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Thursday, July 07, 2011

GPS Mount & Engine RPM vs Speed

After looking at various products and DIY GPS mounts I decided to make a simple bracket to piggyback on the rear master cylinder cover.

It's just a twisted up length of 1"x1/8" aluminum flat stock, sprayed with rattle-can "wrinkle" paint. I replaced the cover screws with M4 x 16mm stainless flat head socket caps. The GPS clip mount is one that had a broken inner clamping finger, Garmin had sent me replacement; I just center drilled it and used a M5 button head cap to mount it to the bracket:


Front view:

Armed with this I set out to reverse engineer the CVT ratios at various vehicle speeds with the clutch locked up. Here is what I found:


Shown here are the tire dimensions (dia = 130 * 0.7 * 2 / 25.4 + 13 = 20.17"), circumference in inches and feet, and the revolutions per mile traveled (interestingly almost exactly 1000 revs/mile). Below that are the K3 AN400 drive ratios, the "primary" being 1:1 as the CVT is driven directly from the crankshaft.

In the table I have calculated the CVT driven sheave speed for each listed vehicle speed; this is the speed in MPH divided by 60 to get miles per minute, this divided by the tire revs/mile to get the tire RPM. The tire RPM is then multiplied by the final and secondary ratios the get the driven sheave RPM.

The engine RPM is that observed on the dash tach at the indicated speed (via GPS), at a steady state speed (with a very slight headwind).

The final column shows the calculated CVT ratio, determined by dividing the engine RPM by the calculated driven sheave RPM--this represents the CVT drive ratio at the selected speed. I found it interesting that there were all very near 1:1, as this is the natural point at which drive efficiency and belt life would be optimised.

One other issue I studied was the function of the cam/spring torque multiplier mechanism of the driven sheave. The cam spiral is such that the movable face moves inward toward the fixed face (pitch increases) when the movable face rotates anti-clockwise relative to the fixed face. You can see in this photo that the movable face has rotated as far as possible anti-clockwise, relative to the fixed face, and therefore the sheave's pitch is at its maximum:


But under what conditions would this happen?

It happens when the belt slips on the fixed face but does not slip on the movable face, which has less rotational resistance unless anti-clockwise to the extreme. So under high loads/hard acceleration, when the belt slips on the fixed driven face, the cam forces the faces together increasing the pitch of the driven sheave and pulling the belt deeper into the drive sheave--against the centrifugal force of the rollers--and increasing the CVT drive ratio--numerically higher, a "lower" gear.

Pretty neat!

BTW and FWIW, I bent up the bracket using a set of these from Grizzly. Had 'em from years and in addition to sheet metal I have bent 1/4" x 1" aluminum and 1/8" x 1" steel using a heavy duty 5" vise.
Posted by CliffyK at 3:46 PM
Edited on: Thursday, October 18, 2012 8:44 AM
Categories: Burgman 400