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Date:         Wed, 31 Jan 2001 10:18:37 -0800
Reply-To:     pensioner <al_knoll@PACBELL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         pensioner <al_knoll@PACBELL.NET>
Subject:      Tire rollout, per jbrandt@hpl
Content-type: text/plain; charset="iso-8859-1"

Are we tired yet?

Some many years ago Klaus at Palo Alto Speedometer gave me the wizard's secret to getting that Tripmaster DFO.

Grasshopper, sez he, the lowly tripmaster knows not of tire height, even more it knows not of tire at all. It only knows axle rotations.

So...per the jbrandt of 356/torsion bar and bicycle fame. Use mustard.

Gitchaself on over to yer fave fast food store and snap up two or three packages of good ol' bright yellow mustard.

Find a straight place in a parking lot or quiet street

Aim the vehicle in question in a straight line.

Place a thick line of mustard in front of the wheel to be measured.

Drive forward slowly through the mustard.

When you think you've gone twenty feet or so go another two feet.

Set the parking brake.

Inspect your tracks for THREE mustard marks. The original, the first revolution and the second revolution.

If only two marks check to see if you have completed two wheel rotations. If not get back in, release parking brake and move forward until you have three mustards.

Using a laser interferometer or other measuring device ( A 30' Craftsman Steel tape is my battery and pollution free portable measuring tool of choice for this application) measure the first edge to first edge between #1 MM and #2 MM (Mustard Mark). One percent of thutty feet is like a quarter inch so get it to a quarter inch. Measure twice. Cut nonce.

Now convert the measurement to your units of choice, nanometers, furlongs, light years. I tend to use feet to tenths.

Now you know the "rollout" for two revolutions of the wheel.

With this secret knowledge written in the back of the owners manual with date and tire size and inflation pressure and ambient or tire temperature you can venture into the world of adjusting your gearing to suit your fancy.

Call this constant 2RO1.

Derek's fine work can calculate wheel rotations as a basis of rpm and gearing. Knowing your RO1 (half of the 2RO1, calculated) you can predict revolutions per mile, revolutions per minute versus an approximate miles per hour AND you can use common math to get an actual wheel diameter or radius for your particular wheelset (sorry JB).

In metrics we learned that any quantity contains a linear error of measurement, call it "error" If you multiply the quantity by a constant greater than one you also increase the absolute error. If you divide the quantity by a constant greater than one you reduce the absolute error, hence three mustards.. Four is impractical as consumer grade mustard only marks distinctly thrice. You DID mark the REAR WHEEL, right? Else you are subject to recursive and multiple mustards. To do the front wheel you use reverse gear, natch?

You can use a measured highway marker set or a gps or other method to achieve much the same accuracy at greater bother. One percent is as good an engineering estimate as is needed for these calculations as they are range limited. EG: one does not have a continuous set of solutions as gear sets are integral devices. 32:11 gives one ratio solution 33:11 gives a second and 33:10 gives one in the middle. Machinists even good machinists balk at cutting a gear with ten and a half teeth.

now back to your regularly scheduled programming.

pensioner


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