Stewart Speedo Problem

[drjefb@tpg.com.au]

I live in Melbourne, Australia and have a lovely 1914 Speedster.

I have fitted a Stewart Model 100 speedo and swivel and gears (60 / 16 teeth) on the front wheel. I have serviced both the speedo and the swivel using the MTFCA book Speedometers as a resource. The speed drum rotates freely and smoothly and returns to zero. The odometer registers correctly and shows 1 mile travelled after 1009 revolutions of the input.

Unfortunately, when installed, spinning the road wheel produces no speed on the speedo.

I have found that the swivel ratio is 4:1 when it should be 2.5:1 and I think I can fix this by obtaining the correct swivel from a friend locally.

However the speedo head is the main problem. Under bench test using a drill input, to produce 5mph, the rotation speed of the input must be 225rpm, instead of 84 rpm which I would have expected (based on 1009revs for a mile at 60mph).

I suspect the revolving magnet may be weak.

Have I missed another explanation? Is there a way of testing the magnet? Is there a clever solution to my problem?

Jeff Brown

[Russ_Furstnow]

Jeff, The hair spring may be too strong or the magnetic drum may be too weak. Those are the only things that would make a magnetic speedometer not register the correct speed. The 4:1 swivel is also a problem and you should get the 2.5:1 RH swivel from your friend and try your speedometer with that swivel. Good luck, Russ Furstnow

[drjefb@tpg.com.au]

Hi Russ

Thanks for your reply. Your comment about the spring gave me an idea. My working speedo is made of a body, frame, mechanism and magnetic cylinder from one speedo, but the speed drum and hair spring mechanism are from another speedo. So maybe the hair spring and magnet are not matched because they are from different speedos. This combination was necessary because the first speedo had a broken slender spindle shaft, just below the spring.

Can I buy a replacement shaft?

Is there a way of reducing the strength of the hair spring?

Jeff

[Russ_Furstnow]

Jeff,
You are correct in that the spring and magnet were matched at the factory. I don't know if a spring can be modified and the shaft is not available or reproduced. Why not take the spring from the first speedometer and install it on the second speed cup? Russ Furstnow

[drjefb@tpg.com.au]

Hi Russ

Thanks for your note

I am daunted by the job of handling such delicate and small items.

Are there guidelines on how to change the hairspring?

Jeff

[Russ_Furstnow]

Jeff,
I've never seen any guidelines or how-to's on removing the hair spring. Just gently pry it off with a couple of small screwdrivers and reinstall. The correct adjustment for the hair spring is to have the speed cup return to zero with VERY little tension. I hope this helps, Russ Furstnow

[drjefb@tpg.com.au]

Hi Russ

Thanks again for your reply.

I managed to swap the hair springs. In fact I became so confident that I removed and replaced it several times to fine adjust the speedo's performance.

Your point about setting the hair spring 'zero' load to be very light, was the key to problem. I found the performance of the speedo is extremely sensitive to this setting - it is by far the most important variable to optimise.

Jeff