Y= MX + B
I'm an avid track-day rider and I have a chassis geometry question. My pace at the track has really improved over the last year, and I'm trying to understand more about my suspension and chassis. To make a bike turn in better and feel lighter, you can either lower the front or raise the rear. I don't believe it matters which way you go, as the end result is the same chassis geometry (except that if you lower the front, the bike's chassis is lower, and if you raise the rear, the bike's chassis is higher).
As I experiment with either of these adjustments, what should I be looking for if I adjust too far? What will the bike start doing badly on the track when I've adjusted the front too far down (or the rear too high)? I know tires can also affect a bike's turn-in and feel, but I just want to know about the chassis side of the equation.
You're correct that lowering the front of the motorcycle or raising the rear decreases rake and trail to make the bike steer quicker, and that the bike overall will be higher or lower depending on which change you make. Raising the whole bike will generally help it turn into a corner more easily, as the center of gravity is higher. Get it too high and you'll run into problems with wheelies under acceleration and the rear tire coming off the ground under heavy braking. Also keep in mind that for a given corner speed and turn radius, the higher the bike the more it will have to be leaned over.
Another factor to consider is swingarm angle and squat: Raising the whole bike increases the swingarm angle, resulting in more antisquat. Squat is the tendency of the bike to transfer weight to the rear end under acceleration. Antisquat is the mechanical advantage of the chain/swingarm relationship that offsets that tendency, allowing just enough weight transfer to give optimum traction at both the front and rear ends. With too much squat, the bike will run wide on corner exits as the front tire loses traction. You can raise the rear end (or the whole bike) to fix this. With too much antisquat, the rear tire will tend to spin up abruptly and snap sideways; lower the rear end (or the whole bike) to address this.
I have a '99 Honda CBR900RR. Unless I give it a lot of gas when pulling out, I hear this slow-speed scraping or clop-clop-clop noise that seems to be coming from the clutch/engine area. It doesn't do it when the engine is cold. The clutch is adjusted properly. I even checked the plates and discs, and they're within spec. The cam chain also seems fine. I can usually hear the noise until I've been in second for a little bit. The bike has about 13,000 miles on it. I let the service manager ride it at a dealership, and he just thought the chain was probably a little loose. I knew that was within spec, but I tightened it a little more, and I still hear it. Any ideas?
via SR Mail
The noise you describe sounds like the clutch basket or its bearing is worn out. The basket is riveted to the gear behind, and a spring mechanism in the gear acts as a damper. The rivets sometimes loosen up, allowing the basket to move on the gear; if that is the case, the noise you hear is the clutch rocking on the gear. Another potential source of the noise may be worn tabs on the basket, which allow the plates to move around excessively inside the basket. The springs in the damper may be worn out, and a bad bearing or thrust washer will also let the clutch move around. In an extreme case part of the clutch may be rubbing on the cover or other parts inside.
You can check this by taking the clutch cover off. Inspect the tabs for wear and see if you can rock the clutch from side to side or easily rotate it in either direction. Any play indicates something is worn out, and from there you can disassemble the clutch until you find the culprit.
Tapered Or Roller?
I am rebuilding a wrecked bike for a track bike, and I need to replace the steering-head bearings. From what I have read, tapered bearings offer longer life, but I don't know if they would affect steering feedback. I have looked for tech articles on the subject and found none. Would OEM-style ball bearings or aftermarket tapered bearings be better for a track bike when handling is more important than maintenance?
Tapered rollers are stronger and last longer because they have more contact area. The disadvantage is that they need regular attention and can affect steering if they are adjusted too tight. Ball bearings, with less contact area, are not so critical for adjustment and have less friction with less play, making steering lighter and improving feedback and stability. This is not something we've been able to test for ourselves, but if I were building a track bike I'd stick with ball bearings.
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