13 replies to this topic

[SolarB]

I'm replacing the original mechanical speedometer drive in a center speedo with an electronic drive but keeping the original face.
I plan on running 10" wheels with A008 165/70/10 tires. Any chance someone has the rolling diameter of a wheel with this tire?
If not, can some kind sole measure the rolling diameter for me? Bicycle style would be easiest - mark the bottom of the tire with a bit of tape or pen, roll it forward for a few revolutions and measure the distance.

[lrostoke]

tyre height is
70 % of 165 thats where the 70 comes in, 70 being 70% of the width of 165

So 115.5 mm height x 2 = 231 , divide by 25.4 to give inches = 9.09 add rim diameter of 10" = 19.09"

circumference is Pi x D = 3.142 x 19.09 = 59.98 " rolling circumference.

Thats the theory anyway but I've just measured the overall diameter of my wheel and it looks closer to 18.5" (165/70/10) that was cross ways , this gives a circumference of 56.55"

Edited by lrostoke, 08 April 2008 - 01:01 PM.

[fastcarl]

tyre height is
70 % of 165 thats where the 70 comes in, 70 being 70% of the width of 165

So 115.5 mm height x 2 = 231 , divide by 25.4 to give inches = 9.09 add rim diameter of 10" = 19.09"

circumference is Pi x D = 3.142 x 19.09 = 59.98 " rolling circumference.

the above is fine if you dont want to put the tyre on the car, as when fitted will have the flat spot where it mets the ground, so the above does not work,
you need to work out your rolling circumference, . chalk a mark on the tyre fitted to car , and another mark on the floor, roll tyre one full turn untill mark is adjacent to floor again, mark spot, now measure distance from first mark to the second, result is rolling circumference.


carl

[dklawson]

I have the Sigma bike computer in my car and the current calibration factor I'm using is a circumference of 1565mm for a 165/70/10 tire. (EDIT: Keep in mind that's just the wheel circumference. If you're planning on using a pickup that replaces the speedo cable you'll also have to account for the final drive gearing AND the speedo drive gearing in the gearbox).

I have done the gauge conversion you're contemplating. You can fit an electronic speedo behind the existing face, you can make the needle swing match what the mechanical speedo does/did. However, you cannot simultaneously make the odometer read correctly. The electronic speedos use an internally saved (not user accessible) variable that ties the analog needle and the odometer. Both cannot be displayed correctly unless you have a way to access and reprogram the internal variable.

You can see a gauge I converted in this picture:
http://home.mindspri...es/Senders2.jpg

There is a VDO movement behind the face. The face is new. It's a decal transferred over the original face with the tick marks spaced to match the VDO calibration.

I have also done this conversion for a friend who owns an early Jeep. My friend is a machinist and he decided in his case the best solution was to CNC machine a new gauge face from anodized black aluminum plate. VERY sharp looking when we were done.

Edited by dklawson, 08 April 2008 - 01:06 PM.

[lrostoke]

the above is fine if you dont want to put the tyre on the car, as when fitted will have the flat spot where it mets the ground, so the above does not work,

I totally agree the most accurate method is actually to roll the car and measure how far it travels before a set mark comes back, but the above got me thinking if you knew the actually diameter of the tyre would the flat spot on the ground change the circumference ?? you would still have same amount of rubber in the casing it would just change shape ??

MMmmm 2nd thoughts it will spread sideways as well, so I guess that would make a difference oh well gave me a thinking moment

Edited by lrostoke, 08 April 2008 - 01:41 PM.

[dklawson]

The tire circumference is influenced by more than the spot contacting the ground. At speed a tire changes shape quite a bit. The circumference also changes through the life of the tire as it wears. You'll note that my value of 1565mm [61.61"] is actually larger (even with wear) than the nominal 59.98" value posted by Irostoke.

That's why I provided the circumference value I use with my Sigma bike computer. That value is based on calibrating the speedo over a measured distance and is quite accurate... for the speeds I drive and the current state of wear for my tires (which is typically around 50 MPH with about 50% of the tread worn off my Yoko A008s).

[lrostoke]

The tire circumference is influenced by more than the spot contacting the ground. At speed a tire changes shape quite a bit

Can see this really clearly on drag cars now I think of it...just look how those big babys get higher when they take off.

[SolarB]

Quick reply (I should be working at the moment).

Intend to take the wheel RPM from an inner CV joint, therefore no problems with gearing.
Mileometer can be driven separately with a motor and doesn't need to be 100% accurate, within +/- 10% would do (could make clocking a doddle with a forward/reverse switch).
I want the speed as accurate as possible so need the precise rolling diameter/circumference of a 10" wheel with an A008 fitted.
Does a flat spot change the circumference? Yep, I guess the side walls bulge and the distance between wheel center and ground reduces. But hold on, a bulge in the side wall doesn't account for the reduction in the overall circumferences. If the circumference does reduce because of the flat spot (straight line between two points iis shorter than a curve) where does the extra tread go?

[matty...]

Ive just tried measuring a wheel 112.5mm with a rubber tyre on it so that equals 353.4mm Circumfrance

I marked the wheel and a table rolled it along (putting pressure on it causing a flat spot) untill the mark was at BDC and measured the marks and it came out at 349mm.

I guess the additional thread or dia will be loss in compression accross the dia.

[Guess-Works.com]

Simple thing is to use a calculated value ( as above ) and then fine tune with the use of a reliable GPS speedometer... I think you'll find the calculated amount is within 1 or 2% of actual.

these are the figures I use to calculate gearbox speedo ratio's

1527mm - 145 x 10
1523mm - 165/70 x 10
1595mm - 145/70 x 12
1579mm - 165/60 x 12
1587mm - 175/50 x 13
1588mm - 195/45 x 13

[minimender]

Those cheep speedo s that you can buy for a bycycle are easy to calibrate and are digital

[SolarB]

I have the Sigma bike computer in my car and the current calibration factor I'm using is a circumference of 1565mm for a 165/70/10 tire. (EDIT: Keep in mind that's just the wheel circumference. If you're planning on using a pickup that replaces the speedo cable you'll also have to account for the final drive gearing AND the speedo drive gearing in the gearbox).

I have done the gauge conversion you're contemplating. You can fit an electronic speedo behind the existing face, you can make the needle swing match what the mechanical speedo does/did. However, you cannot simultaneously make the odometer read correctly. The electronic speedos use an internally saved (not user accessible) variable that ties the analog needle and the odometer. Both cannot be displayed correctly unless you have a way to access and reprogram the internal variable.

Very nice looking. Did you have a mileometer?

[dklawson]

Very nice looking. Did you have a mileometer?

Since I don't know what one is... I'm guessing my answer will be "no". At the risk of sounding dense... what's a mileometer?

[lrostoke]

The number dials that tell you how many miles the car as covered, or a device that will do the same thing.

basically a measure of distance travelled

Just remembered the other name Odometer

http://en.wikipedia.org/wiki/Odometer

Edited by lrostoke, 11 April 2008 - 12:24 PM.