Speedo computers

[mvadventure]

Because I often live in never never land sometimes I have a situation and don't even know about it. It's possible I have a little situation although so far it hasn't really meant anything.

On a little one mile road from the entrace of one of my golf courses to the clubhouse a friend of mine pulled alongside me in his automobile. I had the cruise control set at 20 MPH and he, through an open window, said he'd been following me and I was doing 25 MPH. I immediately checked BOTH speedometers and both said 19.7 MPH. (I'm kind of a redundancy freak)

Even though I gave it no thought at the time later I got to thinking. When I set the computers up initially I went simply by the instructions and used a 26" wheel as a setpoint. Fact is I don't have little tires on the bike, they are, I think, 2.125" tires and are substantially bigger than a regular tire in both width and diameter. So...the first thing I though of doing was a Time-Speed-Distance test but further thinking brought to mind that if my speedometer was off my odometer would also be off and therefore a mile wouldn't be a mile. It would prove something wrong if indeed in three minutes I went more than or less than a mile at 20 MPH but I don't really know how far that mile would be if the odometer is truly off.

Most likely I need to concentrate on more important things like saving the world, ending hunger or simply playing better golf but I was wondering if anyone else has experienced this situation. It would likely be a simpler solution had I saved the instruction booklets that came with the odometers.
Mike

 

[wasp]

the only way to set them properly is to measure the wheel in mm
i mean take one of those soft tape measures(like used for your waist)
and measure or mark your wheel and the floor(tape works) and measure
1 revolution...hope this helps
i had the same prob...remember a road tire and knobbie are both on 26" rim
but the knobbies are taller...just like truck tires...they vary quite a bit

 

[Link]

Damn, I thought this thread was going to be about adding machines made out of swimsuits. :?

Are these Wal-Mart odometers?

 

[dogman dan]

When I first set my speedo, the code the book told me to use was not correct for my tire, or rim or something. I figured out which 26" wheel code to use by checking the odometer against a measured mile of road.

 

[Zoot Katz]

Do a proper roll out measurement.

You need to measure one revolution of the wheel on the ground with your weight on the handle bar so the tire deforms normally.
You can mark the tire with chalk or just line up the valve stem and mark the ground.
Then roll forward until the valve stem or chalk mark comes around again.
Make another mark on the ground and measure between them.

It's easier with two people.

 

[mvadventure]

Thankyou, I will attempt to try this ASAP, hopefully this weekend. In answer to Link, one is a Sony bike radio, the other is of indeterminate origin and on its second bicycle. I went searching and deep within a waterproof bag I carry on the bike I found the directions for the radio/odometer/speedo and will do the appropriate measuring. Naturally my thrill at finding it did not include actually reading it yet.
Fortunately this is only a minor problem and is much, much better than a sharp stick in the eye.
Mike

 

[dogman dan]

Better than a stick for sure, but it can be an eye opener. When I got my speedo calibrated I began to understand why I could never get home all the way on my sla pack. My 12 mile ride was really 15. I kept thinking that I should only have to peadle the last mile or so, and ended up running out of power with 4 miles to go. The little twists and turns the bike path takes add up to more that I thought when I drove next to the route in my car to get the mileage.

 

[D-Man]

Yeah, tire pressure and weight are the biggest factors. I did multiple rollouts at different pressures. So if I change pressure, I'll change the speedo setting.

 

[Link]

How much variation did you get with the different pressures?

 

[D-Man]

The variations are:

At 16 psi I have a rollout of 80.25 inches.
At 30 psi = 81.5 inches.
At 60 psi = 82.5 inches.
At 75 psi = 83.0 inches.

 

[D-Man]

.

 

[Link]

Well, I run 55psi on my tires, so I think I'm good, there.

Never actually done a proper roll test to calibrate my speedos, though. :?

 

[D-Man]

Actually, from my speedo handbook, the values they give are all too high. Obviously taken with no weight on the tire and at max pressure. So if you use those values, your really going slower then what it says.

 

[dogman dan]

I found the book codes pretty useless. Just tried a few till I found one that matched a mile by the highway mile markers.

 

[mikereidis]

Keep in mind car speedometers are not paragons of accuracy. They usually read high; last I checked Jap cars are about 2-3% high and others about 5%. This keeps the auto makers from being sued when someone gets a speeding ticket.

Also, cars often have wheels which are not the calibrated size.

Also, bikes tend to waggle a bit when riding (esp. when pedalling), so knowing your road is exactly 1 mile doesn't mean your odo will show exactly that.

I use thread taped on wheel every 90 degrees, then I measure the thread. My 20" kids mtn bike tire is actually 19.1 inches or 1625 mm in circumference.