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.
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.
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.
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.
Endo much? Once you understand the geometry issues, the disadvantages of conventional downsizing wisdom become obvious. Note stem length on rider at right.
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.
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.”
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.
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.
“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!”
—Chris Porter, Mojo Suspension UK
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?
But here are some things we do know about shorter stems, both from rider input and some simple calculations.
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.
= >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?
“If I could just put a dimple on the bar,
I could shorten my stem another 10mm.”
And that’s how PDent™ was born.
Patent-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.
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.
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.
Q: How does PDent actually perform?
A: Ultimately, the proof is in the riding. Just like always.
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.