AC Speedometer calibration mid 20's Buick

[Hubert_25-25]

I bought a $20 RPM indicator off Amazon to see if it will help with my speedometer calibration.  I made a cardboard wheel for the end of the speedometer cable and installed a reflector strip on the wheel.  My friend drove the car because the first matter of business is to determine the cable RPM relative to the speedometer input.  He held the speed of the car constant and we used my phone APP GPS as our speed indicator.  I used an Acco clip and a small piece of running board material as a shelf for my smart phone to sit on the dashboard.   We achieved the following results.

520 RPM = 30 MPH

600 RPM = 35 MPH

680 RPM = 40 MPH

760 RPM = 45 MPH 

So every 5 MPH is an additional 80 RPM.  This method also does correct for my slightly larger than stock tires.  The original size is no longer available.

Then I put 2 holes in a piece of angle Iron to hold my speedometer backing plate in the bench vise.  I put a piece of reflective tape on the drill chuck and I can run the drill at the same speed as the input from the car.  

 

Then I adjusted the spring tension that is located above the speed wheel to have the speedometer read 40 MPH with the drill at 680 RPM.   So far so good.  I think that I could also calibrate the speedometer if I just left the bezel, face, and glass off during a trial run using my GPS.  I think that is my next step before I secure the glass to see how close I came to the target, and I can still make any necessary adjustments before sealing the bezel.  

 

  The best part is that we put 45 miles on the car and gave it a good run for the afternoon.  

This is a link to the RPM tool.  Kind of handy.     Hugh

https://www.amazon.com/gp/product/B001N4QY66/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1

 

[ply33]

Hmmm. 5 MPH is 80 RPM? At 5 MPH it will take 12 minutes to go a mile. So 12 min/mi * 80 rev/min = 960 rev/mi.

 

Back in '73 when I first got my '33 Plymouth the speedo was way off and my clumsy attempt to adjust resulted in a broken speedometer. I took it to a local automotive speedometer repair shop (there were such things in those days) and a week or so later it was repaired. I asked the old fellow who owned and ran the shop about calibration, if it needed to be done on the car, etc. He said no, that all American cars used a standard of 1000 turns per mile on the cable. I checked the manual for my car and once I did the math saw that the transmission gearing for the speedometer table and the optional wheel/tire sizes worked out to pretty close 1000 turns per mile for the different options available from the factory.

 

Sounds like your mid-20’s Buick fits the pattern as you are only 4% off. Some of which is, as you noted, due to your slightly larger than stock tires.

[Morgan Wright]

That's great, I did not know you could adjust the spring tension on these things, that's good information for when I get my cable in the mail and calibrate mine.

 

Funny your speedo maxes out at 75, mine maxes out at 60.

[27donb]

2 hours ago, Morgan Wright said:

 

Funny your speedo maxes out at 75, mine maxes out at 60.

 

And for 1927,  80.

[Hubert_25-25]

I found this article in a google search.  This is from "Motor Age" dated Dec 22, 1921.  It heralds the new AC speedometer.  

Next is a photo of my test mule for the speedometer with a pointer made so I can work without the face plate on the speedometer.  I am using my worst looking speed wheel at the moment.  I did locate the decals for Chevrolet to 80 MPH (same AC unit as Buick for 1927).   I also found a Stewart Warner wheel decal for 75 MPH, but not sure if this fit is correct for the AC speed wheel yet.  I JB welded together an old speed wheel yoke and put it in the car.  It is running about 13 MPH slow relative to the GPS.  I noticed that the growth of the original repaired yoke does not allow the speed wheel to adjust lower than mid way in the adjustment slots and therefore does not drop deep into the spinning magnetic spider.  The second attempt at a 3D printed speed wheel yoke arrived so I put that in the mule.  Since it is mainly a copy it had similar dimensions to the repaired one.  I had to grind the bottom legs to get it to drop lower on the adjustment scale.  

At 430 RPM I was reading 29 MPH at the top of the adjustment slot, and 34 MPH if I moved it to the bottom of the slot.  According to the early cable readings, 460 RPM should be about 20 MPH.  I also understand that not having the housing on the back (and maybe even the face plate on the front) will effect the magnetic field.  I need to do more testing to find out.     Hugh

Edited December 16, 2019 by Hubert_25-25 (see edit history)

[Mark Kikta]

Great information Hugh!!!

[27donb]

My speedometers are not accurate. 

 

I was happy to get one cobbled together for each car, that will show mph when moving (not accurate) and will accumulate milage. 

 

Perhaps my magnets are weak.

 

I found if I move the mph drum up or down, the change is not that great.  If I lessen the return spring tension on the mph drum, I can get a better speed reading out of the drum, or it allows the drum to move farther, making it more accurate, but once in the car the real life experience is different than the bench experiment.  With the car vibration, road bumps, and any cable fluctuations cause the mph drum to bounce wildly all over the place, not giving a steady reading.

 

From experience, it sucks to spend so much time on this instrument, get it back in the car without scratching the dash too much, only to go out for a test drive and look down to see I am going 10 no 35 no 40 no 25 no 15 mph as I hit each bump in the road.   Turns out that return spring is very important to dampen the mph drum.  Don't take too much tension off it.

 

An instrument restoration company restored a speedometer for my touring car, accomplished by the previous owner, at a cost close to $1000 I was told.   They abandoned the horseshoe magnet and epoxied new magnets to the spinning wheel within the mph drum.   It was inaccurate, and the mph drum bounced uncontrollably.  I put original spare parts back in and it functions better and the drum is fairly stable, is just reads about 5 mph too slow, as all mine do, as checked with a GPS app on my phone. 

Edited December 16, 2019 by 27donb (see edit history)

[Crazyfamily]

Hugh as far as calibration, in the ad would their statement help at least on the bench?

 

 

[dibarlaw]

Hugh:

 It is funny that your repaired yoke's sweet spot for operation produced the same results as what my repaired (JB weld) AC unit did. 13 MPH lower.

 All I used to check mine was the 25 MPH traffic sensor sign near my neighborhood. (Sign indicated speed of 28 MPH. The speedometer showed 15 MPH.) Again, the one that came in my car was spot on up to 30 mph when binding on the drum caused it to go crazy. That yoke had an old repair that was epoxied in place and was missing one leg.

 Another point to take into consideration is that the permanent horseshoe magnet may loose it's magnetic strength over the years.

 When I was restoring some early Briggs and Stratton engines and early 1920s horn type radio speakers the magnets for the ignition on the Briggs and the driver on the speakers had to be "recharged".

[dibarlaw]

Don: 

It see great minds think alike! I was searching for my photo while you made your weak magnet comment. So that is your point....

[Hubert_25-25]

So I have been making a little progress with my test mule.  I have a variable speed drill with the trigger held with a velcro strap.  I have the drill on a foot pedal to make shutting it on and off easier.  I have a spare speedometer cable.  When it is on, I am turning 530 RPM.  If the bucket is off the speedometer, it reads 34 MPH.  Once I put the bucket on, the most I can get out of the speedometer is 19 MPH and with the speed wheel as low into the armature as I can set it.  I grabbed another backing (I have 5).  I put the speed wheel in it, and not all the way down, put it in the bucket, and it reads 30 MPH.  It should read 30 MPH at 504 RPM per the 1921 posting as William pointed out.  That is pretty close and I did not do a lot of tweaking yet, just happy to be in the ball park for once.  Don, Your point is well taken about the spring.  The wheel is still returning to 0 when the drill is off, so it is close.  

So how do I check the power of my magnets?  Obviously I have  at least one that is low.   Can I recharge them?  How would I know the amount of charge to add.?  Are there other good options.  Can I just add magnets to what I have rather than putting them in different places as they did with Don's speedometer?

 

This is so Pre War Buick.  It's not just one part that is broken, but multiple parts as you dig into it.  No wonder Buick chose AC as the supplier.  More equal minds thinking alike.  

[michealbernal]

Hubert 25-25 yes you can recharge the magnets.  People who restore the antique hit and miss single cylinder engines restore magnets on magnetos frequently.  Magnet chargers are for sale on ebay from time to time.  You could google magneto restoration services and contact a magneto restorer for info on prices,  It is usually not very expensive.

[Hubert_25-25]

A lot of progress on the AC speedometers.  I have received both styles of 3D printed speedwheel yokes, this time printed in aluminum.  They are about twice the cost of glass filled nylon.  In all honesty they really work the same but an even more original looking part.  Both should last another century or more.  To me, the best approach is to replace the yoke that holds the speedwheel rather than repair them.  These should not continue to grow like the original pot metal ones.  After these are printed, I do have to tap all the holes - and all the holes are special taps of course.   Then I mount them in a fixture and "line bore them" in my drill press to get the bearing fit correct.  The photo shows the 2 style speedwheel yokes in aluminum.   One photo shows the glass filled nylon speed wheel yoke (white).    

Necessity number 2 is to have the magnet "remagnetized" by a person that recharges magnetos.  Thank you for the suggestion as I was not thinking that this would be another problem that would need to be addressed.  Not many places would remagnetize them either.  Of my original magnets, Only 1 of the 5 magnets was close to having enough strength, and the others did not have enough magnetism left that the calibration kept them behind by 13 mph or more.      

So I was able to build 2 speedometers and both calibrated when clamped to the vise and checked with the hand held tachometer.  Then I put them in the car and checked them with my GPS.  They were right on.   The photo with the green dashboard is a photo taken on an open stretch of road so it is hard to know that we are moving down the road.   It's nice to have an odometer now too.  

Now I have to put the face plate and cover on.  I hope not to have issues.  The covers do not come off easily so I expect nothing less frustrating than putting them back on.     Hugh

   

Edited February 17, 2020 by Hubert_25-25 (see edit history)

[Crazyfamily]

9 minutes ago, Hubert_25-25 said:

A lot of progress on the AC speedometers.  I have received both styles of 3D printed speedwheel yokes, this time printed in aluminum.  They are about twice the cost of glass filled nylon.  In all honesty they really work the same but an even more original looking part.  Both should last another century.  To me, the best approach is to replace the yoke that holds the speedwheel rather than repair them.  These should not continue to grow like the original pot metal ones.  After these are printed, I do have to tap all the holes - and all the holes are special taps of course.   The photo shows the 2 style speedwheel yokes in aluminum.   

Necessity number 2 is to have the magnet "remagnetized" by a person that recharges magnetos.  Thank you for the suggestion as I was not thinking that this would be another problem that would need to be addressed.  Not many places would remagnetize them either.  Of my original magnets, Only 1 of the 5 magnets was close to having enough strength, and the others did not have enough magnetism left that the calibration kept them behind by 13 mph or more.      

So I was able to build 2 speedometers and both calibrated when clamped to the vise and checked with the hand held tachometer.  Then I put them in the car and checked them with my GPS.  They were right on.   The photo with the green dashboard is a photo taken on an open stretch of road so it is hard to know that we are moving down the road.   It's nice to have an odometer now too.  

Now I have to put the face plate and cover on.  I hope not to have issues.  The covers do not come off easily so I expect nothing less frustrating than putting them back on.     Hugh

   

Shoot me a text tomorrow and let me know if we are good so I can finalize everything and settle up with Ron on all his hard work for us Hugh.

 

Ill get my parts to you as well.

[bradsan]

Hugh

 

That new yoke looks awesome!

As far as the magnets go, I can't imagine that remagentising is any different than the process for the magnets on a Model T magneto.

Lots of info for DIY magnet chargers for Model T's which have the same u-shaped magnet , just bit more hairspring shaped than the U shaped speedo magnets.  Check the MTFCA website and forum. Some guys even redo the magnets on the car by pulling the hogshead.

I made one and did our Model T ( while it was apart !) . The tool is currently at the other shop or I'd send you a photo but, as I recall , it wasn't much more that a couple of 4" pieces of 1" dia steel rod and some wire.  You make two electromagnetic coils with opposite windings  so when you tape them together side by side you get opposite polarity  at the adjacent 1" dia ends .  Figure out which end of your magnet is N/S  with a cheap compass. Same for your coll set up . Make sure you mark it with felt because it does get confusing...trust me!. Remember opposites attract so you want N to S between the the two magnet poles and your coil ends . Use a 12V car battery to apply some current to your coils. while each pole of the magnet is in contact with the appropriate end of each coil  You just have to be judicious with the length of time you have the current flowing; it doesn't take much to over heat it!

 

I'll bet it will be more than enough to restore function!

 

If you can't find anything online , let me know

 

Brad