Making Sense of Bicycle Geometry Part 3: How Components Can Modify Ride

Part One explained most of the frame dimensions.

Part Two explained trail.

Frame materials, frame shaping, frame construction, and bicycle component selection can somewhat modify the ride qualities that the frame geometry is designed for. As the first three are pretty complex in their own right, I’m going to stick to component selection for this article.

At the end of Part Two, it’s mentioned that BMC has chosen a relatively long trail dimension for their road racing bikes because of both their bikes’ front-end stiffness and the popularity of deep-dish wheels (rim depths of 50mm and greater). BMC believes that a road bike’s trail should be lengthened a bit to better counteract both the increased stiffness in the front end of the frame and the lateral forces that crosswinds place on deep dish front wheels.

Whether or not lengthening trail a good idea is certainly open for debate. But the thinking behind the decision has widespread agreement. Frames are believed to be much stiffer than they were back in the steel era, which is when ideas on geometry were refined, even at the dawn of the carbon era. Just look at how the steerer tube has gone from an inch to an inch and an eighth, to a tapered 1 1/8” to 1 ¼” or even 1 1/2”. And most wheels in the 50mm depth range and greater do seem to get pushed around by side winds more than the classic, almost completely gone, 20mm deep box-section rims that were common at the dawn of the carbon era. Further, contemporary stems and handlebars are likewise much stiffer than their predecessors a generation ago, thanks to better design and wider clamp diameters. I should add that steel and titanium frames have also gotten stiffer since the 1990’s, with larger downtubes as well as a similar increase in steerer tube diameter, and most steel and titanium frames are finished with carbon-fiber forks as well.

As a result, if you had a classic steel road bike from the 1980s or 1990s replicated today in carbon or aluminum, maybe even in steel and titanium, the new bikes, when outfitted with identical components as the original, would seem to steer faster.

The components that can change the behavior of trail is typically seen as tire size, weight, and pressure, wheel weight, and stem length.

Tires

If you read the article on trail, you should already have a sense that wheel radius, as measured from the hub to the ground, has an effect on trail. To recap, a bike with a 73-degree head angle, fairly typical for bikes in the road racing realm, and a 43mm fork offset, also very common, and 700c x 23mm tires, the trail is 58mm. Smaller tires reduce trail, larger tires increase trail. Put on a 20mm tire, and it goes down to 57mm. A 25mm tire moves it up to 59mm, and a 28mm tire, if you can fit it, pushes it up to 60mm. As most people are (and should be) reducing tire pressure as tire width goes up, the increase in trail is not quite as large as measured, but it still will make a difference, not that one can necessarily feel it.

Reducing tire pressure also slows down steering sensations, even if trail is reduced. If you’ve ever ridden a tire that’s slowly losing pressure, the front end starts to feel heavier as the tire leaks air.

Tires for road bikes were once supposedly sufficiently large that they dramatically slowed down the super-short trail that was once common. In an interesting article by Dave Moulton, a racer turned framebuilder, he posits that road bikes from the 1930s to 1950s were commonly designed with both much longer wheelbases, much more generous fork offsets—twice as long as they are today–and almost no trail. While a bike today built this way might be almost unrideably twitchy, the old bikes weren’t because of three factors: large tires, low pressures, and bad, slow roads.

Wheels

Wheels likewise have an effect. Wheels that have more mass at the rim and tire, will feel slower when starting out than wheels with less mass. And the faster the wheels rotate, the more wheels will want to keep going in a straight line. Deeper wheels can mitigate this somewhat depending on how fast one is going and how strong the crosswinds are.

Stems and bars

Further up the steering axis, the stiffness of the stem can make a difference in how the steering feels. A stiff stem feels faster, both because more of the road vibrations and bumps are transmitted to your hands, and because it reacts faster, though the faster reaction might be hard to sense. A lighter stem should feel faster because there is less mass to move, a heavier a bit slower, more mass.

The same goes for handlebars. Yes to stiffness, yes to weight, but there is more still. The width of the bars matter, as does the reach. Wider bars will feel slower, narrower faster. The longer the reach, the farther hands on the hoods and drops are from the steering axis, effectively lengthening the stem, the lever.

With all of the above, you can play with tire pressure, swap in and out various parts fairly easily, though the main goal might not be trail first but rather performance. Still having an understanding of the secondary effects is helpful in knowing why your bike seems to ride the way it does.

Stem length

Stem length can have a big effect on how you sense a bike’s ride qualities. All the same, the length is pretty much set either at the point of purchase or that of fit. So this knowledge is most important when you’re looking for a new bike or considering changing the position on a bike you already ride.

All other things being equal, longer stem slows steering, vis a vis trail, down; a shorter stem speeds it up. When comparing one bike to another, or one frame size to another, consider stem length when thinking about how fast a bike might steer. Litespeed’s Brad DeVaney believes a “nervous” rider, one who seems to have trouble holding a straight line, can be calmed by putting her on a bike, all other fit points being equal, with a longer stem.

The difficulty is that rarely are all other things equal. Yes, if you could find two bikes that were identical in all aspects save the reach to the head tube, the one with the 1cm shorter reach and 1cm longer stem would seem to steer slower than the one with the 1cm longer reach and 1cm shorter stem. But this is rare. More common is finding a model of a bike you’re interested in and checking out the size run. For one, you might need 1cm of stack spacing under the stem and can run a 13cm stem, and for the size up, you need no stack spacing and a 12cm stem. Here, the shorter stem will steer faster, though the spacers under the 13cm stem might introduce other issues, and the greater amount of seatpost between the clamp and the saddle will also change how the bike rides. With the frame weight a bit lower, the bike might feel a bit lighter when riding out of the saddle. The greater length of unsupported post will also provide a slightly greater damping effect on vibrations coming from the wheels rolling over rough pavement and potholes.

In all cases, the principle is basically the same, shorter stems will give the sensation of steering faster than longer stems, and this has the potential to somewhat exacerbate or counteract what’s going on with a bike’s trail numbers. Arguably, the long trail that is sometimes found on small bikes, sometimes upward of 70mm, can be minimized with a short stem. Conversely, short trail bikes with long stems might mean the reverse, something that was commonly found on steel bikes from the Merckx through LeMond eras. Then pro riders were often installing 130-140mm stems on their bikes.

Pro riders are often seen as liking long stems. The thinking behind this is not necessarily about performance, flexibility, or even fitness, though those factors play into the thinking. They, and their team directors are just as susceptible to trends and myths and history and aesthetics as the rest of us. In the steel era, a smaller frame was often seen as lighter and stiffer. Scott Warren, who worked with the French Cofidis team when he was at Orbea in the 2110’s, found riders wanted both longer stems and shorter trail. He saw it as a push-pull; they could get a faster handling bike if they would be willing to ride a bike one or two sizes taller and a 10-20mm shorter stem, but they wanted a bike that looked small with a longer stem.

But to the point of flexibility, yes there is truth that some riders are more flexible than others. And pro riders are generally thinner than the rest of the riding population, so they can probably get their torsos lower without extra stomach mass to hit their knees. And some are incredibly flexible (as this Peter Sagan demonstration shows). At the same time, in earlier eras, pro riders typically rode higher stems relative to the upper race of the headset and bent their elbows more to get into similarly low positions.  While pro riders of today might be in better shape than in earlier eras, as a group, they’re probably no more flexible.

To figure out how to replicate your current handlebar position on a bike you feel comfortable on to a new bike, you should understand stack and reach measurements, something that can make figuring out whether or not the bike you’re interested in can fit you pretty easy. It’s worth an article in it’s own right, and that will be Part Four.

 

I want to thank Edwin Bull of Van Dessel, Brad DeVaney of Litespeed, Dan Empfield of Slowtwitch, Steve Fairchild of Fuji, Tom Kellogg of Spectrum Cycles, Damon Rinard of Cannondale, and Scott Warren for their time and insights.  And Bikecad.ca for the drawings.

 




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