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Speedometer error

weh8127
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Speedometer error

Post by weh8127 »

On my R60/2 sidecar rig with standard mph speedometer I'm running the solo transmission and 25/8 final drive. I find this overall gearing a little high so I have a 27/7 final drive on the way. I'm at a loss on how to calculate speedometer error with this gearing change. If I knew what the speedo would read when my actual speed is 25, 40 and 55 mph that would be good enough.
Bill Husted
Barre, MA USA
1963 R60/2 w/ 1955 Steib S500
1973 R75/5

cwf
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Re: Speedometer error

Post by cwf »

If you look up the archived magazine of the BMW VMCA, vol.2, no.3 (2005), you'll find an article on just that. It says the standard final gearing for a 60/2 solo is 25:8, for s/car 26:6. Towards the bottom of your speedo dial is a number. For solo 25:8 gearing, the corresponding number is W=1.2, for 26:6 the number is missing and no other postwar BMW to 1969 in the article has the same gearing, so you'll have to work it out.

Good luck, Charlie.
75/7+ offroad sidecar; 50/2; R 35; XR125V; XR200A; Solex; 1939 Hillman Minx DHC.

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schrader7032
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Re: Speedometer error

Post by schrader7032 »

If you can find some sections of measured miles on roads near you, run through 2-3 miles while timing yourself, then do the math. Mostly I find these sections on freeways so that doesn't help with slower speeds. ;)
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.

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Rodolfo850
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Re: Speedometer error

Post by Rodolfo850 »

Use smart phone GPS velocimeter APP...

weh8127
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Re: Speedometer error

Post by weh8127 »

"If you look up the archived magazine of the BMW VMCA, vol.2, no.3 (2005), you'll find an article on just that. It says the standard final gearing for a 60/2 solo is 25:8, for s/car 26:6. Towards the bottom of your speedo dial is a number. For solo 25:8 gearing, the corresponding number is W=1.2, for 26:6 the number is missing and no other postwar BMW to 1969 in the article has the same gearing, so you'll have to work it out."

Charlie: Let me see if I have this right. My R60/2 speedo ratio with 25/8 drive is 1.2. With a 27/7 drive it would have be 1.44. So, when installing the 27/7 final drive and keeping the W=1.2, the speedometer will display a 17% error (1.2 divided by 1.44 = .83 or 17%). So, if my actual speed is 50mph my speedometer will read 58.5 (50 x 1.17 = 58.5) Do I have it right? Many thanks for pointing me in the right direction.
Bill Husted
Barre, MA USA
1963 R60/2 w/ 1955 Steib S500
1973 R75/5

cwf
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Re: Speedometer error

Post by cwf »

The W numbers are only to one or two decimal places, so might not be very accurate. That article describes how to measure off a distance (1/100 of a mile), disconnect the speedo and attach a marker to the cable, push the bike over the distance counting the revolutions and work it out. Bit of a faff but it's accurate.

Is your final drive 27:7 ? On the 2005 list this is for an R 60 s/car (W=1.44), R 60/2 s/car has 26:6.

I think these speedos work by a revolving magnet dragging a cup around it, to which the needle is attached. Simple and accurate enough but if you want better accuracy, you'd better use a digital bicycle one and then start worrying about the circumference of your tyre. Have fun.
75/7+ offroad sidecar; 50/2; R 35; XR125V; XR200A; Solex; 1939 Hillman Minx DHC.

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niall4473
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Re: Speedometer error

Post by niall4473 »

The MPH speedometer head to go with the 26:6 rear drive is W=1.6.
Oil is always cheaper than metal

weh8127
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Re: Speedometer error

Post by weh8127 »

Right, but I want to use the 27/7 drive which is W-1.44.
Bill Husted
Barre, MA USA
1963 R60/2 w/ 1955 Steib S500
1973 R75/5

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wa1nca
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Re: Speedometer error

Post by wa1nca »

I bought a wireless bicycle speedometer and it didn't work well over 20 mph
I will buy a none wireless one this spring and will test that
tire size doesn't matter as you can calibrate to match distance traveled in relation to the magnetic sensor placed on the wheel spoke
Cost 20-30 dollars
Tommy
Tommy Byrnes
54 R51/3, 55 R50/Velorex 560 sidecar, 64 R27, 68 R69US, 75 R75/6
Ashfield, Ma
USA

Daves79x
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Re: Speedometer error

Post by Daves79x »

All this is why I'm going with a sidecar transmission on my R69 with TR500. Also helps that my early R69 has that transmission anyway. It just sucks that the correct speedometers are NLA for the various sidecar final drives. I must say though that the correct ratio NOS speedos are still available for the 27/7 final drives, but they are in KM.

Dave
Dave

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