Glossary of Chassis Terms
[LEFT] To ensure an “apples to apples” comparison of chassis specs and terms, one must assume that the motorcycle is held level (the contact patch of both tires being parallel to level) with the suspension not compressed at all. In effect, “hanging in mid air.”
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[LEFT] Rake-When talking about rake on a motorcycle, it’s used to describe the rake (angle) of the forks/steering head. There are other “rakes” on a bike, but it’s universally understood that rake refers to the front end. The rake angle of the forks/steering head on every superbike I know of is the same. If the forks are at 24 deg, the steering head is at 24 deg, too. There are some “cruisers,” most notably the Harley V-Rod that have different fork and steering head angles, but that’s not a factor for superbikes, so you don’t need to consider that a variable. [/LEFT]
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- Increased rake (higher number) will make the motorcycle turn slower
- Decreased rake (smaller number) will make the motorcycle turn slower
- Increased rake will add trail
- Decreased rake will subtract trail
- Increased rake will lower front ride height
- Decreased rake will raise front ride height
- Increased rake will lengthen wheelbase
- Decreased rake will shorten wheelbase
- Increased rake will move the weight bias toward the rear
- Decreased rake will move the weight bias toward the front
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[LEFT]Trail-Trail is the measurement taken between where a straight line drawn down through the center of the front axle intersects the ground (work surface) and where a line that’s drawn down the center of the steer tube intersects the ground. In my opinion, trail is the most important measurement/spec the bike has. No matter where anything else on the bike is set up, you’ll find that there’s a “magic” trail number to try and meet. This holds true for every bike made.
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[LEFT] Trail can easily be called “front end feel.” The more trail you have, the more front end feel you’ll get. It’s as simple as that. The magic trail number is 100mm. You will, of course, find that some people, mostly racers, will want more trail than 100mm, but that’s to get a specific level of feel they’re looking for. The advantage to more trail is more feel. The downside to trail is that, the more you have, the more bar effort you need to flick the bike in. Of course, with a lot of trail, you can flick that sucker in hard and have the feel to be confident about doing it. The bar effort is never what I would consider high, though.
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[LEFT] Most superbikes don’t have enough trail off of the showroom floor anymore. Manufacturers are building bikes to “turn in fast” to impress magazine hacks who think that quick turn in defines the “handling” of the bike. The manufacturers claim high trail numbers, but, just like horsepower and weight, the figures are “optimistic.”
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[LEFT] The second benefit of trail is that it increases the “mechanical grip” of the front tire. When you have good trail numbers, the front tire is far less likely to tuck under when it loses traction. Instead, the tire, when it loses traction, will push across the top of the track surface. I’m sure your first reaction is “well that doesn’t sound any better” but I assure you, it is. When the tire pushes across the surface of the track, it still has grip; it’s not as much as you want, but it’s not “on ice” either. If the front end tucks under, you lose all traction from the front tire and, in a lot of cases, it will twist the bars out of your hands. When you have good trail numbers and the tire pushes across the top of the track, you can still save most of them with your knee. As long as you keep looking through the corer, stay neutral on the throttle, or, better yet, get on the gas and keep that knee dug in, you’ll save a majority of your front end slides. I’ve saved hundreds of front end slides by simply not giving up and because all my bikes are set up with good trail numbers. There’s nothing amazing about it, it’s just a product of having the additional mechanical grip from the front tire and the confidence in it to attempt the save. Another added benefit of more trail is that the self centering characteristics of the front wheel will increase. This adds to the bike’s stability and allows you to run your steering damper “softer,” which will allow you to flick the bike in with a little more speed, feel, and a little less bar effort (because the damper is looser).
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[LEFT] I’ll bet dollars to donuts that you could measure the trail of every bike in the superbike paddock and you won’t find one with less than 100mm of trail. You’ll find a lot with more, though. For example, I know that when James Toseland won the WSBK championship on the Ducati, he was running upwards of 108mm of trail. Trail is the main reason that you want to get different offset clamps for a bike.[/LEFT]
[LEFT] Trail can be increased by using a set of clamps with less offset (the distance measured center to center between the steer tube and the fork caps). 1mm less offset equals 1mm more trail.
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[LEFT] Trail can also be increased by raising the front ride height by pulling the forks down through the clamps so less fork shows above the top triple or the forks are recessed into the upper triple clamp. This method doesn’t increase trail at a 1:1 rate like the triple clamp offset does, though. This method will increase trail approximately .2mm for every 1mm the forks are moved.
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[LEFT] Trail can also be increased by lowering the rear ride height. If you lower the rear ride height 1mm, you’ll get an approximate trail increase of .25mm.
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[LEFT] Now, keep I mind that you’ll decrease trail by lowering the front ride height, raising the rear ride height, and using clamps that have more offset. I don’t think I’ve ever seen anyone put clamps on with more offset, but just about every bike I get my hands on has the front lowered and the rear raised. Not good for your trail numbers at all. And considering it’s your most important dimension, it means there are a lot of bikes running around with very little trail.
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[LEFT] I consider very little trail to be in the low 90mm range. Dangerous trail is anything under 90mm. Lack of trail takes away feel. In all cases, that will slow the rider down simply because of a lack of confidence. If the rider continues to load the front end, the tire will start to tuck, because it doesn’t have much of a “self centering” characteristic. The rider won’t feel these little front end tucks and will eventually tuck the front end with no warning and go down. I’ve only crashed once because of a front end tuck, and I had zero warning. I was riding a newly released bike, and there wasn’t any data out on it yet in regards to chassis setup. The icing on the cake is that the bike wasn’t mine. I got the bike back to the shop and measured the trail and guess what? It was 88mm. Yeah, I shouldn’t have tried to ride it that hard with the little feel I had, but there’s no excuse for a superbike to have 88mm of trail. It’s simply dangerous.[/LEFT]
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- Increased trail (bigger number) will give more front end feel
- Decreased trail (smaller number) will take away front end feel
- Increased trail will slow down the steering/increase bar effort
- Decreased trail will speed up the steering/reduce bar effort
- Increased trail will move the weight bias toward the front of the motorcycle
- Decreased trail will move the weight bias toward the rear of the motorcycle
- Increased trail will make the front tire more likely to push than tuck when it looses traction
- Decreased trail will make the front tire more likely to tuck than push when it looses traction
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[LEFT]Ride Height-Ride height is a pretty simple one. It’s just the height of the motorcycle usually measured against some sort of baseline. Ride height is, in most cases, broken down into front and rear ride height. Front ride height is generally adjusted by moving where the forks are set in the triple clamps. The more fork showing above the top triple clamp, the lower the ride height. The less fork showing above the top triple clamp, the higher the ride height. In some cases, when trying to get more trail with some motorcycles, people have made custom spacers so the forks are set below the level of the top triple clamp. Generally, this is resorted to when using offset triple clamps isn’t legal for the class they’re racing (supersport, for example). Rear ride height can be changed by increasing or decreasing the length of the shock if the bike has an aftermarket length adjustable shock. If the bike still has an OEM shock, in most cases the ride height can be increased by adding spacers between the frame and shock. If the bike has an OEM shock, it’s rare that anything can be done to reduce the rear ride height. On Ducatis and a few other rare motorcycles, rear ride height is changed by adjusting the length of part of the shock linkage. On Ducati superbikes, part of the shock linkage doubles as the ride height adjuster. It’s commonly referred to as the ride height adjuster or ride height rod. Another factor that can affect a bike’s ride height a lot is tire height. The difference in height between a run of the mill DOT race tire and a 16.5" superbike slick can be massive. I’ve seen tire heights differ by as much as 40mm, and 40mm is a whole lot when dealing with ride height. If the ride height isn’t returned to “normal” after mounting a lot larger or smaller tire, the chassis setup would go out the window. This is a bit out of left field, but I thought it should be mentioned.
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For a long time now it’s been common practice to change ride height to compensate for different height tires. For example, if someone were to switch to a front tire that’s 30mm higher than the one that came off, standard operating procedure would be to lower the front ride height 15mm (remember, only 15mm of the 30mm difference is affecting the ride height at any one time) to “even things up.” On a basic level, this makes sense, but the 1:1 ratio change would only hold true if the two tire profiles were identical, which is doubtful. If the 1:1 ratio were to be used, you would be correcting ride height for when the bike is straight up and down. Which, I’m sure we agree is probably when geometry matters the least. The geometry is going to be a lot more important when the bike is leaned over. But solving the ride height change for a different profile is next to impossible unless you brought a math professor with you. In these situations, I cut the 1:1 ratio change in half and start there. I came up with this process because I like “averages.” They always seem to serve me well. Obviously, if the tire you’re putting on is 30mm taller, you wouldn’t leave the ride height the same; it needs to be adjusted. But you don’t want to move it 30mm because of the profile change. I figure, why not start in the middle and adjust from there.
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- Adding front ride height will increase trail
- Lowering front ride height will decrease trail
- Adding front ride height will transfer the weight bias to the rear
- Lowering front ride height will transfer the weight bias to the front
- Adding front ride height will make the steering slower/more bar effort
- Lowering front ride height will make the steering faster/less bar effort
- Adding front ride height will increase the wheelbase
- Lowering front ride height will decrease the wheelbase
- Adding front ride height will increase the steering head angle
- Lowering front ride height will decrease the steering head angle
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[LEFT]Swingarm Length-Swingarm length is measured from the swingarm pivot to the rear axle. Measurements are to be taken center to center. Swingarm length is fairly simple in regards to the complexity of its contributions to the chassis setup. Right now, the forefront of superbike technology can’t seem to find a downside to increasing the swingarm length as long as it doesn’t push the wheelbase past acceptable levels. For years, manufacturers have been making their motors as compact as they possibly can by stacking transmissions on top of themselves, etc. to reduce the length of the engine cases. When they decrease the length of the engine cases, this allows them to add that length back in as swingarm length. Some of the current production motorcycles (R6, for example) have incredibly short cases. The MotoGP cases, due to being prototype engines in nature, have taken this to the extreme.[/LEFT]
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- Increasing swingarm length will transfer weight to the front wheel
- Decreasing swingarm length will transfer weight to the rear tire
- Increasing swingarm length will raise your ride height (assuming your swingarm angle is not flat or negative, which it shouldn’t be)
- Decreasing swingarm length will lower your ride height (assuming your swingarm angle is not flat or negative, which it shouldn’t be)
- Increasing swingarm length will increase your wheelbase
- Decreasing swingarm length will decrease your wheelbase
- Increasing swingarm length will slow the steering down
- Decreasing swingarm length will speed the steering up
- Increasing swingarm length will provide more stability in the chassis
- Decreasing swingarm length will take stability out of the chassis
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[LEFT]Swingarm Angle-Swingarm angle is exactly what it sounds like, it’s the angle of the swingarm. The swingarm angle is measured from the swingarm pivot to the rear axle through the center of each point. A swingarm should angle downward from the swingarm pivot to the rear axle. There’s no hard and fast “perfect” swingarm angle, but there’s a window that most superbikes will fall into. Most motorcycles will work best with a swingarm angle between 12 and 13 deg. The Ducati superbike chassis likes to run a shallower angle, though. This is most likely because the rear suspension linkage is a lot different than everything else out there. I’ve found that Ducatis work best in the 10-11 deg range. Swingarm angle is crucial. Swingarm angle will dictate how the chassis will behave from the moment you get on the gas at the apex of a corner to when you have the bike straight up and down to accelerate down the next straight. Getting off of the apex well and maximizing drive is arguably the best way to reduce lap times. The sooner you get on the gas and the faster you drive onto the next straight directly impacts your top speed down the following straight. When I went from an Armature level racer to Expert, I quickly learned what “separated the men from the boys.” I could brake with the fast guys, I could trail brake with the fast guys, and I had the same corner speeds that the fast guys had. But when it came to getting out of a corner and accelerating down the next straight, I quickly realized I had something to learn. If the rider in front of you gets just a slightly better drive coming off a corner, it can easily turn into multiple bike lengths at the end of the following straight. Of course, part of the solution to getting a better drive is simply growing “a pair” and screwing the gas on sooner. But, if the rear end of the bike doesn’t want to work with you, it doesn’t matter how big your pair is that weekend, you can’t outride a bad rear end when it comes to drive.
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[LEFT] It’s difficult to do a “if you do this, this will happen” list like I’ve done for the glossary items above when talking about swingarm angle. This is due to the fact that, if you have too much or too little swingarm angle, what you feel from the saddle will be nearly the same. For example, if the swingarm angle is too flat, the bike will want to step out sideways when the tire loses traction. But, the bike will also want to step out sideways when you lose traction if the swingarm angle is too steep. The only difference between the two situations is that you’ll have more feel and a better chance of saving the slide if the swingarm angle is on the low side opposed to high. A shallow swingarm angle will step out sideways slower, be less likely to hook back up while sideways, and will more likely be easier to get back in line than a swingarm setup with a little too much angle. Because of the above, swingarm angle is one of those things that will ultimately come down to rider preference. But, unless the rider has a goofy riding style like a dirt tracker, ice racer, speedway rider, etc., the swingarm angle will want to be in a certain range, and the range is not very wide so you won’t need to spend a lot of time setting it up to see what works best for you. There are a few things about swingarm angle that can be covered in a bullet list though.[/LEFT]
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- Too much swingarm angle will reduce the threshold where the rear end wants to step out sideways.
- Too little swingarm angle will reduce the threshold where the rear end wants to step out sideways.
- If you have too much swingarm angle and the rear does step out sideways, you’re much more likely to hook the tire up while sideways (opposed to a low swingarm angle), which, in most cases, results in a spectacular highside.
- If you have too little swingarm angle and the rear does step out sideways, it’s likely that you’ll hook the tire up while sideways. But if you stay in the gas, you’ll have a much better chance of saving it than if you had too much swingarm angle.
- Adding swingarm angle will increase rear ride height. You can increase swingarm angle on some motorcycles without changing the rear ride height, but the frame needs to have provisions for an adjustable swingarm pivot height. If you have the ability to adjust the swingarm pivot height, you can raise the swingarm pivot to get more swingarm angle, and it won’t affect the ride height.
- Subtracting swingarm angle will reduce rear ride height. You can reduce swingarm angle on some motorcycles without changing the rear ride height, but the frame needs to have provisions for an adjustable swingarm pivot height. If you have the ability to adjust the swingarm pivot height, you can lower the swingarm pivot to get less swingarm angle and it won’t affect the ride height.
- Increasing swingarm angle will add rear ride height, which will, in turn, transfer weight to the front tire.
- Decreasing swingarm angle will reduce rear ride height, which will, in turn, transfer weight to the rear tire.
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[LEFT]Capsize-Capsize is a rather abstract term that refers to chassis setup. I’m sure everyone knows what capsize means, everyone has rolled a canoe over at summer camp when we were kids. You capsized the canoe. Obviously, you don’t want to capsize your bike; it’s expensive and impacts lap times dramatically. When using the term capsize in reference to motorcycle chassis performance, it’s to describe the speed at which the bike will go from full lean one way to full lean the other way. On the surface, you’d think that the quickness of the bike’s steering would affect this more than anything else. The speed of the steering does contribute, but the capsize shoulders the bulk of the responsibility. In simple terms, if you raise the bike’s center of gravity, it will capsize faster and with less effort. The reason it works like this is because the bike’s center of gravity is basically being turned into a pendulum. The center of gravity is the weight at the end of the pendulum, and the “lever” of the pendulum goes straight down through the center of the bike to the ground. The higher the center of gravity, the faster the center of gravity (pendulum) will move. If you want to feel the effect of this, take a 4’ length of rope and swing it around in a circle over your head. Now attach a 5lb weight to the end of that rope and repeat. After it gets going, the test with the weight moves a lot faster and packs more of a punch. Of course, capsize has a downside, (everything with suspension is give and take), and that’s the initial input that you’ll need to put into the bike to get it to flop from one knee to another. It will require a little more oomph to get the bike moving, but you’ll get paid back 10 times over once it does start to flop. If you have your capsize in the optimum spot, you’ll go from one knee to another faster than you can imagine. Eventually, if you raise the center of gravity too high, the center of gravity will impact other functions of the motorcycle. A good example of that being the bike will want to push across the top of the track instead of digging in when you initiate a turn. Another example of having the center of gravity too high is the bike will want to wheelie all over the place on the gas. That said, the ride height of the bike would have to be extremely high to create these circumstances. And it’s not even close to that high with my suggested chassis numbers.[/LEFT]
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- Adding capsize will increase the speed at which your bike will transition from side to side to a certain point
- Reducing capsize will decrease the speed at which your bike will transition from side to side
- Adding capsize involves increasing the motorcycles ride height so all the added advantages/disadvantages of that will apply
- Decreasing capsize involves decreasing the motorcycles ride height so all the added advantages/disadvantages of that will apply
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[LEFT]Wheelbase-Wheelbase may be the most basic term here. Wheelbase is simply the measurement taken from the center of the front axle to the center of the rear axle. All superbikes fall into a “wheelbase window” and that’s because staying within that window is what’s needed to stay competitive with the competition.[/LEFT]
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- A longer wheelbase will make the bike turn slower
- A shorter wheelbase will make the bike turn faster
- A longer wheelbase will improve the bike’s stability
- A shorter wheelbase will lessen the bike’s stability
- A longer wheelbase will lessen the bike’s tendency to wheelie
- A shorter wheelbase will increase the bike’s tendency to wheelie
- A longer wheelbase will allow the bike to run a longer swingarm, which is an advantage
- A shorter wheelbase will require the bike to run shorter components and compromises would need to be made
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[LEFT]Geometry-Geometry is the “hard” suspension settings in your bike. Geometry can also be referred to as “chassis numbers.” Geometry covers every length, height, angle, etc. a chassis part is set up at. After geometry is set up, it doesn’t change while you’re riding. Geometry is the most important part of setting up your bike by a large margin. No amount of suspension components or their expense will solve geometry problems. Geometry is the foundation of the motorcycle. Everything is built outward from the geometry.
Thanks for reading, SOURCE here
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want to get some more universal tech resources up for bikes