Can speedo be calibrated?
Thread Starter
Race Director
Joined: Sep 1999
Posts: 11,361
Likes: 383
From: Plano TX
Update:
I got an optical tach so I can measure the rpm of my drill. I found that the spring adjustment would not do the trick, it still read low and would not return to zero if I changed the adjustment.
So the problem was the magnets have demagnetized over the last 42 years. I don't have the equipment to remagnetize but found a way. I butchered a junk hard drive and extracted the neodymium magnet. It's flat and very thin. I broke it into pieces and stuck 2 pieces on the speedo magnet (#1 in willcox pic above). First attempt made it read way high, those little neodymium magnets are strong! Used smaller pieces and right now it reads correct at 60mph and 30mph.
I'm going to let it sit for a few hours and check again. Want to make sure the mag field doesn't get stronger (magnetizing the anchor).
I got an optical tach so I can measure the rpm of my drill. I found that the spring adjustment would not do the trick, it still read low and would not return to zero if I changed the adjustment.
So the problem was the magnets have demagnetized over the last 42 years. I don't have the equipment to remagnetize but found a way. I butchered a junk hard drive and extracted the neodymium magnet. It's flat and very thin. I broke it into pieces and stuck 2 pieces on the speedo magnet (#1 in willcox pic above). First attempt made it read way high, those little neodymium magnets are strong! Used smaller pieces and right now it reads correct at 60mph and 30mph.
I'm going to let it sit for a few hours and check again. Want to make sure the mag field doesn't get stronger (magnetizing the anchor).
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Joined: Sep 2003
Posts: 25,393
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Update:
I got an optical tach so I can measure the rpm of my drill. I found that the spring adjustment would not do the trick, it still read low and would not return to zero if I changed the adjustment.
So the problem was the magnets have demagnetized over the last 42 years. I don't have the equipment to remagnetize but found a way. I butchered a junk hard drive and extracted the neodymium magnet. It's flat and very thin. I broke it into pieces and stuck 2 pieces on the speedo magnet (#1 in willcox pic above). First attempt made it read way high, those little neodymium magnets are strong! Used smaller pieces and right now it reads correct at 60mph and 30mph.
I'm going to let it sit for a few hours and check again. Want to make sure the mag field doesn't get stronger (magnetizing the anchor).
I got an optical tach so I can measure the rpm of my drill. I found that the spring adjustment would not do the trick, it still read low and would not return to zero if I changed the adjustment.
So the problem was the magnets have demagnetized over the last 42 years. I don't have the equipment to remagnetize but found a way. I butchered a junk hard drive and extracted the neodymium magnet. It's flat and very thin. I broke it into pieces and stuck 2 pieces on the speedo magnet (#1 in willcox pic above). First attempt made it read way high, those little neodymium magnets are strong! Used smaller pieces and right now it reads correct at 60mph and 30mph.
I'm going to let it sit for a few hours and check again. Want to make sure the mag field doesn't get stronger (magnetizing the anchor).
Thread Starter
Race Director
Joined: Sep 1999
Posts: 11,361
Likes: 383
From: Plano TX
More experimentation:
By moving my magnets closer to the center of the anchor I can decrease the force on the speed cup. I can then adjust (loosen) the spring to still show correctly @ 60 mph. But then it reads higher @ 30mph.
So this makes it possible to calibrate the linearity. Strong magnet & spring = lower readings at low speed, higher at high speeds (a speed increase moves the needle further).
I have verified the speed at 20, 30 and 60 mph and have it within 0-2 mph high. I think I'll call it good. Unfortunately my drill maxes out at 1080 rpm so I can't check higher speeds but this should be close enough.
Had a hard time making the camera focus on the right spot, but you can see the magnets:
By moving my magnets closer to the center of the anchor I can decrease the force on the speed cup. I can then adjust (loosen) the spring to still show correctly @ 60 mph. But then it reads higher @ 30mph.
So this makes it possible to calibrate the linearity. Strong magnet & spring = lower readings at low speed, higher at high speeds (a speed increase moves the needle further).
I have verified the speed at 20, 30 and 60 mph and have it within 0-2 mph high. I think I'll call it good. Unfortunately my drill maxes out at 1080 rpm so I can't check higher speeds but this should be close enough.
Had a hard time making the camera focus on the right spot, but you can see the magnets:
Last edited by zwede; Oct 7, 2012 at 02:24 PM.
Thread Starter
Race Director
Joined: Sep 1999
Posts: 11,361
Likes: 383
From: Plano TX
That would mean your magnet is too strong. You can take a neodymium magnet and rub it on the anchor and it will demagnetize it. But if you go too far you'll have to do what I did and add magnets to it.
Once you have weakened the magnet you would loosen the spring to make it match up at high speed. The combination of weaker magnet & spring will make it read higher at low speed.
Once you have weakened the magnet you would loosen the spring to make it match up at high speed. The combination of weaker magnet & spring will make it read higher at low speed.
