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|01-20-2016, 03:40 PM||#1|
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All About Geometry
Mechanics check the front and rear weight of Ben Spies’ Yamaha M1 during a MotoGP test. This data can be used to calculate the horizontal position of the combined bike/rider center of gravity. A slightly more elaborate method, using scales at different heights, can be used to find the vertical CG position.
Quite often in Sport Rider road tests and comparisons we refer to various aspects of a bike’s geometry—rake, trail, wheelbase, center of gravity, anti-squat, and so on—without offering an explanation as to what we are actually referring to or why it’s important. Just as it means in the mathematics world, geometry here refers to the physical measurements of a motorcycle’s chassis. There are a handful that go a long way to determining how a bike handles, with some—like rake and trail—being direct measurements but others—such as anti-squat—being a combination of dimensions rolled into one handy reference value.
When it comes to setting up a motorcycle, the difficulty we usually run into is that changing just one of these variables impacts many others down the line, and it’s easy to lose your way. In this issue we’ve put together a guide to the important dimensions and attributes that make up a motorcycle’s geometry, with explanations as to what aspects of handling those attributes affect and how to make adjustments to change them. This knowledge can go a long way to keeping you from wandering down those dead-end setup paths we are trying to avoid. In our next issue we’ll put that information into practice with a setup guide detailing handling symptoms and how to address them with adjustments based on the material covered here.
Trail is measured from the tire’s contact patch to a point where the steering axis intersects the ground. Other important dimensions shown here that directly affect trail are rake, fork triple clamp offset, and front-tire diameter. Rake refers to the angle of the steering axis with respect to vertical, while offset is the distance from the steering axis to the front axle. On most bikes, the steering axis is set at the same angle as the fork tubes and the axle is on the fork centerline; offset can be measured from the steering stem to the fork centers.
Front Geometry: Rake And Trail
At the front of the bike, it’s trail that largely determines how a bike steers and how stable it is on the road. Trail is defined as the distance the front tire’s contact patch is behind (or “trails”) the intersection of the steering axis with the ground. More trail generally gives heavier steering effort but more stability, while reducing trail makes steering lighter with less stability. Rake is important in that it is one of the parameters that sets trail.
While we cannot adjust trail directly, we can by changing one of the three dimensions that directly affect trail: rake, offset, or front-tire diameter. Fitting a larger front tire, increasing rake, or decreasing offset will all add trail to some degree. On a streetbike with limited adjustments, unfortunately, we can’t change these dimensions directly and have to resort to even more indirect methods. The most common way to adjust trail is to change rake by raising or lowering the front or rear of the bike, which alters the angle of the entire chassis, including the steering axis. This is typically accomplished by sliding the fork tubes up or down in the triple clamps, but some bikes have an adjuster on the rear shock; extending or shortening the shock length will change ride height, an amount determined by the linkage ratio.
A change to trail can be made by raising or lowering the front or rear of the motorcycle, which changes the rake of the chassis. Ride height can be adjusted by sliding the fork tubes up or down in the triple clamps or adjusting the length of the shock. On most bikes, a 4 to 5mm change in ride height equates to a 1mm change in trail. Note that as the bike moves on its suspension, trail will change just as if you were making a ride-height adjustment.
We refer to these changes as ride-height adjustments, which you can measure in a number of ways. Typically we track front ride height by measuring how much fork tube protrudes from the top triple clamp and rear ride height by measuring from the rear axle vertically to a point on the subframe. In summary: You can add trail by raising the front of the bike or lowering the rear of the bike, either of which increases rake; note that if you raise the front end, the fork tubes would be lower in the triple clamps, giving a smaller number if you track ride height this way.
On racebikes, there are usually alternatives to consider: Adjustable triple clamps allow the offset to be changed, inserts in the frame can be used to adjust the steering head angle to directly affect rake, or a different tire could be fitted.
(Left) An easy way to track front ride height is to measure how much fork tube protrudes from the top triple clamp. (Right) Race teams use the fancy “hockey stick” to accurately measure rear ride height, but you can also use a point on the subframe.
Rear Geometry: Anti-Squat
As bikes become increasingly powerful, the effects squat and anti-squat have on handling become more pronounced. Squat refers to the tendency for the rear suspension to compress on acceleration, as load transfers to the rear of the bike. Countering that tendency is anti-squat, a characteristic of the chassis that tries to extend the rear suspension on acceleration.
The anti-squat effect is a combination of two forces. The first is the driving force of the rear tire, which acts to extend the rear suspension provided the swingarm slopes up from the rear wheel to the pivot. Picture the front of the motorcycle blocked against a solid wall; if you were to push directly on the rear axle forward, the upward angle of the swingarm would cause the rear suspension to extend. The second force is from the chain pulling on the axle in a direction parallel to the top chain run, which also acts to extend the suspension in most cases. The magnitude of the chain force’s anti-squat contribution depends on the angle of the chain relative to the swingarm and how far the chain run is from the swingarm pivot.
Anti-squat is a function of two forces acting on the rear axle. The first is driving force (red arrow), which pushes the motorcycle forward. If the swingarm is at an upward angle, this force extends the suspension. The second is the chain force (blue arrow), which pulls the axle forward and down in a direction parallel to the chain. In most cases, this also extends the suspension. The combination of these two forces acts to offset weight transfer from acceleration and prevent the rear suspension from compressing. The magnitude of anti-squat depends on swingarm angle, chain angle, and the chain’s distance from the swingarm pivot.
There are many ways to express the magnitude of the anti-squat effect. The simplest is to use a percentage to indicate how much weight transfer is offset by anti-squat. For example, if anti-squat is 75 percent, its effect offsets 75 percent of the weight transfer on acceleration and the suspension would still compress some amount. At 100 percent anti-squat, the weight transfer is perfectly offset and the suspension would not move due to weight transfer. And at more than 100 percent anti-squat, the suspension would extend on acceleration. This number can be calculated if you know the exact location of the front sprocket in the chassis along with some other dimensions.
Anti-squat is important for handling, as it can be used to change the chassis’ behavior on the exit of a turn. In general, we want some squat to load the rear tire for traction but not so much that the front tire loses traction and causes you to run wide.
As the rear suspension compresses, the swingarm angle and chain position can change considerably with a resulting change in anti-squat characteristics. On most bikes, anti-squat is just over 100 percent with the suspension fully extended, meaning the suspension will extend on acceleration as the anti-squat effect more than offsets any weight transfer. The typical chassis layout causes anti-squat to gradually decrease as the suspension moves through its travel. At full compression, some chassis will exhibit a pro-squat tendency (negative anti-squat) if the swingarm goes much past horizontal.
Adjusting anti-squat is a matter of changing the parameters that affect the swingarm angle or chain angle. That means raising or lowering the rear end to change swingarm angle, changing gearing to alter the chain angle or distance from the pivot, or adjusting the pivot location itself to change swingarm angle. To increase the anti-squat effect, you would: Raise rear ride height to increase swingarm angle; fit shorter gearing to change the chain/swingarm relationship; or raise the swingarm pivot to do both. Note that you must fit gearing with a different ratio to affect anti-squat—and not just bigger or smaller sprockets with the same ratio. It might sound counterintuitive, as the chain run does move significantly if you go from 15–45 to 16–48, for example. But even though the chain angle increases with that swap, the chain is farther away from the pivot, almost exactly offsetting the angle increase in terms of anti-squat.
The center of gravity (marked by the crosshair symbol) is the point at which forces can be considered to act for many calculations. In most cases, the CG’s physical location on the motorcycle cannot be changed, but there are some alternatives: Raising or lowering the entire bike on its suspension effectively raises or lowers the CG. In a similar manner, moving the rear wheel forward or rearward in the swingarm changes the horizontal CG position with respect to the wheelbase. And there is also the rider’s weight, which can be repositioned for a change in CG.
Center Of Gravity
Thus far, we have referred to raising or lowering the front or rear of the bike to change rake, trail, or anti-squat. But doing so raises or lowers the whole bike to a certain extent, and such a change on its own can also affect handling. To put numbers on these attributes, we often refer to the center of gravity (CG), which is the point where forces like load transfer, acceleration, braking, and cornering can be considered to act; it’s easiest imagined as the point from which you could balance the entire motorcycle, as you would balance a hammer by supporting it near the head end rather than near its physical center. On most sportbikes, the CG with rider on is roughly midway between the front and rear wheel (hence the 50/50 weight bias of most bikes) and a few inches below seat height.
Here we are concerned with both the height of the CG as well as its position front to rear. If the bike is higher, wheelies under both acceleration and braking are easier as weight transfer is magnified; however, taller motorcycles tend to turn from side to side quicker for a given effort from the rider, and less lean angle is required for a given cornering force. A more-forward CG helps keep the front end down on acceleration and can help with front grip, but the rear end will more easily rise during braking and will not have much grip; the opposite applies if the CG is more to the rear of the bike.
To use an extreme example, dragracers fit extended swingarms to effectively move the center of gravity position forward in the wheelbase and prevent wheelies on acceleration. Moving the rear axle in the chain adjustment blocks can have a similar (albeit much less pronounced) effect. Here, even though the CG has moved slightly rearward in the chassis due to moving the rear wheel further back, it is significantly further forward as a portion of the wheelbase, which determines weight bias.
There are no direct adjustments for CG, though on some bikes—like the Aprilia RSV4—the engine can be raised or lowered. And there is also the rider’s weight to be considered; moving the seating position can have a significant effect. As mentioned, a ride height change will raise or lower almost the whole bike, with a corresponding change to the CG. In the horizontal direction, adjusting the rear wheel in the swingarm does affect the CG to a certain extent. Many people are under the impression that raising or lowering the front or rear of the motorcycle can add or take away weight from that end, but that is not the case—a ride-height change moves hardly any weight front to back, and the horizontal position of the CG changes very little. Not convinced? Do the math, or make the actual adjustments and record the weights using scales.
Courtesy of MotoSpec
Race teams keep track of setup and geometry changes using some form of software that calculates important characteristics based on a number of input variables. These range from simple apps, which calculate change in rake and trail for given ride height adjustments, to more elaborate packages that track multiple setups and variables. This is a screenshot from MotoSpec (motospec.ca), a high-end program used by many world-level teams.
The trouble that most people encounter with setup is that making a change to adjust one variable affects another of the many geometry numbers and so on down the line. This is especially true on streetbikes that have limited adjustments, which forces us to sometimes make changes in a roundabout way. So, for example, while Valentino Rossi’s mechanics can change his M1’s swingarm pivot height for more anti-squat, we have to raise the rear end for the same thing. But this also changes rake, trail, and center of gravity height. Often making a single change can introduce more problems than it fixes.
Adding to the confusion, everything that we have referred to so far is with the bike vertical and the suspension fully extended—a position that almost never occurs with the bike on track. Geometry numbers constantly change as the suspension compresses. Under braking, for example, the front suspension almost fully compresses and the rear suspension extends; just as if you were to change ride heights, this significantly reduces rake and trail and also lowers the CG. In terms of anti-squat, the swingarm and chain angles change with suspension movement; anti-squat typically decreases with suspension travel and can even turn into pro-squat near the bottom of the stroke.
These dynamic aspects of geometry mean that spring rates and preload can affect how your bike works on track. Making just a single change to ride height, spring rate, gearing, or any other adjustment on your bike can affect the entire setup and undo sometimes hours of work in terms of track and shop time. In our next issue we’ll look at some common issues and ways to isolate and address them while minimizing those secondary effects.
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