02-09-2020, 09:02 PM
(This post was last modified: 02-09-2020, 09:11 PM by Parazine.
Edit Reason: additional info
)
This is the same speedo problem that I encountered with my Chummy. The speedo is (what I call, for obvious reasons) an "A" type, fitted up to early 1927 and it was originally driven by the propshaft via a pulley system. It is geared for 3360 TPM.
The later 3 speed cars used a speedo directly driven from the gearbox, usually a Smiths "PA" type. This was geared for 2240 TPM.
The four speed gearbox drives a "Magmo" or a needle speedometer, these are geared for 1040 TPM (1060 for later types with a different axle ratio).
You will see that driving a speedo geared for 3360 TPM with gears delivering 1040 TPM will incur a reading of about 1/3 of the true speed.
Your solution?
Special gears were made at one time for the 4 speed box but these were aimed at driving a PA I think. Vince Leek will know more....
You could mount a later speedometer but it won't have "Vintage" looks.
You can use a "speed up" gearbox in line with a ratio of 3360/1040 = 3.23:1 but you might have to get someone to make it up specially. (££££)
I believe Willy McKenzie had some Smiths faces made that turn the "A" type into a 100 MPH speedo and put the figures in roughly the right place for you. Might be worth a call....
I use a three speed box on my Chummy, so I needed an "A" type speedo with 2240 TPM gearing so I built a "special", using a 2240 TPM governor from a PN speedo. It was a bit of a jiggle, but works well.
The later 3 speed cars used a speedo directly driven from the gearbox, usually a Smiths "PA" type. This was geared for 2240 TPM.
The four speed gearbox drives a "Magmo" or a needle speedometer, these are geared for 1040 TPM (1060 for later types with a different axle ratio).
You will see that driving a speedo geared for 3360 TPM with gears delivering 1040 TPM will incur a reading of about 1/3 of the true speed.
Your solution?
Special gears were made at one time for the 4 speed box but these were aimed at driving a PA I think. Vince Leek will know more....
You could mount a later speedometer but it won't have "Vintage" looks.
You can use a "speed up" gearbox in line with a ratio of 3360/1040 = 3.23:1 but you might have to get someone to make it up specially. (££££)
I believe Willy McKenzie had some Smiths faces made that turn the "A" type into a 100 MPH speedo and put the figures in roughly the right place for you. Might be worth a call....
I use a three speed box on my Chummy, so I needed an "A" type speedo with 2240 TPM gearing so I built a "special", using a 2240 TPM governor from a PN speedo. It was a bit of a jiggle, but works well.