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Old June 18th, 2013, 09:08 AM   #1
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14 tooth sprocket question

So I did my sprockets yesterday and wow! The bike doesn't feel faster per say, but it feels like its in the power all the time. Second gear wheelies are now a twist of the wrist away. Like others have said the constant shifting kinda sucks as does the lower speed per gear. Would be awesome with an additional 500-1000 rpms. With that being said, before I did the swap, my chain was stretch pretty bad so I had the chain adjust almost as far as the bike would let me. This in turn lowered the rear of the bike giving it a decent head shake while on the throttle. Usually when I do the chain the problem goes away but I'm guessing the smaller front sprocket forces you to lower the bike a tad as I still have a slight head shake that actually reminds me of the Panigales I've demo'd in the past. While fun should I raise the rear of the bike or maybe cut a link out of the chain to raise the back up. Or should I tighten the steering dampener and call it even? Won't make a difference to me at the track but I'd rather not fight the bike while I should be relaxing down the straights.

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David
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Old June 18th, 2013, 10:48 AM   #2
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Actually, lowering the rear changes the fork angle and increases trail — so the likelihood of headshake onset is reduced.

The bike's handling characteristics are degraded if the rear ride height is reduced so whenever you change sprockets measure rear ride height first and use the ride height adjuster rod to restore it after the change.

The rear ride hight adjuster rod is initially set to its shortest length so if the sprocket change moves the eccentric adjuster axle position down you'll need to add chain links to move the axle back and up.
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Old June 18th, 2013, 12:56 PM   #3
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Also going with a smaller sprocket you couldve probably went 1 or 2 links less on the chain length so you could keep pretty close to the same wheel base
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Old June 18th, 2013, 05:10 PM   #4
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Shazzam, as always great information! Maybe the head shake this go around is from the DP air filter lol. When I dropped off the bike it was fine and when I picked the bike back up I noticed the intake roar and that any little imperfection in the road lead to the front end shaking. Tho on the dyno the power is no different. Change of tires later it lessen a bit and now with the smaller sprocket its back in full force! Feels like the front is trying to take off. Now on the smooth track I run at there isn't any head shake at all. I guess I'm trying to fix something I don't know to be a problem yet but I suspect more than a few corners are going to end in front tire climbing the sky on corner exit. Usually with a new tire and new chain I have to tip top a bit on the bike. Right now with slightly worn tire and new chain I can pretty much flat foot the ground. Are you saying I should raise the rear to get the proper rake back into the bike?

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Old June 18th, 2013, 05:42 PM   #5
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Are you saying I should raise the rear to get the proper rake back into the bike?
Definitely. The higher you raise the rear the faster the turn-in to corners. Go too high however and you'll start falling into turns and have trouble holding the line in corners.
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Old June 18th, 2013, 05:45 PM   #6
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Stability and Control of Motorcycles

As a starting point, consider that a motorcycle is stabile only under a limited set of steady-state conditions that exist while turning, braking, accelerating and running straight at a constant speed. When you perturb this condition, the dynamics of the system changes, and the result is that either it returns to the original steady state or the system becomes increasingly unstable, and often uncontrollable by the rider.

How sensitive the motorcycle stability is to a perturbation is a direct result of the manufacturer’s design choices that establish geometry, weight distribution suspension stiffness and damping: even tire size plays an important role. Modern design choices are made using sophisticated computer models that allow designers to predict the stability of a bike before it is built.

Sometime the design considerations for stability collide with desired performance characteristics, especially on sportbikes. For example, quick steering is always preferred by the rider, but in order to achieve it, the geometry that gives a bike it’s ability to quickly return to a straight line after hitting a piece of bad pavement needs to be changed. In doing so, the bike turns better but the bike is more prone to wobble after traversing bad pavement. A severe case of wobble is called a tank-slapper.

At some point in the design process, the stability issues cannot be further ignored so a steering damper is added to deal with the increased tendency to wobble. A steering damper provides a force opposing the movement of the handlebars. The faster the bars turn, the more force the dampers provides to resist turning, so there is always some degradation of handling intrinsic to its use.

This is the design trade-off that the rider has inherited and he needs to consider it when encountering a wobble that could evolve into a potentially dangerous tank-slapper.

There are a number of road conditions that will start a wobble but they all result in diverting the front wheel away from its straight ahead position. Once this happens a number of forces come into play. The tire’s slip angle produces a pneumatic trail force that directs the front wheel to move back toward a centered position. This combines with the centering force produced by the steering trail dimension. Unfortunately, the steering doesn’t just return to straight ahead and stop, it overshoots to the opposite side and the cycle continues until it gets damped out by friction, tire carcass flexure and a perhaps a steering damper.

The same condition develops if you wheelie and then set the front end down with the wheel not pointing straight ahead.

A rider can avoid setting up a wobble if he can avoid encountering pavement conditions and road debris that can divert the front wheel from its straight ahead position, but a some point most riders will experience a wobble onset.

So what response should a rider have to a potential tank-slapper situation?

First, the rider should refrain from pushing with his arms to try to help straighten the handlebars. This will add energy to the system and make the magnitude of the oscillation worse. Try to relax your arms, maintain your grip, and let it flail.

Second, reduce the forces pushing the wheel from side to side by reducing the weight on the front tire. Computer simulations have shown that the correct rider response is to roll-on the throttle, and have also shown that applying the brakes will make the wobble worse.

Rolling on the throttle is the same approach the rider should take when approaching a pothole, pavement ridge or any low obstacle that threatens front wheel alignment if hit.
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Old June 18th, 2013, 07:11 PM   #7
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Thanks! Always good to reread good info after you haven't in a while. Just got back from a tour of my town and WOW! The pros of the 14 tooth sprocket out weight the bad. I mean I was idling at 5 mph without having to feather the clutch! Even in second at 15-20 mph hardly resulted in a stutter from the bike. Definitely my favorite mod to date. Back on topic two lol, I've found the head shake only while turning or switching lanes while the road is uneven as I went through a smooth bend full on throttle in second with no drama just the front tire leaving the ground in epic Ducati style. I'm sure once I set up the ride height and retune the suspension everything will fall back in place. In the mean time I'll turn the damper up a few clicks.

Thanks again!
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Old June 18th, 2013, 08:36 PM   #8
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Shazam WHO ARE YOU!!!!!! I love reading your posts. How do you know everything about motorcycles. You always explain everything in such a great manor.
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Old June 18th, 2013, 09:06 PM   #9
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Still trying to understand how the longer wheel base due to a adjusted chain (stretched) lowers the rear of the bike. can anyone explain this or am i just missing something?
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Old June 18th, 2013, 09:10 PM   #10
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I could be wrong but if you pull off the sprocket and look at the cam design, the ride height lowers as the tire is rotated back, and the ride height goes up as the wheel is rotated forward. If not I'm completely off my rockers but I'm sure that's how it works. Its not like a dual swing arm bike where the axle rides in a straight arm that simply moves back and forth.
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Old June 18th, 2013, 10:32 PM   #11
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Quote:
Originally Posted by Skee View Post
Still trying to understand how the longer wheel base due to a adjusted chain (stretched) lowers the rear of the bike. can anyone explain this or am i just missing something?


Before you make any sprocket changes, make sure you measure and record your rear ride height. Once the chain tension is correct, reset the ride height by changing the length of the tie-rod adjuster. Any size changes to the final drive sprockets or chain length will usually require an adjustment of the rear axle eccentric hub.

Single-sided swingarms use eccentric hubs for chain tension adjustment. With an eccentric hub, when you move the axle back to reduce slack in the chain, the axle also is simultaneously moved upward. Consequently, you end up with both a lengthened wheelbase and a reduction in ride height. Rear ride height changes drastically affect the way a bike turns and holds a line. When you reduce the rear ride height, it slows the steering and the accompanying longer wheelbase will slow handling.

The eccentric hub (viewed from the left side) should set the axle axis at the 4-5 o'clock position relative to the hub axis. You should remove or add chain links to obtain this orientation. Otherwise your suspension set-up starting point is incorrect.
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Old June 19th, 2013, 05:11 AM   #12
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Show off!!!!!

Jk I think I figured it out. Shazaam is a bot that jump from forum to forum and dumps tons of good info when an unassuming newbie asks a question lol! Keeps the forum vets happy.
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Old June 19th, 2013, 11:04 AM   #13
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Yupper Kooloreanked. I'm new to Ducati, i keep forgetting about the eccentric hub. Im still thinking like a Jap bike lol...

Shazaam thanks for the info. Any required reading that you suggest to get smart on Ducati maintenance besides the shop manual?
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