry affected the way I was getting around those twisty corners. And the thought just appeared in my head:

“If I could just put a dimple on the bar,

I could shorten my stem another 10mm.”

And that’s how PDent™ was born.

pdent-horizontalPatent-pending PDent technology creates an elegant solution to the short stem/low bar challenge. As the logo implies, dimpling the bar (D) lets us bring it closer to the steerer tube (P). It’s that simple.

In CAD terms, the “dimple” is a swept radial cut forming a pocket that allows the bar to wrap around the steerer tube. The dimple is designed to allow for a range of head tube angles, plus several degrees of fore/aft rotation.

By dimpling the handlebar we can make stems pretty much as short as we want. And unlike current 0-10mm stems, we can also keep handlebars as low as possible. The PDent bar/stem interface also offers the convenience benefit of making initial setup easier by automatically centering the bar. Finally, it allows some margin of safety in the case of stem bolt failure after setup.


Q: The sample PDent stem you’re showing is 25mm. Other stem manufacturers are already as low as 30mm (nominal length). Is a 5mm difference enough to matter?
A: Short answer, yes. The difference between a 100mm stem and a 95 is not so much (5%). But the difference between 30 and 25mm turns out to be a lot (17%). Longer answer: let’s put that 5mm into context:

  • 27.5” vs 29” wheel is only a 5% increase in diameter, yet there’s a very noticeable difference (feel) on the trail.
  • 148×12 hub width is only a 4% increase in over 142×12, but offers substantial improvement to wheel strength/stiffness, as well as increased pivot width (stiffness) and chain ring clearance.
  • A short chainstay (420mm) is only 4.5% shorter than a “long” one (440mm) but most riders believe this makes a big difference.

So now let’s talk about 5mm of stem difference:

Reducing stem length from 30mm to 25mm is a 17% net change. That’s huge.

  • With an 800mm handlebar, this yields a 5% reduction in rider input required to affect a directional change
  • Compared to 80mm, A 25mm stem creates a 40% reduction in amount of rider input (hand movement) needed to affect steering. Multiplied hundreds or thousands of times over the course of a single ride equals more responsive steering and a massive reduction in fatigue.

Q: Will other companies be able to use PDent technology?
A: Yes and no. PDent is patent pending; we expect our patents to be issued soon. But we’re happy to discuss licensing the technology, so theoretically, you could see PDent on other brands as early as 2017 model introductions. Earlier than that as an aftermarket item.

Q: Will standard stems work with PDent bars?
A: Yes. Testing shows PDent bars meet all industry standards. In theory, other manufacturers could start making sub-30mm stems right now, and match them to PDent bars. And we’re OK with that (subject to a license agreement).

Q: Will other bars work with PDent stems?
A: No. PDent stems are too short to accommodate regular bars. They cant make it past the bike’s steerer.

Q: How short can PDent stems go?
A: The current iteration will accept stems as short as 15–20mm. By deepening the dimple, we can go as far as 12mm. Shorter than that involves other intellectual property which is in our patent but we’re not prepared to discuss.

Q: So if 30mm is too long and 0-10mm is too short, what’s the “optimal” stem length?
A: In theory, somewhere between 12mm-27mm. Beyond that, no one really knows, although our best guess currently is something on the order of 15-25mm. The stem currently in production is 25mm, but ultimately, our IP allows us to go as short as we need to.

Q: PDent puts a dimple in the middle of the bar, which seems like a highly stressed area. How does the dimple affect bar strength?
A: We had the same concern, which is why we conducted extensive FEA and laboratory tests before riding prototypes, much less releasing them to the public. We were surprised to learn that for practical purposes, the PDent dimple has zero effect on bar strength, even with the very deep dimple required for a 15mm stem length. Here’s what we learned from the FEA:
FEA testing shows primary load paths at approximately 90° to the PDent dimple along the bar radius. All real loads are concentrated at the edge of the stem where the bar enters the stem.


Q: Will PDent work with my current bike or, do I need a new one with a special geometry?
A: Maybe. Basically, there are three ways riders can use PDent technology. Here they are, starting with the least desirable.

  • If you can comfortably take the difference out of your current riding position, you can put a PDent on your current bike and with a slightly wider bar, it should work well. Your center of gravity will move back with some of the handling and weight distribution benefits we discussed earlier. Obviously, the shorter the stem you’re on now, the easier it’ll be to make the transition
  • You may be able upsize to a larger bike to get a longer top tube, which will compensate for the difference between your current stem and your PDent. This solution can work very well, especially if your “ideal” fit falls somewhere between existing sizes.
  • The best-possible solution properly needs to be done in conjunction with the overall geometry changes we’ve already discussed. This is the optimal solution in all cases. Fortunately, other small builders are already moving in this direction, and we expect the big guys to follow. Here are some of the builders who are making the move:
  • Mondraker
  • Nicolai/Mojo (Chris Porter)
  • GT moving to longer top tubes

Q: How does PDent actually perform?
A: Ultimately, the proof is in the riding. Just like always.

  • First production samples: late April 2015
  • Available for purchase: late 2015
  • Full PDent acceptance and World Domination: ???

Finally, the bottom line of all this is not just about PDent technology, or about making stems shorter. It’s about opening doors to help designers create better bikes.