Dirt Rag Magazine

Inside Line: Pacenti PDent Shorty Stems


With all the tire size and hub width standards getting pushed around lately, it might have been easy to overlook something as simple as a dent in a handlebar. But with the industry ready and willing to throw convention out the window these days, why not rethink bike sizing as well?

The PDent is Kirk Pacenti’s patent pending idea to allow for stems shorter than what is possible with current 31.8 and 35 mm handlebars. Since the bars will run into the steerer tube once the stem is any shorter than 32 mm or so, companies that wanted to experiment with even shorter stems had to resort to placing the bar clamp above the steerer tube. That is a simple solution to the shorter stem problem, but it pushes the clamp height up a good bit, which is an issue for modern bikes where riders want lower bars along with longer travel and bigger wheels.

040715-5 Pat Pending CU

So the PDent was created, an engineered recess or dimple in the center of the bar that allows the bar to wrap around the steerer. Depending on the size of the dent, stems get get as small as 15mm. For now Pacenti is focusing on stems between 15 mm-30 mm. Lab tests proved the dimple doesn’t weaken the bar in any important way, and in fact the bars will break in other places long before the dimple is under enough stress to cause issues.

040715-2 alt Bar+stem top-down.
This idea isn’t so much about the shorter stem, it is more about rethinking geometry. Top-tubes have gotten progressively longer, and with shorter stems, can get longer still.  The long front center that results from long top tubes results in more stability, particularly when combined with modern slack head angles. Pacenti is a proponent of going even further, with trail bikes getting even slacker and longer. In reality, what we are looking at are almost downhill bike numbers, but made rideable up and down with a steep seat angle.

Pacenti isn’t after cornering the short stem market with Pacenti branded stuff, although he will be selling them soon.  Instead he would like to licence this technology to stem and bar manufacturers, and in turn, bike companies, as these short stems are going to need bikes with even longer top tubes than are currently on the market. Although since modern standover heights are so low, most riders could ride a size larger to get the reach needed to make a shorter stem work.

040715-3 Bar Top-Down
Are tiny stems the next big thing? It’s hard to tell at this point. Mondraker has been pushing the tiny stem thing for awhile, and even the Athertons experimented with similar ideas (before going back to more “normal” stem lengths). It should be interesting to see where this goes. Kona proved with its Process line that shorter stems are not a hinderance to all-around riding when paired with a long enough top tube, although those bikes use slightly steeper head angles and super short chainstays, two things Pacenti advocates pushing in the opposite direction.

040715-7 beauty shot

We are on the short list for media samples, so expect more info about how this all works later this spring.

 

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Blast From The Past: Pacenti 650b prototype review from 2007


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Editor’s note: This Pacenti 650B bike review first appeared in Dirt Rag Issue #131 in 2007, the same year that Kirk Pacenti showed the first production 650b mountain bike at the North American Handmade Bike Show.


By Andy Bruno

If you’ve been asking yourself, “How can I rationalize buying yet another bike?”, along comes Kirk Pacenti with your answer—650B. That’s right, the semi-obscure wheel size used mostly on touring and road bikes is being adapted to mountain bikes. And Dirt Rag got a prototype bike and tires from a guy who wants to bring 650B to the mountain bike masses.

Just to be clear, this review is more about wheel size than it is about the actual bike we got from Pacenti. He’s a custom bike builder and he plans on building 650B customs in both steel and Ti, but his main focus now is tires. If you read “The Big Wheel Mountain Bike Story”  in Dirt Rag Issue #130, the genesis of 650B mountain bike wheels will sound familiar with a slightly different twist.

While the absence of a mountain bike-specific 29er tire slowed the development of 29ers, Pacenti was able to score a deal with Panaracer and is currently working with them on a series of 650B Pacenti branded tires. This he hopes will be the incentive other builders need to start producing 650B specific mountain bikes.

pacenti650b-wheels

Size Matters

As far as size goes, 650B’s (or 27.5ers) fit squarely between the standard 26-inch and 29-inch mountain bike wheel sizes. [Editor’s note: We know they don’t precisely split the difference in terms of size, but this is the line from the original story in 2007.] There are currently several 650B bikes, wheels, and tires on the market today, due in a large part to the popularity of the wheel size in Europe. One of the more recent and notable 650B bikes in the US is the Rivendell Bleriot; an all-around road, trail, and touring bike mentioned in DR #126. But up until now there hasn’t been a dedicated 650B mountain bike.

With the market chock full of well-made, high performance 26-inch and 29er mountain bikes, why would we need a 650B? According to Kirk Pacenti, the answer is frame geometry. In designing the 650B, Pacenti wanted to incorporate the performance advantages of the bigger 29-inch wheels but also experienced their inherent limitations when it comes to frame design and geometry. These limitations translate into compromises in frame building like undesirable chainstay length and/or bottom bracket height. Apparently it’s not as much of an issue for 29-inch hardtails, but he says, “With four inches of travel or more on a twenty-nine-inch-wheeled bike you pretty much have to chuck fundamental frame design principals out the window!”

His idea is that you get the best of both worlds with 650B—they’re not too big, they’re not too small. The “right size” allows builders to utilize proven 26-wheeled geometry while taking advantage of the bicycle handling benefits of bigger wheels.

“Frame designers ‘get it’ right away when I talk to them about 650B wheels. I almost never have to explain the advantages of the size as it pertains to frame geometry…” says Pacenti.

He stressed his belief that there are appropriate applications for 24, 26, and 29-inch wheels, but he believes the 650B size offers a greater range of versatility than the other sizes. And judging from press releases that come across my desk and gossip around the industry, a number of other builders have 650B mountain bike projects in the works.

pacenti650b-sig

Prototype

The prototype 650B bike we got from Pacenti is a clean-looking hardtail 3/2.5 titanium frame painted in a pearlescent white. Up front there’s a modified White Brothers Magic 100 (100mm) fork cut down from its 29er version and optimized for 650B wheels. Pacenti used the White Brothers fork because its modular design lends itself to custom configurations, but he said that any off-the-shelf 29er fork and even some 26-inch forks could be used.

pacenti650b-front

The linchpin(s) holding this project together are the tires, and there are currently three Pacenti branded 650B Panaracers in the works: the Neo-Moto, a full-on 2.3 knobby; the Mini-Moto 650B x 40mm randonneuring/touring mini-knobby tire; and the Quasi-Moto, a 650B x 52mm semi-knob, hard pack/marathon tire. Our test bike came with the Neo-Moto, borrowing aspects from both the Rampage and the Nevegal. The tires were mounted on custom Velocity Synergy 650B rims built up as a Cane Creek Zonos Disc Team wheelset. Other interesting bits and pieces include Shimano Deore XT crank/bottom bracket, front and rear derailleur, SRAM Rocket Twist Shifters, and Avid Juicy Seven disc brakes.

pacenti650b-action

Riding the 650B

What’s it like riding on 650Bs? In the spirit of full disclosure, I’ve got to say that the vast majority of my mountain biking has been on 26-inch wheels. I don’t own a 29er but I have ridden a few. And I have to admit, I love big wheels. So were the 650Bs big enough? I felt like I was getting the benefit of bigger wheels—faster and smoother rolling over rocks, logs and roots. Riding 29ers in the past in very tight sections of the trail sometimes I’ve felt like the wheels were almost too big and the bike wasn’t as nimble as I’d like. On the Pacenti I didn’t notice this as the bike felt right at home on tight, twisty singletrack, swoopy trails through the woods, or bombing downhill.

The geometry of the bike is slightly different from other bikes I’ve ridden with twin 72 degree head and seat tube angles. The Neo-Moto’s tread design along with the longer contact patch of the 650B wheel provided great traction climbing and in all kinds of trail conditions. The tires handled particularly well in the corners because of the large, beefy side knobs.

What didn’t I like about the bike? Not much, really. If the bike were custom built for me I’d probably choose a few different components. The White Brothers fork was alright by me, but others around the office seemed to think it didn’t perform as well as expected. Kirk Pacenti also told me that the chainstays on the bike are a bit longer than they need to be, but since it was a prototype they wanted to make sure the tires would fit. I’m not sure if I would notice the less than half-inch difference, but it’s worth mentioning.

The bottom line is that I’m curious to see what else is possible with 650Bs. As far as hardtails go, the prototype I rode was a fun all-around bike. But if the real benefit of 650Bs is on bikes with greater than four inches of rear travel, you’ve got my attention. I’d like to see more.

Keep reading

We’ve published a lot of stuff in 25 years of Dirt Rag. Find all our Blast From the Past stories here.

 

 

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