Making Sense of Bicycle Geometry

Bikes are kind of inscrutable. Standing still, there are lots of clues to the character of the ride, but many of those clues can be deceiving. You can sense the bike’s stance, examine the tubing shapes, take note of components, and intuit the paint job. But the key to what the bike can do is first found in the geometry.

It’s worth asking if you need to understand bicycle geometry. You do and you don’t, just like you don’t need to be able to explain physics to keep a bike upright. If you ride a bicycle, you, at the very least, can direct it and react with it enough to ride somewhat safely.

At the same time, a better understanding has value. If a bike ever doesn’t seem to be quite doing what you want it to do, if a bike is giving you trouble in ways you don’t understand, if you feel it is holding you back, if you wonder why you seemed to have trouble controlling the bike, or crashed for reasons you can’t quite make out, more knowledge of bike geometry might answer questions you have. And, if you ever want to get another bike, it’s good to know both the geometry of what you have and the geometry of what you’re considering to understand how that next bike might ride before you get on it.

Bicycle geometry refers to the tube lengths and angles that comprise a bicycle frameset. They are the primary indicator of how a bike will handle. And that handling is very much what you feel whenever you’re aboard and the wheels are moving.

Geometry is how the bike’s designer both puts a human body in a general position and then from that position gets the bike to handle a certain way.

Or, as Cannondale’s Damon Rinard suggests, there are top and bottom missions. “The top half is seat and hands. And the way it puts weight on the bottom half, weight distribution. Dividing it in top and bottom, the top is the rider’s fit coordinates. On the bottom half, is where road and endurance bikes might differ, like different head angles and fork offsets. And that’s more about steering and handling.”

There are seemingly countless labels bandied about, but even those, can be distracting. They’re shorthand, both useful and deceiving. Just in the road bike realm there are: track bikes, fixies, crit bikes, road bikes, stage race bikes, fondo (aka endurance) bikes, brevet bikes, cyclocross bikes, gravel bikes, adventure bikes, hybrids, and touring bikes. The mtb side has even more labels and niches.

To keep things relatively simple, I’m going to stick to the road realm in this article. And even by limiting the article to that, it’s still possibly too broad.

The geometry for each bike should put the rider in a position where the rider is comfortable and efficient and can control the bike relatively easily for the effort they’re expecting to put out. The classic beach cruiser is great for light efforts; but piloting that cruiser in a bike race, even if you’re able to duplicate a racing position, the bike will present issues that are likely to hold you back.

It turns out that the generalized road racing bike geometry is also a great launching point for discussing geometry, because the road racing bike is often designed to do everything. It’s a bike that gets ridden both fast and slow, uphill and down, it has to be relatively easy to both ride in a straight line and turn, as well as take on a variety of surfaces. This is opposed to touring bikes, which are generally ridden more slowly over longer periods of time, or track bikes ridden quickly for shorter period of time.

While this is an extended preamble, it’s important to set the proverbial table before getting to the meal. Every tube on the bike has a name, of course; some are called different things in geometry tables. Everything matters, but some things matter more than others. While it’s easiest to explain geometry by isolating various aspects, the totality of how a bike rides can’t be reduced to a single number. As Edwin Bull of Van Dessel relates, “You can’t isolate one thing. It’s how you’re centered on the bike. Where the front wheel is in relation to your center of gravity.” And more. Designers are lengthening some dimensions, shortening others, and more, in the pursuit of a particular ride.

Wheelbase might be the easiest concept to understand. The longer the wheelbase, the more room there is for weight between the axles. Think of a limousine vs. a sub-compact car. Shorter wheelbase has less room and a shorter turning radius. Generally, the greater effort the rider is putting out, the shorter the wheelbase is. It’s for many reasons, including weight distribution and power transmission. Longer wheelbases distribute the weight the bike is carrying over a greater length, so just like the limo’, it’s generally a smoother ride. The road racing bike wheelbase is generally around 99cm, though you’ll often see the longer-seeming 990mm.

But wheelbase can be divided by where the bottom bracket is relative to the front and rear axles. Those dimensions are rear center, or chainstay length, and front center. On bikes designed for loaded touring, rear center can be fairly long, but for most other bikes, the length of the rear center is dictated largely by the max tire size designers want to fit in the frame. There is a minimum recommended length to achieve reliable shifting of a bike with 700c tires and 9/10/11/12-speed cassettes; that number, for both Campagnolo and Shimano is 405mm. It’s good for designers that wider tire sizes are also in vogue as well, as fitting wider tires necessitates longer stays. Scott Warren, who has designed bikes both under his own label at Javelin and for Orbea claims, “Short chainstays are much ado about nothing on road bikes.”

rear center

Road racing bike chainstays these days are typically between 405 and 410mm. Some companies keep the same length of chainstay on their endurance bikes, some like to add a few mm because they believe that the weight distribution on endurance bikes tilts a bit more rearward, and the few extra millimeters can smooth out the ride. With a longer rear, the weight on the rear wheel is spread over a greater area, which can have multiple effects, including how the front end feels, and how stiff the bike feels. Among designers, there seems to be a general consensus that most riders won’t notice a difference of 5mm here, but that of 10mm and greater will be noticed.  Cyclocross and gravel bikes have longer chainstays to fit tires and possibly fenders.  Touring bikes are longer still so that when pedaling, heels don’t hit panniers.

 

Front center isn’t quite as important on most road bikes, so long as toe overlap is minimized.   Obviously, the bigger the tire expected to go in the fork, and fenders beyond that, the longer the front center gets to limit the issues of overlap—at the same time, designers worry less about overlap on road racing bikes, and a bit more on slower-moving bikes, like cyclocross and gravel steeds. Front center needs to be a good bit longer on time trial bikes because of how body weight is much more forward thanks to the resting of forearms on aero bars.

Front center isn’t a dimension designers are necessarily focusing on. They’ve got to contend with where they expect the body to be positioned over the wheels and are limited by body dimensions as well as stem lengths and handlebar reach. That written, some designers like even their taller bikes to have a little toe overlap, as they like how it rides; some believe it results in a more aggressive feel.

 

Front center on small bikes tends to be long relative to other fit dimensions because there is an ISO (International Standards Organization) standard limiting the minimum clearance. It’s ISO 4210-2: 4.13.22 Toe Clearance. The center of the pedal spindle cannot be less than 89mm from the back of the front tire or fender when the front wheel is turned enough to be in line with the pedal. Why 89mm? According to Rinard, it’s obviously because the American standard of 3.5” was converted to metric.

It used to be common to discuss frame angles, namely the seat tube angle and head tube angle, as if they were determinants of riding qualities, and they do play a role. Generally, the longer the wheelbase, the less pedaling effort expected, the more relaxed the angles. Track bikes often had 74-76-degree angles for the seat and head angles. Criterium bikes, when they were being built, often were parallel 74-degrees. Road racing bikes were in the 73-74 degree range. Touring bikes often have 70-72 degree angles. Beach cruisers often have parallel 67-degree angles.

But discussion of angles is largely outdated. Designs are also less constrained by angles as lugged construction has largely receded. Nowadays, the seat angle is often seen as a way for the frame designer to get the rider into a general position relative to the bottom bracket and between the wheels. Generally, the steeper the seat angle, the more forward the rider’s weight is expected be relative to the bottom bracket, but as seat rails are designed to allow the rider to customize the position vis a vis the seat angle, the exact number is not terribly important.

But the seat angle advice is general. For road racing bikes, it can go from as shallow as 72-degrees to as steep as 75-degrees, depending on the size and company.  It also gets to an issue of philosophy.  At one time, the dominant way to design frames was set seat tube angle and top tube length first, and design the bike from there.  That design philosophy has faded somewhat for a number of reasons, including: compact frames, sloping top tubes, popular fabricating processes, the rise of computers, and an emphasis on stack and reach dimensions (stack and reach will be addressed later).    Seat angle is now often a decision that comes late in the design process, after the other dimensions are chosen, with where the top of the head tube should be relative to the bottom bracket coming before seat angle and top tube length coming last.  If there’s a reason to be careful about saddle setback, like unusually short or long femurs, unusually short or long legs, or preferring an extreme forward or rearward position, then seat angle can be important.  Otherwise, the designer probably can be trusted to find a seat angle that works.  Even if it doesn’t seem perfect, you can tune the resulting position somewhat with either zero setback post if the angle is slack or a greater offset post if the angle is steep.

Bottom bracket drop is the difference in height between the wheel axles and the bottom bracket. As body position above the ground is dictated by where the bb is placed, this dimension is very important. The greater the bottom bracket drop, conventional wisdom claims the more stable the bike.

Where it gets a bit complicated is that the greater the drop, the lower the bb, the more likely one is to hit a pedal on the ground when pedaling through turns. This is when people sometimes discuss bottom bracket height, or the distance from the ground to the bottom bracket. For some companies, the bottom bracket drop is dictated by the angle at which a pedal at the bottom of its stroke will hit the ground (aka lean angle); the result is that the drop is the result of determining which crank length and what tire size are specified with the frame.

Touring bikes typically have the lowest bottom brackets, as they’re designed for carrying lots of weight and it’s not expected the rider will need to pedal through corners. By low, a drop of 75mm isn’t uncommon. Road racing bikes used to be close, like around 72mm of drop, but now are more likely between 67-70mm. Cyclocross bikes, when toe clips were used, once universally had higher bottom brackets so that the toe clips wouldn’t scrape the ground or exposed roots, but now that no one races with toe clips, ‘cross bikes vary between the bottom bracket drops of toe-clip days, often seen on Belgian-style cross bikes, like around 50-60mm of drop, and road bottom bracket drops, closer to 65-70mm. Endurance bikes sometimes go a bit lower than road racing bikes. Gravel bikes are varied. On the one hand, greater stability is a good thing in loose or slippery conditions, on the other, pedal strike on rocks and roots is not. Track bikes, at least the ones designed for mass-start racing and match sprints have possibly the highest bottom brackets so that the rider can turn up the banking at a low speed without striking her right pedal on the track.

Adding to the complexity of bb drop is that it can change over the size run of some bike models. For example, some manufacturers reduce the drop on their smallest frames.  Yes, the bottom bracket gets higher. They typically do this to have a greater drop from the saddle to the handlebars, as there is a limit to how short a head tube can get. Others increase drop, go for a lower bb, on smaller models because they are using shorter cranks. Meanwhile, on the tall end, many big frames have a little less drop, higher bb, than the smaller sizes because the designers are specifying longer cranks and want to keep the angle at which the bike experiences pedal strike to be the same as the smaller sizes.

Head angle is a bit more complicated to discuss. That’s because it’s part of the trail equation along with fork offset, or rake, and wheel radius as measured to the ground. For many frame designers, trail is the frame dimension that does more than anything else to determine ride.

As this piece has gotten long enough already, and trail can be a bit complicated, I’m saving that for Part Two.

 

 

 

 

I want to thank Edwin Bull of Van Dessel, Brad DeVaney of Litespeed, 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.

 




This post has been updated from the original.

 

most of the dimensions you should think of

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