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Old 12-04-2011, 06:57 PM   #1963
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Joined: Apr 2003
Location: Long Beach, CA
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Smaller Front Sprocket vs. The IICE Air

Originally Posted by DidYouSeeThat View Post

Test Drive completed:

Short version: I like it!

Longer Version: I did not use good engineering practices for my test. I changed two things at the same time, not one. I swapped the 16T for the 17T AND added the Device. It feels like a good setup. Snatchy throttle is gone and response feels like the 15T but with a better ratio for cruising. Did a little dirt mostly mild, but went through enough rough stuff two try to miss the lower gear ratio... Yea, a little longing for the 15T in the technical stuff but far superior to the "Stock" Setup (16T w/o the Device). Leaving the 16T on for now, will report back if I swap the gear back. Stock 16T but a better bike with the device, so its a win. I like it!

Thanks Poolside!
C.N. Ca.
Switching to a smaller front sprocket vs. fitting the IICE Air.

Regarding the improvement in abrupt or 'snatchy' throttle, there's a reason why fitting the smaller sprocket feels the same as fitting an IICE Air and keeping the larger sprocket.

The reason is, the issue that causes abrupt throttle response was never the sprocket in the first place. The abruptness is caused by problems in transient fueling. Changing to a smaller front sprocket does help mask the fueling problem though. I'll explain why that is a little later.

Here's the sequence of events that makes the throttle so abrupt. First the rider opens the throttle and immediately the motor goes lean for a moment. The lean condition causes a drop in power output. The drop in power output is due to insufficient transient fueling. The rider experiences the drop in power output as a momentary lurch.

The momentary lean condition, and corresponding drop in power output, lasts for approximately two to three combustion cycles. The motor doesn't stall during this power interruption, but you sure lurch forward in the saddle from the momentary decrease in power. The lurch is the first half of the abruptness. It occurs during the throttle tip-in.

Throttle tip-in is a transient event, it only lasts a moment. It comprises the period of time that the rider is actually moving the throttle, plus two or three combustion cycles. After that period, normal fueling resumes and normal power is restored.

When normal fueling resumes, the rider experiences the increase in power output as a momentary lunge. And it's the preceding drop in power that makes the normal power feel abrupt.

Did you get that last piece? The abrupt throttle is composed of two events, one after the other. The drop in power when the throttle is tipped in, followed by normal power a moment later. The lurch-followed-by-lunge is the abruptness the rider feels when the throttle is moved.

Why does the throttle abruptness improve a little when I fit a smaller front sprocket? The answer to that is simple. A smaller front sprocket is a lower drive ratio, which means it takes less power from the motor to move the bike forward. The lower drive ratio needs less power, so the reduction in power from the momentary lean condition during throttle tip in is less noticeable.

Fitting a smaller front sprocket only masks the transient fueling problem. And here's something... Fitting an IICE Air also only masks the transient fueling problem.

The IICE Air, and other competitive products, are designed to enrich acceleration, not throttle tip-in.

Now, if you dump enough fuel in during acceleration (setting the IICE Air to -30 for example), it will make a small improvement to the initial lean condition at throttle tip-in. But it will be overly rich during acceleration and will not feel as crisp.

So while there is a small improvement to one, it comes at the expense of the other. It's a blunt instrument approach that lacks refinement. To fix the lurch-followed-by-lunge at throttle tip-in, and really make the throttle responsive, you need an IICE Cool.


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Poolside screwed with this post 12-04-2011 at 11:49 PM
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