VDOs for Penton CMF period

[pklinck]

I have a vdo speedo with I believe is a ratio of "W1,54 and a manufacture date of 4.70 and a vin of: 20109/4.

Is this the right ratio for a 73' penton 125 with the drive in the front wheel?

peter klinck

[Mike Rosso]

I'm not sure if I remember correctly but when I was doing a mechanical odo for the early '80's KTM's we were using a VDO with a 1.42 ratio front wheel odo drive and if you spun the front wheel 142 times (one hundred and forty-two) the odo would move forward 1-tenth.

Now you could calculate the front tire diameter/circumference or mark the tire and measure the distance traveled of one revolution of the wheel, divide that into 1-mile converted from 5,280 feet to inches (63,360) and then determine how many revolutions the cable turns, then add in the 43 beats a second wing speed of the African Swallow needed to maintain air speed velocity and...................way too complicated IMO LOL:D:D


I think if you count the number of revolutions of the front wheel it takes to go from one tenth to the next tenth on the odometer that will give you the ratio needed......or the very easiest why is to have a member chime in and tell you the answer.

[Mike Rosso]

quote:Originally posted by pklinck

I have a vdo speedo with I believe is a ratio of "W1,54 and a manufacture date of 4.70 and a vin of: 20109/4.

Is this the right ratio for a 73' penton 125 with the drive in the front wheel?

peter klinck

To determine the wheel drive ratio you have, you WILL need to determine the number of cable revolutions (i.e., wheel revolutions) in 1/10 of a mile. (6,336 inches). That would require that you measure the distance the front wheel rolls in one revolution and how many times the cable turns in that same distance....sorry.

[pklinck]

Mike,

I do not know the history of this VDO since a purchased it with a bunch of other parts and it does not work, though it moves a bit when the wheel goes around.  Though my bike is a 73 cmf 125, the VDO apparently is made in April, 1970.  I believe that the later steel tankers had the same setup and I think more than less the same hub as the 72-74' cmf bikes.  So this is why I would like to know what the ratios or range of ratios others have on their cmf bikes prior to 1975 before i spend the money to rehab. it.  I could google a fine search on this site.  thanks if anyone has a comment.

peter klinck

[rob w]

Mike is right, 1.42 is the correct speedo ratio to use.

[pklinck]

Rob,

Thanks for the info.  I had just confimed the ratio of 1.42 as per a past msg Doug Wilford indicated when I did a subject search. thanks again.

peter klinck

[rob w]

Reposted from POG search. Feb 18, 2003

I have been messing around with VDO Enduro speedometers in an effort to fit one to my Penton. I have figured out a few things that may be of interest to others trying to fit a Speedometer-Odometer to their motorcycle.

1. The speedometer "ratio" refers to the number of revolutions of the speedometer / odometer input drive required to register one (1) mile. Or conversely the RPM needed to indicate 60 MPH.

For example:

1.15 = 1150 revolutions make 1 mile, or 1150 RPM is indicated as 60 MPH

1.42 = 1420 rev/mile

1.6 = 1600 rev/mile

2.15 = 2150 rev/mile

(yes, I actually spun and counted to figure this out. Too much time on my hands or just nuts, you make the call)

The ratio is usually marked (stamped) on the housing as W1.15, W1.95, etc.

2. In order to get an accurate speed or mileage reading (assuming that the speedometer / odometer units are accurate, I don't know if they actually are) it is necessary to get a speedometer that coordinates with the drive mechanism built into the motorcycle, or fit an external accessory drive unit. How do you determine this? Let me try to explain how I did it.

3. First I figured out how many revolutions my wheel makes in a mile. I measured the circumference of the tire mounted on its wheel. In this case 3.00x21 = 87 inches. One mile is 5280 feet or 63,360 inches.

63,360 divided by 87 = 728 revolutions per mile

4. Next I determined the drive ratio of the speedometer drive mechanism built into the front wheel hub. I had easy access to the drive gears so I just counted them. The drive gear has 13 teeth and the driven gear has 7 teeth. 13/7 = 1.857. For every revolution of the wheel the drive output will spin 1.87 times.

5. Now just a little math and you will know what ratio of speedometer you need. (revolutions of the wheel per mile) x (drive ratio of speedometer drive) = the revolutions of your speedometer drive per mile, which divided by 1000 gives you the desired speedometer ratio.

Ex.1 728 x 1.857 = 1352 / 1000 = 1.35 (W1.35)

Ex.2 728 x 1.57 = 1143 / 1000 = 1.14 (W1.14)



If you don't have easy access, or just don't want to take your front end apart, you can spin the wheel and count the revolutions of the drive output. To make it easier elevate the front wheel. Then put a stripe of tape across the tire. Make an indicator to fit into the cable drive. I used a small (1/8"diameter) piece of wood dowel rod whittled to a square taper with a piece of tape as a flag. Spin the wheel 10 (or better 100) times and count the number of times the flag on your indicator goes around. In my case 100 revolutions of the wheel make 187.5 revolutions of the flag. All of this spinning and counting goes much easier with two people. One to count the spinning wheel, one to count the spinning output.

If you don't have, or can not get, a speedometer/odometer of the correct ratio there is a solution. A speedometer repair facility should be able to make up a gear ratio correction drive box. It is a small gear set (available in many ratios) installed in the drive cable. They are typically used to correct for changes in tire size or rear end gear ratios in cars and trucks. To find a speedometer shop in your area look in the yellow pages under speedometer, drive train, transmission, or look for a shop that services over the road trucks. Often they can also make up and/or repair drive cables. Some of these sources may also be able to change the internal ratio of your speedometer.


Mark Annan

[pklinck]

Rob W.

If the 70-75 penton motorcycles with the 21 wheels had a speedo ratio off the front wheel of W1.42, how far off, in terms of miles per hour, would the reading be from speedo mounted on a penton that is slightly different, such as W1.54?  I have to decide whether or not to rehab (spend $)this vdo for my 73' penton 125.  thanks.

peter klinck

[hankthecrank]

Rob, you are right about the speedo drive correction kits. I worked on 18 wheelers back in the day when employers and employees both wanted accurate mileage readings. Employees because they wanted paid for correct miles driven and employers because they didn't want to pay more than necessary. The DOT also wanted accurate readings for driver hours, breaks etc.  White was the brand name on the kit I used , came in a plastic toolbox, and I had a short cable with an odometer on one end that I would mount on the trans or wherever the speedo drive was. Zero the odometer , drive a measured 1/10 mile and take that reading to the chart supplied with the kit. It told me what gears to use in the drive unit to get the speedo reading that paticular vehicle needed. With digital speedo's it may be different now however I figure this service is still available.

Hank Rinehart

[Mike Rosso]

quote:Originally posted by pklinck

Rob W.

If the 70-75 penton motorcycles with the 21 wheels had a speedo ratio off the front wheel of W1.42, how far off, in terms of miles per hour, would the reading be from speedo mounted on a penton that is slightly different, such as W1.54?  I have to decide whether or not to rehab (spend $)this vdo for my 73' penton 125.  thanks.

peter klinck

Peter, you need another 120 cable revolutions per mile and you could get that by going to a smaller circumference front tire IMO See step 3. of Rob's repost of Mark Annan's excellent research. Mark figured 87" and I know from doing the same thing back in '83 that all 3.00X21 tires are not the same.........If you used a 100/90, 90/90, 90/80 you may end up finding a tire with a smaller circumference than 87" that would end up giving you that 1,540 cable rev's in 1-mile. You're 1/10th or less short every mile now with a 1.42 ratio wheel drive with a 1.54 ratio speedo/odo head.


Rethinking this, Peter are you sure what wheel drive you have now?

Mark the tire and roll the bike one wheel revolution and count the number of cable revolutions as per Mark Annan's step #3.

[pklinck]

Mike/Mark,

My speedo cable does 2 revolutions per 21" wheel revolution so

87"(total inches around 21" wheel w/tire)x2=174/1000 = 1.74(officially W1.73) THis is the drive ratio on my adjacent 125 73' penton.  Apparently, while checking the drive gear in the front wheel of the other in concern, i noticed that the cable does not move.

So thanks, I see the calulations and now I must, as just discovered, fix or adjust the drive unit down by the from wheel.  I got the process now.  Thanks everyone.

peter klinck