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Common Tread

How to read a motorcycle spec sheet

Jan 28, 2019

Spec sheets are like the back of a baseball card.

Insights about a motorcycle can be gleaned from the spec sheet. Nothing can replace riding a motorcycle, of course — I’ve said many times we ride motorcycles, not spec sheets. However, a good look over a spec sheet by a knowledgeable rider can help one flag items of interest or cross a bike off the potential “short list.” Further, understanding the range of permissible values with respect to a certain specification can help one better understand bike construction with respect to certain disciplines, to race hardware, and to quality and technology. Let’s examine a few specifications commonly seen on manufacturer spec sheets, and I’ll tell you what I think you might want to know about them.

Weight

Depending on the style of motorcycle being discussed, this may be the most important datum examined. For many, a value too large may be a deal-breaker right off the bat. In the full-size motorcycle world, the lightest machines are dirt models, hovering around 200 pounds, like the KTM 125 SX. (Trials bikes are even lighter, but they’re a bit of a specialty machine, so I have excluded them.) At the heavy end of the spectrum, one will usually find full-dress touring models, like a Harley-Davidson Electra Glide Ultra Classic, topping out around 900 pounds or so.

Increased weight is a given on physically larger or more powerful motorcycles. The easiest way to make more power is with a larger engine, and bigger engines are not just larger, but also heavier, which in turn necessitates larger, heavier chassis to house them. Conversely, the riding disciplines demanding the most rider input (trials riding, off-road riding, various forms of racing that don’t spec minimum weights) usually are the ones where riders demand the lightest machinery.

Trials bike
The low weight of this trials bike (146 lbs.) is in part what allows acrobatics like this to be performed. It's an extreme example, but it illustrates why low weight is so critical. Motorcycling is a physcial activity! Beta photo.

One useful ratio when comparing dissimilar motorcycles can be power-to-weight. Often, a less powerful motorcycle can outperform one with more power if it’s lighter. Being down on power may not matter if the engine has less mass to move. Lighter bikes will make the rider’s task of handling the motorcycle easier, too.

And if you’re comparing models, be sure you’re comparing wet weights (or at least like-for-like). Sometimes weight is listed with no fuel, oil, battery, or other essential pieces. I don’t know about you, but I’ve never ridden a dry motorcycle.

Takeaway: Racing, aggressive street riding, and off-road riding all require extreme physical inputs, and a lighter motorcycle will preserve a rider’s stamina. All things being equal, a lighter bike will require less effort to accelerate, decelerate, and turn.

Power and torque

I’m listing these together as they both are measures of an engine’s output, and they’re interrelated. Tomes could be written on this, but think of torque as a direct measurement of the amount of work a given engine can do. Horsepower is a calculation specifying the rate of work, that is, how much work is performed over time. Normal horsepower ranges for full-size motorcycles are probably between 15 and 200 horsepower. (Yes, exceptions exist, but most will fall within that range.) Torque specs will usually ring in at 10 to 175 foot-pounds.

In general, the bigger the torque number is, the stronger a bike will feel low in the rev range. The bigger the power number is, the stronger it will feel high in the engine’s rev range. Engines with larger displacements usually move both numbers up.

There is usually some tradeoff between the figures; a very torquey bike, like a Honda Gold Wing making 108 foot-pounds of torque, produces a comparatively mild 108 horsepower. The opposite is true, too. A Suzuki GSX-R600 makes 144 horsepower, which is pretty impressive for having an engine one third the size of that Gold Wing. However, its torque rating is a little anemic: just 44 foot-pounds. It should be noted that the Honda has a redline at 6,000 rpm and the ‘Zuk zooms all the way to 15,500 rpm. It does smaller amounts of work more quickly than the Gold Wing.

Brian on the dyno.
A dynamometer measures torque. This is a rear-wheel dyno. Most manufacturers will quote figures taken from the crankshaft, which are usually about ten percent higher than what will be measured at the wheel. Photo by Lemmy.

If the two engines were theoretically swapped, the Suzuki would be wheelie-prone and run out of breath quickly, and its top speed would be dismal. The Gold Wing would fall on its face off the line; it would be very easy to stall. But as it stands, both bikes are lovely. Flyweight racing machines need to spin the engine fast and make small bits of torque in rapid succession, and heavy motorcycles need bigger doses of muscle, but at a slower pace. All bikes need a nice balance of the two.

One last item of note: seeing dyno charts is nice, too. Figures quoted on spec sheets are peak numbers, but sometimes, as in a two-stroke engine, the peak numbers can be spikes seen in a very narrow part of the total rev range. Think of those as a photo. A dyno chart shows the relationship between power and torque at every speed the engine is designed to operate in — more like a video. A bike with lower peak numbers but much flatter, more consistent power or torque plots on the dyno chart would probably be more rideable in most non-competition scenarios, but the very upper echelon of riders in a race might prefer the optimal peak numbers if they were confident of their ability to keep the engine working in its most productive range.

Takeaway: Power and torque are figures that express the amount of work an engine can perform, and the rate at which it can perform that work. Displacement will cause both numbers to rise, but all motorcycles need some balance between the two figures, depending on the motorcycle’s intended task. The numbers given in a spec sheet are peak values, and sometimes seeing the bike’s output across the entire rev range can give a better idea of an engine’s personality.

Wheelbase

Wheelbase is the measurement of the motorcycle’s length from the front axle to the rear axle. (It’s not tip to tip of the tires!) The shortest full-size motorcycles are sub-55 inches, generally seen in the race-rep world, and they extend out to 72 inches or so, typically the domain of cruisers. (Even longer wheelbases can be found on choppers and drag bikes, but those are usually owner-modified.) For example, a Yamaha YZF-R3 is a compact 54 inches, as compared to a Honda Fury, stretching out at a touch over 71 inches.

In general, wheelbase affects a motorcycle’s handling and feeling of stability. Shorter wheelbases usually feel more lively in terms of cornering, but can feel less stable at high speed. Conversely, a long wheelbase will usually feel very planted when moving fast, but may feel slow to respond to steering inputs. It makes sense if you think about it for one moment: A stable feel comes from the feeling of the bike’s resistance to turning, and light handling comes from its willingness to turn. Additionally, in terms of weight transfer, longer wheelbases permit harder braking and acceleration.

Wheelbase
Wheelbase is the measurement of the bike's length from contact patch to contact patch (which is almost always the same as axle-to-axle.) New Zealand Transportation Authority photo.

Most riders on a race replica want the flickable, nimble steering those bikes are built to exhibit. However, someone piloting a large touring bike will prize both room on the motorcycle for rider, passenger, and gear as well as high-speed stability. A long wheelbase can lend to a high-speed, high-weight motorcycle intended to travel roads that are relatively straight and slow when juxtaposed with a race track.

You might think that with their relatively low speeds and racing pedigree that dirt bikes might have the shortest axle-to-axle measurements, but that’s not the case, in part due to how high they sit. Their higher weight necessitates a little more wheelbase length to keep them feeling calm. This is made more apparent by the large, soft suspension travel off-road motorcycles have as well. The extreme squatting and diving (pitch) that an off-road bike is capable of relative to shorter-travel bikes is tamed some by a longer wheelbase. Compare that to a low, heavy cruiser that generally encounters few obstacles (requiring little suspension travel) and is not called upon to change directions very quickly (meaning that straight-line steadiness is important).

Takeaway: As wheelbase increases, so does straight-line steadiness, as well as effort required to make the bike change direction.

Rake and trail

Rake and trail are related and they also affect wheelbase. Rake is the angle the neck’s headstock tube is set at. Measured in degrees, the number normally ranges from the low 20s (usually sport bikes) to measurements in the mid-30s on production motorcycles. A 2017 Suzuki GSX-R1000, for instance, sports a 23.2-degree fork angle, whereas a Harley-Davidson Softail Breakout made the same model year kicks the front wheel all the way out to 35 degrees.

Dirt bike rake trail
Rake and trail for performance machines are usually steeper (smaller figures) for competition bikes...Honda image.

Rake affects the weight distribution of the motorcycle, too. Assuming the fork length changes (they must be longer on models with more rake) to keep the bike somewhat level from front-to-rear, the motorcycle's weight begins to shift increasingly to the rear. Higher rake numbers also go hand in hand with increases in wheelbase, of course, and also an increase in trail. A sport bike, with its short wheelbase (for quick turning and sharp lean angle) winds up with a more frontward weight bias than a bike with more rake, which would make handling feel slow and heavy, so the fork angle is made steeper. Low rake and trail numbers restore lively steering feel.

Trail, just mentioned without a definition, is actually an imaginary measurement. It’s the theoretical distance between where the front wheel touches the ground and the spot where the steering axis would intersect with the ground. If rake is increased, trail will also increase. Much like with wheelbase and rake, increased trail helps straight line stability and hurts cornering ability. Rake and trail in higher numbers also make steering feel heavier. That Gixxer from before has a trail of 3.7 inches, but the Breakout? That’s got a whole ‘nother two inches. If you’ve ever ridden both bikes, you can easily tell feel how steering effort is affected.

Cruiser rake trail
...than they are on bikes built for leisurely pleasure rides. Honda image.

Steering offset (the distance the triple trees sit ahead or behind the steering stem) and wheel diameter factors into these measurements, as well; they’re interrelated. Rake and trail are not static on suspended motorcycles; they change as the suspension compresses and extends. Fork “flop” (the propensity of the wheel to turn easily as it rotates away from straight) can also be affected by rake, trail, and offset, too. In general, higher rake and trail numbers lead to steering that requires more effort, but also more straight-line stability. It's also important to know that some manufacturers (BMW comes to mind) will express rake as the angle from a line drawn parallel to the ground, not perpendicular, so some quick math is required to compare two motorcycles with their rake measured differently.

Takeaway: Rake, trail, wheel diameter, and fork offset, in conjunction with wheelbase, all have an impact on a motorcycle’s steering, weight distribution, and the effort required to make the bike change direction.

Seat height

Seat height seems (and is) a relatively simple concept: It’s how high the seat is from the ground, so I won’t belabor the point — much. Range is usually from the mid 20s (like the Indian Scout at 25.1 inches) to just shy of 40 (like a Yamaha WR250R at 38 inches even). The seat height figure does not tell the whole story, though. The spec is taken as a single vertical line, but human legs straddling a motorcycle form a bipod. The wider the bike is, the wider the base of that isosceles triangle must be. Given that legs are an item of fixed length, that means the “point” of the triangle (your crotch!) will be closer to the ground if a bike forces your legs apart. The closer the additional width is to the top of the triangle, the more pronounced the effect. It’s why a bike with a low seat height but very wide saddle can feel like a tall bike with a narrow seat.

Seat height
Perhaps a graphic depiction is overkill, but seat height is an important measurement for many riders. GearChic image.

One thing to note is that as a motorcycle's ground clearance and suspension travel figures increase, seat height usually scoots up as well. It's why the very fastest off-road riders are usually on motorcycles with very tall seats.

Takeaway: Everyone’s body is different, and you should have an idea what seat heights are comfortable for you, but don’t be afraid to pop a test squat on a motorcycle that’s slightly outside your normal range.

Compression ratio

If you’d like an in-depth explanation of how compression ratio is calculated, I can offer that, but suffice it to say that CR is a measurement of how much the air and fuel mixture gets squeezed in the combustion chamber, and it can be a good determinant of how much potential there is for an engine to make power. Current overhead valve models generally range from around 8:1 (the Royal Enfield Bullet spits out an 8.5:1) to 14:1 or so for street-ridden, non-competition machines. At the high end of the spectrum, there is the Kawasaki Ninja ZX-10R putting out 13.0:1, and if we look to factory race offerings, a KTM 250 XC-F squeezes its mixture at 14.4:1.

Generally, lower compression ratio indicates a need for a larger engine to make the same amount of power as a smaller one. Higher compression ratios make more power, but can often cost more because many of the internal parts must deal with more stress to make that power. While electronic ignition control can help mitigate some of the effects of preignition and detonation that often come with high compression ratios, to make peak power, the motorcycle will often require premium fuel.

Takeaway: A high compression ratio often indicates the potential to make lots of power, at the expense of higher-cost parts and greater collateral damage when an internal component fails. Usually, more expensive, higher octane fuel is required to make maximum power. Some bikes can tolerate cheaper fuel at the expense of lower power, and some cannot.

Engine architecture

Engine layout is arguably the characteristic that gives the motor in our motorcycles the biggest dose of personality. It dictates the chassis layout, the output, and in many cases, defines the bike. (What would a Gold Wing be without a boxer, or a Street Glide without its V-twin?) Most importantly to some, it largely determines the sounds a motorcycle will make.

Architectural elements may give clues as to what an engine is designed for and how it might behave in its cycle. Four-stroke or two? How many cylinders? Pushrods or overhead cams? Undersquare or oversquare? Firing order? Is it mounted longitudinally or transversely? This guide is far too compressed to be of any real use highlighting the nuances of engine design, but generally speaking, larger engines make more power than small ones, and engines with more cylinders are more powerful than those with fewer. Engines with a larger bore than stroke (oversquare) will tend to rev higher than those with a greater stroke than bore (undersquare), but those oversquare engines will usually provide power less evenly across the rpm range.

Single
A big single cylinder motor, the simplest type of internal combustion engine, is often used where low weight is a priority — but it's not the best design for smooth, balanced running, or high-RPM operation. KTM image.

Some features are associated with certain styles of motorcycle. Cruisers often have V-twins, dirt bikes and dual-sports are almost always single-cylinder machines, and racing motorcycles frequently have inline-four, across-the-frame setups. In this day and age, two-stroke engines have been all but outlawed for street use, so they’ll be seen in off-road or competition bikes. And of course, for every rule, there is almost always at least one notable exception, but these should prove to be useful general guidelines.

Takeaway: Engine layout and construction have a huge bearing on how a motorcycle comports itself when underway. A given layout may lend a motorcycle a quirky, lovable trait, or drive you nuts. Understanding how those characteristics relate to a given model or discipline will help you understand how the bike can be expected to behave.

Brakes and suspension

Lance asked me to tuck this in here, and I squirmed, because these are complex systems with lots of variables. A spec sheet tells us less about brakes and suspension than other figures because something like seat height is pretty cut and dried. Complex systems, like the brake and suspension, can be a crapshoot. Items with “great” specs can feel awful, and so-so specs sometimes turn out lovely, so my commentary on these items can be considered a loose guideline.

XSR brakes
This image shows radially mounted brake calipers, floating rotors, and an ABS tone ring. However, the rotors are not the largest that could be fitted, and I spy a rubber brake line. The system should be somewhat high-performance but not top-of-the-line for track work. And happily, that's exactly the case — this brake setup is from a Yamaha XSR900, a speedy bike, but not an all-out racer. Photo by Lemmy.

In terms of brakes, generally speaking, more is better. Twin discs up front is usually preferable to one, depending on the weight of a motorcycle. Discs are generally preferable to drums, larger discs are usually better than smaller ones, more pistons in a caliper usually stops faster than fewer, and radial-mount calipers and radial master cylinders are preferred over axial pieces. Braided hydraulic lines generally contribute to what your hand will interpret as better feel. Note that these are all generalities; component quality and matching is just as important as the spec.

Suspension is another hugely variable area. Monoshock rears are generally seen as better than twin-shock, and rear linkage styles are often used to provide progressivity to a given suspension system. They often sacrifice low seat height for high ground clearance — undesirable in some machines (like dirt bikes) for some riders (short ones). But on a sport bike, where ground clearance is less critical, something like a linkage-style suspension is a good fit for the majority of riders.

Up front, upside-down suspensions are generally seen as superior to right-side-up forks. Larger diameter tubes should deflect less than smaller ones, and more adjustability (rebound, compression, preload) is seen as making a fork more “tunable” than forks without those adjustments. The caveat here is this indicates the potential to have a great setup, but there is also more opportunity to diddle the clickers all wrong and get something way out of whack. The knowledge on how to use the tools is as important as the tools themselves.

Indian shock
This is one side of an Indian Scout equipped with dual rear shocks. Given the relaxed nature of most rides a Scout owner will likely take, a lower-spec, lower-cost suspension system is an appropriate choice. Photo by Lemmy.

Total travel is also a figure you may want to consider when assessing a motorcycle’s abilities, especially when leaving tarmac. Four to probably about 13 inches is the normal range, with the number climbing on machines meant to venture off the pavement. Lots of travel is helpful when navigating rugged terrain, but less desirable on pavement.

Takeaway: More and bigger is usually better for brakes. For suspensions, lots of adjusters signifies high-quality pieces that will likely work very satisfactorily. For these items, a test ride is probably the best way to assess their quality.

Carrying capacity

The carrying capacity of a motorcycle won’t be a critical figure for every rider, but for some (those touring with lots of gear, those traveling with a pillion, and heavier riders), an inability to carry a given load may be all that is needed to consider a given model or scratch it off the list. Note that the carrying capacity is a calculation. To calculate carrying capacity of not given, find the GVWR. GVWR is the maximum amount of weight a motorcycle can carry including its own weight. Unfortunately, this is not listed on a spec sheet, but is usually available from the VIN plate or owner’s manual. Subtract the motorcycle’s wet weight from the GVWR, and the remainder is the carrying capacity. It’s important to note that some small bikes have surprisingly high capacities, and some large ones are rather small. For instance, a Honda Grom has a 340-pound carrying capacity, yet the much-larger CBR600RR is only around 366 pounds. Motorcycle style and intent often has a significant bearing on these numbers.

Spec table
Note here that the "maximum added weight allowed" (which is carrying capacity) is the GVWR less running weight. Screen capture from Harley-Davidson owner's manual.

Takeaway: Not all bikes can carry all loads. Just because it fits doesn’t mean it’s safe. Heavy riders and bikes with small capacities in combination may have restrictions on the gear or passengers that the motorcycle can safely carry.

There are a number of other significant specifications for all motorcycles, and spec sheets vary from manufacturer to manufacturer. And of course, some specs matter more for some bikes than for others, but understanding the general range of specs you’re likely to see may help you quickly get a feel for a how you can expect a given motorcycle to perform. But as always, the proof is in the pudding — sometimes a test ride confirms a suspicion and sometimes it disproves a hypothesis. Those discoveries are part of the fun of shopping and riding!