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Speedo Query

brianhicks

Joined: 5-05-01

Topics: 4

Replies: 12

Posted: Sun 23rd Oct 2011, 11:23am
Speedo Query

I have just found out that my engine wheels are9.6 inch in radius and not 8.5 inch. I have calculated my speedo head for the 8.5 figure. Have I been going faster or slower than I thought and if so by how much. No doubt there will be a Scotsman somewhere who will put me right!
 

Replies To This Post

Peter Beevers

Joined: 9-10-01

Topics: 3

Replies: 134

Posted: Sun 23rd Oct 2011, 12:26pm

Brian,
I think you're confusing radius and diameter - I can't believe your wheels are 9.6" radius!!!
So, assuming you mean 9.6" diameter against 8.5" diameter, your speedo will read 1.129 times too slow. So, when it reads 8mph, you will, in reality, be doing 9 mph.
Regards
Peter
 

brianhicks

Joined: 5-05-01

Topics: 4

Replies: 12

Posted: Sun 23rd Oct 2011, 5:17pm

Thanks Peter, You now know why I need help! I've taken the wheels off Captain Hook as the flanges were getting thin and was able to check the radius accurately for the first time. Not being an engineer but only an enthusiast I am always grateful to guys like you.
 

Peter Beevers

Joined: 9-10-01

Topics: 3

Replies: 134

Posted: Sun 23rd Oct 2011, 7:12pm

Brian, A pleasure to have helped you.
Regards
Peter
 

Xz

Joined: 1-01-92

Topics: 17

Replies: 244

Posted: Sun 23rd Oct 2011, 10:09pm

I have found the most accurate way of finding the wheel circumference (which Halfords speedos require, not Dia or Radius) is to mark the rim of the wheel and the track at the same point, and roll the wheel several full rotations and then measure the overall distance and divide by the number of turns. The more turns the greater the accuracy, but four or five is plenty.
 

brianhicks

Joined: 5-05-01

Topics: 4

Replies: 12

Posted: Mon 24th Oct 2011, 12:34pm

It occurs to me that I may have information that could be of benifit to others who like me have bought a Sigma Sport speedo on their locomotive. All the examples for calibrating the speedo head with the requisite set of figures are of course for cycle tyre sizes aand the smallest are for 16 inch wheels. Some users are probably dividing the speeds on the head by 2 or even 3. I rang the makers in the States and a very helpful guy said "No problem" when I told him (as I thought at the time !) my wheels were 8 and a half inch diameter He told me the calculation for MPH was 8.5 x25.4 x 3.14divided by 1.61= 421.07204 ie 0421 to be put into the speedo head> A 9 inch wheel would be 9.0x25.4 x 3.14 divided by1.61= 445.84099 so 0445 into the head. I understand the 1.61 division is Kilometers per mile for miles. No doubt better brains than mine will be able to say what the 25.4 and 3.14 are!!Come on Peter what do they mean?!!I'm relying on you!
 

Peter Beevers

Joined: 9-10-01

Topics: 3

Replies: 134

Posted: Mon 24th Oct 2011, 1:27pm

Brian,
multiplying by 25.4 changes the unit from inches to millimetres. 3.14 is an approximation of pi (Circumference of a circle = pi x d). So 8.5*25.4*3.14 is the circumference of the wheel in millimetres.
 

Xz

Joined: 1-01-92

Topics: 17

Replies: 244

Posted: Mon 24th Oct 2011, 7:14pm

I rest my case M'lud! Lol.
 

brianhicks

Joined: 5-05-01

Topics: 4

Replies: 12

Posted: Mon 24th Oct 2011, 7:38pm

Hi Peter. I knew you wouldn't let me down! And thanks Xz. I appreciate your humour. It makes the world go round and hopefully my wheels when I get them back!! Regards Brian.
 
 
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